Quasi-free scattering

Knockout reactions using light targets, e.g., Be or C, have proven in the past to be very useful in gaining information on the wave function of the valence nucleons. However, the strong absorption concentrates the reaction probability at the surface. Similar arguments hold for transfer and Coulomb break-up reactions. Nucleon knockout reactions using protons, on the other side, allow one to determine the spectral functions of protons and neutrons in a wide range from the weakly bound valence nucleons to the deeply bound core states. Thus, in neutron rich nuclei one gains access to the hitherto unknown region of the strongly bound protons and simultaneously to the valence neutrons. Beside the single-particle shell-structure, nucleon-nucleon correlations may be investigated as well as cluster knockout reactions. For stable nuclei and in normal kinematics, (p,pN) reactions have been used in the past as spectroscopic tool [1].

We intend to develop and apply the technique of quasi-free scattering using radioactive beams in inverse kinematics. At energies around 700 MeV/nucleon (which is high enough to ensure that the conditions for quasi-free scattering are met), both outgoing nucleons have energies in the range where the nucleon-nucleon cross section is at minimum, thus maximizing the transparency of the nucleus and minimizing final state interaction. Measurements such as (p,2p), (p,pn), (p,pd) etc. will become possible in a kinematically complete geometry, allowing a background-free measurement and also for a better control of final state interactions. As in the knockout program, a key instrument for the quasi-free scattering program is the high resolution magnetic spectrometer. It will allow the momentum of the recoil fragment to be measured with a relative momentum resolution of 10^-4, which will be essential for studying medium-mass and heavy nuclei. To detect the proton recoils and other charged particles, we foresee two shells of Si microstrip detectors that surround the liquid hydrogen target integrated into a 4π gamma calorimeter. For (p,pn) reactions, part of the LAND neutron detector can be placed at angles around 45 to detect the knocked-out neutrons with energies of few hundred MeV. Thus, the experiment determines the complete kinematics of quasi-free knockout reactions including those to continuum states (by measuring the invariant mass of the decaying system). Even reactions populating states beyond the dripline such as, e.g., ¹⁴Be(p,p´p)¹³Li, ¹⁴Be(p,p´α)¹⁰He, ¹¹Li(p,p´α)⁷H, or ⁸He(p,p´α)⁴n, can be studied. A thick liquid hydrogen target (200 mg/cm², ~3 cm thickness) will be used. The tracking of both protons and the beam will allow reconstruction of the interaction point with accuracy much better than 3 mm, corresponding to an effective target thickness of less than 20 mg/cm². Thus, the relative momentum resolution of 10^-4 for the fragment can be preserved. Experiments can be performed with intensities of 1000 ions/s corresponding to a luminosity of 10²⁶ cm^-2s^-1.

In a further development of our experimental program, we wish to exploit polarized quasi-free hadronic scattering, such as (p,2p) and (p,pn) reactions. The interest is twofold. Firstly, polarized measurements will allow the j-values of the hole states to be determined experimentally, in addition to the l-values. Secondly, such measurements can be used to study in-medium effects. Modification of meson and nucleon properties in the nuclear medium is one of the most interesting topics in current nuclear physics. There have been speculations on modifications of nucleon and meson masses and sizes, and of meson-nucleon coupling constants. These speculations have been motivated from a variety of theoretical points of view, which include renormalization effects due to strong relativistic nuclear fields, deconfinement of quarks, and chiral symmetry restoration. Such modifications cause a density dependence of the nucleon-nucleon interaction and are expected to play an important role in nucleon induced reactions. Hence, exclusive measurements of nucleon quasi-free scattering, and in particular polarized scattering, give a direct way to study the nucleon-nucleon interaction in nuclei and, therefore, to study meson and nucleon properties in the nuclear medium. For example, analyzing power measurements from (p,2p) reactions on several stable nuclei have recently been reported. It is found that A_y is a monotonically decreasing function of the averaged nuclear density, strongly suggesting the existence of a nuclear medium effect on the nucleon-nucleon interaction. Clearly measurements on unstable nuclei where the averaged nuclear density can be varied systematically could be invaluable. Measurements on unstable nuclei will have the possibility to probe the nucleon-nucleon interaction in a nuclear medium with very different values of isospin and also low-density nuclear matter, for example in the diffuse surface of neutron-rich nuclei. Clearly, the polarized quasi-free scattering program will require the development of a polarized hydrogen target, which is one of the future avenues we wish to pursue within R³B.

[1] G. Krein, Th.A.J. Maris, B.B. Rodrigues, E.A. Veit, Phys. Rev. C 51 (1995) 2646.