sources of exposure

8/16/2019 Sources of Exposure

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Sources of Radiation Exposure

Radioactivity is a part of our earth -- it has existed all along. Naturally occurring radioactivematerials are present in its crust, the floors and walls of our homes, schools, or offices and in

the food we eat and drink. There are radioactive gases in the air we breathe. Our own bodies -

- muscles, bones, and tissue -- contain naturally occurring radioactive elements.

Man has always been exposed to natural radiation arising from the earth as well as fromoutside the earth. The radiation we receive from outer space is called cosmic radiation or

cosmic rays.

We also receive exposure from man-made radiation, such as X-rays, radiation used to diagnosediseases and for cancer therapy. Fallout from nuclear explosives testing, and small quantities of

radioactive materials released to the environment from coal and nuclear power plants, are also

sources of radiation exposure to man.

Sources of exposure for the general public

•

Natural radiation of terrestrial origin• Natural radiation of cosmic origin

• Natural internal radioisotopes

• Medical radiation

• Technologically enhanced natural radiation

• Consumer products

• Fallout

• Nuclear power

• Occupational exposure


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Another way to look at the same data is shown in the Table below with the doses expressed in both uSv and mrem. [What information is required to express these different types of dose in

units of Sv or rem?]

In the United States, the annual estimated average effective dose equivalent is 360 mrem peradult. This is broken down as:

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

of the United States.

Source Average annual effective dose equivalent

(µSv) (mrem)

Inhaled (Radon and Decay Products) 2000 200

Other Internally Deposited Radionuclides 390 39

Terrestrial Radiation 280 28

Cosmic Radiation 270 27

Cosmogenic Radioactivity 10 1

Rounded total from natural source 3000 300

Rounded total from artificial Sources 600 60

Total 3600 360

Shown in the table above, 82% of the total average annual effective dose is from natural sourcesof radiation, and of that, most is from radon. Of the other 18%, the majority is from medical

diagnosis and treatments, with


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Summary:

Average effective dose (U.S.) ≈ 360 mrem/year (3.6mSv/y)

NCRP 116; ICRP 60: Recommended dose limits:

Occupational worker

Effective Dose, annual 5 rem /year (50mSv/y)

Pregnant workers 0.05 rem/month (0.5mSv/month)

General public,

Infrequent exposure 0.5 rem (5mSv/y)Continuous exposure 0.1 rem (1mSv/y)


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Radioactivity in Nature

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

1. Primordial - been around since 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

Radionuclides are found in air, water and soil, and additionally in us, being that we are products

of our environment. Every day, we ingest/inhale nuclides in our air we breathe, in the food we

eat and the water we drink. Radioactivity is common in the rocks and soil that makes up our

planet, in the water and oceans, and even in our building materials and homes.

Where do radioactive isotopes come from?

1) Natural (primordial) radionuclides

About 340 nuclides have been found in nature, of which about 70 are radioactive and are found

mainly among the heavy elements. All elements having an atomic number greater than 80 possess radioactive isotopes, and all isotopes of elements heavier than number 83 are radioactive.

2) Artificial radionuclidesResult of bombardment of stable nuclei.

Uranium-238 is the heaviest natural radionuclide, however heavier species can be artificially

made. These are called trans-uranics and are usually short –lived.

1) Primordial radionuclides

When the earth was first formed a relatively large number of isotopes would have been

radioactive. Those with half-lives of less than about 108 years would by now have decayed into

stable nuclides. Radionuclides that exist for more than 30 half-lives are not measurable. Thus,

the primordial (or natural terrestrial) radionuclides are left over from when the world and theuniverse was created. They are typically long lived, with half-lives often on the order of

hundreds of millions of years. The progeny or decay products of the long-lived radionuclides are

also in this heading. A few examples of primordial radionuclides:

