Radiation Basics Energy Deposition in Matter Jeffrey A. Mahn Nuclear Engineer (Retired) Albuquerque, NM USA jamahn47@gmail.com
Ionizing & Non-Ionizing Radiation • Ionizing Radiation: Energy transmitted as particles or electromagnetic waves and having sufficient energy to dislodge orbital electrons in atoms, thereby producing ions. Examples: alpha, beta, gamma, and x-ray • Non-Ionizing Radiation: Electromagnetic waves that do not have sufficient energy to dislodge orbital electrons in atoms; produces a heating effect in body tissue. Examples: visible light, infrared, microwaves, radio waves, and radar
Ionizing Radiation • Alpha particle • Beta particle • Gamma ray
Types of Radiation • Alpha Particle – a – Helium nucleus (2 protons & 2 neutrons)
• Beta Particle – b
– Energetic electron emitted from atomic nucleus when a neutron converts to a proton and an electron
• Gamma Rays – g
– High energy electromagnetic wave emitted when nucleus of atom transitions from higher to lower energy state
Radioactive Decay The process by which an unstable atomic nucleus becomes more stable by emitting energy in the form of particles or ionizing electromagnetic waves
Unstable Atomic Nuclei What makes some atoms radioactive? The ratio of neutrons to protons in the atomic nucleus is either too large or too small, causing the nucleus to be unstable. β- (electron) emission – occurs when a neutron in the nucleus converts to a proton β+ (positron) emission – occurs when a proton in the nucleus converts to a neutron
Alpha & Beta Decay • Original nucleus called parent nucleus
• Nucleus resulting from radioactive decay called daughter nucleus • Alpha and beta emission change original atomic element (parent) into a different atomic element (daughter) – Daughter nucleus has different number of protons than parent nucleus and therefore is different element
Alpha Particle Radiation
Daughter Nucleus Parent Nucleus
a
Alpha Particle (Helium Nucleus)
Beta Particle Radiation
Antineutrino
Daughter Nucleus
Parent Nucleus
b
Beta Particle
Gamma Radiation • Gamma radiation oftentimes accompanies alpha and beta radioactive decay processes in atoms – Has no effect on number of protons or neutrons in daughter nucleus produced from particle decay – Gamma-ray emission changes energy state of daughter nucleus from higher energy state to lower energy state or to ground energy state When transition is immediate, gamma-ray is associated with parent alpha or beta decay Metastable isotopes (e.g., Tc-99m) decay by gamma-ray emission and have associated decay half-lives – known as isomeric transition (IT)
Gamma-Ray Radiation
β
_
Beta Particle
Gamma Rays Parent Nucleus
Daughter Nucleus
Decay Scheme for Cobalt-60
Radioactive Material Half-Life
1200 1000 800 Activity 600 400 200 0 New
The time required for the amount of radioactive material to decrease by one-half
1 2 3 4 Half- Half- Half- HalfLife Lives Lives Lives
Cosmic
Inhaled Radon
Bodies
Plants
Radioactive Elements K-40, Uranium, Radium
We live in a sea of radiation‌ Rocks
Quantifying Radioactivity • Decay rate of radioactive material is λN, where – λ is decay constant for particular radioisotope (usually expressed as disintegrations per second) – N is number of radioisotope atoms present in radioactive material
• Quantity λN known as activity of radioisotope – Activity expressed in units of Becquerel (Bq) or Curie (Ci) – 1 Bq = 1 atomic disintegration per second (dis/sec), 1 Ci = 3.7x1010 dis/sec
Quantifying Radioactivity (cont.) • Decay constant of radioactive isotope can be calculated from its half-life (λ = 0.693/t1/2) – Radioisotope half-life is experimentally observed quantity – The smaller the half-life, the larger the decay constant – Radioisotope activity (i.e., decay rate) directly proportional to decay constant (e.g., large λ → large activity)
• Radioisotope activity can be converted to dose rate for calculating radiation exposure
Average Annual Radiation Exposure • Natural background (cosmic & terrestrial, including radon) varies around the world U.S. Average -- 350 mrem/y (3.5 mSv/y) Cosmic 30 mrem/y (0.3 mSv/y) Terrestrial 30 mrem/y (0.3 mSv/y) Internal (K-40) 40 mrem/y (0.4 mSv/y) Inhaled (Radon) 200 mrem/y (2.0 mSv/y) Medical/Etc. 50 mrem/y (0.5 mSv/y)
Terrestrial Sources of Radiation Isotope
Half-life (yrs)
Radiation α, γ
Ra-226
1622
U-238
4.5 x 109
α
Th-232
1.4 x 1010
α, γ
K-40
1.3 x 109
β, γ
Rb-87
4.7 x 1010
β
In-115
6 x 1014
β
La-138
1.1 x 1011
β, γ
Sm-147
1.2 x 1011
α
Lu-176
2.1 x 1010
β, γ
Typical Radioactivity in Common Building Materials Material Granite Sandstone Cement Limestone concrete Sandstone concrete Dry wallboard By-product gypsum Natural gypsum Wood Clay Brick
U-238 (pCi/g) 1.7 0.2 1.2 0.8 0.3 0.4 5.0 0.4 3
Th-232 (pCi/g) 0.22 0.19 0.57 0.23 0.23 0.32 1.8 0.2 1.2
K-40 (pCi/g) 32 11 6.4 2.4 10 2.4 0.2 4 90 18
Estimates of concentrations of uranium, thorium and potassium in in building materials (NCRP 94, 1987)
Natural Radioactivity in the Body Total Activity of Nuclide Nuclide in the Body Uranium 30 pCi Thorium 3 pCi Potassium 40 120,000 pCi Radium 226 30 pCi Carbon 14 400,000 pCi Tritium 600 pCi Polonium 1,000 pCi Estimated concentrations of radionuclides calculated for a 70 kilogram adult based on ICRP 30 data
Natural Radioactivity in Food Food
40K
(βγ) pCi/kg
226Ra
(α) pCi/kg
Brazil Nuts
5600
1000-7000
Lima Bean
4640
2-5
Banana
3520
1
White Potato
3400
1-2.5
Carrot
3400
0.6-2
Red Meat
3000
0.5
Low-sodium Salt
3000
---
Beer
390
---
---
0-0.17
Drinking Water
Natural Radiation Exposures • Radioactive content of earth’s crust responsible
for about 75% of natural radiation exposure
• Areas in the world where natural background radiation is much higher than in U.S. (360 mrem/yr avg.) average annual dose in Araxa, Brazil is 2,500 mrem annual dose in State of Kerala, India as high as 3,500 mrem annual dose in Guarapari, Brazil as high as 17,500 mrem annual dose in Ramsar, Iran as high as 26,000 mrem
Interaction With Matter -------------------------------------------------Relative Relative Radiation Range LET* -------------------------------------------------Alpha 1 10,000 Beta 100 100 Gamma 10,000 1 -------------------------------------------------* Linear Energy Transfer – energy deposition per unit of distance traveled
What Does This Mean?
