Search

Background: $\ensuremath{\beta}$-decay spectroscopy provides valuable information on exotic nuclei and a stringent test for nuclear theories beyond the stability line.Purpose: To search for new $\ensuremath{\beta}$-delayed protons and $\ensuremath{\gamma}$ rays of $^{25}\mathrm{Si}$ to investigate the properties of $^{25}\mathrm{Al}$ excited states.Method: $^{25}\mathrm{Si}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decays were measured by using the Gaseous Detector with Germanium Tagging system at the National Superconducting Cyclotron Laboratory. The protons and $\ensuremath{\gamma}$ rays emitted in the decay were detected simultaneously. A Monte Carlo method was used to model the Doppler broadening of $^{24}\mathrm{Mg}\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$-ray lines caused by nuclear recoil from proton emission. Shell-model calculations using two newly developed universal sd-shell Hamiltonians were performed.Results: The most precise $^{25}\mathrm{Si}$ half-life to date has been determined. A new proton branch at 724(4) keV and new proton-$\ensuremath{\gamma}$-ray coincidences have been identified. Three $^{24}\mathrm{Mg}\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$-ray lines and eight $^{25}\mathrm{Al}\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$-ray lines are observed for the first time in $^{25}\mathrm{Si}$ decay. The first measurement of the $^{25}\mathrm{Si}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray intensities through the $^{25}\mathrm{Al}$ unbound states is reported. All the bound states of $^{25}\mathrm{Al}$ are observed to be populated in the $\ensuremath{\beta}$ decay of $^{25}\mathrm{Si}$. Several inconsistencies between the previous measurements have been resolved, and new information on the $^{25}\mathrm{Al}$ level scheme is provided. An enhanced decay scheme has been constructed and compared to the mirror decay of $^{25}\mathrm{Na}$ and the shell-model calculations.Conclusions: The measured excitation energies, $\ensuremath{\gamma}$-ray and proton branchings, log $ft$ values, and Gamow-Teller transition strengths for the states of $^{25}\mathrm{Al}$ populated in the $\ensuremath{\beta}$ decay of $^{25}\mathrm{Si}$ are in good agreement with the shell-model calculations, offering gratifyingly consistent insights into the fine nuclear structure of $^{25}\mathrm{Al}$.

Background: Properties of proton-unbound $^{31}\mathrm{S}$ states determine the $^{30}\mathrm{P}(p,\ensuremath{\gamma})^{31}\mathrm{S}$ reaction rate, which has a significant impact on explosive hydrogen burning in classical novae and type-I x-ray bursts. Despite several previous studies, uncertainties still remain with respect to the nuclear structure of $^{31}\mathrm{S}$ near the proton threshold.Purpose: The level structure of $^{31}\mathrm{S}$ has been presently investigated via a charged-particle spectroscopy experiment using the $^{32}\mathrm{S}(p,d)^{31}\mathrm{S}$ reaction.Method: Deuterons corresponding to $^{31}\mathrm{S}$ excited states with $3.285\ensuremath{\le}\phantom{\rule{4pt}{0ex}}{E}_{x}\phantom{\rule{4pt}{0ex}}\ensuremath{\le}10.8$ MeV were momentum analyzed via an Enge split-pole spectrograph at six laboratory angles between ${10}^{\ensuremath{\circ}}$ and ${62}^{\ensuremath{\circ}}$. Differential cross sections of the $^{32}\mathrm{S}(p,d)^{31}\mathrm{S}$ reaction were measured at ${E}_{p}\phantom{\rule{4pt}{0ex}}=34.5$ MeV. Distorted-wave Born approximation calculations were performed to constrain the spin-parity assignments of several of the observed levels.Results: We have detected 72 excited states of $^{31}\mathrm{S}$, out of which 17 are within the astrophysical region of interest corresponding to the temperature range of 0.1--1.5 GK. We have resolved the discrepancy in the spin and parity of an excited state with ${E}_{x}\phantom{\rule{4pt}{0ex}}=6542$ keV, showing that is it not ${J}^{\ensuremath{\pi}}\phantom{\rule{4pt}{0ex}}=3/{2}^{\ensuremath{-}}$, and therefore the contribution of this state to the $^{30}\mathrm{P}(p,\ensuremath{\gamma})$ reaction rate is likely much less significant than previously thought owing to the larger angular-momentum transfer required to populate this excited state. Moreover, our measurement results help consolidate the spin-parity assignments for the 6377 and 6636 keV states in $^{31}\mathrm{S}$.Conclusions: This work presents the most comprehensive spin-parity assignments to date from a single-neutron transfer reaction on $^{32}\mathrm{S}$ to $^{31}\mathrm{S}$ excited states in the region between 6 to 7 MeV excitation energy. This region is significant for the determination of the $^{30}\mathrm{P}(p,\ensuremath{\gamma})^{30}\mathrm{S}$ reaction rate over the temperatures characteristic of explosive hydrogen burning in novae.