Primordial nuclides Nuclide Symbol Half-life Natural Activity

Uranium

235 235

U 7.04 x 108 yr 0.72% of all natural uranium

Uranium

238 238

U 4.47 x 109 yr

99.2745% of all natural uranium; 0.5 to 4.7 ppm

total uranium in the common rock types

Thorium232

Th 1.41 x 1010

yr 1.6 to 20 ppm in the common rock types with a


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232 crustal average of 10.7 ppm

Radium

226 226

Ra 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 222 222

Rn 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 40

K 1.28 x 109 yr soil - 1-30 pCi/g (0.037-1.1 Bq/g)

Primordial radionuclides can be divided into those that occur singly, or those occurring as part of

one of three series or chains of radioactive species decaying sequentially.

Some other primordial radionuclides are50

V,87

Rb,113

Cd,115

In,123

Te,138

La,142

Ce,144

Nd,147

Sm,152

Gd,

174

Hf,

176

Lu,

187

Re,

190

Pt,

192

Pt,

209

Bi.

1) Single primordial nuclides

At least 22 naturally occurring single or nor-series primordial radionuclides have been identified.Most of these have such long half-lives, small isotopic and elemental abundances and small

biological uptake and concentration that they give little environmental dose. The most important

is potassium-40. Potassium is a metal with 3 natural isotopes, 39, 40 and 41. Only40

K isradioactive and it has a half life of 1.3 x 10

9 years.

Natural Radioactivity in soilHow much natural radioactivity is found in an area 1 square mile, by 1 foot deep? The followingtable is calculated for this volume (total volume is 7.894 x 10

5 m

3) and the listed activities.

Activity levels vary greatly depending on soil type, mineral make-up and density (~1.58 g/cm3).

This table represents calculations using typical numbers.

Natural Radioactivity by the Mile

NuclideActivity used

in calculationMass of Nuclide Activity

Uranium 0.7 pCi/gm (25 Bq/kg) 2,200 kg 0.8 curies (31 GBq)

Thorium 1.1 pCi/g (40 Bq/kg) 12,000 kg 1.4 curies (52 GBq)Potassium 40 11 pCi/g (400 Bq/kg) 2000 kg 13 curies (500 GBq)

Radium 1.3 pCi/g (48 Bq/kg) 1.7 g 1.7 curies (63 GBq)

Radon 0.17 pCi/gm (10 kBq/m3) soil 11 µg 0.2 curies (7.4 GBq)


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Potassium is essential to life and is thus found in all living, and formerly living, things.The isotopic abundance of

40K is small, only 0.012% of naturally occurring potassium which

gives a specific activity of 855 pCi/g (31.6 Bq/g) of natural K.

S.A = 0.00012 x λ x A

gatoms x /1002.6 23

S.A = 0.00012 x y x x years x

x A

gatoms x

sec/102.3103.1

693.0/1002.679

23

S.A. = 30.1 disintegrations/sec/gram


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From a biological point of view potassium is everywhere. A 70 kg man contains about 140 g,

mostly in muscle. The K concentration in men is about 20-30% greater than in women of thesame age, largely due to the greater muscle mass and lower fat content in men.

855 pCi/g x 140 g = 0.12 µCi of40

K in a 70 kg male.

While the mean overall concentration of K in the soft tissues of the body is about 0.2%, there is

considerable variability among the various organs and tissues. Highest in muscle, lowest in teeth

and bones. Because the body composition is homeostatically controlled, the K level will notchange as we change diet, residence location, etc. Typically the soft tissues (those with the

greatest concentrations of K such as the muscles and the gonads) will incur a dose of about 19

mrad (190µGy) per year from the40

K decay, about 85% of which is from the beta particlesassociated with the decay of the nuclei and the remainder from photons.

Because it is everywhere in the environment, and because of the 1.46 MeV photon associated

with the decay,40

K also delivers a small external dose.

2) Chain or series decaying primordial radionuclides.

