Search

The nature of the 1 − and 2 − excited states in Be 10 is studied using the Be 11 ( p , d ) transfer reaction in inverse kinematics at 10A MeV at TRIUMF ISAC-II, in particular to assess whether either of them can be considered as an excited halo state. The angular distributions for both states are extracted using deuteron- γ coincidences and analyzed using a transfer model taking into account one-step and two-step processes. A good fit of the angular distributions is obtained considering only the one-step process, whereby an inner p 3 / 2 neutron of Be 11 is removed, leaving the halo neutron intact. Higher-order processes however cannot be rejected. The small spectroscopic factors extracted suggest that the structure of both states is not uniquely halo-like, but rather display a more complex configuration mixing cluster and halo structures. Further insights are limited, as this experiment specifically probed the halo-like (but not cluster-like) Be 11 ( 1 / 2 + ) ⊗ ( ν p 3 / 2 ) − 1 configuration in both states.

A high-count-rate ionization chamber design with position-sensitivity has been developed and deployed at several accelerator facilities. Counting rates of ≥ 500 kHz with good Z -separation (up to 5% energy resolution) for particle identification have been demonstrated in a series of commissioning experiments. A position-sensitive capability, with a resolution of 3 mm, has been implemented for the first time to record position information and suppress pileup. The design and performance of the detectors are described.

A recoil mass separator SECAR has been designed for the purpose of studying low-energy ( p , γ ) and ( α , γ ) reactions in inverse kinematics with radioactive beams for masses up to about A = 65. Their reaction rates are of importance for our understanding of the energy production and nucleosynthesis during explosive hydrogen and helium burning. The radiative capture reactions take place in a windowless hydrogen or He gas target at the entrance of the separator, which consists of four Sections . The first Section selects the charge state of the recoils. The second and third Sections contain Wien Filters providing high mass resolving power to separate efficiently the intense beam from the few reaction products. In the following fourth Section , the reaction products are guided into a detector system capable of position, angle and time-of-flight measurements. In order to accept the complete kinematic cone of recoil particles including multiple scattering in the target in the center of mass energy range of 0.2 MeV to 3.0 MeV, the system must have a large polar angle acceptance of ± 25 mrad. This requires a careful minimization of higher order aberrations. The present system will be installed at the NSCL ReA3 accelerator and will be used with the much higher beam intensities of the FRIB facility when it becomes available.

An active target detector array, ANASEN, has been developed for nuclear reaction studies with rare isotope beams at low energies. It aims at measurements of the excitation functions for proton and α - particle elastic and inelastic scattering and direct measurements of ( α , p) reactions with exotic nuclei in inverse kinematics. ANASEN is composed of three types of charged particle detectors. The length of the active area is 340 mm and the total covered area is 1300 cm 2 (almost 3 π steradian solid angle coverage) providing high efficiency for experiments with low intensity radioactive beams. A mix of 78 conventional electronics channels (for Proportional Counter and CsI-detectors) and 480 dedicated high-density ASICs electronics channels for the silicon detector are used for readout.

This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21–23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9–10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12–13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade.

Results are presented from the first neutron-transfer measurement on $^{80}\mathrm{Ge}$ using an exotic beam from the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. Newly measured spins and spectroscopic factors of low-lying states of $^{81}\mathrm{Ge}$ are determined, and the neutron capture cross section on $^{80}\mathrm{Ge}$ was calculated in a direct-semidirect model to provide a more realistic ($n,\ensuremath{\gamma}$) reaction rate for $r$-process simulations. Furthermore, a region of shape coexistence around $N\ensuremath{\approx}50$ is confirmed and implications for the magic nature of $^{78}\mathrm{Ni}$ are discussed.

The cross sections of nuclear reactions between the radioisotope $^{7}\mathrm{Be}$ and deuterium, a possible mechanism of reducing the production of mass-7 nuclides in big-bang nucleosynthesis, were measured at center-of-mass energies between 0.2 and 1.5 MeV. The measured cross sections are dominated by the $(d,\ensuremath{\alpha})$ reaction channel, towards which prior experiments were mostly insensitive. A new resonance at 0.36(5) MeV with a strength of $\ensuremath{\omega}\ensuremath{\gamma}=1.7(5)\text{ }\text{ }\mathrm{keV}$ was observed inside the relevant Gamow window. Calculations of nucleosynthesis outcomes based on the experimental cross section show that the resonance reduces the predicted abundance of primordial $^{7}\mathrm{Li}$, but not sufficiently to solve the primordial lithium problem.

