arXiv subjects

K. E. Rehm

Publications and source records attributed to K. E. Rehm.

At least 19 recordsLinked to original sources

arXiv2026

Implications for Type Ia Supernova Nucleosynthesis from an Experimentally Constrained $^{16}$O$(p,α)^{13}$N Reaction Rate

The $^{16}$O$(p,α)^{13}$N reaction plays a key role in shaping the $α$-particle abundance during explosive oxygen burning in Type Ia supernovae. By enhancing $α$-production, this reaction directly affects the calcium-to-sulphur (Ca/S) and argon-to-sulphur (Ar/S) ratios, which serves as a tracer of progenitor metallicity. However, recent work suggests that the rate must be enhanced by a factor of up to seven over the standard value to explain observed Ca/S ratios across a range of progenitor metallicities. To explore this impact, available experimental cross-section data for the $^{16}$O$(p,α)^{13}$N reaction have been compiled and critically evaluated. Significant discrepancies are identified in the low-energy region ($E_{\mathrm{cm}}$ = 5.7--7.0 MeV), primarily due to limitations of the activation method. To resolve this, the first direct measurement at astrophysical energies has been performed using the MUSIC active-target detector. The new $^{16}$O$(p,α)^{13}$N thermonuclear reaction rate is found to be approximately 1.5 times higher than the REACLIB rate in the temperature range T = 3--4 GK, with more constrained uncertainties that resolve the previously large spread among existing data. The suggested factor of seven enhancement is excluded and these results indicate that this reaction alone cannot fully explain the variation in the Ca/S and Ar/S ratios observed across different progenitor metallicities. Therefore, future work should focus on reducing the uncertainties in other key oxygen-burning reactions, particularly $^{16}$O+$^{16}$O and $^{12}$C+$^{16}$O. Further reducing the constraints on the $^{16}$O$(p,α)^{13}$N rate is also needed to fully determine to whether a nuclear physics solution to this discrepancy is possible.

nucl-ex↗

arXiv2025

First experimental determination of the $^{40}$Ar($n,2n$)$^{39}$Ar reaction cross section and $^{39}$Ar production in Earth's atmosphere

The cosmogenic $^{39}$Ar(t$_{1/2}$= 268 years) isotope of argon is used for geophysical dating and tracing owing to its appropriate half-life and chemical inertness as a noble gas; $^{39}$Ar serves also in nuclear weapon test monitoring. We measured for the first time the total cross section of the main $^{39}$Ar cosmogenic production reaction in the atmosphere, namely $^{40}$Ar$(n,2n)^{39}$Ar, using 14.8$\pm0.3$ MeV neutrons. The neutrons, produced by a deuterium-tritium generator, impinged on a stainless steel sphere filled with Ar gas highly enriched in the $^{40}$Ar isotope. The reaction yield was measured by atom counting of $^{39}$Ar with noble gas accelerator mass spectrometry and, independently, by decay counting relative to atmospheric argon. A total $^{40}$Ar$(n,2n)^{39}$Ar cross section of 610$\pm100$ mb was determined. This result serves as a benchmark for recent theoretical calculations and evaluations, found to reproduce well the experimental total cross section. We use these energy-dependent theoretical cross sections together with experimental spectra of cosmogenic neutrons at different altitudes to calculate the global average rate of neutron-induced $^{39}$Ar atmospheric production, resulting in $770\pm240$ $^{39}$Ar atoms/cm$^2$/day. The secular equilibrium between the $^{39}$Ar calculated production rate and radioactive decay rate leads to a partial isotopic abundance $^{39}$Ar/Ar$= (5.9\pm 1.8) \times 10^{-16}$, showing that $\approx$73% of atmospheric $^{39}$Ar is produced by cosmogenic neutrons. The $^{40}$Ar($n,2n$)$^{39}$Ar cross section at 14 MeV is also a key parameter for quantifying the anthropogenic contribution to atmospheric $^{39}$Ar produced during the thermonuclear tests of the 1960s. We estimate that anthropogenic $^{39}$Ar accounts for roughly 20% of the present atmospheric inventory.

nucl-ex↗

arXiv2025

First measurement of 87Rb(α, xn) cross sections at weak r-process energies in supernova ν-driven ejecta to investigate elemental abundances in low-metallicity stars

