Laboratory For Underground Nuclear Astrophysics Research Articles

Detectors and Shieldings: Past and Future at LUNA

Nuclear reactions are responsible for the chemical evolution of stars, galaxies and the Universe. Unfortunately, at temperatures of interest for nuclear astrophysics, the cross-sections of the thermonuclear reactions are in the pico- femto-barn range and thus measuring them in the laboratory is extremely challenging. In this framework, major steps forward were made with the advent of underground nuclear astrophysics, pioneered by the Laboratory for Underground Nuclear Astrophysics (LUNA). The cosmic background reduction by several orders of magnitude obtained at LUNA, however, needs to be combined with high-performance detectors and dedicated shieldings to obtain the required sensitivity. In the present paper, we report on the recent and future detector-shielding designs at LUNA.

Recent results and future perspectives with solid targets at LUNA

The stellar evolution and chemical make-up of the Universe are determined by nuclear reactions occurring in a wide variety of stellar sites. Precise determinations of the cross sections of these reactions are crucial for the calculation of reaction rates and for the development of stellar evolution models. The Laboratory for Underground Nuclear Astrophysics (LUNA) collaboration has been at the forefront of the direct measurement of nuclear reactions at the low energies of astrophysical interest for the last 35 years. The many significant results achieved at LUNA have been made possible due to the low background conditions uniquely available thanks to its location deep underground at the Laboratori Nazionali del Gran Sasso. Another key aspect of these successes is due to the experience of the LUNA collaboration in the production and characterization of a variety of solid targets used in reaction measurements. In this review, the main production techniques of solid targets are described, as well as the common methods adopted for target degradation monitoring. We also present the results of recent measurements using these targets and the future plans of the LUNA collaboration for measurements using solid targets at the LUNA400 kV and the new Ion Beam Facility (IBF) 3.5 MV are also presented.

Study of the 20Ne(p,γ)21Na reaction at LUNA

The NeNa-MgAl cycles are involved in the synthesis of Ne, Na, Mg, and Al isotopes. The 20Ne(p,γ)21Na (Q = 2431.68 keV) reaction is the first and slowest reaction of the NeNa cycle and it controls the speed at which the entire cycle proceeds. At the state of the art, the uncertainty on the 20Ne(p,γ)21Na reaction rate affects the production of the elements in the NeNa cycle. In particular, in the temperature range from 0.1 GK to 1 GK, the rate is dominated by the 366 keV resonance corresponding to the excited state of EX = 2797.5 keV and by the direct capture component. The present study focus on the study of the 366 keV resonance and the direct capture below 400 keV. At LUNA (Laboratory for Underground Nuclear Astrophysics) the 20Ne(p,γ)21Na reaction has been measured using the intense proton beam delivered by the LUNA 400 kV accelerator and a windowless differential-pumping gas target. The products of the reaction are detected with two high-purity germanium detectors. The experimental details and preliminary results on the 366 keV resonance and on the direct capture component at very low energies will be shown, together with their possible impact on the 20Ne(p,γ)21Na reaction rate.

Proton-Capture Rates on Carbon Isotopes and Their Impact on the Astrophysical ^{12}C/^{13}C Ratio.

The ^{12}C/^{13}C ratio is a significant indicator of nucleosynthesis and mixing processes during hydrogen burning in stars. Its value mainly depends on the relative rates of the ^{12}C(p,γ)^{13}N and ^{13}C(p,γ)^{14}N reactions. Both reactions have been studied at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy down to the lowest energies to date (E_{c.m.}=60 keV) reaching for the first time the high energy tail of hydrogen burning in the shell of giant stars. Our cross sections, obtained with both prompt γ-ray detection and activation measurements, are the most precise to date with overall systematic uncertainties of 7%-8%. Compared with most of the literature, our results are systematically lower, by 25% for the ^{12}C(p,γ)^{13}N reaction and by 30% for ^{13}C(p,γ)^{14}N. We provide the most precise value up to now of 3.6±0.4 in the 20-140MK range for the lowest possible ^{12}C/^{13}C ratio that can be produced during H burning in giant stars.

