Underground Nuclear Astrophysics Research Articles

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 of the 18O(α, γ)22Ne resonances at JUNA

22Ne(α,n)25Mg is one of the main neutron sources of the s process. 22Ne is produced by the 14N(α, γ)18F(β+)18O(α, γ)22Ne reaction chain in the helium burning, thus, the production rate of 22Ne is dominated by 14N(α,γ)18F and 18O(α,γ)22Ne. At the astrophysical relevant temperatures, the 18O(α,γ)22Ne reaction rates are determined by several low-energy resonances. In this work, the 18O(α,γ)22Ne reaction was measured at the 400 kV accelerator of Jinping Underground Nuclear Astrophysics experiment (JUNA). The γ-ray yields of the resonances between 470 to 770 keV were obtained.

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

First result from the Jinping Underground Nuclear Astrophysics experiment JUNA: precise measurement of the 92 keV 25Mg(p, [formula omitted])26Al resonance

The 25Mg(p, γ)26Al reaction plays an important role in the study of cosmic 1.809 MeV γ-ray as a signature of ongoing nucleosynthesis in the Galaxy. At astrophysical temperature around 0.1 GK, the 25Mg(p, γ)26Al reaction rates are dominated by the 92 keV resonance capture process. We report a precise measurement of the 92 keV 25Mg(p, γ)26Al resonance in the day-one experiment at Jinping Underground Nuclear Astrophysics experiment (JUNA) facility in the China Jinping Underground Laboratory (CJPL). The resonance strength and ground state feeding factor are determined to be 3.8±0.3 ×10−10 eV and 0.66±0.04, respectively. The results are in agreement with those reported in the previous direct underground measurement within uncertainty, but with significantly reduced uncertainties. Consequently, we recommend new 25Mg(p, γ)26Al reaction rates which are by a factor of 2.4 larger than those adopted in REACLIB database at the temperature around 0.1 GK. The new results indicate higher production rates of 26gAl and the cosmic 1.809 MeV γ-ray. The implication of the new rates for the understanding of other astrophysical situations is also discussed.

Direct Measurement of the Astrophysical ^{19}F(p,αγ)^{16}O Reaction in the Deepest Operational Underground Laboratory.

Fluorine is one of the most interesting elements in nuclear astrophysics, where the ^{19}F(p,α)^{16}O reaction is of crucial importance for Galactic ^{19}F abundances and CNO cycle loss in first generation Population III stars. As a day-one campaign at the Jinping Underground Nuclear Astrophysics experimental facility, we report direct measurements of the essential ^{19}F(p,αγ)^{16}O reaction channel. The γ-ray yields were measured over E_{c.m.}=72.4-344 keV, covering the Gamow window; our energy of 72.4keV is unprecedentedly low, reported here for the first time. The experiment was performed under the extremely low cosmic-ray-induced background environment of the China JinPing Underground Laboratory, one of the deepest underground laboratories in the world. The present low-energy S factors deviate significantly from previous theoretical predictions, and the uncertainties are significantly reduced. The thermonuclear ^{19}F(p,αγ)^{16}O reaction rate has been determined directly at the relevant astrophysical energies.

Pinning Down the Fate of Fluorine

The first results from the Jinping Underground Nuclear Astrophysics particle accelerator refine a key reaction rate for the destruction of fluorine in stars.

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

Low event rate neutron detector array using the coincidence between plastic scintillator and Helium-3 proportional counters

A neutron detector array consisting of a cylindrical plastic scintillator and 24 Helium-3 proportional counters is developed for the detection of neutrons with an extremely low rate. This detector array is designed for the JUNA (Jinping Underground Nuclear Astrophysics laboratory) project to measure the 13C (α, n) 16O reaction cross-section at stellar energies in CJPL (China Jinping Underground Laboratory). In this paper, we introduce the design of this detector array. The responses of the plastic scintillator to gamma rays and neutrons are tested at the ground level by shielding the detector array with a lead room. The neutron detection efficiency is obtained with a 252Cf source and a PuC source. The measured result is reproduced by Monte Carlo simulation. The coincidence measurement between plastic scintillator and Helium-3 counters is also performed and the coincidence times are obtained. These tests lay the foundation for the future measurement of the 13C (α, n) 16O reaction cross-section in CJPL.

