The y-ray spectra following neutron capture in silicon have been recorded in the neutron energy range 2.7-6.2 MeV and partial cross sections for y-rays to the 2si/2 ground state and id3/2 first excited states in 29Si determined. The results indicate considerable fluctuations with neutron energy with a prominent resonance peak at 4.6 MeV in the (n,y o) cross section. The existence of fluctuations is predicted in a recent theoretical calculation based on a model designed to include singleparticle resonances in nuclear reaction processes.This experiment was performed to study resonancelike structure in the reaction 28Si(n,y)29Si for incident neutron energies between 2.7 and 6.2 MeV. Several experiments (I-3) have previously localized considerable p-wave resonance strength below En=l MeV. These data have been taken as evidence for a P3/2 doorway state (4) common to the reaction channels 28Si+n and 29Si+y. We have observed similar fluctuations around 5 MeV in a previous (n,y) work (5). It is the purpose of the present experiment to study this structure in more detail.A particular incentive to this study was the theoretical work on doorway structures by Micklingheff and Castel (6). They developed a method to include single-particle resonances in the microscopic treatment of the neutron capture process. Calculations were performed for the reactions ZSSi(n,y)29Si and 32S(n,y)33S and comparisons were made with our previous experimental results. Below the giant dipole region, i e for neutron energies below I0 MeV, the model predicts rapid fluctuations of the cross sections for y-ray transitions to the ground states (yo) and to the first excited states (y1). This is consistent with the observed results for (n,Yo) and (n,y I) in 28Si whereas the experimental cross sections and y-ray spectra for 32S vary rather smoothly with neutron energy. However, we felt that the experimental results were too crude to provide an adequate test of the model calculations. Therefore, we repeated the measurements for 28Si(n,y) with better neutron energy resolution and with much shorter neutron energy intervals between the experimental points.The measurements were carried out at the 6 MV tandem accelerator at Uppsala. Monoenergetic neutrons were produced by means of a nanosecond pulsed proton beam onto a 2.5 cm long gas target cell containing tritium at a pressure of 1.6 atm. The sample was a 460 g metallic silicon cylinder placed 16 cm from the target cell with its axis along the incident beam direction. With this arrangement one obtains a neutron energy spread varying from about lO0 keV at 3.0 MeV to 80 keV at 6.0 MeV. The y-rays were detected by a 22.6 cm diameter and 20.8 cm long Nal(Tl) crystal located at 90 ~ with respect to the incident neutron beam. The detector resolution is about 9 % which is sufficient to resolve the y-rays to the ground state and to the first excited state at i.27 MeV in 29Si. Other experimental details have been described previously (5).The yield curves for (n,y o) and (n,y I) are shown in fig I together w...
