Understanding Neutron Radiography Reading VIII Part 2(a)of 2 16th August 2016 Post Exam Reading
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Spallation Source
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Spallation Source
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Spallation Source
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http://www.gizmodo.com.au/2014/01/27-amazing-images-from-the-depths-of-scientific-labs/
Spallation Source
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http://www.gizmodo.com.au/2014/01/27-amazing-images-from-the-depths-of-scientific-labs/
The Magical Book of Neutron Radiography
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数字签名 者：Fion Zhang DN：cn=Fion Zhang, o=Technical, ou=Academic, email=fion_zhang @qq.com, c=CN 日期：2016.08.19 23:29:54 +08'00' Charlie Chong/ Fion Zhang
ASNT Certification Guide NDT Level III / PdM Level III NR - Neutron Radiographic Testing Length: 4 hours Questions: 135 1. Principles/Theory • Nature of penetrating radiation • Interaction between penetrating radiation and matter • Neutron radiography imaging • Radiometry 2. Equipment/Materials • Sources of neutrons • Radiation detectors • Non-imaging devices
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3. Techniques/Calibrations
• Electron emission radiography
• Blocking and filtering
• Micro-radiography
• Multifilm technique
• Laminography (tomography)
• Enlargement and projection
• Control of diffraction effects
• Stereoradiography
• Panoramic exposures
• Triangulation methods
• Gaging
• Autoradiography
• Real time imaging
• Flash Radiography
• Image analysis techniques
• In-motion radiography • Fluoroscopy
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4. Interpretation/Evaluation • Image-object relationships • Material considerations • Codes, standards, and specifications 5. Procedures • Imaging considerations • Film processing • Viewing of radiographs • Judging radiographic quality 6. Safety and Health • Exposure hazards • Methods of controlling radiation exposure • Operation and emergency procedures Reference Catalog Number NDT Handbook, Third Edition: Volume 4, Radiographic Testing 144 ASM Handbook Vol. 17, NDE and QC 105 Charlie Chong/ Fion Zhang
Fion Zhang at Copenhagen Harbor 16th August 2016
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SME- Subject Matter Expert http://cn.bing.com/videos/search?q=Walter+Lewin&FORM=HDRSC3 https://www.youtube.com/channel/UCiEHVhv0SBMpP75JbzJShqw
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Gamma- Radiography TABLE 1. Characteristics of three isotope sources commonly used for radiography. Source
T½
Energy
HVL Pb
HVL Fe
Specific Activity
Dose rate*
Co60
5.3 year
1.17, 1.33 MeV
12.5mm
22.1mm
50 Cig-1
1.37011
Cs137
30 years
0.66 MeV
6.4mm
17.2mm
25 Cig-1
0.38184
Ir192
75 days
0.14 ~ 1.2 MeV (Aver. 0.34 MeV)
4.8mm
?
350 Cig-1
0.59163
Th232
Dose rate* Rem/hr at one meter per curie
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0.068376
八千里路云和月
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闭门练功
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http://greekhouseoffonts.com/
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Whole Chapter 5 Radiation Measurement
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PART 1. Principles of Radiation Measurement Emissions from radioactive nuclei and radiation from that portion of the electromagnetic spectrum beyond the ultraviolet energies can cause the ionization of atoms and molecules. Ionizing radiation occurs as three forms: (1) charged particles such as alpha particles, beta particles and protons, (2) uncharged particles such as neutrons and (3) electromagnetic radiation in the form of X-rays and gamma rays.
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Radiation Detection Systems Some forms of radiation, such as light and heat, can be detected by human sense organs; ionizing radiation, however, can be detected only by the after effect of itsionizing properties. If ionizing radiation does not interact with matter, its detection and measurement is impossible. For this reason, the detection process uses substances that respond to radiation, as part of a system for measuring the extent of that response. The ionization process isused by a large class of detection systems, including: ■ ion chambers, ■ proportional chambers, ■ geiger-müller counters and ■ semiconductor devices (Table 1). ■ Some systems depend on the excitation and molecular dissociation (分子离解) that occur with ionization. These processes are useful in (1) scintillation counters and (2) chemical dosimeters. Although other types of detection systems exist, they are not generally used in radiation survey instruments.
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TABLE 1. Effect of detected and measured ionization.
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PART 2. Ion Chambers and Proportional Counters Principles of Ionization The mechanism most widely used in radiation survey applications is the ionization principle: charged particles producing ion pairs by direct interaction. These charged particles may (1) collide with electrons and remove them from their atoms or (2) transfer energy to an electron by the interaction of their electric fields (Fig. 1). If the energy transfer is not sufficient to completely remove an electron, the atom is left in a disturbed or excited state. Gamma and X-ray photons interact with matter mainly by: ■ photoelectric absorption, ■ compton scattering and ■ pair production, each of which produces electrons and ions that may be collected and measured.
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The average energy expended in the creation of an ion pair, in air and most gases, is about 34 eV) . The number of ion pairs produced per unit of path length is called specific ionization. Specific ionization is affected by the energy of the particle or photon by its change and by the nature of the ionized substance.
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FIGURE 1. Ion pair (showing ejected electron and vacancy in electron orbit of atom).
pair 34 eV for Air
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Ionization Chambers In an ionization chamber, an electric field is applied across a volume of gas, between two electrodes. Often the chamber’s geometry is cylindrical, a cylindrical cathode enclosing the gas and an axial, insulated rod anode (Fig. 2). Charged particles, photons or both pass through the chamber and ionize the enclosed gas. When an electric field is applied to the gas, ions drift along the electrical lines of force to produce an ionization current. Under normal conditions, electrons drift at speeds of about 104 m·s–1 (22 000 mi·h–1). The drift velocity of positive ions is many orders of magnitude less. When the electric field is increased slightly from zero and a detector is placed in the constant radiation field the collected ions still will be few in number because many recombine. As the voltage is further increased, recombination yields to ionization, where all ions are collected (Fig. 3).
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FIGURE 2. Basic ionization chamber with high value resistance R and voltage V.
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Electromagnetic Energy Interactions Photons interact with subatomic structures in one of the following three ways: •Photoelectric absorption •Compton Scatter •Pair production The particular type of interaction reflects probability statistics based on both the energy of the photon and the atomic number of the traversed atom. For most tissues of the body, average atomic number does not vary greatly – though cortical bone has the highest effective atomic number.
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-02.html
Photoelectric Absorption An atom completely absorbs a photon, which then disappears; this excess energy provided to the atom ultimately results in ejection of an orbital electron. The ejected electron is known as a photo electron. Electrons have binding energies of orbit, with outer shells having less energy than those closer to the positive nucleus. When the initial orbital vacated is not that of an outer valance electron, the atom remains in a high-energy state until an outer orbital electron shifts to fill its incomplete inner shell. This shift is accompanied by emission of a characteristic X-ray. Photoelectric Absorption is an important interaction for low energy photons (<100 KeV) (<0.1Mev) . Note: <0.34MeV (Exam Q)
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
True or False? The "photo electron" and free radical can interact with other molecules -ultimately leading to ionizations and bond breakage, which are the biologically-important molecular manifestations of radiation damage.
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Compton Scatter Collision of a photon with an electron increases the kinetic energy of the electron. Thus set in motion, the electron is known as a recoil electron. The incident photonâ&#x20AC;&#x2122;s energy is not necessarily depleted, but it will diverge from its path and have lower energy -- i.e., it has a longer wavelength after collision. In diagnostic radiology, such scattered photons may lower contrast and thus degrade quality of the radiographic image. Important interaction for intermediate-energy photons (100 KeV to 10 MeV) Note: 0.34 ~ 1.2 MeV (Exam Q)
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Compton Scatter
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Pair Production Note pair production requires relatively high photon energies that are generally not produced in diagnostic imaging. Photons with quantum energy in excess of 1.02 MeV (usually >10 MeV) may interact with matter to produce a negative electron and its anti-particle (positron). The value of 1.02 MeV equals the combination rest mass energy of an electron and a positron.
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Pair Production 0.51MeV
0.51MeV
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Linear Energy Transfer [LET] LET is the amount of energy transferred to the local environment in the form of ionizations and excitations. Thus, LET indicates the potential for biologically important damage from radiation. Linear Energy Transfer can be thought of in two ways: â&#x20AC;˘ an average energy for a given path length traveled or â&#x20AC;˘ an average path length for a given deposited energy. The standard unit of measure is keV/um.
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Ionization tracts. When particulate or electromagnetic energy penetrates a cell, one or more ionizations will likely take place. While the precise site of interaction is somewhat random, ionizations will distribute along distinct paths. The density of ionizations along a given path relates inversely to kinetic energy of the particle or photon. Thus a decelerating particle produces the greatest number of ionizations just before coming to rest. Comparing particles or photons, it follows also that LET for a gamma ray may be smaller than LET for an x-ray.
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Simulation of various radiation energies passing through a medium â&#x20AC;&#x201C; each hatch mark represents an ionization. The heavy ion is a very high-LET particle; the delta ray represents secondary electrons with sufficient energy to make a separate ionization tract. The 5 keV electron is the typical energy of a secondary electron produced by X-ray photons used in diagnostic imaging. Note that absorption (and attenuation) of a photon beam is related to the atomic number of the impinged mass and inversely related to the energy of the incident photon beam. The medium shown is approximately 200 nm in width â&#x20AC;&#x201C; a DNA double helix width is about 2 nm. (Developed after Cox J.D. and Ang K.K., eds. Radiation Oncology Rationale, Technique, Results. 8th edition. St Louis, MO: Mosby, 2003. p44.)