Background: Dust grains condensed in the outflows of presolar classical novae should have been present in the protosolar nebula. Candidates for such presolar nova grains have been found in primitive meteorites and can in principle be identified by their isotopic ratios, but the ratios predicted by state-of-the-art one-dimensional hydrodynamic models are uncertain due to nuclear-physics uncertainties.Purpose: To theoretically calculate the thermonuclear rates and uncertainties of the $^{34}\mathrm{S}(p,\ensuremath{\gamma})^{35}\mathrm{Cl}$ and $^{34g,m}\mathrm{Cl}(p,\ensuremath{\gamma})^{35}\mathrm{Ar}$ reactions and investigate their impacts on the predicted $^{34}\mathrm{S}/^{32}\mathrm{S}$ isotopic ratio for presolar nova grains.Method: A shell-model approach in a ($0+1$) $\ensuremath{\hbar}\ensuremath{\omega}$ model space was used to calculate the properties of resonances in the $^{34}\mathrm{S}(p,\ensuremath{\gamma})^{35}\mathrm{Cl}$ and $^{34g,m}\mathrm{Cl}(p,\ensuremath{\gamma})^{35}\mathrm{Ar}$ reactions and their thermonuclear rates. Uncertainties were estimated using a Monte Carlo method. The implications of these rates and their uncertainties on sulfur isotopic nova yields were investigated using a postprocessing nucleosynthesis code. The rates for transitions from the ground state of $^{34}\mathrm{Cl}$ as well as from the isomeric first excited state of $^{34}\mathrm{Cl}$ were explicitly calculated.Results: At energies in the resonance region near the proton-emission threshold, many negative-parity states appear. Energies, spectroscopic factors, and proton-decay widths are reported. The resulting thermonuclear rates are compared with previous determinations.Conclusions: The shell-model calculations alone are sufficient to constrain the variation of the $^{34}\mathrm{S}/^{32}\mathrm{S}$ ratios to within about 30%. Uncertainties associated with other reactions must also be considered, but in general we find that the $^{34}\mathrm{S}/^{32}\mathrm{S}$ ratios are not a robust diagnostic to clearly identify presolar grains made from nova ejecta.

Certain astrophysical environments such as thermonuclear outbursts on accreting neutron stars (Type-I X-ray bursts) are hot enough to allow for breakout from the Hot CNO hydrogen burning cycles to the rapid proton capture (rp) process. An important breakout reaction sequence is 15O(α,γ)19Ne(p,γ)20Na and the 19Ne(p,γ)20Na reaction rate is expected to be dominated by a single resonance at 457 keV above the proton threshold in 20Na. The resonance strength and, hence, reaction rate depends strongly on whether this 20Na state at an excitation energy of 2647 keV has spin and parity of 1 + or 3 + . Previous 20Mg ( J π = 0 + ) β + decay experiments have relied almost entirely on searches for β-delayed proton emission from this resonance in 20Na to limit the log ft value and, hence, J π . However there is a non-negligible γ-ray branch expected that must also be limited experimentally to determine the log ft value and constrain J π . We have measured the β-delayed γ decay of 20Mg to complement previous β-delayed proton decay work and provide the first complete limit based on all energetically allowed decay channels through the 2647 keV state. Our limit confirms that a 1 + assignment for this state is highly unlikely.

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.

Background: Isobaric quintets provide the best test of the isobaric multiplet mass equation (IMME) and can uniquely identify higher order corrections suggestive of isospin symmetry breaking effects in the nuclear Hamiltonian. The generalized IMME (GIMME) is a novel microscopic interaction theory that predicts an extension to the quadratic form of the IMME. Only the $A=20,32$ $T=2$ quintets have the exotic ${T}_{z}=\ensuremath{-}2$ member ground state mass determined to high precision by Penning trap mass spectrometry.Purpose: We aim to establish $A=36$ as the third $T=2$ isobaric quintet with the ${T}_{z}=\ensuremath{-}2$ member ground state mass measured by Penning trap mass spectrometry and provide the first test of the predictive power of the GIMME.Method: A radioactive beam of neutron-deficient $^{36}\mathrm{Ca}$ was produced by projectile fragmentation at the National Superconducting Cyclotron Laboratory. The beam was thermalized and the masses of $^{36}\mathrm{Ca}^{+}$ and $^{36}\mathrm{Ca}^{2+}$ were measured by the time-of-flight ion cyclotron resonance method in the LEBIT 9.4 T Penning trap.Results: We measure the mass excess of $^{36}\mathrm{Ca}$ to be $\mathrm{ME}=\ensuremath{-}6483.6(56)$ keV, an improvement in precision by a factor of 6 over the literature value. The new datum is considered together with evaluated nuclear data on the $A=36$, $T=2$ quintet. We find agreement with the quadratic form of the IMME given by isospin symmetry, but only coarse qualitative agreement with predictions of the GIMME.Conclusion: A total of three isobaric quintets have their most exotic members measured by Penning trap mass spectrometry. The GIMME predictions in the $T=2$ quintet appear to break down for $A=32$ and greater.

