Part 1 Exposures from natural and man-made radiation sources

Naturally Occurring Radioactivity

Natural radiation sources are classified into two categories-:

1 -external sources of extraterrestrial sources such as cosmic radiation and cosmogenic and radiation of terrestrial origin. i.e radiation in air, earth crust and building materials.

2 -Internal sources , comprising naturally occurring radio-nuclides that is taken into a human body.

There are three sources for naturally occuring of radiation

1 -Cosmic Radiation2 -Cosmogenic Radiation

3 -Premordial Radiation –three series

Origin and kind of Cosmic rays.

Cosmic rays are very high-energy particles from extraterrestrial sources that bombard the earthOne source is the sun, which emits alpha particles and protons.

The other radiation , consisting mainly of electrons and proton originates beyond our solar system and is called galactic radiation.

Exposure by Cosmic radiation and Cosmogenic radionuclidesThe primary particles enter the earth atmosphere and collide with the atmospheric molecules and produce the secondary cosmic molcules that bombard the earth surface and high sufficient energy to penetrate deeply into the ground and sea.

Exposure by Cosmic radiation

A-COSMIC RAYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .

. 1-Exposure at ground level. the population-weighted average absorbed dose rate from the directly ionizing and photon components of cosmic radiation at sea level corresponds to 31 nGy h.

Since mostly muons are involved, a radiation weighting factor of unity , yielding the same values for the effective dose rate, i.e. 31 nSv /h or 270 μSv per yearCosmic ray intensity increases with altitude because of the decreased shielding effect of the atmosphere. For example, the cosmic ray intensity in Denver, CO, which is at an altitude of about 1609 meters (5280 feet), is about twice that in New York, which is at sea level.

2.Exposures at aircraft altitudes .

In a jet aircraft at an altitude of 30,000 feet (9140 meters), the cosmic ray intensity is about six to seven times that at sea level. Cosmic-ray intensity increases with increasing latitude north and south of the equator because the earth’s magnetic field deflects the high-velocity charged particles that are cutting across the magnetic force field.

The results of recent measurements and recentcalculations are broadly consistent. For altitudes of 912 km at temperate latitudes, the effective dose rates are in the range 58 μSv/ h, such that for a transatlantic flight from Europe to North America, the route dose would be 3045 μSv. At equatorial latitudes, the dose rates are lower and in the range of 24 μSv/ h.Effective dose rates of 1012 μSv/ h were normally found for passengers and flight crews travel at higher altitudes (~18 km) on supersonic transports. Calculated dose equivalent rates for thisevent at 20 km are of the order of 1 mSv/ h

However, no events of this magnitude have taken place since then. It requires both high solar particle flux densities and high energies (1 GeV) for an event to produce high dose rates at aircraft altitudes, and this is a rare occurrence

B. COSMOGENIC RADIONUCLIDES

Interactions that occur between cosmic radiation and the atmosphere lead to the production of numerous cosmogenic radionuclides Two of these, tritium (3H) and radiocarbon (14C), are of interest to health physicists because anthropogenic tritium and radiocarbon are widely used in

research and the presence of the naturally occurring isotopes

must be considered in interpreting health physics measurements when dealing with these radionuclides. For 3H, the worldwide steady state inventory is estimated to be 34 × 106 Ci (1.26 × 1018 Bq) and the estimated global inventory for

14C is 31 × 107 Ci (1.15 × 1019 Bq) .

The long half-life of 14C (5730 years) makes it an excellent tool for “radiocarbon” dating of organic artifacts from the historical past. The production of 14C is still going on due to nuclear transformations induced by the cosmic-ray bombardment of 14N. The environmental burden of 14C before the advent of nuclear bombs was about 1.5 × 1011 MBq (4 MCi) in the atmosphere, 4.8 × 1011 MBq (13 MCi) in plants, and 9 × 1012 MBq (243 MCi) in the oceans. Testing of nuclear weapons has resulted in an increase in the atmospheric level of radiocarbon. It is estimated that about 1.1 × 1011 MBq (3 MCi) 14C were injected into the air by all weapons tests conducted up to 1963, when atmospheric testing was halted. The atmospheric radiocarbon exists as 14CO2. It is therefore inhaled by all animals and utilized by plants in the process of photosynthesis. Because only living plants continue to incorporate 14C along with nonradioactive carbon, it is possible to determine the age of organic matter by measuring the specific activity of the carbon present.

If 2 g of carbon from a piece of wood found in an ancient temple is analyzed and found to have an activity of 10 transformations/minute/gram, what is the age of the wood if the current specific activity of 14C in carbon is assumed to have been constant at 15 transformations/minute/gram?SolutionThe fraction of the original 14C remaining today is, according to Eq. (4.18),A/A0 = 10/15= e−λt.

Since the half-life for 14C is 5730 yearsλ = 0.693/5730 years= 1.21×10−4

10/15 =e−1.21×10−4tt = 3.35 × 103 years.

It consists of three series1 -Uranium series , U-238 and U-234, 4n+2

,2 -Actium series , U-235, 4n+33 -Thorium series Th-232, 4n

And an artifical series called neptunium series, Pu-241, 4n+1 على الكتلى العدد الباقى 4بقسمة واجمع

Terrestrial Radiation

Uranium, the most abundant of the radioactive elements in this mixture, consists of three different isotopes: about 99.3% of naturally occurring uranium is 238U, about 0.7% is 235U, and a trace quantity (about 5 × 10−3%) is 234U. The 238U and 234U belong to one family, the uranium series, while the 235U of uranium is the first member of another series called the actinium series.

