Interaction of Positive and Negative Pions withAl27andSi

Abstract

The interaction of low-energy positive and negative pions with 27 A1 and 28 Si was studied by the detection of y radiation emitted from residual nuclei formed in the reaction. Large cross sections for the removal of "a" clusters were found for both nuclei and both pion charges. A large asymmetry is observed between the cross sections for the (ir" t ir"p) and (7r + ,7r + w) on 28 Si leading to mirror states in 27 A1 and 27 Si. The ratio of 7r + to IT' cross sections for proton knockout is about a third of the value predicted by a simple impulse approximation.Several studies in which the interaction of negative kaons and negative pions with nuclei was investigated have recently been reported. These studies were done for stopped 1 and for fast 2 ' 3 (220, 380 MeV) negative pions and for stopped negative kaons. 4 The method used in these studies was the detection of prompt y radiation emitted from residual nuclei formed in the reaction. No such experiments were reported to date in which positive pions were used. One of the most striking results of the IT" and K" studies was the relatively strong population of some low-lying states in nuclei formed by the removal of a particles or two protons and two neutrons from the target nucleus. The removal of two or more such "a" particles was also observed. The stopped-K" and -ir" experiments were interpreted in terms of the possible absorption of the incoming particle by a correlated substructure in the nucleus. 1 * 4 The fast-7r~ experiments were interpreted in terms of the knockout of such a correlated substructure or of the individual nucleons from which it could be composed, 2 ' 3 although contributions from pion absorption could not be ruled out.We report here experimental results for the interaction of both positive and negative pions with 27 Al and 28 Si. These are two neighboring nuclei and the self-conjugate nucleus 28 Si could be considered as composed of a substructures. The experiment was performed using the secondary pion beam from the electron linear accelerator at Saclay. Negative pions of 70 MeV and positive pions of 25, 70, and 100 MeV were used for the bombardment of natural aluminum (100% 27 A1) and silicon (92.2% 28 Si) targets. The target thickness was 3 g/cm 2 and the beam momentum spread was ±3%. A 45-cm 3 Ge(Li) detector was placed •below the target about 110° to the beam axis. The detector was shielded by lead and boron. Coincidence (2r^20 nsec) was required between the Ge(Li) detector and a coincidence pulse from two plastic scintillators placed about 50 cm in front of the target. An additional anticoincidence requirement between the Ge(Li) detector and a plastic scintillator placed on the Ge(Li) detector eliminated any background resulting from charged particles. The y spectrum (from 0.180 to 3.260 MeV) was recorded on a 4096-channel analyzer. The y-energy resolution obtained was about 3 keV at 511 keV. The coincidence between the two plastic scintillators was used to monitor the beam, The protons present in the beam were stopped with an absorber in ...