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Simulation of various radiation energies passing through a medium
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https://www.med-ed.virginia.edu/courses/rad/radbiol/01physics/phys-03-03.html
Sources of Attenuation The attenuation that results due to the interaction between penetrating radiation and matter is not a simple process. A single interaction event between a primary x-ray photon and a particle of matter does not usually result in the photon changing to some other form of energy and effectively disappearing. Several interaction events are usually involved and the total attenuation is the sum of the attenuation due to different types of interactions. These interactions include the photoelectric effect, scattering, and pair production. The figure below shows an approximation of the total absorption coefficient, (Âľ), in red, for iron plotted as a function of radiation energy. The four radiation-matter interactions that contribute to the total absorption are shown in black. The four types of interactions are: photoelectric (PE), Compton scattering (C), pair production (PP), and Thomson or Rayleigh scattering (R). Since most industrial radiography is done in the 0.1 to 1.5 MeV range, it can be seen from the plot that photoelectric and Compton scattering account for the majority of attenuation encountered.
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https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuation.htm
Total Absorption Coefficient, (µ),
µ
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photoelectric (PE), Compton scattering (C), pair production (PP), and Thomson or Rayleigh scattering (R).
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Coherent scattering (also known as unmodified, classical or elastic scattering) is one of three forms of photon interaction which occurs when the energy of the X-ray or gamma photon is small in relation to the ionisation energy of the atom. It therefore occurs with low energy radiation. Upon interacting with the attenuating medium, the photon does not have enough energy to liberate the electron from its bound state (i.e. the photon energy is well below the binding energy of the electron) so no energy transfer occurs. The only change is a change of direction (scatter) of the photon, hence 'unmodified' scatter. Coherent scattering is not a major interaction process encountered in radiography at the energies normally used. Coherent scattering varies with the atomic number of absorber (Z) and incident photon energy (E) by Z2 / E.
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http://radiopaedia.org/articles/coherent-scattering
Charlie Chong/ Fion Zhang
http://radiopaedia.org/articles/coherent-scattering
Photoelectric effect, or photoelectric absorption (PEA) is a form of interaction of X-ray or gamma photon with the matter. A low energy photon interacts with the electron in the atom and removes it from its shell. The probability of this effect is maximum when: ď Ž the energy of the incident photon is equal to or just greater than the binding energy of the electron in its shell ('absorption edge') and ď Ž the electron is tightly bound (as in K shell)
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http://radiopaedia.org/articles/photoelectric-effect
The electron that is removed is then called a photoelectron. The incident photon is completely absorbed in the process. Hence it forms one of the reason for attenuation of X-ray beam as it passes through the matter. PEA is related to the atomic number of the attenuating medium (Z), the energy of the incident photon (E) and the physical density of the attenuating medium (p) by: Z³ p / E³. Therefore, if Z doubles, PEA will increase by a factor of 8 (because 2³ is 8) and if E doubles, PEA will reduce by 8. As small changes in Z can have quite profound changes in PEA this has practical implications in the field of radiation protection and is why materials with a high Z such as lead (Z = 82) are useful shielding materials.
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http://radiopaedia.org/articles/photoelectric-effect
The incident photon is completely absorbed in the process.
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Photoelectric effect
ABSORBED Charlie Chong/ Fion Zhang
Completely Heroes
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Compton effect or Compton scatter is one of three principle forms of photon interaction. It is the main cause of scattered radiation in a material. It occurs due to the interaction of the X-ray or gamma photon with the outermost (and hence loosely bound) valence electron at the atomic level. The resultant incident photon gets scattered (changes direction) as well as ejects the electron (recoil electron), which further ionizes other atoms. Therefore the Compton effect is a partial absorption process and as the original photon has lost energy, this is known as Compton shift (the shift being a shift of wavelength/frequency). Probability of Compton effect: directly proportional to • number of outer shell electrons, i.e. the electron density • physical density of material inversely proportional to • photon energy does not depend on • atomic number (unlike photoelectric effect and pair production)
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http://radiopaedia.org/articles/compton-effect
History and etymology Named after Professor Arthur Holly Compton (18921962), US physicist, who was awarded the Nobel Prize in Physics in 1927 for his discovery of Compton effect. Charlie Chong/ Fion Zhang
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Pair production (PP), like the photoelectric effect, results in the complete attenuation of the incident photon. Pair production can only occur if the incident photon energy is at least 1.022 MeV. As the photon interacts with the strong electric field around the nucleus it undergoes a change of state and is transformed into two particles (essentially creating matter from energy): â&#x2013; one electron â&#x2013; one positron (antimatter equivalent of the electron) These two particles form the pair referred to in the name of the process. It is noteworthy that other 'pairs' of leptons (of which the electron is a type) can be created such as muon - antimuon and tau - antitau pairs, however the type of lepton pair would dictate the energy of the incident photon necessary to create them as both have far higher resting energy masses (1776 MeV for the tau and 105 MeV for the muon) than the electron and positron.
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http://radiopaedia.org/articles/pair-production
The reason at least 1.022 MeV of photon energy is necessary is because the resting mass (using E=MC² ) of the electron and positron expressed in units of energy is 0.511 MeV (or 9.1 x 10-31 kg) each, therefore unless there is at least 0.511 MeV *2 (i.e., 1.022 MeV) it is not possible for the electron-positron pair to be created. If the energy of the incident photon is greater than 1.022 MeV, the excess is shared (although not always equally) between the electron and positron as kinetic energy. PP is related to the atomic number (Z) of attenuator, incident photon energy (E) and physical density (p) by Z E p.
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http://radiopaedia.org/articles/pair-production
The electron and positron, once liberated within the medium are dissipated through successive interactions within the medium. The electron is quickly absorbed, however the fate of the positron is not so straight forward. As it comes to a rest, it combines with a neighbouring electron and the two particles neutralise each other in a phenomenon known as annihilation radiation. Here, the two particles are converted back into two photons of electromagnetic radiation, each of 0.511 MeV energy travelling at 180 degrees to each other (a concept utilised in positron emission tomography PET). These photons are then absorbed or scattered within the medium. Pair production in reality does not become the dominant process in water below about 30 MeV (due to its dependence on the 'Z' of absorber) and is therefore of less importance in the low atomic number soft tissue elements. In industrial radiography where high atomic number elements are irradiated, pair production can become the major attenuation process assuming the incident radiation energy exceeds 1.022 MeV.
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http://radiopaedia.org/articles/pair-production
Pair Production
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Summary of different mechanisms that cause attenuation of an incident x-ray beam Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is (totally) absorbed, resulting in the ejection of electrons from the outer shell of the atom, and hence the ionization of the atom. Subsequently, the ionized atom returns to the neutral state with the emission of an x-ray characteristic of the atom. This subsequent emission of lower energy photons is generally absorbed and does not contribute to (or hinder) the image making process. Photoelectron absorption is the dominant process for x-ray absorption up to energies of about 500 KeV (<0.34Mev ASNT Exam Q) . Photoelectron absorption is also dominant for atoms of high atomic numbers.
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https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuation.htm
Photoelectric (PE) absorption
Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is (totally) absorbed, resulting in the ejection of electrons from the outer shell of the atom.
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Effect of Photon Energy on Attenuation Absorption characteristics will increase or decrease as the energy of the xray is increased or decreased. Since attenuation characteristics of materials are important in the development of contrast in a radiograph, an understanding of the relationship between material thickness, absorption properties, and photon energy is fundamental to producing a quality radiograph. A radiograph with higher contrast will provide greater probability of detection of a given discontinuity. An understanding of absorption is also necessary when designing x-ray and gamma ray shielding, cabinets, or exposure vaults. The applet below can be used to investigate the effect that photon energy has on the type of interaction that the photon is likely to have with a particle of the material (shown in gray). Various materials and material thicknesses may be selected and the x-ray energy can be set to produce a range from 1 to 199 KeV. Notice as various experiments are run with the applets that low energy radiation produces predominately photoelectric events and higher energy xrays produce predominately Compton scattering events. Also notice that if the energy is too low, none of the radiation penetrates the material.
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This second applet is similar to the one above except that the voltage (KVp) for a typical generic x-ray tube source can be selected. The applet displays the spectrum of photon energies (without any filtering) that the x-ray source produces at the selected voltage. Pressing the "Emit X-ray" button will show the interaction that will occur from one photon with an energy within the spectrum. Pressing the "Auto" button will show the interactions from a large number of photos with energies within the spectrum.
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More Reading on: Photoelectric effect The photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays. Upon exposing a metallic surface to electromagnetic radiation that is above the threshold frequency or threshold wavelength (absorption edge?) (which is specific to the type of surface and material), the photons are absorbed and current is produced. No electrons are emitted for radiation with a frequency below that of the threshold, as the electrons are unable to gain sufficient energy to overcome the electrostatic barrier presented by the termination of the crystalline surface. By conservation of energy, the energy of the photon is absorbed by the electron and, if sufficient, the electron can escape from the material with a finite kinetic energy. A single photon can only eject a single electron, as the energy of one photon may only be absorbed by one electron. The electrons that are emitted are often termed photoelectrons. Charlie Chong/ Fion Zhang
https://www.sciencedaily.com/terms/photoelectric_effect.htm
Photoelectric effect, or photoelectric absorption (PEA) is a form of interaction of X-ray or gamma photon with the matter. A low energy photon interacts with the electron in the atom and removes it from its shell. The probability of this effect is maximum when: ď Ž the energy of the incident photon is equal to or just greater than the binding energy of the electron in its shell ('absorption edge') and ď Ž the electron is tightly bound (as in K shell) (?)