Background: The $^{15}\mathrm{O}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{Ne}$ bottleneck reaction in Type I x-ray bursts is the most important thermonuclear reaction rate to constrain experimentally, to improve the accuracy of burst light-curve simulations. A proposed technique to determine the thermonuclear rate of this reaction employs the $^{20}\mathrm{Mg}(\ensuremath{\beta}p\ensuremath{\alpha})^{15}\mathrm{O}$ decay sequence. The key $^{15}\mathrm{O}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{Ne}$ resonance at an excitation of 4.03 MeV is now known to be fed in $^{20}\mathrm{Mg}(\ensuremath{\beta}p\ensuremath{\gamma})^{19}\mathrm{Ne}$; however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future $^{20}\mathrm{Mg}(\ensuremath{\beta}p\ensuremath{\alpha})^{15}\mathrm{O}$ measurements.Purpose: To determine the energy of the proton transition feeding the 4.03 MeV state in $^{19}\mathrm{Ne}$.Method: A fast beam of $^{20}\mathrm{Mg}$ was implanted into a plastic scintillator, which was used to detect $\ensuremath{\beta}$ particles. 16 high purity germanium detectors were used to detect $\ensuremath{\gamma}$ rays emitted following $\ensuremath{\beta}p$ decay. A Monte Carlo method was used to simulate the Doppler broadening of $^{19}\mathrm{Ne}\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$-ray lines and compare to the experimental data.Results: The center of mass energy between the proton and $^{19}\mathrm{Ne}$, feeding the 4.03 MeV state, is measured to be $1.21{}_{\ensuremath{-}0.22}^{+0.25}\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$, corresponding to a $^{20}\mathrm{Na}$ excitation energy of $7.44{}_{\ensuremath{-}0.22}^{+0.25}\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$. Absolute feeding intensities and $\ensuremath{\gamma}$-decay branching ratios of $^{19}\mathrm{Ne}$ states were determined including the 1615 keV state, which has not been observed before in this decay. A new $\ensuremath{\gamma}$ decay branch from the 1536 keV state in $^{19}\mathrm{Ne}$ to the ground state is reported. The lifetime of the 1507 keV state in $^{19}\mathrm{Ne}$ is measured to be $4.3{}_{\ensuremath{-}1.1}^{+1.3}$ ps resolving discrepancies in the literature. Conflicting $^{20}\mathrm{Mg}(\ensuremath{\beta}p$) decay schemes in published literature are clarified.Conclusions: The utility of this Doppler broadening technique to provide information on $\ensuremath{\beta}$-delayed nucleon emission and excited-state lifetimes has been further demonstrated. In particular, knowledge of the proton energies feeding the 4.03 MeV $^{19}\mathrm{Ne}$ state in $^{20}\mathrm{Mg}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay will facilitate future measurements of the $\ensuremath{\alpha}$-particle branching ratio.

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.

An accreting compact star in a binary system can generate periodic thermonuclear runaways on its surface. In the case of a white dwarf star, the result is a classical nova, which enriches the interstellar medium with newly synthesized nuclides. In the case of a neutron star, a detectable burst of X-rays is emitted. Nucleosynthesis and energy generation in these events depends on resonant thermonuclear reaction rates, which are especially challenging to measure directly in the laboratory when they involve radioactive reactants. Fortunately, the resonances can be discovered and their relevant properties can be constructed using nuclear structure experiments. We will describe a program of beta decay experiments the National Superconducting Cyclotron Laboratory to this end. In particular, the Gaseous Detector with Germanium Tagging (GADGET) system, developed to measure very weak low-energy beta delayed proton emission branches and gamma rays, is now operational. Recent results and future opportunities at the Facility for Rare Isotope Beams will be discussed.

The two rp-reactions 34S(p,γ)35Cl and 34 g,m Cl(p,γ)35Ar were studied via a shell-model approach. At energies in the resonance region near the proton-emission threshold many negative-parity states appear. We present results of calculations in a full (0+1)ℏω model space which addresses this problem. Energies, spectroscopic factors and proton-decay widths are calculated for input into the reaction rates. Comparisons are also made with a recent experimental determination of the reaction rate for the 34S(3He,d)35Cl reaction. The thermonuclear 34 g,m Cl(p,γ)35Ar reaction rates are unknown because of a lack of experimental data. The rates for transitions from the ground state of 34Cl as well as from the isomeric first excited state of 34Cl are explicitly calculated taking into account the relative populations of the two states.

