40
CHAPTER 3
SAMPLING AND PROCESSING METHODS,
INSTRUMENTATION AND EXPERIMENTAL
TECHNIQUES
3.1 PROFILE OF THE STUDY AREA – KANYAKUMARI
DISTRICT
Studies have been carried out in twenty villages from Chinnavilai to Neerodi
with in stretch of 35 km in the Natural High Background Radiation Areas situated on the
western coast of Tamil Nadu in Kanyakumari district and located between 77030’22”and
77008’35”of east of longitude and 8
014’71” and 8
015’40” north of latitude. The southern
region of the district is surrounded by the Indian Ocean .The mineral deposits are
extended to a width of 400 to 1000 meters from the sea. The monazite content of the raw
sand varies from 0.5 to 3 percent. The deposits are present as beach washings and placer
deposits and extend to a depth of approximately 5 metre. The present work was carried
out from August 2008 to June 2012. A special radiation survey is carried out to get first
hand information for selecting representative locations and sampling. A few places are
identified as high background radiation areas which are very rich in monazite, a prime ore
of thorium that is being separated by M/S Indian Rare Earths Limited, Manavalakurichi
under the administrative control of the Department of Atomic Energy in Kanyakumari
district (Paul et al., 1994 ). The coastal villages around Manavalakurichi are naturally
high background radiation areas. Each village is clearly demarcated. Nearly 60% of the
houses have plastered brick walls and tiled roofs. Approximately 35% houses are brick–
concrete structures. Fully thatched houses are still seen in these villages though most of
these have been replaced by solid structures over the years. Some of these houses are built
41
with locally available red mud having high monazite content. One of the distinguishing
features of the study villages is the type of settlement comprising cluster of houses. Each
village has 500-2000 houses. The population of nearly three lakhs comprises mainly of
fishermen. They have identical social, religious, economic and other characteristics,
i.e.they form a homogeneous population group. Most of the able bodied adult men are
engaged in fishing. These villages have churches, schools and community centers. The
people spend a lot of outdoor time and on the beaches by way of recreation and
occupation in addition to the men folk going to sea for fishing. Some of the men in these
villages are also engaged in collection of beach sand as part-time employment. Major
crop grown here is coconut. Most of the food items like cereals; pulses etc. come from
outside the NHBRA. Wells are the major source of drinking water. Public water supply is
also available in some areas. Fish is the common ingredient of the diet.
The Kanyakumari district map showing the sampling locations is provided as
Fig.3.1
Fig 3.1 Sampling Locations
42
3.1.1 Monazite Minerals in Kanyakumari district
The beach sands of Midalam-Muttom coastal belt in Tamil Nadu are endowed
with rich deposits of heavy minerals like ilmenite (46%), zircon (4-6%), rutile (2-7%)
monazite (1-5%), garnet (7-14%) and sillimanite (2-3%). Indian Rare Earths Ltd., an
undertaking of the Department of Atomic Energy has a plant at Manavalakurichi where
mining and mineral separation is carried out. Surface mining, collection of beach
washings and dredge mining are the mining methods adopted. The mineral separation
plant makes use of the differences in the electrical and magnetic properties and
differences in specific gravity of the constituent minerals to separate them. The dried
concentrate is passed through a series of high tension electric separators and magnetic
separators of varying intensities. Fine separation of some minerals is effected by wet
tabling and froth flotation also. During the final stages of monazite separation, air tabling
is also adopted (Pillai et al., 2000). Monazite is radiologicallly the most significant
mineral as it contains about 8-9% thorium as ThO2 and 0.35% uranium. The presence of
monazite can give rise to occupational radiation exposure to the workers.
In the Indian peninsular region, some coastal locations show significantly higher
concentrations of monazite (0.3-3%). There are even some pockets having natural
deposits upto 7-8%. These regions are Chavara-Neendakara of Kerala, Muttom-Midalam
belt of Manavalakurichi, Chatrapur beaches of Odisha. Many more beaches of Kerala
and TamilNadu show general concentrations 0.05-1 %.
3.1.2 Significance of Monazite Mineral
Monazite is the ortho phosphate of thorium, uranium and rare earths. It is found in
higher concentrations at many parts of the globe, where Muttom-Midalam region of
Manavalakurichi and Chavara-Neendakara region of Karunagappalli are some of those. It
43
contains radioactivity of 310 Bq of thorium and 40 Bq of uranium with their daughter
decay products.
It is believed that monazite is getting accumulated in selective coastal regions due
to sea waves and coastal wind. Monazite is present in Manavalakurichi regions for more
than ten thousand years.
Monazite is used for separation of strategic materials thorium, uranium and rare
earths elements. Monazite mineral being radioactive emits a number of alphas, beta
particles and gamma photons. Hence monazite rich areas are classified as Natural High
Background Radiation Areas (NHBRA). The region near Manavalakurichi is classified as
NHBRA. From beach washing and inland deposits monazite and other minerals are
collected by surface mining. The mined sand is subjected to a series of physical
separation process like spirals, magnetic and electric separators so that all the valuable
heavy minerals like ilmenite, rutile, garnet, zircon and monazite are separated. The
monazite is stored in either as finished product of 96% purity inside the godowns or as
semi finished product of 45% purity in earthen pits within fenced site boundary for the
exclusive use of Department of Atomic Energy. Monazite separation is done by physical
separation only. Hence no chemical is used in any stage of the programme. No chemical
waste is generated from IREL, MK. The silica reject void of any heavy minerals is
pumped back to sea shore for backfilling mined out areas. The only gaseous waste is
smoke from oil fired dryer and minimum quantity of silica dust.
Due to removal of radioactive monazite rich sand from inlands as well as beaches
the radiation levels are drastically reduced. An effective reduction of 2-4 folds is found
after mining and reduction of 5-8 folds is effected after back filling with mineral free
silica sand IREL is preserving monazite mineral within the fenced site boundary at Main
plant/ Kootumangalam for exclusive use of the Department of Atomic Energy.
44
The environmental and radiological impact of IREL activities are monitored by
Health Physics Unit deputed by Bhabha Atomic Research Center. This unit carries out
scheduled/random environmental radiation surveys, sampling and other measurements to
ensure that IREL mining and plant operation do not adversely change environmental and
radiological conditions in any part of public domain. This unit submits report to the
Atomic Energy Regulatory Board and State Pollution Control Board periodically.
There are actually at least four different kinds of monazite, depending on relative
elemental composition of the mineral:
Monazite- (Ce, La, Pr, Nd, Th, Y)PO4 -Main Indian composition
Monazite- (La, Ce, Nd, Pr)PO4
Monazite- (Nd, La, Ce, Pr)PO4
Monazite- (Sm, Gd, Ce, Th)PO4
The elements in parentheses are listed in the order in which they are in relative
proportion within the mineral, so that lanthanum is the most common rare earth in
monazite. Thorium and uranium are present in trace amounts in all the classes of
monazite. Due to the alpha decay of thorium and uranium, monazite contains a significant
amount of helium, which can be extracted by heating.
Monazite is an important ore for thorium, lanthanum, and cerium. It is often found
in beach or placer deposits. The deposits in India are particularly rich in monazite. It has a
hardness of 5.0 to 5.5 and is relatively dense, about 4.6 to 5.7 g/cm3.
3.1.3 Composition of Monazite
• P2O5-25-30%
• CeO2- 27-28%
• La2O3- 3%
45
• Pr6O11- 3%
• Nd2O3- 11%
• ThO2- 8-10% , Th-7.5%
• U3O8-0.35% U-0.3%
• Total Rare Earths Oxide- 60%,
3.1.4 Availability of Monazite
The mineral monazite is present in significantly large quantities in eastern and
western coasts of India. It is believed that the rocks in Eastern and Western Ghats contain
a low concentration of monazite. Over millions of years by the action of rivers and floods,
this monazite has reached sea bed and gradually getting concentrated in some pockets
(Sea waves push heavy minerals along with silica sand to shore where lighter silica is
pulled back to sea by returning wave). Monazite is generally found on sea beaches (called
beach washing) or inlands near beaches. The brother minerals found along with monazite
are ilmenite, rutile, garnet, zircon and silliminite. They are treated as heavy minerals as
they settle down in bromoform (density >2.9 g/cc) separation.
3.1.5 Mining and Milling of Monazite.
In India processing of monazite for extraction of its components (RE, thorium and
uranium) is done by IREL. Other mining industries who are engaged in separation of
ilmenite, rutile, garnet or any other associated heavy mineral may receive monazite as
their feed input material, but required to preserve monazite for exclusive processing by
DAE. Monazite mineral separation is done at IREL plants at Chavara, Manavalakurichi
and Chatrapur. The chemical processing for separation of RE and Th-U is done at IREL,
Aluva.
