Nuclear reactions and the double beta decay

Charge-exchange reactions of ( n , p ) and ( p , n ) types at intermediate energies are introduced as a tool for the study of nuclear matrix element in β β  decay. Here, the ( n , p ) type reactions are realized through ( d , 2 He ) , where 2He refers to two protons in a singlet 1 S 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. The ( 3 He , t )  reaction is of ( p , n ) type and was developed at the RCNP facility in Osaka (JP) at incident energies of 420 MeV. Using both reaction types one can extract the Gamow–Teller transition strengths B ( G T + )  and B ( GT − ) , which define the two “legs” of the β β decay matrix elements for the 2 ν β β  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.