1.3 Naturally Occurring Radioactive Material (NORM)

NPRE 441, Principles of Radiation Protection

Chapter 1: Radioactivity

1.3 Naturally Occurring Radioactive Material (NORM)

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An Overview of Radiation Exposure to US Population

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Naturally Occurring Radioactivity

Radioactivity of building materials

An example of environmental radiation

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• The existence of the radioactive gas in the three series is one of the chiefreasons for the presence of naturally occurring environmental radioactivity.


• The probable atmospheric radon concentration is on the order of 2x10‐6 Bq/mL.Significant increase in background radiation level can be expected in the case ofrain fall.

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Terrestrial NORM consists of radioactive material that comes out of the Earth’s crustand mantle, and where human activity results in increased radiological exposure.The materials may be original (such as uranium and thorium) or decay productsthereof, forming part of characteristic decay chain series, or potassium‐40. The twomost important chains providing nuclides of significance in NORM are the thoriumseries and the uranium series.

Terrestrial NORM

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http://www.world‐nuclear.org/info/inf30.html258


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Radioactivity of building materials

An example of environmental radiation

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• All of the heavy elements (Z>83) found in nature are radioactive and decay byalpha and beta emission. 209

83Bi is the only stable nuclide with an atomicnumber greater than that of lead (Z=82).

Terrestrial NORM

• All of the naturally occurring heavy radionuclides belong to one of the threeseries, the uranium series, the thorium series and the actinium series.

• Several other elements, outside the uranium, actinium and thorium series, arealso found to have radioactive isotopes, such as 40K, 87Rb etc.

• Other naturally occurring radionuclides are of cosmogenic origin. Only thoseproduced as a results of cosmic ray interactions with constituents of theatmosphere results in any mentionable exposure to man: 3H, 7Be, 14C and 22Na.

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Common characteristics of radioactive series:• The first member of each series is very long‐lived – 232Th: 1.391010 years, 238U:

4.51109 years and 235U: 7.13108 years.

The Indoor Radon Problem

Actinon called is and series actiniumin appears

Thoron called is and series in thorium appears

Radon called is and series uraniumin appears

21986

22086

22286

Rn

Rn

Rn

series actiniumin appears

series in thorium appears

series uraniumin appears

20782

20882

20682

Pb

Pb

Pb

• All three naturally occurring series each has a gaseous member.

Artificially created radioactive series, such as the neptunium series has nogaseous member.

• The end product of all three naturally occurring radioactive series is lead.

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All three isotopes of radon has radioactive daughters, so they are all potentiallyhazardous.

Naturally Occurring Radioactivity– Other Isotopes of Radon

seconds 4T U,from :(Actinon)

seconds 56T Th,from :(Thoron)

3.81daysT U,from :(Radon)

23521986

23222086

23822286

Rn

Rn

Rn

The health concerns of these isotopes are determined by two factors:• The rate of production from their parent nuclides.• The probability of decay before get airborne.

The contributions from the daughters of 220Rn and 219Rn to internal exposure areusually negligible compared with that from 222Rn.

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Radon gas in the environment:

• The noble gas 222Ra produced in the uranium series can becomeairborne before decaying.

Naturally Occurring Radioactivity– Radon and Radon Daughters

• The soil and concrete in building material contribute to the presence ofindoor radon.

• Some of the daughters of radon atoms in air are absorbed onto micron‐or submicron aerosols or dust particles. Others remain in the air asessentially free ions and other forms.

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Sources of indoor radon in a typical house265



• Airborne radon itself poses little health hazard. It is not retained in significantamounts by the body.

• The health hazard is closely related to the short‐lived daughters of radon.

Naturally Occurring Radioactivity– Health Concerns of Radon Gas

particles

particles

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The WHO predicts that “The risk of lung cancer increased by 8% per 100 Bq/m3 increase in measured radon concentration (95% confidence interval).” (from the WHO Indoor Radon Handbook)

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Example of Secular Equilibrium

From <<Radiation Protection and Dosimetry>>, by Michael Stabin.

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From this relationship,

one can see that1. As the time goes by, e‐tdecreases and QB approaches QA.At equilibrium, we have

BA QQand

2. Since A has a relatively long half life, QA may be considered as aconstant. So the total activity converges to a constant.

Secular Equilibrium: TA >> TB (λA << λB)

TA >> TB

7TB

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The radon problem, is that of alpha‐particle irradiation of sensitive lung tissues bythe short‐lived daughter of radon and the associated risk of lung cancer.