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http://radiopaedia.org/articles/photoelectric-effect
The electron that is removed is then called a photoelectron. The incident photon is completely absorbed in the process. Hence it forms one of the reason for attenuation of X-ray beam as it passes through the matter. PEA is related to the atomic number of the attenuating medium (Z), the energy of the incident photon (E) and the physical density of the attenuating medium (ρ) by: PEA ∝ Z³ ·ρ / E³. Therefore, if Z doubles, PEA will increase by a factor of 8 (because 2³ is 8) and if E doubles, PEA will reduce by 8. As small changes in Z can have quite profound changes in PEA this has practical implications in the field of radiation protection and is why materials with a high Z such as lead (Z = 82) are useful shielding materials.
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http://radiopaedia.org/articles/photoelectric-effect
First principle mechanism of ionization The photoelectric effect of ionization involves the complete absorption of the photon energy during the process of knocking an electron out of orbit. This process primarily occurs with low energy photons ranging in energy between 10 Kev and less than 500 Kev. (0.01~0.5MeV) Notice in the above illustration that an ion pair is created in the interaction between the radiation photon and the atom. During this process, when the photon liberates the electron, all of the photons energy is transferred to create the ion pair and total absorption has occurred. Remember, there is a binding force that the holds the electron in its orbital shell. The amount of energy required to create the ion pair must be at least equal to this binding force.
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https://www.nde-ed.org/EducationResources/HighSchool/Radiography/photoelectric_popup.htm
If during the ionization process, only part of the photons energy is needed to liberate the electron, the rest of the energy is transferred to the electron in the form of speed (velocity). Now that all of the photon's energy is accounted for, the photon ceases to exist and total absorption has occurred. Remember that a photon is not a particle, but acts like one. When the energy of the photon is used, there is nothing left to cause further ionization. Keep in mind that electrons orbit in various shells of the atom and not all electrons have the same binding energy. This binding energy is dependent upon the elements (Z) number and the position of the electron in the atom. Those electrons nearer the nucleus possess greater binding energy and will require greater photon energy to remove them than will electrons in the outer shells.
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https://www.nde-ed.org/EducationResources/HighSchool/Radiography/photoelectric_popup.htm
Photoelectron Spectroscopy
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http://jahschem.wikispaces.com/Photoelectron+Spectroscopy
Photoelectron Spectroscopy
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http://www.met.reading.ac.uk/pplato2/h-flap/phys8_3.html
Photoelectron Spectroscopy
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https://en.wikibooks.org/wiki/Basic_Physics_of_Digital_Radiography/The_Source
Photodisintegration (PD) is the process by which the x-ray photon is captured by the nucleus of the atom with the ejection of a particle from the nucleus when all the energy of the x-ray is given to the nucleus. Because of the enormously high energies involved, this process may be neglected for the energies of x-rays used in radiography. (this photodisintegration to be distinguishes from photo annihilation of positron/electron pair)
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https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuation.htm
Thomson scattering (R), also known as Rayleigh, coherent, or classical scattering, occurs when the x-ray photon interacts with the whole atom so that the photon is scattered with no change in internal energy to the scattering atom, nor to the x-ray photon. Thomson scattering is never more than a minor contributor to the absorption coefficient. The scattering occurs without the loss of energy. Scattering is mainly in the forward direction.
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https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuation.htm
FIGURE 3. Pulse size as function of voltage in gas ion chamber.
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Ion current chambers have a response magnitude proportional to the absorbed energy and are therefore widely used for making dose measurements. When (1) recombination is negligible, (2) gas amplification does not occur and (3) all other charges are efficiently collected, then the steady state current produced is an accurate measurement of the rate at which ion pairs are formed within the gas. Measurement of this ionization current is the principle behind the direct current ion chamber. Ion chambers may be constructed of several different materials and, because radiation must penetrate the wall of the chamber to ionize the gas volume, chambers are chosen for the specific radiation energy being evaluated. When considering a particular instrument the energy response curve should always be consulted (Fig. 4). Some instruments may also have an angular dependence (more sensitivity in some directions), which should also be considered when making measurements. Radio frequency shielded ionization chambers are available for measurements made near high level radio frequency sources.
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FIGURE 3. Pulse size as function of voltage in gas ion chamber.
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FIGURE 4. Energy and directional response of typical ion chamber survey meters: (a) example of response curve; (b) comparison of several response curves.
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FIGURE 4. Energy and directional response of typical ion chamber survey meters: (a) example of response curve; (b) comparison of several response curves.
Legend
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Ionization Chambers
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Ionization Chambers
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Compton scattering (C) (incoherent scattering) occurs when the incident x-ray photon is deflected from its original path by an interaction with an electron. The electron gains energy and is ejected from its orbital position. The x-ray photon loses energy due to the interaction but continues to travel through the material along an altered path. Since the scattered x-ray photon has less energy, it, therefore, has a longer wavelength than the incident photon. The event is also known as incoherent scattering because the photon energy change resulting from an interaction is not always orderly and consistent. The energy shift depends on the angle of scattering and not on the nature of the scattering medium
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https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuation.htm
Pair production (PP) can occur when the x-ray photon energy is greater than 1.02 MeV, but really only becomes significant at energies around 10 MeV. Pair production occurs when an electron and positron are created with the annihilation of the x-ray photon. Positrons are very short lived and disappear (positron annihilation) with the formation of two photons of 0.51 MeV energy. Pair production is of particular importance when high-energy photons pass through materials of a high atomic number. (single photon for single pair production or single high energy photon for multiple pair production?)
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https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuation.htm
Output Current Measurements The ionization current collected in the ion chamber flows through an external circuit for measurement. Although in principle an ammeter could be placed in the external circuit to read the ion current, in practice the ammeter is not placed there, for the current is very small. A 440 cm3 (27 in.3) ion chamber typically produces about 4 × 10–15 A·μSv–1 (4 × 10–14 A·mR–1) at standard temperature and pressure. A high valued load resistor (on the order of 1010 Ω) is placed in the circuit and the voltage drop across the resistor is measured with a sensitive electrometer. A metal oxide silicon field effect transistor (MOSFET) is used in some electrometers. The metal oxide silicon field effect transistor produces an input impedance on the order of 1015 Ω to amplify the collected current (Fig. 5).
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FIGURE 5. Operational configuration of current amplifier.
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Vibrating Reed Electrometers An alternative approach to ion current measurement is to convert the signal from direct current to alternating current at an early stage. This allows a more stable amplification of the alternating current signal in subsequent operations. The conversion is accomplished in a dynamic capacitor or vibrating reed electrometer, by collecting the ion current across a resistive capacitive circuit. The capacitance is then changed rapidly, compared to the time constant of the circuit. The induced alternating current voltage is proportional to the ionization current (Fig. 6).
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FIGURE 6. Principle of vibrating reed electrometer; oscillations of capacitance induce alternating current voltage proportional to steady state signal current.
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Vibrating Reed Electrometers Cary Vibrating Reed Electrometer with Ionization Chamber ( late 1950s)
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https://www.orau.org/PTP/collection/ionchamber/vibratingreedion.htm
Cary Vibrating Reed Electrometer with Ionization Chamber ( late 1950s) The spherical ion chamber, electrometer head, and amplifier were made by Applied Physics Corporation of Pasadena, California. The Model 31, which replaced the Model 30, was introduced in 1957/1958 and seems to have been superceded by the Model 32 in 1959 This system would have been used to measure the activity of chemically unreactive gases such as krypton, xenon, CO2 and HT. Either the gas being analyzed would flow through the chamber or be held inside the chamber for the duration of the measurement.
Charlie Chong/ Fion Zhang
https://www.orau.org/PTP/collection/ionchamber/vibratingreedion.htm
The weak current generated in the chamber (less than 10-12 amperes) was converted into an alternating current by the vibrating reed in the electrometer head. The AC current was then amplified and fed to a strip chart recorder. The electrometer readout employed multiple scales and measured up to 30 volts. The spherical ion chamber, shown to the right, is connected directly to the electrometer head. Made of stainless steel, it is approximately 3" in diameter and has a 250 ml volume. The original version of the chamber was made of Pyrex. The reed, a thin metal plate, was vibrated by an electromagnet at frequency of 450 cycles per second. The reed also formed part of a capacitor onto which the current from the chamber was sent. The cyclical movement of the reed resulted in a fluctuating capacitance and the generation of an alternating current. The advantage the AC signal had over the original DC signal was that the former could be amplified much more reliably.
Charlie Chong/ Fion Zhang
https://www.orau.org/PTP/collection/ionchamber/vibratingreedion.htm
An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical handmade mechanical instruments to high-precision electronic devices. Modern electrometers based on vacuum tube or solidstate technology can be used to make voltage and charge measurements with very low leakage currents, down to 1 femtoampere. A simpler but related instrument, the electroscope, works on similar principles but only indicates the relative magnitudes of voltages or charges.
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Older electrometers Gold-leaf electroscope The gold-leaf electroscope was one of the first sensitive instruments used to indicate electric charge. It is still used for science demonstrations but has been superseded in most applications by electronic measuring instruments. The instrument consists of two thin leaves of gold foil suspended from an electrode. When the electrode is charged by induction or by contact, the leaves acquire similar electric charges and repel each other due to the Coulomb force. Their separation is a direct indication of the net charge stored on them. On the glass opposite the leaves, pieces of tin foil may be pasted, so that when the leaves diverge fully they may discharge into the ground. The leaves may be enclosed in a glass envelope to protect them from drafts, and the envelope may be evacuated to minimize charge leakage. A further cause of charge leakage is ionizing radiation, so to prevent this, the electrometer must be surrounded by lead shielding. This principle has been used to detect ionizing radiation, as seen in the quartz fibre electrometer and Kearny fallout meter.
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This type of electroscope usually acts as an indicator and not a measuring device, although it can be calibrated. The Braun[dubious electroscope replaced [when?] the gold-leaf electroscope for more accurate measurements. The instrument was developed in the 18th century by several researchers, among them Abraham Bennet and Alessandro Volta.