Radiation
Range in Air
Alpha (4-5 MeV)a
1-2 inches
Beta – 0.1 MeV 1.0 MeVb 3.0 MeV
4 inches 12 feet 43 feet
Notes: a. Alpha energy associated with Uranium, Thorium, and Plutonium decay
b. Average maximum energy of beta particles from fission products is about 1.2 MeV
Penetrating Properties of Ionizing Radiation
Focus of Radiation Concerns Based on Penetrating Power Internal radiation hazards • result from inhalation or ingestion of alpha and beta sources • concern about skin and clothing contamination with alpha and beta sources is proximity to pathways for inhalation and ingestion
External radiation hazards – gamma- and xrays
Absorbed Dose • Radiation Absorbed Dose (rad) – Energy deposition per unit mass of material – Describes concentration and not amount of energy absorbed in exposed mass
International Standard – Gray [Gy] 1 Gy = 100 rad
Radiation Effects • Depend on absorbed dose and LET • Effect on biological systems quantified using relative biological effectiveness [RBE] Radiation X-rays Gamma radiation Beta particles Thermal neutrons Fast neutrons Alpha particles
RBE Factor 1 1 1 5 10 20
Equivalent Dose • Historical/conventional unit
Dose (rem) = RBE x Dose (rad) • Provides common reference and allows for additive doses • Represents POTENTIAL for biological effects
Equivalent Dose Units • International (SI) unit – Sievert [Sv] Dose (Sv) = RBE x Dose (Gy) 1 Sv = 100 rem
Radiation Terms and Units
Activity Absorbed Dose
Traditional
S.I.*
Curie (Ci) 3.7x1010 dis/sec
Becquerel (Bq) 1 dis/sec
Rad 100 ergs/gm
Gray (Gy) 1 joule/gm
Rem
Sievert (Sv)
Committed Effective Dose
* Abbreviation for International System of Units
Dose Factors • Acute Dose – Dose delivered over a short period of time
• Chronic Dose – Dose delivered over an extended period of time
• Dose Rate Effect – Acute dose generally more damaging than chronic dose of same magnitude because short term cell damage overwhelms body’s natural repair mechanisms
Nuclear Regulatory Commission Administrative Limits Subjects Exposed
Time Frame
Dose (mrem)
Nuclear Worker
1 year
5000
General Public (from nuclear facility)
1 year
100
9 months
500
Pregnant Woman
Radiation Damage • Biological Tissue – Dose & dose rate (acute or chronic) – Direct effects → break important molecular chains – Indirect effects → break less critical molecules into reactive parts
Radiation Damage (cont.) • Hierarchy of susceptibility – – – – – –
Lymph nodes (most susceptible to damage) Blood Bone Nerve Brain Muscle (least susceptible to damage)
Human Response • LD 50/30: Lethal acute dose for 50 percent of exposed population within 30 days without medical attention (~ 450 rem)
• Threshold for detectable medical effects (red & white blood cell count): ~ 25 rem
Human Response (cont.) • Potential delayed effects may include – Leukemia/Cancers – Cataracts – Genetic Effects
– Blood Disorders – Lifespan Shortening
Correcting a Radiation Exposure Misconception • Exposure to gamma radiation does not make
humans, animals, or food radioactive • Only way to make an atom radioactive is to change ratio of neutrons to protons in atomic nucleus • Gamma-rays only interact with atomic electrons and have no effect on numbers of neutrons and protons in atomic nucleus
Dose Reduction and Control • Basic principles – Restrict proximity TIME Dose = Dose Rate x Time
– Increase DISTANCE from the source Point source: “1-over-R-squared” reduction
– Use SHIELDING material
Radiation Basics Summary • Types of ionizing radiation – alpha, beta, gamma-ray, x-ray • Our radiation environment – cosmic, terrestrial, inhaled (radon), internal • Radiation health concerns based on penetrating power External hazard – gamma-ray and x-ray Internal hazard – alpha and beta
Radiation Basics Summary (cont.) • Dose factors – biological effect of acute exposure more serious than chronic exposure of same magnitude • Radiation protection principles Time – minimize exposure time Distance – maximize distance from source Shielding – insert appropriate shielding material between people and source
Contact Information Jeffrey A. Mahn Nuclear Engineer (Retired) Albuquerque, NM USA jamahn47@gmail.com