2003 10 Measurement of Radon Exhalation Rate From Indian Granite Tiles

8/9/2019 2003 10 Measurement of Radon Exhalation Rate From Indian Granite Tiles

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Proceedings o f the 2003 International Radon Symposium

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Volume I1

American Association of Radon Scientists and Technologists Inc.

October 5

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8 2003

MEASUREMENT OF RADON EXHALATION RATE FROM INDIAN GRANITE

TILES

Sundar S. Bala; Ajoy K.C; Dhanasekaran A. Gajendiran V Santhanam R.

S Bala Sundar. K.C. Aiov A. Dhanasekaran. V. Gaiendiran and

R.

Santhanam

Radiation Safety Section Radiological Safety Division

Indira Gandhi Centre for Atomic Research Kalpakkam-603 102,Tamil Nadu India.

ABSTRACT

Measurement o f radon exhalation rate from different varieties of granite tiles

quarried from various parts of India has been carried out. Seventeen varieties of widely

exported granite tiles of dimensions 5 x 5 x 2.5 cm have been tested. Five samples were

received for each variety. The s ample s were exposed for 24 hours in a leak tight mild

steel chamber. Gas sa mples were then collected using Lucas cell and counted for alpha

activity to evaluate using standard procedure the concentration of radon from which

radon exhalation rate w as estimated. Powder sam ples of the tiles were subjected to

gamma spectral analysis. The results obtained are presented and discussed.

Key words : radon granite Luca s cell

Corresponding author: S.Ba la Sunda r email

:

sbs^?.iecar.ernet.in

INTRODUCTION

Human beings have always been exposed to ionizing radiation from various

natural source s

of

radiation and one of the major routes of internal exposures is through

inhalation of radioactivity present in the atmosphere. The three primordial radionuclides

viz ^K ^U a n d 2 3 2 ~ hhat are present in the building materials in varying

concentrations cause both internal and external exposures to the residents. External

exposure is caused by the gamma radiation from *OK and the da ughter products of

238

and ^ ~ h . t is known that as a result of inhalation of ^ ~ n daughter product of decay

chain of ^u and its daugh ter products the equivalent dos e to the entire lung is 20

and 45 higher than the equivalent dose in other tissues.


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Soil, natural g as, water, construction materials are so me o f the sources of radon.

From the epidemiological study made on the mortality rate among uranium and non-

uranium miners, the correlation between cumulative exposure to the short-lived radon

daughters and the excess incidence o f lung cancer has vividly demonstrated the existence

of a positive relation Ahem ed,J.U.,1992). For this reason, an assessme nt of radon,

exhaling from building m aterials assumes significance.

Several studies have been undertaken to evaluate the radon exhalation rate from

building materials Maged and Borhem, 1997; Abu-Jarad e t al 1980). The Austrian

Standard

ONOR

S 5200 has proposed that a type of building material is considered

acceptable if in a room the annual effective dose does not exceed 2.5 mSv Steger.F et

al. 1999).

Granite is a form of igneous rock, which is composed primarily of Quartz,

Alkalie, and Felspar. India stands third in export of granite stone and first in granite stone

production. In this paper, the results of radon exhalation measurements of different

varieties of granites mined in southern parts of India are presented. Th ese results are of

general interest since granites are globally used as building, ornamental and monumental

materials especially due t o its elegant look, durability and scratch resistant properties.

MATERIALS AND METHODS

Seventeen varieties of granite tile samples of dimensions 5 x 5 x 2.5 cm each

weighing about 120 - 150 g were c ollected. Five samples w ere rec eived f or each variety.

Of these, two samp les were crushed and sieved through one mm sieve and preserved for

gamma spectral analysis and remaining three tiles were used for exhalation rate

measurements.