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Volta Electrometers
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Kolbe electrometer, precision form of goldleaf instrument. This has a light pivoted aluminum vane hanging next to a vertical metal plate. When charged the vane is repelled by the plate and hangs at an angle.
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Gold-leaf electroscope
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Modern electrometers A modern electrometer is a highly sensitive electronic voltmeter whose input impedance is so high that the current flowing into it can be considered, for most practical purposes, to be zero. The actual value of input resistance for modern electronic electrometers is around 1014Ί, compared to around 1010Ί for nanovoltmeters. Owing to the extremely high input impedance, special design considerations must be applied to avoid leakage current such as driven shields and special insulation materials. Among other applications, electrometers are used in nuclear physics experiments as they are able to measure the tiny charges left in matter by the passage of ionizing radiation. The most common use for modern electrometers is the measurement of radiation with ionization chambers, in instruments such as Geiger counters.
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Vibrating reed electrometers Vibrating reed electrometers use a variable capacitor formed between a moving electrode (in the form of a vibrating reed) and a fixed input electrode. As the distance between the two electrodes varies, the capacitance also varies and electric charge is forced in and out of the capacitor. The alternating current signal produced by the flow of this charge is amplified and used as an analogue for the DC voltage applied to the capacitor. The DC input resistance of the electrometer is determined solely by the leakage resistance of the capacitor, and is typically extremely high, (although its AC input impedance is lower). For convenience of use, the vibrating reed assembly is often attached by a cable to the rest of the electrometer. This allows for a relatively small unit to be located near the charge to be measured while the much larger reed-driver and amplifier unit can be located wherever it is convenient for the operator.
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Integrating Instruments The instruments described above (Fig. 7) are generally rate meters; that is, they indicate the radiation at the time of exposure and, depending on its time constant, will return to background levels as the source is removed. Some instruments may have an integration switch that introduces a capacitor to the circuit to accumulate the charge. Leaving such an instrument at an operatorâ&#x20AC;&#x2122;s location will indicate the total amount of ionizing radiation that area has received, from the time the instrument is engaged.
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FIGURE 7. Examples of ionization chambers located externally on survey instruments. Protective caps are removed, showing thin windows for low energy X-ray or beta detection.
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Personnel Monitoring Instruments Pocket Chambers Personnel monitoring instruments, some the size of a ball point pen, are usually the integrating type and contain an ionization chamber. One version, the pocket chamber, requires the application of an initial charge of 150 to 200 V by an external instrument. Zero dose is then indicated on a scale contained in the charging unit. Exposure of the chamber to ionization decreases the initial charge. When the chamber is reconnected to the charging unit the reduced charge is translated to the level of exposure (Fig. 8).
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FIGURE 8. Cross section of quartz fiber pocket dosimeter. Legend 1. Low atomic number wall 2. Graphite coated paper shell 3. Aluminum terminal head 4. Aluminum terminal sleeve 5. Polystyrene support bushing 6. Central electrode, graphite coated 7. Polyethylene insulating washer 8. Polystyrene fixed bushing 9. Electrode contact 10. Retaining ring 11. Aluminum base cap 12. Polyethylene friction bushing
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Quartz Fiber Pocket Dosimeter.
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Quartz Fiber Pocket Dosimeter.
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https://www.nde-ed.org/EducationResources/CommunityCollege/RadiationSafety/Graphics/DOSE.gif
Quartz Fiber Pocket Dosimeter.
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Quartz Fiber Pocket Dosimeter.
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Pocket Chambers and Pocket Dosimeters Paul Frame, Oak Ridge Associated Universities Pocket chambers and pocket dosimeters are small ionization chambers that, as the name implies, are usually worn in the pocket. While they were designed to measure x-rays and gamma ray exposures, they would also respond to betas above 1 MeV. Neutron-sensitive versions were also available. The terms pocket chamber and pocket dosimeter are often used interchangeably. The original distinction between the two terms, used here, is rarely made anymore. In part, this is due to the fact that the devices that I call pocket chambers are rarely used any more. Note: pocket dosimeter is an ion chamber type.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Quartz Fiber Pocket Dosimeter.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
1. Pocket Chambers Pocket chambers go by a variety of names: indirect-reading dosimeters, nonself-reading dosimeters and condenser-type pocket dosimeters. Prior to WW II, they were only used to a limited extent, primarily in medical facilities and around accelerators. The Manhattan Project however created a huge demand and they were worn by almost everyone who might be exposed to radiation. A pocket chamber acts as an air-filled condenser (capacitor) much like the thimble chambers used in radiology. Prior to being worn, it is given a charge with a charger-reader, e.g., the Victoreen Minometer. Any subsequent exposure to radiation ionizes the air inside the chamber and this reduces the stored charge. In order to quantify the exposure, the charge is measured and the decrease is related to the exposure.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Manhattan Project
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http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Manhattan Project
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Manhattan Project
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Manhattan Project
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Manhattan Project
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Manhattan Project
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Manhattan Project Director J Robert Oppenheimer
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WWII
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Ion Chamber Charger & Reader PP-630(A)/PD Dosimeter Charger (ca. 1954-1961) The PP-630 (A)/PD Dosimeter Charger is the military equivalent of the Keleket Model 430A chargerreader which was advertised as early as 1954. Since this PP-630(A) has "August 29 1961" stamped on it, I assume that it probably dates from 1954 to 1961. Unfortunately I have not been able to locate any specific references to the PP-630A/PD.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/radiac/PP630.htm
Ion Chamber Charger & Reader Radiac Computer-Indicator CP-95A/PD is designed as a portable radiac computer-indicator, for computing & indicating the total amount of X and gamma radiation to which Radiac Detector DT-60()/PD has been exposed (and thus revealing the X and gamma radiation to which the wearer of the DT-60()/PD had been exposed). The CP-95A/PD operates in conjunction with Radiac Detector DT-60()/PD (not supplied) which contains a specially compounded silver-actuated phosphor glass. When the total radiation dosage of a DT-60()/PD is to be measured, the DT-60()/D is placed in Radiac Computer-Indicator C-95A/PD and exposed to a source of ultraviolet light. The ultraviolet light causes the silver-activated glass to emit an orange luminescence, the intensity of which is proportional to the total amount of radiation the glass has received. The intensity of the orange luminescence is measured by a photomultiplier tube which is fitted with a filter to prohibit the passage of blue and green light. The photomultiplier tube employs the principle of secondary emission to amplify the initial electron emission caused by the filtered orange illumination of the light-sensitive cathode. The output of the photomultiplier tube is applied to an indicating circuit to indicate the total amount of radiation to which the wearer of the DT60()/PD has been exposed.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/radiac/CP95APD.htm
Pocket chambers were approximately 4 - 5" long and 0.5" in diameter. An aluminum rod (ca. 0.0625â&#x20AC;? in diameter) running along the chamber axis served as one electrode, while the outer wall of the chamber served as the other electrode. The central electrode was suspended at each end with a polystyrene insulator and at one end it penetrated the insulator to serve as the charging contact. One problem with the early models involved the threaded caps that were used to protect the charging contact - they would wear and the metal fragments would get on the insulator. The graphite coating on the inside of the chamber wall caused a similar type of problem with some of the early models because it would sometimes flake off and short out the chamber. The early models were also susceptible to discharge as a result of mechanical shock because the central electrode would flex and contact the chamber wall. To solve this problem, later versions used a thicker central electrode and/or positioned a small insulating disk in the center of the electrode. Because of these problems, it was usual for a worker to wear two dosimeters and the lower of the two readings was considered the most accurate.
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http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Quartz Fiber Pocket Dosimeter.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
2. Pocket Dosimeters (quartz fiber electroscopes) Like pocket chambers, pocket dosimeters are known by a number of other names, e.g., direct-reading dosimeters, self-reading pocket dosimeters and pocket electroscopes. They are actually quartz fiber electroscopes the sensing element of which is a movable bow-shaped quartz fiber that is attached at each end to a fixed post. The latter is also shaped like a bow (or horseshoe). The dose is determined by looking through the eyepiece on one end of the dosimeter, pointing the other end towards a light source, and noting the position of the fiber on a scale. Until 1950 or so, the vast majority of pocket dosimeters had a range up to 200 mR, although a few high range versions were available for emergency situations. Higher range versions became more readily available in the 1950s for military and civil defense purposes.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Pocket dosimeters tended to be slightly larger than pocket chambers. Their walls might be made of aluminum, bakelite, or some other type of plastic. If the material was not conductive, the inner surface of the chamber was coated with Aquadag (graphite). The central electrode was usually a phosphor bronze rod. This made pocket dosimeters more energy dependent than pocket chambers whose central electrodes were usually aluminum. Some dosimeters (e.g., Keleket Model K-145) employed boron-lined chambers which made them sensitive to thermal neutrons. Pocket dosimeters must be charged (ca. 150 ~ 200 volts) with some sort of charger, but they do not require another device to read them. This allows the worker to determine his or her exposure at any time, an important advantage when working in high radiation fields. The first direct reading pocket dosimeters were built by Charlie Lauritsen at the California Institute of Technology.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
3. Pocket Chambers (indirect-reading) vs Pocket Dosimeters (direct reading) 1. Pocket chambers were far less expensive (ca. $5 vs $40 in 1950) 2. Pocket chambers, despite their problems, were more reliable. 3. Pocket chambers did not permit the wearer to know their exposure, for military purposes, this was sometimes desirable. 4. Pocket dosimeters allowed the worker to check their exposure during a particular task and to take corrective actions when appropriate. 5. Pocket dosimeters did not have to be recharged every time they were read. 6. Pocket dosimeters could use very small chargers, small enough to easily fit into a pocket.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Quartz Fiber Pocket Dosimeter. CDV-750 Dosimeter Charger and THREE (3) CDV-742 Dosimeters $260.00 The direct-reading pocket dosimeter is a portable instrument designed to measure the total dose of moderate and high levels of gamma radiation. The instruments make use of a small quartz fiber electroscope as an exposure detector and indicator. An image of the fiber is projected onto a film scale and viewed through the eyepiece lens. The scale is calibrated in roentgens (R) and may be read by looking through the eyepiece toward a lamp or other source of light. A CDV-750 dosimeter charger must be used in conjunction with the dosimeter to set the instrument to zero. The charger may also be used to read the scale or you can hold the dosimeter up to any light source and look through it. NOTE: the CDV-750 uses 1 D cell Battery (not included). The CDV-742 Dosimeter is an 'electroscope' it is electro statically charged by the CDV-750 charger. The Dosimeter does NOT use a battery. No battery to replace, no battery to go bad, EVER!
