SSNTD study of the probable in¯uence of alpha activityon the mass distribution of 252Cf ®ssion fragments

Debasish Paul a, 1, Subrata Sena, Debasis Ghoseb, R.C. Sastri a, *aDepartment of Physics, Jadavpur University, Calcutta 700 032, India

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

Received 15 April 1998; accepted 24 July 1998

Abstract

The SSNTD has come a long way in its application for the study of nuclear phenomena. Spontaneous ®ssion oftransuranic elements is one such phenomena wherein use of SSNTD o�ers easy registration of the signature of the

®ssion fragments. The object of the present study is to explore whether any one of the track parameters such as thediameter can be used to estimate the atomic mass ratios of the spontaneous ®ssion fragments. The spontaneous®ssion data from 252Cf recorded almost at the end of one and four half-life periods for alpha decay are analysed,taking a plot of the number of tracks versus the track diameter. From these plots it is seen that initially, when

signi®cant alpha activity of 252Cf persists, the ®ssion fragments appear to cluster into two predominant groups asindicated by two peaks. The ratio of the diameters at these peak positions appear to be related to the ratio ofaverage mass numbers of the light and heavy groups of ®ssion fragments. However, absence of two peaks for

similar plots at the end of about four half-life periods for alpha decay suggests that presumably the presence ofalphas in¯uence the mass distribution of the ®ssion fragments. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

Starting from Libby (1939) the study of spontaneous

®ssion has attracted the attention of many investi-

gators (Bohr and Wheeler, 1939; Seaborg, 1952; Segre,

1952; Huizenga, 1954). Petrzhak and Flerov (1940),

followed soon after by Chatterjee and Sarkar (1944),

had clearly demonstrated the phenomena of spon-

taneous ®ssion from uranium. These studies gained

momentum and atoms of elements having half-lives for

spontaneous ®ssion over a wide range of values were

detected (Fleischer et al., 1975; Gay and Sher, 1975;

Ho�man, 1979). Since the introduction of SSNTD in

the 1960s (Fleischer et al., 1965; Benton and Henke,

1969) there has been a steady growth in the study of

nuclear phenomena using these detectors. The study of

spontaneous ®ssion is one such phenomena where the

®ssion fragments register their signatures in these

detectors with ease, even in the presence of alpha par-

ticles. Of particular interest in this study is the sequen-

tial etching method developed by Pandey et al. (1993)

where the property of the di�erential appearance times

for ®ssion fragments and alpha particles stands out

distinctly. However, the di�erence in the diameters of

the etched tracks also display the distinct identity of

these particles. Much interest has been focused on the

spontaneous ®ssion activity of 252Cf since its discovery

in 1952 (Ghiorso et al., 1954; Fleischer et al., 1965;

Fraenkel, 1967). In this study we have analysed the

data for 252Cf ®ssion fragments recorded on a CR-39

detector at the end of about one half-life period and

also at the end of about four half-lives for alpha emis-

sion when the alpha activity (half-life 2.65 years) is

reduced signi®cantly while the spontaneous ®ssion ac-

tivity (half-life 85 years) has only been slightly reduced

(Paul et al., 1998, in press).

Radiation Measurements 30 (1999) 127±129

1350-4487/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S1350-4487(98 )00078-X

PERGAMON

1 On study leave from Bangladesh Atomic Energy

Commission.

* Corresponding author.

2. Experimental method

The experimental arrangement is the same as

reported by the authors in their earlier communi-

cation (Paul et al., 1998, in press). CR-39 (standardgrade) detectors, obtained from Page Mouldings

(Pershore) Ltd, England, are exposed using 2p geo-

metry on 12 August 1997 to a deposited source of252Cf that had an alpha activity of 3.08 micro curie

as at 1 May 1987. The exposed CR-39 detectors

were etched following the sequential etching methodof Pandey et al. (1993). After 60 min etching time

only distinguishable ®ssion tracks appear while

alpha tracks begin to be discernible after 90 minetching time using 6 N NaOH solution at 708C in

a constant temperature bath. However, scanning

under an optical microscope for the ®nal measure-ments was done at the end of 3.5 h and 6 h of

etching times when ®ssion tracks along with the

alpha tracks are simultaneously distinguishable. Thediameters of the tracks along the minor and major

axes are measured under the optical microscope

with a 675� magni®cation. For the CR-39(Pershore) detectors exposed earlier to the same252Cf deposited source on 3 June 1989, data were

recorded for 3 h and 6 h etching times using 6.25 NNaOH solution at 708C in a constant temperature

bath. Track diameters along the minor and major

axes were measured under an optical microscope

with a 675� magni®cation.

3. Results and discussion

Depending on the angle at which the ®ssion frag-

ments are entering the detectors, we get circular orelliptical shapes for the tracks at the surface of the

detectors. For the detector exposed to 252Cf at the endof one half-life period for alpha emission, we measured

the diameters of the tracks along the minor as well asthe major axes for each of the 3 h and 6 h etchingtimes. In all cases the plots of the number of tracks

against track diameter show that ®ssion fragmentsappear to cluster into two groups showing two peaks

when the 252Cf source has signi®cant alpha activity. Atypical case for 6 h etching data along the major axis

is given in Fig. 1. The ratio, 1.4020.03 on an average,of the track diameters at the peak positions is more or

less related to the ratio of average values of light andheavy groups as reported in the literature (Knoll,1989). However, such distinct clustering for ®ssion

fragments is not discernible as found from the plots ofa number of tracks versus track diameter for the data

taken at the end of about four half-life periods foralpha emission. The track diameters were measured

along the minor and major axes for each of the etchingtimes of 3.5 h and 6 h and a typical plot for 6 h etching

time data for major axis is presented in Fig. 2. It ispossible that, in the absence of signi®cant alpha emis-sion after about four half-life periods of alpha activity,

the ®ssion fragments tend to have a mass distributionwith the most probable value indicated by a single

peak in the plot of the number of tracks as a functionof track diameter. Further, it should be interesting to

Fig. 1. Plot of the number of tracks versus track diameter

along the major axis for the ®ssion fragments from 252Cf at

the end of about one half-life period for alpha activity, for

6 h etching time.

Fig. 2. Plot of the number of tracks versus track diameter

along the major axis for the ®ssion fragments from 252Cf at

the end of about four half-life periods for alpha activity, for

6 h etching time.

D. Paul et al. / Radiation Measurements 30 (1999) 127±129128

investigate the dynamics of ternary ®ssion using suchsources.

Nonetheless, it needs to be mentioned that plots ofthe number of tracks against track depths from thepresent data have indicated only a single peak in all

the cases, con®rming the ®ndings of Fleischer et al.(1964).

Acknowledgements

One of the authors, Debasish Paul, is grateful to the

Indian Council for Cultural Relations (ICCR) and theJawaharlal Nehru Memorial Fund for ®nancial sup-port.

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