Mining starts with collection of beach washing/ inland sand having heavies
concentration 10-40%. It is brought to an intermediate upgrading plant for enhancing total
46
heavies concentration to 96-97%.This is done by passing wet sand (mined out sand)
through a series of spirals at predetermined speed. In this process heavies get
concentrated due to higher inertia.
Table 3.1 Predominant Gamma Energies from Monazite Mineral
Radio
nuclide
Energy
MeV Ei
Yield
fraction Yi
MeV/ 232
Th
disintegration EiYi
228 Ac 0.129 0.024 0.003096
228 Ac 0.209 0.0389 0.00813
228 Ac 0.338 0.11 0.03718
228 Ac 0.911 0.258 0.235038
228 Ac 0.964 0.0499 0.048104
212Pb 0.238 0.43 0.10234
212 Bi 0.727 0.0658 0.047837
208 Tl 0.583 0.30 0.1749
208 Tl 2.614 0.35 0.9149
Total ΣEiYi 1.571524
The sand containing 96-97 % total heavy minerals is subjected to sun/furnace
drying for moisture removal. Then it is fed to mineral separation plant for each mineral
separation.
Table 3.1 provides predominant gamma energies of radionuclide monazite
mineral.
47
3.2 SAMPLING PROGRAMME
3.2.1 General
The collection of valid samples is the vital first step. Sampling should be done
with the same care as the analysis, and both should be done with a rigor that is
appropriate for the project at hand. A variety of samples may be required for the purpose
of obtaining data in different matrices which will serve the objectives. Sampling must be
carried out based on certain specific criteria so that representative samples, can be
collected. Usually, the crucial decisions in planning a sampling programme are sampling
locations, sampling frequency, sample matrices of relevance etc. For instance Fig 3.2 to
3.8 (photographs) show some of the sampling locations from where the samples are
collected.
48
Fig. 3.2 A natural high background radiation area at Manavalakurichi
Fig. 3.3 A natural highbackground radiation area in Chinnavilai village
49
Fig. 3.4 An area with heavy mineral deposits at Kurumbanai beach
Fig. 3.5 An area with heavy mineral deposits at Enayam beach
50
Fig.3.6. A view of the Manavalakurichi beach having mineral deposits
Fig. 3.7 An area having heavy mineral placer deposit in Mel Midalam beach
51
Fig 3.8 An area having heavy mineral deposit in Mel Midalam beach
Fig. 3.9 An area having heavy mineral deposit at Midalam beach
52
The sampling locations need to be identified based on the wind pattern,
environmental usage and other utilizations of the environment. Fig. 3.2 to 3.9 indicates
the locations from where season wise samples are collected. The locations up to a
distance of 30 km from the Manavalakurichi environment are exploited. In general the
samples from nearby sources may be more frequent as compared to samples from far off
distances. Five numbers of samples are collected in a particular season for each species
(e.g. 5 spinach samples within 30 km radius and the corresponding soil samples). The
collected samples are identified and logged in the record book according to the dates of
collection and locations.
Sampling locations, matrices and frequencies have been selected on the basis of,
a. Distance from the source
b. Predominant wind affected sector
c. Down stream water flow from discharge point
d. Discharge point use/consumption of matrix and its contribution and
importance in internal exposure
e. Production center and availability of matrix
f. Population using the matrix
g. Coverage of all sectors with appropriate frequency
h. Frequency and number reduced with distance
3.2.2 Types of Samples
Samples collected can be categorized as atmospheric, terrestrial, and aquatic
(samples of marine or fresh water origin). Samples from atmospheric environment
53
Fig. 3.10 Schematic Diagram of Radiation Emitting Samples
3.3 AMBIENT GAMMA ACTIVITY
3.3.1 General
The radioactivity status of the environment is defined mainly by gamma radiation
in a particular region since gamma rays contribute most of the external exposure to
population. Hence, the ambient gamma radiation survey constitutes the first and
important stage in monitoring a region for the background radiation. In the present
investigation the ambient gamma radiation level has been measured by using micro R
survey meter or Radiation survey meter and the survey readings are recorded with GPS
coordinates.
Meat
s
Radiation Emitting
Samples
Air Water
Soil Sand &
Sediments
Plants Animals
Milk
Aquatic
Animals
Aquatic
Plants
Fishing
Gears
Human
54
3.3.2 Micro R Survey Meter
Micro R survey meter type supplied by Nucleonix System Pvt Ltd. Hyderabad,
India has been used in the present study. It consists of a halogen quenched G.M.Detector
(Ind. Inc.USA) powered by a rechargeable battery. The dosimeter/survey meter is
designed to read exposure rate in two accuracy ranges of 0.1µR/h and 1 µR/h. It has an
excellent flat energy response from 20 KeV to 2 KeV.The survey meter is calibrated
regularly using a standard source (137
Cs), before starting survey work.
3.3.3 Ambient Survey
The gamma dose rate measurement has been done at 1 meter above the ground
level. At each location, a total of 5 readings have been recorded. Geometric mean value of
the measured readings is calculated to reduce the small-scale variations of the level at a
site. Finally, the survey meter readings, recorded as exposure rate (µR/h), have been
converted into nGy/h using appropriate conversion factor.
3.4 INDOOR GAMMA ACTIVITY
3.4.1 General
The indoor gamma activity on the population depends on the concentrations of
238U series,
232Th series and
40K in the earth crust, materials used for the construction of
houses and also on the cosmic radiation in the environment. The external doses received
by the residents were estimated using TLDs, replaced on a quarterly basis. CaSO4 powder
as TL phosphor is used in TLDs which are developed in Babha Atomic Research Centre
(BARC), Mumbai (Basu et al., 1983). During the replacement time the external gamma
doses were also measured inside and outside houses using a scintillation survey meter and
the estimated doses are compared. The indoor radiation dose to population is widely
measured by passive thermoluminescent dosimeter (TLD) Becker added TLD to the
55
family of radiation measuring devices in 1973 (Becker, 1973).The TLD records
background radiation dose at a particular location for a period typically three months.
3.4.2 Thermoluminescent Dosimeter (TLD)
Thermoluminescent dosimeters are used for measurement of environmental
gamma radiation dose rate (Fig. 3.11). When a thermo-luminescent material is exposed to
ionizing radiation; electrons are given sufficient energy to move around in the material.
Many of these freed electrons are trapped at small imperfection points inside the material.
These electrons remain trapped for appreciable length of the time if the material is kept at
normal room temperature. If the temperature is later increased (6000C), the electrons
leave the traps and get rid of their surplus energy by the emission of light. With a
photomultiplier, one can measure the amount of light emitted during heating. It gives a
means of estimating the number of electrons originally released by the ionizing radiation
and this in turn depends on the size of the radiation dose of the material. The heat
required for releasing the trapped electrons and for producing the luminescent light is
emitted when the electrons get rid of their surplus energy.
Top – Only Gamma
Middle – Pelsper (Plastic) Hard Beta and Gamma
Bottom – Open Both Beta and Gamma
Fig: 3.11. Thermoluminescent Dosimeter
56
The advantages of passive TLDs for environmental monitoring are that they are
small, cheap and do not require power supply during application (Ranogajec – Komor,
M., 2003). The CaSO4, Dy. teflon TLD discs, specifically designed for environmental
thermoluminescent dosimetric purpose have been used. These discs have very high TL
sensitivity, a negligible fading rate and a stable TL response. The TLD used in the present
survey comprises of teflon embedded discs. The disc contains 70 mg of CaSO4 : Dy. and
210 mg of teflon powder. Two such discs are mechanically clipped over two symmetrical
circular holes on nickel plated aluminum. The TLD badge is covered with and sealed in a
polythene pouch to protect the card from rough environment. This system has been
thoroughly characterized and the results of this badge have been proved to be highly
satisfactory for environmental gamma radiation monitoring (Chougaonkar, et al., 2008).
These annealed TLD discs are deployed on a quarterly basis. Each TLD, bearing an
identification number is deployed at a pre-designated sampling location for a period of
three months is replaced by a new TLD of next batch.
The TLD after exposure for about three months are evaluated using standard
protocol (Nambi, et al., 1987). The data obtained from above technique are subjected to
various statistical analyses. Non parametric techniques are used for evaluation at 95%
confidence intervals. These techniques require no assumptions regarding the statistical
distribution of the underlying population and hence can be applied to variety of situations.
In practice, parametric assumptions are difficult to justify, especially in environmental
applications. The data are also analyzed for any seasonal variation in gamma radiation
field.
3.4.3 Effective Radiation Dose
Indoor exposure to gamma rays, mainly determined by the materials of
construction is greater than outdoor exposure if earth materials are used. The source
57
geometry changes from half-space to a more surrounding configuration. When the
duration of occupancy is taken into account, indoor exposure becomes even more
significant. Buildings constructed of wood add little to indoor exposures, which may then
be comparable to outdoor exposures.
