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First generation, or Population III, stars have a different evolution than those of later generations owing to their initial primordial abundance composition. Most notably, the lack of carbon, oxygen, and nitrogen means that primordial massive stars must rely on the less efficient p-p chains, thereby requiring the star to contract to reach temperatures high enough to eventually trigger 3α reactions. Even small amounts of the C12(α,γ)O16 reactions begin feeding the CNO mass range and enable the CNO cycle to generate energy, but this occurs at higher temperature compared to later stellar generations. It is currently controversial if the observed enhanced abundances of Ca in the most metal-poor stars could be a result of the high temperature H-burning conditions in the first massive stars. The level of this enrichment depends on the hot breakout path from the CNO cycles via the F19(p,γ)Ne20 reaction. In this work, the rates of both the F19(p,γ)Ne20 and competing F19(p,α)O16 reactions are re-evaluated using the phenomenological R-matrix approach, simultaneously considering several F19(p,γ)Ne20, F19(p,α)O16, and F19(p,p)F19 data sets, to better characterize the rate uncertainties. It is found that the rate uncertainty for F19(p,γ)Ne20 reaction is considerably larger than previously reported. This is the result of undetermined interferences between observed resonances, a possible threshold state, possible subthreshold states, direct capture, and background levels. Additional experimental measurements are therefore needed to determine if F19(p,γ)Ne20 CNO breakout is responsible for Ca enrichment in metal-poor stars. Astrophysically, the breakout reaction revision makes it less likely that Ca observed in the most Fe-poor stars can originate in hot CNO breakout H-burning nucleosynthtesis, thereby casting doubt on the prevailing faint supernova scenario to explain the abundances observed in these stars.

The CNO cycle is the main energy source in stars more massive than our sun, it defines the energy production and the cycle time that lead to the lifetime of massive stars, and it is an important tool for the determination of the age of globular clusters. One of the largest uncertainties in the CNO chain of reactions comes from the uncertainty in the $^{14}$N$(p,\gamma)^{15}$O reaction rate. This uncertainty arises predominantly from the uncertainty in the lifetime of the sub-threshold state in $^{15}$O at $E_{x}$ = 6792 keV. Previous measurements of this state's lifetime are significantly discrepant. Here, we report on a new lifetime measurement of this state, as well as the excited states in $^{15}$O at $E_{x}$ = 5181 keV and $E_{x}$ = 6172 keV, via the $^{14}$N$(p,\gamma)^{15}$O reaction at proton energies of $E_{p} = 1020$ keV and $E_{p} = 1570$ keV. The lifetimes have been determined with the Doppler-Shift Attenuation Method (DSAM) with three separate, nitrogen-implanted targets with Mo, Ta, and W backing. We obtained lifetimes from the weighted average of the three measurements, allowing us to account for systematic differences between the backing materials. For the 6792 keV state, we obtained a $\tau = 0.6 \pm 0.4$ fs. To provide cross-validation of our method, we measured the known lifetimes of the states at 5181 keV and 6172 keV to be $\tau = 7.5 \pm 3.0$ and $\tau = 0.7 \pm 0.5$ fs, respectively, which are in good agreement with previous measurements., Comment: 10 pages, 13 figures

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.

The $^{13}\mathrm{C}(\ensuremath{\alpha},n)^{16}\mathrm{O}$ reaction is the main source of neutrons for the $s$ and $i$ processes in asymptotic giant branch stars and carbon-enhanced metal-poor stars, respectively. The reaction rate over the relevant temperature range from 0.1 to 0.3 GK translates into a center-of-mass energy range of 150 to 540 keV. Current measurements extend down to 300 keV, still requiring an extrapolation of the cross section. At these low energies, the high-energy tail of a $1/{2}^{+}$ state near the reaction threshold makes a significant contribution to the cross section, but its amplitude is still highly uncertain. In this paper the uncertainties associated with the low-energy cross-section extrapolation are investigated, in particular the sensitivity to the energy, width, and Coulomb renormalized asymptotic normalization coefficient of the near-threshold resonance. Recently it has been suggested that the energy of the near-threshold level may have a large impact on the extrapolation, but this is not found to be the case compared with the other sources of uncertainty.

Nucleosynthesis in the first generation of massive stars offers a unique setting to explore the creation of the first heavier nuclei in an environment free of impurities from earlier stellar generations. In later generations of massive stars, hydrogen burning occurs predominantly through the CNO cycles, but without the carbon, nitrogen, and oxygen to catalyze the reaction sequence, first stars would have to rely on the inefficient $pp$ chains for their energy production. Observations of second and third generation stars show pronounced abundances of carbon and oxygen isotopes, which suggests a rapid conversion of the primordial abundances to heavier elements. While the triple-alpha-process primarily facilitates this conversion, there are alternative reaction sequences, such as $^{2}\mathrm{H}(\ensuremath{\alpha},\ensuremath{\gamma})^{6}\mathrm{Li}(\ensuremath{\alpha},\ensuremath{\gamma})^{10}\mathrm{B}(\ensuremath{\alpha},n)^{13}\mathrm{N}$, that may play a significant role. To study such alternate reaction pathways for production of carbon and heavier nuclei, a number of new measurements are needed. In this work, new measurements are reported for the $^{10}\mathrm{B}(\ensuremath{\alpha},n)^{13}\mathrm{N}$ reaction, extending the cross section down to 575 keV incident $\ensuremath{\alpha}$-particle energy. The measurements were made using a state-of-the-art deuterated liquid scintillator and a spectrum unfolding technique. An $R$-matrix analysis was performed in order to facilitate a comparison of the underlying nuclear structure with the reaction measurements. An unexpected upturn is observed in the low-energy $S$ factor that indicates the presence of a new low-energy resonance. A revised reaction rate is determined that takes into account the present data as well as other previous measurements from the literature that were previously neglected.

