Progress in Jinping Underground Nuclear Astrophysics Experiment

Nuclear astrophysics is an interdisciplinary subject of nuclear physics and astrophysics. As the forefront of fundamental research, it has played an important role in the process of deepening understanding of the universe and stellar evolution. One of the primary tasks of nuclear astrophysics is the precise measurement of the cross-sections of the thermonuclear reactions which are responsible for stellar evolution and the synthesis of the chemical elements. Deep underground laboratory, where the flux of cosmic muons is reduced by several orders of magnitude, provides a great opportunity of conducting the measurements of the crucial reaction cross section directly within their Gamow energy regions. At present, there are three ongoing underground nuclear astrophysics experiments, including LUNA in National Laboratory of Gran Sasso (LNGS), CASPER in Sanford Underground Research Facility (SURF), and Jinping Underground Nuclear Astrophysics (JUNA) in China Jinping Underground Laboratory (CJPL). As a pioneer, LUNA has carried out experimental research on several key reactions in the last three decades. LUNA’s result deepens our understanding of the properties of the neutrino, of the sun, and of the universe itself. Recently, with the development of CASPAR and JUNA, the underground experiment has breathed new life. JUNA project is carried out in the second phase of CJPL (CJPL-Ⅱ). CJPL was built under Jinping Mountain with 2 400 m of rock overburden (6 720 m of water equivalent, m.w.e.) leading to the muon flux, the most penetrating component of the cosmic rays, is about 100 times lower compared with LNGS and SURF. More importantly, JUNA has developed a 400 kV intense current accelerator, the beam intensity is more than 10 times that of LUNA. Corresponding underground measurement techniques have also been developed. With the advantage of the ultra-low background of CJPL-Ⅱ and intense beam current at mA level, JUNA’s experimental platform becomes the underground accelerator facility with the highest measurement accuracy in the world. In 2021, JUNA started its first campaign of experimental measurements. Within the 4-month of operation, JUNA measured several important reaction cross sections for the studies of hydrostatic hydrogen and helium burnings, including 25Mg(p, γ)26Al, 19F(p, αγ)16O, 19F(p, γ)16O, 13C(α, n)16O, 18O(α, γ)22Ne and 12C(α, γ)16O reactions. JUNA’s experiments achieved the lowest energy and the highest accuracy in all the above measurements, bring