The indoor and outdoor results may be derived in separate surveys in locations not
closely coordinated. The outdoor levels generally refer to open, undisturbed ground, but
sometimes street locations may be used. The indoor to outdoor ratios range from 0.6 to
2.3, with a population-weighted value of 1.4. Thus indoor exposures (absorbed dose rate
in air from terrestrial gamma radiation) are in general 40% greater than the outdoor
exposures. Values less than one are determined only in Thailand, the United States of
America and Iceland, where wood frame construction is common. High values of the
ratio (>2) result from high level indoors (in Sweden and Hong Kong) relative to outdoors
or from low value outdoors (in the Netherlands) relative to indoors.
To estimate annual effective doses, (a) the conversion coefficient from absorbed
dose in air to effective dose and (b) the indoor occupancy factor must be taken into
account. The average numerical values of those parameters vary with the age of the
population and the climate at the location. In the UNSCEAR (1993) Report, the
Committee used 0.7 Sv Gy-1
as the conversion coefficient from absorbed dose in air to
effective dose received by adult and 0.8 for the indoor occupancy factor, i.e. the fraction
of time spent indoors and outdoors are 0.8 and 0.2, respectively. These values are retained
in the present analysis. The components of the annual effective dose are determined as
follows
Indoors: 84 nGy h-1
× 8,760 h × 0.8 × 0.7 Sv Gy-1
=0.41 mSv
Outdoors: 59 nGy h-1
× 8,760 h × 0.2 × 0.7 SvGy- 1
=0.07 mSv
58
The worldwide average of the annual effective dose is 0.48 mSv, while for
individual countries the effective dose is generally within the 0.3-0.6 mSv range. For
children and infants, the values are about 10% and 30% higher, in direct proportion to an
increase in the value of the conversion coefficient from absorbed dose in air to effective
dose.
3.5 SOIL ACTIVITY
3.5.1 General
In the natural environment, rocks undergo a continuous process of weathering
owing to several geological processes which eventually results in soil formation. The
resulting soil type has the characteristics of the parent rock i.e., the radioactivity of soil is
that of the rock from which the soil has been derived. Hence, soil becomes the best
representative of radioactivity of any environment. In the present study, the activity
concentrations of 238
U series, 232
Th series and 40
K have been measured from soil samples
using gamma spectrometric analysis.
3.5.2 Collection and Processing of Soil Samples
The soil samples for analysis have been collected from natural, undisturbed and
uncultivated ground surfaces and beaches in conformity with the IAEA recommendations
(IAEA, 1989). Each soil sample has been obtained from nine sub-samples collected in an
area of approximately 100 m2.
The samples for analyses are collected using spear, auger
and Conrad bunker. Samples from various depths ranging 10-20 cm are collected.
Extraneous materials like plant parts, pebbles, stones etc have been removed from the soil
samples and they have been oven-dried at 105ºC for 24 hours to remove the water content
from soil. The dried samples are crushed in motar and allowed to pass through micro
sieves to maintain the uniform soil grain size. The fine samples are then packed in a 250
ml polythene vessel and weighed to obtain the activity concentration of radionucluides in
59
Bq/kg. The bottle containing processed samples are sealed hermetically and externally, so
that the over pressure produced inside by the 222
Rn decay should not result in leakage of
gas. These samples are kept for one month period so as to ensure secular equilibrium
between 226
Rn and its daughter products and are then subjected to analysis. The samples
collected are separated using splitter and sieve. The separated samples are further
analysed using optical microscope.
3.5.3 Heavy Mineral Distribution and Microscopic Study of Heavy Minerals
The sand samples containing heavy minerals are collected from the
Manavalakurichi-Midalam beach placer deposit by the grab sampling method (using
spear, augar and Conrad bunker). Samples from various depths ranging from 0-6 meter
are collected at an interval of 1km. About 1 kg of sand sample is collected from each
location. In the laboratory, the samples are cleaned with warm water, dried and subjected
to heavy mineral separation by gravity method using bromoform (tribromo methane,
specific gravity = 2.8.9). The light fraction of mineral sand contains essentially quartz,
potash felspar and some mica. The total heavy mineral (THM) population by weight
varies between 45 and 65 percentage. The THM concentrates indicated above does not
reflect the average grade of the entire beach placer deposit, which is considerably lower.
The heavy mineral concentrates are then separated to different individual mineral
fractions with a hand magnet and Frantz is dynamic separator. In the latter, the setting is
used for the separation of minerals with a forward and side slope of 15” to 25”,
respectively. Sample splitting is conducted meticulously to ensure that the mineral
fractions are statistically representative. The following current settings are used for the
separation of heavy fraction minerals (i) Ilmentite 0.1-0.35A, (ii) Garnet 0.35—0.45A and
(iii) Monazite 0.40-0.8A. However, due to overlapping of magnetic susceptibilities of
sillimanite separator, high force magnetic separator and lift roll magnetic susceptibility
60
separator, these three mineral fractions produced in this manner seemed to be not entirely
homogeneous and are used for the radiometric analysis.
Microscopic study reveals that the heavy mineral assemblage is consisting of
ilmenite, garnet, sillimanite, rutile, zircon and monazite and they are the dominant heavy
minerals. It is also noticed that the concentrations of zircon and garnet are significant.
Ilmenites are mostly subrounded and show the textural patterns of seriate, granular,
nyrmekititic, and emulsion. Lauexene and anatase occur as patches along margin fracture
and with ilmentite due to alternation. Subrounded grains of rutile are found in small
amounts. Monazite grains are subrounded to rounded and show some pitted marks on the
surface due to action of chemical leaching. Zircon grains are elliptical to subhedral.
Some zircon grains show zoning and some metamict varieties are also found. Garnet
grains are coarser in size, angular to subangular in shape and some metamict varieties are
also found. Garnet grains are coarser in size, angular to subangular in shape and buff red
to brown red in colour. Sillimanite grains show prismatic forms with smooth edges.
3.5.4 Measurement of Gamma Dose Rate
The primordial radionuclides existing in the soil continuously emit gamma
radiation. The gamma dose rate due to primordial radionuclides present in the soil
samples at 1 m above ground level is also calculated. The conversion factor given by
UNSCEAR (1998) is used in this study programme.
3.6 COUNTING SYSTEMS
3.6.1 General
The most common counting systems used in nuclear research laboratory are: (i)
Alpha Counting System, (ii) Beta Counting System and (iii) Gamma Spectrometry
61
3.6.2 Alpha Counting System
The functional layout of the alpha counting system is shown in Fig 3.12.
Electronic Corporation of India Ltd (ECIL) make alpha model SP647A which is used for
the measurement (Fig.3.12). It uses ZnS (Ag) scintillator powder spread over a
transparent polymethylmethacrylate (PMMA) support. The alpha probe is connected to a
radiation counting system model RCS 4027. When an alpha source is placed in a drawer
assembly, the alpha particles emitted from the source taken in aluminum planchet,
interact with ZnS, to produce scintillation (light photons). This scintillation is picked up
by a photocathode tube. These narrow tail pulses are picked from the anode of the PMT
through a load resistor. The pulses are sent to Radiation Counting System.
Fig: 3.12 Functional layout of alpha counting system.
62
Fig 3.13 Alpha Counting System
3.6.3 Background count of the system
The first step involved in counting the radioactivity is to find the background
count. Because of existence of natural radioactivity in the environment and due to several
reasons committed with the detector and electronic circuit of the instrument, all radiation
detectors produce some background signal. The magnitude of the background ultimately
decides the minimum detectable activity. So it is most important to reduce the
background count to the minimum. At present, the background count is estimated for a
duration of 1000 seconds and repeated four times and the average taken as the
background count for the particular counting period.
3.6.4 Determination of Efficiency of Alpha Counting System
The efficiency of the scintillation counter is determined by placing U-235
standard electroplated source that has an activity of 335 disintegrations per minute
63
(DPM). The counts with the source for 1000 seconds are noted. The process is repeated
and the average of the counts is taken. The efficiency is calculated by using the equation
CPM = (Net counts /Counting time) x 60
Efficiency = (CPM/DPM) x 100 %
3.6.5 Estimation of Gross Alpha Activity of a Soil Sample
Approximately 10mg of powdered sample is taken in a previously cleaned
aluminum planchet.
The sample along with the planchet is kept in a drawer assembly of the
alpha system and counted for 1000 seconds.
The background count of counting system is also determined in a similar
way by counting the empty planchet for 1000 seconds.
The net count is obtained by subtracting the background count from the
sample count.
From the measured count rate, the gross alpha activity is calculated using
the formula.
