Radiation Measurements 33 (2001) 167–169www.elsevier.com/locate/radmeas

Short Communication

An SSNTD study of spontaneous �ssion fragments from thesoil-gas samples of Bakreswar thermal springs

Debasish Paula ; 1, Debasis Ghoseb, R.C. Sastria ; ∗aDepartment of Physics, Jadavpur University, Calcutta 700 032, India

bVariable Energy Cyclotron Centre, 1=AF Bidhannagar, Calcutta 700 064, India

Received 21 January 2000; accepted 25 July 2000

Abstract

During the course of investigations on the radon and stable gas migration around the hot spring zone at Bakreswar, Birbhumin India, it was noticed that CR-39 plastic detectors used for the detection of radon revealed tracks with much bigger diametersthan usual for alpha particle tracks. Exposed CR-39 detectors etched adapting sequential etching technique con�rmed thepresence of bigger diameter tracks similar in nature to the tracks formed by spontaneous �ssion fragments. This paper presentsthe results of these observations along with the histogram plots of the track number versus track diameter that indicatean asymmetric distribution as was seen for mass distribution of spontaneous �ssion fragments. c© 2001 Elsevier Science Ltd.All rights reserved.

1. Introduction

It is recognized that natural radioactivity in hot springsis, principally, due to the presence of radon released withthe bubbling emanations and its seepage through adjacentsoil surface. Indeed, a number of workers had reported thepresence of radon in thermal spring emanations in India(Schynoll and Chatterjee, 1958) and elsewhere (Leonardand Janzer, 1978). At Agnikunda, Bakreswar in India, thereported radon concentrations in the emanations from ther-mal springs were found to be higher by two orders of mag-nitude as against the soil gas average values (Ghose et al.,1993). In recent times the spring has attracted considerableattention because of extraction of helium gas from its em-anations (Ghose et al., 1996). The substantially large uxof gaseous volatiles from the spring vents, it appears, gives

∗ Corresponding author. Tel.: +91-33-472-0484; fax:+91-33-472-0964.E-mail address: res@juphys.ernet.in (R.C. Sastri).1 Present address: Radioactivity Testing & Monitoring Labora-

tory, Bangladesh Atomic Energy Commission, G.P.O. Box 1352,Chittagong, Bangladesh.

rise to a corresponding enhanced radon leakage. This leak-age takes place not only from the ori�ces of issue but alsofrom the surrounding soil. For the purpose of this study wewere initially interested to �nd out the relative concentra-tions of radon with respect to stable gases and to record thepattern of their changes in the soil gas around the springs.CR-39 having the unique property of registering chargedparticles ranging from proton to �ssion fragments and asone of the most sensitive polymer detectors was used in thisstudy (Khan et al., 1983; Somogyi et al., 1984). During thecourse of these investigations it was noticed that distincttracks with diameters much bigger than are usual for alphaparticle tracks were revealed, where these bigger size tracksare found to be similar to those due to spontaneous �ssionfragments.

2. Experimental

For estimating ionizing emanations from the soil aroundthe main ori�ce of thermal springs CR-39 (standard grade)plastic track detector was used. The detector comprises asmall square of 10 mm× 10 mm taped to the inner bottom

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168 D. Paul et al. / Radiation Measurements 33 (2001) 167–169

of a plastic cup of height 45 mm and median diameter70 mm. The cup was placed inverted on the soil at a depth of600 mm from the surface. A GI cylinder of diameter 200 mmand height 300 mm was inverted over the cup. The cylinderwas provided with a nipple for drawing soil gas for analysis.The cup and cylinder are retained in position for about 140 h.This particular exposure time was arrived at, after a few ini-tial trials involving shorter durations of 35–70 h, to enableappropriate density of tracks to be formed as also to allowequilibration of soil gases to occur within the GI canopy. Along exposure duration smoothes changes and uctuationsdue to short time e�ects such as barometric pressure, tem-perature and moisture-content variations. During the initialtrials, track etching was carried out using 6 NNaOH solutionat 70◦C in a constant temperature bath, for 4 h. Interestingly,the tracks revealed had two fairly distinct sizes suggestingthe presence of �ssion fragment tracks along with the alphaparticle tracks (Bhanu and Iyer, 1986; Paul et al., 1999b).This has prompted us to follow the sequential etching pro-cedure developed by Pandey et al. (1993) to distinguishthe nature of tracks from the di�erences in their appearancetimes. So, CR-39 (standard grade) detectors exposed to soilgas at di�erent locations around the main ori�ce of thermalsprings for 140 h, as described above, were etched in 6 NNaOH solution at 70◦C for 60 min and the same sampleswere further etched for another 30 min under the sameconditions. Scanning under the optical microscope with400×magni�cation was done at the end of each etchingtime. It was found that the sizes of observed tracks for 60min as well as for a total of 90 min etching time are compa-rable in size, as was seen earlier for �ssion tracks of 252Cf(Paul et al., 1998). On further etching for another 2.5 h twofairly distinct sizes of tracks are revealed. Later, all theseCR-39 detectors were etched in 6 N NaOH solution at 70◦Cfor a total of 6 h only to ensure improved distinguishablefeatures of the tracks (Abu-Jarad et al., 1991; Randhawa etal., 1997). Measurements of the track diameters for �ssionfragments along the minor and major axes were done underan optical microscope with 675× magni�cation.

