Pionic Double Charge Exchange on N  =  Z Doubly Closed Shell Nuclei

The (π⁺,π^-) reaction on the isoscalar, doubly magic nuclei ¹⁶O and ⁴⁰Ca has been measured in the energy range T_π  =  45–90 MeV. The forward angle cross sections of the ground state transitions (GST) show a very pronounced resonancelike energy dependence. The observed energy and angle dependences are well described by the recent hypothesis of the formation of a πNN resonance in the course of the double charge exchange process. The deduced spreading widths are in accordance with estimates of the d^′ collision damping in the nuclear medium. In the reaction on ¹⁶O in addition to the GST the transition to a 0⁺ state at E_x  =  2.1(2) MeV in ¹⁶Ne has been observed, which is interpreted as the quadruple isobaric analog state (QIAS) of the 0₂⁺ state in ¹⁶C at E_x  =  3.03 MeV.