Gross activity = (Net count/T) x (100/E) x (1/W) Bq/g
Net counts= Sample counts- Background counts
W= Weight of the sample in grams
T= Counting time in seconds
E= Efficiency of the counting system
3.6.6 Low Beta Counter
3.6.6.1 Coincidence Unit
It consists of two champers of main and guard counters, two identical
discriminators, a number of mono stable multi-vibrators coincidence and anticoincidence
circuit. Discriminator level is fixed for the typical input sensitivity of 0.5 volt. If input is
64
present from main GM detector only, output is obtained at anticoincidence. If input at
main detector follows the input at guard counter of GM detector within 0.7 milliseconds,
output is obtained at coincidence.
3.6.6.2 Beta Counting Set up
It operates at GM region. The counter consists of an aluminised Mylar foil as
window. (Mylar thickness is not more than 0.9 m/cm2
and 100 m dia tungsten wire is
used as anode). The entire area of the counter is 16 cm2. Sample holder accepts planchet
diameter of maximum 30mm and thickness of maximum 4 mm. Gas flow type guard
counter also operates in G.M region. This is a metal counter with 100m dia tungsten
wire as anode. The active area of the counter is 169 cm2. Argon gas bubbles through
isopropyl alcohol which is kept at zero degree centigrade temperature. The counting gas
flows through the counters continuously at the rate of 3-4 bubbles per second as measured
in bubbler 2 with low viscous oil which is non-solidifying in winter. Counting gas first
enters main counter then into guard counter and comes out to the atmosphere through
bubbler 2. The schematic diagram of the beta counting system is shown in Fig 3.14. The
photograph of ECIL make low beta counting set up is given in Fig 3.15.
65
Fig 3.14 Schematic Diagram of Beta Counting System
Fig: 3.15 ECIL make Low Beta Counting Setup
3.6.7 Preparation of K- source for Low Beta Counting Set up
Heated and powdered 100 mg of KCl is taken in a planchet. Dissolve colloidine
solution as ethyl acetone in the ratio 1:1 [1g of KCl gives activity of 990 dpm]
66
3.6.7.1 Estimation of Gross Beta Activity of a Soil Sample
Approximately 50mg of powdered sample is taken in a previously cleaned
stainless steel planchet
The sample along with a planchet is kept in a drawer assembly of the beta
counting system.
The sample count is determined for 600 seconds
The background of counting system is determined by counting the empty
planchet for 600 seconds
The net count is found out by subtracting the background count from the
sample count.
From the measured count rate the gross beta activity is calculated using
the formula
Gross activity = (Net count/T) x (100/E) x 1/W Bq/g
Net count= Sample count- Background count
W= Weight of the sample in grams
T= Counting time in seconds
E= Efficiency of the counting system
3.6.7.2 Calibration
Analytical grade potassium chloride crystals are powdered after drying at 110 ºC
for 1 hour and uniformly spread and fixed with gelatin or collodion on an aluminum
planchet. The size of the planchet is properly chosen to match the detector and window
size. Strength of around 2 Bq of K-40 is sufficient to give significant count rate. Natural
potassium contains about 0.012% of K-40. Higher strengths will increase the thickness of
standard source causing self absorption. The efficiency of the detector is mostly
independent of energy in GM mode of operation but attenuation due to sample thickness
67
needs to be corrected. Gas filled GM counters normally gives 15-20 % efficiency and
about 0.4 cps (counts per second) background. The specific activity of KCl is 16 Bq/g.
3.6.7.3 Background
The background due to cosmic radiation and environmental radiation is reduced to
some extent by employing good quality lead shielding of about 5 cm thicknesses with Al
or Cu lining. Background radiation of detector materials cannot be reduced and therefore
detectors giving low background should be chosen. Detector of size 25 mm x 50 mm (dia
x height) would have background in the region of about 15 cpm to 50 cpm in 5 cm Pb
shielding depending on the type of detector.
3.7 RADIOACTIVITY DETERMINATION WITH SPECTRO
METRIC SYSTEMS
3.7.1 General
Spectrometry is a system of several devices, which helps in identification and
estimation of mostly gamma or alpha emitting radionuclides. It broadly comprises of a
detector, a high voltage unit, signal shaping electronics and multichannel analyser.
3.7.2 Gamma Ray Spectrometry
Fig. 3.15 shows the multichannel analyser gamma spectrometer. Gamma
spectrometry is a nondestructive technique used to identify and quantify gamma-emitting
radionuclides. It is mainly carried out using NaI(Tl), for gamma energies primarily in the
range of 100 keV to 3 MeV. Thin crystals of both types are used for low energy gamma
emitter analysis. The following paragraphs describe the theory of detectors and their
characteristic parameters used in gamma spectrometry.
68
Fig: 3.16 Multichannel Analyser- Gamma spectrometer
3.7.2.1 Energy Calibration
Energy calibration of NaI(Tl) detector is performed by using sources like 137
Cs
and 60
Co. For lower energies 192
Ir and 108
Ag can be used. Sources should be chosen in
such a way that they have long half-lives and are mono energetic or have multiple
energies of wide separation. Initially, after setting up of the spectrometer, the 137
Cs source
is placed and spectrum is acquired. The peak position due to 137
Cs source is noted. If the
peak is not in the desired position, the gain of the linear amplifier is increased or
decreased and the peak at desired position is obtained. For example if a 10 keV/channel
calibration is required, the peak should be positioned at 66th
channel for 662 keV gamma
lines of 137
Cs. Now different sources of known energies are placed one by one and their
channel positions are noted. Various gamma energies and corresponding channel
69
positions are tabulated and a linear graph is drawn. A linear equation y = mx + c can be
easily fitted. The equation can later be used to find the energy of unknown peak channel.
3.7.2.2 Efficiency Calibration
The geometry in which the samples are analysed should be ascertained to be the
same as that of calibration. The spectrometric system should be calibrated for its overall
sensitivity. Sources obtained from recognized laboratories mostly in liquid form are to be
filled into container of selected geometry after suitable dilution. Spectrum should be
acquired until sufficient number of counts is registered in the peak region.
3.7.2.3 Minimum Detectable Activity (MDA)
MDA is designed to measure the detection capability of the spectroscopic system.
The MDA is defined as the smallest amount of sample activity that would yield a net
count rate for which there is pre-determined level of confidence. MDA is a function of
standard error of the background count rate of the peak and mathematical expression used
to convert the count rate into MDA is as given below.
MDA =
Bq/kg or l …………(1)
Background counts of the photo peak of nuclide of interest, R%. Radiochemical
recovery if any pre-concentration method is employed on the bulk sample, 4.66 value
corresponds to a 95% Confidence Level and 5% chance of assuming a false detection.
MDA =
Bq/kg or l …………(2)
Expression (2) is used for samples having a constant (stable) background count
rates such as sea water samples, fresh water samples, milk samples which do not contain
large number of peaks that may contribute to the increased Compton background.
Expression (1) can be used for samples such as soil, sediment and even vegetation
samples containing high K-40 concentration. The number of channels chosen for Nb
70
should be a function of FWHM and is ~ 2.5 times the FWHM in KeV. Therefore for an
HPGe detector of 2 KeV resolutions, the number of channels considered should be 2.5 x 2
= 5 KeV energy band or 10 channels around the peak position if the ECF is 0.5 keV. The
method of determining Nb can be one of the following:
3.8 DETERMINATION OF RADIUM-226 AND RADIUM-228 IN
GROUND WATER USING BARIUM-133 AS TRACER IN
RADIOCHEMICAL ANALYSIS
3.8.1 General
The determination of 226
Ra and 228
Ra in ground water is carried out using
radiochemical separation coupled with gamma spectrometry, alpha counting and beta
counting. The standardized procedures of 226
Ra and 228
Ra determination using 133
Ba as
radiochemical recovery monitor (pseudo tracer) are discussed here. 226
Ra is the decay
product of 238
U and is an α emitter with half life of 1600 years and is the most radiotoxic
among the radium isotopes. 228
Ra is the decay product of 232
Th and is a ß- emitter with
half-life of 5.75 years. The International standard value for 226
Ra and 228
Ra together is
0.185 Bq/l.
3.8.2 Experimental Programme
226Ra and
228Ra are commonly detected by alpha counting and beta counting
separetley after following a series of radiochemical separation steps. Ra isotopes have
also been determined using liquid scintillation counting (LSC) after separation on a
membrane loaded element selective empore disk. 226
Ra is also determined by measuring
the daughter nuclide 222
Rn after reaching equilibrium with mother using LSC. The
method assumes 100% recovery as it involves evaporation of 200 ml water.
