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Employing the Argonne Fragment Mass Analyzer and the implantation-decay-decay correlation technique, a weak 0.50(21)% proton decay branch was identified in 108I for the first time. The 108I proton-decay width is consistent with a hindered l = 2 emission, suggesting a d 5 2 origin. Using the extracted 108I proton-decay Q value of 597(13) keV, and the Q α values of the 108I and 107Te isotopes, a proton-decay Q value of 510(20) keV for 104Sb was deduced. Similarly to the 112,113Cs proton-emitter pair, the Q p ( I 108 ) value is lower than that for the less-exotic neighbor 109I, possibly due to enhanced proton-neutron interactions in N ≈ Z nuclei. In contrast, the present Q p ( Sb 104 ) is higher than that of 105Sb, suggesting a weaker interaction energy. For the present Q p ( Sb 104 ) value, network calculations with the one-zone X-ray burst model Mazzocchi et al. (2007) [18] predict no significant branching into the Sn-Sb-Te cycle at 103Sn.

At Argonne we have enhanced the AMS capabilities of the Argonne Tandem Linac Accelerator System (ATLAS) to attempt to minimize crosstalk between samples and to quickly move from one (M/Q) setting to another in order to make measurements on a large number of samples provided by the MANTRA (Measurement of Actinide Neutron Transmutation Rates by Accelerator mass spectrometry) project. Those improvements include the use of a picosecond laser to ablate actinide material into the source, a new 20-sample holder that can switch samples within 1–2 min, and a number of accelerator configuration improvements that allow quick and precise switching between species. In principle, AMS can provide production yields of actinide isotopes produced during the irradiation period at a sensitivity exceeding other mass spectrometry techniques. A total of 27 irradiated samples of a variety of actinides have been provided for measurement. We discuss our experience with these facility improvements and how well we have met our performance goals. In addition, we present preliminary results on a number of the irradiated actinide samples with this approach and compare those results to Multi-Collector ICPMS measurements.

Radiative neutron capture reactions play an important role in nuclear astrophysics. In some cases direct neutron capture reaction studies are not possible and neutron transfer reactions have been suggested as a surrogate approach. We have performed a detailed study of the $^{25}\mathrm{Mg}(d,p)^{26}\mathrm{Mg}$ reaction at a beam energy of 56 MeV as a surrogate reaction to the radiative neutron capture reaction $^{25}\mathrm{Mg}(n,\ensuremath{\gamma})^{26}\mathrm{Mg}$. A large number of neutron bound and unbound states between 10.6 and 12.1 MeV excitation energy in $^{26}\mathrm{Mg}$ were observed. Angular distribution analysis provided information about the orbital momentum transfer populating these levels. The comparison with resonances observed in the $^{25}\mathrm{Mg}(n,\ensuremath{\gamma})^{26}\mathrm{Mg}$ reaction indicate that different levels in $^{26}\mathrm{Mg}$ are being populated through the two reaction mechanisms, causing substantial discrepancies in the reaction-rate prediction. This result demonstrates that neutron transfer reaction studies may not necessarily lead to reliable predictions for neutron capture reaction rates.

Low-lying negative-parity states in $^{11}\mathrm{Be}$ having dominant $p$-wave neutron configurations were studied using the $^{12}\mathrm{B}(d,^{3}\mathrm{He})\phantom{\rule{0.16em}{0ex}}^{11}\mathrm{Be}$ proton-removal reaction in inverse kinematics. The $1/{2}_{1}^{\ensuremath{-}}$ state at 0.32 MeV, the $3/{2}_{1}^{\ensuremath{-}}$ state at 2.56 MeV, and one or both of the states including the $5/{2}_{1}^{\ensuremath{-}}$ level at 3.89 MeV and the $3/{2}_{2}^{\ensuremath{-}}$ level at 3.96 MeV were populated in the present reaction. Spectroscopic factors were determined from the differential cross sections using a distorted wave Born approximation method. The $p$-wave proton removal strengths were well described by the shell model calculations while the Nilsson model calculation underestimates the spectroscopic factors for the higher excited states. Results from both variational Monte Carlo and no-core shell-model calculations were also compared with the experimental observations.