Towards a direct measurement of the E cm = 65 keV resonance strength in 17O(p, γ)18F at LUNA

The 17O(p, γ)18F reaction plays a crucial role in several stellar scenarios where the hydrogen burning phases takes place. In particular, in the temperature energy range of interest for AGB nucleosynthesis (20 MK< T <80 MK) the main contribution to the astrophysical reaction rate comes from the elusive 65 keV resonance. Indeed, this resonance strength is at the moment determined only through indirect measurements, with a reported value of ωγ = (1.6 ± 0.3) × 10−11 eV. With typical experimental quantities for beam current, isotopic enrichment and detection efficiency, this strength yields an expected count rate of less than one count per Coulomb, making the direct measurement of this resonance extremely challenging. The Laboratory for Underground Nuclear Astrophysics (LUNA) 400kV accelerator installed in Laboratori Nazionali del Gran Sasso (Italy) provides a unique possibility to directly measure this low resonance thanks to the reduction of cosmic ray background by six orders of magnitude with respect surface laboratories and thanks to an intense, narrow proton beam. To improve the experimental sensitivity, the environmental background was further reduced designing a lead and borated (5%) polyethylene shielding and the absorption of γ − rays emitted by the reaction was minimised by the installation of target chamber and holder made of aluminum.With about 400 Coulomb accumulated on Ta2O5 targets, with nominal 17O enrichment of 90%, the LUNA collaboration has performed the first direct measurement of the 65 keV resonance strength.

Study of proton capture on carbon-13 isotope at LUNA

13C(p,γ)14N is a key reaction in different stellar environments for which both accurate and precise measurements of its cross section are required to constraint stellar models. This is currently ongoing at the Laboratory for Underground Nuclear Astrophysics where in addition to the low-background environment, the use of different detection systems (HPGe and BGO detectors) and analysis techniques (Peak Shape Analysis and Sum Peak Method) allowed to perform reaction cross section measurements at the lowest energy ever. Both the setup employed in the measurements and the analysis performed are presented in this paper.

Updated reaction rate of Mg25(p,γ)Al26 and its astrophysical implication

Al26 with a half-life of 7.17×105 years is one of the most significant nuclides in γ-ray astronomy and presolar grains of meteorites. Its main production mechanism in the H-burning MgAl cycle is the Mg25(p,γ)Al26 reaction. In the temperature region of 0.05–0.3 GK of astrophysical interest, the astrophysical Mg25(p,γ)Al26 reaction rate is dominated by the resonant capture of several low-energy resonances. In this work, we report the results of a complete experimental investigation of the Ec.m.=92, 130, and 189 keV resonances in the Mg25(p,γ)Al26 reaction with the Jinping Underground Nuclear Astrophysics Experimental Facility. The updated thermonuclear Mg25(p,γ)Al26 reaction rate is (32–39)% higher than that obtained at the Laboratory for Underground Nuclear Astrophysics around 0.07–0.09 GK, mainly due to the 32% enhancement of the 92-keV resonance strength. The astrophysical impact of our new rate on the Al26 yield in a 5 M⊙ low-metallicity asymptotic giant branch star is investigated, in which an increase of (45–79)% in the Al26 yield is found by adopting our new Mg25(p,γ)Al26 rates.5 MoreReceived 19 December 2022Revised 22 March 2023Accepted 24 April 2023DOI:https://doi.org/10.1103/PhysRevC.107.065801©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasH & He burningH & He induced nuclear reactionsHydrostatic stellar nucleosynthesisNuclear astrophysicsRadiative captureProperties20 ≤ A ≤ 38Nuclear Physics

Advances in radiative capture studies at LUNA with a segmented BGO detector

Studies of charged-particle reactions for low-energy nuclear astrophysics require high sensitivity, which can be achieved by means of detection setups with high efficiency and low backgrounds, to obtain precise measurements in the energy region of interest for stellar scenarios. High-efficiency total absorption spectroscopy is an established and powerful tool for studying radiative capture reactions, particularly if combined with the cosmic background reduction by several orders of magnitude obtained at the Laboratory for Underground Nuclear Astrophysics (LUNA). We present recent improvements in the detection setup with the Bismuth Germanium Oxide (BGO) detector at LUNA, aiming to reduce high-energy backgrounds and increase the summing detection efficiency. The new design results in enhanced sensitivity of the BGO setup, as we demonstrate and discuss in the context of the first direct measurement of the 65 keV resonance (E x = 5672 keV) of the 17O(p,γ)18F reaction. Moreover, we show two applications of the BGO detector, which exploit its segmentation. In the case of complex γ-ray cascades, e.g. the de-excitation of E x = 5672 keV in 18F, the BGO segmentation allows to identify and suppress the beam-induced background signals that mimic the sum peak of interest. We demonstrate another new application for such a detector in form of in situ activation measurements of a reaction with β + unstable product nuclei, e.g. the 14N(p,γ)15O reaction.

