Charge-changing cross section measurements of atomic nuclei and charge radii

Abstract

<p indent=0mm>The atomic nucleus is a finite open quantum many-body system that is composed of two basic building blocks, proton and neutron. Like many systems governed by the laws of quantum mechanics, a nucleus is an object full of mysteries. Nuclear properties and structure are much more difficult to characterize than those of macroscopic objects and it is almost impossible to build an exact replica of this Fermi-scale nuclear system. In reality, physicists have developed an approach to reveal nature by collisions of two atomic nuclei, i.e., nuclear reactions, since the very beginning of nuclear physics. One famous example is the Rutherford scattering experiment performed at around 1910, in which the nuclide as a core of atom was discovered. Since the late 1980s, precision measurements of reaction cross section or interaction cross sections played a crucial role in nuclear size determination. This was entwined with the realization of radioactive ion beams (RIB). Such measurements contributed to discovering, for example, the neutron skin and neutron halo, when moving far away from the stability line. The new phenomena and properties found along the experimental advance are meanwhile among the strongest motivations for initiating, constructing, and operating the new generations of nuclear physics facilities worldwide. HIAF, the High-Intensity heavy-ion Accelerator Facility, is now under construction in Huizhou, China. In the last few years, systematic measurements of charge-changing cross sections (CCCS) of exotic nuclei have attracted much attention. It offers a complementary way to study exotic structures that could exist in exotic nuclei. By definition, CCCS refers to the total probability of removing at least one proton from the projectile nuclide. It reflects the geometric overlapping area during the collision. In particular, by combining with the Glauber-type model calculations, precise CCCS data have been used as an effective probe to investigate the proton density distribution of unstable atomic nuclei. This method is suitable for the investigation of exotic nuclei with low production yields, and the measurements accordingly were proposed in almost all the major nuclear physics facilities in the last years. In this paper, we first attempted to give a review on the progress of CCCS worldwide, from the pioneering work of RIB physics in the 1980s to the recent advances towards neutron-rich nuclei. Then we introduced the experimental work performed at the Heavy Ion Research Facility at Lanzhou (HIRFL). So far, CCCSs of more than 30 light nuclei have been determined and most of them are along the β-stability line. Based on the same setup, the Beihang group will focus on the CCCS measurements of sd-shell nuclei, aiming to study the (sub-)shell structure at 14 and 16. After surveying the existing CCCS data worldwide, it is found that there are often large deviations among various measurements in different laboratories even for the simplest reaction system. Precision data are therefore urgently called to clarify the deviations in existing data and to constrain the reaction mechanism on hydrogen targets. From my point of view, simultaneous measurements of both CCCS and reaction cross section in particular on hydrogen targets, the simplest target composed of only protons, are expected to enhance dramatically the journey to reveal the nature of very exotic nuclei. In the last part of this paper, perspectives at the High-energy FRagment Separator (HFRS) at HIAF are discussed. After a careful design of the detector setup and data taking system, it is possible to open a new era of nuclear physics experiments, in which various types of experiments can be carried out simultaneously.