This paper describes the production and characterization of a 26Al beam comprised of both, its 5 + ground state, and its 0 + isomeric state. The 26Al beam was produced in-flight via the p (26Mg, 26Al)n reaction. The isomer fraction of the 26Al beam was maximized by choosing a bombarding energy of 158.5 MeV for the 26Mg primary beam. The resulting beam had an energy of 120 MeV, a total intensity of 2 × 10 5 particles/sec, a purity of 98% and an isomer content of 70%. This high-quality 26Al isomeric beam was used to study the 26Al m (d, p)27Al reaction relevant for understanding the nucleosynthesis of 26Al in the Galaxy.

A large number of (α,p)(α,p) and (α,n)(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 direct measurements of the previously measured reactions 17O (α,n)(α,n)20Ne, 23Na (α,p)(α,p)26Mg, and 23Na (α,n)(α,n)26Al. These reactions were investigated in inverse kinematics using 4He gas in the detector to study the excitation functions in the energy range of about 2–6 MeV in the center of mass. We found good agreement between the cross sections of the 17O (α,n)(α,n)20Ne reaction measured in this work and previous measurements. Furthermore we have successfully performed a simultaneous measurement of the 23Na (α,p)(α,p)26Mg and 23Na (α,n)(α,n)26Al reactions.

We report the first observation of the ^{108}Xe→^{104}Te→^{100}Sn α-decay chain. The α emitters, ^{108}Xe [E_{α}=4.4(2) MeV, T_{1/2}=58_{-23}^{+106} μs] and ^{104}Te [E_{α}=4.9(2) MeV, T_{1/2}

New experimental data for the $^{38}$Ar($\alpha$,n)$^{41}$Ca and $^{38}$Ar($\alpha$,p)$^{41}$K reactions were used to find a best-fit set of parameters for statistical model calculations. The very good agreement between the experimental data and the best-fit calculation confirms the applicability of the statistical model for nuclei in the vicinity of the doubly-magic $^{40}$Ca despite of their relatively low level densities. The present study investigates the sensitivities and finds that the $\alpha$-nucleus and the nucleon-nucleus potentials are the most important ingredients for the calculation of ($\alpha$,n) and ($\alpha$,p) reactions in the statistical model. Furthermore, the width fluctuation correction plays an essential role in the peculiar case of $^{38}$Ar. The best-fit parameters from $^{38}$Ar are applied to the mirror nucleus $^{38}$Ca and the neighboring $^{36}$Ar and $^{40}$Ar nuclei. For $^{38}$Ca this results in an astrophysical reaction rate of the $^{38}$Ca($\alpha$,p)$^{41}$Sc reaction which has a flatter temperature dependence compared to all previous calculations. For $^{40}$Ar a better reproduction of $^{40}$Ar($\alpha$,p)$^{43}$K data from literature is obtained. The disagreement between calculation and an early experimental data point for the $^{36}$Ar($\alpha$,p)$^{39}$K reaction persists.

The ^{36}Ar(n,γ)^{37}Ar (t_{1/2}=35 d) and ^{38}Ar(n,γ)^{39}Ar (269 yr) reactions were studied for the first time with a quasi-Maxwellian (kT∼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) 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.

The single-neutron configurations of several low-lying states in $^{22}\mathrm{F}$ have been determined from neutron-adding cross sections of a ($d,p$) reaction using a $^{21}\mathrm{F}$ radioactive beam in inverse kinematics. Final states in $^{22}\mathrm{F}$ consisting of a proton in the $\ensuremath{\pi}0{d}_{5/2}$ orbital coupled to a neutron in either the $\ensuremath{\nu}0{d}_{5/2}$, $\ensuremath{\nu}1{s}_{1/2}$, or $\ensuremath{\nu}0{d}_{3/2}$ orbitals were observed up to an excitation energy of $\ensuremath{\sim}5$ MeV. Spectroscopic factors and strengths were determined from the angular distribution of the cross sections using a distorted wave Born approximation method. The distribution of the $\ensuremath{\nu}0{d}_{5/2}$ and $\ensuremath{\nu}1{s}_{1/2}$ strength was well described by all USD effective shell model interactions, while only the more recent USDA/USDB interactions showed a marked improvement in describing the observed $\ensuremath{\nu}0{d}_{3/2}$ strength. Diagonal two-body matrix elements of the ${(0{d}_{5/2})}_{J=0--5}^{2}$ effective nucleon-nucleon interaction were extracted from the data and compared with previous determinations of the same matrix elements from particle-particle and hole-hole spectra, as well as the calculated results of the USDA interactions. No significant discrepancies were observed. Inspection of the monopole energies showed that an improved agreement between the different empirical matrix elements is found if a mass dependence is included.

