Charge-exchange reactions and nuclear matrix elements for ββ decay.

Charge-exchange reactions of (n,p) and (p,n) type at intermediate energies are a powerful tool for the study of nuclear matrix element in beta beta decay. The present paper reviews some of the most recent experiments in this context. Here, the (n,p) type reactions are realized through (d,He-2), where He-2 refers to two protons in a singlet S-1(0) state and where both of these are momentum analyzed and detected by the same spectrometer and detector. These reactions have been developed and performed exclusively at KVI, Groningen (NL), using an incident deuteron energy of 183 MeV. Final state resolutions of about 100 keV have routinely been available. On the other hand, the (He-3,t) reaction is of (p,n) type and was developed at the RCNP facility in Osaka (JP). Measurements with an unprecedented high resolution of 30 keV at incident energies of 420 MeV are now readily possible. Using both reaction types one can extract the Gamow-Teller transition strengths B(GT(+)) and B(GT(-)), which define the two "legs" of the beta beta decay matrix elements for the 2 nu beta beta decay. The high resolution available in both reactions allows a detailed insight into the excitations of the intermediate odd-odd nuclei and, as will be shown, some unexpected features are being unveiled.