Hiranandani, M R

Iyengar, K N

Rangarajan, R

IACS Institutional Repository

MASS DISTRIBUTION STUDIES IN NUCLEAR FISSION USING LEXAN DETECTORR. RANGARAJAN*, K. N. IYENGAR a» d M. R. HIRANANDANIT a t a  I k s t it u t e  o r  F o n d a h b n t a i. R b sb a r o h , CoiuIb a , B o u b a t  6 , I n d ia .{Roecived August 31, 1067).ABSTRACT. An investigation of the ranges of fission fragments from thermal fission of 2SSU has been made using “ Lexan” polycarbonate detector. A method of sandwiching the Uranium between two layers of “ Lexan” enabled a simultaneous measurement on the two fragments. The average ranges of the fragments of the light and heavy groups in Lexan wore found to be 20.22±0.027 microns and 16.38±0.021 microns respectively. I t  is found that from the correlated range measurements, the fragment masses can be estimated faiily accurately using an iteration procedure.I N T R O D U C T I O NThe determination of fragment masses in fission by physical methods requires a simultaneous measurement of the kinetic energies or the velocities of the two fragments. In the past, attempts have been made to obtain the fragment mass distribution by measurement of the ranges of fragments using radiochemical methods (Aras et al, 1965) and employing known range-energy relationships. Ranges of fission fragments have also been measured using nuclear emulsions (Manley, 1962). In recent years, solid state track detectors are being used in­creasingly in the detection of fission fragments (Fleischer et cH, 1966) especially for recording events of low probability such as in photofission. The present work was undertaken to assess the accuracy with which fragment mass distributions could be determined by measuring fragment ranges in the thermal fission of with a “Lexan” polycarbonate (Fleischer et al, 1964) detector. “Lexan” being a primary ionisation threshold detector, records alpha particles of energy only less than 0.6 MeV (Fleischer ef oi, 1967). The study of fission events in the presence of the background of alpha particles was thus possible. Moreover, the ranges of yfission fragments in this detector are larger than those in nuclear emulsions because the stopping power of the polycarbonate is smaller than that of nuclear emulsion. Simplicity of handling and ease of processing of these detectors are added advantages. In the present work the ranges of the two fragments are found simultaneously by using a sandwich technique and fragment masses are derived from the measured ranges by a method of iteration.108* Bhabha Atomio Beseaioh Oantre, Trombay, Bombay.902M ass D istrthutkm  Studies in  Nuclear F ission mE X P E R I M E N TA sandwich of natural uranium was prepared by pressing a drop of 10% uranium nitrate between two layers of Lexan (density 1.25, composition HiaCieOa) joined together at one end. The layers which wore held pressed between two perspex plates with screws were exposed to a beam of neutrons in the thermal column of “CIRUS” for about a day where the flux of neutrons is about 3 x 10’ w/om*/sec. After exposure, the sandwich was washed and etched in a 6N solution of NaOH at TO'^ C for about 30 minutes. The fragment tracks were measured by viewing the sandwich under a microsc^e with a magnification of 1000 X. I t  was found that a drop of sandal wood all between the layers kept the surfaces in close contact. A dye, ‘Nigrosinc’ wasj introduced into the tracks to enhance the contrast characteristics. This enabled better accuracy in the measurements. A track density of between 10 and 25 per field of view was maintained. The two surfaces of the sandwich were found to have a lateral displacement of between 6 and 40 microns. The correlation could b('- obtained easily here from the simi­larity in direction because of the low track density. A total of 2000 events were scanned and the data analysis was performed on the computer CDC 3600. Mea­sured ranges were corrected for a slight change in the length by a factor connected with the dip angle of the track (Price).r e s u l t s  a n d  a n a l y s i s(a) RdTigB measurements of fission fragmentsThe range distributions of light and heavy fragments were obtained from the measurements on correlated fragments making use of the fact that in general lighter fragment has a longer range. These distributions are shown m fig. <1).Fig. 1. Banga-frequenoy plot of correlated fragment tracks in Lexan from thermal fission of *»»U.The average ranges of the Ught and the heavy groups were found to 20.22±0 827 m io L . i d  16.38±0.021 re.peclirdy, T l.