In massive stars, the $^{41}\mathrm{Ca}(n,\ensuremath{\alpha})^{38}\mathrm{Ar}$ and $^{41}\mathrm{K}(p,\ensuremath{\alpha})^{38}\mathrm{Ar}$ reactions have been identified as the key reactions governing the abundance of $^{41}\mathrm{Ca}$, which is considered as a potential chronometer for solar system formation. So far, due to experimental limitations, the $^{41}\mathrm{Ca}(n,\ensuremath{\alpha})^{38}\mathrm{Ar}$ reaction rate is solely based on statistical model calculations. In the present study, we have measured the time-inverse $^{38}\mathrm{Ar}(\ensuremath{\alpha},n)^{41}\mathrm{Ca}$ and $^{38}\mathrm{Ar}(\ensuremath{\alpha},p)^{41}\mathrm{K}$ reactions using an active target detector. The reactions were studied in inverse kinematics using a 133-MeV $^{38}\mathrm{Ar}$ beam and $^{4}\mathrm{He}$ as the active-gas target. Both excitation functions were measured simultaneously in the energy range of $6.8\ensuremath{\le}{E}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\le}9.3$ MeV. Using detailed balance the $^{41}\mathrm{Ca}(n,\ensuremath{\alpha})^{38}\mathrm{Ar}$ and $^{41}\mathrm{K}(p,\ensuremath{\alpha})^{38}\mathrm{Ar}$ reaction rates were determined, which suggested a 20% increase in the $^{41}\mathrm{Ca}$ yield from massive stars.

A beam containing a substantial component of both the J^{π}=5^{+}, T_{1/2}=162 ns isomeric state of ^{18}F and its 1^{+}, 109.77-min ground state is utilized to study members of the ground-state rotational band in ^{19}F through the neutron transfer reaction (d,p) in inverse kinematics. The resulting spectroscopic strengths confirm the single-particle nature of the 13/2^{+} band-terminating state. The agreement between shell-model calculations using an interaction constructed within the sd shell, and our experimental results reinforces the idea of a single-particle-collective duality in the descriptions of the structure of atomic nuclei.

International audience; Carbon burning is a critical phase for nucleosynthesis in massive stars. The conditions for igniting this burning stage, and the subsequent isotope composition of the resulting ashes, depend strongly on the reaction rate for C12+C12 fusion at very low energies. Results for the cross sections for this reaction are influenced by various backgrounds encountered in measurements at such energies. In this paper, we report on a new measurement of C12+C12 fusion cross sections where these backgrounds have been minimized. It is found that the astrophysical S factor exhibits a maximum around Ecm=3.5–4.0 MeV, which leads to a reduction of the previously predicted astrophysical reaction rate.

Reaction rates involving α-particles are known to be important in different astrophysical scenarios. Direct measurements of these reactions at relevant astrophysical energies are experimentally challenging. This is due to the typically small cross sections of these reactions and the experimental difficulties associated with low intensity radioactive beams needed to study them. As a consequence, most of these reaction rates are still unknown. The MUlti-Sampling Ionization Chamber (MUSIC) detector, a simple and highly efficient active target system, has been developed for measurements of (α, n) and (α, p) reactions. Recent results on the direct measurement of (α, p) and (α, n) reactions in the MUSIC detector and future possibilities are presented in this work.

The existence of $^{26}\mathrm{Al}$ (${t}_{1/2}=7.17\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{ }\mathrm{yr}$) in the interstellar medium provides a direct confirmation of ongoing nucleosynthesis in the Galaxy. The presence of a low-lying ${0}^{+}$ isomer (${^{26}\mathrm{Al}}^{m}$), however, severely complicates the astrophysical calculations. We present for the first time a study of the ${^{26}\mathrm{Al}}^{m}(d,p)^{27}\mathrm{Al}$ reaction using an isomeric $^{26}\mathrm{Al}$ beam. The selectivity of this reaction allowed the study of $\ensuremath{\ell}=0$ transfers to $T=1/2$, and $T=3/2$ states in $^{27}\mathrm{Al}$. Mirror symmetry arguments were then used to constrain the ${^{26}\mathrm{Al}}^{m}(p,\ensuremath{\gamma})^{27}\mathrm{Si}$ reaction rate and provide an experimentally determined upper limit of the rate for the destruction of isomeric $^{26}\mathrm{Al}$ via radiative proton capture reactions, which is expected to dominate the destruction path of ${^{26}\mathrm{Al}}^{m}$ in asymptotic giant branch stars, classical novae, and core collapse supernovae.