International Conference on Nuclear Data for Science and Technology, ND 2019, Bejing, China, 19 May 2019 - 24 May 2019; The European physical journal / Web of Conferences 239, 01006 (2020). doi:10.1051/epjconf/202023901006, Published by EDP Sciences, Les Ulis

The National Ignition Facility (NIF) provides the opportunity to study nuclear reactions under controlled conditions at high temperatures and pressures at a level never before achieved. However, the timescale of the deuterium-tritium (DT) implosion is only a few nanoseconds, making data collection and diagnostics very challenging. One method that has been proposed for obtaining additional information about the conditions of the implosion is to activate a dopant material using the $\ensuremath{\alpha}$ particles produced from the DT fuel as a diagnostic. The yield of the activated material can give a measure of the mixing that occurs in the capsule. One of the reactions that has been proposed is $^{10}\mathrm{B}(\ensuremath{\alpha},n)\phantom{\rule{0.16em}{0ex}}^{13}\mathrm{N}$ as it produces a radioactive reactant product with a convenient half-life of $\ensuremath{\approx}10\phantom{\rule{0.16em}{0ex}}\mathrm{min}$. Although this reaction has several advantages for the application at hand, it has not seen much study in the present literature, resulting in large uncertainties in the cross section. Furthermore, for the current application, the cross section must be well characterized. With this motivation, the $^{10}\mathrm{B}(\ensuremath{\alpha},n)\phantom{\rule{0.16em}{0ex}}^{13}\mathrm{N}$ cross section has been remeasured for $2.2l{E}_{\ensuremath{\alpha}}l4.9\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ with the angle-integrated ground-state cross section reported for the first time. The present results, combined with previous measurements, allow for a determination of the cross section to a significantly higher degree of accuracy and precision than obtained previously and are shown to be consistent with thick-target measurements. Preliminary calculations are performed to test the feasibility of this reaction as a diagnostic for a NIF implosion.

Reactions that populate the 15N system have implications for nucleosynthesis through the 11B(α, n)14N and 14N(n, p)14C reactions and the 14N(n, p)14C reaction is also a key component in modeling atmospheric 14C production. A convenient characteristic of this system is that the α-particle, proton, and neutron separation energies are all within ≍1 MeV of one another. Further, it has been observed that 11B+α, 14N+n and 14C+p induced reactions all populate many of the same resonances near their reaction thresholds. This strongly facilitates the simultaneous analysis of data for all three of these entrance partitions using a global R-matrix analysis, which in turn provides a method of comparing the consistency among the different experimental measurements. In this work, a new measurement has been performed for the 11B(α, n)14N reaction, which gives a more accurate description of the cross section, in particular over an important interference region. This new data is combined with results from previous measurements, which populate a similar excitation energy range in the 15N system, to produce a global fit that includes 11B(α, n)14N reaction data for the first time.

Background: The reaction $^{20}\mathrm{Ne}(p,\ensuremath{\gamma})^{21}\mathrm{Na}$ influences the nucleosynthesis of Ne, Na, and Mg isotopes while contributing to hydrogen burning in several stellar sites, such as red giants, asymptotic giant branch (AGB) stars, massive stars, and oxygen-neon (ONe) novae. In the relevant temperature range for these environments ($T$ = 0.05--0.5 GK), the main contributions to this reaction rate are from the direct capture process as well as the high-energy tail of a subthreshold resonance in the ground-state transition at ${E}_{x}$ = 2425 keV in the $^{21}\mathrm{Na}$ compound nucleus.Purpose: The previous measurement of this reaction reports cross sections with large uncertainties for the ground-state transition. At higher energies, where the subthreshold resonance makes a smaller contribution to the total cross section, only upper limits are provided. This work aims to reduce the uncertainty in the cross section where direct capture dominates, as well as provide cross-section data in previously unmeasured regions.Method: The $^{20}\mathrm{Ne}(p,\ensuremath{\gamma})^{21}\mathrm{Na}$ reaction was measured over a wide proton energy range (${E}_{p}$ = 0.5--2.0 MeV) at ${\ensuremath{\theta}}_{\text{lab}}$ = ${90}^{\ensuremath{\circ}}$. Transitions to the ground state and to the 332 and 2425 keV excited states were observed. The primary transitions to these three bound states were utilized in an $R$-matrix analysis to determine the contributions of the direct capture and the subthreshold resonance to the total cross section.Results: The cross sections of the present measurements have been found to be in good agreement with the previous data at low energy. Significantly improved cross-section measurements have been obtained over the ${E}_{p}$ = 1300--1900 keV region. The narrow resonance at ${E}_{\text{c.m.}}$ = 1113 $\mathrm{keV}({E}_{x}$ = 3544.3 keV) has also been remeasured and its strength has been found to be in good agreement with previous measurements.Conclusions: An extrapolation of the $S$ factor of $^{20}\mathrm{Ne}(p,\ensuremath{\gamma})^{21}\mathrm{Na}$ has been made to low energies using the $R$-matrix fit. The reaction rate from the subthreshold resonance was found to be the main contributor to the reaction rate at temperatures below about 0.1 GK. The present rate is lower in the temperature range of interest than those presented in current reaction rate libraries by up to 20%.

It is crucial to measure reaction cross sections relevant to the astrophysical $\ensuremath{\gamma}$ process so that theoretical reaction rates can be tested and validated with experimental data. The total cross sections for the $^{162}\mathrm{Er}(p,\ensuremath{\gamma})^{163}\mathrm{Tm}$ and the $^{162}\mathrm{Er}(p,n)^{162}\mathrm{Tm}$ reactions have been measured by the activation method in center-of-mass energies from 3.973 to 8.944 MeV and from 5.962 to 8.944 MeV, respectively. The nucleus $^{162}\mathrm{Er}$ is the heaviest $p$ nuclide to be measured by the activation method using $\ensuremath{\gamma}$-ray spectroscopy, so far. It is important to note that the energy range for the $(p,\ensuremath{\gamma})$ reaction measurement covers a large fraction of the astrophysically relevant energy region between 2.71 and 5.34 MeV. The targets were prepared by evaporating $28.2%$ isotopically enriched $^{162}\mathrm{Er}_{2}{\mathrm{O}}_{3}$ powder onto carbon backing foils, and bombarded with proton beams provided by the FN Tandem Accelerator at the University of Notre Dame. The reaction yields have been determined by the observed activity of produced radioactive isotopes, which was detected offline by a high-purity germanium detector. The results are presented and compared with calculations from two statistical model codes: non-smoker and talys.