Deuterium burning measurement at LUNA and its astrophysical and nuclear implications

The deuterium burning reaction D(p,γ)3He at low energies is of primary importance in cosmology because it affects the primordial deuterium abundance, that in turn is very sensitive to fundamental cosmological parameters such as the baryon density and the amount of relativistic species permeating the early Universe. This reaction is also of a particular interest in theoretical nuclear physics because it offers a unique opportunity to test ab initio calculations. In the following a recent study of the D(p,γ)3He reaction in the 30 ≲ Ecm[keV] ≲ 280 energy range will be presented. This measurement has been performed in the underground Gran Sasso Laboratory by the LUNA (Laboratory for Underground Nuclear Astrophysics) collaboration. In addition, the need of having new and precise measurement of the D(d,n)3He and D(d,p)3H reactions will also be discussed.

Towards a direct measurement of the 17O(p, γ)18F 65 keV resonance strength at LUNA

The 17O(p, γ)18F reaction plays a crucial role in the hydrogen burning phases of different stellar scenarios. At temperature of interest for AGB nucleosynthesis (20 MK &lt; T &lt; 80 MK) the main contribution to the astrophysical reaction rate comes from the poorly constrained 65 keV resonance. The strength of this resonance is presently determined only through indirect measurements, with a reported value of ωγ = (1.6 ± 0.3) 10−11 eV. With typical experimental quantities for beam current, isotopic enrichment and detection efficiency, this strength yields to an expected count rate of less than one count per Coulomb, making the direct measurement of this resonance extremely challenging. A new high sensitivity setup has been installed at LUNA (Laboratory for Underground Nuclear Astrophysics) of Laboratori Nazionali del Gran Sasso. The high performance LUNA 400kV accelerator underground location guarantees, indeed, a reduction of cosmic ray background by several orders of magnitude. The residual background was further reduced by a devoted shielding of lead and borated (5%) polyethylene. On the other hand, the 4π BGO detector efficiency was optimized installing aluminum target chamber and holder. With about 400 C accumulated on Ta2O5 targets, with nominal 17O enrichment of 90%, the LUNA collaboration has performed the first direct measurement of the 65 keV resonance strength.

Nuclear Astrophysics at the Low-Energy Frontiers: Updates from underground laboratories

Nuclear fusion reactions are the heart of nuclear astrophysics: they sensitively influence the nucleosynthesis of the elements in the earliest stages of the Universe and in all the objects formed thereafter; control the associated energy generation and neutrino luminosity; influence the evolution of stars. Unfortunately, measuring reaction cross sections at astrophysically relevant energies is exceptionally challenging due to Coulomb repulsion between nuclei, resulting in cross section values as low as fbar. Laboratorial measurements of these cross sections are often unfeasible due to overwhelming cosmic-ray-induced backgrounds. One effective solution to this problem is to conduct experiments in underground laboratories. The Laboratory for Underground Nuclear Astrophysics (LUNA) is an experimental approach based on an underground accelerator focusing on studying nuclear fusion reactions. Its primary objective is to accomplish direct measurement of cross sections for nuclear reactions that have significance in stellar and primordial nucleosynthesis. This article will present the latest findings and future objectives.

Low-energy Cross Section Measurements of 12C(p, γ) Deep Underground at LUNA

The 12C(p, γ)13N reaction cross section is currently under investigation in the low-background environment of the Laboratory for Underground Nuclear Astrophysics (LUNA). It is being studied using different types of solid targets, and employing two complementary detection techniques: HPGe spectroscopy and activation counting. To reduce systematic uncertainties, targets have been accurately characterized and their degradation under the intense beam of the LUNA-400 accelerator monitored. We present the experimental techniques and the corresponding analyses used to extract the reaction cross section.