3. Results and discussion

The detectors exposed to the soil gases at Bakreswarclearly revealed the presence of �ssion tracks along withtracks generated by alphas as shown by the photomi-crographs in Fig. 1. Ghose et al. (1989) had observedanomalous values for Xe and Kr abundance in the gaseousemanations from Bakreswar. They obtained 0.262 as valuefor Kr=Xe which was markedly di�erent from the atmo-spheric value of 11.1, and closely resembling �ssionogenicvalue of 0.261. It is well known that natural radionuclides129I, 40K, 244Pu and 238U decay, respectively, to nobledaughters 129Xe, 40Ar, �ssion Xe and �ssion Xe. Fissionis observed in natural decay. It is therefore not unlikelythat soil gases driven o� from deep underground in some

Fig. 1. Typical photomicrographs of the tracks from soil gassamples from Bakreswar thermal springs, exposed during (a) 4thquarter of May, 1997 (b) 4th quarter of July, 1997, at the samelocation, etched for 6 h.

thermal spring areas, contain �ssionogenic components andgive away their signatures as tracks in Solid State NuclearTrack Detectors. Ghose Debasis (1999) has informed thepresence of �ssion Kr isotopes in a mass analysis of theBakreswar gas, which was chemically treated for enrich-ment of the noble gas fractions.The observed tracks in CR-39 (standard grade) exposed

to soil gas and etched for a total of 4 and 6 h clearly re-vealed two distinct sizes that showed the presence of alphaand spontaneous �ssion tracks. It is indeed signi�cant thatthe application of sequential etching technique developed byPandey et al. (1993) has unmistakably con�rmed the signa-ture of spontaneous �ssion fragments in the soil gas ema-nations. Further, typical histogram plot of the frequency oftracks versus track diameter, given in Fig. 2, for minor axisdata, shows an asymmetric distribution similar to the distri-bution obtained from spontaneous �ssion fragments data for252Cf where the �ssion fragments clustered into two groups(Paul et al., 1999a). It is seen from Fig. 2 that the ratio of

D. Paul et al. / Radiation Measurements 33 (2001) 167–169 169

Fig. 2. Histogram plot of the number of tracks versus track diameteralong the minor axis for the spontaneous �ssion fragments from thesoil gas emanation at Bakreswar thermal springs, for 6 h etchingtime.

the track diameters at the peak positions corresponding toaverage mass distribution of the �ssion fragments clustersis about 1:26 ± 0:03 which is comparable to that of 252Cfas seen from the literature (Knoll, 1989). Again, signi�cantnumber density of �ssion fragments recorded during the ex-posure time of 140 h indicate the presence of radionuclidesof transuranic nature with half-lives for spontaneous �ssionclose enough to the half-life for alpha emission. However, noattempt has been made to interpret the alpha-to-spontaneous�ssion fragment ratio since the observed alpha tracks couldbe due to the presence of 222Rn isotope along with �ssionalphas. A more systematic and detailed measurement of iso-topic abundnance of both the Kr as well as Xe componentsin the thermal spring emanations are required to establishbeyond doubt the degree and extent of the occurrence ofspontaneous �ssion fragments.

Acknowledgements

One of the authors, Debasish Paul, is grateful to IndianCouncil for Cultural Relations (ICCR) and Jawaharlal NehruMemorial Fund for the �nancial support.

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