Charlie Chong/ Fion Zhang
http://josephdanielassociates.us/index.php?main_page=product_info&products_id=356
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http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Charlie Chong/ Fion Zhang
Ion Chamber Charger & Reader
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http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Ion Chamber Charger & Reader
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http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
Quartz Fiber Pocket Dosimeter.
Charlie Chong/ Fion Zhang
http://www.orau.org/ptp/collection/dosimeters/pocketchamdos.htm
WWII Heroes
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Direct Reading DosimetersThe direct reading dosimeter operates on the principle of the gold leaf electroscope (Fig. 9). A quartz fiber is displaced electrostatically by charging it to a potential of about 200 V. An image of the fiber is focused on a scale and viewedthrough a lens at one end of the instrument. Radiationexposure of the dosimeter discharges the fiber, allowing itto return to its original position.Personnel dosimeters may have a full scale reading of 1 to 50 mSv (100 mR to 5 R) (0.1R ~ 5R) and may have other scales according to applicable regulations. Chambers are available with thin walls for sensitivity to beta radiation over 1 MeV and may be coated on the inside with boron for neutron sensitivity. Figure 10 demonstrates the energy response of self- reading pocket dosimeters. Table 2 lists performance specifications of dosimeters in general. Keypoints: â&#x2013; Chambers are available with thin walls for sensitivity to beta radiation 1 MeV â&#x2013; may be coated on the inside with boron for neutron sensitivity.
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FIGURE 8. Cross section of quartz fiber pocket dosimeter.
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FIGURE 9. Cross section of pocket (direct reading) ionization chamber.
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FIGURE 10. Energy dependence of response of different commercial selfreading dosimeters.
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TABLE 2. General performance specifications for dosimeters.
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Proportional Counters If the electric field in an ion chamber is raised above the ionization potential but below saturation potential, enough energy is imparted to the ions for production of secondary electrons by collision and gas amplification. Operation at this electric potential overcomes the difficulty of the small currents in the ionization region yet takes advantage of pulse size dependence for separating various ionizing energies. When an ionization chamber is operated in this region it is called a proportional counter. The size of the output pulse is determined by, and proportional to, the number of electrons collected at the anode (n) and the voltage applied (V) at the detector (Outputâ&#x2C6;?n¡V) . By careful selection of gases and voltages, a properly designed proportional counter can detect alphas in the presence of betas, or higher energy beta and gamma radiation in the presence of lower energies. Proportional counters are often used in X-ray diffraction applications.
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REM (Roentgen equivalent man) – Dose Equivalent One of the two standard units used to measure the dose equivalent (or effective dose), which combines the amount of energy (from any type of ionizing radiation that is deposited in human tissue), along with the medical effects of the given type of radiation. For beta and gamma radiation, the dose equivalent is the same as the absorbed dose. By contrast, the dose equivalent is larger than the absorbed dose for alpha and neutron radiation, because these types of radiation are more damaging to the human body. Thus, the dose equivalent (in rems) is equal to the absorbed dose (in rads) multiplied by the quality factor of the type of radiation [see Title 10, Section 20.1004, of the Code of Federal Regulations (10 CFR 20.1004), "Units of Radiation Dose"]. The related international system unit is the sievert (Sv), where 100 rem is equivalent to 1 Sv. For additional information, see Doses in Our Daily Lives and Measuring Radiation. 先有 rad 才有 rem / 先有 Gray 才有 Sievert
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http://www.nrc.gov/reading-rm/basic-ref/glossary/rem-roentgen-equivalent-man.html
Townsend Discharges (Avalanches) The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and free additional electrons. The result is an avalanche multiplication that permits electrical conduction through the gas. The discharge requires a source of free electrons and a significant electric field; without both, the phenomenon does not occur. The Townsend discharge is named after John Sealy Townsend, who discovered the fundamental ionisation mechanism by his work between 1897 and 1901.
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https://en.wikipedia.org/wiki/Townsend_avalanche
Sir John Sealy Townsend
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https://en.wikipedia.org/wiki/Townsend_avalanche
General description of the phenomenon The avalanche occurs in a gaseous medium that can be ionised (such as air). The electric field and the mean free path of the electron must allow free electrons to acquire an energy level (velocity) that can cause impact ionisation. If the electric field is too small, then the electrons do not acquire enough energy. If the mean free path is too short, the electron gives up its acquired energy in a series of non-ionising collisions. If the mean free path is too long, then the electron reaches the anode before colliding with another molecule. A positive charge q is placed in a uniform electric field E set up between two charged parallel plates. If the particle is at a distance s from the negative plate, its electrical potential energy is qEs joules (C¡N¡C-1¡m = N.m) http://www.physchem.co.za/OB11-ele/charge3.htm
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https://en.wikipedia.org/wiki/Townsend_avalanche
Note: If the electric field is too small, then the electrons do not acquire enough energy. If the mean free path is too small, then the electrons do not acquire enough kinetic energy. If the mean free path is too short, the electron collides too early with low energy, gives up its acquired energy in a series of non-ionising collisions. If the mean free path is too long, then the electron reaches the anode before colliding with another molecule.
Charlie Chong/ Fion Zhang
https://en.wikipedia.org/wiki/Townsend_avalanche
The avalanche mechanism is shown in the accompanying diagram. The electric field is applied across a gaseous medium; initial ions are created with ionising radiation (for example, cosmic rays, X radiation and gamma ray). An original ionisation event produces an ion pair; the positive ion accelerates towards the cathode while the free electron accelerates towards the anode. If the electric field is strong enough, the free electron can gain sufficient velocity (energy) (qEs) to liberate another electron when it next collides with a molecule.
Charlie Chong/ Fion Zhang
https://en.wikipedia.org/wiki/Townsend_avalanche
The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause further impact ionisations, and so on. This process is effectively a chain reaction that generates free electrons. The total number of electrons reaching the anode is equal to the number of collisions, plus the single initiating free electron. (n+1) Initially, the number of collisions grows exponentially. (?) The limit to the multiplication in an electron avalanche is known as the Raether limit. The Townsend avalanche can have a large range of current densities. In common gas-filled tubes, such as those used as gaseous ionisation detectors, magnitudes of currents flowing during this process can range from about 10â&#x2C6;&#x2019;18 amperes to about 10â&#x2C6;&#x2019;5 amperes.
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https://en.wikipedia.org/wiki/Townsend_avalanche
Visualization of Proportional Counter Gas Magnification Event Single gas avalanche near the anode?
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https://en.wikipedia.org/wiki/Townsend_avalanche
Visualization of Proportional Counter Gas Magnification Event Single gas avalanche near the anode?
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https://en.wikipedia.org/wiki/Townsend_avalanche
Multiple Gas Amplification due to multiple ionization
Single gas avalanche near the anode
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dose equivalent (in rems) is equal to the absorbed dose (in rads) multiplied by the quality factor of the type of radiation
rems Charlie Chong/ Fion Zhang
rads
http://www.nrc.gov/reading-rm/basic-ref/glossary/rem-roentgen-equivalent-man.html
PART 3. Geiger-M端ller Counters Operating Voltage Level Increasing voltage beyond the proportional region (Fig. 3) will eventually cause the gas avalanche to extend along the entire length of the anode wire. When this happens, the end of the proportionalregion is reached and the geiger-m端ller region begins. An instrument operating in this voltage range, using a sealed gas filled detector, is referred to as a geiger-m端ller counter, a GM counter or simply a geiger tube. This instrument was introduced in 1928 and its simplicity and low cost have made it the most popular radiation detector since then. Geiger-m端ller counters complement the ion chamber and proportional counter and comprise the third category of gas filled detectors based on ionization.
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Operating Voltage Level
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Visualization of Geiger Muller Gas Magnification Event- Townsend Avalanches Multiple gas avalanche
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Visualization of Geiger Muller Gas Magnification Event- Townsend Avalanches
Visualization of the spread of Townsend avalanches by means of UV photons. This mechanism allows a single ionising event to ionise all the gas surrounding the anode by triggering multiple avalanches.
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Properties Extension of the gas avalanche increases the gas amplification factors so that 109 to 1010 ion pairs are formed in the discharge. This results in an output pulse large enough (0.25 to 10 V) to require no sophisticated electronic amplification circuitry for readout. At this voltage, the size of all pulses, regardless of the nature of the ionization, is the same. When operated in the geiger-mĂźller region, a counter cannot distinguish among the several types of radiation and therefore is not useful for spectroscopy or for the detection of one energy event in the presence of another. An external shield is often used to filter out alpha and beta particles in the presence of gamma energies. Note: The amplification factor: 109 to 1010 ion pairs are formed in the discharge for single ionization of multiple events avalanches
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Resolving Time As an ionizing event occurs in the counter, the avalanche of ions paralyzes the counter. The counter is then incapable of responding to another event until the discharge dissipates and proper potential is established. The time it takes to reestablish the electric field intensity is referred to as the resolving time. Average resolving time for a geiger-mĂźller counter is about 100 ms, which must be corrected at high level readings. Resolving time Ď&#x201E; of a counter may be determined by counting two sources independently (R1 and R2), then together (R1, R2). The background count is Rb.