The creation of carbon and oxygen in our Universe is one of the forefront questions in nuclear astrophysics. The determination of the abundance of these elements is key to our understanding of both the formation of life on Earth and to the life cycles of stars. While nearly all models of different nucleosynthesis environments are affected by the production of carbon and oxygen, a key ingredient, the precise determination of the reaction rate of C12(α, 3)O16, has long remained elusive. This is owed to the reaction's inaccessibility, both experimentally and theoretically. Nuclear theory has struggled to calculate this reaction rate because the cross section is produced through different underlying nuclear mechanisms. Isospin selection rules suppress the E1 component of the ground state cross section, creating a unique situation where the E1 and E2 contributions are of nearly equal amplitudes. Experimentally there have also been great challenges. Measurements have been pushed to the limits of state-of-the-art techniques, often developed for just these measurements. The data have been plagued by uncharacterized uncertainties, often the result of the novel measurement techniques that have made the different results challenging to reconcile. However, the situation has markedly improved in recent years, and the desired level of uncertainty ‰10% may be in sight. In this review the current understanding of this critical reaction is summarized. The emphasis is placed primarily on the experimental work and interpretation of the reaction data, but discussions of the theory and astrophysics are also pursued. The main goal is to summarize and clarify the current understanding of the reaction and then point the way forward to an improved determination of the reaction rate.

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.

Background: The $^{11}\mathrm{B}{(p,2\ensuremath{\alpha})}^{4}\mathrm{He}$ reaction is being discussed as a prime candidate for advanced aneutronic fusion fuel systems. Particular interest in this reaction has recently emerged for laser driven plasma systems for energy generation and jet-propulsion systems. The lack of long-lived radioactive reaction products has been suggested as the main advantage of proton-boron fusion fuel. However, 19% of natural boron is $^{10}\mathrm{B}$, with the $^{10}\mathrm{B}(p,\ensuremath{\alpha})^{7}\mathrm{Be}$ fusion reaction producing long-lived $^{7}\mathrm{Be}$ as a side product.Purpose: A detailed measurement of the $^{10}\mathrm{B}(p,\ensuremath{\alpha})^{7}\mathrm{Be}$ reaction over the critical energy range of hot fusion plasma environments will help to determine the amount of $^{7}\mathrm{Be}$ radioactivity being produced. This information can be used in turn to monitor the actual fusion temperature by offline measurement of the extracted $^{7}\mathrm{Be}$ activity. The goal of the here presented experiment is to expand on the results of earlier experiments, covering a wider energy range of interest for aneutronic plasma fusion applications, including also both $^{10}\mathrm{B}(p,{\ensuremath{\alpha}}_{0})^{7}\mathrm{Be}$ and the $^{10}\mathrm{B}(p,{\ensuremath{\alpha}}_{1})^{7}\mathrm{Be}$ reaction channels.Method: The reaction cross section was measured over a wide energy range from ${E}_{p}=400$ to 1000 keV using particle detection and from ${E}_{p}=80$ to 1440 keV using $\ensuremath{\gamma}$-ray spectroscopic techniques. Reaction $\ensuremath{\alpha}$ particles were measured at different angles to obtain angular distribution information. The results are discussed in terms of an $R$-matrix analysis.Results: The cross section data cover a wider energy range than previously investigated and bridge a gap in the previously available data sets. The cross sections show good agreement with previous results in the low energy region and show that the $^{10}\mathrm{B}(p,{\ensuremath{\alpha}}_{0})^{7}\mathrm{Be}$ channel is considerably larger than that of the $^{10}\mathrm{B}(p,{\ensuremath{\alpha}}_{1})^{7}\mathrm{Be}$ channel up to ${E}_{p}\ensuremath{\approx}1\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$.Conclusions: The new reaction data provides important new information about the reaction cross section over the entire energy range of plasma fusion facilities. This data, when coupled with previous measurement of the competing $^{10}\mathrm{B}(p,\ensuremath{\gamma})^{11}\mathrm{C}$ reaction, will provide the opportunity for an extensive $R$-matrix analysis of the rather complex level structure in the $^{11}\mathrm{C}$ compound nucleus system.

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.

Ultra-weak resonances in nuclear reactions impose a challenge on experimentalists. The present work describes the setup used in the first direct measurement of extremely weak low energy resonances of the 18O(α, γ)22Ne reaction. A Clover HPGe detector in coincidence with specified energy windows in BGO counters covering a large solid angle, and combined with large plastic veto counters led to a significantly improved sensitivity, thus allowing for the clear identification of specific γ-transitions. Different modes of operation have been tested for optimizing the final experimental setup.

High demand for nuclear reaction cross-section measurements relevant for astrophysical processes requires the development of new experimental techniques that allow for investigation of, often very low, cross-sections. For this purpose, a new 4 π Summing NaI(Tl) detector (SuN) has been constructed, which is an 8-fold segmented NaI(Tl) barrel read by 24 photomultipliers. The design of the detector will be presented and detailed results of the commissioning experiments utilizing standard calibration sources and known Al 27 ( p , γ ) Si 28 resonances will be discussed.