Some nuclides like232

Th have several members of its decay chain. You can roughly follow thechain starting with

232Th

232Th -->

228Ra -->

228Ac -->

228Th -->

224Ra -->

220Rn -->

216Po -->

212Pb -->

212Bi -->

212Po -->

208Pb (stable)

Radioactive series refers to any of four independent sets of unstable heavy atomic nuclei that

decay through a sequence of alpha and beta decays until a stable nucleus is achieved. These four


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chains of consecutive parent and daughter nuclei begin and end among elements with atomic

numbers higher than 81, which is the atomic weight of thallium; the members of each set aregenetically related by alpha and beta decay. Three of the sets, the thorium series, uranium series,

and actinium series, called natural or classical series, are headed by naturally occurring species

of unstable nuclei that have half-lives comparable to the age of the elements. By 1935 these three

radioactive series had been fully delineated. The fourth set, the neptunium series, is headed byneptunium-237, which has a half-life of 2,000,000 years. Its members are produced artificially

by nuclear reactions and do not occur naturally; all their half-lives are short compared with the

age of the elements.

Because the two pertinent decay processes result either in no change or in a change of four units

in the mass number, the mass numbers of all the members of each series are divisible by four,with a constant remainder. Within each series, therefore, the mass number of the members may

be expressed as four times an appropriate integer (n) plus the constant for that series; thus, the

thorium series is sometimes called the 4n series; the neptunium series, 4n + 1; the uranium series,

4n + 2; and the actinium series, 4n + 3.

The thorium series begins with thorium-232 and ends with the stable nuclide lead-208. Theneptunium series is named for its longest-lived member, neptunium-237; it ends with bismuth-

209. The uranium series begins with uranium-238 and ends with lead-206. The actinium series,named for its first-discovered member, actinium-227, begins with uranium-235 and ends with

lead-207.

Alpha decay, symbolized by a larger arrow in the accompanying diagram, involves the ejection

from an unstable nucleus of a particle composed of two protons and two neutrons. Thus alpha

emission lowers the atomic number (number of protons) by two units, the neutron number bytwo units, and the mass number (total of neutrons and protons) by four units. At the head of the

thorium series, for example, thorium-232 undergoes alpha decay to radium-228.

Negative beta decay, symbolized by a smaller arrow, involves the ejection from an unstable

nucleus of an electron and an antineutrino that are produced by the decay of a neutron into a

proton. This process lowers the neutron number by one unit, raises the atomic number by oneunit, and leaves the mass number unchanged. At the end of the neptunium series, for example,

lead-209 undergoes negative beta decay to bismuth-209.

Branching (the decay of a given species in more than one way) occurs in all four of the

radioactive series. For example, in the actinium series, bismuth-211 decays partially by negative

beta emission to polonium-211 and partially by alpha emission to thallium-207.


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Important points about series-decaying radionuclides

• 3 main series

• the fourth exists only with man-made isotopes, but probably existed early in the life of

the earth

•

the 3 main series decay schemes all produce radon (but primary radon source, the longesthalf-life, is the uranium series).

Series name Begins T1/2 Ends Gas (T1/2)

Thorium232

Th 1.4 x 1010

yr208

Pb220

Rn (55.6 sec) thoron

Uranium238

U 4.5 x 109 yr

206Pb

222Rn (3.8 days) radon

Actinium235

U 7.1 x 108 yr

207Pb

219Rn (4.0 sec) actinon


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Uranium 238 decay scheme. Note the tendency of decay to push the series back towards the line of stability.

Branching occurs when the radionuclide is unstable to both alpha and beta decay, for example,218

Po.

Gamma emission occurs in most steps.

High Background Radiation Areas

Background radiation levels are from a combination of terrestrial (from the40

K,232

Th,226

Ra,

etc.) and cosmic radiation (photons, muons, etc.). The level is fairly constant over the world,

being 8-15 µrad/hr. Around the world though, there are some areas with sizable populations thathave high background radiation levels. The highest are found primarily in Brazil, India and

China. The higher radiation levels are due to high concentrations of radioactive minerals in soil.