In classical novae simulations, the uncertainty in the reaction rate of \(^{ 38 }\)K(p,\(\gamma \)) has been shown to affect the abundances of endpoint nuclides significantly. To better understand the reaction rate, we have done a spectroscopic study on \(^{ 39 }\)Ca. The reaction \(^{ 40 }\)Ca(d,t)\(^{ 39 }\)Ca at a beam energy of 22 MeV was used to populate excited states of \(^{ 39 }\)Ca. Tritons were momentum analyzed using a high resolution quadrupole-dipole-dipole-dipole (Q3D) magnetic spectrograph at 4 angles. Preliminary resonance energies for \(^{ 39 }\)Ca within the energetic region of interest for classical novae - 6.0–6.4 MeV - were determined.

Background: $^{32}\mathrm{Cl}$ is a neutron-deficient isotope with a $\ensuremath{\beta}$-decay half-life of 298 ms and a spin and parity of ${J}^{\ensuremath{\pi}}={1}^{+}$. Previous measurements of $^{32}\mathrm{Cl} \ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays have yielded a $\ensuremath{\beta}$-decay scheme with twelve $\ensuremath{\beta}$-decay transitions, contributing to studies of nuclear structure and fundamental symmetries. Those experiments have been limited to the observation of $^{32}\mathrm{S}$ states with ${J}^{\ensuremath{\pi}}={0}^{+},{1}^{+},{2}^{+}$.Purpose: Our goal is to search for new $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays and $\ensuremath{\beta}$-decay transitions of $^{32}\mathrm{Cl}$ to $^{32}\mathrm{S}$.Methods: A measurement of $^{32}\mathrm{Cl} \ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ decay has been performed using the Clovershare array of high-purity germanium detectors at the National Superconducting Cyclotron Laboratory.Results: By acquiring the highest-statistics $^{32}\mathrm{Cl} \ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray spectrum to date and exploiting a new sensitivity to $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences, this experiment has enabled the observation of nine previously unobserved $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray transitions, leading to the inference of five $\ensuremath{\beta}$-decay transitions never before observed in $^{32}\mathrm{Cl} \ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ decay. The set of observed states includes negative-parity states for the first time. By combining the new information with data from previous work, the lifetimes and partial widths of the 8861- and 9650-keV states of $^{32}\mathrm{S}$ have been determined. In addition, the $^{31}\mathrm{P}(p,\ensuremath{\alpha})\phantom{\rule{0.16em}{0ex}}^{28}\mathrm{Si}$ resonance strength of the 9650-keV state has been limited to $\ensuremath{\omega}\ensuremath{\gamma}l9.8$ meV, which is an improvement over direct measurements.Conclusion: An enhanced decay scheme has been constructed. Most of the excited bound $^{32}\mathrm{S}$ states that would correspond to allowed and first-forbidden $\ensuremath{\beta}$-decay transitions have been observed, demonstrating the potential of $\ensuremath{\beta}$-decay experiments to approach complete spectroscopy measurements at the next generation of radioactive beam facilities. The observed positive-parity levels are well matched by $sd$ shell-model calculations.

It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle (χ) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single χ along with an e^+e^− pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with ∼4π acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). The summed kinetic energy (E_(e^+e^−)) from such events is used to set limits, as a function of the χ mass, on the branching fraction for this decay channel. For χ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at ≫ 5σlevel for 100 keV < E_(e^+e^−) < 644 keV. If the χ + e^+e^− final state is not the only one, we set limits on its branching fraction of 90% confidence level.

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.

Background: The neutron β-decay asymmetry parameter A_0 defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for A_0 permit an extraction of the ratio of the weak axial-vector to vector coupling constants, λ≡gA/gV, which under assumption of the conserved vector current hypothesis (gV=1) determines gA. Precise values for gA are important as a benchmark for lattice QCD calculations and as a test of the standard model. Purpose: The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron β-decay angular correlation performed with UCN. This article reports the most precise result for A_0 obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response. Methods: UCN produced via the downscattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of A_0. Results: The UCNA experiment reports a new 0.67% precision result for A_0 of A_0=−0.12054(44)_(stat)(68)_(syst), which yields λ=gA/gV=−1.2783(22). Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces A0=−0.12015(34)stat(63)syst and λ=gA/gV=−1.2772(20). Conclusions: This new result for A0 and gA/gV from the UCNA experiment has provided confirmation of the shift in values for gA/gV that has emerged in the published results from more recent experiments, which are in striking disagreement with the results from older experiments. Individual systematic corrections to the asymmetries in older experiments (published prior to 2002) were >10%, whereas those in the more recent ones (published after 2002) have been of the scale of

Classical novae are common thermonuclear explosions in the Milky Way galaxy, occurring on the surfaces of white-dwarf stars that are accreting hydrogen-rich material from companion stars. Nucleosynthesis in classical novae depends on radiative proton-capture reaction rates on radioactive nuclides. Many of these reactions cannot be measured directly at current accelerator facilities due to the lack of intense, high-quality, radioactive-ion beams at the relevant energies. Since most of these reactions proceed via resonant capture, their rates can be determined indirectly by measuring the properties of the resonances. At the National Superconducting Cyclotron Laboratory, we have used the β-delayed γ decays of 26 P and 31 Cl to populate resonances in 26 Si and 31 S and study the radiative proton captures on 25 Al and 30 P, respectively. These were two out of the three most important nuclear-physics uncertainties associated with the observable products of nova nucleosynthesis. The 26 P experiment has enabled a more accurate estimate of the nova contribution to the long-lived Galactic 26 Al detected with γ-ray telescopes. The 31 Cl experiment, currently under analysis, will calibrate potential nova thermometers and mixing meters based on elemental abundance ratios, and facilitate the identification of pre-solar nova grain candidates found in primitive meteorites based on isotopic ratios.