This is the accepted version of the following article: Z. Halasz, E. Somorjai, Gy. Gyurky, Z. Elekes, Zs. Fulop, T. Szucs, G. G. Kiss, N. T. Szegedi, T. Rauscher, J. Gorres, and M. Wiescher (2016), Experimental study of the astrophysical γ -process reaction Xe 124 (α,γ) Ba 128, Physical Review C 94, 045801, which has been published in final form at https://doi.org/10.1103/PhysRevC.94.045801. ©2016 American Physical Society

The {beta}{sup +}/EC decay of mass-separated A=80 nuclei was studied by means of {beta}- and {gamma}-ray spectroscopy after the bombardment of a {sup 24}Mg target with {sup 58}Ni ions at 190 MeV. A new 1{sup {minus}} isomer at 228.5(1) keV has been identified in {sup 80}Y with a half-life of 4.7(3) s. This new isomer decays to the 4{sup {minus}} ground state with an 81(2){percent} branch and by {beta}{sup +}/EC transitions to levels in {sup 80}Sr with a 19(2){percent} branch. The half-life of the {sup 80}Y ground-state decay has been remeasured to be 30.1(5) s. A level scheme for the low-lying states in {sup 80}Y is presented. Hartree-Fock-Bogolyubov calculations show a large prolate deformation for these states, and two-quasiparticle + rotor model calculation results suggest that the low-lying states can be reproduced by inclusion of an effective proton-neutron residual interaction. The dominating Nilsson configurations for the 4{sup {minus}} ground state and the 1{sup {minus}} isomeric state have been found to originate from the parallel and antiparallel coupling of the proton [422]5/2{sup +} and the neutron [301]3/2{sup {minus}} orbitals. {copyright} {ital 1998} {ital The American Physical Society}

Background: The CNO cycle is the main energy source in stars more massive than our sun; it defines the energy production and the cycle time that lead to the lifetime of massive stars, and it is an important tool for the determination of the age of globular clusters. In our sun about 1.6% of the total solar neutrino flux comes from the CNO cycle. The largest uncertainty in the prediction of this CNO flux from the standard solar model comes from the uncertainty in the $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ reaction rate; thus, the determination of the cross section at astrophysical temperatures is of great interest.Purpose: The total cross section of the $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ reaction has large contributions from the transitions to the ${E}_{x}=6.79\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ excited state and the ground state of $^{15}\mathrm{O}$. The ${E}_{x}=6.79\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ transition is dominated by radiative direct capture, while the ground state is a complex mixture of direct and resonance capture components and the interferences between them. Recent studies have concentrated on cross-section measurements at very low energies, but broad resonances at higher energy may also play a role. A single measurement has been made that covers a broad higher-energy range but it has large uncertainties stemming from uncorrected summing effects. Furthermore, the extrapolations of the cross section vary significantly depending on the data sets considered. Thus, new direct measurements have been made to improve the previous high-energy studies and to better constrain the extrapolation.Methods: Measurements were performed at the low-energy accelerator facilities of the nuclear science laboratory at the University of Notre Dame. The cross section was measured over the proton energy range from ${E}_{p}=0.7$ to 3.6 MeV for both the ground state and the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ transitions at ${\ensuremath{\theta}}_{\text{lab}}={0}^{\ensuremath{\circ}}$, ${45}^{\ensuremath{\circ}}$, ${90}^{\ensuremath{\circ}}$, ${135}^{\ensuremath{\circ}}$, and ${150}^{\ensuremath{\circ}}$. Both TiN and implanted-$^{14}\mathrm{N}$ targets were utilized. $\ensuremath{\gamma}$ rays were detected by using an array of high-purity germanium detectors.Results: The excitation function as well as angular distributions of the two transitions were measured. A multichannel $R$-matrix analysis was performed with the present data and is compared with previous measurements. The analysis covers a wide energy range so that the contributions from broad resonances and direct capture can be better constrained.Conclusion: The astrophysical $S$ factors of the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ and the ground-state transitions were extrapolated to low energies with the newly measured differential-cross-section data. Based on the present work, the extrapolations yield ${S}_{6.79}(0)=1.29\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.09(\mathrm{syst})\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b}$ and ${S}_{\text{g.s.}}(0)=0.42\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b}$. While significant improvement and consistency is found in modeling the ${E}_{x}=6.79\phantom{\rule{4.pt}{0ex}}\mathrm{MeV}$ transition, large inconsistencies in both the $R$-matrix fitting and the low-energy data are reaffirmed for the ground-state transition. Reflecting this, a systematic uncertainty of ${}_{\ensuremath{-}0.19}^{+0.09}\phantom{\rule{4pt}{0ex}}\mathrm{keV}\phantom{\rule{0.16em}{0ex}}\mathrm{b}$ is recommended for the ground-state transition.

