Abstract
The emission of two nucleons from a nucleus following the absorption of a low energy pi-meson (10 MeV < T[pi] < 120 MeV) is studied in an impulse approximation. Some of the simplifications which are usually made in such studies are investigated and calculations are presented for the Li[6] (pi[+], 2p)He[4] reaction using cluster model wave functions for the target nucleus. The matrix element of the transition Hamiltonian is found to be sensitive to the long-range behaviour of the overlap integral between the wave functions of the target and the residual nucleus. The overlap integral must therefore be given the correct asymptotic form as required by the general theory of transfer reactions. An eikonal approximation is used to treat the distortions of the pi-meson field by the target nucleus and of the outgoing nucleon wave functions by the residual nucleus. The results obtained with distorted waves are compared with those obtained using plane waves and distortion is found to play an important part. It is concluded that one must treat with caution results which have been obtained in calculations which do not take into account the above corrections, in view of the significant errors involved. In particular, it must be considered doubtful whether the (pi,NN) reaction can be used to yield information about nuclear pair correlations without a detailed analysis.