No abstract

During the recent Variable Energy Cyclotron experiment involving the elastic scattering of protons on 12C, the complete energy spectrum of the scattered protons was obtained by means of a 100-channel analyser (McKenna, Baxter, and Shute 1961, henceforth referred to as I). Two prominent peaks were obtained: the higher energy one corresponding to elastic scattering and the lower energy one to inelastic scattering with the residual carbon nucleus being left in its first excited state, 4· 43 MeV. In I the elastic scattering results were reported briefly. The purpose of this short communication is to discuss the main features of the inelastic scattering.Angular distributions for ~his inelastic process have been determined at approximately 5° intervals, between 170° and 25° scattering angles, over the energy range llt-8 MeV in 100 keY steps. Moreover, complete angular dis. tributions were made, in 25 keY steps, over the known resonance region at 9·2 MeV, while runs at nine preferred angles, with 40 keY spacing, were later made over the 7'6, 8'2, and 10·5 MeV resonance regions. Below 8 MeV only fragmentary angular distributions were obtained. This was due to the low energy of the inelastic peak, especially at backward angles, which prevented correct reproduction by the kick-sorter. From 8 to 7 MeV, angular distributions were obtained only for angles less than 100°. Below 7 MeV no useful data were obtained, both because of the low proton energy at all angles, and because the inelastic cross section had become too small for the peak to be observed above the background.None of the angular distributions exhibit any information between 55° and 25°. This is due to the masking of the inelastic peak, at these angles, by protons elastically scattered from the hydrogen content of the polythene target. This process has a relatively high cross section, and the energy of the scattered proton is a cos 2 function of the dete(Jtion angle. In order to keep the analyser counting rate low without using excessively small beam currents, the display of the non-elastic region was biased out; thus only elastic distributions are available between these angles. However, at 25° a separate run was made, using a reduced beam and no analyser bias, so that inelastic scattering information is available at this angle. This 25° point is of low accuracy (~10%), due to poor statistics and high dead-time counting losses, but was persisted with because information at this forward angle is of assistance in judging the possible direct interaction contribution to the inelastic scattering process.

The latest tabulation of energy levels in 13N (Ajzenberg-Selove and Lauritsen 1959) shows that there is a region from 8 ·08 to 22·7 MeV which previously has been inaccessible to thorough investigation. Since the mirror nucleus, 130, has been thoroughly investigated and shows a well-defined level scheme in this region, it was decided to make a search for similar levels of the 13N nucleus in the excitation range 6· 5-12 . 5 MeV in this laboratory by means of elastic scattering of protons from 12 0.Protons with energies continuously variable from 5 to 11·5 MeV were available from the University of Melbourne Variable Energy Oyclotron (Oaro, Martin, and Rouse 1955), the energy being determined by a 60° magnetic analyser which was nuclear-resonance controlled. Accurate energy calibration of this analyser at the high end of its range has not yet been completed, but its stability and resolution are known to be better than 0·1 %.After magnetic analysis the proton beam entered a large scattering chamber (16 in. radius), and was scattered from a thin (1 mg cm-2 ) polythene target. Scattered protons were detected at angles between 10° and 170° by a rotatable scintillation counter using a thin OsI(Tl) crystal scintillator, the output pulses being fed into a 100-channel analyser. Unscattered beam was collected in a Faraday cup and measured by a vibrating reed type current integrator. Angular distributions were taken at 100 keV steps throughout the machine energy range, at approximately 5° intervals from 25° to 170°. The absolute values of differential cross section thus obtained have been compared with data of other experimenters at machine energies of 5 MeV (Reich, Phillips, and Russell 1956), 7 MeV (Schneider 1956), 9·5 MeV (Greenlees, Kuo, and Petravic 1957), and 10 MeV (Fischer 1954). Agreement better than 5 % is obtained at 5 and 7 MeV both as to absolute cross section and angular distribution. At the higher energies, while quite good agreement is obtained as to the absolute cross section values, the angular distributions differ somewhat at back angles from the curves published at 9·5 MeV (Greenlees, Kuo, and Petravic 1957) and 10 MeV (Fischer 1954). This is probably due to the critical dependence of cross section upon energy at back angles, which we have found to occur in the neighbourhood of the newly discovered resonances at these energies. Typical angular distributions over the energy range are shown in Figure 1. Figure 1 (a) shows a typical distribution at t Supported in part by a grant from the Australian Atomic Energy Commission. t Manuscript