Background: The $^{22}\mathrm{Ne}(\ensuremath{\alpha},n)^{25}\mathrm{Mg}$ reaction is the dominant neutron source for the slow neutron capture process ($s$ process) in massive stars, and contributes, together with $^{13}\mathrm{C}(\ensuremath{\alpha},n)^{16}\mathrm{O}$, to the production of neutrons for the $s$ process in asymptotic giant branch (AGB) stars. However, the reaction is endothermic and competes directly with $^{22}\mathrm{Ne}(\ensuremath{\alpha},\ensuremath{\gamma})^{26}\mathrm{Mg}$ radiative capture. The uncertainties for both reactions are large owing to the uncertainty in the level structure of $^{26}\mathrm{Mg}$ near the $\ensuremath{\alpha}$ and neutron separation energies. These uncertainties affect the $s$-process nucleosynthesis calculations in theoretical stellar models.Purpose: Indirect studies in the past have been successful in determining the energies and the $\ensuremath{\gamma}$-ray and neutron widths of the $^{26}\mathrm{Mg}$ states in the energy region of interest. But, the high Coulomb barrier hinders a direct measurement of the resonance strengths, which are determined by the $\ensuremath{\alpha}$ widths for these states. The goal of the present experiments is to identify the critical resonance states and to precisely measure the $\ensuremath{\alpha}$ widths by $\ensuremath{\alpha}$-transfer techniques.Methods: The $\ensuremath{\alpha}$-inelastic scattering and $\ensuremath{\alpha}$-transfer measurements were performed on a solid $^{26}\mathrm{Mg}$ target and a $^{22}\mathrm{Ne}$ gas target, respectively, using the Grand Raiden Spectrometer at the Research Center for Nuclear Physics in Osaka, Japan. The $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}})$ measurements were performed at $0.{45}^{\ensuremath{\circ}}, 4.{1}^{\ensuremath{\circ}}, 8.{6}^{\ensuremath{\circ}}$, and $11.{1}^{\ensuremath{\circ}}$ and the $(^{6}\mathrm{Li},d)$ measurements at ${0}^{\ensuremath{\circ}}$ and ${10}^{\ensuremath{\circ}}$. The scattered $\ensuremath{\alpha}$ particles and deuterons were detected by the focal plane detection system consisting of multiwire drift chambers and plastic scintillators. The focal plane energy calibration allowed the study of $^{26}\mathrm{Mg}$ levels from ${E}_{x}$ = 7.69--12.06 MeV in the $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}})$ measurement and ${E}_{x}$ = 7.36--11.32 MeV in the $(^{6}\mathrm{Li},d)$ measurement.Results: Six levels (${E}_{x}$ = 10717, 10822, 10951, 11085, 11167, and 11317 keV) were observed above the $\ensuremath{\alpha}$ threshold in the region of interest (10.61--11.32 MeV). The $\ensuremath{\alpha}$ widths were calculated for these states from the experimental data. The results were used to determine the $\ensuremath{\alpha}$-capture induced reaction rates.Conclusion: The energy range above the $\ensuremath{\alpha}$ threshold in $^{26}\mathrm{Mg}$ was investigated using a high resolution spectrometer. A number of states were observed for the first time in $\ensuremath{\alpha}$-scattering and $\ensuremath{\alpha}$-transfer reactions. The excitation energies and spin-parities were determined. Good agreement is observed for previously known levels in $^{26}\mathrm{Mg}$. From the observed resonance levels the ${E}_{x}$ = 10717 keV state has a negligible contribution to the $\ensuremath{\alpha}$-induced reaction rates. The rates are dominated in both reaction channels by the resonance contributions of the states at ${E}_{x}$ = 10951, 11167, and 11317 keV. The ${E}_{x}$ = 11167 keV state has the most appreciable impact on the $(\ensuremath{\alpha},\ensuremath{\gamma})$ rate and therefore plays an important role in the prediction of the neutron production in $s$-process environments.

The natural neutron background influences the maximum achievable sensitivity in most deep underground nuclear, astroparticle and double-beta decay physics experiments. Reliable neutron flux numbers are an important ingredient in the design of the shielding of new large-scale experiments as well as in the analysis of experimental data. Using a portable setup of He-3 counters we measured the thermal neutron flux at the Kimballton Underground Research Facility, the Soudan Underground Laboratory, on the 4100 ft and the 4850 ft levels of the Sanford Underground Research Facility, at the Waste Isolation Pilot Plant and at the Gran Sasso National Laboratory. Absolute neutron fluxes at these laboratories are presented., Preparation version for submission

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 setup and experimental techniques for measurements of zero-degree inelastic scattering and reactions involving light ions with the K=600 magnetic spectrometer at iThemba LABS are described. Measurements were performed for inelastic proton scattering at an incident energy of 200 MeV for targets ranging from 27Al to 208Pb. An energy-resolution of 45 keV (FWHM) was achieved by utilizing the faint-beam dispersion-matching technique. A background subtraction procedure was applied and allowed for the extraction of excitation energy spectra with low background. Measurements of the (p,t) reaction at zero degrees for Ep=100 and 200 MeV benefited from the difference in magnetic rigidity between the reaction products and the beam particles, resulting in background-free spectra with an excitation energy-resolution of 32 and 48 keV (FWHM), respectively, and a scattering angle resolution of 0.55° (FWHM). The addition of Double Sided Silicon Strip Detectors (DSSSD) at backward scattering angles allowed for coincident measurements of particle-decay of states excited in the (p,t) reaction at E p = 200 MeV .

The neutron long counter NERO was built at the National Superconducting Cyclotron Laboratory (NSCL), Michigan State University, for measuring β -delayed neutron-emission probabilities. The detector was designed to work in conjunction with a β -delay implantation station, so that β decays and β -delayed neutrons emitted from implanted nuclei can be measured simultaneously. The high efficiency of about 40%, for the range of energies of interest, along with the small background, are crucial for measuring β -delayed neutron emission branchings for neutron-rich r-process nuclei produced as low intensity fragmentation beams in in-flight separator facilities.

Implanted neon targets were produced for nuclear astrophysics experiments at the Ruhr-Universitat Bochum, and their characteristics were studied at the University of Notre Dame. The Ne ions were implanted sequentially at two different energies to create a more uniform depth distribution. The targets were stable under high beam loads. To reduce the amount of oxygen contamination on the target’s surface, a procedure of chemical cleaning and thermal outgassing of targets was developed. The impact of this treatment on implanted Ne targets was investigated and found to reduce the oxygen amount by a factor of 4. The depth profile of the implanted Ne atoms was studied via narrow (p, γ) resonances while the oxygen contamination was monitored using the Deuteron-Induced γ -ray Emission (DIGE) method.

The study of low energy radiative capture reactions is of prime interest for many nuclear astrophysics scenarios. While many measurements have been made using various setups in direct kinematics, the very small cross section of these reactions at astrophysically interesting energies, along with the beam induced background, limit the possible range of measurements. A facility for measuring cross sections of radiative 4He capture in inverse kinematics has been designed. This 20 m long recoil mass separator (St. George) will be coupled to the KN Van de Graaff accelerator at the University of Notre Dame. The goal of St. George is to separate a low mass ( A ⩽ 40 ) , low energy ( E 0.5 MeV / A ) , high intensity ( ∼ 100 μ A ) beam from the few reaction products emerging from a windowless 4He jet gas target. The desired acceptance is Δ θ = ± 40 mrad and Δ E / E = ± 7.5 % .

