Knockout reactions

If we are to understand exotic nuclei and use them as a tool to progress our description of the nuclear many- body problem, it will be essential to explore their single-particle structure in detail. The most direct way to do this is by exploiting reactions where we add or remove single nucleons. The technique of knockout reactions induced by high-energy beams of exotic nuclei has recently been developed. It allows the exploration of ground-state configurations and of excited states. Due to limitations in beam intensity for heavier nuclei, this method has been restricted mainly to light nuclei so far. In particular, the method has been used to map the halo-nucleon wave function in momentum space, from which their spatial distribution is derived via Fourier transformation [1]. By γ-coincidence measurements, different single-particle configurations (including core-excited states) can be identified and corresponding spectroscopic factors may be obtained [2]. The coincident measurement of neutrons allows the method to be utilized even if unbound states are involved. As an example, we mention the one-neutron knockout from the two-neutron halo nucleus ¹¹Li populating states in the unbound nucleus ¹⁰Li, from which the occupancy of the (s_1/2)² and (p_1/2)² configurations of the two halo neutrons in the ¹¹Li ground state were deduced [3]. Recently, knockout has been developed into a tool to allow the extraction of absolute spectroscopic factors and hence the determination of the spectroscopic strength for single-particle states. This has confirmed the findings of (e,e’p) reactions that the spectroscopic strength in valence orbits is only 60% of that expected in the independent-particle shell model (IPSM). This fundamental result indicates the presence of short- and long-range correlations in the nuclear wavefunction. Unlike (e,e’p) reactions, knockout reactions can also probe the neutron wavefunction and it has now been found that the spectroscopic strength for valence neutrons is also 60% of the value of the IPSM. Another very intriguing result is that, when the spectroscopic strength for a halo neutron is measured using knockout, it has a value closer to 100% of the IPSM as would be expected for an almost free nucleon. Extending such measurements to more and heavier exotic nuclei to fully explore the effects of correlations in the nuclear system is clearly of fundamental interest.

At the new facility heavy neutron-rich nuclei, produced by in-flight fission can be studied. With high detection efficiency, secondary beam intensities in the order of ten ions/s are sufficient to extract detailed spectroscopic information, thus making nuclear structure studies possible even very far from stability. The key instrument in this programme is the high resolution magnetic spectrometer. This is because in order to determine the angular momentum l of the knocked out nucleon, the momentum of the recoil fragment has to be measured with high precision and this device will provide a relative momentum resolution of about 10^-4, which is essential for studying medium-mass and heavy nuclei. Other important devices will be efficient high-resolution γ-ray spectrometers, like the proposed cooled CsI (or NaI) array or AGATA in cases where the ultimate energy resolution is essential.

Another promising field is the study of resonant states in the continuum [4] or even nuclei beyond the drip lines, see [5] for a recent study of the unbound ⁵H nucleus.

[5] M. Meister et al. Phys. Rev. Lett. 91 (2003) 162504.