Volume 156B, number 3,4 PHYSICS LETTERS 20 June 1985 NEUTRON EMISSION FROM THE COMPOUND NUCLEUS 26A! M. BALDO, G. LANZANO, A. PAGANO, A. PALMERI, S. AIELLO, G. PICCITTO Istttuto Naztonale dl Fsstca Nucleare, Seztone dt Catanta, Catanta, Italy and Istttuto Dlparttmentale dl Ftslca, Unwersttil dl Catanta, Catanta, Italy Y. CASSAGNOU, R.A, DAYRAS, J. FOUAN and L. RODRIGUEZ Serwce de Physique Nuclbatre, Basse Energle, CEN Saclay, 91191 Gtf-sur-Yvette Cedex, France Received 2 January 1985 Neutron ume-of-fhght spectra have been measured m comodence with mdlvldual nucle~ produced in the fusmn reaction 14N + 12C induced by the bombardment of a 12C target with a 48 MeV 14N beam. Whereas for the residual nuclei 21Na(an), 2°Ne(apn) and 23Na(2pn), the observed neutron spectra are well reproduced m intensity and shape by statistical model calculations, ttusts not the case for the restdual nucleus 24Mg(pn) In order to reproduce the experimental neutron spectra, ~t seems necessary to assume that around an exotatlon energy of 16 to 18 MeV m 24Mg, gamma-ray emission is able to compete favourably agmnst particle decay. 1. Introductior~ At low bombarding energy (Elab ~< 4 MeV/A), and for light systems, fusion between heavy ions exhausts most of the reaction cross section. In this energy regime, it is usually assumed that fusion proceeds through the formation of an equilibrated compound nucleus, the decay of which is well de- scribed by statistical model calculations. However, in most cases, the calculations have been aimed at repro- ducing the experimental yields of evaporation residues and in few cases their angular distributions [1]. As many decay paths can lead to the same final nucleus these quantities are not very sensitive to a detailed knowledge of the level densities or the precise de- scription of the yrast lines in the different nuclei par- ticipating in the decay of the compound nucleus. Recently, emission of light particles in fusion in- duced reactions, has been the object of intensive in- vestigations [2-6]. However, such measurements were essentially performed at incident energies rang- ing from 5 MeV/A to 12 MeV/A and were aimed at probing the limit of stability of the compound nu- cleus and at determining the onset of preequilibrium effects by looking at deviations from statistical model predictions. 0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) In order to provide a more stringent test of the sta- tistical model predictions, we have made a detailed study of the decay of the compound nucleus 26A1 formed in the bombardment of a 12C target with a 14N beam at 48 MeV. For this purpose, the neutrons produced m the decay of the compound nucleus 26A1, were detected in coincidence with evaporation resi- dues Identified by their characteristic q-rays. Neutron energy spectra are not affected by the Coulomb bar- rier and are well suited to test the decay path from the compound nucleus to the final evaporation resi- dues as predicted by the statistical model. 2. Experimental method. We used the pulsed beam facility at the Saclay FN tandem to produce the 48 MeV 14N beam. The beam bursts had an energy reso- lution of ~300 keV and a time width of ~60 ps with a repetition rate of 27 MHz. The 200 #g/cm 2 thick natural carbon target depos- ited onto a 0.2 mm thick, 2 cm diameter tantalum disk made up the closed end of a 1 cm internal diame- ter, 12 cm long brass tube. This tube, electrically insu- lated from the beam pipe was used as a Faraday cup. Before impinging on the target tilted at 45 ° relative to 181