One such mineral, Monazite, is a highly insoluble rare earth mineral that occurs in beach sandtogether with the mineral ilmenite, which gives the sands a characteristic black color. The

principal radionuclides in monazite are from the232

Th series, but there is also some uranium and

its progeny,226

Ra.


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In Brazil, the monazite sand deposits are found along certain beaches. The external radiation

levels on these black sands range up to 5 mrad/hr (50 µGy/hr), which is almost 400 times normal background in the US. Some of the major streets of the surrounding cites have radiation levels as

high as 0.13 mrad/hr (1.3 µGy/hr), which is more than 10 times the normal background. Another

high background area in Brazil is the result of large rare earth ore deposits that form a hill that

rises about 250 meters above the surrounding area. An ore body near the top of the hill is verynear the surface, and contains an estimated 30,000 tons of thorium and 100,000 tons of rare earth

elements. The radiation levels near the top of the hill are 1 to 2 mrad/hr (0.01 to 0.02 mGy/hr)

over an area of about 30,000 m2. The plants found there have absorbed so much

228Ra, that they

can produce a self "x-ray" if placed on a sheet of photographic paper (an autoradiogram).

On the Southwest coast of India, the monazite deposits are larger than those in Brazil. The dosefrom external radiation is, on average, similar to the doses reported in Brazil, 500-600 mrad/yr (5

- 6 mGy/yr), but individual doses up to 3260 mrad/yr (32.6 mGy/yr) have been reported.

An area in China, has dose rates that are about 300-400 mrad/yr (3-4 mGy/yr). This is also from

monazite that contains thorium, uranium and radium.

[From BEIR V, National Research Council report on Health Effects of Low Levels of Ionizing

Radiation: In areas of high natural background radiation, an increased frequency of chromosomeaberrations has been noted repeatedly. The increases are consistent with those seen in radiation

workers and in persons exposed at high dose levels, although the magnitudes of the increases are

somewhat higher than predicted. No increase in the frequency of cancer documented in populations residing in areas of high natural background radiation. ]

Internal Radiation

What makes a radionuclide biologically important?

• Abundance (both elemental and isotopic)

• Half-life

• Decay scheme (emission type and energy)

• Chemical state

• Chemical behavior in the body

• Does it concentrate?

• Ultimate location

• Rate of excretion

How do the series radionuclides contribute to our dose?

• Inhalationo Isotopes of radon (inert gas, but may decay in the lung)

o Dust; e.g., our main source of uranium is due to resuspension of dust particlesfrom the earth. Uranium is ubiquitous, a natural constituent of all rocks and soil.


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• Externally- gamma emission occurs in most decay steps.

• Consumption- food and drinking water

Example:

Alpha activity in various foodstuffs (pCi/g)

Brazil nuts 14Cereals 0.06

Teas 0.04

Organ meats 0.015Flours 0.014

Peanuts 0.012

Chocolate 0.008Cookies 0.002

Milk (evaporated) 0.002

Fish 0.002Cheeses 0.0009

Eggs 0.0009

Vegetables 0.0007Meat 0.0005

Fruits 0.0001

N.B.: The Brazil nut tree concentrates barium, which is chemically similar to radium. Brazil also

has areas of very high natural concentrations of thorium and radium in ores and soil.

In most places on earth, natural radioactivity varies only within relatively narrow limits. In some

places there are wide deviations from these limits due to the presence of abnormally highconcentrations of radioactive minerals in local soils.

The important contributors to our exposure:

• Uranium:

• found in all rocks and soil, and thus in both our food and in dust.