«e vriue. i«e .bout 13than the correeponding »nget h> »uele» ( V ^ ,61950) and about 1.1 times larger than those in the celluloid backing of normal photographic films (Gerhardt et al, 1966).Fig. (2) shows the distribution in the range of one of the fragments for various fixed ranges of the complimentary fragment. It is not necessary here to associate the longer range with the lighter fragment and vice versa. The results are found to be aimiln,r to those obtained by Brunton and Hanna (1950) for the kinetic energies904 B ,  B a n g a r a j a n ^  K .  N .  I y e n g a r  a n d  M .  B .  S i r a n a n d a n iRange in Miorone0 N(R)Range in MicronsFig. 2. The distribution in range of one fragment for various fixed ranges of the oomplixnentary fragment. The number on the left hand top oorner, of represents the range of the fixed fragment. The mean of the distribution obtained is represented on the right hand top comer. The error given is statistical.of fission fragments. This suggests the validity of a direct correspondence between range and energy. However, since most of the contribution to the spectra of fig. (2) comes from the most probable masses, any breakdown of the direct cor* respondence in other mass regions may not be apparent here.Fig. (3) shows the obtained distribution in the total range of the two fragments. The experimental points have been fitted to a Gaussian distribution by the least square method. The average value of the total range and its variance are found to be 36.62 microns and 3.23 respectively.(b) Determination of mass distributions from range measurementsLindhard et al (1963) have used the Thomas-Fermi model of the atom to cal­culate theoretically the range energy r^ationships for heavy ions. These cal­culations are based on the Bloch’s r(^tioii I  =: /o-Z^, where I  is the average ionisation potential, 1  ^is the Bloch’s consent and Zq the charge number of the medium. This relation is not strictly valid for the detector used hero (Brandt, 1966). The mass distribution was djbtained here using the range-energy relation given by Barkas and Berger (1^4) and applying an iteration procedure as described below.M obs D is tr ib v i io n  S titd ie s  in  N u c lea r F is e w n  905N(Rjt)Fig. 3.31 37 43Rr in MicronsTho distribution in total range of the two fragments. The curve <lrawn is a gaussian with a mean of 36.62 microns and steuidard deviation 1.80 microns.The range of a fragment of mass M  and charge Z, moving with a velocity fiC ia given by(i)where A is the range of a proton of velocity /?0 and the term B, takes into account the range extension caused by the capture of electrons by a positive ion of oha^e Ze. For the case of “Lexan” used, the value of A was obtained by a numerical integration of the stopping powers, of the constituent atoms as given by Whaling906 B . R a n g a ra ja n  K .  N .  l y e i ^ a r  a n d  M . B . H ira n a n d a n i(1958) and the value of Bg was evaluated according to Barkas and Berger (1964). These values of A and Bg are given by(2)A =  84.1264 xy?i®/®gms/om5^Bg =  7.717 X 10-^ X gms/cm* (3)A constant charge density of 0.394, equal to that of the parent nucleus, was used in arriving at the value of Bg given by equation 3. Assuming a linear relation between energy and range of the fission fragment, approximate fragment masses were first calculated from the relationshipsandJlfj . — ^ 2  • ^2M^+M^ -  M,(4)(5)where M  is the mass of the fissioning nucleus. These masses and ifg fed back in equations 1,2 and 3 and the values of and were estimated. Betterestimates of Mi and Jlf2 were then obtained using the rigorous relationshipMifii =  M2^2These now values of Mi and M 2 were again fed into equations 1, 2 and 3 and three such iterations were made using the computer. In all the cases the results of the last two iterations were found to be converging to within five mass units. The resultant mass ratio distribution is shown in fig. 4. For the sake of com­parison, the mass ratio distributions derived from measurements of fragmentN(Mi/Ma)1.0 2.21.4 1.8Mass RatioFig. 