I)Radon exhalation measurements

Lucas cells Scintillation cells) were used for the estimation of radon exhalation

rate RER) from these materials Somalai J et al, IRPA, Poffijin A. et al, 1983). A

cylindrical shaped of mild-steel container was fabricated for carrying out the

measurement. The containe r was 10 cm in diameter with an internal volume 750 cc.

Nozzles fitted with va lves were provided for collecting sample s with the L ucas cells.

The back diffusion process and leak rate which hamper the exhalation

measurement results, are significant for the sampling time more than 30 hours Stranden,

1983). Hence, the residence time o f the sample in the container was restricted to about 24


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Proceedings of the 2003 International Radon Symp osium Volum e I

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October

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hours, to minimize back diffusion. The ratio of volumes of the contain er and sample was

more than 10, which fu rther reduced the probability of bac k diffusion (Hafez.A.F, 2001).

In order to establish the leak rate of the container, a separate study was carried out. The

chambers were pressurized upto 1.2 kg/cm2 and sealed. The reduction in pressure was

continuously monitored for about a month. A graph wa s plotted with the pressure against

time (Fig 1 . It was observed th at the leak rate was 0.5% by volum e per day.

The samples were kept in the containers and sealed. After allowing a growth time

of

24

hours, gas samples were collected from the container in Lucas cell of volume 150

ml. After a delay of three hours to allow radon gas to attain equilibrium with its

daughters, the Lucas cells were counted using a PM tube assembly. The efficiency of the

counting system used in the study w as 70%. From the coun ts, the concentration of radon

gas collected in the Lucas cell was estimated using the following equation (Jha, et al,

200 1):

cy

......................

1)

3EV e'

1

e'?lT)

Where,

Q

is the radon concentration (Bq m 3)

is the net coun ts in seconds

A

is the deca y constant o f radon (s )


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Proceedings of the 2003 International Radon Symposium Volume I

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E is the efficienc y of the coun ting system (fraction)

V is the volume o f the Lucas cell (m3)

t is the counting delay (s)

As stated above, it may be noted that radon is in equilibrium with its daughters; for the

decay of each radon atom there will be simultaneous disintegration of its two daughters

and hence the factor 3 is used in the equation.

From the sample surface area and residence time in the container, radon

exhalation rate was calculated using the equation giv en below.

Q Vem

E

(2)

A (1- e^)

Where,

E is the radon exhalation rate (Bq m 2h- )

is the concentration of radon

(Bq

m 3)

Vcm is the effective volume of the co ntaine r (m3)

s the decay constant of radon (h- )

A is the surface area of the sample (m 2)

is the growth tim e

(h)

The MDA for the exhalation rate for the above experimental conditions was estimated to

be 0.10 Bq mV2h- .


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American Association of Radon Scientists and Technologists, Inc.

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II)

Gamm a spectral analysis

The crushed samples were homogenized and dried in an oven at 100 lo 0 C for

about 24 hours. The sample s were then filled in standard 25 0 ml air tight

PV

containers. The containers were sealed hermetically and externally using a adhesive tape

and kept aside for abou t 30 days. This will ensure the equilibrium between Ra and its

daughters.

The containers were subjected to gamma ray spectral analysis using 3 3 NaI

(TI) detector. The detecto r is shielded on all the four sid es as well a s at the top portion by

15-cm thick lead. Alum inum, cadm ium and cop per sheets in that order (graded lining) are

also provided in between the lead shield and the detector so as to decrease the intensity of

characteristic X-rays emitted by the high atomic number shield materials. 95% of

background reduction is achieved by this arrangement. This system is situated in a

nuclear counting facility, which is constructed using soil with low natural radioactivity.

The system was calibrated using IAEA reference standar ds sim ilar in geom etry as that of

the sample containers.

Each sample was counted for 20000 sec and the natural primordial radionuclides

present in the tiles were identified. For the quantification, the peaks corresponding to


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Proceedings of the 2003 International Radon Symp osium Volume I1

American A ssociation of Radon Sc ientists and Technologists, Inc.