Observed abundances of Z ~ 40 elements in metal-poor stars vary from star to star, indicating that the rapid and slow neutron capture processes may not contribute alone to the synthesis of elements beyond iron. The weak r-process was proposed to produce Z ~ 40 elements in a subset of old stars. Thought to occur in the ν-driven ejecta of a core-collapse supernova, (α, xn) reactions would drive the nuclear flow toward heavier masses at T = 2-5 GK. However, current comparisons between modelled and observed yields do not bring satisfactory insights into the stellar environment, mainly due to the uncertainties of the nuclear physics inputs where the dispersion in a given reaction rate often exceeds one order of magnitude. Involved rates are calculated with the statistical model where the choice of an α-optical-model potential (αOMP) leads to such a poor precision. The first experiment on 87Rb(α, xn) reactions at weak r-process energies is reported here. Total inclusive cross sections were assessed at Ec.m. = 8.1 - 13 MeV (3.7 - 7.6 GK) with the active target MUlti-Sampling Ionization Chamber (MUSIC). With a N = 50 seed nucleus, the measured values agree with statistical model estimates using the αOMP Atomki-V2. A re-evaluated reaction rate was incorporated into new nucleosynthesis calculations, focusing on ν-driven ejecta conditions known to be sensitive to this specific rate. These conditions were found to fail to reproduce the lighter-heavy element abundances in metal-poor stars.

nucl-ex↗

arXiv2024

Direct cross-section measurement of the weak r-process 88Sr(α,n)91Zr reaction in ν-driven winds of core collapse supernovae

About half of the heavy elements beyond iron are known to be produced by the rapid neutron capture process, known as r-process. However, the astrophysical site producing the r-process is still uncertain. Chemical abundances observed in several cosmic sites indicate that different mechanisms should be at play. For instance, the abundances around silver measured in a subset of metal-poor stars indicate the presence of a weak r-process. This process may be active in neutrino-driven winds of core collapse supernovae where ($α$,n) reactions dominate the synthesis of Z ~ 40 elements in the expelled materials. Scarcely measured, the rates of ($α$,n) reactions are determined from statistical Hauser-Feshbach calculations with $α$-optical-model potentials, which are still poorly constrained. The uncertainties of the ($α$,n) reaction rates therefore make a significant contribution to the uncertainties of the abundances determined from stellar modeling. In this work, the $^{88}$Sr($α$,n)$^{91}$Zr reaction which impacts the weak r-process abundances has been probed at astrophysics energy for the first time; directly measuring the total cross sections at astrophysical energies of 8.37 - 13.09 MeV in the center of mass (3.8 - 7.5 GK). Two measurements were performed at ATLAS with the electrically-segmented ionization chamber MUSIC, in inverse kinematics, while following the active target technique. The cross sections of this $α$-induced reaction on $^{88}$Sr, located at the shell closure N = 50, have been found to be lower than expected, by a factor of 3, despite recent statistical calculations validated by measurements on neighboring nuclei. This result encourages more experimental investigations of ($α$,n) reactions, at N = 50 and towards the neutron-rich side, to further test the predictive power and reliability of such calculations.

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arXiv2023

Measuring the cross section of the $^{15}$N($α$,$γ$)$^{19}$F reaction using a single-fluid bubble chamber

$^{15}$N($α$,$γ$)$^{19}$F is believed to be the primary means of stellar nucleosynthesis of fluorine. Here we present the use of a single-fluid bubble chamber to measure the cross section of the time-inverse photo-dissociation reaction. The method benefits from a luminosity increase of several orders of magnitude due to the use of a thicker liquid target -- when compared to thin films or gas targets -- and from the reciprocity theorem. We discuss the results of an experiment at the Thomas Jefferson National Accelerator Facility, where the cross section of the photodisintegration process $^{19}$F($γ$, $α$)$^{15}$N was measured by bombarding a superheated fluid of C$_3$F$_8$ with bremsstrahlung $γ$-rays produced by impinging a 4 - 5.5 MeV electron beam on a Cu radiator. From the photodissociation yield the cross section was extracted by performing a convolution with a Monte Carlo-generated $γ$-ray beam spectrum. The measurement produced a cross section that was then time inverted using the reciprocity theorem. The cross section for the $^{15}$N($α$,$γ$)$^{19}$F reaction was determined down to a value in the range of hundreds of picobarns. With further improvements of the experimental setup the technique could potentially push cross section measurements down to the single picobarn range.