4. Plot of mass ratio versus frequency. The histogram refers to the present experi­ment. The dotted curve is computed from the data of Walter etal (1063) and the continuous curve is f^om Bnmton and Hanna (1950)energies by double ionisation chamber method (Brunton et al, 1960) and slid-state detectors (Walter et al, 1963) are also shown. I t  is seen that the present data com­pare favourably with the results of Brunton and Hanna ^ 1960) except for the regionof near symmetry. The agreement of the present results with that of Walter et al (1963) is not, however, very good. This discrepancy could be due to not including in our calculations the effect of nuclear stopping also termed “Ionisa­tion defect’’ (Lamphere, 1960). I t  must be mentioned here that while Brunton and Hanna (1950) did not correct for the ionisation defect in their results, Walter et al (1963) have corrected for the same |)y an elaborate method of calibration (Schmitt et al, 1965) for the solid-state detectors. In any case, the present results show that the range measurements on coi^elated fragments can be used for ap­proximate fragment mass ratio determina^ons and therefore this mesthod can bo cases where low counting ^ates prohibit the use of conventional methods. The method could be improved^ further by using a better range-energy relationship which takes into account a|om-atom elastic collisions as well as experimentally observed fragment charge'distributions in fission (Reisdorf et al, 1967).M a s s  D i s t r i b u t i o n  S t u d i e s  i n  N u c l e a r  F i s s i o n  907A C K N O W L E D G E M E N T SWe are thankful to Dr. R. Ramanna for his continued interest and guidance and to Dr. S. S. Kapoor and Shri D. M. Nadkarni for helpful discussions. Dr.P. B. Price was kind enough to give us the Lexan used in this (experiment. Our thanks are also due to Km. K. D. Prabhu for careful microscopic work and to Messrs Imperial Chemical Industries for supplying the Nigrosino dye.R E F E R E N C E SAras, N. K., Menon, M. P., and Gordon, G. E., 1965, NucL Phys,, 69, 337,Barkos, W. H. and Berger, M. J ., 1964, N A S A - E e p o r t  no, SP-3013.Brandt, W., 1956, Phya. Bev., 104, 691.Brunton, D. C. and Hanna, G. C., 1950, Can. Jr. Rea., 2BA, 190.Fleischer, R. L., Price, P. B. and Walker, R. M., 1965, Ann. Rev. Nncl. Sc., 15, 1. Fleischer, R. L., Price, P. B., Walker, R. M. and Hubbard, E. L., 1964, Phya. Rev., 188A, 1443.Fleischer, R. L., Price, P. B., Walker, R. M. and Hubbard, E. L., 1967, Phya. Rev,, 166, 353.Gerhardt, H., Gonnenwein, F., Heinrich, H. and Hipp, H., 1966, Proc. Coll. Inter. Foto. Coaep. Firenze. (Preprint)Lamphere, R. W., 1960, Fast neutron physics. Inter Science Publisher, New York, Vol. 1. 449.Lindhard, J ., SoharfP, M. and Schiott, H. E., 1963, Mat. Fya. Medd. Dan. Vid. Sdsk, 88, no. 14.Manley, J . H., 1962, N ud. Phya., 88, 70.Price, P. B. (Private Communication).Reiadorf, W. and Annbruster, P.. 1967, Phya. Lettera, 24B, 601.Schmitt, H. W., Gibson, W. M., Noiler, J . H., Walter, F. .T. and T. jD...lM5,SM-60/40, Proceedings of the Symposium on Physios and Chmustry of Jfisswn,Saixburg.Vigneron, L., 1950, Comp. Rendu, 281, 1473.Walter, F. J., Schmitt, H. W. and Neiler, J . H., 1963, AJVL-6797, p. 441,Whaling, Wf  ^ 1058  ^Handhuoh dor Physic, 84^  198»

Mass Distribution Studies in Nuclear Fission Using Lexan Detector

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Mass Distribution Studies in Nuclear Fission Using Lexan Detector

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