Date of Award

Summer 1995

Document Type


Degree Name

Doctor of Philosophy (PhD)



Committee Director

Andreas Klein

Committee Member

Rocco Schiavilla

Committee Member

J. Wallace Van Orden


The medium and long range part of the nucleon-nucleon interaction can be described within the framework of pion exchange. In order to understand this fundamental interaction, it is therefore important to study the interaction of pions with nucleons and nuclei, which can either be elastic or inelastic scattering or absorption. Whereas the elastic and inelastic channels can be well described by different models, our understanding of pion absorption is still rather poor. There are still a lot of open questions, for instance on the strength of three and more nucleon absorption and the role of initial and final state interactions. In this thesis an experiment carried out at the Paul Scherrer Institut (PSI) in Villigen, Switzerland, to measure the cross section for the ppd final state from pion absorption on 4He is reported. The experiment was carried out during 1991 using the Large Acceptance Detector System (LADS) with a beam momentum of 355 MeV/c. LADS is a detector system which covers almost 4 π sr of the solid angle. Due to the complexity of the detector a Monte Carlo Program based on the CERN code Geant to simulate the response of the various components was written. To get a better understanding of the physics involved, different event generators modeled the various physics processes. The goal of the present treatise is to analyze the ppd final state and separate the different reactions leading to its formation. The data analyzed consisted of events from 4H as a calibration tool and events from 4He for this examination. It was found that three different mechanisms could be identified which would lead to this state. The first one is the two nucleon absorption with a recoiling spectator deuteron. In the second process, a deuteron is knocked out of the nucleus by the incoming pion which is then subsequently absorbed (Initial State Interaction or ISI). In the third reaction, one of the energetic outgoing protons from the two nucleon absorption picks up a neutron in a final state interaction (FSI). The cross sections for these three different processes were extracted and determined to be 5.6 mb, 4.8 mb and 1.9 mb respectively. The total integral of the cross section for the ppd final state is 12.3 mb.



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