Neutron stars in close binary star systems often accrete matter from their companion stars. Thermonuclear ignition of theaccretedmaterialintheatmosphereof theneutronstarleadstoathermonuclearexplosion,whichisobservedas an X-ray burst occurring periodically between hours and days depending on the accretion rate. The ignition conditions are characterized bya sensitive interplay between the accretion rate of the fuel supply and its depletion rate by nuclear burning in the hot CNO cycle and the rp-process. For accretion rates close to stable burning the burst ignition therefore depends critically on the hot CNO breakout reaction 15 O(�; � ) 19 Ne that regulates the flow between the hot CNO cycle and the rapid proton capture process. Until recently, the 15 O(�; � ) 19 Ne reaction rate was not known experimentally and the theoretical estimates carried significant uncertainties. In this paper we perform a parameter study of the uncertainty of this reaction rate and determine the astrophysical consequences of the first measurement of this reaction rate. Our results corroborate earlier predictions and show that theoretically burning remains unstable up to accretion rates near the Eddington limit, in contrast to astronomical observations. Subject headingg nuclear reactions, nucleosynthesis, abundances — stars: neutron — X-rays: bursts

Thermonuclear stellar explosions depend critically on the available fuel and the ignition through nuclear reactions. In this paper we intend to demonstrate the sensitive interplay between nuclear fuel supply and depletion by nuclear reaction processes at high temperature and densities. We demonstrate this on the example of X-ray bursts which are explained as thermonuclear explosions in the atmosphere of an accreting neutron star. These bursts occur periodically between hours and days depending on the accretion rate. The burst itself is triggered by the 15O(α,γ)19Ne reaction regulating the flow between the β -limited hot CNO cycle and the rapid proton capture process. The 18Ne(α,p)21Na reaction on the other hand provides a continuous processing flow of light 4He fuel towards heavier elements. The reaction rates of both α capture processes carry significant uncertainties. In this paper we report on the measurements of the critical nuclear parameters for the determination of the rate of the 15O(α,γ)19Ne and the 18Ne(α,p)21Na reactions. We demonstrate and analyze the impact of these experimental results on the burst pattern and burst periodicity for a range of accretion rates.

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.

This paper reports on the first cross section measurement of the $^{72}\mathrm{Ge}(p,\ensuremath{\gamma})^{73}\mathrm{As}$ reaction. The proton capture reaction on $^{72}\mathrm{Ge}$ is relevant to the astrophysical $p$-process and was identified in a sensitivity study as one of the important reactions required to estimate the abundances of the light $p$ nuclei. The $\ensuremath{\gamma}$-summing technique was employed using the summing NaI(Tl) detector (SuN) from the National Superconducting Cyclotron Laboratory. The experiment was performed at the University of Notre Dame using a 1.8--3.6 MeV proton beam. In order to test the predictive power of different theoretical calculations in the region, experimental values were compared to the results given by the nuclear reaction code talys1.6. The theoretical uncertainties in the cross section arising from different combinations of nuclear level densities, $\ensuremath{\gamma}$-ray strength functions, and optical model potentials were reduced to 10--18% by the experimental data. The recommended reaction rates from the standard astrophysical libraries, BRUSLIB and REACLIB, are found to be in good agreement with the experimental results.

Background: The slowest reaction in the first CNO cycle is $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$, therefore its rate determines the overall energy production efficiency of the entire cycle. The cross section presents several strong resonance contributions, especially for the ground-state transition. Some of the properties of the corresponding levels in the $^{15}\mathrm{O}$ compound nucleus remain uncertain, which affects the uncertainty in extrapolating the capture cross section to the low energy range of astrophysical interest.Purpose: The $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ cross section can be described by using the phenomenological $R$ matrix. Over the energy range of interest, only the proton and $\ensuremath{\gamma}$-ray channels are open. Since resonance capture makes significant contributions to the $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ cross section, resonant proton scattering data can be used to provide additional constraints on the $R$-matrix fit of the capture data.Methods: A 4 MV KN Van de Graaff accelerator was used to bombard protons onto a windowless gas target containing enriched $^{14}\mathrm{N}$ gas over the proton energy range from ${E}_{p}=1.0$ to 3.0 MeV. Scattered protons were detected at ${\ensuremath{\theta}}_{\text{lab}}=90, {120}^{\ensuremath{\circ}}, {135}^{\ensuremath{\circ}}, {150}^{\ensuremath{\circ}}$, and ${160}^{\ensuremath{\circ}}$ using ruggedized silicon detectors. In addition, a 10 MV FN Tandem Van de Graaff accelerator was used to accelerate protons onto a solid Adenine (${\mathrm{C}}_{5}{\mathrm{H}}_{5}{\mathrm{N}}_{5}$) target, of natural isotopic abundance, evaporated onto a thin self-supporting carbon backing, over the energy range from ${E}_{p}=1.8$ to 4.0 MeV. Scattered protons were detected at 28 angles between ${\ensuremath{\theta}}_{\text{lab}}=30.{4}^{\ensuremath{\circ}}$ and $167.{7}^{\ensuremath{\circ}}$ by using silicon photodiode detectors.Results: Relative cross sections were extracted from both measurements. While the relative cross sections do not provide as much constraint as absolute measurements, they greatly reduce the dependence of the data on otherwise significant systematic uncertainties, which are more difficult to quantify. The data are fit simultaneously using an $R$-matrix analysis and level energies and proton widths are extracted. Even with relative measurements, the statistics and large angular coverage of the measurements result in more confident values for the energies and proton widths of several levels; in particular, the broad resonance at ${E}_{\text{c.m.}}=2.21$ MeV, which corresponds to the $3/{2}^{+}$ level at ${E}_{x}=9.51$ MeV in $^{15}\mathrm{O}$. In particular, the $s$- and $d$-wave angular-momentum channels are separated.Conclusion: The relative cross sections provide a consistent set of data that can be used to better constrain a full multichannel $R$-matrix extrapolation of the capture data. It has been demonstrated how the scattering data reduce the uncertainty through a preliminary Monte Carlo uncertainty analysis, but several other issues remain that make large contributions to the uncertainty, which must be addressed by further capture and lifetime measurements.