• High concentrations in phosphate rocks (and thus in commercial fertilizers)

• Is absorbed by the skeleton which receives roughly 0.3 mrem/year (3 µSv/year) from

uranium

•

Radium:• Also present in all rocks and soils, but food is a more important source of intake

• 226

Ra and its daughter products (beginning with222

Rn) contribute the major dosecomponents from naturally occurring internal emitters

• dissolves readily

• chemically similar to calcium, and is absorbed from the soil by plants and passed upthe food chain to humans

• variations in Ra levels in soil lead to variations in Ra levels in food


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• 80% of the total body Ra is in bone (~7mrem/year). The rest is uniformly distributedin soft tissue.

• Thorium

• Lots in dust but little is incorporated in food

•

Thorium is present in the highest concentrations in pulmonary lymph nodes and lung,indicating that the principle source of exposure is due to inhalation of suspended soil

particles.

• Ultimately a bone seeker with a long residence time

• Since it is very slowly removed from bone, concentration increases with age.

• Lead

• Also a bone seeker, half-life in bone is ~ 104 days

• Polonium

Unlike other naturally occurring α-emitters,210

Po deposits in soft tissue not bone.

Two groups exist for which the dose from 210Po is apt to be exceptionally high.• Cigarette smokers

• Residents of the north who subsist on caribou and reindeer. Reindeer eat lichens that

absorb trace elements in the atmosphere (210

Po and210

Pb). The 210Po content ofLapps living in northern Finland is ~12 times higher than the residents of southern

Finland. Liver dose in the Laplanders is 170 mrem/year compared to 15 mrem/yearfor those in the south.

Human body

The human body is made up of many elements, and it should be of no surprise that some of them

are radionuclides, many of which are ingested daily in water and food. The following table liststhe estimated concentrations of radionuclides calculated for a 70 Kg adult based on ICRP 30data:

Natural Radioactivity in your body

NuclideTotal Mass of Nuclide

Found in the Body

Total Activity of Nuclide

Found in the BodyDaily Intake of Nuclides

Uranium 90 µg 30 pCi (1.1 Bq) 1.9 µg

Thorium 30 µg 3 pCi (0.11 Bq) 3 µg

Potassium

40

17 mg 120 nCi (4.4 kBq) 0.39 mg

Radium 31 pg 30 pCi (1.1 Bq) 2.3 pg

Carbon 14 95 µg 0.4 µCi (15 kBq) 1.8 µg

Tritium 0.06 pg 0.6 nCi (23 Bq) 0.003 pg

Polonium 0.2 pg 1 nCi (37 Bq) ~0.6 µg

It would be reasonable to assume that all of the radionuclides found in your environment wouldexist in the body in some small amount. The internally deposited radionuclides contribute about

11% of the total annual dose.


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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. The following table lists three common cosmogenic nuclides:

Cosmogenic Nuclides Nuclide Symbol Half-life Source Natural Activity

Carbon

14 14

C 5730 yr Cosmic-ray interactions,14 N(n,p)

14C; 6 pCi/g (0.22 Bq/g)

Tritium3H 12.3 yr

Cosmic-ray interactions with N and O;

spallation from cosmic-rays,6Li(n,alpha)

3H

0.032 pCi/kg (1.2 x10

-3 Bq/kg)

Beryllium

7 7Be 53.28 days Cosmic-ray interactions with N and O;

0.27 pCi/kg (0.01Bq/kg)

Some other cosmogenic radionuclides are10

Be,26

Al,36

Cl,80

Kr,14

C,32

Si,39

Ar,22 Na,

35S,

37Ar,

33P,

32P,

38Mg,

24 Na,

38S,

31Si,

18F,

39Cl,

38Cl,

34mCl.

Track structure of a cosmic ray collision in a nuclear emulsion

The atmosphere and the Earth's magnetic fields act as shields against cosmic radiation, reducing

the amount that reaches the Earth's surface. Thus, the annual dose from cosmic radiation

depends on altitude. From cosmic radiation in the U.S., the average person will receive a dose of27 mrem per year and this roughly doubles for every 6,000 foot increase in elevation.