The $^{15}\mathrm{O}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{Ne}$ reaction is expected to trigger the initial path for breakout from the CNO hydrogen-burning cycles to the rapid proton capture $(rp)$ process in type I x-ray bursts on accreting neutron stars. The thermonuclear reaction rate has a major impact on models of type I x-ray burst observables and it depends on the small $\ensuremath{\alpha}$-particle branching ratio, ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\alpha}}/\mathrm{\ensuremath{\Gamma}}$, of the 4.03 MeV state in $^{19}\mathrm{Ne}$. Attempts to measure ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\alpha}}/\mathrm{\ensuremath{\Gamma}}$ by populating the 4.03 MeV state using nuclear reactions have only led to strong upper limits. In the present work, we report the first experimental evidence that the 4.03 MeV $^{19}\mathrm{Ne}$ state is populated in $^{20}\mathrm{Mg} \ensuremath{\beta}$-delayed proton emission. This new channel has the potential to provide the necessary sensitivity to detect a finite value of ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\alpha}}/\mathrm{\ensuremath{\Gamma}}$.

We report the independent experimental confirmation of an isomeric state in the proton drip-line nucleus $^{26}$P. The ${\gamma}$-ray energy and half-life determined are 164.4 $\pm$ 0.3 (sys) $\pm$ 0.2 (stat) keV and 104 $\pm$ 14 ns, respectively, which are in agreement with the previously reported values. These values are used to set a semi-empirical limit on the proton separation energy of $^{26}$P, with the conclusion that it can be bound or unbound., Comment: 5 pages, 3 figures

Background: A previous measurement of the $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ directional coefficient in $^{22}\mathrm{Na}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay was used to extract recoil-order form factors. The data indicate the requirement of a significant induced-tensor matrix element for the decay. This conclusion largely relies on a standard-model-allowed weak magnetism form factor which was determined using an unpublished value of the analog ${2}_{1}^{+}\ensuremath{\rightarrow}{3}_{1}^{+}$ $\ensuremath{\gamma}$ branch in $^{22}\mathrm{Na}$, with the further assumption that the transition is dominated by its isovector $M1$ component.Purpose: Our aim is to determine the ${2}_{1}^{+}\ensuremath{\rightarrow}{3}_{1}^{+}$ width in $^{22}\mathrm{Na}$ in order to obtain an independent measurement of the weak magnetism form factor for the $\ensuremath{\beta}$ decay.Methods: A $^{21}\mathrm{Ne}(p,\ensuremath{\gamma})$ resonance reaction on an implanted target was used to produce the first ${2}^{+}$ state in $^{22}\mathrm{Na}$ at ${E}_{x}=1952$ keV. Deexcitation $\ensuremath{\gamma}$ rays were registered with two 100% relative efficiency high purity germanium detectors.Results: We obtain for the first time an unambiguous determination of the ${2}_{1}^{+}\ensuremath{\rightarrow}{3}_{1}^{+}$ branch in $^{22}\mathrm{Na}$ to be $0.45(8)%$.Conclusions: Using the conserved vector current (CVC) hypothesis, our branch determines the weak magnetism form factor for $^{22}\mathrm{Na}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay to be $|b/A{c}_{1}|=8.9(1.2)$. Together with the $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ angular correlation coefficient, we obtain a large induced-tensor form factor for the decay that continues to disagree with theoretical predictions. Two plausible explanations are suggested.

Nucleosynthesis in classical novae on oxygen-neon white dwarfs is sensitive to the poorly constrained thermonuclear rate of the 30 P(p, $\gamma$ )31 S reaction. In order to improve this situation, a variety of experiments have been performed over the past decade to determine the properties of proton unbound 31S levels up to an excitation energy of $\approx 6.7$ MeV. Inconsistencies in the energies and $ J^{\pi}$ values for these levels have made it difficult to produce a useful 30 P(p, $ \gamma$ )31 S reaction rate based on experimental information. In the present work, we revisit a subset of published data on the structure of 31S in order to shed light on these problems. First, we present an alternative calibration of 31 P(3 He, t)31 S spectra using newly available high-precision data in order to address discrepant 31S excitation energies. Second, we apply a similar method to a recently acquired 32 S(d, t)31 S spectrum. Third, for a different 31 P(3 He, t)31 S experiment in which angular distributions were acquired, we present alternative fits to the experimental data in order to address discrepant 31S $J^{\pi}$ values. Finally, we compare the $J^{\pi}$ values from 31 P(3 He, t)31 S to those reported from in beam $ \gamma$ -ray spectroscopy experiments in order to search for potential resolutions to the inconsistencies. Overall, viable new solutions to some of the problems emerge, but other problems persist.

