Nuclear Radiation 101 (Bonne Posma) USofA

NUCLEAR RADIATION 101 Bonne Posma September 13, 2011 1. Radiation refers to particles or waves traveling through space which transfer part or all of their energy upon contact with matter. It includes the electromagnetic spectrum such as radio and light waves. Nuclear radiation is radiation with sufficient energy to dislodge electrons from atoms to create positive ions or free radicals, which are chemically reactive. It is therefore also referred to as ionizing radiation and can penetrate living cells to create ions in the cell structure. The most common source of nuclear radiation is the decay of the nuclei of certain unstable elements such as Radium, which undergoes spontaneous change to a new and different element, and in the process, emits this ionizing radiation.

2. Types of ionizing radiation due to nuclear decay.  α (alpha) rays - helium nuclei: 2 protons and 2 neutrons; approx 8,000 X electron mass  β (beta) rays – which are electrons with a mass of 9 X kg  γ (gamma) rays – high energy electromagnetic waves, higher in frequency than X-rays.

Note: X-rays are also a type of ionizing radiation, but are not as energetic as gamma rays. X-rays are created as a result of activity of the electron orbits of atoms, whereas gamma rays are created as a result of activity in the nucleus of an atom.

3. Nuclear Decay - a tumultuous story of the relationship between parents and daughters.

99.3 % of naturally-occurring Uranium is Uranium-238 (U-238), with the remaining 0.7% consisting of U-235 which, if concentrated from 0.7 % to over 80% can be used for nuclear weapons. These two manifestations of Uranium are called isotopes, which are forms of the same element that are chemically identical, since they have the same number of protons (92) in their nuclei and 92 electrons in their atomic orbits, but have different numbers of neutrons: 146 and 143 for U-238 and U-235 respectively. Both isotopes are unstable, and emit alpha rays or particles, as their nuclei disintegrate or decay to new and different elements.

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We shall look at the decay of the more abundant isotope: U-238 , which emits alpha particles (2 neutrons and 2 protons) to form a new element called Th-234 ( Thorium) which has a nucleus of 90 protons and 144 neutrons ( 90 + 144 = 234).

Nuclear physics convention denotes U-238 as the parent and Th-234 as the daughter.

Th-234 is also unstable, and spontaneously disintegrates to Pa-234 (Protactinium) by the emission of an electron or beta ray. The electron or beta ray comes from one of the protons in the nucleus of Th-234 to convert it to a neutron, so the sum of the neutrons and protons does not change. However, the isotope of the new element Pa-234 has one more proton than its parent Th-234 but one less neutron.

The sum of the newly formed element consist of 91 protons plus 143 neutrons = 234.

In this last transmutation (another word for change to a new element through decay) Th-234 became the parent, and Pa-234 the daughter.

But the nucleus of Pa-234 is also unstable and spontaneously changes to yet another element, and so it goes on until eventually, after 12 more decays, Pb-208 is created (Lead) which is stable.

According to the periodic table, the number of protons in the nucleus of an element establishes its atomic number. In the case of these first three elements in this spontaneous nuclear process of decay, the atomic numbers of the first three elements are: Uranium : 92 Protactinium: 91 Thorium: 90 The spontaneous change from one element to another element is called nuclear decay, and the resulting process is called a nuclear decay chain. There exist a number of decay chains, and the one for U-238 as the primordial parent is depicted in Fig 1.

U-238 occurred 4.5 billions of years ago when the earth was formed, and has a halflife of, co-incidentally, 4.5 billion years. A half-life is the amount of time it takes an element to decay until half its mass is left. Thus, the total amount of U-238 on earth is about a half of what it was when the earth was formed. The number below each element in Fig 1 indicates its half-life. For the first three decays starting with U238, we have: U-238: 4.5 billion years Th-234: 24 days Pa-234: 1.2 minutes

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Fig 1 Nuclear Decay chain from Radium (element discovered by Madame Curie in 1898) 226Ra (Radium) (1602 y)

α

230Th (80 ky)

α

234U (247 ky)

β

234Pa (1.2 m)

β

234Th (24 d)

Parent

α

238U (Uranium) (4.5 by)

α 222Rn (Radon) (3.825 d)