Typical Cosmic Radiation Dose rates:

4 µR/hr in the Northeastern US

20 µR/hr at 15,000 feet

300 µR/hr at 55,000 feet

There is only about a 10% decrease at sea level in cosmic radiation rates when going from poleto the equator, but at 55,000 feet the decrease is 75%. This is due to the effect of the earth's and

the Sun's geomagnetic fields on the primary cosmic radiations.


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Flying can add a few extra mrem to your annual dose, depending on how often you fly, how high

the plane flies, and how long you are in the air.

Variations in cosmic ray intensity at the earth’s surface are due to:

• Time

• Latitude

• Altitude

(i) Time

In general the galactic component is constant with time. Cosmic ray fluence has remained moreor less constant for at least 200 years (and may have varied only by a factor of 2 over the last 10

9

years).

Temporal variations in the solar component have been directly observed for about a century andoccur in cycles of 11 years (seemingly associated with or following the sunspot cycle), 1 year, 27days and 1 day. This effect is small, less than 10% at sea level (but it is very important for air

and space travel).

A sunspot is a magnetically disturbed area on the surface of the sun that is cooler than its

surroundings.

It appears darker only because its gases, at 4000 - 4500º K, radiate less than the surrounding gasat 5700º K.

Around 1830 an obscure German amateur astronomer, H. Schwabe, began observing sunspots asa hobby. In 1851 he announced a “solar cycle” – the number and positions of sunspots vary in a

cycle.

A year later it was discovered that terrestrial magnetic compass deviations followed the same

cycle.

The cycle’s duration averages 22 years and consists of two 11year cycles.


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The sunspot cycle occurs because the sun rotates faster at its equator than near its poles. This

causes a shearing and twisting of the magnetic field that controls the motion of the solar gas.

During years of maximum sunspot activity, solar particles shooting off the sun affect the

magnetic field and upper atmosphere of the earth, disturbing radio communications and causing

aurorae.

The effect on cosmic ray dose to persons living at sea level is small (~ 10% change) but solar

flare activity is of major concern to space travel and, to some extent, air travel.

(ii) Latitude

The source of the latitude effect is basically geomagnetic and is related to the location of theearth’s magnetic poles.

On entering the earth’s magnetic field, some of the primary particles are deflected toward the

polar regions, resulting in a somewhat lower radiation flux at the equator. This phenomenon becomes more accentuated with altitudes above a few kilometers. The difference in the dose rate

due to geomagnetic latitude varies from 14% at sea level to 33% at 4360 meters.


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(iii) Altitude

The most important factor from the radiation dose point of view.

The cosmic ray dose approximately doubles with each 1800 meter rise in altitude, at least for the

first 10 km or so above the surface of the earth. This variability is largely due to the change inattenuation brought about by the decreasing thickness and density of air, which results in reduced

shielding as one ascends from the surface.

(However for the first 1000 m, the total dose rate actually decreases with altitude above thesurface, because attenuation of the γ rays from terrestrial sources occurs more rapidly than the

increase in cosmic radiation).

Residents of Denver (altitude 1600 m) receive nearly twice the dose than at sea level, and in

Leadville, Colorado (altitude 3200 m) the residents receive about 125 mrem/year from cosmicrays, more than four times the annual dose at sea level.


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The van Allen Radiation BeltsThe trapping regions of high-energy charged particles surrounding the Earth are called radiation(or van Allen) belts. The inner one, located between about X = 1.1 - 3.3 Re (Earth radii,

geocentric) in the equatorial plane, contains primarily protons with energies exceeding 10 MeV.

Flux maximum is at about X = 2 Re. (Distances given here are approximate, since the location of particles is energy dependent.) This is a fairly stable population but it is subject to occasional

perturbations due to geomagnetic storms, and it varies with the 11-year solar cycle. The source

of protons in this region is the decay of cosmic ray induced albedo from the atmosphere.