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1.46 MeV ( 'K) .76 MeV (' ~i ) and 2.61 MeV ( ~1 )were considered in arriving at

the activity levels of OK,

238 ,

6

Ra and ^ ~ h espectively. The activity of each

radionuclide in the sample was determined using the total net counts under the selected

photo peaks after subtracting appropriate background counts and applying appropriate

factors for photo peak efficiency and weight of the sample. The MDA (30) for each

radionuclide wa s established from the background radiation spectrum f or a counting time

of 20000 sec and the values were 8.5,

1

and 13.25 ~~k~~ for U ( ^ ~ a ) , ~ h nd ^K

respectively.

RESULTS

ND

DIS USSION

T o estima te the leak rate of the chamber, plot between pressure and

number of days is given Fig. 1 Th e average volumetric leak rate wa s calculated using the

formula (V.Barashko et.al, 2002).

Where

Q

is average volumetric flo w rate in cm3m

H

is initial and final pressure difference in bar

B is initial and final atmospheric pressure change during

observation in bar

V is the volume of the cham ber in cm3

~t is the time of observation in min

The volumetric leak rate w as calcula ted to be 4.2k0.33 cm3d'l

The radon co ncen tra tio n in side the conta iner v aried f ro m 6 0 to 4 85 ~ ~ m ' ~ .he

exhalation rate of the tiles varied from 0.24

0.01 to 2.07

0.07 Bq m^h which are

tabulated in Table I

Out of seventeen samples 11 were hav in ^ ~ h ontent above 1 ~ ~ k ~ ' ,0

samples were having

2 3 8 ~

ontent above 8 5 Bqkg' and all the samples were having ' K.


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The activity concentration of the primordial radionuclides in the tiles are given table.2

The ^ ~ h oncentration varied from 17 to 212 Bqkg ' , 2 3 8 ~ ( 2 2 6 ~ a )oncentration

varied from 17

to 195~ ~ k ~ ' 'nd the

4 ~

oncentration varied from 132

5

to

2 169 ~ ~ k ~ - ' ,

In order to compa re the specific activities of the radionuclides, a common index is

required to obtain the sum of activities. The index, which is called the radium equivalent

activity or the samples, was calculated using the formula (Beretka and Mathew , 1985).

where, AK,Apa and A n are the activities of Potassium, Radium and Thorium

respectively, in Bq kg '. Thi s formula is based on the estimation th at 370 Bq kg ' of

'~ a, 259 Bq kg of * ~ h r 48 10 Bq kg of K produce the same ga mma dose rate.

The radium equivalent for the tested samples varied from 10 to 56 3 Bqkgm '.

Fig

2

gives the correlation between radon exhalation rate and uranium content of

the tile. An exponential fitting was done which yielded equation with a correlation

coefficient 0.9469 given below.

E

(Bq m'2h = 0.107 Exp (-0.0 147 X)

(5)

where

X

is uranium activity c onc entra tion of th e sa mple in ~ ~ k ~ ' ' .he calculated

values using the above equation and the measured values ar e given in table 3. The

variation between calculated and experimental values of radon exh alation rate may be

attributed to the inadequacy o f the number of samples with de tectable uranium content

and also to the fact that th e porosity and the density of the samples, wh ich influence the

radon exhalation rate, were not normalized for the plot.

CONCLUSION

Out of seventeen samples nine samples were found to ha ve higher thorium

content upto a maximum of 10 times the uranium content. Due to this, estimation of

thoron exhalation rate als o assum es importance. Attempts are being m ade to study the

thoron exhalation too.

Six samples showed radon exhalation rate above detecta ble level and these values

are in good agreement with the reported values for Indian granite tiles (Mentazul I.

Choudhury, JRNC 998, Al Jarallah, 2001). There exists a good correlation between the

uranium content of the sam ple and radon exhalation rate. This implies that there exists a

possibility to use the concentration of

38

u ( ^ R ~ ) as an indicator for the extent of radon

exhalation.


