Nuclear Tracks, Vol. 12, Nos I-6, pp. 333-336, 1986. 0191-278X/86 $3.00+.00 Int. J. Radiat. Appl. Instrum., Part D Pergamon Journals Ltd. Printed in Great Britain. MEASUREMENT OF PROTON INDUCED FISSION CROSS-SECTION OF 235U AT SUB-COULOMB BARRIER ENERGIES K.N.Iyengar,R.P.Anand,N.N.Ajitanand,D.M.Nadkarni,A.K.Mohanty Nuclear Physics Division, B.A.R.C.,Bombay 400085, India ABSTRACT It has been observed that in heavy ion induced fission the sub coulomb cross section does not decrease with energy as rapidly as expected. In order to study the situation p~[ailing in proton induced fission, measurements of fission cross-sections for protons on ~--U were carried out using plastic track detectors in a close geometry for Ep= 2 to 4 MeV. Care was taken to measure the background tracks due to neutron induced fission at each energy. The results show a much lower fall off of cross-section with energy than is indicated by an optical model prediction. At 2.5 MeV the fission cross-section is several orders of magnitude higher than the predicted reaction cross-section. An interpretation of the results as fission following coulomb excitation is~gggested which requires the existence • • . ZJ3 of a thlrd well In the potentlal energy surface for U. KEYWORDS sub-coulomb fission cross-section, lexan track recorder, 235U INTRODUCTION Sub-coulomb fusion has been studied extensively in heavy ion reactions ( Vaz 1981) and it has been established that for heavy projectiles the cross-section is enhanced with respect to the predictions based on a a model with a static reaction barrier. By invoking a dynamic model wherein the relative motion is coupled to the internal degrees of freedom giving rise to a fluctuation in the barriers, the data have been satisfactorily explained Jn most cases. However, because of the complexity of the situation it is very difficult to take into account such effects explicitly by means of full f]edged coupled chamle! c~Ic~]lations With a view to investigate a case more amenable to such calculations, in the present study, we have carried out an experimental investigation of the sub-coulomb fission cross scctJo;~ of protons on 235U to look for deviations, if any, from the predictions of a ~impl<. reaction model such as the optical model. EXPERIMENTAL METHOD As shown schematically in Fig.l, a 200 pgm/cm2 235U target was bombarded with the proton beam from a Van de Graaff accelerator. The target was confronted with an annular lexan detector of thickness 75 pm which detected fission fragments due to fission induced by both, protons and background neutrons generated by ~ proton beam. The beam was stopped in 2mm thick aluminium after which another target of ---U of similar dimensions, faced by a similar annular lexan detector, was used to measure the background fission due to the neutrons alone. The assembly containing the two targets also served as the Faraday cup to measure the total number of protons seen by the target. 333