Reaction mechanism and nuclear correlations study by low energy pion double charge exchange.

摘要

In pion double-charge-exchange (DCX) reactions, a positive (negative) pion is incident on a nucleus and a negative (positive) pion emerges. These reactions are of fundamental interest since the process must involve at least two nucleons in order to conserve charge. Although two nucleon processes are present in many reactions they are usually masked by the dominant single nucleon processes. DCX is unique in that respect since it is a two nucleon process in lowest order and thus may be sensitive to two-nucleon correlations. Measurements of low energy pion double-charge-exchange reactions to the double-isobaric-analog-state (DIAS) and ground-state (GS) of the residual nucleus provide new means for studying nucleon-nucleon correlations in nuclei. At low energies (T(sub (pi)) <65 MeV) the (pi)N interaction is relatively weak thus simplifying the theoretical treatment of the problem. Also, isobaric-analog-state (IAS) transitions are suppressed near 50 MeV due to destructive s-p wave interference in the (pi)N scattering amplitude leading to enhancement of DIAS transitions through non-analog intermediate states. Theoretical studies by several groups have shown that, while transitions through the analog route involve relatively large nucleon-nucleon distances, those through the non-analog intermediate states involve distances of the order 1 to 2 fm. Therefore, DCX reactions at low incident pion energies provide a way to study the short-range part of the nucleon-nucleon correlations. The author presents measured DCX cross sections of DIAS and GS transitions for several nuclei in the f(sub 7/2) shell at energies ranging from 25 to 65 MeV. Cross sections were measured on (sup 42,44,48)Ca, (sup 46,50)Ti and (sup 54)Fe. The calcium isotopes make a good set of nuclei on which to study the effects of correlations in DCX reactions.