Uranium forms extremely stable compounds with phosphorous. Phosphate-rich soil, therefore, contains uranium at concentrations much higher than average, from about 7 ppm to about 125 ppm Medium-grade uranium ore contains about 1000–5000 ppm uranium, while the uranium concentration in high-grade ore is about 10,000–40,000 ppm. Uranium concentration in most surface waters in the United States of America are in the range of about 1–10 μg/L, and in most of the groundwater, the concentrations vary from about 1–120 μg/L.

Thorium, another ubiquitous naturally occurring radioactive element, is about 4 times more abundant in nature than uranium. The most abundant thorium isotope,232Th, is the first member of still another long chain of successive radionuclides. All of the isotopes that are members of a radioactive series are found in the upperportion of the periodic table; the lowest atomic number in these groups is 81, while the lowest mass number is 207.All the radioactive series have several common characteristics. The first member of each series is very long-lived, with a half-life that may be measured in geological time units. That the first member of each must be very long-lived is obvious because, if the time since the creation of the world is considered, relatively short-lived radioactive

material would have decayed away during the 4.5 billion years that the earth isbelieved to have been in existence. This point is well illustrated by considering the artificially produced neptunium series. In this case, the first member is the transuranic element 241Pu, which is produced in the laboratory by neutron irradiation of reactor produced 239Pu. The half-life of 241Pu, however, is only 13 years. Because of this short half-life, even a period of a century is long enough to permit most of the 241Pu to decay away. Even the half-life of the longest-lived member of this series, 237Np, which is 2.2 × 106 years, is sufficiently short for this element to have essentially disappeared if it had been created at the same time as all the other elements of the earth

A second characteristic common to all three naturally occurring series is that each has a gaseous member and, furthermore, that the radioactive gas in each case is a different isotope of the element radon. In the case of the uranium series, the gas(Rn) is called radon; in the thorium series, the gas is called thoron; and in the

actinium series (Table 4-4), it is called actinon .

Radon Risk

Three common three characteristics of premordial series

Terrestrial radiation includes external exposure, internal exposure other than radon and radon decay products

.AA.External exposure .B1 .Outdoors

External exposures outdoors arise from terrestrialradionuclides present at trace levels in all soils. The specificlevels are related to the types of rock from which the soilsoriginate. Higher radiation levels are associated with igneousrocks, such as granite, and lower levels with sedimentaryrocks. There are exceptions, however, as some shales and phosphate rocks have relatively high content of radionuclides.There have been many surveys to determine the backgroundlevels of radionuclides in soils, which can in turn be related tothe absorbed dose rates in air

The radionuclides in the uranium and thorium decay

chains cannot be assumed to be in radioactive equilibrium.The isotopes 238U and 234U are in

approximate equilibrium, as they are separated by two much shorter-lived nuclides, 234Th and

234Pa. The decay process itself may, however, allow some dissociation of the decay radionuclide from the source material, The radionuclide 228Ra

has a sufficiently long half-life that may allow some separation from its parent, 232Th. The gaseous

element of the chain, 220Rn, has a very short half-life and no long-lived decay products.

The activity concentration of 40K in soil is an order ofmagnitude higher than that of 238U or 232Th.

IOn the basis of the higher levels reported for China and the United States, the Committee revised the values for both 238U and 232Th to 40 Bq kg1 in the UNSCEAR 1993 Report . A more recently completed country-wide survey in China indicatessomewhat lower values . The median values are 400, 35, and 30 Bq kg1, and the population weighted values are 420, 33, and 45 Bq kg1 for 40K, 238U,

The population-weighted values givean average absorbed dose rate in air outdoors from terrestrial gamma radiation of60 nGyh

Direct measurements of absorbed dose rates in air have been carried out in the last few decades in many countries of the world. The population-weighted average is 59 nGy/ h, compared with 57 nGy/ h in the previous assessment .The average values range from 18 to 93 nGy h1. A typical range of variability for measured absorbed dose rates in air is from 10 to 200 nGy/ h. Washout and rainout of radon progeny from air may result in the short-term enhancement, by 50%100%, of the gamma ray dose rate in air. The extent of the elevation depends on rain interval as well as the rainfall amount.

The absorbed dose rate in air outdoors, the lowest are in Cyprus, Iceland, Egypt, the Netherlands, Brunei, and the United Kingdom, all less than 40 nGy h1, and the highest values are in Australia, Malaysia, and Portugal, all greater than 80 nGy/ hExposures inferred from the soil concentrationThe median for the population included(788 million persons in the 25 countries) is in the 50-59 nGy/ h range. A relatively large population group in the Russian Federation is reported to be in the 60-69 nGy/ h

There are various causes of the elevated exposure levels. Some result from monazite sand deposits, which have high levels of thorium, including Guarapari in Brazil, Yangiang in China, the states of Kerala and Madras in India, and the Niledelta in Egypt. Some have volcanic soils such as Mineas Gerais in Brazil, Niue Island in the Pacific, and parts of Italy. The central massive in France has granitic and schistic rocks and sands, and an area in the southwest of that country is one