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Natural Terrestrial Radiation

Day 3 – Lecture 7

Sources of Radiation

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OBJECTIVE

To discuss about the natural terrestrial radiation, radioactive decay chains, important radionuclides, radon and its significance, NORM, TENORM and NARM

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Content

Terrestrial decay chains

Important radionuclides

Radon and its health significance

NORM

TENORM

NARM

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Radioactivity in Nature

Primordial – existing since the creation of the Earth

Cosmogenic – formed as a result of cosmic ray interactions

Human produced – enhanced or formed due to human actions

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Primordial Nuclides

Nuclide Half-life Natural Activity235U 7.04 x 108 yr 0.711% of all natural

uranium238U 4.47 x 109 yr 99.275% of all

natural U; 0.5 to4.7 ppm total U in common rocks

232Th 1.41 x 1010 yr 1.6 to 20 ppm in common rocks

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Nuclide Half-life Natural Activity226Ra 1,600 yr 16 Bq/kg in limestone and

48 Bq/kg in igneous rock

222Rn 3.82 days Noble gas; average annual air concentrations in US range from 0.6 to 28 Bq/m3

40K 1.28 x 109 yr 0.037 to 1.1 Bq/g in soil

Primordial Nuclides

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Background Radiation

There are three decay “chains” that occur in nature:

the uranium series, beginning with 238U the thorium series, which originates with 232Th the actinium series, which originates with 235U

Once upon a time there was also a neptunium series, which originated with 241Pu, that has a half-life of only 14 years. The only remaining member of this series is 209Bi with a half-life of 2E18 years.

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238U Decay Chain

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Natural Radioactivity in Soil

Element*Assumed Activity**

Mass of Element* Activity

Uranium 25 Bq/kg 2,200 kg 31 GBq

Thorium 40 Bq/kg 12,000 kg

52 GBq

Potassium-40

400 Bq/kg 2,000 kg 500 GBq

Radium 48 Bq/kg 1.7 g 63 GBq

Radon 10 kBq/m3 11 g 7.4 GBq* Potassium-40 is a radionuclide ** per kg of soil

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Natural Radioactivity inBuilding Materials (mBq/g)

Material Uranium Thorium Potassium

Granite 63 8 1184

Sandstone 6 7 414

Concrete 31 8.5 89

Wallboard 14 12 89

Gypsum 186 66 5.9

Clay Brick 111 44 666

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Natural Radioactivityin the Oceans

Nuclide*Assumed Activity Pacific Atlantic All Oceans

Uranium 33 mBq/L 22 EBq 11 EBq 41 EBq

40K 11 Bq/L 7400 EBq 3300 EBq 14000 EBq

3H 0.6 mBq/L 370 PBq 190 PBq 740 PBq

14C 5 mBq/L 3 EBq 1.5 EBq 6.7 EBq

87Rb 1.1 Bq/L 700 EBq 330 EBq 1300 EBq

*Uranium is an element

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Background Radiation - Radon

Radon is a noble gas (also called “inert”).

Radon is chemically like other members of this group of the periodic table, namely He, Ne, Ar, Kr, and Xe, gas. The noble gases do not readily form compounds due to their stable electron shell configuration. With the exception of helium, they all have 8 electrons in their outer shell (ns2np6 for you chemists in the audience).

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Background Radiation - Radon

The average dose from radon-222 (222Rn) is approximately 2000 Sv per year. Radon is an alpha emitter. Many of the radon decay products are also alpha emitters.

Radon is produced from the radioactive decay of 238U an isotope of uranium which is naturally present in the environment. In fact, in some areas of the western US, the concentration of natural uranium is high enough that is it mined to provide a source of uranium for reactors.

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Production of 222Rnby Decay of 238U

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Background Radiation - Radon

Just as helium can diffuse through a balloon, radon can diffuse through the soil and foundations of homes. The diffusion is greater when the soil has low moisture content.

Radon is a radiological hazard because the decay products are alpha emitters. Since they are formed by emission of alpha particles, the resulting decay products have an electrostatic charge and are attracted to particulates in the air. These may become deposited in the lung. These particles then decay by alpha emission, which results in a dose to the lung.

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Radon Diffusion Through Soil

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Radon Transport Into Homes

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Background Radiation - Radon

The concern is really not with the radioactive decay of radon, but with the radon progeny (also called “daughter products”), that are produced when radon decays. Since radon decays by alpha emission, the radon “daughters” have a ++ charge and so are electrostatically attached to particulates in the air. These decay products are also radioactive, and many decay by alpha particle emission.

The alpha particle energy delivers a dose to the lung where the particles are deposited. The dose to the lung is attributed to the development of lung cancer for uranium miners (hence the regulatory limits for radon and radon daughter concentrations).

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Relative Risk from Radon

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Radionuclides Foundin Your Body

Nuclide*Total Mass of

Nuclide Found in the Body

Total Activity of Nuclide Found in

the Body

Daily Intake of Nuclides

Uranium 90 g 30 pCi (1.1 Bq) 1.9 g

Thorium 30 g 3 pCi (0.11 Bq) 3 g

40K 17 mg 120 nCi (4.4 kBq) 0.39 mg

Radium 31 pg 30 pCi (1.1 Bq) 2.3 pg

14C 95 g 0.4 Ci (15 kBq) 1.8 g

*Uranium, Thorium and Radium are elements

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Natural Radiation Exposure Around the World

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Effective Dose Equivalent to a Member of the Population of the United States

Source

Average AnnualEffective Dose Equivalent Sv mrem

Inhaled (radon and decay products) 2000 200

Other Internally Deposited Radionuclides

390 39

Terrestrial Radiation 280 28

Cosmic Radiation 27 3

Cosmogenic Radioactivity 10 1

Rounded Total from Natural Sources 3000 300

Rounded Total from Artificial Sources 600 60

Total 3600 360

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Natural Sources2.4 mSv (79%)

Others× 0.005 mSv (<1%)

Medical Exposures0.64 mSv (21%)

Per Capita Annual Dose – UNSCEAR 2008

×Others include Fallout, Nuclear Power Production, Occupational exposures etc.

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Per Capita Annual Doses from Natural Sources – UNSCEAR 2008

Radon Inhalation1.15 mSv (48%)

Cosmic Radiation0.39 mSv (16%)

Ingestion 0.29 mSv (12%)

Thoron Inhalation0.1 mSv (4%)

External GammaRadiation0.48 mSv (20%)

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NORM and NARM

NORM Naturally Occurring Radioactive Material

TENORM Naturally Occurring Radioactive Material enhanced by

processing is called TENORM NARM Naturally Occurring and

Accelerator-Produced Radioactive Material

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Summary

Natural terrestrial radiation was discussed

Important radionuclides and their distribution was discussed

Radon in particular and its significance was discussed

NORM, TENORM and NARM were defined

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Where to Get More Information

Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)

UNSCEAR, Sources and Effects of Ionizing Radiation, 2008 Report to the General Assembly with Scientific Annexes, United Nations, New York, 2008

International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources(PGEC), Training Course Series 18, IAEA, Vienna (2002)