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these have been replaced by solid structures over the years.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

94

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

95

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) =