The $\ensuremath{\nu}0{h}_{9/2}$ and $\ensuremath{\nu}0{i}_{13/2}$ strength at $^{137}\mathrm{Xe}$, a single neutron outside the $N=82$ shell closure, has been determined using the $^{136}\mathrm{Xe}(\ensuremath{\alpha},^{3}\mathrm{He})^{137}\mathrm{Xe}$ reaction carried out at 100 MeV. We confirm the recent observation of the second $13/{2}^{+}$ state and reassess previous data on the $9/{2}^{\ensuremath{-}}$ states, obtaining spectroscopic factors. These new data provide additional constraints on predictions of the same single-neutron excitations at $^{133}\mathrm{Sn}$.

International audience; The 12C+12C fusion reaction is one of the most important for nuclear astrophysics since it determines the carbon ignition in stellar environments. Two experiments which make use of the gamma-particle coincidence technique to measure the 12C+12C S-factors at deep sub barrier energies are discussed. Results are presented showing a decrease of the S-factor below Ec.m. = 3 MeV.

CITATION: Long, A. M., et al. 2017. Indirect study of the stellar ³⁴ Ar(α,p) ³⁷K reaction rate through ⁴⁰Ca(p,t) ³⁸Ca reaction measurements. Physical Review C, 95(5):1-9, doi:10.1103/PhysRevC.95.055803.

Carbon burning plays a crucial role in stellar evolution, where this reaction is an important route for the production of heavier elements. A particle-γ coincidence technique that minimizes the backgrounds to which this reaction is subject and provides reliable cross sections has been used at the Argonne National Laboratory to measure fusion cross-sections at deep sub-barrier energies in the 12 C+12 C system. The corresponding excitation function has been extracted down to a cross section of about 6 nb. This indicates the existence of a broad S-factor maximum for this system. Experimental results are presented and discussed.

The γ - summing technique was successfully implemented for the first time in inverse kinematics using the NSCL Summing NaI(Tl) detector at the Nuclear Science Laboratory of the University of Notre Dame. This new method offers a way to measure (p,γ) and (α,γ) reactions on unstable nuclei, which are critical for an improved understanding of nucleosynthesis. As a proof of principle, the well-known resonances at E c . m . = 956 keV in the p(27Al,γ)28Si reaction and at E c . m . = 1400 keV in the p(58Ni,γ)59Cu reaction were measured and their resonance strengths were deduced. The values of the present work agree well with the reported literature values, demonstrating the viability of the technique.

The $^{23}\mathrm{Na}(\ensuremath{\alpha},p)^{26}\mathrm{Mg}$ and $^{23}\mathrm{Na}(\ensuremath{\alpha},n)^{26}\mathrm{Al}$ reactions are important for our understanding of the $^{26}\mathrm{Al}$ 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 $^{4}\mathrm{He}$ 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 $^{23}\mathrm{Na}(\ensuremath{\alpha},p)^{26}\mathrm{Mg}$ and the $^{23}\mathrm{Na}(\ensuremath{\alpha},n)^{26}\mathrm{Al}$ reactions are in good agreement with previous experiments and with statistical-model calculations. The astrophysical reaction rate of the $^{23}\mathrm{Na}(\ensuremath{\alpha},n)^{26}\mathrm{Al}$ reaction has been reevaluated and it was found to be larger than the recommended rate.

A high density supersonic helium jet gas target has been developed for the newly installed St. George Recoil Mass Separator, at the Nuclear Science Laboratory, University of Notre Dame. The jet's properties for both helium and nitrogen have been studied by means of elastic scattering and energy loss experiments. The helium jet's full width at half maximum was found to be 2.1 mm, assuming axially symmetric gas expansion with a maximum target thickness of (2.67±0.16)×10 17 atoms/cm 2 at 1500 mbar of inlet pressure, and well confined within the jet region.