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Correct counting rate R can be calculated from observed counting rate Ro andresolving time Ď&#x201E; in the following equation for nonparalyzable systems:
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Dead Time The relationship of resolving time to dead time and recovery is illustrated in Fig. 11.Resolving time may be a function of the detector alone or of the detector and its signal processing electronics. Its effect on the real counting rate depends on whether the system design is paralyzableor nonparalyzable.
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FIGURE 11. Resolving time, dead time and recovery time for geiger-mĂźller system.
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Nonparalyzable Systems In Fig. 12, a time scale is shown indicating six randomly spaced events in the detector. At the bottom of the illustration is the corresponding dead time behavior of a detector assumed to be nonparalyzable. A fixed time Ď&#x201E; follows each event that occurs during the live period of the detector. Events occurring during the dead time have no effect on the detector, which would record four counts from the six interactions.
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FIGURE 12. Processing of detector interactions in paralyzable and nonparalyzable systems.
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Paralyzable Systems The top line of Fig. 12 illustrates a paralyzable system. Resolving time τ follows each interaction, whether it is recorded or not. Events that occur during resolving time τ are not recorded and further extend the dead time by another period τ. The chart indicates only three recorded events from the six interactions. In this case, τ increases with increased number of interactions. It can be demonstrated that with a paralyzable system (at increasingly higher interaction rates), the observed counting rates can actually decrease with an increased number of events. When using a counting system that may be paralyzable, extreme caution must be taken to ensure that low observed counting rates correspond to low interaction rates, rather than very high interaction rates with accompanying, long dead time. It is possible for a paralyzable system to record the first interaction and then be paralyzed, recording zero counts in high radiation fields.
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Quenching As positive ions are collected after a pulse, they give up their kinetic energy by striking the wall of the tube; Ultraviolet photons and/or electrons are liberated, producing spurious counts. Prevention of such counts is called quenching. Quenching may be accomplished electronically (by lowering the anode voltage after a pulse) or chemically (by using a self-quenching gas). â&#x2013; Electronic Quenching (by lowering the anode voltage after a pulse) Electronic quenching is accomplished by introducing a high value of resistance into the voltage circuit. This will drop the anode potential until all the positive ions have been collected.
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â&#x2013; Self-Quenching Gas (absorb ultraviolet (UV) photons) A self-quenching gas is one that can absorb ultraviolet (UV) photons without becoming ionized. One way to use this characteristic is to introduce a small amount of organic vapor, such as alcohol or ether, into the tube. The energy from the ultraviolet photons is then dissipated by dissociating the gas molecule. Such a tube is useful only as long as it has a sufficient number of organic molecules to dissociate, generally about 108 counts. To avoid the problem of limited lifetime, some tubes use halogens (chlorine or bromine) as the quench gas. The halogen molecules also dissociate in the quenching process but they are replenished by spontaneous recombination at a later time. Halogen quench tubes have an infinite lifetime and are preferred for extended applications. Reaction products of the discharge often produce contamination of the gas or deposition on the anode surface and generally limit the lifetimes of geigermĂźller tubes.
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FIGURE 13. Assortment of geiger-mĂźller counters demonstrating availability of sizes and shapes. Smallest counter shown isabout 30 mm (1 in.) long.
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Geiger Muller
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http://solderpad.com/solderpad/mightyohm-geiger-counter/board/embed
Geiger Muller
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http://www.jonshobbies.com/civil-defense-lionel-cd-v-700-model-6b-geiger-counter.html
Geiger Muller
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http://www.jonshobbies.com/civil-defense-lionel-cd-v-700-model-6b-geiger-counter.html
Geiger Muller
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http://www.jonshobbies.com/civil-defense-lionel-cd-v-700-model-6b-geiger-counter.html
Geiger Muller
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http://www.jonshobbies.com/civil-defense-lionel-cd-v-700-model-6b-geiger-counter.html
Geiger Muller
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http://www.jonshobbies.com/civil-defense-lionel-cd-v-700-model-6b-geiger-counter.html
Geiger Muller
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http://www.jonshobbies.com/civil-defense-lionel-cd-v-700-model-6b-geiger-counter.html
Geiger Muller
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http://www.jonshobbies.com/civil-defense-lionel-cd-v-700-model-6b-geiger-counter.html
Smart Geiger Counter
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http://akihabaranews.com/2015/10/05/article-en/app-enabled-geiger-counter-smartphones-113561601
Design Variations Geiger-müller counters (Fig. 13) are available in various shapes and sizes. The most common form is that of a cylinder with a central anode wire. If low energy beta or alpha particles are to be counted, a unit with a thin entrance window (1 to 4 mg·cm–2) should be selected. For surveying large surfaces, pancake or large window counters are available. High count rate instruments, greater than 0.14 mSv·s–1 (50 mR·h–1), generally contain a small tube to minimize resolving time of the system; large volume detectors may require significant correction. Note: small volume minimize resolving time A geiger-müller counter response to gamma rays occurs by way of gamma ray interaction with the solid wall of the tube. The incident gamma ray interacts with the wall and produces a secondary electron that subsequently reaches the gas. The probability of gamma ray interaction generally increases with higher density wall material.
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A geiger-müller counter response to gamma rays occurs by way of gamma ray interaction with the solid wall of the tube. The incident gamma ray interacts with the wall and produces a secondary electron that subsequently reaches the gas. The probability of gamma ray interaction generally increases with higher density wall material. Note: ■ Smaller chamber volume, minimize resolving time ■ Gamma ray interact with the chamber wall ■ Higher the density higher the probability of interaction ■ Higher the atomic number higher the interaction
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FIGURE 14. Dose rate ratio versus effective energy for personnel radiation monitor.
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FIGURE 14. Dose rate ratio versus effective energy for personnel radiation monitor. Ir-192
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TABLE 1. Characteristics of three isotope sources commonly used for radiography.
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Alarming Rate Meters (Personnel Monitors) Small geiger-müller tubes are used in pocket-sized units for personnel monitoring. They generally emit a high frequency chirp at a rate proportional to the subjected dose rate. United States regulations specify an alarm threshold of 500 mSv·h–1 (500 mR·h–1) (500 μR·h–1) for field gamma radiography. The energy dependence curve for one such instrument is shown in Fig. 14.
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Mini Geiger Muller Alarm Meter
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Mini Geiger Muller Alarm Meter
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Mini Geiger Muller Alarm Meter
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Smart Geiger Counter
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http://xronosclock.com/home/?p=4238
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Applications Geiger-müller counters are the most widely used, general purpose radiation survey instruments. It must be remembered that geiger-müller counters, unlike current ionization chambers, read pulses (regardless of their energy or ionizing potential) and register in counts per minute. Some instruments have a scale calibrated in milliroentgens per hour (mR·h–1); however, this is an arbitrary scale calibrated on the radiation from radium-226, cesium-137 or some other energy (Fig. 15). Another scale is microsieverts per second (μSv·s–1). A sensitivity versus energy table should always be consulted before making measurements with a geiger-müller instrument.
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FIGURE 15. Typical energy response curves for geiger-mĂźller counters (a) shielded versus unshielded; (b) radiation incident on side versus front; (c) exposure ratio close to ideal with radiation incident normal to long axis of probe; (d) radiation incident normal to long axis of probe.
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(a) shielded versus unshielded;
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(b) radiation incident on side versus front;
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(c) exposure ratio close to ideal with radiation incident normal to long axis of probe;
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(d) radiation incident normal to long axis of probe.
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PART 4. Scintillation Detectors Soon after the discovery of X-rays and radioactivity, it was observed that certain materials emit visible light photons after interacting with ionizing radiation. These light photons appear to flash or sparkle and the materials are said to scintillate. Scintillators commonly used with radiation survey instruments are solid materials. Being denser than gases, these scintillators have greater detection efficiencies and are useful for low level measurements. For gamma photons, scintillators have detection efficiencies 106 times greater than typical gas ionization chambers. Detection of alpha and beta particles, neutrons and gamma photons is possible with various scintillator systems (Table 3).
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TABLE 3. Common scintillators.
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Scintillation Process Radiation interactions with matter produce excitation as well as irrigation. Ionization refers to the removal of an electron from an atom and excitation refers to the elevation of an electronâ&#x20AC;&#x2122;s energy state. The return of excited electrons to their normal, lower energy state is called deexcitation. Scintillators excited by ionizing radiation return to lower energy states quickly and emit visible light during the deexcitation process. Radiation detection is possible by measuring the scintillatorâ&#x20AC;&#x2122;s light output (Fig. 16).
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FIGURE 16. Energy diagram of scintillation process.
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Materials and Characteristics Scintillation materials come in gaseous, liquid and solid forms. Organic liquids and solids, as well as inorganic gases and solids, are common scintillators. Organic, solid scintillators are available as crystals, plastics and gels. Inorganic solid scintillators are usually alkali halide crystals. The scintillation process in inorganic materials requires the presence of small amounts of an impurity, or activator. Inorganic solid scintillators are commonly used with radiation survey instruments and are listed in Table 3.
