Search

We shall discuss the nuclear spin response as measured in charge exchange reactions at intermediate energies. We shall especially be interested in the spin-longitudinal part of the response. This is where the largest effects from particle-hole correlations at larger momentum transfer are expected. The attraction in the spin-isospin channel as seen in NN scattering and dominantly coming from one pion exchange would give very dramatic effects inside a nucleus. That is, if we just took the particle-hole interaction equal to the free NN-interaction in this \(\overrightarrow \sigma \overrightarrow q\) spin-longitudinalchannel. Similar dramatic effects could be expected in the Δ-region. The question is then, what are the effective particle-hole or Δ-hole interactions at finite momentum transfers.

Put rather high energy w (30~120MeV) and high momentum q (1.0~2.5 fm−1) together with spin and isospin into nucleus and see how it responds. This is one of the main subjects of this meeting. Inelastic and charge exchange scatterings of nuclear probes are very promising tools for this purpose. If a probe interacts a nucleon inside the nucleus almost freely, one expects the broad bump with the peak at ω = q2/2m (m being the nucleon mass). This bump region is called the quasi-free region.

The recent (p, n) experiments at intermediate energy at the Indiana University Cyclotron1–6 have led to a major breakthrough in the understanding of spin correlations in nuclei. The (p,n) reaction for bombarding energies between 100 and 200 MeV has proven to be a unique tool for the study of spin excitations in nuclei. Fig.1 shows zero degree neutron spectra for Ep=200 MeV. 5 The spectra are seen to be dominated by a single peak which is interpreted as a collective l+-state, the giant Gamow-Teller state, the isospin-spin nuclear sound. The collective state carries a large fraction of the total Gamow-Teller strength, i.e. strength associated with the operator σμt_ that governs Gamow-Teller β-decay. The concentration of strength into a collective state was first discussed by K. Ikeda, S. Fujii and J.I. Fujita as early as 1963.7 In a shell model picture of e.g. 208Pb the G.T. operator has non-zero matrix elements with all 44 excess neutrons (and the 12 h11/2 neutrons in the next lower shell). The collective state then corresponds to a transition where all these nucleons coherently change direction of spin and isospin.