Generally, isotopes of Ra are determined earlier using standardized radiochemical
procedures without using appropriate tracers. Now the procedures are standardized with
known activity226
Ra and/or 228
Ra in repeated analyses and an average obtainable
71
recovery is applied. The variations in chemical recovery in the methods can be as high as
30-40% which if applied in the estimation, combined uncertainty becomes the major
source of uncertainty along with the counting error. Hence addition of appropriate tracer
is essential to reduce the level of uncertainty and to increase the accuracy and
precision.223
Ra,224
Ra and 225
Ra are the generally used tracers of which 225
Ra is a beta
emitter. The half lives of these isotopes are 11.4, 3.66 and 14.9 days respectively. Apart
from having low half-life values they or their daughters are determined by alpha
spectrometric measurements. 225
Ra is the most commonly used tracer as 223
Ra and 224
Ra
are also naturally occurring and its evaluation is carried out by measuring the progeny of
217At after its growth (17 days) using 7.07 MeV alpha particles. The availability of this
tracer is rather scanty and also involves electroplating and alpha spectrometry. Keeping
all these in view, a procedure is standardized using 133
Ba as radiochemical tracer for
simultaneous determination of 226
Ra and 228
Ra in ground water. The basic procedure
involves calcium phosphate co-precipitation and Ba (Ra)SO4 counting.
3.8.3 Analytical Procedure
i) 10 l of water (ground water/PDW) is acidified with 15 ml of Conc. HNO3.
133Ba standard (2 Bq in 10 l), 300 mg Ca and 100 mg of Pb carrier are added
to water.
ii) 100 ml of ammonia is added and stirred to obtain Ca phosphate precipitate
and then allowed for settling overnight.
iii) Ca3(PO4)2 is dissolved in 10 ml HNO3, 5 mg Ba carrier is added and
evaporated to dryness.
iv) Nitrate precipitation is carried out by adding 30 ml Conc. HNO3 (70%) under
ice cool conditions to precipitate Ba(NO3)2, Ra(NO3)2 and Sr(NO3)2. The
precipitate is dissolved in 5\ml distilled water. pH is adjusted to 5 with
72
ammonia and acetic acid. BaCrO4 is precipitated with 5 ml of 10% Na2CrO4 in
2ml of 1 N HCl.
v) 5 ml of 1:4 H2SO4 is added to the precipitate of (Ra)BaSO4, centrifuged,
supernatant discarded; the precipitate is dissolved in ammonical EDTA.
BaSO4 is precipitated by adjusting pH 4-5 with acetic acid.
vi) BaSO4 is dissolved in HClO4. 5 mg each of La, Pb, Bi carriers are added and
kept for 60 h for 228
Ac growth to attain equilibrium with 228
Ra.
vii) During the equilibration period, the solution is counted in 35% NaI(Tl)
detector of a gamma spectrometer in a pre-calibrated geometry for 80,000
seconds to estimate 133
Ba and there from the radiochemical recovery for Ra
using 356 keV gamma line.
viii) 1:4 H2SO4 is added to precipitate Ba(Pb)SO4 for the determination of 226
Ra
and the supernatant containing 228
Ac is preserved for 228
Ra determination. The
ppt containing 226
Ra(BaSO4) is transferred to stainless steel planchet and
counted in low background ZnS(Ag) alpha counter. HCl and HF are added to
the supernatant of step (ix) to precipitate La(Ac)F3 and transferred to
aluminium planchet and counted in low background beta counter to determine
228Ra.
ix) The samples consist of different building materials such as river sand, brick,
cement, jelly, soil, asbestos, wood etc. which are used in and around
Kalpakkam for construction of buildings. After collection, each sample is
dried in an oven at 100 -110⁰C for about 24h and sieved through a 2-mm
mesh-sized sieve to remove stone, pebbles and other macro-impurities. The
homogenized sample is placed in a 250 ml airtight PVC container. The inner
lid is placed in and closed tightly with outer cap. The container is sealed
73
hermetically and externally using cellophane tape and kept aside for about a
month to ensure equilibrium between 226
Ra and its daughters and 224
Ra and its
daughters before being taken for gamma spectrometric analysis.
x) The concentrations of primordial radionuclides (228
U, ²³²Th &⁴⁰K ) in the
sample are determined by employing a high resolution hyper pure germanium
(HPGe) gamma ray spectrometer system consisting of a p-type intrinsic
germanium
xi) Coaxial detector (Type : EGPC 150 P 15-R, volume 151cc; Eurisys Measures
make) is mounted vertically and coupled to an 8K PC based multichannel
analyzer (APTEC make). The detector is housed inside a massive lead shield
to reduce the background of the system. IAEA standard reference material
uranium ore RGU1, Thorium ore RGTh1 and KCl powder of known activity
are used for calibration of the system. Each sample after equilibrium is kept on
top of the HPGe detector and counted for a period of 50,000 sec. The
minimum detectable activity (MDA) for each radionuclide is determined from
the background radiation spectrum for the counting time of 50,000 sec, The
estimated (3σ) values are 1 Bq/kg for ²²⁶Ra, 4 Bq/kg for ²³²Th, and 38 Bq/kg
for ⁴⁰K.
3.9. ENVIRONMENTAL AIR MONITORING
3.9.1 General
Air samples are collected on quarterly basis from air sampling stations located at
coastal areas of Kanyakumari district. Short duration samples (1 hour) collected at 50
litres per minute are analysed for thoron daughter products activity. Long duration
samples are collected for dust concentration and long lived alpha activity.
74
3.9.2 Static Air Sampling
Air samples are collected for evaluation of gaseous pollutants or pollutants in the
form of aerosols. In the context of natural radiation this leads to assessment of inhalation
and submersion radiation doses. Air particulate samples are collected to estimate gross
alpha, gross beta and gamma emitting radionuclides present in the atmospheric air. The
samples are collected by filtering a known volume of air through glass fiber filters using a
suction pump.
Essential equipments used for air sampling are:
a. Air mover, which is essentially a suction pump, capable of continuous operation
for several hours at high flow rates. Stationary pumps at defined locations near the
laboratory can be used for a continuous weekly averaged air sampling. The
sampling is carried out by running the pump round the clock, generally with a
capacity of 20 to100 liters per minute.
b. Flow meter is used to measure the rate of passage of air. Air pumps available with
constant rates are to be used or alternatively, flow meters are to be used for the
purpose. Battery operated pumps have to be used for field sampling. Sampled air
volume is expressed in m3 and air concentration as mg per m
3.
c. A time totalizing device is used to note the duration of air sampling.
d. Filter head containing filter paper and filter holder is used for particulates or
absorbents for gaseous or liquid form on activated charcoal cartridges, or air
moisture on silica gel. In specific cases like tritium, devices like cold finger can be
used to collect air moisture samples. Measurement of wet and dry temperature for
relative humidity while sampling is required to obtain air concentration from
measured concentration in condensed air moisture.
75
e. Air filter paper should be of good surface deposition characteristics. Air filters can
be made of glass fiber filter paper with efficiency of >99.97 % for 0.3 µm particle
size. Glass fiber filter papers show deeper penetration, but with the recommended
face velocity (<100 cm/sec) and short sampling period, particle burial in the paper
is negligible. Filters of small pore size, though good surface collectors, generally
give resistance to flow, necessitating high capacity pump. The connecting tube
between the filter holder and the flow meter should be thick walled pressure type
tubing.
The static air sampling set up consists of an air mover (vacuum/pressure pump) an
open face suction head and the connecting PVC tubing. A flow meter (plastic body
rotometer) is connected in line after the suction head to measure the suction rate. Pre-
weighed GFA filter circles (2.5cm dia) are loaded with a backing wire mesh to the suction
head. The duration of sampling and flow rate are noted for each sample. The pumps
used have flow rates of 40-60 liters per minutes (lpm) with filter.
3.9.2.1 Measurement of Dust Concentration
The filter paper has to be weighed carefully before and after the sampling. For
this purpose the filters should be heated at 150⁰C for 10 minutes and transferred to
desiccators for cooling. There after the filter should be weighed using analytical balance.
The process of heating, cooling and weighing should be repeated after collecting the
sample. The difference in weight should be noted. The weight of the sample should be
used to calculate the APM in mgm-3
of the air.
Calculation of dust concentration:
Dust concentration = (Mx103)/ (VxT) mg.m
-3
where, M= Mass of dust collected in mg.
V= Sampling rate, lpm
76
T= Sampling duration, minutes
The dust concentrations obtained by the above method represents the gross APM
(respirable and non-respirable). In order to find the respirable APM, respirable air
samplers, HASL cyclone and Anderson sampler are employed. However, some of these
equipments such as cyclones and Anderson samplers are not routinely used. The APM
has to be characterized periodically with these samplers and the ratio obtained with
respect to the gross APM to the respirable APM is used to routinely define the latter
(Respirable APM =<10µm)
3.9.2.2 Air Activity due to Thoron Daughters
As a measure of the concentration of short-lived radon daughters in the air in
ilmenite mines, the unit Working Level (WL) is introduced. Initially, the WL is taken to
represent the maximum concentration of airborne short-lived radon daughters to which
the ilmenite miners could safely be exposed. Subsequently, this unit has been applied to
the concentration of short lived airborne radon and thoron daughters in buildings
occupied by the general population.
