Underground laboratory JUNA shedding light on stellar nucleosynthesis Enhanced Publication

Underground laboratory JUNA shedding light on stellar nucleosynthesis 1 2 3 corresp first-author * kajino@buaa.edu.cn * kajino@buaa.edu.cn kajino@buaa.edu.cn https://orcid.org/0000-0002-0233-9900 School of Physics, and International Research Center for Big-Bang Cosmology and Element Genesis, Beihang University, Beijing 100083, China School of Physics, and International Research Center for Big-Bang Cosmology and Element Genesis, Beihang University, Beijing 100083, China Division of Science, National Astronomical Observatory of Japan, Tokyo 181-8588, Japan Division of Science, National Astronomical Observatory of Japan, Tokyo 181-8588, Japan Graduate School of Science, The University of Tokyo, Tokyo 113-033, Japan Graduate School of Science, The University of Tokyo, Tokyo 113-033, Japan Extremely low background experiments to measure key nuclear reaction cross sections of astrophysical interest are conducted at the world’s deepest underground laboratory, the Jingping Underground laboratory for Nuclear Astrophysics (JUNA). High precision measurements provide reliable information to understand nucleosynthetic processes in celestial objects and resolve mysteries on the origin of atomic nuclei discovered in the first generations of Pop. III stars in the universe and meteoritic SiC grains in the solar system. Nuclear astrophysics is a growing interdisciplinary research field of nuclear physics and astrophysics. It investigates the origin and evolution of hundreds of atomic nuclei in the universe to understand the evolution of stars and galaxies. In addition to the events of the century, SN1987A and GW170817, many complementary data have been accumulated from the high-resolution spectroscopic observations of old faint stars and precise meteoritic analysis of isotopic anomalies and correlations among them. The origin of atomic nuclei is classified into three astrophysical sites and epochs in cosmic evolution [ 1 1 ]: Big-Bang nucleosynthesis in the first 3–10 min of the early universe for the production of light mass nuclei, thermonuclear and explosive nucleosyntheses in stars for the production of intermediate-to-heavy mass nuclei after galaxies formed, and Galactic cosmic-ray interactions with interstellar medium for the production of rare abundant nuclei. In nuclear astrophysics [ 2 2 ], identifying key nuclear reactions and precisely determining their rates [ 3 3 ] have been a long-standing challenge for several nucleosynthetic processes, as shown in Figure 1 Figure 1 . Several nucleosynthetic paths, such as the rapid-neutron capture process (r-process), involve many radioactive unstable nuclei, which are challenging to study experimentally because of their short lifetimes and low beam intensities. This difficulty remains even for the measurements of the cross sections of stable nuclei because most cosmic or stellar nucleosynthesis occurs at temperatures of T = 10 8 -10 9 K under extreme conditions. Although these temperatures are relatively lower than those of other astronomical phenomena, their energy scale is as low as E = 10 - 100 keV. This energy scale is too low to directly measure the reaction cross sections of charged particles in laboratories because extremely small Coulomb penetrability significantly reduces the cross section[ 4 4 ]. In addition, the cosmic-ray background prevents the precise measurement of small reaction cross-sections. Hence, eliminating the cosmic-ray background is critical for the detection of neutral particles like photons and neutrons to measure the reliable reaction rates of radiative-capture <math id="M1"><mrow><mo stretchy="false">(</mo><mi>α</mi><mn>,</mn><mi>γ</mi><mo stretchy="false">)</mo></mrow></math> , (p, γ ) and neutron emission ( α , n) reactions, which are key to the nucleosynthetic processes in Fig. 1 Fig. 1 . The JUNA Collaboration is directed by CIAE, jointly supported by NSFC, CNNC, and CAS, and has established a unique underground facility equipped with a high-intensity accelerator for the direct measurements of extremely small cross sections at low astrophysical energies. JUNA is 2400 m underground at the China JinPing Laboratory complex (CJPL-II) established in 2014 and is the deepest underground laboratory in the world ( Fig. 2 Fig. 2 ). The laboratory is beneath rocks with a thickness of several kilometers; the rocks shield cosmic rays and their corresponding background. Since the first beam was delivered in Dec. 2020, experiments on several key reactions have been carried out with a high-intensity accelerator in an ultra-low background environment. From the onset, JUNA was dedicated to experiments at 19 F(p, αγ ) 16 O at very low energies of E cm = 72.4–344 keV, covering the Gamow window [ 5 5 , 6 6 ]. Fluorine is one of the most mysterious monoisotopic elements whose predicted abundance has a large uncertainty for the reaction rate. To have multiple origins, that is, asymptotic giant branch stars (AGB stars) in a relatively recent epoch with solar metallicity [ 7 7 ], and the core-collapse type II supernova (cc-SN II) in the early galaxy at low metallicity [ 8 8 ]. The JUNA experiment reduces theoretical uncertainties significantly, and the breakout condition of the carbon–nitrogen–oxygen cycle (CNO cycle) in AGB stars is more precisely constrained. The SN ν nucleosynthesis of 19 F is also revisited by considering flavor oscillation effects to infer nucleosynthetic constraints on an unknown mass hierarchy [ 9 9 ]. As part of the inception objectives of the JUNA collaboration, 25 Mg(p, γ ) 26 Al, which plays an important role in the production of 26 Al ( τ 1/2 =7.17×10 5 y) in massive srars [ 10 10 ], was successfully studied. γ rays with an energy of 1.809 MeV emitted from 26 Al were observed by the Gamma-ray Satellite INTEGRAL that provides the distribution and total accumulated mass of 26 Al in the Milky Way. The estimated frequency of cc-SN II and Ib/c was 1.9 ± 1.1 events per century [ 11 11 ], providing a firm basis for the theoretical study of the Galactic chemical evolution of r-process elements [ 12 12 ]. The JUNA Collaboration team determined the reaction rate of 25 Mg(p, γ ) 26 Al with high accuracy by measuring resonance parameters at 92 keV and contributed to establishing a scheme of estimating the rate of SN events. The production mechanism of 40 Ca observed in the oldest ultra-metal-poor star [ 13 13 ] remains an unresolved mystery in recent astronomical observations. In the standard stellar evolution model, most 19 F produced by the hot CNO cycle are recycled back to 16 O by the 19 F(p, α ) 16 O reaction; therefore, the flow cannot reach the production of 40 Ca. However, this scenario is subject to the error bars of its competing reaction rate for 19 F(p, γ ) 20 Ne that breaks the hot CNO cycle as the temperature increases. The JUNA Collaboration team directly measured this reaction down to E cm = 186 keV and found that the resonance at 225 keV contributes to the enhancement in the thermonuclear reaction rate of 19 F(p, γ ) 20 Ne [ 14 14 ]. The hydrostatic burning in Pop. III stars calculated using new rate results of 40 Ca abundance are consistent with those observed in the oldest known ultra-metal-poor stars [ 14 14 ]. The JUNA Collaboration team studied the 13 C( α , n) 16 O reaction and successfully measured its cross section in the range of E cm = 240 keV-1.9 MeV, removing uncertainty from previous experimental data [ 15 15 ]. Since this reaction and 22 Ne( α , n) 25 Mg are a major neutron source for the s-process in AGB stars, the precise determination has