214Po (Polonium) (1.6 X 10-6 s)

α α 214Pb (Lead) (26.8 m)

β

β

218Po (Polonium) (3.05 m)

214Bl (19.7 m)

210Po (Polonium) (138.4 d)

210Bl (5.01 d)

α

Legend

α

β

β 210Pb (Lead) (22 y)

y- year ky- 1,000 years by- 1billion years s- seconds m- minutes h- hours

208Pb (Lead) (STABLE)

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4. Penetrating power of ionizing radiation

α rays β rays γ rays

5 cm of Air

0.2 cm of Skin

5-100 cm of Lead

5. Other Sources of Ionizing Radiation  medical X-rays  neutrons (from nuclear reactions, NOT from spontaneous decay)  cosmic rays from outer space 6. Initial method used to measure Ionizing Radiation.

In 1928, the roentgen (R), named after the discoverer of X-rays, Wilhem Roentgen, was adopted as a measure of the amount of ionizing radiation released in one cm3 of dry air by a radioactive source. One roentgen/ is equal to 1 esu (electro static unit) which corresponds to 2.08 X ion pairs. In 1975, the SI unit for ionizing radiation, C/kg (coulombs/kg) where “C” is a unit of charge equal to one ampere.second equal to the charge of 6.28 X was adopted instead of the roentgen. The conversion is 1 R = 2.58X C/kg and 1 C/kg = 3.875 X R. R and C/kg mainly indicate the effects of gamma radiation in air, and do not fully take into account the effects of alpha and beta rays. Moreover, most of us are interested in the effects of all three types of radiation on living tissue, and the above units are therefore not generally useful in describing the effects of radiation on humans. This led to the adoption of other units such as the rem and Sievert described in sections 7 and 8..

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As a side note, it was determined early in the 1900’s that exposure to gamma radiation of 500 R/hour intensity is fatal to humans.

7. SI (International Systems for Units) radiation units of measurement – adopted in 1975.  becquerel (Bq) = 1 one decay per second – a decay consists of a nucleus spontaneously expelling a β or α particle (or ray) , sometimes accompanied by the emission of γ rays. In a “fixed” mass of radioactive material, the number of becquerels/second decreases with time as the material decays.

(There also occurs a miniscule decrease in mass – but it is so small that it cannot be conveniently measured.)  gray (Gy) = 1 J/kg (1 joule per kilogram) - the amount of radioactive energy absorbed by 1 kg of matter.  Sievert (Sv) the amount of radioactive energy absorbed by 1 kg of living tissue. It is equal to Q times the number of grays where Q is the quality factor, which varies from 1 to 20. It is 1 for β , γ and X-rays, in which case 1 Sievert = 1 gray. It is 20 for α rays, so that a living tissue exposed to 1 gray of α rays will be exposed to 20 Sieverts.  millisievert ( mSv) and centigray (cGy). The Sv and Gy are unwieldy large units and for convenience, smaller units are usually found in articles about nuclear radiation. They are the mSv ( and the cGy ( Gy) .

8. Units of Radiation Measurement used in the USA  The US NRC (Nuclear Regulatory Commission), successor to the AEC (Atomic Energy Commission) founded in 1956, requires the use of the following units in the Code of Federal Regulations 10CFR20 . Note that these units were in use by these agencies before SI units were adopted in 1975.  curie (Ci) = 3.7 X decays per second. This is approximately equivalent to the decay activity of 1 gram of fresh Radium-226. (pCi = .000 001 Ci)  rad = 0.01J/kg (.01 joules per kilogram) - the amount of radioactive energy absorbed by 1 kg of matter  rem is the amount of radioactive energy absorbed by 1 kg of living tissue. It is equal to Q times the number of rads, so that 1 rem = 1 rad for β, γ and X-rays; and 1 rad of α rays equals 20 rems.  ( 1 mrem = .001 rem)

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9. Converting between Old and New Units  1 Bq = 2.70 X 1 Ci = 3.7 X

