Intermediate Energy Charge-Exchange Physics

The intermediate energy charge-exchange physics carried out by the researchers at The Ohio state University and their collaborators can be simply described as the study of the spin and isospin dependence of the nucleon-nucleon force. Primarily these investigations over the last decade and a half have been carried out using the (p,n) and, occasionally, the (n,p) probes. These methods have the advantage over composite probes, such as (d,2p) and (3He,t) of being easier to interpret, and an advantage over the electromagnetic probe of coupling strongly to the nucleus.

The (p,n) studies as carried out at LAMPF and IUCF have proved most useful simply because it is much easier to produce a monoenergetic, intense source of protons. Such an ideal source of neutrons has not been acheived and so energy resolution in the outgoing channel of a (n,p) reaction has not been competitive with (p,n). Using an appropriate flight path for neutron time-of-flight measurements the resolution of the (p,n) technique has been comparable to that of a medium resolution spectrometer (~0.1%). In addition, for measurements of the spin dependence a high-luminosity polarized proton source is required, which working in conjunction with a sophisticated neutron polarimeter enables the measurement of polarization transfer observables in our reactions.



The spectrum for 12C(p,n) at 12 degrees shown above displays the various regions of nuclear excitation which we have explored in our experiments. These regions are:

  1. The Nuclear Sector - Small energy loss regime, excitation of nuclear states.
  2. The Quasifree Region - Energy loss between 50 and 150 MeV, and momentum transfer between 1 and 3 fm^{-1}.
  3. The Delta Region - Excitation of a Delta resonance in the nuclei. Energy loss between 200 and 400 MeV and momentum transfer between 1 and 5 fm^{-1}.

The Nuclear Sector
At small momentum transfer this region is characterized by states with low angular momentum transfer and giant resonances, such as Gamow-Teller (GT) resonances. Strong GT transitions to 1+ states such as 12N(g.s.) and 14N(3.95 MeV) have been used to study the energy dependence of the spin-isospin term in the nucleon-nucleus interaction, while studies of isobaric analog or Fermi (F) transitions,such as 14C(p,n)14N(2.31 MeV), have been used to study the isospin term. In addition, measurement of polarization transfer observables have provided a means for probing the spin character of these transitions. For example, spin observables have been used in studies of the location of GT strength seeking to explain the shortfall of observed strength with respect to sum rule predictions. In addition, GT strengths obtained by charge-exchange reactions are used as input in models of weak interaction processes, such as neutrino capture, needed to analyze data from various solar neutrino detectors.

Higher momentum transfer studies in this region are generally characterized by the excitation of stretched states, or those states in which a particle-hole pair are in a stretched subshell. The simple nature of this configuration these states make good candidates for the study of reaction mechanisms and effective interactions.

The Quasifree Region
The large energy transfer involved in quasifree scattering allows one to overcome the uncertainty of having to know the nuclear wave functions (as is necessary in the nuclear region) by treating the target as a Fermi gas and the scattering as a single scattering off of one of the constituent nucleons. This simplification has allowed continuum studies of medium-modification effects to the free nucleon-nucleon interaction, collectivity in the nucleus brought on by coupling 1p-1h to 2p-2h effects, and the role of mesonic fields in the nucleus.

The Delta Region
The quasifree pion production or delta region of the (p,n) spectrum is associted with the production of a delta particle within the nucleus. Study of this region also provides information on the role of mesonic fields in the nucleus but in this case through the study of delta-hole correlations.


Back to Nuclear Charge-Exchange Homepage.
************

Created by Daniel A. Cooper , (dcooper@mps.ohio-state.edu )
Last modified 3/27/96.