nucl-ex↗

arXiv2022

Study of the $^{22}$Mg waiting point relevant for x-ray burst nucleosynthesis via the $^{22}$Mg($α$,$p$)$^{25}$Al reaction

The $^{22}$Mg($α$,$p$)$^{25}$Al reaction rate has been identified as a major source of uncertainty for understanding the nucleosynthesis flow in Type-I x-ray bursts (XRBs). We report a direct measurement of the energy- and angle-integrated cross sections of this reaction in a 3.3-6.9 MeV center-of-mass energy range using the MUlti-Sampling Ionization Chamber (MUSIC). The new $^{22}$Mg($α$,$p$)$^{25}$Al reaction rate is a factor of $\sim$4 higher than the previous direct measurement of this reaction within temperatures relevant for XRBs, resulting in the $^{22}$Mg waiting point of x-ray burst nucleosynthesis flow to be significantly bypassed via the ($α,p$) reaction

nucl-ex↗

arXiv2022

First direct measurement of the $^{13}$N($α$,$p$)$^{16}$O reaction relevant for core-collapse supernovae nucleosynthesis

Understanding the explosion mechanism of a core-collapse supernova (CCSN) is important to accurately model CCSN scenarios for different progenitor stars using model-observation comparisons. The uncertainties of various nuclear reaction rates relevant for CCSN scenarios strongly affect the accuracy of these stellar models. Out of these reactions, the $^{13}$N($α$,$p$)$^{16}$O reaction has been found to affect various stages of a CCSN at varying temperatures. This work presents the first direct measurement of the $^{13}$N($α$,$p$)$^{16}$O reaction performed using a 34.6 MeV beam of radioactive $^{13}$N ions and the active-target detector MUSIC (MUlti-Sampling Ionization Chamber) at Argonne National Laboratory. The resulting total $^{13}$N($α$,$p$)$^{16}$O reaction cross sections from this measurement in the center-of-mass energy range of 3.26 - 6.02 MeV are presented and compared with calculations using the Hauser-Feshbach formalism. Uncertainties in the reaction rate have been dramatically reduced at CCSN temperatures.

nucl-ex↗

arXiv2018

Stellar $^{36,38}$Ar$(n,γ)^{37,39}$Ar reactions and their effect on light neutron-rich nuclide synthesis

The $^{36}$Ar$(n,γ)^{37}$Ar ($t_{1/2}$ = 35 d) and $^{38}$Ar$(n,γ)^{39}$Ar (269 y) reactions were studied for the first time with a quasi-Maxwellian ($kT \sim 47$ keV) neutron flux for Maxwellian Average Cross Section (MACS) measurements at stellar energies. Gas samples were irradiated at the high-intensity Soreq applied research accelerator facility-liquid-lithium target neutron source and the $^{37}$Ar/$^{36}$Ar and $^{39}$Ar/$^{38}$Ar ratios in the activated samples were determined by accelerator mass spectrometry at the ATLAS facility (Argonne National Laboratory). The $^{37}$Ar activity was also measured by low-level counting at the University of Bern. Experimental MACS of $^{36}$Ar and $^{38}$Ar, corrected to the standard 30 keV thermal energy, are 1.9(3) mb and 1.3(2) mb, respectively, differing from the theoretical and evaluated values published to date by up to an order of magnitude. The neutron capture cross sections of $^{36,38}$Ar are relevant to the stellar nucleosynthesis of light neutron-rich nuclides; the two experimental values are shown to affect the calculated mass fraction of nuclides in the region A=36-48 during the weak $s$-process. The new production cross sections have implications also for the use of $^{37}$Ar and $^{39}$Ar as environmental tracers in the atmosphere and hydrosphere.

nucl-ex↗

arXiv2016

Study of $\boldsymbol{(α,p)}$ and $\boldsymbol{(α,n)}$ reactions with a Multi-Sampling Ionization Chamber

