Progress on internal gas-jet target nuclear reaction at the HIRFL-CSR

Abstract

Direct nuclear reactions induced by light ions, e.g., proton and alpha particles, have been investigated for about a century since the Geiger-Marsden experiment in 1908. It provides important information on nuclear structure and nuclear astrophysics. For instance, neutron skin thicknesses of nuclei determined by using proton and α elastic scattering play an important role in constraining the equation of state (EOS) of isospin asymmetric nuclear matter. These reactions were mostly conducted in direct kinematics, that is to say, the light-ion beam from accelerator interacts with a target made of the nuclei of interest. Obviously, such kinds of experimental investigations are limited to stable solid isotopes without active chemical property and radioactivity, due to the difficulty to make unstable targets. To test theoretical models developed from stable nuclei and explore new physical phenomenon on exotic nuclei, it is essential to investigate direct reactions on nuclei far from stability. As a result, experimental method based on inverse kinematics, namely active gas target technique, has been developed, where the unstable nuclei of interest produced by radioactive ion beam facilities collide with hydrogen and helium gases in a bubble chamber. The direct reactions are performed in inverse kinematics. One advantage is that low energy recoils at small center of mass angles can be measured easily at relatively large laboratory angles. However, this experimental method is insensitive to detect scattering recoils with energies of a few hundred keV, due to measurement based on gas detector. In recent years, a novel experimental method, namely study with stored beam in storage ring interacting with internal gas-jet target, has attracted much interest. It was proposed firstly for the exotic nuclei studied with light-ion induced reactions in storage rings (EXL) project. The technique is based on the internal gas-jet target instead of active gas target. It is an important supplementary experiential method for studying direct nuclear reactions induced by light ions, which is characterized by low-momentum sensitivity, high detection efficiency and low background. It is an ideal tool to study the isotopes with active chemical property and radioactivity. The cooler storage ring at the heavy ion research facility in Lanzhou (HIRFL-CSR) consists of a main cooler storage ring (CSRm) and an experimental cooler storage ring (CSRe). The two rings are connected by a radioactive ion beam line 2 (RIBLL2). The CSRe is equipped with an internal gas-jet target. As one of the existing facilities, the HIRFL-CSR provides an opportunity for performing in-ring direct nuclear reactions induced by light ions with the internal gas-jet target. Since 2016, a silicon strip detector compatibility with ultra-high vacuum in heavy-ion storage ring has been developed to build the in-ring direct nuclear reaction spectrometry at the HIRFL-CSR. A feasibility experiment on proton elastic scattering on 58Ni nuclei has been operated successfully by using the in-ring nuclear reaction spectrometry at the HIRFL-CSR. The nuclear matter distribution radius of 58Ni was extracted from measured differential cross sections at low momentum transfer. Proton elastic scattering experiment on 78Kr nuclei was also carried out to extract the nuclear matter radius at the beginning of 2021. In this paper, we report the development of in-ring direct nuclear reaction spectrometry at the HIRFL-CSR and corresponding experiments. Research plans in the (near) future, especially for measuring nuclear matter distribution radius of 133Cs, are also introduced.