3.9.2.3 Definition of working level (WL)
Short lived radon or thoron daughters present in a liter air with the potential of
emitting 1.3×105 MeV of alpha particle energy during their decay is known as one
working level.
Air activity due to thoron daughters in the village area is determined by analyzing
GFA (Glass fiber air) filter samples collected using static air sampler for short duration of
30 min. The filter in a counter ZnS (Ag) of known efficiency (30%) and background after
a delay of 300 minutes from and of sampling is counted.
Activity due to thoron daughters
Th(B) =
= mWL
77
CPM= Net counts (Gross counts- Background) per minute
Alpha factor = 0.0762
E= Efficiency
V= Volume of air sampled
T= Duration of sampling
Correction for self-absorption in the sample matrix has to be applied to the counts,
depending on the weight of the dust collected.
3.9.2.4 Measurement of Air Activity due to Long Lived Thorium -232
The sample collected is preserved as above for one month. The sample is counted
in an alpha counter ZnS (Ag) of known background and efficiency (E %) for long lived
air activity due to 232
Th, 228
Ra, 228
Th. etc (“Th” Chain Nuclides)
Calculation:
C is the counts obtained for t seconds and B is the background for the same period
Net counts, N = C-B
Total air activity collected on filter = (N/t) x100/E) Bq
Gross air borne activity, A = (Nx100x1x103)/ (t x E x V x T) Bqm
-3
Where, t = Period of counting (Seconds)
E= Efficiency (%) of the instrument
V= Sampling rate, lpm
T= Sampling duration, min
Air activity due to 232
Th = A/6 Bqm-3
(Applicable to minerals industry)
An equal activity is assigned in all cases for 228
Th.
3.9.3 Air Quality of Samples
Air samples are collected for determining conventional parameters like suspended
particulate matter (SPM), respirable particulate matter (RPM) and 232
Th, in atmospheric
78
air. The samples are collected through filters followed by absorption in bubblers
containing absorbing media.The samples are collected at the terraces of all selected
houses in the study area. The SPM samples are collected at a height of 20 meter above the
ground level and RPM sampling is carried out in parallel using a microprocessor
controlled high volume sampler (Envirotech high volume sampler) shown in Fig.3.17
operating at the flow rate of 0.9 m3/minute. The sampler has a size selective inlet to
remove all the particulates larger than 10mm. EPM 2000 glass fiber filter paper is used
for SPM and sample holder is used for RPM. Eight hour samples are collected throughout
the year. Mass measurements are carried using micro balance for weighing the filters
before and after sampling. Filters are desiccated in an environmental chamber with
constant air humidity (50% air humidity) and temperature between 20 and 40º C before
weighing. After exposure filters are again desiccated for 24 hours before final weighing.
79
Fig: 3.17 Envirotech High Volume Sampler
Safety Precautions:
1. Ensure the electric line is properly earthen to keep away from electric shock.
(1) Calculation of Suspended particular matter
SPM =
= μgmm
-3
Net weight = Gross weight – Filter paper weight + Gross bottle weight- bottle
weight.
(2) Calculation of respirable particulate matter:
SPM =
= μgmm
-3
Net weight = Gross weight – Filter paper weight
80
(3) Calculation of Long lived activities:
Th =
= Bqm
-3
Alpha factor = 6
3.10 BREATH ANALYSIS FOR RADON-220 (THORON)
3.10.1 General
Thoron emanating from 232
Th can be estimated by using thoron breath
measurement. The thoron breath analyser layout is shown in Fig.3.18.
3.10.2 Procedure
All the interconnections of the delay chambers and double filter are ensured to be
leak proof. The suction rate is adjusted through the system between 30 to 40 litres per
minute (lpm) with the exit and inlet filters in position. The inlet and exit filters (GFA) are
then changed. The subject is assisted for sitting on a chair to wear the respirator and adjust
leak tightness and breathing comfort by adjusting the head straps. The system is operated
10 min for equilibrium, with the person wearing the respirator in position. The pump is
switched off after 10 min and waited for 2 min for the vacuum in the system to break. The
exit filters are removed and discarded and a fresh exit filter is loaded. The system is made
to run for 30 minutes for sampling the thoron in the exhaled breath of subject. At the end
of 30 min the pump stops and the respirator is removed from the person. After 2 min the
exit filter is removed into a clean petri dish using a clean forceps. The alpha counter is
calibrated to find out its background with GFA filter disc by counting for a minimum
period for 1 hour. The exit filter in the calibrated alpha counter of known efficiency and
background is counted for 16 hours from 4th hours reckoned from the end of sampling and
noted the counts.
81
Fig 3.18 Thoron Breath Analyser
Calculation:
Thoron emanating from 220
Ra can be calculated using the relation, QRa =KDE-
1Z-1 Bq
where, K is a non-dimensional factor.
D is the total number of counts on the second filter paper for the entire counting
period.
E is the efficiency of the counter (Decimal fraction)
Z is a theoretical parameter derived based on the sampling duration and counting
intervals for the set up used,
K= 2.72, Z= 900 (for 16 hours counting)
Z= 930 (for 18 hours counting)
3.11. INDOOR RADON, THORON AND DAUGHTER ACTIVITIES
3.11.1 General
Radon, thoron and their short lived daughter products present in man’s natural
environment can pose radiation hazard if such sources are concentrated in enclosed areas
like poorly ventilated houses/buildings and underground mines. This radon, thoron and
their daughter products contribute the maximum to the natural radiation dose to general
82
public. Large scale and long-term measurement of these radionuclides in houses has been
receiving considerable attention.
Several techniques are in use to measure the radon, thoron and their progeny
levels in indoor air (Subba Ramu et al., 1992). These include active spot sampling and
time integrated passive method. The active method involves the collection of atmospheric
particulate matter on a millipore filter disc sucking air through it using a vacuum pump
for a known period of time and by counting the alpha activity of the filter paper using
ZnS counting system.
The concentration of radon and thoron gas in indoors is a very fluctuating one and
it depends on barometric pressure, humidity, temperature, porosity of soil, building
materials etc. Because of this, it is very difficult to interpret the results of active
measurements, which measure only for a few hours, whereas, passive detectors avoid this
problem as they give a result in terms of the average radon gas concentration for the
duration of exposure (Mishra et al., 1995).The Solid State Nuclear Track Detector
(SSNTD), a passive-integrating detector has also been employed in the present
investigation at a few air sampling locations.
3.11.2 Distribution of Indoor Radon and Thoron Progeny Levels in a Two Count
Method
Exposure to 222
Rn and 220
Rn and their progeny present in air is the largest
contributor to the average effective dose received by human beings. The present study is
aimed at assessing the inhalation dose received by the population living in the coastal
areas of Kanyakumari district of Tamil Nadu, by the progeny level of 222
Rn and 220
Rn. A
two count method has been used for the determination of 222
Rn and 220
Rn progeny levels
in the present study.
83
Air samples are collected on glass fiber filter papers (GFA) at flow rate of 50 l/
minutes . After sample collection, the filter paper was counted for alpha activity for 1000
seconds duration using a ZnS(Ag) scintillation counter. This was taken as the first count
C1.After a further delay of 300 minutes post sampling for the second count C2 was
taken for duration of 1 hour using these counts the concentrations are estimated. For the
second count the short lived radon daughter product would have completely decayed and
subsequent counts obtained are expected to be due to the decay and buildup of thoron
progeny. The theoretical computation for the build and decay of radon and thoron
progeny was possible from unit activity of individual radionuclides. Different techniques
are used in measuring the 222
Rn and 220
Rn progeny level in the indoor air.
WLRn= The working levels of radon and thoron are computed from the following
formulae V is the volume of air collected. k is the ratio of the counting rate for 220
Rn
daughters in counting period (1) and (2).
The values for FRn, FTn and k obtained for 30 minutes sampling and counts after
30 and 500 minutes
3.11.3 Solid State Nuclear Track Detector (SSNTD) Dosimeter
Solid State Nuclear Track Detector (SSNTD) based dosimeters have been used for
this survey. These are simple to use and less expensive as compared to some continuous
measurement systems like the Alpha guard. The latter is useful for occasional
comparisons with the SSNTD based dosimeters. In view of this, SSNTD based
dosimeters have been developed and calibrated for the survey. Since the sampling is
passive and integrated for long duration, the diurnal and seasonal variations in radon /
thoron concentrations have been accounted.
