Articles techniques et présentations

Studying the properties of nuclei produced in various types of nuclear reactions at accelerator facilities is challenged by small production yields and short half-lives of isotopes of interest, therefore making high efficiency and fast timing essential parameters of the experimental setups. One of the commonly used approaches to stop and thermalize nuclear reaction products is to use gas cells filled with high-purity noble gases, such as helium or argon.

At the Ion Guide Isotope Separator On-Line (IGISOL) facility [1] in the JYFL Accelerator Laboratory, a number of ongoing projects involve usage of various gas cells for stopping, thermalization and transportation of nuclear reaction products by subsonic gas flow. Multinucleon-transfer (MNT) reaction studies and Mass Analysing Recoil Apparatus Low-Energy Branch (MARA-LEB) are amongst these projects. MNT reactions have gained a lot of attention in the last decade following the observation of a merger of two neutron stars (GW170817 event) [2]. A dedicated MNT gas cell has been designed at the IGISOL facility using numerical calculations in the Computational Fluid Dynamics (CFD) Module of COMSOL Multiphysics®. In MNT reaction studies products of nuclear reactions, e.g. 136Xe+209Bi, enter into a gas cell through a thin entrance window, after which they are transported by helium gas flow out from the gas cell and subsequently by electrical fields towards the detector stations. The first experiments with the new MNT gas cell were successfully performed, and showed an increase of the measured count rates compared to our earlier results [3]. Within another ongoing project, MARA-LEB group aims to produce short-lived radioactive isotopes via different types of nuclear reactions and to perform high-resolution laser spectroscopy, mass measurements and nuclear decay spectroscopy on these isotopes [4]. In order to achieve that, the parameters of the MARA-LEB gas cell were carefully optimized in the CFD Module, after which the gas cell was commisioned and characterized at the IGISOL facility [5]. For both gas cells, MNT and MARA-LEB, the evacuation time and efficiency were measured and compared to numerical calculations performed in the CFD Module (see Fig. 1 for numerical calculations of MARA-LEB gas cell). Besides calculations of subsonic flow inside the gas cell, the numerical calculations of supersonic free jets formed by cylindrical exit holes inside the gas cell were performed for the future Super-FRS stopping gas cell [6] to obtain velocity and density profiles. These results were then compared with experimental characterization using Pitot tubes and Schlieren method.

Simulations using the CFD Module of the COMSOL Multiphysics® software were applied for the optimization of the parameters of the gas cells. The Laminar Flow and Transport of Diluted Species physics interfaces were used for the cases of stationary and time dependent studies. The calculations of supersonic jets were performed using the High Mach Number Flow physics interface. Results of the aforementioned design and characterization of gas cells, as well as characterization of supersonic free jets, will be reported in this contribution.