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REFEREN E

Abu-Jarad F., Frem1in.J.H. and Bull.R.,1980. A study of radon emitted from building

materials using plastic alpha track detectors. Physic s of Medical B iology 25, pp.

683-694.

Ahemed.J.U. 1992) Regulatory Approach Toward Controlling Exposure to Radon in

Dwellings, Radiation protection and Dosimetry vol.45, pp 745-750.

Al Jarallah, 2001. Radon exhalation from granites used in Saudi Arabia Journal of

Environmental Research 53, pp 9 1-98.

Barashko.V, Dolinsky.S, Ignatenko.M, K0rytov.A and Procofiev.0, 2002. EMU CSC

systems chamb er leak rate measurements, internal report, university o f Florida.

Beretka J. and M athew P.J, 1985. Natural radioactivity o f Australian building materials,

industrial wastes and by products. Health Physics 48, pp.87-95,

Erling Stranden, 1.983). Assessm ent Of The Radiological Impact Of Using Fly Ash In

Cement, Health Physics vol. 44 no.2,

pp

145 153.

Hafez.A.F, Hussein.A.S. and Rasheed.N.M., 200 1. A study of radon and thoron release

from Egyptian building materials using polymeric nuclear track detectors.

Applied Radiation and Isotopes.54, pp 291-298.

Jha.S, Khan A.H, M ishra U.C. 2001), A Study Of The Technologically Modified

Sources Of Radon And Its Environmental Impacts In An Indian U Mineralized

Belt. Journal of Environmental Radioactivity Vo l5 3, pp no. 183 197.

Maged.A.F. a nd Borham.E., 1997. A study of the radon emitted from various building

materials using alpha trac k detectors. Radiation Me asurem ents 28, p p 61 3-6 17.

Mentazul I . Choudhury, 1998) Concentration Of R adio Nuclides In Building A nd

Ceramic Materials Of Bungladesh And Evaluation Of Radiation Hazard. Jrnl.

Radioanalytical and Nuclear Chemistry, vol23

1

Poffijin A., Bourgoignie.R, Marijns.R, Uyttenhove.J, Ja ns se ns A and. Jac0bs.R 1984);

Laboratory measurements of radon exhalation and diffusion, Radiation Protection

Dosimetry, vol7, p 77-79.


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Somalai.J, Nemeth Cs, Kanyar.B, K0vacs.J; Variations o f Radon Emanation o f Coal-

Slags with the Burning Temperature, Proceedings o f IRPA

10.

Steger

F.

And Grun K. 1999),Proceedings of Radon in the Living Environment, Athens,

Greece.


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5 1 15

2 25

3 35

M I M K R O D YS

FIG GRAPH SHOWING PRESSURE DECREASE WITH TIME


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w 50.00 100 w 150.00

200.00 250.00

U 238

Bafkg)

FIG 2

CORRELEATION BETWEEN URANIUM CONTENT AND

EXHALTION RATE OF THE SAMPL ES


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TABLE RADON EXHALATION RATE (RER)

OF

GRANITE TILES

SAMPLE CODE RER (Bq m^h )

S

0 0

S8

0.

S


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TABLE 2 NATURAL RA DIOACTIVITY CONCENT RATION PRES ENT IN THE TILES

SAMPLE

^ ~ h

238

U ^ ~ a ) 4 0 ~

Ra e

CODE ~ ~ k g '

~ ~ k g '

~ ~ k ~ qkg

S

74 195

9891 377


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TABLE 3 COMPARISON OF RADON EXHALATION RATES

SAMPLE

CODE

8

w 2 2 6 ~ a )

RER (Bq m-'h )

B '

XPERIMENTAL CALCULATED

ERROR

2003 10 Measurement of Radon Exhalation Rate From Indian Granite Tiles

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