Ci Bq

 1Gy = 100 rad 1 rad = 0.01 Gy = 1 cGy  1 Sv = 100 rem 1 rem = 0.01Sv = 10 mSv  1 R = .876 rad (only for gamma exposure to dose in air – it varies considerably for other materials and human tissue) 1 R/hour = 0.12 Gy 1 Gy = 8.38 R/hour 10. Emissions of some natural radioactive nuclei (* indicates artificially created radioactive nuclei) Table 1 Plutonium Plutonium Thorium Uranium

Ra-226 Rn-222 Po-210 Am-241*

Radium Radon Polonium Americium

β ONLY

H-3* Sr-90* C-14 Cs-137*

Tritium Strontium Carbon Cesium

Ba-133* Co-60*

Barium Cobalt

Cs-134* I-131* K-40 I-131* K-40

Cesium Iodine Potassium Iodine Potassium

α ONLY

Pu-238* Pu-239* Th-232 U-238

γ ONLY

Element

β AND γ

Symbol

half life 87.7 years 24,200 years 14 billion years 4.5 billion years 1,602 years 3.8 days 138 days 432 years 12.3 years 28.5 years 5,730 years 30.2 years 10.7 years 5.27 years 2.06 years 8.02 days 1.25 billion years 8.02 days 1.25 billion years

decay energy (keV) 5,593 5,245 4,083 4,267 4,871 5,590 5,304 5,486 18.6 546 49.5 1,174 356 1,332

power/kg (W/kg) 570 2 X 10-3

140,000 100 900

17,900

606 (β) 1,310 (β) 364 (γ) 1,505 (γ)

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Figure 1: β (electron) emissions decay 10000

1,174 1000

546 Man-made sources of β radiation 70

keV 100

49.5

30 18.6 10

1 H-3 (Tritium)

C-14 (Carbon)

Sr-90 (Strontium)

Cs-134 (Cesium)

2000 Vintage Medical XColor TV Ray

Figure 2: Sources of Natural Radiation

Cosmic 11% Terrestrial 11%

Radon 78%

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11. Background radiation throughout the world  Average in the USA - 3 mSv/year  Colorado Plateau – 6 mSv/year  Gerais, Brazil – 23 mSv/year  Ramasari, Iran – 48 mSv/ year  Nuclear industry workers: 50 mS/year is the maximum allowable exposure  Hormesis Optimum (Luckey) : 100 mSv/year (this will be explained later) Figure 3: Background Radiation 120 100 100 80 mSv / year 60

48

50

40 23 20 3

6

0 Average in Colorado the USA Plateau

Gerais, Ramasari, Nuclear Hormesis Brazil Iran Industry Optimum Workers * (Luckey)

*Maximum allowable exposure

12. Dangerous Radiation Levels RS50: 1,000 mSv/day will result in radiation sickness for 50% of humans : 8,000 mSv/day is lethal for 100% of humans

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Figure 4: Radiation Exposure Effects

RS50

L50

L100

**Not visible: .008/mSv/Day (3 mSv/Year) - average US background radiation 0.273 mSv/Day (100 mSv/year) is Hormesis Optimum

13. Nuclear Disasters - Radiation Levels -

Three Mile Island within 16 km radius - .08 mSv/year Fukushima – 0.87 mSv/year and declining (as of August 2011) Chernobyl Forced Evacuation area - 5mSv/year which below the Colorado Plateau level of 6 mSv/year.

Recall: average US background radiation level is 3 mSv/year

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Figure 5: Nuclear Disasters - Radiation Levels

6 5 5 Three Mile Island within 16 k radius.08 mSv/year 4 3

Fukushima- 0.87 mSv/year and declining (as of August 2011)

mSv/year 3 Chernobly Forced Evacuation area5 mSv/year (below the Colorado Plateau level of 6mSv/year)

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Average US background radiation level is 3 mSv/year 0.87

1 0.08 0

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Injuries/Deaths due to nuclear power plant disasters a. Three Mile Island ( March 1979) – deaths: 0; injuries: 0; long-term health or genetic defects: none found b. Fukushima (March 2011) – deaths: 0. Injuries ; 3 with beta burns, who have fully recovered ( As of August 2011, no other adverse health effects have been found) c. Chernobyl (April 1986) – confirmed deaths according to 2006 UNSCEAR report: 57. Predicted cancer deaths: 4,000. No unusual birth defects have been found.

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