The $^{90}\mathrm{Zr}(\ensuremath{\alpha},\ensuremath{\gamma})^{94}\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}^{92}\mathrm{Zr}(\ensuremath{\alpha},\ensuremath{\gamma})^{96}\mathrm{Mo}$, and $^{74}\mathrm{Ge}(\ensuremath{\alpha},\ensuremath{\gamma})^{78}\mathrm{Se}$ reaction cross sections were measured for the first time in an effort to expand the existing experimental database for $(\ensuremath{\alpha},\ensuremath{\gamma})$ reactions relevant for the production of $p$ nuclei in the universe. In particular, the $^{90}\mathrm{Zr}(\ensuremath{\alpha},\ensuremath{\gamma})^{94}\mathrm{Mo}$ reaction was identified by a sensitivity study for its potential impact on the $\ensuremath{\gamma}$-process mass flow in the region of light $p$ nuclei. The measurements were performed for energies ${E}_{\ensuremath{\alpha}}=9.5--12.0$ MeV at the University of Notre Dame using the SuN detector and the $\ensuremath{\gamma}$-summing technique. The results are compared to theoretical calculations from the talys and non-smoker nuclear reaction codes, and it is shown that the data greatly reduce the uncertainty in the cross section for the measured energies. The talys parameters that provide the best description of the experimental data are reported.

A systematic measurement of the $(\ensuremath{\alpha},\ensuremath{\gamma})$ reaction for all the stable nickel isotopes has been performed using the $\ensuremath{\gamma}$-summing technique. For two of the isotopes, $^{60}\mathrm{Ni}$ and $^{61}\mathrm{Ni}$, the $\ensuremath{\alpha}$-capture cross sections have been experimentally measured for the first time. For $^{58,62,64}\mathrm{Ni}$, the current measurement is in excellent agreement with earlier results found in the literature, and additionally extends the energy range of the measured cross sections up to 8.7 MeV. The data provided a tool for testing the cross section predictions of Hauser-Feshbach calculations. The experimental results were compared to the cross sections calculated with the talys 1.6 code and commonly used databases non-smoker and bruslib. For each of the investigated isotopes a combination of input parameter for talys was identified that best reproduces the experimental data, and recommended reaction rate has been calculated. Additionally, a set of inputs for Hauser-Feshbach calculations was given that, simultaneously for all the isotopes under consideration, reproduces the experimental data within the experimental uncertainties.

A highly efficient MUlti-Sampling Ionization Chamber (MUSIC) detector has been developed for measurements of fusion reactions. A study of fusion cross sections in the 10,12,13,14,15 C+ 12 C and 20,22 Ne+ 20,22 Ne systems has been performed at ATLAS. Experimental results and comparison with theoretical predictions are presented. Furthermore, results of direct measurements of the 17 O( α, n ) 20 Ne, 23 Ne( α, p ) 26 Mg and 23 Ne( α, n ) 26 Al reactions will be discussed.

The explosive nuclear burning of hydrogen at high temperatures and densities on the surface of accreting neutron stars, known as the rp-process, gives rise to a number of observable phenomena related to X-ray bursts. Recent astronomical observations provide unprecedented information, e.g. on atomic abundances in ejecta and time structure in X-ray bursts. To interpret these data requires an understanding of the nuclear processes during the explosive events and, therefore, information on the structure of unstable, proton-rich nuclei. Network model calculations show that the dominant burning processes, after breakout from the hot CNO cycles at sufficiently high temperatures, proceed via proton- and α-induced reactions. Since the reaction rates are very sensitive to the nuclear structure, shell- and statistical-model calculations are often insufficient in predicting exact reaction paths. Therefore, we have been conducting experiments using high-resolution spectrometers to search for resonances that play a role in the rp-process and determine the nuclear-structure, most importantly excitation energies, accurately above the particle thresholds. The techniques and examples of experimental results will be presented and discussed. The status and outlook of direct measurements of stellar reactions rates in inverse kinematics using planned recoil separators will be outlined.

International audience; We present a status report of recent neutron capture experiments performed with the mA-proton beam (1.92 MeV, 3 kW) of the Soreq Applied Research Accelerator Facility (SARAF) and the Liquid-Lithium Target (LiLiT). Experiments and preliminary results for (n) reactions on 36,38Ar, studied for the first time with 30-keV neutrons, on natKr, natCe and on radioactive targets 147Pm and 171Tm are described.