Background: Measurements of $\ensuremath{\beta}$ decay provide important nuclear structure information that can be used to probe isospin asymmetries and inform nuclear astrophysics studies.Purpose: To measure the $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ decay of $^{26}\text{P}$ and compare the results with previous experimental results and shell-model calculations.Method: A $^{26}\text{P}$ fast beam produced using nuclear fragmentation was implanted into a planar germanium detector. Its $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray emission was measured with an array of 16 high-purity germanium detectors. Positrons emitted in the decay were detected in coincidence to reduce the background.Results: The absolute intensities of $^{26}\text{P}\phantom{\rule{4.pt}{0ex}}\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays were determined. A total of six new $\ensuremath{\beta}$-decay branches and 15 new $\ensuremath{\gamma}$-ray lines have been observed for the first time in $^{26}\text{P}\phantom{\rule{4.pt}{0ex}}\ensuremath{\beta}$ decay. A complete $\ensuremath{\beta}$-decay scheme was built for the allowed transitions to bound excited states of $^{26}\text{Si}$. $ft$ values and Gamow-Teller strengths were also determined for these transitions and compared with shell-model calculations and the mirror $\ensuremath{\beta}$ decay of $^{26}\text{Na}$, revealing significant mirror asymmetries.Conclusions: A very good agreement with theoretical predictions based on the USDB shell model is observed. The significant mirror asymmetry observed for the transition to the first excited state $(\ensuremath{\delta}=51(10)%)$ may be evidence for a proton halo in $^{26}\text{P}$.

We have designed and built a lithium target station for the production of 6He atoms for fundamental research. The system relies on the reaction 7Li(2H,3He)6He using a deuteron beam provided by the tandem Van de Graaff accelerator available at the Center for Experimental Nuclear Physics and Astrophysics of the University of Washington, which can reach a maximum intensity and energy of 10 μ A and 18 MeV. The extracted intensity of gaseous 6He atoms was measured to be ∼ 10 9 atoms/s in a low-background experimental area.

The preparation of ^(114)mIn sources of conversion electrons in the energy range 162–190 keV and β continuum with a 1989 keV endpoint via ion implantation of ^(113)In into Al substrates and subsequent irradiation by thermal and epi-thermal neutrons in a nuclear reactor is described

In January, 2018, Fornal and Grinstein proposed that a previously unobserved neutron decay branch to a dark matter particle (χ) could account for the discrepancy in the neutron lifetime observed in two different types of experiments. One of the possible final states discussed includes a single χ along with an e+e− pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with ∼ 4π acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). We use the timing information of coincidence events to select candidate dark sector particle decays by applying a timing calibration and selecting events within a physically-forbidden timing region for conventional n → p + e- + ν̅e decays. The summed kinetic energy (Ee+e−) from such events is reconstructed and used to set limits, as a function of the χ mass, on the branching fraction for this decay channel.

Ten types of 23Na implanted targets have been fabricated for the purposes of investigating the effects of proton beam bombardment on the implanted sodium distribution. Targets were implanted at energies of ENa = 10–30 keV using copper, tantalum, and nickel as host materials. Thin layers (100–200 A) of chromium and gold were also evaporated over some of the targets to provide a protective layer for the implanted sodium. The 23Na(p, γ) resonance at a lab proton energy of Ep = 309 keV was used to determine the implanted distribution. Successive resonance profile measurements are presented for each implanted target, and the concurrent loss of 23Na resulting from beam bombardment is reported. The calculated temperature rise of the targets indicates that beam heating has a negligible effect on the implanted sodium distribution, and that the principal mechanism for 23Na loss during beam bombardment is sputtering.

The thermonuclear ^{30}P(p,γ)^{31}S reaction rate is critical for modeling the final elemental and isotopic abundances of ONe nova nucleosynthesis, which affect the calibration of proposed nova thermometers and the identification of presolar nova grains, respectively. Unfortunately, the rate of this reaction is essentially unconstrained experimentally, because the strengths of key ^{31}S proton capture resonance states are not known, largely due to uncertainties in their spins and parities. Using the β decay of ^{31}Cl, we have observed the β-delayed γ decay of a ^{31}S state at E_{x}=6390.2(7) keV, with a ^{30}P(p,γ)^{31}S resonance energy of E_{r}=259.3(8) keV, in the middle of the ^{30}P(p,γ)^{31}S Gamow window for peak nova temperatures. This state exhibits isospin mixing with the nearby isobaric analog state at E_{x}=6279.0(6) keV, giving it an unambiguous spin and parity of 3/2^{+} and making it an important l=0 resonance for proton capture on ^{30}P.