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Desirable Scintillator Characteristics A useful and practical scintillator needs to have most of the characteristics listed below. Not all of these characteristics are ideally satisfied by each scintillator and often a compromise is acceptable. 1. The scintillator should be of high density and large enough to ensure adequate interaction with the ionizing radiation. 2. Efficient conversion of the electronâ&#x20AC;&#x2122;s kinetic energy into visible light is required and the light yield should be linearly related to the deposited electron kinetic energy. 3. The scintillator should be of good optical quality, transparent to its emitted light and free of hydroscopic effects, and should have an index of refraction close to that of glass. 4. The wavelength of the emitted light should be appropriate for matching to a photomultiplier tube.
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Photomultiplier Tubes Before the advent of photomultiplier tubes (PMTs), scintillation light photons had to be visually counted. This limited the use and development of scintillators. In the 1940s, the photomultiplier tube was developed and dramatically increased the use of scintillators, to the point where scintillators are preferred over other radiation detectors for many survey applications. The photomultiplier tubeâ&#x20AC;&#x2122;s function is to convert the scintillatorâ&#x20AC;&#x2122;s light output into a electrical pulse. The photomultiplier tube is composed of a photosensitive layer, called the photocathode, and a number of electron multiplication structures called dynodes. Conversion of the scintillation light into photoelectrons is accomplished by the photocathodes through the photoelectric effect. To maximize the information contained in the scintillation light, the photomultiplier tube photocathode should be matched to the scintillator; the scintillator and photomultiplier tube should be optically coupled to minimize light losses.
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Electron multiplication, or gain, is accomplished by positively charging the dynodes in successive stages, so that the total voltage applied to the photomultiplier tube is around 1000 V. Electrons emitted by the photocathode are focused toward the first dynode; more electrons are emitted than were initially incident on the dynode. This is repeated at each dynode stage. The photocathode and dynodes are positioned in a glass enclosed vacuum so that air molecules will not interfere with the collection of electrons. The net result of the photomultiplier tube may be an electron gain up to 1010 per emitted photoelectron. Figure 17 illustrates the structure of a photomultiplier tube.
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FIGURE 17. Cutaway drawing of photomultiplier tube, showing crystal, photocathode, collecting dynodes and voltage divider network.
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PMT
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PMT
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PMT
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PMT - Photomultiplier tubes lining the walls of the Daya Bay neutrino detector. The tubes are designed to amplify and record the faint flashes of light that signify an antineutrino interaction. This experiment aims to measure the final unknown mixing angle that describes how neutrinos oscillate â&#x20AC;&#x201D; another chapter in Brookhaven National Laboratory's long history of neutrino research over the last several decades.
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https://www.bnl.gov/newsroom/news.php?a=24055
System Electronics Once the output pulse from a photomultiplier tube is generated, it is amplified and analyzed. The pulse height, or amplitude, is proportional to the amount of energy deposited within the scintillator and can be correlated to a count rate or scale of microsievert per second (μSv·s–1) or milliroentgen per hour (mR·h–1) when calibrated against a known energy source. (See Fig. 18.)
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FIGURE 18. Comparison of sodium iodide (thallium activated) and germanium detectors for gamma spectroscopy.
Energy (MeV) Charlie Chong/ Fion Zhang
PART 5. Luminescent Dosimetry Thermoluminescent Dosimetry Thermoluminescence is the emission of light from previously irradiated materials after gentle heating. The radiation effect in thermoluminescent (TL) materials is similar to that observed in scintillators, except that light photon emission does not occur in thermoluminescent materials until some heat energy is supplied (Fig. 19). Measurement of the light photons emitted after heating permits correlation to the amount of ionizing radiation energy that was absorbed in the thermoluminescent material. Thermoluminescent dosimetry (TLD) is possible for beta, gamma and neutron (alpha?) radiations, if the appropriate thermoluminescent material is used.
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Thermoluminescence
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Thermoluminescence
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Lithium Fluoride Properties The most common thermoluminescent phosphor used in gamma and neutron personnel dosimetry is lithium fluoride (63LiF for neutron detection)) . Other thermoluminescent phosphors are available for personnel dosimetry but, for various reasons, are not as well suited as lithium fluoride. The advantages of lithium fluoride include its: 1. 2. 3. 4. 5. 6. 7.
usefulness over a wide dose range, linear dose response, near dose rate independence, reusability, stability, short readout time and near tissue equivalence.
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Disadvantages include the loss of information after readout and lack of information about the incident radiation energy. Both gamma photons and neutrons produce ionization indirectly. Gamma photons interact with matter, releasing electrons that in turn cause ionization. Lithium fluoride undergoes interactions with gamma photons and is therefore used in gamma dosimetry. Slow neutrons require the presence of the lithium fluoride enriched with lithium-6 for detection of the (n, α) nuclear reaction. 6
3Li
+ n → 42He + 21D + γ
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Fast neutron detection with lithium fluoride would only be possible if the fast neutrons were slowed down to thermal energies before reaching the lithium fluoride thermoluminescent dosimeter. Nearly complete elimination of neutron response in lithium fluoride is possible with lithium fluoride enriched with lithium-7. In a mixed gamma and slow neutron field, distinction of gamma and neutron doses is possible by comparing the readings of two lithium fluoride thermoluminescent dosimeters with different lithium-6 contents. Keywords: ď Ž Nearly complete elimination of neutron response in lithium fluoride is possible with lithium fluoride enriched with lithium-7. ď Ž In a mixed gamma and slow neutron field, distinction of gamma and neutron doses is possible by comparing the readings of two lithium fluoride thermoluminescent dosimeters with different lithium-6 contents.
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Thermoluminescent Dosimetric Readout Systems Thermoluminescent dosimetric readout systems are commonly made up of a sample holder, heating system, photomultiplier tube (light detector), high voltage supply, signal amplifier and a recording instrument. The thermoluminescent dosimetric sample is heated indirectly, using electrical resistance heat applied to a pan or planchette. The photomultiplier tube converts the light output into an electronic pulse that is then amplified before recording. The recording instrument may be a plotter or any other instrument that can measure the amplified photomultiplier tube output signal. A plot of the output signal versus time is equivalent to emitted light intensity versus heat and results in a glow curve. The area under the glow curve is proportional to the absorbed dose (Fig. 20). Uses of thermoluminescent measurement of radiation include personnel dosimetry, medical dosimetry, environmental monitoring and archeological and geological dating.
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FIGURE 19. Thermoluminescence process.5
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FIGURE 20. Typical glow curve. Integrated area under curve is measure of radiation exposure.
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Optically Stimulated Luminescence Dosimetry Optically stimulated luminescence dosimeters typically have aluminum oxide detectors and are available in plastic holders, or body badges, that are worn at collar level to measure full body dose. They can measure gamma ray and X-ray doses from 10ÎźSv to 10Sv (1 mrem to 1 krem).
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TDL
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http://www.rpe.org.in/article.asp?issn=0972-0464;year=2011;volume=34;issue=1;spage=6;epage=16;aulast=Bhatt;type=3
TDL
Charlie Chong/ Fion Zhang
http://www.rpe.org.in/article.asp?issn=0972-0464;year=2011;volume=34;issue=1;spage=6;epage=16;aulast=Bhatt;type=3
TDL
Charlie Chong/ Fion Zhang
http://www.rpe.org.in/article.asp?issn=0972-0464;year=2011;volume=34;issue=1;spage=6;epage=16;aulast=Bhatt;type=3
TDL
Charlie Chong/ Fion Zhang
http://www.rpe.org.in/article.asp?issn=0972-0464;year=2011;volume=34;issue=1;spage=6;epage=16;aulast=Bhatt;type=3
PART 6. Neutron Detection Characteristics The neutron is a part of the nucleus, has no charge and is somewhat larger in mass than the proton. It is similar to the photon in that it has no charge and produces ionization indirectly; it is different from the photon because it is a nuclear particle and not a unit of electromagnetic energy. Because the neutron is an uncharged particle, its interactions with matter are different from those of charged particles or photons. Ionization by neutrons is indirect: as a result of neutron interactions with matter, recoil nuclei, photons or charged particles are produced and then interact with matter by various mechanisms that cause ionization.
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Neutron Sources Neutrons are classified according to their energies as shown in Table 4. Some radionuclides (such as californium-252) may decay by spontaneousfission and emit neutrons with fission fragments, photons and electrons. Induced fission reactions, such as those occurring in a nuclear reactor with uranium, emit about 2.5 neutrons per fission. Fission neutrons range in energy from 0.025 eV to about 16 MeV. Other neutron sources are the result of various nuclear reactions and produce either a spectrum of neutron energies or monoenergetic neutrons. Common neutron producing nuclear reactions are the (γ, n), (α, n), (p, n), (d, n) and (α, 2n) reactions and may use radionuclide emissions or accelerated particles to initiate the reaction. Neutron radiography usually uses radionuclides that emit alpha or gamma photons and produce neutrons by (α, n) and (γ, n) reactions with various target materials.
