Above in momentum transfer a broad resonance-like structure becomes prominent in the continuum spectrum at an energy loss, , between 50 and 150 MeV. This is the quasifree region and the broad bump is the result of scattering of energetic particles off of individual nucleons in the nucleus with the subsequent knock-out of the struck nucleons. This process becomes noticeable above about 1.0 when the momentum of the recoiling nucleon is large enough to overcome the Pauli blocking and escape from the nucleus. The quasifree peak is generally near (where is energy loss, q is the momentum transfer, and is the nucleon mass) which is the energy transfer in free nn scattering. The width of the peak is due primarily to Fermi motion of the nucleons in the nucleus. An example of a spectrum showing a quasifree peak is shown in figure .
Figure: A energy spectrum showing the position of the quasifree peak. The spectrum was taken at and MeV. The cutoff at 100 MeV is an artifact of the experiment.
The quasifree (QF) region of nuclear scattering is the ideal place to look for possible changes in the isovector spin longitudinal (IVSL) to isovector spin transverse (IVST) response ratio for a couple of reasons. First of all, the quasifree peak is the most prominent feature in the spectrum at the momentum transfers for which the calculations of Alberico et al. predict a significant affect on the IVSL to IVST ratio. Secondly, the nature of quasifree scattering -- scattering off of a single nucleon in the nucleus -- makes it a relatively direct method of investigating modifications to the nn interaction due to mesonic fields in the nucleus without sensitivity to the initial and final states of the nucleus itself. This means the experimenter can, to some extent, avoid complications which might arise from the nuclear structure, and make a fairly direct measurement of the medium-modified nn (nucleon-nucleon) potential.