Study on Differential Structure and Thermal Effects of Windowless Gas Target for Deep-underground Nuclear Astrophysical Experiments

Abstract

The JUNA (Jinping Underground Nuclear Astrophysics) platform has the facility with the world’s most intense beams, and provides a crucial opportunity for China to strive for a leading international position in nuclear astrophysics. The first phase of JUNA’s experiments has demonstrated a promising prospect for accurately measuring nuclear astrophysical reactions in underground laboratories. The windowless gas target is a critical component for directly measuring low-energy nuclear astrophysical reactions involving specific isotopes, thereby effectively expanding the scope of deep-underground experiments. Windowless gas targets have been employed in studies of low-energy nuclear reactions in several international laboratories. Represented by the work of Gran Sasso group from Italy, many achievements have been made in nuclear astrophysics research on windowless gas target equipment, including those on Big Bang nucleosynthesis and solar neutrino physics. However, these studies have focused on experiments with relatively low beam intensities. To meet the high-power, high-intensity beam requirements of the JUNA experiment, a dedicated gas target design is needed. In this work, a gas target device that can be used for high intensity beams was designed. Through modeling with ANSYS Fluent and COMSOL software, the feasibility of a multi-stage differential structure was validated. According to the simulation results, it is possible to achieve a total gradient decrease of about 6 orders of magnitude in gas pressure from the target chamber to the accelerator end. This indicates that the current gas target setup is sufficient for our future nuclear reaction measurement experiments. In addition, the heating effect proves to be important for gas target experiment. On one hand, when the beam passes through the gas target, some energy will be deposited, then the target density (i.e. target thickness) near the beam will correspondingly decrease. In nuclear physics experiments, precise measurement of target thickness is an important step in extracting key physical quantities (such as reaction cross-section). Therefore, systematic research is needed on the target thickness changes induced by beam to guide our future experimental set up. On the other hand, due to the possible changes in charge state when the beam passes through a gas target, which may not be detected by current measurement devices (such as Faraday cup), it is necessary to develop methods for normalizing the beam intensity that is insensitive to charge. The calorimeter is an optional solution. The response of calorimeter to thermal power input needs to be carefully studied to guide the future design of calorimeter. Both the beam induced heating effects on target thickness and thermal response of the calorimeter have been investigated by simulation. A design optimization for windowless gas target experiments has been proposed, which can effectively alleviate the beam heating effect and improve the accuracy of calorimeter measurement of beam intensity, providing a reliable foundation for the development of JUNA’s deep-underground experimental research in the future.

Keywords

• nuclear astrophysics
• deep-underground laboratory
• windowless gas target
• low-energy nuclear reaction