A large number of $(α,p)$ and $(α,n)$ reactions are known to play a fundamental role in nuclear astrophysics. This work presents a novel technique to study these reactions with the active target system MUSIC whose segmented anode allows the investigation of a large energy range of the excitation function with a single beam energy. In order to verify the method, we performed a direct measurements of the previously measured reactions $^{17}$O$(α,n)^{20}$Ne, $^{23}$Na$(α,p)^{26}$Mg, and $^{23}$Na$(α,n)^{26}$Al. These reactions were investigated in inverse kinematics using $^{4}$He gas in the detector to study the excitation function in the range of about 2 to 6 MeV in the center of mass. We found good agreement between the cross sections of the $^{17}$O$(α,n)^{20}$Ne reaction measured in this work and previous measurements. Furthermore we have successfully performed a simultaneous measurement of the $^{23}$Na$(α,p)^{26}$Mg and $^{23}$Na$(α,n)^{26}$Al reactions.

nucl-ex↗

arXiv2016

Study of the astrophysically important $\boldsymbol{^{23}\mathrm{Na}(α,p)^{26}\mathrm{Mg}}$ and $\boldsymbol{^{23}\mathrm{Na}(α,n)^{26}\mathrm{Al}}$ reactions

The $^{23}$Na$(α,p)^{26}$Mg and $^{23}$Na$(α,n)^{26}$Al reactions are important for our understanding of the $^{26}$Al abundance in massive stars. The aim of this work is to report on a direct and simultaneous measurement of these astrophysically important reactions using an active target system. The reactions were investigated in inverse kinematics using $^{4}$He as the active target gas in the detector. We measured the excitation functions in the energy range of about 2 to 6 MeV in the center of mass. We have found that the cross sections of the $^{23}$Na$(α,p)^{26}$Mg and the $^{23}$Na$(α,n)^{26}$Al reactions are in good agreement with previous experiments, and with statistical model calculations.

nucl-ex↗

arXiv2015

High-Sensitivity Measurement of 3He-4He Isotopic Ratios for Ultracold Neutron Experiments

Research efforts ranging from studies of solid helium to searches for a neutron electric dipole moment require isotopically purified helium with a ratio of 3He to 4He at levels below that which can be measured using traditional mass spectroscopy techniques. We demonstrate an approach to such a measurement using accelerator mass spectroscopy, reaching the 10e-14 level of sensitivity, several orders of magnitude more sensitive than other techniques. Measurements of 3He/4He in samples relevant to the measurement of the neutron lifetime indicate the need for substantial corrections. We also argue that there is a clear path forward to sensitivity increases of at least another order of magnitude.

physics.ins-det↗

arXiv2015

Bubble Chambers for Experiments in Nuclear Astrophysics

A bubble chamber has been developed to be used as an active target system for low energy nuclear astrophysics experiments. Adopting ideas from dark matter detection with superheated liquids, a detector system compatible with gamma-ray beams has been developed. This detector alleviates some of the limitations encountered in standard measurements of the minute cross sections of interest to stellar environments. While the astrophysically relevant nuclear reaction processes at hydrostatic burning temperatures are dominated by radiative captures, in this experimental scheme we measure the time-reversed processes. Such photodisintegrations allow us to compute the radiative capture cross sections when transitions to excited states of the reaction products are negligible. Due to the transformation of phase space, the photodisintegration cross sections are up to two orders of magnitude higher. The main advantage of the new target-detector system is a density several orders of magnitude higher than conventional gas targets. Also, the detector is virtually insensitive to the gamma-ray beam itself, thus allowing us to detect only the products of the nuclear reaction of interest. The development and the operation as well as the advantages and disadvantages of the bubble chamber are discussed.

nucl-ex↗

arXiv2012

First determination of an astrophysical cross section with a bubble chamber: the 15N(α,γ)19F reaction