As a result of the offset between the Earth's geographical and magnetic axes, the inner belt

reaches a minimum altitude of about 250 km above the Atlantic Ocean off the Brazilian Coast.This South Atlantic Anomaly occupies a region through which low-orbiting satellite frequently

pass. Energetic particles in this region can be a source of problems for the satellites and

astronauts.


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The outer belt contains mainly electrons with energies up to 10 MeV. It is produced by injection

and energization events following geomagnetic storms, which makes it much more dynamic thanthe inner belt (it is also subject to day-night variations). It has an equatorial distance of about 3 -

9 Re, with maximum for electrons above 1 MeV occurring at about X = 4 Re. 'Horns' of the outer

belt dip sharply in towards the polar caps.Recently a new belt has been found within the inner belt. It contains heavy nuclei (mainly

oxygen, but also nitrogen and helium, and very little carbon) with energies below 50 MeV/nuc.

The source of these particles are the so called "anomalous cosmic rays" of interstellar origin.The radiation belts are of importance primarily because of the harmful effects of high energy

particle radiation for man and electronics:

• it degrades satellite components, particularly semiconductor and optical devices

• it induces background noise in detectors

• it induces errors in digital circuits

• it induces electrostatic charge-up in insulators

• it is also a threat to the astronauts

Auroras

Earth itself is a gigantic magnet. The solar wind confines Earth's magnetic field to a comet-shaped cavity known as the magnetosphere. As the solar wind flows past the magnetosphere, itacts like a cosmic generator, producing millions of amps of electric current. Some of this electric

current flows into Earth's upper atmosphere which can light up like a neon tube to create the

aurora. People living in the arctic or antarctic regions can witness the aurora-- beautifulshimmering curtains of light appearing in the night sky. The aurora takes its name from the

Roman goddess of dawn, but its cause has nothing to do with the Sun's light. Earth's magnetotail

deflects solar wind toward Earth's polar regions. An aurora is produced when the energetic


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charged particles comprising the solar wind collide with neutral gas molecules in the upper

atmosphere. The electrical discharge occurs about 70 miles above Earth's surface.


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Summary

Estimated per caput annual effective dose equivalent

from natural sources in areas of normal background

(Estimates from the UNSCEAR 1982 Report are given in parentheses.)

Annual effective dose equivalent (μSv)

Source of irradiation

External Internal Total

irradiation irradiation

Cosmic rays

Ionizing component 300 (280) 300 (280) Neutron component 55 (21) 55 (21)Cosmogenic radionuclides 15 (15) 15 (15)

Primordial radionuclides

K-40 150 (120) 180 (180) 330 (300)Rb-87 6 (6) 6 (6)

U-238 series:

U-238 → U-234 5 (10)

Th-230 7 (7)

Ra-226 100 (90) 7 (7) 1300 (1040)

Rn-222 → Po-214 1100 (800)

Pb-210 → Po-210 120 (130)Th-232 series:

Th-232 3 (3)

Ra-228 → Ra-224 160 (140) 13 (13) 340 (330)

Rn-220 → Tl-208 160 (170)

Total (rounded) 800 (650) 1600 (1340) 2400 (2000)


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3) Human Produced

Humans have used radioactivity for just over 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. A few examples are listed below.

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 131 131

I 8.04 daysFission product produced from weapons testing and fissionreactors, used in medical treatment of thyroid problems

Iodine 129 129

I 1.57 x 107 yr

Fission product produced from weapons testing and fission

reactors

Cesium 137 137

Cs 30.17 yr Fission product produced from weapons testing and fissionreactors

Strontium 90 90

Sr 28.78 yr Fission product produced from weapons testing and fission

reactors

Technetium

99m 99m

Tc 6.03 hr Decay product of99

Mo, used in medical diagnosis

Technetium

99 99

Tc 2.11 x 105 yr Decay product of

99mTc

Plutonium

239 239

Pu 2.41 x 104 yr

Produced in reactors by neutron bombardment of238

U(

238U + n-->

239U-->

239 Np +ß-->

239Pu+ß)

sources of exposure

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