The astrophysical $S$-factor for the radiative proton capture reaction on $^7$Be ($S_{17}$) at low energies is affected by the $s$-wave scattering lengths. We report the measurement of elastic and inelastic scattering cross sections for the $^7$Be+p system in the center-of-mass energy range 0.474 - 2.740 MeV and center-of-mass angular range of 70$^\circ$- 150$^\circ$. A radioactive $^7$Be beam produced at Oak Ridge National Laboratory's (ORNL) Holifield Radioactive Ion Beam Facility was accelerated and bombarded a thin polypropylene (CH$_{2}$)$_\text n$ target. Scattered ions were detected in the segmented Silicon Detector Array. Using an $\textit{R}$-matrix analysis of ORNL and Louvain-la-Neuve cross section data, the $s$-wave scattering lengths for channel spins 1 and 2 were determined to be 17.34$^{+1.11}_{-1.33}$ and -3.18$^{+0.55}_{-0.50}$ fm, respectively. The uncertainty in the $s$-wave scattering lengths reported in this work is smaller by a factor of 5-8 compared to the previous measurement, which may reduce the overall uncertainty in $S_{17}$ at zero energy. The level structure of $^8$B is discussed based upon the results from this work. Evidence for the existence of 0$^+$ and 2$^+$ levels in $^8$B at 1.9 and 2.21 MeV, respectively, is observed.

The $^{15}$O($\alpha$,$\gamma$)$^{19}$Ne reaction is responsible for breakout from the hot CNO cycle in Type I x-ray bursts. Understanding the properties of resonances between $E_x = 4$ and 5 MeV in $^{19}$Ne is crucial in the calculation of this reaction rate. The spins and parities of these states are well known, with the exception of the 4.14- and 4.20-MeV states, which have adopted spin-parities of 9/2$^-$ and 7/2$^-$, respectively. Gamma-ray transitions from these states were studied using triton-$\gamma$-$\gamma$ coincidences from the $^{19}$F($^{3}$He,$t\gamma$)$^{19}$Ne reaction measured with GODDESS (Gammasphere ORRUBA Dual Detectors for Experimental Structure Studies) at Argonne National Laboratory. The observed transitions from the 4.14- and 4.20-MeV states provide strong evidence that the $J^\pi$ values are actually 7/2$^-$ and 9/2$^-$, respectively. These assignments are consistent with the values in the $^{19}$F mirror nucleus and in contrast to previously accepted assignments.

The Silicon Array for Branching Ratio Experiments (SABRE) has been developed for use to study reactions of interest to nuclear structure and astrophysics . The array has been incorporated into the Super Enge Split-Pole Spectrograph (SE-SPS) experimental setup at Florida State University’s John D. Fox accelerator laboratory to detect charged-particle decays in coincidence with reaction products detected by the SE-SPS focal plane detector. Its construction and electronics processing are discussed, as well as the commissioning data used to validate its performance.

GODDESS is a coupling of the charged-particle detection system ORRUBA to the gamma-ray detector array Gammasphere. This coupling has been developed in order to facilitate the high-resolution measurement of direct reactions in normal and inverse kinematics with stable and radioactive beams. GODDESS has been commissioned using a beam of 134 Xe at 10 MeV/A, in a campaign of stable beam measurements. The measurement demonstrates the capabilities of GODDESS under radioactive beam conditions, and provides the first data on the single-neutron states in 135 Xe, including previously unobserved states based on the orbitals above the N=82 shell closure.

The 14O(α,p)17F reaction is an important trigger reaction to the α-p process in X-ray bursts. The most stringent experimental constraints on its astrophysical rate come from measurements of the time-inverse reaction, 17F(p,α)14O. Previous studies of this inverse reaction have sufficiently characterized the high-energy dependence of the cross section but there are still significant uncertainties at lower energies. A new measurement of the 17F(p,α)14O cross section is underway at the Twin Solenoid (TwinSol) facility at the University of Notre Dame using an in-flight secondary 17F beam. The initial results are promising but improvements are needed to complete the measurement. The initial data and plans for an improved measurement are presented in this manuscript.

