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UBC Theses and Dissertations

The pion double charge exchange reaction on ¹⁸O at 50 MeV Hessey, Nigel P.


This thesis discusses the pion double charge exchange (DCX) reaction ¹⁸0(π⁺,π⁻)¹⁸Ne at 50 MeV. Transitions to the ground state of ¹⁸Ne, which is the double-isobaric-analogue state (DIAS) of ¹⁸0, have been isolated. The differential cross sections for DIAS transitions have been measured at 6 scattering angles from 18.2° to 122.6°. The experiment was performed at TRIUMF in December 1984 using the QQD low energy pion spectrometer [26]. The differential cross section angular distribution is forward peaked, falling from 4.7±0.5 μb/sr at 0° (by extrapolation) to 0.61±0.11 μb/sr at 122.6°. The total (angle-integrated) cross section is 16.2±1.2 μb. DCX measurements are expected to give information on nuclear structure that is hard to obtain by other reactions. This information includes short range correlations and neutron-proton density differences. However, before such information can be extracted the mechanism for DCX must be understood. The aim of this experiment was to provide more data to test the various theories of the DCX mechanisms. The implications of the results for several theories of DCX are discussed. The forward peaking of DCX angular distributions at 50 MeV was unexpected. 50 MeV single charge exchange (SCX) angular distributions are forward dipped e.g. [14], a result of the cancellation of the 0° s and p wave scattering amplitudes for the reaction p(π⁺,π⁰)n. Early DCX calculations were based on the simple sequential mechanism. This assumes DCX proceeds via 2 successive SCX reactions, with the isobaric analogue as the intermediate state. These calculations predicted forward dipping and small cross sections for DCX [13,15]. The data shows this mechanism is an over-simplification. The standard model for π-nucleus scattering is the optical potential. Johnson and Siciliano are developimg a potential with which to calculate elastic, SCX and DCX cross sections [48,38,22]. They include second, order terms, important in DCX because the reaction must involve scattering by at least two nucleons. By using a general form for the optical potential they include contributions from excited intermediate states. Miller has suggested the forward peaking is due to the presence of six-quark clusters in the nucleus [16]. His model reproduces the data for 50 MeV DCX on ¹⁸0 and ¹⁴C at forward angles. Karapiperis and Kobayashi have used the Δ-hole model to calculate elastic, SCX and DCX cross sections [19]. They obtain fair agreement with data for a range of nuclei and energies. Jennings et al. [22] are developing a model in which short range correlations produce the forward peaking. This work is at an early stage. More DCX measurements are needed to choose between the various models. Measurements at 50 MeV are particularly valuable because the simple sequential mechanism is small, allowing other mechanisms to be observed. Further data such as excitation functions below 80 MeV and angular distributions for other nuclei are needed.

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