long been wanted to predict reliable nuclear abundances from iron to 209 Bi theoretically. The JUNA experiment covers the Gamow window for the intermediate-neutron capture process (i-process) recently identified as a new process in metal-deficient AGB stars and collapsars, which are major r-process sites of massive stars collapsing to a black holes [ 16 16 ]. Comprehensive theoretical studies of the s-, i-, and r-processes in multiple astrophysical sites are being conduncted with JUNA’s new precise data. Recently, the JUNA Collaboration team reported an experimental result [ 17 17 ] on 18 O <math id="M2"><mrow><msup><mrow><mo stretchy="false">(</mo><mi>α</mi><mn>,</mn><mi> </mi><mi>γ</mi><mo stretchy="false">)</mo></mrow><mrow><mn>22</mn></mrow></msup></mrow></math> Ne. For the first time, the team successfully determined the energy E lab = 474.0 ± 1.1 keV, spin-parity J π = 1 - , and γ -width ω γ = 0.25 ± 0.03 μ eV of the most effective resonance dominating the total reaction rate at the Gamow window. 22 Ne is produced via <math id="M3"><mrow><msup><mrow/><mrow><mn>14</mn></mrow></msup><mtext>N</mtext><msup><mrow><mrow><mo>(</mo><mrow><mi>α</mi><mo>,</mo><mtext> </mtext><mi>γ</mi></mrow><mo>)</mo></mrow></mrow><mrow><mn>18</mn></mrow></msup><mtext>F</mtext><msup><mrow><mo stretchy="false">(</mo><msup><mi>e</mi><mo>−</mo></msup><mi>ν</mi><mo stretchy="false">)</mo></mrow><mrow><mn>18</mn></mrow></msup><mtext>O</mtext><msup><mrow><mrow><mo>(</mo><mrow><mi>α</mi><mo>,</mo><mtext> </mtext><mi>γ</mi></mrow><mo>)</mo></mrow></mrow><mrow><mn>22</mn></mrow></msup><mtext>Ne</mtext></mrow></math> , which is key to identifying the conversion efficiency of the CNO cycle, whose main product is 14 N, to the advanced burning stage where the s-process occurs using neutrons produced via the reaction 22 Ne( α , n) 25 Mg in both AGB and supergiant stars. The improved JUNA data also contributes to probing the unknown origin of the isotopic abundance ratio of 21 Ne/ 22 Ne observed in meteoritic stardust SiC grains. The so-called mainstream components of SiC grains arise from AGB stardust. Therefore, both 18 O <math id="M4"><mrow><msup><mrow><mo stretchy="false">(</mo><mi>α</mi><mn>,</mn><mi> </mi><mi>γ</mi><mo stretchy="false">)</mo></mrow><mrow><mn>22</mn></mrow></msup></mrow></math> Ne and its competing reaction 18 O( α , n) 21 Ne affect the resultant isotopic ratio. The improved JUNA data remove the uncertaity in this isotopic ratio. The observed 21 Ne/ 22 Ne ratios in meteoritic stardust SiC grains are used to constrain the physical conditions of nuclear burning processes that depend on the initial mass of the parent AGB star and other astronomical parameters in modeling stellar evolution. Phase I of the JUNA collaboration has almost completed the high precision measurements of several key reactions at the lowest cosmic-ray background. Phase II of the JUNA collaboration has began and will study the following reactions: 12 C( α , γ ) 16 to improve knowledge on stellar evolution and the explosion mechanism of massive stars and resolve the mystery of the massive black-hole mass-gap [ 18 18 ]; 12 C( 12 C, α ) 20 Ne and 12 C( 12 C, p) 23 Na to identify the unknown heat source of X-ray Superburst and clarify the double-degenerate explosion mechanism of SN Ia [ 19 19 ]; 3 He( α, γ ) 7 Be and 3 H( α, γ ) 7 Li to understand the overproduction of Big-Bang lithium and precisely constrain neutrino-mixing parameters from missing solar neutrino flux [ 20 20 ]; 11 B( α , n) 14 N, 15 N( α, γ ) 19 F, and 17 O(n, α ) 14 C and its reverse reaction to elucidate the physical conditions of α -rich freezeout and the r-process nucleosynthesis in magneto-hydrodynamic jets from cc-SN II, collapsars, and binary neutron star mergers [ 21 21 , 12 12 ]. 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