The Versatile Array of Neutron Detectors at Low Energy (VANDLE) is a new, highly efficient plastic-scintillator array constructed for decay and transfer reaction experimental setups that require neutron detection. The versatile and modular design allows for customizable experimental setups including beta-delayed neutron spectroscopy and (d,n) transfer reactions in normal and inverse kinematics. The neutron energy and prompt-photon discrimination is determined through the time of flight technique. Fully digital data acquisition electronics and integrated triggering logic enables some VANDLE modules to achieve an intrinsic efficiency over 70% for 300-keV neutrons, measured through two different methods. A custom Geant4 simulation models aspects of the detector array and the experimental setups to determine efficiency and detector response. A low detection threshold, due to the trigger logic and digitizing data acquisition, allowed us to measure the light-yield response curve from elastically scattered carbon nuclei inside the scintillating plastic from incident neutrons with kinetic energies below 2 MeV.

A direct test of nova explosion models comes from the observation of γ rays created in the decay of radioactive isotopes produced in the nova. One such isotope, 18F, is believed to be the main source of observable γ rays at and below 511 keV. The main destruction mechanism of 18F is thought to be the 18F(p,α)15O reaction, and uncertainties in the reaction rate arise from uncertainties in the energies, spins, and parities of the nuclear levels in 19Ne above the 18F+p threshold. To measure the properties of these levels, the 19F(3He,t)19Ne∗(γ) reaction was studied at Argonne National Laboratory and the Nuclear Science Laboratory at the University of Notre Dame.A direct test of nova explosion models comes from the observation of γ rays created in the decay of radioactive isotopes produced in the nova. One such isotope, 18F, is believed to be the main source of observable γ rays at and below 511 keV. The main destruction mechanism of 18F is thought to be the 18F(p,α)15O reaction, and uncertainties in the reaction rate arise from uncertainties in the energies, spins, and parities of the nuclear levels in 19Ne above the 18F+p threshold. To measure the properties of these levels, the 19F(3He,t)19Ne∗(γ) reaction was studied at Argonne National Laboratory and the Nuclear Science Laboratory at the University of Notre Dame.

The cross sections of nuclear reactions between the radioisotope ^{7}Be and deuterium, a possible mechanism of reducing the production of mass-7 nuclides in big-bang nucleosynthesis, were measured at center-of-mass energies between 0.2 and 1.5 MeV. The measured cross sections are dominated by the (d,α) reaction channel, towards which prior experiments were mostly insensitive. A new resonance at 0.36(5) MeV with a strength of ωγ=1.7(5) keV was observed inside the relevant Gamow window. Calculations of nucleosynthesis outcomes based on the experimental cross section show that the resonance reduces the predicted abundance of primordial ^{7}Li, but not sufficiently to solve the primordial lithium problem.

Detection of nuclear-decay γ rays provides a sensitive thermometer of nova nucleosynthesis. The most intense γ-ray flux is thought to be annihilation radiation from the β^{+} decay of ^{18}F, which is destroyed prior to decay by the ^{18}F(p,α)^{15}O reaction. Estimates of ^{18}F production had been uncertain, however, because key near-threshold levels in the compound nucleus, ^{19}Ne, had yet to be identified. We report the first measurement of the ^{19}F(^{3}He,tγ)^{19}Ne reaction, in which the placement of two long-sought 3/2^{+} levels is suggested via triton-γ-γ coincidences. The precise determination of their resonance energies reduces the upper limit of the rate by a factor of 1.5-17 at nova temperatures and reduces the average uncertainty on the nova detection probability by a factor of 2.1.

The astrophysical F 18 ( p , α ) O 15 rate determines, in large part, the extent to which the observable radioisotope 18 F is produced in novae. This rate, however, has been extremely uncertain owing to the unknown properties of a strong subthreshold resonance and its possible interference with higher-lying resonances. The new Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas-jet target has been used for the first time to determine the spin of this important resonance and significantly reduce uncertainties in the F 18 ( p , α ) O 15 rate.

To take full advantage of advanced exotic beam facilities, target technology must also be advanced. Particularly important to the study of astrophysical reaction rates is the creation of localized and dense targets of hydrogen and helium. The Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas-jet target has been constructed for this purpose. JENSA was constructed at Oak Ridge National Laboratory (ORNL) where it was tested and characterized, and has now moved to the ReA3 reaccelerated beam hall at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University for use with radioactive beams.

A recoil separator SECAR has been designed to study radiative capture reactions relevant for the astrophysical rp-process in inverse kinematics for the Facility for Rare Isotope Beams (FRIB). We describe the design, layout, and ion optics of the recoil separator and present the status of the project.

The Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas-jet target was used to perform spectroscopic studies of $^{20}\mathrm{Ne}+p$ reactions. Levels in $^{19}\mathrm{Ne}$ were probed via the $^{20}\mathrm{Ne}(p,d)^{19}\mathrm{Ne}$ reaction to constrain the astrophysical rate of the $^{18}\mathrm{F}(p,\ensuremath{\alpha})^{15}\mathrm{O}$ reaction. Additionally, the first spectroscopic study of the $^{20}\mathrm{Ne}(p,^{3}\mathrm{He})^{18}\mathrm{F}$ reaction was performed. Angular distribution data were used to determine or confirm the spins of several previously observed levels, and the existence of a strong subthreshold $^{18}\mathrm{F}(p,\ensuremath{\alpha})^{15}\mathrm{O}$ resonance was verified.

Background: The $^{17}\mathrm{F}(p,\ensuremath{\gamma})^{18}\mathrm{Ne}$ reaction is part of the astrophysical ``hot CNO'' cycles that are important in astrophysical environments like novas. Its thermal reaction rate is low owing to the relatively high energy of the resonances and therefore is dominated by direct, nonresonant capture in stellar environments at temperatures below 0.4 GK.Purpose: An experimental method is established to extract the proton strength to bound and unbound states in experiments with radioactive ion beams and to determine the parameters of direct and resonant capture in the $^{17}\mathrm{F}(p,\ensuremath{\gamma})^{18}\mathrm{Ne}$ reaction.Method: The $^{17}\mathrm{F}(d,n)^{18}\mathrm{Ne}$ reaction is measured in inverse kinematics using a beam of the short-lived isotope $^{17}\mathrm{F}$ and a compact setup of neutron, proton, $\ensuremath{\gamma}$-ray, and heavy-ion detectors called resoneut.Results: The spectroscopic factors for the lowest $l=0$ proton resonances at ${\mathrm{E}}_{\mathrm{c}.\mathrm{m}.}=0.60$ and 1.17 MeV are determined, yielding results consistent within $1.4\ensuremath{\sigma}$ of previous proton elastic-scattering measurements. The asymptotic normalization coefficients of the bound ${2}_{1}^{+}$ and ${2}_{2}^{+}$ states in $^{18}\mathrm{Ne}$ are determined and the resulting direct-capture reaction rates are extracted.Conclusions: The direct-capture component of the $^{17}\mathrm{F}(p,\ensuremath{\gamma})^{18}\mathrm{Ne}$ reaction is determined for the first time from experimental data on $^{18}\mathrm{Ne}$.

Background: The $^{31}\mathrm{S}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction becomes important for sulfur production in novae if the $^{31}\mathrm{P}(p,\ensuremath{\alpha})^{28}\mathrm{Si}$ reaction rate is somewhat greater than currently accepted. The rate of the $^{31}\mathrm{S}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction is uncertain, primarily due to the properties of resonances at ${E}_{\mathrm{c}.\mathrm{m}.}=156$ and 549 keV.Purpose: We precisely determined the excitation energies of states in $^{32}\mathrm{Cl}$ through high-resolution $\ensuremath{\gamma}$ spectroscopy including the two states most important for the $^{31}\mathrm{S}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction at nova temperatures.Method: Excited states in $^{32}\mathrm{Cl}$ were populated using the $^{10}\mathrm{B}(^{24}\mathrm{Mg},2n)\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction with a $^{24}\mathrm{Mg}$ beam from the ATLAS facility at Argonne National Laboratory. The reaction channel of interest was selected using recoils in the Fragment Mass Analyzer, and precise level energies were determined by detecting $\ensuremath{\gamma}$ rays with Gammasphere.Results: We observed $\ensuremath{\gamma}$ rays from the decay of six excited states in $^{32}\mathrm{Cl}$. The excitation energies for two unbound levels at ${E}_{x}=1738.1$ (6) keV and 2130.5 (10) keV were determined and found to be in agreement with a previous high-precision measurement of the $^{32}\mathrm{S}(^{3}\mathrm{He},t)^{32}\mathrm{Cl}$ reaction [1].Conclusions: An updated $^{31}\mathrm{S}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{32}\mathrm{Cl}$ reaction rate is presented. With the excitation energies of important levels firmly established, the dominant uncertainty in the reaction rate at nova temperatures is due to the strength of the resonance corresponding to the 2131-keV state in $^{32}\mathrm{Cl}$.

