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What is Radioactivity ?

Our world is radioactive and has been since it was created. Over 60 radionuclides (radioactive elements) can be found in nature, and they can be placed in three general categories:

  1. Primordial - from before the creation of the Earth
  2. Cosmogenic - formed as a result of cosmic ray interactions
  3. Human produced - enhanced or formed due to human actions (minor amounts compared to natural)

Radionuclides are found naturally in air, water and soil. They are even found in us, being that we are products of our environment. Every day, we ingest and inhale radionuclides in our air and food and the water. Natural radioactivity is common in the rocks and soil that makes up our planet, in water and oceans, and in our building materials and homes. There is nowhere on Earth that you can not find Natural Radioactivity and the overdose or overexposure of radioactivity is very dangerous for human life.

Radioactivity Allowed Limit

Health Risk Chart

Quantities of Source Material Requiring Labeling

Radioactive elements are often called radioactive isotopes or radionuclides or just nuclides. There are over 1,500 different radioactive nuclides. Often, radionuclides are symbolized based on the element and on the atomic weight, as in the case of radioactive hydrogen or tritium with an atomic weight of 3 is shown as H-3 or 3H. As another example, Uranium with the atomic weight of 235 would be shortened to U-235 or 235U.

Note: Many of the units used in science are broken down into smaller units or expressed as multiples, using standard metric prefixes. As examples, a kilobecquerel (kBq) is 1000 becquerels, a millirad (mrad) is 10-3 rad, a microrem (µrem) is 10-6 rem, a nanogram is 10-9 grams, and a picocurie is a 10-12 curies. These are examples of units used frequently throughout this short paper.

Common abbreviations used on this page are: m - meter, m3 - cubic meter, g - gram, kg - kilogram, Bq - becquerel, Sv - sievert, Gy - gray, Ci - curie, ppm - parts per million, yr - year, hr - hour, L - liter.

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In the United States, the annual estimated average effective dose equivalent from natural radiation is 310 per adult. This is broken down as:

Annual estimated average effective dose equivalent received by a member of the population of the United States.

Average annual effective dose equivalent

Source

NCRP 95 Pub-1987

NCRP 160 Pub-2006

(µSv)

(mrem)

(µSv)

(mrem)

Inhaled (Radon and Decay Products)

2000

200

2290

229

Other Internally Deposited Radionuclides

390

39

310

31

Terrestrial Radiation

280

28

190

19

Cosmic Radiation

270

27

270

27

Rounded total from natural source

3000

300

3100

310

Rounded total from artificial Sources (Medical, industrial, etc)

600

60

3100

310

Total

3600

360

6200

620

Note - the above are average dose to the US. This is calculated by taking the total collective dose in the US and dividing by the US population. Shown in the table above, in 1987, there was 82% of the total average annual effective dose is from natural sources of radiation, and of that, most is from radon. Of the other 18%, the majority is from medical diagnosis and treatments, with <1% from nuclear power and fallout. In 2006, the medical exposures dominate the average. The increase was due mostly to the higher utilization of computed tomography (CT) and nuclear medicine. Therefore, unless you were undergoing CT scans or nuclear medicine, your annual background dose did not change significantly.

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Primordial radionuclides

Primordial radionuclides are left over from when the world and the universe were created. They are typically long lived, with half-lives often on the order of hundreds of millions of years. Radionuclides that exist for more than 30 half-lives are not measurable. The progeny or decay products of the long lived radionuclides are also in this heading. Here is some basic information on some common primordial radionuclides:

Primordial nuclides

Nuclide

Symbol

Half-life

Natural Activity

Uranium 235

235U

7.04 x 108 yr

0.72% of all natural uranium

Uranium 238

238U

4.47 x 109 yr

99.2745% of all natural uranium; 0.5 to 4.7 ppm total uranium in the common rock types

Thorium 232

232Th

1.41 x 1010 yr

1.6 to 20 ppm in the common rock types with a crustal average of 10.7 ppm

Radium 226

226Ra

1.60 x 103 yr

0.42 pCi/g (16 Bq/kg) in limestone and 1.3 pCi/g (48 Bq/kg) in igneous rock

Radon 222

222Rn

3.82 days

Noble Gas; annual average air concentrations range in the US from 0.016 pCi/L (0.6 Bq/m3) to 0.75 pCi/L (28 Bq/m3)

Potassium 40

40K

1.28 x 109 yr

soil - 1-30 pCi/g (0.037-1.1 Bq/g)

Some nuclides like 232Th have several members of its decay chain. You can roughly follow the chain starting with 232Th

232Th --> 228Ra --> 228Ac --> 228Th --> 224Ra -->
      220Rn --> 216Po --> 212Pb --> 212Bi --> 212Po --> 208Pb (stable)

You can see how the decay process works with this interesting Decay Science Trek from University of Colorado.

Some other primordial radionuclides are 50V, 87Rb, 113Cd, 115In, 123Te, 138La, 142Ce, 144Nd, 147Sm, 152Gd, 174Hf, 176Lu, 187Re, 190Pt, 192Pt, 209Bi.

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Cosmogenic

Cosmic radiation permeates all of space, the source being primarily outside of our solar system. The radiation is in many forms, from high speed heavy particles to high energy photons and muons. The upper atmosphere interacts with many of the cosmic radiations, and produces radioactive nuclides. They can have long half-lives, but the majority have shorter half-lives than the primordial nuclides. Here is a table with some common cosmogenic nuclides:

Cosmogenic Nuclides

Nuclide

Symbol

Half-life

Source

Natural Activity

Carbon 14

14C

5730 yr

Cosmic-ray interactions, 14N(n,p)14C

6 pCi/g (0.22 Bq/g) in organic material

Hydrogen 3 
(Tritium)

3H

12.3 yr

Cosmic-ray interactions with N and O, spallation from cosmic-rays, 6Li(n, alpha)3H

0.032 pCi/kg 
(1.2 x 10-3 Bq/kg)

Beryllium 7

7Be

53.28 days

Cosmic-ray interactions with N and O

0.27 pCi/kg (0.01 Bq/kg)

Some other cosmogenic radionuclides are 10Be, 26Al, 36Cl, 80Kr, 14C, 32Si, 39Ar, 22Na, 35S, 37Ar, 33P, 32P, 38Mg, 24Na, 38S, 31Si, 18F, 39Cl, 38Cl, 34mCl.

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Human Produced

Humans have used radioactivity for one hundred years, and through its use, added to the natural inventories. The amounts are small compared to the natural amounts discussed above, and due to the shorter half-lives of many of the nuclides, have seen a marked decrease since the halting of above ground testing of nuclear weapons. Here are a few human produced or enhanced nuclides:

Human Produced Nuclides

Nuclide

Symbol

Half-life

Source

Tritium

3H

12.3 yr

Produced from weapons testing and fission reactors; reprocessing facilities, nuclear weapons manufacturing

Iodine 131

131I

8.04 days

Fission product produced from weapons testing and fission reactors, used in medical treatment of thyroid problems

Iodine 129

129I

1.57 x 107 yr

Fission product produced from weapons testing and fission reactors

Cesium 137

137Cs

30.17 yr

Fission product produced from weapons testing and fission reactors

Strontium 90

90Sr

28.78 yr

Fission product produced from weapons testing and fission reactors

Technetium 99

99Tc

2.11 x 105 yr

Decay product of 99Mo, used in medical diagnosis

Plutonium 239

239Pu

2.41 x 104 yr

Produced by neutron bombardment of 238
238U + n--> 239U--> 239Np +ß--> 239Pu+ß)

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