Several lifetimes in $^{23}\text{Mg}$ have been determined for the first time using the Doppler-shift attenuation method. A Monte Carlo simulation code has been written to model the $\ensuremath{\gamma}$-ray line shape. An upper limit of $\ensuremath{\tau}l12$ fs at the 95% C.L. has been obtained for the astrophysically important 7787 keV state.

We report here on experiments with radioactive 21 Na and 7 Be beams performed by Colorado School of Mines students at the ISAC facility of TRIUMF and the Holifield Radioactive Ion Beam Facility (HRIBF) of ORNL. At TRIUMF, the DRAGON recoil separator and its segmented BGO array were used to investigate higher energy resonances in the reaction H( 21 Na, γ) 22 Mg. Using the HRIBF we performed an experiment with a 7 Be ion beam to measure scattering off Hydrogen and Carbon. Both elastic 7 Be + p scattering and for the first time resonant inelastic scattering 7 Be(p, p′) 7 Be ∗ were observed.

Background: The Doppler broadening of gamma-ray peaks is due to nuclear recoil from beta-delayed nucleon emission can be used to measure the energies of the nucleons. This method has never been tested using beta-delayed proton emission or applied to a recoil heavier than A = 10. Purpose: To test and apply this Doppler broadening method using gamma-ray peaks from the P-26(beta p gamma)Al-25 decay sequence. Methods: A fast beam of P-26 was implanted into a planar Ge detector, which was used as a P-26 beta-decay trigger. The SeGA array of high-purity Ge detectors was used to detect gamma rays from the P-26(beta p gamma)Al-25 decay sequence. Results: Radiative Doppler broadening in beta-delayed proton-gamma decay was observed for the first time. Moreover, the Doppler broadening analysis method was verified using the 1613-keV gamma-ray line for which the proton energies were previously known. The 1776-keV gamma ray de-exciting the 2720 keV Al-25 level was observed in P-26(beta p gamma)Al-25 decay for the first time and used to determine that the center-of-mass energy of the proton emission feeding the 2720-keV level is 5.1 +/- 1.0 (stat.) +/- 0.6 (syst.) MeV, corresponding to a Si-26 excitation energy of 13.3 +/- 1.0 (stat.) +/- 0.6more » (syst.) MeV for the proton-emitting level. Conclusions: Finally, the Doppler broadening method has been demonstrated to provide practical measurements of the energies for beta-delayed nucleon emissions populating excited states of nuclear recoils at least as heavy as A = 25.« less

The performance of the DRAGON recoil spectrometer at TRIUMF-ISAC has been studied using radiative capture reactions with stable beams of 12C, 20Ne, 21Ne, 23Na, 24Mg and 26Mg. Calibration of the deflection magnet measuring the beam energy was established and the beam suppression factors of the separator were investigated. Yields from six narrow resonances were measured and compared with previous results. For the 1,112.6 keV resonance in Ne 20 ( p , γ ) Na 21 our result is in disagreement with the NACRE database assignment but agrees with another previous result.

A series of astrophysical measurements was recently completed at TRIUMF, related to the unknown total rate of the 21 Na(p, γ ) 22 Mg reaction. With a high intensity 21 Na beam from the ISAC facility, the DRAGON recoil mass spectrometer was used to directly measure seven resonances at center of mass energies from E c.m. = 200 to 1135 keV and determine their respective contributions to the 21 Na(p, γ ) 22 Mg reaction rate in novae and x-ray bursts, as well as their impact on 22 Na production in novae. This study also allowed the investigation of different excited states in 22 Mg; proposed decays and spin assignments are given for the 6246, 6329, and 6609 keV levels.

The 21Na(p,γ)22Mg reaction is an important link in the synthesis of 22Na in oxygenneon novae, and serves as one of the first steps of the rp-process in x-ray bursts. This reaction has recently been studied at the TRIUMF-ISAC radioactive beam facility with the new DRAGON recoil separator. Resonant contributions have been measured for seven 22Mg states, from near the proton threshold to about 6 MeV in excitation energy. We report the results and their impact on the 21Na(p,γ)22Mg reaction rate in novae and x-ray bursts.

The DRAGON (detector of recoils and gamma-rays of nuclear reactions) setup at the ISAC radioactive ion beam facility of TRIUMF, Vancouver, was used to measure the energy loss of stable neon and magnesium as well as of radioactive sodium ions (energy range 200–1150 keV/u) in hydrogen gas. Stopping power values were determined and (as no previous experimental data around the stopping power maximum existed) compared to the available semi-empirical codes SRIM 2000, SRIM 2003, ATIMA and MSTAR. The experimental data seems to favor the new SRIM 2003 approach and will hopefully provide input to a further improvement of the parameter set.