Background: An assessment done under the auspices of the Nuclear Energy Agency in 2007 suggested that the $\ensuremath{\beta}$ decays of abundant fission products in nuclear reactors may be incomplete. Many of the nuclei are potentially affected by the so called pandemonium effect and their $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ decay heat should be restudied using the total absorption technique. The fission products $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ were assigned highest priority for restudy due to their large cumulative fission branching fractions. In addition, measuring $\ensuremath{\beta}$-delayed neutron emission probabilities is challenging and any new technique for measuring the $\ensuremath{\beta}$-neutron spectrum and the $\ensuremath{\beta}$-delayed neutron emission probabilities is an important addition to nuclear physics experimental techniques.Purpose: To obtain the complete $\ensuremath{\beta}$-decay pattern of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ and determine their consequences for reactor decay heat and ${\overline{\ensuremath{\nu}}}_{e}$ emission. Complete $\ensuremath{\beta}$-decay feeding includes ground state to ground state $\ensuremath{\beta}$ feeding with no associated $\ensuremath{\gamma}$ rays, ground state to excited states $\ensuremath{\beta}$ transitions followed by $\ensuremath{\gamma}$ transitions to the daughter nucleus ground state, and $\ensuremath{\beta}$-delayed neutron emission from the daughter nucleus in the case of $^{137}\mathrm{I}$.Method: We measured the complete $\ensuremath{\beta}$-decay intensities of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory. We describe a technique for measuring the $\ensuremath{\beta}$-delayed neutron energy spectrum, which also provides a measurement of the $\ensuremath{\beta}$-neutron branching ratio, ${P}_{n}$.Results: We validate the current Evaluated Nuclear Structure Data File (ENSDF) evaluation of $^{137}\mathrm{Xe}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay. We find that major changes to the current ENSDF assessment of $^{137}\mathrm{I}\ensuremath{\beta}$-decay intensity are required. The average $\ensuremath{\gamma}$ energy per $\ensuremath{\beta}$ decay for $^{137}\mathrm{I}\ensuremath{\beta}$ decay ($\ensuremath{\gamma}$ decay heat) increases by 19%, from 1050--1250 keV, which increases the average $\ensuremath{\gamma}$ energy per $^{235}\mathrm{U}$ fission by $0.11%$. We measure a $\ensuremath{\beta}$-delayed neutron branching fraction for $^{137}\mathrm{I}\ensuremath{\beta}$ decay of $7.9\ifmmode\pm\else\textpm\fi{}0.2(\mathrm{fit})\ifmmode\pm\else\textpm\fi{}0.4(\mathrm{sys})%$ and we provide a $\ensuremath{\beta}$-neutron energy spectrum.Conclusions: The Modular Total Absorption Spectrometer measurements of $^{137}\mathrm{I}$ and $^{137}\mathrm{Xe}$ demonstrate the importance of revisiting and remeasuring complex $\ensuremath{\beta}$-decaying fission products with total absorption spectroscopy. We demonstrate the ability of the Modular Total Absorption Spectrometer to measure $\ensuremath{\beta}$-delayed neutron energy spectra.

The 15O(α,γ)19Ne reaction is an important trigger reaction leading to the rapid proton (rp) capture process in X-ray bursts. The primary uncertainty in determining its astrophysical rate is the uncertain α branching ratios of levels near Ex = 4.1 MeV in 19Ne. These states have been populated in a study of the 20Ne(p, d)19Ne reaction, and α branching ratios are reported in this manuscript.

The SuperORRUBA (Oak Ridge Rutgers University Barrel Array) of double-sided silicon strip detectors has been constructed at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL). The array will primarily be used to study single-nucleon transfer reactions in inverse kinematics at exotic beam facilities. The detector exhibits good intrinsic energy resolution ( ∼ 25 keV ) and large solid-angle coverage over the polar angular range 55–125°. The detector is now in routine use and has been used for several measurements at the HRIBF.

When a neutron star accretes hydrogen and helium from the outer layers of its companion star, thermonuclear burning enables the α p-process as a break out mechanism from the hot CNO cycle. Model calculations predict ( α , p) reaction rates significantly affect both the light curves and elemental abundances in the burst ashes. The Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas jet target enables the direct measurement of previously inaccessible ( α ,p) reactions with radioactive beams provided by the rare isotope re-accelerator ReA3 at the National Superconducting Cyclotron Laboratory (NSCL), USA. JENSA is going to be the main target for the Recoil Separator for Capture Reactions (SECAR) at the Facility for Rare Isotope Beams (FRIB). Commissioning of JENSA and first experiments at Oak Ridge National Laboratory (ORNL) showed a highly localized, pure gas target with a density of ∼10 19 atoms per square centimeter. Preliminary results are presented from the first direct cross section measurement of the 34 Ar( α , p) 37 K reaction at NSCL.