We have devised a technique for measuring some of the most important nuclear reactions in stars which we expect to provide considerable improvement over previous experiments. Adapting ideas from dark matter search experiments with bubble chambers, we have found that a superheated liquid is sensitive to recoils produced from γ-rays photodisintegrating the nuclei of the liquid. The main advantage of the new target-detector system is a gain in yield of six orders of magnitude over conventional gas targets due to the higher mass density of liquids. Also, the detector is practically insensitive to the γ-ray beam itself, thus allowing it to detect only the products of the nuclear reaction of interest. The first set of tests of a superheated target with a narrow bandwidth γ-ray beam was completed and the results demonstrate the feasibility of the scheme. The new data are successfully described by an R-matrix model using published resonance parameters. With the increase in luminosity of the next generation γ-ray beam facilities, the measurement of thermonuclear rates in the stellar Gamow window would become possible.

astro-ph.IM↗

arXiv2012

A Shorter 146Sm Half-Life Measured and Implications for 146Sm-142Nd Chronology in the Solar System

The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its alpha-decay daughter 142Nd. Based on analyses of 146Sm/147Sm alpha-activity and atom ratios, we determined the half-life of 146Sm to be 68 \pm 7 (1sigma) million years (Ma), which is shorter than the currently used value of 103 \pm 5 Ma. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm_0 = 0.0094\pm0.0005 (2sigma), than previously estimated. Terrestrial, Lunar and Martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)_0 values.

astro-ph.EP↗

arXiv2011

Upper Limit on the molecular resonance strengths in the ${}^{12}$C+${}^{12}$C fusion reaction

Carbon burning is a crucial process for a number of important astrophysical scenarios. The lowest measured energy is around E$_{\rm c.m.}$=2.1 MeV, only partially overlapping with the energy range of astrophysical interest. The currently adopted reaction rates are based on an extrapolation which is highly uncertain because of potential resonances existing in the unmeasured energy range and the complication of the effective nuclear potential. By comparing the cross sections of the three carbon isotope fusion reactions, ${}^{12}$C+${}^{12}$C, ${}^{12}$C+${}^{13}$C and ${}^{13}$C+${}^{13}$C, we have established an upper limit on the molecular resonance strengths in ${}^{12}$C+${}^{12}$C fusion reaction. The preliminary results are presented and the impact on nuclear astrophysics is discussed.

nucl-ex↗

arXiv2011

Fusion reactions with the one-neutron halo nucleus 15C

The structure of 15C, with an s1/2 neutron weakly bound to a closed-neutron shell nucleus 14C, makes it a prime candidate for a one-neutron halo nucleus. We have for the first time studied the cross section for the fusion-fission reaction 15C + 232Th at energies in the vicinity of the Coulomb barrier and compared it to the yield of the neighboring 14C + 232Th system measured in the same experiment. At sub-barrier energies, an enhancement of the fusion yield by factors of 2-5 was observed for 15C, while the cross sections for 14C match the trends measured for 12,13C.

nucl-ex↗

arXiv2008

Nuclear Structure Relevant to Neutrinoless Double Beta Decay: the Valence Protons in 76Ge and 76Se

The possibility of observing neutrinoless double beta decay offers the opportunity of determining the effective neutrino mass if the nuclear matrix element were known. Theoretical calculations are uncertain and the occupations of valence orbits by nucleons active in the decay are likely to be important. The occupation of valence proton orbits in the ground states of 76Ge, a candidate for such decay, and 76Se, the corresponding daughter nucleus, were determined by precisely measuring cross sections for proton-removing transfer reactions. As in previous work on neutron occupations, we find that the Fermi surface for protons is much more diffuse than previously thought, and the occupancies of at least three orbits change significantly between the two 0+ ground states.

nucl-ex↗

arXiv2007

Nuclear Structure Relevant to Neutrinoless Double Beta Decay: 76Ge and 76Se

The possibility of observing neutrinoless double beta decay offers the opportunity of determining the neutrino mass IF the nuclear matrix element were known. Theoretical calculations are uncertain and measurements of the occupations of valence orbits by nucleons active in the decay can be important. The occupation of valence neutron orbits in the ground states of 76Ge and 76Se were determined by precisely measuring cross sections for both neutron-adding and removing transfer reactions. Our results indicate that the Fermi surface is much more diffuse than in theoretical (QRPA) calculations. We find that the populations of at least three orbits change significantly between these two ground states while in the calculations the changes are confined primarily to one orbit.

nucl-ex↗