Range of232Th (γ,f) fragments in metallic Th

Nuclear Instruments and Methods in Physics Research B 82 (1993) 7-8 North-Holland

B e a m Interactions with Materials & Atoms

Range of 232Th(~,f) fragments in metallic Th M. Piessens, E. J a c o b s l, S. P o m m 6 a n d D. D e F r e n n e

Nuclear Physics Laboratory, Proefiuinstraat 86, B-9000 Gent, Belgium Received 22 December 1992

The range in metallic Th of fission fragments produced in the photofission of 232Th with 12 MeV bremsstrahlung has been determined by measuring fission product yields using ),-spectrometric methods. The range of Th fission fragments in Th is about 10% lower than the range of U fragments in U. A lower range is expected for the Th fragments because of their lower kinetic energy. Knowledge of the range of fission fragments in the target material is important for many experimental and technical applications. For example, when performing catcherfoil experiments a range correction, proportial to one over the range, has to be applied to the measured photopeak intensities when deducing fission fragment yields from these intensities (see e.g. ref. [ 1 ] ). The fission fragment range is a function of the fragment charge, mass and energy. Good knowledge of the range of the fragments as a function of their mass is thus necessary for the determination of mass distributions in catcherfoil experiments. The range of 235U(nth,f) fragments in U is well studied [2]. The range of Th fission fragments in Th is much less known. Only for about l0 fission products, from reactor neutron induced fission on Th, has the range in metallic Th been measured [3]. We have determined the range of 232Th(),,f) fragments in metallic Th for about 30 masses. To do so, we compared the areas of the y-peaks obtained under exactly the same conditions (beam intensity, measuring time, catcheffoil-detector geometry, etc.) in catcherfoil experiments on 232Th(),,f) with 12 MeV bremsstrahlung, once with a thick (thickness greater than the range of the fragments) and once with a thin Th target. The irradiations were performed on the 15 MeV linac of Gent University. The bremsstrahlung was produced in a graphite block with a central hole in it. The graphite block acted simultaneously as beam stop and as bremsstrahlung convertor. This unusual shape was needed to handle the heat produced by the very intense electron beam (up to 1 mA) used in our experiments. Gamma-ray

spectra were measured using an n-type 29% efficiency Ge detector with conventional electronics and a PCbased data handling system. The computer codes MARKER and CAOS [4] were used to analyse the y-spectra. The thickness of the thin Th target, 0.5 mg/cm 2, was chosen in such a way that in practice it can be assumed that the number of fragments caught in the catcherfoil, even for those fragments with the shortest range, is independent of their range. The number of fission fragments escaping from the surface of the thick fission target into a catcherfoil on the other hand is proportional to the range of the fragments in the target material [2]. From this comparison of the )'-peak areas obtained once with a thin and once with a thick Th target the relative average ranges for about 30 mass chains were obtained. It should be remarked here that it is also assumed that each fragment leaving the target is caught in the catcherfoil. To determine the absolute value of the fission fragment range the following procedure was followed. A thick Th target, with known thickness of 96 mg/cm 2 closely surrounded by aluminium catcherfoils, was irradiated with 12 MeV bremsstrahlung. Afterwards )'spectra of target and catcherfoils are measured separately, and the yields of fission fragment i, N~ r~ and N: °il are calculated from the ),-spectra from the target and from the foil. We know that (see ref. [2] ) the ratio of the total number of fission products with range Ri caught in an aluminium foil in front of it, to the total number of fission fragments produced in a target with thickness d, is given by

Nfi °il N : °il -~- N~ars -

Correspondence to: E. Jacobs, Nuclear Physics Laboratory, Proeftuinstraat 86, B-9000 Gent, Belgium. 1 Research Director National Fund for Scientific Research.

Ri 4d"

So from the measured N f°il and N~ar8 and the known thickness of the target we obtain the absolute value of

Ri.

0168-583X/93/$ 06.00 (~) 1993 - Elsevier Science Publishers B.V. All rights reserved

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M. Piessens et al. /Range of 232Th(7,.Ofragments in metallic Th 15

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Fig. 1. Range of 232Th(7,f) fission fragments in metallic Th (our results), compared to the range of 232Th(n,f) fragments in Th [3] and 235U(nth,f) fragments in U [2]. Due to the highly disturbing background in the irradiated Th target this procedure could only be applied unambiguously for about 10 isotopes. Using these absolute range values the 30 earlier obtained relative range values are transformed into absolute range values. The results are given in fig. 1. The indicated uncertainties are only those due to the uncertainties on the areas of the y-peaks in the measured spectra. In addition to these uncertainties, one has to include some additional contributions to the uncertainties on the absolute values of the obtained ranges. For example, in our measurements we did not include a correction for the inevitable oxide layer on the Th targets. From the experiments of Niday [2] it follows that this layer reduces the range of 99M0 fragments by 3%. The surfaces of the

targets were not specifically treated and polished, as was done in the experiments of refs. [2,3]. Also, we did not correct, as was done by Prakash et al. [3], for the difference in surface scattering between the aluminium foil and the Th target. Consequently, it may be that the measured ranges in our experiments are too high by about 5% (see ref. [2] ). The results of Niday [2] and Prakash et al. [3] are also included in the figure. From fig. I it is clear that the range of the Th fission fragments in Th, as a function of the fragment mass, is, except for a displacement of,~ 10%, the same as for the U fragments in U. The ranges obtained by Prakash et al. [3] are in good agreement with our results. A lower range for Th fragments is expected due to the lower kinetic energy of Th fragments.

Acknowledgements Thanks are expressed to Ir. W. Mondelaers and the linac crew for the reliable operation of the accelerator. This work is part of the research program of the InterUniversity Institute of Nuclear Sciences - National Fund for Scientific Research.

References [ l ] H. Thierens, D. De Frenne, E. Jacobs, A. De Clercq, P. D'Hondt and A.J. Deruytter, Nucl. Instr. and Meth. 134 (1976) 299. [2] J.B. Niday, Phys. Rev. 121 (1961) 1471. [3]S. Prakash, S.B. Manohar, C.L. Rao and M.V. Ramaniah, J. Inorg. Nucl. Chem. 31 (1969) 1217. [4] W. Westmeier, private communication (1980).