This paper provides a detailed analysis of the main component of the slow neutron capture process (the s-process), which accounts for the solar abundances of half of the nuclei with 90, Comment: 43 pages, 17 Figures, 4 Tables, Supporting Information

One of the two breakout reactions from the hot CNOcycle is 15O(a,g)19Ne, which at low temperatures depends strongly on the resonance strength of the 4.033 MeV state in 19Ne. An experimental upper limit has been placed on its strength, but the lower limit on the resonance strength and thereby the astrophysical reaction rate is unconstrained experimentally. However, this breakout reaction is crucial to the thermonuclear runaway which causes type I X-ray bursts on accreting neutron stars. In this paper we exploit astronomical observations in an attempt to constrain the relevant nuclear physics and deduce a lower limit on the reaction rate. Our sensitivity study implies that if the rate were sufficiently small, accreting material would burn stably without bursts. The existence of type I X-ray bursts and superbursts consequently suggests a lower limit on the 15O(a,g)19Ne reaction rate at low temperatures., Comment: 10 pages, 4 figures, uses apj.sty, accepted for publ. in Astrophys. J

We study how the production of the magnesium and aluminum isotopes in intermediate mass Asymptotic Giant Branch stars is affected by uncertainties in the 22Ne + α-capture reaction rates. We use new estimates of these rates with smaller uncertainties compared to NACRE and consequently find smaller uncertainties on the production of the magnesium and aluminum isotopes. The production of the aluminum isotopes and in particular the radioactive isotope 26Al is not greatly affected by variations in the α-capture rates and will be more sensitive to uncertainties in the proton-capture MgAl chain reactions. We compare our results to data from pre-solar spinel grains, where the magnesium isotopic ratios have been accurately measured.

The 112Sn(α, γ)116Te reaction cross-section has been measured to test the applicability of statistical models, especially NON-SMOKER in the energy range of importance for the astrophysical p-process nucleosynthesis. The measurements were carried out at the Notre Dame FN Tandem Van de Graaff accelerator by means of the activation method. Enriched self-supporting foils were irradiated with alpha beams over the alpha bombarding energy range of 8MeV to 12MeV in 0.5MeV steps. The induced activity was measured with a pair of large volume Ge Clover detectors in close geometry to maximize the detection efficiency. The preliminary results are compared with recent statistical model predictions using the code NON-SMOKER.

The 19F(α, p)22Ne reaction is considered to be the main source of fluorine depletion during the Asymptotic Giant Branch phase in stars. The reaction rate still retains large uncertainties due to the lack of experimental studies available. The yields for both the ground p0 and first excited state p1 exit channels of 22Ne have been measured with the University of Notre Dame KN van de Graaff accelerator. Ten resonances were found in the energy range E c m = 990 – 1570 keV and their strengths have been determined. A comparison between the rate calculated from the experimental data and other standard rates is provided.

In order to test the reliability of reaction rate calculations in the framework of the Hauser-Feshbach statistical model, the cross sections of Cd-106(alpha,gamma)Sn-110 radiative capture reaction were determined in the energy range close to the Gamow window for the astrophysical p-process scenario. In addition, to reduce the errors in the predictions of the statistical model calculations, the relevant optical potential parameters are to be determined from the angular distribution measurement of the Cd-106(alpha, alpha) Cd-106 elastic scattering reaction at low energies.

We report on new measurements of the 14 C(n, γ ) 15 C cross section with averaged neutron energies of 30, 150, and 500 keV using the fast cyclic activation technique.

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge up and the extent of the partial mixing zone. Then we perform a detailed analysis of the reaction rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(alpha,gamma)18O and the 19F(alpha,p)22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. The importance of the partial mixing zone is reduced when using our estimate for the 14C(alpha,gamma)18O rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, also in relation to reducing the uncertainties of the 14C(alpha,gamma)18O reaction rate. At the same time the possible effect of Cool Bottom Processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could in principle help matching the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

Precise mass measurements of nuclei along the N = Z line are important input parameters for simulations of the rp-process. Of particular interest is the mass of the 6834Se34 rp-process waiting point nucleus for determining the possibility of a two-proton capture branch bypassing its slow β-decay. The mass of 68Se was measured via the β-decay endpoint. 68Se was produced by the 12C(58Ni, 2n)68Se reaction and subsequently implanted onto a moving tape system using the fragment mass analyzer at the ATLAS facility of Argonne National Laboratory. A mass excess value of ( − 54189 ± 240 ) keV was determined from the β-endpoint measurement of Q EC = 4710 ( 200 ) keV . Evaluations of proton separation energies based on the measured mass were used in a one zone type I X-ray burst model. It is concluded that the N = Z = 34 isotope 68Se is a waiting point for the rp-process.

The fusion–evaporation reaction 10 B( 12 C, 2 n ) was used to make the first observation of in-beam γ decays from the astrophysically important nucleus 20 Na, lying adjacent to the proton drip-line. All states below the proton threshold in 20 Na were populated and identified in the experiment. These include new levels, previously unresolved levels, and states located with improved energy precision. The level structure of 20 Na, and its γ transitions, are compared to the mirror partner 20 F measured simultaneously in this experiment. In particular, a high degree of energy stability is found for all negative parity states. These results are discussed in the context of the nuclear shell model.

Low-energy resonances in the reaction 18 O(α,γ) have been measured using a highly efficient γ ray coincidence detector system. These resonances dominate the reaction rate for temperatures typical for TP-AGB stars (0.1–0.3 GK). Preliminary results reduce the reaction rate by up to a factor of three at these temperatures. While this leads to an enhancement of the fluorine production in TP-AGB stars, it is not sufficient to explain the observed fluorine abundances in the surface of red giants.

Total cross section measurements for the 102 Pd(p, γ ) 103 Ag and 116 Sn(p, γ ) 117 Sb reactions have been performed in the proton energy range 2.6 to 4.25 MeV, and for the 112 Sn( α , γ ) 116 Te reaction over the alpha beam energy range 7.0 to 10.5 MeV. An activation technique was used in which gamma rays from decays of the reaction products were detected off-line by two hyper-pure germanium detectors in a low background environment. Where possible, reaction rates are derived and the results compared to those of calculations generated by the NON-SMOKER and the MOST statistical model codes so as to judge their applicability for describing the cross sections needed for network calculations of nucleosynthesis in explosive astrophysical environments via the γ - and rp-processes.