Strong and durable fluorine-implanted targets developed for deep underground nuclear astrophysical experiments

Nine fluorine targets of various types were developed for the 19F(p, α)16O experiment to be performed at the Jinping Underground Nuclear Astrophysics Experiment (JUNA), China. Two targets were produced by evaporating CaF2 on Ta backings: one was produced by sputtering MgF2 on the Cr+Fe backings, and six were produced by implanting 20–80 keV 19F ions into pure Fe and Cu backings. Thin Cr protective layers were covered over these targets. We assessed each target’s stability by monitoring the γ-ray yields of the 19F(p, αγ)16O reaction over the well-known 340 keV resonance. Our results indicate that the traditional evaporated and sputtered targets exhibit 0.6%–6% deterioration (or 19F material loss) per Coulomb, while the implanted targets exhibit relatively small deterioration due to the proton beam bombardment. For the optimum target #8, the 19F target material loss is only approximately 0.05% per Coulomb proton beam bombardment, and such target is much more stable than the traditional targets. The fluorine depth distribution of the implanted target #8 was precisely analyzed by the atom probe tomography (APT) technique. The obtained depth distribution can well reproduce the experimental γ-ray yield curve. Furthermore, as test experiment, the 19F(p, αγ)16O cross section was measured with the implanted target #8 in a center-of-mass energy region of Ec.m. = 174–358 keV, which is consistent with the previous results. In conclusion, traditional fluorine targets produced with the evaporation and sputtering techniques are not suitable for the high-current experiment even with a protective layer. For the upcoming high-current JUNA experiment, this work offers an optimum target scheme: first, implanting 19F ions into the pure Fe backings with an implantation energy of 40 keV, and then sputtering a 50 nm thick Cr layer to further prevent the fluorine material loss.

The Study of Key Reactions Shaping the Post-Main Sequence Evolution of Massive Stars in Underground Facilities

The chemical evolution of the Universe and several phases of stellar life are regulated by minute nuclear reactions. The key point for each of these reactions is the value of cross-sections at the energies at which they take place in stellar environments. Direct cross-section measurements are mainly hampered by the very low counting rate and by cosmic background; nevertheless, they have become possible by combining the best experimental techniques with the cosmic silence of an underground laboratory. In the nineties, the LUNA (Laboratory for Underground Nuclear Astrophysics) collaboration opened the era of underground nuclear astrophysics, installing first a homemade 50 kV and, later on, a second 400 kV accelerator under the Gran Sasso mountain in Italy: in 25 years of experimental activity, important reactions responsible for hydrogen burning could have been studied down to the relevant energies thanks to the high current proton and helium beams provided by the machines. The interest in the next and warmer stages of star evolution (i.e., post-main sequence and helium and carbon burning) drove a new project based on an ion accelerator in the MV range called LUNA-MV, able to deliver proton, helium, and carbon beams. The present contribution is aimed to discuss the state of the art for some selected key processes of post-main sequence stellar phases:12C(α,γ)16O and12C+12C are fundamental for helium and carbon burning phases, and13C(α,n)16O and22Ne(α,n)25Mg are relevant to the synthesis of heavy elements in AGB stars. The perspectives opened by an underground MV facility will be highlighted.

The baryon density of the Universe from an improved rate of deuterium burning.

Light elements were produced in the first few minutes of the Universe through a sequence of nuclear reactions known as Big Bang nucleosynthesis (BBN)1,2. Among the light elements produced during BBN1,2, deuterium is an excellent indicator of cosmological parameters because its abundance is highly sensitive to the primordial baryon density and also depends on the number of neutrino species permeating the early Universe. Although astronomical observations of primordial deuterium abundance have reached percent accuracy3, theoretical predictions4-6 based on BBN are hampered by large uncertainties on the cross-section of the deuterium burning D(p,γ)3He reaction. Here we show that our improved cross-sections of this reaction lead to BBN estimates of the baryon density at the 1.6 percent level, in excellent agreement with a recent analysis of the cosmic microwave background7. Improved cross-section data were obtained by exploiting the negligible cosmic-ray background deep underground at the Laboratory for Underground Nuclear Astrophysics (LUNA) of the Laboratori Nazionali del Gran Sasso (Italy)8,9. We bombarded a high-purity deuterium gas target10 with an intense proton beam from the LUNA 400-kilovolt accelerator11 and detected the γ-rays from the nuclear reaction under study with a high-purity germanium detector. Our experimental results settle the most uncertain nuclear physics input to BBN calculations and substantially improve the reliability of using primordial abundances to probe the physics of the early Universe.