Reactions on certain proton-rich, radioactive nuclei have been shown to have a significant influence on X-ray bursts. We provide an overview of two recent measurements of important X-ray burst reactions using in-flight radioactive ion beams from the RESOLUT facility at the J. D. Fox Superconducting Accelerator Laboratory at Florida State University. The 17 F(d,n) 18 Ne reaction was measured, and Asymptotic Normalization Coefficients were extracted for bound states in 18 Ne that determine the direct-capture cross section dominating the 17 F(p, γ ) 18 Ne reaction rate for T≲ 0.45 GK. Unbound resonant states were also studied, and the single-particle strength for the 4.523-MeV (3 + ) state was found to be consistent with previous results. The 19 Ne(d,n) 20 Na proton transfer reaction was used to study resonances in the 19 Ne(p, γ ) 20 Na reaction. The most important 2.65-MeV state in 20 Na was observed to decay by proton emission to both the ground and first-excited states in 19 Ne, providing strong evidence for a 3 + spin assignment and indicating that proton capture on the thermally-populated first-excited state in 19 Ne is an important contributor to the 19 Ne(p, γ ) 20 Na reaction rate.

The $^{19}\mathrm{Ne}(p,\ensuremath{\gamma})^{20}\mathrm{Na}$ reaction is the second step of a reaction chain which breaks out from the hot CNO cycle, following the $^{15}\mathrm{O}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{Ne}$ reaction at the onset of x-ray burst events. We investigate the spectrum of the lowest proton-unbound states in $^{20}\mathrm{Na}$ in an effort to resolve contradictions in spin-parity assignments and extract reliable information about the thermal reaction rate. The proton-transfer reaction $^{19}\mathrm{Ne}(d,n)^{20}\mathrm{Na}$ is measured with a beam of the radioactive isotope $^{19}\mathrm{Ne}$ at an energy around the Coulomb barrier and in inverse kinematics. We observe three proton resonances with the $^{19}\mathrm{Ne}$ ground state, at 0.44, 0.66, and 0.82 MeV c.m. energies, which are assigned ${3}^{+}$, ${1}^{+}$, and (${0}^{+}$), respectively. In addition, we identify two resonances with the first excited state in $^{19}\mathrm{Ne}$, one at 0.20 MeV and one, tentatively, at 0.54 MeV. These observations allow us for the first time to experimentally quantify the astrophysical reaction rate on an excited nuclear state. Our experiment shows an efficient path for thermal proton capture in $^{19}\mathrm{Ne}(p,\ensuremath{\gamma})^{20}\mathrm{Na}$, which proceeds through ground state and excited-state capture in almost equal parts and eliminates the possibility for this reaction to create a bottleneck in the breakout from the hot CNO cycle.

We report total absorption spectroscopy measurements of $^{92}\mathrm{Rb}$, $^{96\mathrm{gs}}\mathrm{Y}$, and $^{142}\mathrm{Cs}$ $\ensuremath{\beta}$ decays, which are the most important contributors to the high energy ${\overline{\ensuremath{\nu}}}_{e}$ spectral shape in nuclear reactors. These three $\ensuremath{\beta}$ decays contribute 43% of the ${\overline{\ensuremath{\nu}}}_{e}$ flux near 5.5 MeV emitted by nuclear reactors. This ${\overline{\ensuremath{\nu}}}_{e}$ energy is particularly interesting due to spectral features recently observed in several experiments including the Daya Bay, Double Chooz, and RENO Collaborations. Measurements were conducted at Oak Ridge National Laboratory by means of proton-induced fission of $^{238}\mathrm{U}$ with on-line mass separation of fission fragments and the Modular Total Absorption Spectrometer. We observe a $\ensuremath{\beta}$-decay pattern that is similar to recent measurements of $^{92}\mathrm{Rb}$, with a ground-state to ground-state $\ensuremath{\beta}$ feeding of 91(3)%. We verify the $^{96\mathrm{gs}}\mathrm{Y}$ ground-state to ground-state $\ensuremath{\beta}$ feeding of 95.5(20)%. Our measurements substantially modify the $\ensuremath{\beta}$-decay feedings of $^{142}\mathrm{Cs}$, reducing the $\ensuremath{\beta}$ feeding to $^{142}\mathrm{Ba}$ states below 2 MeV by 32% when compared with the latest evaluations. Our results increase the discrepancy between the observed and the expected reactor ${\overline{\ensuremath{\nu}}}_{e}$ flux between 5 and 7 MeV, the maximum excess increases from $\ensuremath{\sim}10%$ to $\ensuremath{\sim}12%$.