The new DRAGON recoil separator facility, designed and built to measure directly the rates of radiative proton and alpha capture reactions important for nuclear astrophysics, is now in operation at the TRIUMF-ISAC radioactive beams facility in Vancouver, Canada. Experiments have been conducted for the first time on the 21 Na(p,γ) 22 Mg reaction. The evolution of nova explosions, and particularly their 22 Na abundance, depends sensitively on this reaction rate. Commissioning studies using the well-known stable beam reactions 21 Ne(p,γ) 22 Na, 20 Ne(p,γ) 21 Na, and 24 Mg(p,γ) 25 Al have shown that the recoil separator performs within its design specifications both in suppression power and acceptance. The first radioactive beam studies were done using a beam of 5 × 10 8 21 Na atoms/s. Yield measurements recording simultaneously singles and coincident heavy-ion and gamma signals were performed, scanning in energy over the known resonance reported previously in 22 Mg at E cm = 212 keV, and in addition, over a strong resonance observed at E cm ≈822 keV.

The rate of the 21 Na(p,γ) 22 Mg reaction is expected to play a major role in the nucleosynthesis of 22 Na in Oxygen-Neon novae, leading to the emission of a characteristic 1.28 MeV gamma-line. This paper reports on preliminary results of the first direct measurements of this reaction and its astrophysical implications.

In the modelling of nucleosynthesis in nova explosions, temperature and density are important parameters to describe the hydrodynamics. Those parameters are not easy to observe, but specific gamma-ray emitters produced in the explosion provide constrains on the models, such as 22 Na, produced via 21 Na ( p , γ ) 22 Mg ( β + ) 22 Na . The new DRAGON recoil separator facility, designed and built to measure directly the rates of radiative proton and alpha capture reactions, important for nuclear astrophysics, is now operational. Experiments have been conducted on the 21 Na(p , γ) 22 Mg reaction using a radioactive 21 Na beam incident onto a windowless hydrogen gas target. Yield measurements have been performed detecting the prompt gamma and the reaction recoils at E cm ≈ 821 keV and 204 keV.

The newDRAGON facility (detector of recoils and gammas of nuclear reactions), located at the TRlUMF-ISAC Radioactive Beams facility in Vancouver, Canada is nowoperational. This facility is used to study radiative proton capture reactions in inverse kinematics (heavy ion beam onto a light gaseous target) with both stable beams and radioactive beams of mass A ¼ 13–26 in the energy range 0.15–1.5 MeV/u. A double sided silicon strip detector (DSSSD) has been used to detect recoil ions. Tests have been performed to determine the performance of this DSSSD. 2002 Elsevier Science B.V. All rights reserved.

A facility for measuring cross-sections (resonance strengths) for reactions of astrophysical importance involving short-lived, radioactive reactants has been designed, built and installed at the new TRIUMF-ISAC Radioactive Beams Laboratory in Canada. Named DRAGON (Detector of Recoils And Gamma-rays of Nuclear reactions), it has been successfully commissioned with stable and radioactive heavy ion beams from ISAC. This report presents the main components of the facility, namely, the windowless gas target, the surrounding g detector array, the subsequent electromagnetic recoil mass separator, the focal plane detectors for recoils, the detection system for elastics, and the modular electronics and computer software used for the data acquisition. Examples of the operation of the facility for both stable beam reactions and the first radioactive beam reaction study, 21 Naðp;gÞ 22 Mg are also presented, along with future plans for the program. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.0

Background: The thermonuclear Cl-34g,Cl-m(p,gamma)Ar-35 reaction rates are unknown due to a lack of experimental nuclear physics data. Uncertainties in these rates translate to uncertainties in S-34 production in models of classical novae on oxygen-neon white dwarfs. S-34 abundances have the potential to aid in the classification of presolar grains.; Purpose: Determine resonance energies for the Cl-34g,Cl-m(p,gamma)Ar-35 reactions within the region of astrophysical interest for classical novae to a precision of a few keV as an essential first step toward constraining their thermonuclear reaction rates.; Method: Ar-35 excited states were populated by the Ar-36(d,t)Ar-35 reaction at E(d) = 22 MeV and reaction products were momentum analyzed by a high resolution quadrupole-dipole-dipole-dipole (Q3D) magnetic spectrograph.; Results: Seventeen new Ar-35 levels have been detected at a statistically significant level in the region E-x approximate to 5.9-6.7 MeV (E-r < 800 keV) and their excitation energies have been determined to typical uncertainties of 3 keV. The uncertainties for five previously known levels have also been reduced substantially. The measured level density was compared to those calculated using the WBMB Hamiltonian within the sd-pf model space.; Conclusions: Most of the resonances in the region of astrophysical interest have likely been discovered and their energies have been determined, but the resonance strengths are still unknown, and experimentally constraining the Cl-34g,Cl-m(p,gamma)Ar-35 reaction rates will require further experiments.

The preparation of isotopically pure 3 – 6 μ g / cm 2 targets of 20Ne, 24Mg, 28Si, 32S, and 36Ar by the implantation of 25–70 keV ions into 30 μ g / cm 2 carbon foils is described.