The Study of the 6Li(p,γ)7Be Reaction at LUNA

The 6Li(p,γ)7Be reaction is mainly involved in two astrophysical scenarios: the primordial nucleosynthesis and 6Li consumption in pre-main and main sequence stars. A recent measurement of 6Li(p,γ)7Be reaction S-factor reported a resonance-like structure at Ecm = 195 keV, which has not been confirmed neither by other direct measurements nor by theoretical calculations.A new experiment was performed at the Laboratory for Underground Nuclear Astrophysics (LUNA). The extremely low background environment allowed to measure the 6Li(p,γ)7Be cross section down to low energies with unprecedented sensitivity leading to clarify the existence of the claimed resonance. Details on the experimental setup and the preliminary results of the ongoing analysis are reported in this work.

Low energy cross section of 18O(p,γ)19F

The observation of oxygen isotopes in giant stars sheds light on mixing processes operating in their interiors. Due to the very strong correlation between nuclear burning and mixing processes it is very important to reduce the uncertainty on the cross sections of the nuclear reactions that are involved. In this paper we focus our attention on the reaction 18O(p,γ)19F. While the 18O(p, α)15N channel is thought to be dominant, the (p,γ) channel can still be an important component in stellar burning in giants, depending on the low energy cross section. So far only extrapolations from higher-energy measurements exist and recent estimates vary by orders of magnitude. These large uncertainties call for an experimental reinvestigation of this reaction.We present a direct measurement of the 18O(p,γ)19F cross section using a high-efficiency 4π BGO summing detector at the Laboratory for Underground Nuclear Astrophysics (LUNA). The reaction cross section has been directly determined for the first time from 140 keV down to 85 keV and the different cross section components have been obtained individually. The previously highly uncertain strength of the 90 keV resonance was found to be three orders of magnitude lower than an indirect estimate based on nuclear properties of the resonant state and a factor of 20 lower than a recently established upper limit. This result excludes the possibility that the 90 keV resonance can contribute significantly to the stellar reaction rate.In addition the strengths and branching ratios of resonances between 150 and 400 keV have been determined with much improved precision and sensitivity using a HPGe detector, including a first measurement of branching ratios of the 216 keV resonance. Preliminary results are presented.

Nuclear Astrophysics in underground laboratories: the LUNA experiment

One of the main ingredients of nuclear astrophysics is the knowledge of the thermonuclear reactions responsible for powering the stellar engine and for the synthesis of the chemical elements. At astrophysical energies the cross section of nuclear processes is extremely reduced by the effect of the Coulomb barrier. The low value of cross sections for charged particles prevents their measurement at stellar energies on Earth surface and often extrapolations are needed. The Laboratory for Underground Nuclear Astrophysics (LUNA) is placed under the Gran Sasso mountain and thanks to the cosmic-ray background reduction provided by its position can investigate cross sections at energies close to the Gamow peak in stellar scenarios. Many crucial reactions involved in hydrogen burning have been measured directly at astrophysical energies with both the LUNA-50kV and the LUNA-400kV accelerators, and this intense work will continue with the installation of a MV machine able to explore helium and carbon burnings. Based on this progress, currently there are efforts in several countries to construct new underground accelerators. In this talk, the typical techniques adopted in underground nuclear astrophysics will be described and the most relevant results achieved by LUNA will be reviewed. The exciting science that can be probed with the new facilities will be highlighted.

Preparation of large-area isotopic magnesium targets for the 25Mg(p,γ)26Al experiment at JUNA

To study the $${^{25}\hbox{Mg}}(p,\gamma ){^{26}\hbox{Al}}$$ reaction at the Jinping Underground Nuclear Astrophysics laboratory, a large-area $${^{25}\hbox{Mg}}$$ target with a uniform thickness is needed. A rotating unit is used to ensure the uniformity of the target thickness during evaporation. After many attempts, 19 targets with diameters of 40 mm and a non-uniformity of 8.4% were prepared simultaneously. The rate of material utilization was approximately 4.7 times higher than that obtained using a conventional evaporation method.

