Download Radiation occurs naturally throughout the biosphere

Radiation occurs naturally throughout the biosphere due to radioactive
elements and their decay products present in the earth and through other
natural processes. These natural sources expose human beings to radiation
both outdoor and indoor. The exposure due to technologically enhanced
radiations such as from thermal and nuclear power plants, uranium milling
and phosphate fertilizer industries etc. may also contribute significantly. Of
all the sources of environmental radiation, radon and its progeny are
considered to be responsible for a significant radiation dose to man.
Uranium occurs in dispersed state and has a heterogeneous distribution
in earth due to the geochemical processes which have slowly recycled the
crystal material to and from the earth's mantle. Although considered as the
rare element it has higher content in earth's crust than other toxic elements
such as K. Sb. Cd, Bi, and Hg. Its concentration varies significantly from
rock to rock. It is more abundant in acidic rocks than in basic rocks and is
also found in lignite, monazite and phosphate deposits. These rocks come in
contact with underground water and uranium is transferred from bed rocks
by its leaching action to the water and then to the soil. Human activities like
mining and milling are also the chennels for transfer of underlying uranium
to surface soil.
Radon, a radioactive inert gas having a fairly long half life is the decay
product of uranium series and can migrate from the subsoil to indoor and
outdoor atmosphere. Building materials may also be the one of the potential
sources of indoor radon which causes the build up of radon progeny in
dwellings. Now it is well established that higher levels of radon and its
progeny pose a health hazard to the general public. Also long term exposure
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to low levels of radon in dwellings may lead to cause lung cancer. Therefore,
the study of uranium levels in various materials, radon and its progeny in
dwellings and radon exhalation from building materials is of great
importance.
Chapter I of the thesis presents a general introduction to naturally
occurring radiations, external and internal exposure, technologically
enhanced radioactivity, naturally occurring radon, characteristics of radon
and its decay products, indoor radon levels, sources of indoor radon, radon
exhalation from building materials and relation of radon and its progeny
with lung cancer risk.
The techniques used for the measurement of radon and its decay
products and for the microanalysis of uranium have been discussed in chapter
II. Most of the methods detect alpha particles, some y-emissions while a
few detect beta decays. Solid State Nuclear Track Detectors (SSNTD's) have
been widely used for the measurement of the passive time integrated radon
and its progeny, radon exhalation and for microanalyses of uranium.
The third chapter of the thesis gives a general idea about SSNTD's,
their applications and advantage. The SSNTD's have been extensively used
in almost all branches of Nuclear Science and Technology, Health Physics,
Environmental science. Earth sciences etc. Different type of track detectors
and their relative merits and demerits, various track formation mechanisms
and models, track registration criterion, methodologies of track revelation
and their visulisation and main applications of etched track detectors have
been discussed.
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The fourth chapter of the thesis depicts the results of the study of
radon and its progeny. Indoor radon measurement have been undertaken in
about 150 dwellings in hilly regions such as Kohima (Nagaland), Palampur
and Baijnath (Himachal Pradesh) and Dehradun (Uttar Pradesh). Kohima,
Palampur and Baijnath are supposed to be high background areas. Kohima
in Nagaland, the south-eastern region of India is situated at major plateau
boundary (Subduction zone). It has mainly sedimentary rocks and is expected
to have rich uranium deposits. Palampur and Baijnath in Himachal Pradesh
are situated in hills of northern Indian state of Himachal Pradesh which is
close to Himalaya. The life style and building construction of these regions
are different from the other part of the country. Dehradun is supposed to be
a normal background area.
Radon measurements have also been carried out in 143 dwellings of
Udaipur, Bikaner and Banswara towns situated in Rajasthan province of
Northern India. According to Geological Survey of India, these three places
fall in pre-cambrian rocks in Aravalli mountain range which can be grouped
in Banded Gneissic Comlpex and Aravalli super group. In the Aravalli super
group workable phosphorite deposits are present in the significant amount.
These phosphorites are invariably associated with dolostones and their P.,G*5
contents vary from 5 to 23%. Trace element analysis indicate higher
percentage of natural uranium.
Indoor radon measurements were also made in about 150 dwellings of
oil refinery area at Mathura and in dwellings in nearby normal background
areas of Mathura and Agra towns of the state of Uttar Pradesh. In nature,
sedimentary rocks are the sources of oil and gas. There are occasional cases
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of igneous and metamorphic rocks serving as the source of petroleum. The
sedimentary and igneous rocks also contain substantial trace amount of
uranium-238 in varying concentrations and in the process of oil exploration,
uranium-238 may also be extracted and be present in crude oil and natural
gas.
Annual effective dose, lung cancer risk and attributable loss of life
expectancy have been calculated on the basis of models and compared with
the available data for different countries.
Fifth chapter of the thesis presents the results of the radon exhalation
rate measurements in a large number of soil, fly ash and cement samples.
The soil samples were collected from different places of two major Indian
states viz. Uttar Pradesh and Rajasthan. Fly ash samples were obtained from
the three large thermal power plants operating in our region i.e. Kasimpur,
Parichha and Obra. Various brands of cement samples from different
manufacturing companies commonly used in constuction activities in the
region, were collected from the market.
The purpose of the study was to observe the contribution of building
materials to radon levels in dwellings. Over the last two decades it has been
realised that some materials used for building construction may raise the
airborne radioactivity levels in indoor air to unacceptable levels especially
when the radium content in the materials is high. Various waste materials
produced by power plants, chemical and metallurgical industry are commonly
used in building materials. Some of them such as fly ash. furnace slag,
byproduct gypsum etc. contain appreciable amount of natural radionuclides
from uranium and thorium series and may pose radiation risk to the
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population.
The results of the measurements of the radon exhalation rates with
effective dose equivalents, uranium concentration and coefficient of
permeability in soil, fly ash and cement samples point out that the uranium
concentrations of fly ash samples are much higher than the soil samples but
their radon exhalation rates are found to be lower or of the same order. A
positive correlation has been found between the radon exhalation rate and
the uranium concentration as well as the permeability of these samples.
In addition, the radon exhalation rates have been measured in
geological samples from the Mosabani copper mine and the Narwapahar and
Jaduguda uranium mines areas of the Singhbhum Shear Zone in the state of
Bihar, India. The values of radon exhalation rates from crushed rock and
soil samples are found to show a positive correlation with the measured values
of uranium in the corresponding samples. High values of radon exhalation
in subsurface mines like Jaduguda (depth ~ 800m) and Mosabani (depth
~ 1000m) seem to emphasise the need for adequate ventilation for the removal
of radon and its progenies from the mining area.
In order to study the effect on the radon exhalation rates by adding
different amounts of fly ash in the soil, a separate experiment was performed
to measure the radon exhalation from intimate mixtures of fly ash and soil
samples in different concentrations. From the results gradual decrease in
exhalation rate has been observed with the addition of fly ash in soil samples.
The last chapter of thesis has been devoted to the microanalysis of
uranium in solid phase materials such as soil, fly ash, cement and geological
samples from the Mosabani copper mine and the Narwapahar and the
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Jaduguda uranium mine areas by "fission track registration technique". From
the analysis of the samples no significantly high amount of uranium has
been observed. But fly ash and rock samples from mining areas have higher
levels of uranium and show un-even, uniform and non-uniform distribution.