We report total absorption spectroscopy measurements of ^{92}Rb, ^{96gs}Y, and ^{142}Cs β decays, which are the most important contributors to the high energy ν[over ¯]_{e} spectral shape in nuclear reactors. These three β decays contribute 43% of the ν[over ¯]_{e} flux near 5.5 MeV emitted by nuclear reactors. This ν[over ¯]_{e} energy is particularly interesting due to spectral features recently observed in several experiments including the Daya Bay, Double Chooz, and RENO Collaborations. Measurements were conducted at Oak Ridge National Laboratory by means of proton-induced fission of ^{238}U with on-line mass separation of fission fragments and the Modular Total Absorption Spectrometer. We observe a β-decay pattern that is similar to recent measurements of ^{92}Rb, with a ground-state to ground-state β feeding of 91(3)%. We verify the ^{96gs}Y ground-state to ground-state β feeding of 95.5(20)%. Our measurements substantially modify the β-decay feedings of ^{142}Cs, reducing the β feeding to ^{142}Ba states below 2 MeV by 32% when compared with the latest evaluations. Our results increase the discrepancy between the observed and the expected reactor ν[over ¯]_{e} flux between 5 and 7 MeV, the maximum excess increases from ∼10% to ∼12%.

The chemical elements are made at astrophysical sites through a sequence of nuclear reactions often involving unstable nuclei. The overarching aim of this project is to construct a system that allows for the inverse process of nucleosynthesis (i.e. breakup of heavier nuclei into lighter ones) to be studied in high efficiency. The specific problem to be overcome with this grant is inadequate dynamic range and (triggering) threshold to detect the products of the breakup which include both heavy ions (with large energy and large deposited energy in a detector system) and protons (with little energy and deposited energy.) Early on in the grant we provided both TAMU and RIKEN (the site of the eventual experiments) with working systems based on the existing technology. This technology could be used with either an external preamplifier that was to be designed and fabricated by our RIKEN collaborators or upgraded by replacing the existing chip with one we designed. The RIKEN external preamplifier project never can to completion but our revised chip was designed, fabricated, used in a test experiment and performs as required.

The ^{19}Ne(p,γ)^{20}Na reaction is the second step of a reaction chain which breaks out from the hot CNO cycle, following the ^{15}O(α,γ)^{19}Ne reaction at the onset of x-ray burst events. We investigate the spectrum of the lowest proton-unbound states in ^{20}Na in an effort to resolve contradictions in spin-parity assignments and extract reliable information about the thermal reaction rate. The proton-transfer reaction ^{19}Ne(d,n)^{20}Na is measured with a beam of the radioactive isotope ^{19}Ne at an energy around the Coulomb barrier and in inverse kinematics. We observe three proton resonances with the ^{19}Ne ground state, at 0.44, 0.66, and 0.82 MeV c.m. energies, which are assigned 3^{+}, 1^{+}, and (0^{+}), respectively. In addition, we identify two resonances with the first excited state in ^{19}Ne, one at 0.20 MeV and one, tentatively, at 0.54 MeV. These observations allow us for the first time to experimentally quantify the astrophysical reaction rate on an excited nuclear state. Our experiment shows an efficient path for thermal proton capture in ^{19}Ne(p,γ)^{20}Na, which proceeds through ground state and excited-state capture in almost equal parts and eliminates the possibility for this reaction to create a bottleneck in the breakout from the hot CNO cycle.

As one of a series of physics cases to demonstrate the unique benefit of the new Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas-jet target for enabling next-generation transfer reaction studies, the $^{14}\mathrm{N}$ ($p, t$)$^{12}\mathrm{N}$ reaction was studied for the first time, using a pure jet of nitrogen, in an attempt to resolve conflicting information on the structure of $^{12}\mathrm{N}$. A potentially new level at 4.561-MeV excitation energy in $^{12}\mathrm{N}$ was found.

International audience; The changing of the nuclear shells for light, neutron-rich nuclei, and the single-particle nature of 26Na, has been explored by studying 25Na(d, p)26Na in inverse kinematics, using a beam of 25Na ions at 5 MeV per nucleon, provided by the ISAC-II facility at TRIUMF, Vancouver. Charged particles were detected with a highly-segmented silicon array that surrounded the 0.5 mg/cm2 (CD2)n target. Gamma rays from the recoiling 26Na nucleus were detected using eight Compton-suppressed HPGe clover detectors. Recoil tagging was provided by an in-beam scintillation foil, downstream of the germanium array. A novel technique of utilising pγ- and pγγ-gating to extract proton angular distributions from states populated close in energy was employed with success. New states in 26Na that are populated directly have been identified, using γ-decay patterns. Shell model calculations for comparison to experimental results are ongoing, using different model bases.