Setup commissioning for an improved measurement of the D(p,$$\gamma $$)$$^3$$He cross section at Big Bang Nucleosynthesis energies

Among the reactions involved in the production and destruction of deuterium during Big Bang Nucleosynthesis, the deuterium-burning D(p,gamma)3He reaction has the largest uncertainty and limits the precision of theoretical estimates of primordial deuterium abundance. Here we report the results of a careful commissioning of the experimental setup used to measure the cross-section of the D(p,gamma)3He reaction at the Laboratory for Underground Nuclear Astrophysics of the Gran Sasso Laboratory (Italy). The commissioning was aimed at minimising all sources of systematic uncertainty in the measured cross sections. The overall systematic error achieved (< 3 %) will enable improved predictions of BBN deuterium abundance.

A new approach to monitor $$^{13}\hbox {C}$$-targets degradation in situ for $$^{13}\hbox {C}(\alpha ,\hbox {n})^{16}\hbox {O}$$ cross-section measurements at LUNA

Direct measurements of reaction cross-sections at astrophysical energies often require the use of solid targets able to withstand high ion beam currents for extended periods of time. Thus, monitoring target thickness, isotopic composition, and target stoichiometry during data taking is critical to account for possible target modifications and to reduce uncertainties in the final cross-section results. A common technique used for these purposes is the Nuclear Resonant Reaction Analysis (NRRA), which however requires that a narrow resonance be available inside the dynamic range of the accelerator used. In cases when this is not possible, as for example the 13C(alpha,n)16O reaction recently studied at low energies at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy, alternative approaches must be found. Here, we present a new application of the shape analysis of primary gamma rays emitted by the 13C(p,g)14N radiative capture reaction. This approach was used to monitor 13C target degradation {\em in situ} during the 13C(alpha,n)16O data taking campaign. The results obtained are in agreement with evaluations subsequently performed at Atomki (Hungary) using the NRRA method. A preliminary application for the extraction of the 13C(alpha,n)16O reaction cross-section at one beam energy is also reported.

22Ne(p,γ)23Na cross section measurement at astrophysical energies.

A large number of uncertain resonances in the Gamow window make the 22Ne(p, γ)23Na reaction rate the most uncertain in the NeNa cycle. A new direct measurement of the 22Ne(p, γ)23Na reaction has been performed at LUNA (Laboratory for Underground Nuclear Astrophysics), with a windowless gas target. Two complementary setups have been used, to obtain both high resolution and high efficiency measurements. Three new resonances have been discovered at 156.2, 189.5 and 259.7 keV and their decay scheme has been determined. Two tentative resonances at 71 and 105 keV have not been detected and improved upper limits have been put on their strength. Thanks to the high-efficiency setup, it was possible to measure the non-resonant cross section at unprecedented low energies.

Nuclear astrophysics at Gran Sasso : the study of BBN and post-main sequence fusion reactions at LUNA

The first nuclei were formed a few minutes after the Big Bang, through the process called Big Bang nucleosynthesis, that left our universe containing about 75% hydrogen, 24% helium by mass, with small traces of other elements such as lithium and the hydrogen isotope deuterium. Heavier nuclei are produced during the different characteristic phases of the star evolution. At astrophysical energies the cross section of nuclear processes is usually extremely small and the cosmogenic background prevents their measurement at stellar energies on Earth surface. Deep underground in the Gran Sasso laboratory, several crucial reactions involved in hydrogen burning has been measured directly at astrophysical energies by the LUNA (Laboratory for Underground Nuclear Astrophysics) Collaboration with both the LUNA-50kV and the LUNA-400kV accelerators. This intense work will continue with the installation of a new LUNA-MV machine able to provide hydrogen, helium and carbon high current beams: the new facility will allow to explore the helium and carbon burning processes, by studying the key reactions shaping the evolution of massive stars towards their final fate. The present contribution is aimed to summarise the most recent results achieved by LUNA Collaboration and to highlight the rich experimental program connected to the new facility.