LUNA measurement found no evidence of a low-energy resonance in 6Li(p, γ)7Be reaction

The 6Li(p, γ)7Be reaction is mainly at work in three nucleosynthesis scenarios: Big Bang Nucleosynthesis, 6Li depletion in pre-main and in main sequence stars and cosmic ray interaction with interstellar matter. The 6Li(p, γ)7Be S-factor trend was poorly constrained at astrophysical energies because of conflicting experimental results reported in literature. A recent direct measurement, indeed, found a resonance-like structure at Ec.m. = 195 keV, corresponding to an excited state at Ex ∼ 5800 keV in 7Be which, however, has not been confirmed by either other direct measurements or predicted by theoretical calculations. In order to clarify the existence of this resonance, a new experiment was performed at the Laboratory for Underground Nuclear Astrophysics (LUNA), located deep underground in Gran Sasso Laboratory. Thanks to the extremely low background environment, the 6Li(p, γ)7Be cross section was measured in the center-of-mass energy range E = 60-350 keV with unprecedented sensitivity. No evidence for the alleged resonance was found. LUNA results was confirmed by latest published indirect determination of 6Li(p, γ)7Be S-factor and it is supported by a recent theoretical study.

LUNA (Laboratory for Underground Nuclear Astrophysics): A Long Success Story with New Goals in Sight

LUNA (Laboratory for Underground Nuclear Astrophysics): A Long Success Story with New Goals in Sight

&amp;lt;sup&amp;gt;22&amp;lt;/sup&amp;gt;Ne(&amp;alpha;, n)&amp;lt;sup&amp;gt;25&amp;lt;/sup&amp;gt;Mg: The key neutron source in massive stars

<p indent="0mm">The origin of heavy elements from iron to uranium in the universe is an important frontier direction for nuclear astrophysics. It is considered to be one of the 11 greatest unanswered questions of physics in this century. The neutron capture reaction can overcome the Coulomb barrier and is the key way to generate heavy elements in the universe. The average life of free neutrons is only about <sc>15 min.</sc> Therefore, the neutron source is a hot topic for understanding the origin of heavy elements. There are two main neutron sources in the s-process nucleosynthesis, one is the ¹³C(α, n)¹⁶O reaction, the other is the ²²Ne(α, n)²⁵Mg reaction. The ¹³C(α, n)¹⁶O reaction mainly provides neutrons for the s-process nucleosynthesis of small or medium mass stars, and affects the nucleosynthesis of nuclides with the mass numbers from 90 to 209; while the ²²Ne(α, n)²⁵Mg reaction mainly provides neutrons for the s-process nucleosynthesis of massive stars and affects the nucleosynthesis of nuclides with the mass number less than 90. The ²²Ne(α, n)²⁵Mg reaction takes place in the core helium burning phase and the subsequent shell carbon burning phase of massive stars. Due to the difficulty of experiment measurement, the data of the ²²Ne(α, n)²⁵Mg reaction rate is still lacking, which leads to the problem of understanding effective neutron supply in the nucleosynthesis of the heavy elements in massive stars. Limited by cosmic ray backgrounds, a large number of key reactions in hydrostatic nuclear burning phases cannot be accurately measured. Therefore, direct measurement of these reactions has gradually moved from the earth-surface to underground laboratories. The Laboratory for Underground Nuclear Astrophysics (LUNA) experimental platform in Italy is a mature underground nuclear astrophysics laboratory in the world. Since the 1990s, direct measurement of many nuclear reactions has been carried out. Jinping Underground Nuclear Astrophysics Laboratory (JUNA) is a new experimental platform. JUNA is located in the world’s deepest underground laboratory—China JinPing Underground Laboratory (CJPL). JUNA team has successfully developed a <sc>10 mA</sc> low-energy accelerator and experimental terminal, which aims to conduct experimental research on a number of key nuclear reactions in hydrostatic hydrogen and helium burnings of stars in an underground environment with very low background. Since its operation in the underground laboratory at the end of 2020, JUNA has completed the first phase of experimental tasks and successfully studied several key nuclear reactions including the neutron source reaction¹³C(α, n)¹⁶O. In this paper, we review the motivation of the key neutron source ²²Ne(α, n)²⁵Mg in the origin of heavy elements and in the massive stars, the latest research progress of this neutron source and future research plan, especially the underground direct measurement experimental plan. As the key neutron source reaction of the s-process, the ²²Ne(α, n)²⁵Mg reaction is of great significance for understanding the origin of heavy elements in the universe. As the first experiment in the second phase of JUNA, the ²²Ne(α, n)²⁵Mg reaction is expected to achieve world-class research results and provides the crucial nuclear input for understanding the nucleosynthesis in weak s-process.