The Galactic 1.809-MeV $\ensuremath{\gamma}$-ray signature from the $\ensuremath{\beta}$ decay of $^{26g}\mathrm{Al}$ is a dominant target of $\ensuremath{\gamma}$-ray astronomy, of which a significant component is understood to originate from massive stars. The $^{26g}\mathrm{Al}(p,\ensuremath{\gamma})^{27}\mathrm{Si}$ reaction is a major destruction pathway for $^{26g}\mathrm{Al}$ at stellar temperatures, but the reaction rate is poorly constrained due to uncertainties in the strengths of low-lying resonances in $^{27}\mathrm{Si}$. The $^{26g}\mathrm{Al}(d,p)^{27}\mathrm{Al}$ reaction has been employed in inverse kinematics to determine the spectroscopic factors, and hence resonance strengths, of proton resonances in $^{27}\mathrm{Si}$ via mirror symmetry. The strength of the 127-keV resonance is found to be a factor of 4 higher than the previously adopted upper limit, and the upper limit for the 68-keV resonance has been reduced by an order of magnitude, considerably constraining the $^{26g}\mathrm{Al}$ destruction rate at stellar temperatures.

International audience; The Galactic 1.809-MeV γ-ray signature from the β decay of $^{26g}$Al is a dominant target of γ-rayastronomy, of which a significant component is understood to originate from massive stars. The$^{26g}$Al(p; γ)$^{27}$Si reaction is a major destruction pathway for $^{26g}$Al at stellar temperatures, but the reactionrate is poorly constrained due to uncertainties in the strengths of low-lying resonances in $^{27}$Si. The$^{26g}$Al(d; p)$^{27}$Al reaction has been employed in inverse kinematics to determine the spectroscopic factors,and hence resonance strengths, of proton resonances in $^{27}$Si via mirror symmetry. The strength of the127-keV resonance is found to be a factor of 4 higher than the previously adopted upper limit, and the upperlimit for the 68-keV resonance has been reduced by an order of magnitude, considerably constraining the$^{26g}$Al destruction rate at stellar temperatures.

Proton elastic scattering, proton inelastic scattering, proton transfer reactions and the (d, p) neutron transfer reaction have been measured in inverse kinematics with radioactive ion beams at the HRIBF to determine the properties of nuclear excited states that are important in stellar explosions. The experimental techniques and some initial results are briefly summarized.

The reaction mechanisms of the two-neutron transfer reaction ${}^{12}$C(${}^{6}$He,${}^{4}$He) have been studied at ${E}_{\mathrm{lab}}=30$ MeV at the TRIUMF ISAC-II facility using the Silicon Highly-segmented Array for Reactions and Coulex (SHARC) charged-particle detector array. Optical potential parameters have been extracted from the analysis of the elastic scattering angular distribution. The new potential has been applied to the study of the transfer angular distribution to the 2${}_{2}^{+}$ 8.32 MeV state in ${}^{14}$C, using a realistic three-body ${}^{6}$He model and advanced shell-model calculations for the carbon structure, allowing to calculate the relative contributions of the simultaneous and sequential two-neutron transfer. The reaction model provides a good description of the 30-MeV data set and shows that the simultaneous process is the dominant transfer mechanism. Sensitivity tests of optical potential parameters show that the final results can be considerably affected by the choice of optical potentials. A reanalysis of data measured previously at ${E}_{\mathrm{lab}}=18$ MeV, however, is not as well described by the same reaction model, suggesting that one needs to include higher-order effects in the reaction mechanism.

A method has been developed that can provide beams of many short-lived nuclei of interest in nucleosynthesis along the rp process path. With a {sup 17}F beam (T{sub 1/2}=64 s ) the excitation function of the {sup 17}F( p,thinsp{alpha}){sup 14} O reaction was measured to determine properties of excited states in {sup 18}Ne . These states influence the rate of the {sup 14}O( {alpha},thinspp){sup 17} F reaction which is important for understanding energy generation and nucleosynthesis in x-ray bursts. The present direct measurements yield a pattern of resonances and cross sections which differ substantially from previous estimates. {copyright} {ital 1999} {ital The American Physical Society}