84
3.11.3.1 SSNTD Based Dosimeter System
The functional diagram of a SSNTD is shown in Fig.3.19. The components are
noted below. The internal dose due to radon, thoron and their progeny was estimated
using SSNTD films, in three different modes (Barooah et al., 2003).
1. Bare mode SSNTD film
2. Radon cup mode SSNTD film
3. Radon+ Thoron Cup mode filter
The dosimeter system developed is a cylindrical plastic chamber divided into
equal compartments, each having an inner volume of 135cm3 and diameter 4.5cm.
Dimensions of the dosimeter are chosen, based on the ratio of the effective volume of the
cup to its total volume to achieve maximum track registration for the cylindrical cup. The
design of the dosimeter is well suited to discriminate radon and thoron in mixed field
situations, where both the gases are present as in the monazite deposited areas.
Fig. 3.19 Functional Diagram of SSNTD
Cellulose nitrate films (LR-115 type II) manufactured by the Kodak Pathe are
used as detectors. The 12cm thick film cut into 2.5cm x 2.5cm size is affixed at the
bottom of each cup as well as on the outer surface of the dosimeter. The exposure of the
85
detector inside the cup is termed as cup mode and the one exposed open is termed as the
bare mode. One of the cups has its entry covered with a glass fiber filter paper that
permeates both radon and thoron gases into the cup and is called the filter cup. The other
cup is covered with a semi-permeable membrane sandwiched between two glass fiber
filter papers and is called the membrane cup. These membranes has permeability
constant in the range of 10.8-10.7 cm2/s and allow more than 95% of the radon gas to
diffuse while it suppresses the entry of thoron gas almost completely. Thus, the SSNTD
film inside the membrane cup registers tracks contributed by radon only, while that in the
filter cup records tracks due to radon and thoron. The third SSNTD film exposed in the
bare mode registers alpha tracks contributed by the concentrations of both the gases and
their alpha emitting progeny.
The dosimeter is kept at a height of 1.5m from the ground and care is taken to
keep the bare card at least 10cm away from any surface. This ensures that errors due to
tracks from deposited activity from nearby surfaces are avoided, since the ranges of alpha
particles from radon/thorn progeny fall within 10cm distance. After the exposure period
of 90 days, the SSNTD films are retrieved. The SSNTD films are counted in alpha and
beta counters, to see whether there is any deposited or placed out activity. The counting
is ensured that there is no deposited activity. Further the SSNTD films are chemically
etched in 2.5N NaOH solution at 60ºC for 60 minutes with mild agitation throughout.
The tracks recorded in all the three SSNTD films are counted using a spark counter.
3.11.3.2 Spark Counter
The instrument has provisions to produce sparks, which will pass through the
holes produced by the radiation, etches and by the chemical process. An aluminum sheet
(Mylar) with one side conducting and having a thickness of 90 mm with its conducting
surface faced to the SSNTD and touching the electrode is placed over the SSNTD. When
86
the instrument is sparked the one that passes through the holes make the aluminum sheet
area nonconducting by melting and the same is registered. The total holes are thus
counted and registered.
One may expect deposition of activity on the SSNTD film in the bare mode
exposure, which may pose as an unknown parameter in the calibration factor. But it has
been proved that the LR-115 (12mm) film does not register tracks from deposited
activity. This is because the Emax for LR-115 film is 4MeV and all the progeny isotopes
of radon/thorn emit alpha with energies greater than 5MeV.
Calibration Factors
Calibration factors (concentration conversion factors) for radon and thoron are
required to convert the recorded tracks in the exposed SSNTD films into radon and thoron
concentrations. Calibration factors are estimated experimentally as well as theoretically
for all the three modes of exposures. These are discussed in the following section.
Calibration factors (CFs) for radon and thoron gases in the cup mode are
determined through a series of experiments. CFs for radon (kR) and for thoron (kT) in
terms of tr.cm-2
per Bq.d.m-3
can be obtained as
kR = (24xT) / (CRxH)
kT = (24xT) / (CTxH)
where,
T is the tracks per unit area (tr.cm-2
)
CR is concentration of the radon gas (Bq.m-3
)
CT is the concentration of thoron gas (Bq.m-3
)
H is the exposure time (hours)
The calibration factor for the bare detector is defined as the track density rate
obtained per unit WL. The track formation rate in the bare mode is not a unique function
87
of WL, but would depend on the equilibrium factor (F). If one defines the bare detector
calibration factor as kB (tr.cm-2
/Bq.d.m-3
) of each species, it may be easy to show that this
quantity is independent of the equilibrium factor as well as the incident energy of the
alpha particle.
For a given track density rate T (tr.cm-2
d-1
) and working level (WR for radon and
WT for thoron in mWL units) and the corresponding equilibrium factors, FR and FT, the
calibration factors can be obtained for radon (kBR) and thoron (kBT) respectively in
terms of tr.cm-2
/Bqdm-3
using the following equations.
kBR = (T/3.7WR) (FR/(1+2FR)
kBT = (T/0.275WT) (FT/(1+2FT)
Based on this concept, CFs are derived for the species matrix for radon,
thoron and their progeny concentrations.
3.12 RADIO CHEMICAL ANALYSIS
3.12.1. General
The sample preparation for radioactivity estimation depends upon the type of
sample and radionuclides to be analysed and the activity levels. Gamma emitters are
estimated in fresh, dried or ashed samples after filling in a container of suitable geometry
by direct gamma spectrometry depending upon activity levels. Volatile radionuclides
such as radioiodine are estimated in fresh samples or with special precautions to avoid
loss by volatilization. Beta and alpha emitters are estimated after radiochemical
separation. For the purpose of radiochemical separation, it is necessary to first solubilise
the sample to mobilise all detectable radionuclides from the sample matrix. Generally the
following methods or a combination of them are adopted depending upon the sample
matrix and objective of the analysis.
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3.12.2. Sampling and Methodology
For the present study twenty sites are identified in Kanyakumari District of South
West India. From each site four to five soil samples of approximately 1 kg (wet weight)
are collected and analysed for radioactivity. About 2kg (fresh weight) of each food crops
selected are also collected and analysed for radioactivity.
3.12.3. Acid Leaching Method
Strong nitric acid and hydrochloric acid leaching suffices to mobilise most of the
radionuclides from environmental samples (e.g. marine and fresh water sediment, ash of
tissue, vegetation, crop, milk etc.) .
3.12.4. Methods for Determination of Uranium
Fluorometry is one of the most suitable instrumental methods for uranium analysis
in trace level. The solid sample is converted into an aqueous solution and then the
solution containing uranium is extracted with ethyl acetate in the presence of saturated
solution of aluminum nitrate. After extraction, an aliquot of the organic layer is taken in
platinum dish and the solvent evaporated under UV lamp, the residue is fused with
sodium fluoride and sodium carbonate flux at 800ºC, for 3 minutes in a muffle furnace.
Then the fluorescence of the resultant mass is measured. The liquid samples are analysed
by applying the same procedure.
3.12.4.1 Fluorometric estimation of Uranium
1. 3 grams of soil is taken and 5 ml of Con. HNO3 added and evaporated to dryness
2. 1-2 drops of perchloric acid is added and evaporated again
3. The evaporation is repeated with Con. HNO3 till the organic matter is destroyed
4. The residue is dissolved in a minimum volume of 8N HCl (about 10 to 20ml) and
transferred to a polythene beaker. Then passed through the ion exchange columns.
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Glass wool should tighten the column, then 2 grams of dowex resinis added and
washed two or three times with distilled water then washed two to three times with 8N
HCl. Then the entire solution is passed through the column and the column is washed
with 25 to 30ml of 8N HCl.
The column is eluted with sample and 15ml of 1N HNO3, it is collected in a
100ml of beaker, evaporated the effluent to dryness and dissolved in exactly (pipette out)
5ml of 1N HNO3. From that 0.2 ml is taken to estimate uranium fluorometrically as
follows
Taken clean platinum planchet in porcelain plate dried under infrared lamp
Plank 0.2ml sample + flux 0.2mlsample+0.2 uranium std+flux 0.2 ml
uranium std+ flux Flux alone reading in the fluoro meter.
Take clean platinum planchat in porcelain plate dry under infrared lamp
Plank 0.2ml
sample + flux
0.2mlsam
ple+0.2 uranium
std+flux
0.2 ml
uranium std+
flux
Flux
alone
Then carefully heated on a burner using platinum tipped forceps to hold the
planchet and the reading is taken in the fluorometer.
3.12.4.2 Reagents and Chemicals
Water samples (i.e. tap water, bore well water, river water etc.) can be directly
analysed. A standard stock solution of 0.973 g-1
uranium (Aldrich make) is diluted to
working concentrations for regular calibration of the system. Sodium pyrophosphate (5%)
is used as the fluorescence enhancement agent and for the formation of uranyl complex
since uranyl phosphate complexes are stable.
