锦屏深地核天体物理实验进展

Progress in Jinping Underground Nuclear Astrophysics Experiment

深地极低辐射本底环境是核天体物理关键反应直接测量的基础。1990年世界首个深地核天体物理实验项目LUNA在意大利LNGS启动,此后美国和中国也开展了深地实验研究。为实现核天体物理关键反应的精确测量,我国在中国锦屏地下实验室建立了首个深地核天体物理实验项目,即JUNA项目。JUNA研制了强流高稳定加速器装置,束流强度超过意大利LUNA装置的10倍,同时发展了一系列极低辐射本底环境下的测量技术。2021年,JUNA试运行期间完成了多项重元素合成核天体物理关键反应的直接测量,取得重要研究成果。本文对JUNA项目的深地测量技术和开展的实验研究进行综述,并展望以束流强度和测量技术的突破为主要目标的新一代深地实验计划。

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 2400 m of rock overburden(6720 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-Ⅱand 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 25 Mg(p,γ)^(26)Al^(19)F(p,αγ)^(16)O,^(19)F(p,γ)^(16)O,13 C(α,n)^(16)O,^(18)O(α,γ)^(22)Ne and^(12)C(α,γ)^(16)O reactions.JUNA’s experiments achieved the lowest energy and the highest accuracy in all the above measurements,bringing the underground nuclear astrophysics experiment into a new stage.After nearly a century of development,people have a deeper understanding of stellar evolution and the synthesis of chemical elements.In particular,the deep underground experiment has brought nuclear astrophysics into a new stage of accurate measurement.However,there are still many mysteries to be resolved in nuclear astrophysics.For example,the most important reaction^(12)C(α,γ)^(16)O,known as the“Holy Grail”,still has no accurate cross-section data in its Gamow window.Looking to the future,measurements over a wider energy range and with higher beam current intensity will be the development direction of underground nuclear astrophysics experiments.Now CASPAR has temporarily withdrawn from SURF due to laboratory construction,and improving the accelerator beam intensity will be its primary task for future restarts.LUNA-MV accelerator is in the commissioning phase at LNGS,and its high voltage reaches 3.5 MV.JUNA also proposed a new plan,which will be focused on improving beam intensity and developing new underground techniques to maintain the advantage of measurement accuracy.With the new plan,JUNA will continue to occupy a dominant position in underground nuclear astrophysics experiments.The future of underground nuclear astrophysics experiments can be expected.