Background: The $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ reaction is of critical importance in determining the flux of solar neutrinos through the $pp$-II and $pp$-III chains. For this reason and others, the description of the cross section and its extrapolation towards low-energy has always been a matter of intense debate. While large systematic differences have been present in the past, several recent measurements are all in excellent statistical agreement.Purpose: The convergence of the recent individual experimental measurements of the $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ reaction prompts a global analysis of the reaction data. From the combined data, a more precise estimate of the low-energy cross section can be determined.Results: A global $R$-matrix fit is used to describe the $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ data as well as scattering data over a similar energy range. The $R$-matrix fit is then subjected to a Monte Carlo analysis to extract the uncertainties on the cross section and corresponding reaction rate.Conclusion: By combining several recent measurements of the $^{3}\mathrm{He}(\ensuremath{\alpha},\ensuremath{\gamma})^{7}\mathrm{Be}$ reaction, the combined data yield a zero energy $S$ factor of $S(0)=0.542\ifmmode\pm\else\textpm\fi{}0.011(\text{MC}\phantom{\rule{4.pt}{0ex}}\text{fit})\ifmmode\pm\else\textpm\fi{}0.006{(\text{model})}_{\ensuremath{-}0.011}^{+0.019}(\text{phase}\phantom{\rule{4.pt}{0ex}}\text{shifts})$ keV b. This gives a total uncertainty in $S(0)$ of $+0.023/\ensuremath{-}0.017$ keV b.

Background: Optical potentials are crucial ingredients for the prediction of nuclear reaction rates needed in simulations of the astrophysical $\ensuremath{\gamma}$ process. Associated uncertainties are particularly large for reactions involving $\ensuremath{\alpha}$ particles. This includes $(\ensuremath{\gamma},\ensuremath{\alpha})$ reactions which are of special importance in the $\ensuremath{\gamma}$ process.Purpose: The measurement of $(\ensuremath{\alpha},n)$ reactions allows for an optimization of currently used $\ensuremath{\alpha}$-nucleus potentials. The reactions $^{165}\mathrm{Ho}$$(\ensuremath{\alpha},n)$ and $^{166}\mathrm{Er}$$(\ensuremath{\alpha},n)$ probe the optical model in a mass region where $\ensuremath{\gamma}$ process calculations exhibit an underproduction of $p$ nuclei which is not yet understood.Method: To investigate the energy-dependent cross sections of the reactions $^{165}\mathrm{Ho}$$(\ensuremath{\alpha},n)$ and $^{166}\mathrm{Er}$$(\ensuremath{\alpha},n)$ close to the reaction threshold, self-supporting metallic foils were irradiated with $\ensuremath{\alpha}$ particles using the FN tandem Van de Graaff accelerator at the University of Notre Dame. The induced activity was determined afterwards by monitoring the specific $\ensuremath{\beta}$-decay channels.Results: Hauser-Feshbach predictions with a widely used global $\ensuremath{\alpha}$ potential describe the data well at energies where the cross sections are almost exclusively sensitive to the $\ensuremath{\alpha}$ widths. Increasing discrepancies appear towards the reaction threshold at lower energy.Conclusions: The tested global $\ensuremath{\alpha}$ potential is suitable at energies above 14 MeV, while a modification seems necessary close to the reaction threshold. Since the $\ensuremath{\gamma}$ and neutron widths show non-negligible impact on the predictions, complementary data are required to judge whether or not the discrepancies found can be solely assigned to the $\ensuremath{\alpha}$ width.

Background: The ${}^{22}$Ne${(\ensuremath{\alpha},n)}^{25}$Mg reaction is an important source of neutrons for $s$-process nucleosynthesis. The neutron production from the reaction is quite sensitive to the low-energy cross section, which is dominated by narrow resonances. The high level density of the ${}^{26}\mathrm{Mg}$ compound nucleus above the $\ensuremath{\alpha}$ separation energy prevents simple extrapolations from higher energy and the high Coulomb barrier makes the direct measurements extremely difficult. For this reason, indirect methods must be employed to study the level properties of ${}^{26}\mathrm{Mg}$.Purpose: The current measurement utilizes the reaction ${}^{26}\mathrm{Mg}$${(\ensuremath{\gamma},n)}^{25}$Mg to probe the level structure of the ${}^{26}\mathrm{Mg}$ compound nucleus from the neutron-separation energy at 11.093 MeV up to ${E}_{x}\ensuremath{\approx}$ 12 MeV.Methods: The High-Intensity $\ensuremath{\gamma}$-ray Source of the Triangle Universities Nuclear Laboratory was used to bombard a $\ensuremath{\sim}$16 g sample of enriched ${}^{26}\mathrm{Mg}$ oxide and the resulting decay neutrons were detected with an array of nine liquid scintillator detectors. Neutron time-of-flight peaks with corresponding energies as low as $\ensuremath{\sim}$50 keV were detected. An efficiency measurement of the detectors was made at the University of Notre Dame's nuclear science laboratory to energies as low as 45 keV.Results: Five resonances were observed at ${E}_{\ensuremath{\gamma}}$ = 11.150, 11.289, 11.329, 11.506, and 11.749 MeV and their strengths have been extracted.Conclusion:. The resulting strengths at ${E}_{\ensuremath{\gamma}}$ = 11.289, 11.329, 11.506, and 11.749 MeV are in good agreement with previous measurements. The strength of the resonance at ${E}_{\ensuremath{\gamma}}$ = 11.150 MeV is somewhat lower than previously measured but is in reasonable agreement when systematic uncertainties are considered. The results are also consistent with those of ${}^{25}$Mg${(n,\ensuremath{\gamma})}^{26}$Mg studies where a comparison shows that many of the resonances observed here are the result of multiple unresolved narrow resonances.

Funding Details: PHY 08-22648, NSF, National Science Foundation; PHY 0969058, NSF, National Science Foundation; PHY 1102511, NSF, National Science Foundation