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TABLE 4. Neutron classification. Class Thermal Epithermal Slow Intermediate Fast Relativistic greater than
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Energy < 0.3 meV >1 eV 0 meV to 100 eV 100 eV to 10 keV 10 keV to 10 MeV 10 MeV
Neutron Detectors There are several mechanisms and devices used to detect neutrons of various energies. Ionization chambers, proportional counters, scintillators, activation foils, track etch detectors, film emulsions, nuclear emulsions and thermoluminescent phosphors are some of the many devices used to detect neutrons. The main mechanisms used to detect neutrons in these devices are the (n, α), (n, p), (n, d), (n, f) and (n, γ) nuclear reactions. (n, f ) f= fragments Proportional Neutron Detectors Many fast and slow neutron counters use proportional counting chambers filled with boron trifluoride (BF3) gas, often enriched in boron-10. The interaction of thermal (slow) neutrons with boron gas releases an alpha particle of several mega electron volts that is easily detected in the proportional mode. Fast neutrons are detected by a similar counter, in which thermal neutrons are absorbed in an external cadmium shield; the fast neutrons that pass through the shield are thermalized in hydrogen rich material and counted in the proportional chambers. 10
5B
+ n → 42He2+ + 73Li + γ
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Scintillation Scintillators containing lithium-6, boron-10 and hydrogenous plastics have been used as neutron detectors. Lithium-6 is used as lithium iodide (europium activated) and in lithium glasses to detect slow and fast neutrons. Scintillators loaded with boron-10 are used for slow neutron detection. Plastic scintillators with high hydrogen content are used in fast neutron detection and spectroscopy by measuring the energy deposited by recoil protons. Activation Foils Introducing certain materials to an incident neutron flux will result in these materials becoming radioactive. The process is called activation and gaining information about the incident neutron flux and energy is possible by analyzing the radiations emitted from the activated foil. Activation foils rely on (n, Îł), (n, p), (n, Îą), (n, f ) and other nuclear reactions to cause the activation. Selection of the proper activation foil can give a rough estimate of the neutron energy spectrum. In high neutron flux fields, where instruments would fail, activation foils are used as integrating detectors.
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Miscellaneous Neutron Detectors Track etch detectors, nuclear emulsions and film have all been used to detect neutrons. Various neutron interactions with the detector material or foils in intimate contact with the detectors allow these systems to operate as integrating dosimeters.
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TABLE 6. Properties of Some Thermal Neutron Radiography Conversion Materials Material
Useful Reactions
Lithium
6Li(n,α) 3H
910
prompt
Boron
10B(n,α) 7Li
3,830
prompt
Rhodium
103Rh(n)104mRh
11
45 min
103Rh(n)104Rh
139
42 s
107Ag(n)108Ag
35
2.3 min
109Ag
91
24 s
Silver
(n)110 Ag
Cadmium
113Cd((n,γ)114Cd
Indium
115
In(n)116n
115 In(n)116mln
Samarium
149Sm(n,γ) 150Sm I52 Sm(n)153Sm
Cross Section for Thermal Neutrons (barns)
Life
20,000
prompt
157
54 min
42
14 s
41,000
prompt
210
47 h
Europium
151
Eu(n)152Eu
3,000
9.2 h
Gadolinium
155 Gd(n,γ) I56Gd
61,000
prompt
157 Gd(n.γ)158Gd
254,000
prompt
164 Dy(n)165mDy
2,200
1.25 min
800
140 min
99
2.7 days
Dyprosium
164 Dy(n)165Dy
Gold
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197
Au(n)198Au
PART 7. Semiconductors Certain semiconductor crystals, when exposed to ionizing radiation, become conductors and may be used as radiation detectors. Semiconductors are most often used for low level spectroscopic measurements of alpha particles, beta particles and gamma rays in laboratory settings and in X-ray diffraction equipment (Table 5). The most widely used semiconductor devices are diffused p-n junction, surface barrier and lithium drifted detectors. Semiconductor detectors have found their broadest application in the field of spectroscopy, although lithium drifted detectors are also being used for gamma ray detection.
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Detector The diffused p-n junction detector (Fig. 21a) gets its name from its manufacturing process. A slice of p type (electron depleted) silicon or germanium crystal, with a layer of n type (electron rich) impurity (usually phosphorus) deposited on the surface, is heated to form a p-n junction just below the surface. The phosphorus may also be painted onto the silicon and made to diffuse into it by applying heat. Because the n type material has an excess of electrons and the p type has an excess of holes (holes may be thought of as unit positive charges), the natural action of the combined materials tends to align the electrons on one side of the junction and the holes on the other. Thus a difference of potential is built up across the junction. By applying an external voltage to the crystal of such polarity as to oppose the natural movement of electrons and holes (reverse bias), the potential barrier across the junction is increased and a depletion region is produced.
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This depletion region is the sensitive part of the detector and is analogous to the gas volume in a gas ionization detector. Charged particles, on entering the depletion region, produce electron hole pairs analogous to the ion pairs produced in gas ionization chambers. Because an electric field exists in this region, the charge produced by the ionizing particle is collected, producing a pulse of current. The size of the pulse is proportional to the energy expended by the particle.
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Surface Barrier Detectors The operation of surface barrier and lithium drifted detectors is the same as for the p-n junction: a depletion region is produced, in which there exists an electric field. The means of producing the depletion region (as well as its dimension and location within the crystal) vary from one type of detector to another. The operation of a surface barrier detector (Fig. 21b) depends on the surface conditions of the silicon or germanium. At the surface of a piece of pure crystal, an electric field exists such that both holes and electrons are excluded from a thin region near the surface. For n type crystals, the field repels free electrons. If a metal is joined to the crystal, the free electrons are still repelled but a concentration of holes is produced directly under the surface. If a reverse bias is then applied, a depletion region is produced. Surface barrier detectors give better resolution for particle spectroscopy than p-n junctions but wider depletion regions are possible with the latter. (The wider the depletion region, the higher the energy of particles can be analyzed because a particle must expend all its energy in a depletion region.)
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Lithium Drifted Detectors The lithium drifted detector (Fig. 22) is produced by diffusing lithium into low resistivity p-type silicon or germanium. When heated under reverse bias, the lithium ions serve as an n type donor. These ions drift into the silicon or germanium in such a way that a wide layer of the p type material is compensated by the lithium, yielding an effective resistivity comparable to that of the intrinsic material. Wider depletion regions can be obtained with the lithium drift process than by any other means. Consequently, lithium drifted detectors are most useful in gamma spectroscopy work. Silicon detectors can be operated at room temperatures but exhibit low efficiency for gamma rays. Germanium detectors have higher gamma efficiencies but must be operated at liquid nitrogen temperatures. For these reasons, coupled with the small sensitive volumes obtainable to date, semiconductor detectors have not received widespread application in radiation survey instruments.
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TABLE 5. Radiation detector types.
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FIGURE 21. Cross sections: (a) diffused p-n junction detector; (b) surface barrier detector.
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FIGURE 21. Cross sections: (a) diffused p-n junction detector; (b) surface barrier detector.
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FIGURE 22. Cross section of lithium drifted detector.
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PART 8. Film Badges One of the most important uses of radiographic film as a means of measuring radiation is in film badges. Individuals who work with isotope radiation sources and X-ray machines are required by codes to wear badges indicating cumulative exposure to ionizing radiation. Film badges are discussed in this volumeâ&#x20AC;&#x2122;s chapter on radiation safety and elsewhere
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Latent Image Formation Latent image formation is a very subtle change in the silver halide grain of film. The process may involve the absorption of only one or, at most, a few photons of radiation and this may affect only a few atoms out of some 109 or 1010 atoms in a typical photographic grain. Formation of the latent image, therefore, cannot be detected by direct physical or analytical chemical means. The process that made an exposed photographic grain capable of transformation into metallic silver (by the mild reducing action of a developer) involved a concentration of silver atoms at one or more discrete sites on the photographic grain. In industrial radiography, the image forming effects of Xrays and gamma rays, rather than those of light, are of primary interest. The agent that actually exposes a film grain (a silver bromide crystal in the emulsion) is not the X-ray photon itself but rather the electrons (photoelectric and compton) resulting from an absorption event. The most striking difference between X-ray and visible light exposures arises from the difference in the amounts of energy involved.
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The absorption of a single photon of light transfers a very small amount of energy to the crystal â&#x20AC;&#x201D; only enough energy to free a single electron from a bromide (Brâ&#x20AC;&#x201C;) ion. Several successive light photons are required to make a single grain developable (to produce within it, or on it, a stable latent image). The passage of an electron through a grain can transmit hundreds of times more energy than the absorption of a light photon. Even though this energy is used inefficiently the amount is enough to make the grain developable. In fact, a photoelectron or compton electron can have a fairly long path through a film emulsion and can render many grains developable. The number of grains exposed per photon interaction varies from one (for X-radiation of about 10 keV) to 50 or more (for a 1 MeV photon). Because a grain is completely exposed by the passage of an energetic electron, all X-ray exposures are, as far as the individual grain is concerned, extremely short. The actual time that an electron is within a grain depends on the electron velocity, the grain dimensions and the squareness of the hit. A time on the order of 10â&#x20AC;&#x201C;13 s is representative. (In the case of light, the exposure time for a single grain is the interval between the arrival of the first photon and the arrival of the last photon required to produce a stable latent image.)
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Development Many materials discolor with exposure to light (some kinds of wood and human skin are examples) and could be used to record images. Most of these materials react to light exposure on a 1:1 basis â&#x20AC;&#x201D; one photon of light alters one molecule or atom. In the silver halide system of radiography, however, a few atoms of photolytically deposited silver can, by development, be made to trigger the subsequent chemical deposition of some 109 or 1010 additional silver atoms, resulting in an amplification factor on the order of 109 or greater. This amplification process can be uniform and reproducible enough for quantitative radiation measurements. Development is essentially a chemical reduction in which silver halide is converted to metallic silver. To retain the photographic image, however, the reaction must be limited largely to those grains that contain a latent image; that is, to those grains that have received more than a prescribed minimum radiation exposure.
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Compounds that can be used as photographic developing agents are those in which the reduction of silver halide to metallic silver is catalyzed (speeded up) by the presence of metallic silver in the latent image. Those compounds that reduce silver halide, in the absence of a catalytic effect by the latent image, are not suitable developing agents because they produce a uniform overall density on the processed film.
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Closing More information on the radiographic latent image, its formation and processing are available elsewhere. The correct use of film badges is especially important for safety in the conduct of radiographic testing programs and is discussed in this bookâ&#x20AC;&#x2122;s chapter on radiation safety and elsewhere.
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