For example, An alpha particle emitted by 214Po deposits 7.69MeV within ~70m.In comparison, an 1MeV beta particle emitted by 214Bi deposits its energy over adistance of ~4000 m.

Indoor radon is the second leading cause for lung cancer in the US, only aftersmoking.

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Cosmogenic Radioactivity

• Cosmogenic NORM is formed as a result of interactions between certaingases in the Earth’s atmosphere and cosmic rays.

• The production of cosmogenic radionuclides is an ongoing process and asteady state has been established. The radionuclides are produced at thesame rate as they decay.

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Examples of Cosmogenic Radioactivity – 14C• Carbon‐14 is produced in the atmosphere by thermal neutrons absorbed by

nitrogen atoms. When cosmic rays enter the atmosphere, they undergo varioustransformations, including the production of neutrons. The resulting neutrons (1n)participate in the following reaction:

1n + 14N → 14C + 1H

• Carbon‐14 decay through the following scheme (half‐life: 5730 years)

• The carbon‐14 dating technique?

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Examples of Cosmogenic Radioactivity – 3H• The production of cosmogenic radionuclides is an ongoing process and a

steady state has been established. The radionuclides are produced at thesame rate as they decay.

• Tritium occurs naturally due to cosmic rays interacting with atmosphericgases. In the most important reaction for natural tritium production, a fastneutron (greater than 4MeV) interacts with atmospheric nitrogen:

14N + n→ 12C + 3H

• Tritium is produced in nuclear reactors by neutron activation of lithium‐6:

6Li + n→ 4He ( 2.05 MeV ) + 3H ( 2.75 MeV )

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Examples of Cosmogenic Radioactivity – 3H

• Tritium decays through beta emission and releases 18.6 keV of energy in theprocess.

3H→ 3He + e− + νanti

• The electron has an average kinetic energy of 5.7 keV, while the remaining energyis carried off by the nearly undetectable electron and antineutrino.

• The low‐energy beta radiation from tritium cannot penetrate human skin, sotritium is only dangerous if inhaled, ingested, or—if it is in water molecules, as withtritiated water—absorbed through pores in the skin. Its low energy also createsdifficulty detecting tritium labelled compounds except by using liquid scintillationcounting.

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Examples of Cosmogenic Radioactivity – 14C• Carbon‐14 is produced in the atmosphere by thermal neutrons absorbed by

nitrogen atoms. When cosmic rays enter the atmosphere, they undergo varioustransformations, including the production of neutrons. The resulting neutrons (1n)participate in the following reaction:

1n + 14N → 14C + 1H

• Carbon‐14 decay through the following scheme (half‐life: 5730 years)

• The carbon‐14 dating technique?

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Radiocarbon (14C) formation and decay

1 14 14 10 7 6 1n N C H

‐formed by interaction of cosmic ray spallation products with stable N gas

14 146 7C N Q

‐radiocarbon subsequently decays by β‐ decay back to 14N with a half‐life of 5730y

Radiocarbon dating was first explored by W.R. Libby (1946), who later won the Nobel Prize. Most published dates still use the “Libby” half‐life of 5568y to enable comparison of 14C dates.

The activity of radiocarbon in the atmosphere represents a balance of its production, its decay, and its uptake by the biosphere, weathering, etc.

Which of these threethings might change

through time,and why? 278

Convention:The atmosphericradiocarbonanomaly with respect to a standardis defined as D14C

14 12

1414 12

/1 *1000

/spl

std

C CC

C C

Atmospheric radiocarbon variability through time

‐addition of isotopically lightfossil fuel C to atmosphere

‐solar activity changes

Note:the D14C is 0during 1890,b/c that’sthe activity of theoxalic acid standard

time

But how did somebodyconstruct this curve?

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data from:corals (bright red)lake varves (green)marine varves (blue)speleothems (orange)tree rings (black)

The Radiocarbon Calibration Curve (atmospheric 14C history)Principle: compare radiocarbon dates with independent dates examples of independent dating: tree‐ring counting, coral U‐Th dates, varve counting,

correlation of climate signals in varves with ice core

Hughen et al., 2004

So was atmospheric14C larger or

smaller at 20k (LGM)than today?

Observation:radiocarbon datesare consistently younger than calendar ages

time

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• The coal industry (mining and combustion)• The oil and gas industry (production)• Metal mining and smelting

• Mineral sands (rare earth minerals, titanium and zirconium).• Fertilizer (phosphate) industry • Building industry

• Recycling

Industries Producing NORM

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1.3 Naturally Occurring Radioactive Material (NORM)

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