Pion Absorption Processes.

A study of pion absorption reactions is described in this thesis. Proton and deuteron production from low energy pion absorption in light nuclei have been measured leading to discrete and continuum states. The LEP beam line at LAMPF was used with a stack of 8 intrinsic germanium crystals developed by LBL and CMU for this experiment. The high resolution and large dynamic range of this detector allow us to measure a variety of reactions at the same time. The proton energy spectra are in general characterised by a broad bump at an energy approximately corresponding to (pi)('+)d (--->) pp reaction kinematics, suggestive of pion absorption on 2 nucleons. A simple picture based on pion absorption on 2 nucleons is able to explain the systematics of inclusive proton cross-sections measured elsewhere. The energy-integrated cross-section for production of deuterons has an angular distribution similar to that for production of protons. The dependence of the total pion absorption cross-section on A is explained using a semi-classical model for pion transport in nuclei. The ((pi)('+),p) as well as ((pi)('+),d) reactions generally favor transitions involving larger angular momentum transfer to the residual nucleus when states of similar nuclear structure are considered. The low energy excitation spectra from the ((pi)('+),p) reaction are similar to the spectra from (p,d) reaction on ('12)C and ('13)C. However, a calculation of the ((pi)('+),p) cross-section using the measured (p,d) reaction with the formulation of Wilkin to relate the two reactions is in moderate disagreement with the measured ((pi)('+),p) cross-sections, indicating a possible problem in the model for low energies. The excitation spectra from the ((pi)('+),p) reaction indicate the importance of two step processes for the reaction. The ((pi)('+),d) reaction leading to the ground state of the residual nucleus has been seen for ('7)Li, ('12)C and ('13)C targets. The measured cross-section for ('12)C((pi)('+),d)('10)C(0('+) + 2('+)) reaction has a shape similar to that predicted by Betz and Kerman. The cross-section to the 2('+) state is much higher than that for the ground state, whereas the calculation of Betz and Kerman gives them similar strengths. For the case of ('18)O, we see no counts for excitation energy of < 10 MeV, at a sensitivity of (TURN) 100 nb/sr. count, contrary to the predictions of Betz and Kerman. These features indicate a possible failure of their model for the ((pi)('+),d) reaction. The ('13)C target has the largest cross-section ((TURN)2 (mu)b/sr at 35 degrees) among all the nuclei studied.