Underground measurement at LUNA found no evidence for a low-energy resonance in the 6Li(p, γ)7Be reaction

The 6Li(p,γ)7Be reaction is involved in all three main nucleosynthesis scenarios: Big Bang Nucleosynthesis, the interaction of cosmic rays with interstellar matter, and stellar nucleosynthesis. Conflicting experimental results have been reported in literature for the 6Li(p,γ)7Be reaction cross section trend at astrophysical energies. A recent direct measurement found a resonance-like structure at Ec.m. = 195 keV, corresponding to an excited state at Ex ~ 5800 keV in 7Be which, however, has not been confirmed by either theoretical calculations or other direct measurements. In order to clarify the existence of this resonance, a new experiment was performed at the Laboratory for Underground Nuclear Astrophysics, located deep underground at Laboratori Nazionali del Gran Sasso (Italy). The 6Li(p,γ)7Be cross section was measured in the energy range Ec.m. = 60-350 keV with unprecedented sensitivity and no evidence for the alleged resonance was found.

Measurement and analysis techniques for a study of 12C(p,γ) and 13C(p,γ) deep underground

The 12C(p,γ) and 13C(p,γ) reaction cross sections are currently under investigation in the low-background environment of the Laboratory for Underground Nuclear Astrophysics. Both reactions are being studied using different types of solid targets, and employing complementary detection techniques (HPGe spectroscopy, total absorption spectroscopy and activation counting). To reduce systematic uncertainties, targets must be accurately characterized and their degradation monitored under the intense (~ 400 µA) beam of the LUNA400 accelerator. We present the experimental techniques employed, and the analyses developed for the study of these reactions.

Underground Measurements of Nuclear Reaction Cross-Sections Relevant to AGB Stars

Nuclear reaction cross sections are essential ingredients to predict the evolution of AGB stars and understand their impact on the chemical evolution of our Galaxy. Unfortunately, the cross sections of the reactions involved are often very small and challenging to measure in laboratories on Earth. In this context, major steps forward were made with the advent of underground nuclear astrophysics, pioneered by the Laboratory for Underground Nuclear Astrophysics (LUNA). The present paper reviews the contribution of LUNA to our understanding of the evolution of AGB stars and related nucleosynthesis.

Enrico Bellotti: a leader in underground physics

Enrico Bellotti: a leader in underground physics

Low-energy resonances in the O18 ( p,γ)19F reaction

Background: Shell hydrogen burning during the asymptotic giant branch (AGB) phase through the oxygen isotopes has been indicated as a key process that is needed to understand the observed O18/O16 relative abundance in presolar grains and in stellar atmospheres. This ratio is strongly influenced by the relative strengths of the reactions O18(p,α)N15 and O18(p,γ)F19 in low-mass AGB stars. While the former channel has been the focus of a large number of measurements, the (p,γ) reaction path has only recently received some attention and its stellar reaction rate over a wide temperature range rests on only one measurement. Purpose: Our aim is the direct measurement of states in F19 as populated through the reaction O18(p,γ)F19 to better determine their influence on the astrophysical reaction rate, and more generally to improve the understanding of the nuclear structure of F19. Method: Branchings and resonance strengths were measured in the proton energy range Eplab=150–400keV, using a high-purity germanium detector inside a massive lead shield. The measurement took place in the ultra-low-background environment of the Laboratory for Underground Nuclear Astrophysics (LUNA) experiment at the Gran Sasso National Laboratory, leading to a highly increased sensitivity. Results: The uncertainty of the γ branchings and strengths was improved for all four resonances in the studied energy range; many new transitions were observed in the case of the 334keV resonance, and individual γ decays of the 215keV resonance were measured for the first time. In addition a number of transitions to intermediate states that decay through α emission were identified. The strengths of the observed resonances are generally in agreement with literature values. Conclusions: Our measurements substantially confirm previous determinations of the relevant resonance strengths. Therefore the O18(p,γ)F19 reaction rate does not change with respect to the reaction rate reported in the compilations commonly adopted in the extant computations of red-giant branch and AGB stellar models. Nevertheless, our measurements definitely exclude a nonstandard scenario for the fluorine nucleosynthesis and a nuclear physics solution for the O18 depletion observed in Group 2 oxygen-rich stardust grains.3 MoreReceived 18 June 2020Revised 12 April 2021Accepted 6 July 2021DOI:https://doi.org/10.1103/PhysRevC.104.025802©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasH & He burningNuclear astrophysicsNuclear reactionsRadiative captureProperties6 ≤ A ≤ 19Nuclear Physics