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3.12.4.3 Analytical Procedure
About 5 ml of water sample is taken in dry and clean cell and 0.5 ml of 5%
sodium pyrophosphate (pH ~ 7) is added and measured. The instrument is calibrated with
standard uranium solution of a known concentration. Standard addition method is
followed for analysis of field samples in order to avoid the matrix effect. Both
micropipettes and analytical balance are used simultaneously to avoid any error in
pipetting.
The concentration of uranium (ppb) in samples is calculated by using the formula,
U (ppb) = D1 / (D2-D1) x (V1C / V2)
Where, D1 – fluorescence due to sample only
D2 – fluorescence due to sample and U-standard spiked
V1 – volume of U-standard added (ml)
V2 - volume of sample taken (ml)
C - Concentration of U-standard solution (ppb)
The advantage of the laser photometry against the other methods is the high
measuring precision with the possibility of detecting fairly low concentrations down to
0.0002 mg/l. Laser photometry has accuracy comparable to that of liquid scintillation, it is
quick and avoids organic waste as in the case of LSC. Furthermore, there is no need for a
specific sample preparation. It uses modern technology and is easy to use even in the
field. For fairly low concentrations, liquid scintillation and laser photometry are equally
applicable and the results do not show significant differences.
3.12.5 Estimation Of Polonium Activity
The chemical deposition method is employed for the determination of 210
Po both
for soil and sand samples. The sand and soil samples are dried in an oven at 110ºC till a
constant dry weight is obtained. From the fresh and dry weight, moisture content is
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calculated. The dried 20 g of sample is leached with 4M HNO3 and then organic matter
present in the sample is destroyed by digestion with HNO3+H2O2 mixture in small
increments to get a white residue. The residue is then dissolved in 0.5M HCl medium and
210Po in the solution is deposited into the silver disc at 97C for 6 hours. The disc is
washed with distilled water, rinsed with alcohol, dried under an infrared lamp and the
activity is counted on both the sides in ZnS (Ag) alpha counter of 30% efficiency
Procedure:
1. To 3 grams of soil, 5 ml of con. HNO3 is added and evaporated to dryness
2. 1-2 drops of perchloric acid is added and evaporated again
3. The evaporation with con. HNO3 is repeated till the organic matter is destroyed
4. 90 ml of water and 10 ml of con .HCl (10N) are added.
5. The solution is heated in a constant temperature bath at 90oC
6. Fresh silver disc is put in the solution and mechanically stirred with a glass stirrer
so that the silver disc spins inside the beaker. The silver disc should be initially
counted to determine the background alpha activity on either side.
7. The stirring and heating for 2 h is continued making up for the loss of solution due
to evaporation at regular intervals.
8. The sliver planchet is taken out rinsed in distilled water, allowed to dry and
counted on both sides of alpha counting.
3.13. RADIOACTIVITY IN FOOD STUFFS
3.13.1 General
Food is essential for growth and other activities of human beings and other living
organisms. The food habit of human beings widely changes depending on the living
region and life style. Vegetables are the major constituent of food consumed by Indian
population. In Manavalakurichi environment, people consume vegetables, which are
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cultivated locally i.e., near their houses. For normal growth of plants and vegetables,
nutrients like potassium, calcium, copper, magnesium, manganese, chlorine, Zinc etc.,
are essential. The uptake of these elements from soil to different parts of plants takes
place through roots. Radioactive isotopes coming under three radioactive series 238
U,
232Th,
40K and singly occurring radionuclides in soil are also transferred to plants. The
uptake of naturally existing radionuclides 210
Po and 210
Pb in to vegetables are (1) through
roots uptake from soil and (2) atmospheric deposition on plant part from air and
subsequent absorption by plants. The accumulation of 210
Po and 210
Pb through root
transfer depends on concentration of the radionuclides in the underlying soil,
physicochemical nature of the radionuclides, use of biochemical fertilizers and
morphological aspects of plants.
Thus there is a possibility of contamination of food stuffs, following soil-plant
transfer as well as getting into the human body (Chen et al.,., 2005). The release of
radionuclides into environment contaminates food materials according to the type of soil,
its chemical characteristics, and the physical and chemical forms of the radionuclides in
the soil, radionuclide uptake by particular plant and finally the level of accumulation by a
particular foodstuff (Samavat et al., 2006) such as tapioca, vegetables and fruits to
estimate the annual ingestion dose from these natural isotopes for the south Indian adults.
3.13.2 Food Samples from Terrestrial Environment
Soil, vegetation, food crops, fruits, milk, vegetables etc. are collected from pre
designated locations. Ground water samples from wells and bore wells are analyzed for
radionuclides to study terrestrial subsoil movement of radioactivity.Soil sampling is
carried out with an intention to mainly evaluate root uptake leading to environmental
transfers. Soil sample should be collected from an undisturbed area. For study of transfer
factors, area has to be nearly covering the root spread. Samples from different spots
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covering a depth of 15 to 20 cm up to which the ingrowth of nourishing roots is expected,
has to be covered. A small pickaxe or a hand-scoop can be used for sampling.
3.13.2.1 Vegetables/Vegetation/Food crops
For evaluation of contribution of a pollutant to environment from a source,
vegetables and vegetation have to be collected from the fields located in the environment.
A composite sample, about 4 kg in weight is to be collected from the locality, from
different plants distributed at the locality. 2 kg is normally sufficient for analyses, the rest
being used for storage. Samples can be collected in perforated polythene bags and stored
under refrigeration. Fresh weight is to be taken at the earliest. Vegetables and vegetation
vary from place to place and the sample chosen should be representative for the location.
Rice, wheat, millets and pulses are the main food crops in the country. For study
of transfer factors the samples have to be collected from the field along with soil sample.
For dose evaluation, they can be collected from the fields or from granaries known to
store crop from the locality. About 2 kg of sample should be adequate for radiochemical
estimation and storage.
3.13.2.2 Milk
Milk should be collected from dairy farms where milk is processed for distribution
or pooled from 5 milk producers and pooled to make a representative sample. 2 litres of
milk is needed to be sampled, 1 litre for immediate analysis and 1 litre as standby. 5 ml of
5% formalin is added per litre milk is to be preserved for long periods. For short duration,
refrigeration is enough.
3.13.2.3 Ground Water
Main ground waters to be studied in terrestrial environment are well waters and
bore well waters. For study of contribution to dose about 20 litres of water needs to be
analyzed since the levels are likely to be very low. In case of monitoring bore wells and
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open wells new waste storage facility, a volume of 1 litre is adequate for studying ground
water movement or seepage of the pollutant.
3.13.2.4 Aquatic Food Monitoring
Fresh water, sea water, aquatic organisms like fish (fresh water and marine) shore
sediment, bottom sediment and bottom cores, aquatic biota and aquatic plants cover the
spectrum of samples of aquatic origin.
3.13.2.5 Water Samples
10 - 50 litres of water from each water body at desired locations should be
collected in plastic containers. At locations where treated effluents are discharged into the
aquatic system, it is desirable to have a continuous sampler which pumps small quantity
of water from the location to a container. This will give the time averaged concentration.
3.13.2.6 Sediment
Shore sediment is collected from top layer, using a procedure same as that of
surface soil. Lake bed, river bed and sea bed are sampled using grab samplers (Ekman
Dredge). From each location, two or three rabs should be collected and pooled. 1 to 2 kg
samples are collected and a composite sample is prepared to represent the sample of that
location,
3.13.2.7
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3.13.2.8 Fish and Aquatic Organisms
Fish samples are collected from fish landing centres or brought directly from the
boats or trawlers. These samples have to be collected depending on the availability, with
the help of local fishermen. It would be better if variety of samples can be collected so
that study on preferential uptake of a particular radionuclide may be carried out. About 1
kg of fish is normally sufficient for a single analysis. Marine organisms like oysters,
crabs, clams and sponges which concentrate radionuclides and are good indicators for
specific radionuclide should also be collected. Samples collected in plastic bags should be
transferred to ice-box and subsequently preserved in deep freeze prior to analysis.
3.14. TRANSFER FACTOR
Transfer factor (TF) is calculated as the ratio of the radionuclide concentration in
food crop (Bq/kg) to its concentration in soil (Bq/kg)
T F =
The uptake factor of radionuclides from soil to different parts of plants has been
analysed .Ten plants are analysed for this study. The plants are separated into fruit, grain,
leaf and stem. The activity concentrations of radionuclides are analysed for each plant
part.
Concentration of radionuclide in plant when normalized for the potassium content
of the soil and plant gives the discrimination against its uptake as compared to that for
potassium by the plant. The observed ratio can be calculated as
Observed ratio (OR) =