Search

Neutron-capture cross sections of neutron-rich nuclei are calculated using a Hauser–Feshbach model when direct experimental cross sections cannot be obtained. A number of codes to perform these calculations exist, and each makes different assumptions about the underlying nuclear physics. We investigated the systematic uncertainty associated with the choice of Hauser-Feshbach code used to calculate the neutron-capture cross section of a short-lived nucleus. The neutron-capture cross section for $$^{73}\hbox {Zn}$$ 73 Zn (n,$$\gamma $$ γ )$$^{74}\hbox {Zn}$$ 74 Zn was calculated using three Hauser-Feshbach statistical model codes: TALYS, CoH, and EMPIRE. The calculation was first performed without any changes to the default settings in each code. Then an experimentally obtained nuclear level density (NLD) and $$\gamma $$ γ -ray strength function ($$\gamma \hbox {SF}$$ γ SF ) were included. Finally, the nuclear structure information was made consistent across the codes. The neutron-capture cross sections obtained from the three codes are in good agreement after including the experimentally obtained NLD and $$\gamma \hbox {SF}$$ γ SF , accounting for differences in the underlying nuclear reaction models, and enforcing consistent approximations for unknown nuclear data. It is possible to use consistent inputs and nuclear physics to reduce the differences in the calculated neutron-capture cross section from different Hauser-Feshbach codes. However, ensuring the treatment of the input of experimental data and other nuclear physics are similar across multiple codes requires a careful investigation. For this reason, more complete documentation of the inputs and physics chosen is important.

New half-lives for exotic isotopes approaching the neutron drip-line in the vicinity of N∼28 for Z=12-15 were measured at the Facility for Rare Isotope Beams (FRIB) with the FRIB decay station initiator. The first experimental results are compared to the latest quasiparticle random phase approximation and shell-model calculations. Overall, the measured half-lives are consistent with the available theoretical descriptions and suggest a well-developed region of deformation below ^{48}Ca in the N=28 isotones. The erosion of the Z=14 subshell closure in Si is experimentally confirmed at N=28, and a reduction in the ^{38}Mg half-life is observed as compared with its isotopic neighbors, which does not seem to be predicted well based on the decay energy and deformation trends. This highlights the need for both additional data in this very exotic region, and for more advanced theoretical efforts.

The $\ensuremath{\beta}$ decays of $^{103,104m}\mathrm{Nb}$ were studied with the Summing NaI(Tl) (SuN) detector at the National Superconducting Cyclotron Laboratory. The $\ensuremath{\beta}$-decay feeding intensity distribution ${I}_{\ensuremath{\beta}}(E)$ for each isotope was extracted by measuring $\ensuremath{\gamma}$ rays in coincidence with an emitted electron. The ${I}_{\ensuremath{\beta}}(E)$ was extracted via the total absorption spectroscopy technique. The ${I}_{\ensuremath{\beta}}(E)$ for each nucleus was compared to predictions made by the quasiparticle random-phase approximation (QRPA) model which is commonly used to calculate $\ensuremath{\beta}$-decay properties for astrophysical applications. The main goal was to provide experimental data for neutron-rich nuclei, relevant to the astrophysical $r$ process. In addition, the extracted $\ensuremath{\beta}$-decay feeding intensity distributions can lead to a better understanding of nuclear structure in a region of rapid structure changes around $A=100$. Finally, experimental data for $^{104m}\mathrm{Nb}$ are also of interest to antineutrino studies of nuclear reactors.

The $\ensuremath{\beta}$ decay of $^{101,102}\mathrm{Zr}$ and $^{109}\mathrm{Tc}$ was studied using the technique of total absorption spectroscopy. The experiment was performed at the National Superconducting Cyclotron Laboratory using the Summing NaI(Tl) (SuN) detector in the first-ever application of total absorption spectroscopy with a fast beam produced via projectile fragmentation. The $\ensuremath{\beta}$-decay feeding intensity and Gamow-Teller transition strength distributions were extracted for these three decays. The extracted distributions were compared to three different quasiparticle random-phase approximation (QRPA) models based on different mean-field potentials. A comparison with calculations from one of the QRPA models was performed to learn about the ground-state shape of the parent nucleus. For $^{101}\mathrm{Zr}$ and $^{102}\mathrm{Zr}$, calculations assuming a pure shape configuration (oblate or prolate) were not able to reproduce the extracted distributions. These results may indicate that some type of mixture between oblate and prolate shapes is necessary to reproduce the extracted distributions. For $^{109}\mathrm{Tc}$, a comparison of the extracted distributions with QRPA calculations suggests a dominant oblate configuration. The other two QRPA models are commonly used to provide $\ensuremath{\beta}$-decay properties in $r$-process network calculations. This work shows the importance of making comparisons between the experimental and theoretical $\ensuremath{\beta}$-decay distributions, rather than just half-lives and $\ensuremath{\beta}$-delayed neutron emission probabilities, as close to the $r$-process path as possible.

The interpretation of observations of cooling neutron star crusts in quasipersistent x-ray transients is affected by predictions of the strength of neutrino cooling via crust Urca processes. The strength of crust Urca neutrino cooling depends sensitively on the electron-capture and $\ensuremath{\beta}$-decay ground-state-to-ground-state transition strengths of neutron-rich rare isotopes. Nuclei with a mass number of $A=61$ are predicted to be among the most abundant in accreted crusts, and the last remaining experimentally undetermined ground-state-to-ground-state transition strength was the $\ensuremath{\beta}$ decay of $^{61}\mathrm{V}$. This Letter reports the first experimental determination of this transition strength, a ground-state branching of ${8.1}_{\ensuremath{-}3.1}^{+4.0}%$, corresponding to a log $ft$ value of ${5.5}_{\ensuremath{-}0.2}^{+0.2}$. This result was achieved through the measurement of the $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays using the total absorption spectrometer SuN and the measurement of the $\ensuremath{\beta}$-delayed neutron branch using the neutron long counter system NERO at the National Superconducting Cyclotron Laboratory at Michigan State University. This method helps to mitigate the impact of the pandemonium effect in extremely neutron-rich nuclei on experimental results. The result implies that $A=61$ nuclei do not provide the strongest cooling in accreted neutron star crusts as expected by some predictions, but that their cooling is still larger compared to most other mass numbers. Only nuclei with mass numbers 31, 33, and 55 are predicted to be cooling more strongly. However, the theoretical predictions for the transition strengths of these nuclei are not consistently accurate enough to draw conclusions on crust cooling. With the experimental approach developed in this work, all relevant transitions are within reach to be studied in the future.

Excited states in ${}^{73,75}\mathrm{Ni}$ were investigated through the $\ensuremath{\beta}$ decay of ${}^{73,75}\mathrm{Co}$ in an experiment performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). The experimental results extended the level scheme of $^{73}\mathrm{Ni}$ to 3.2-MeV excitation energy and provided the experimental information on excited states in $^{75}\mathrm{Ni}$. The $\ensuremath{\beta}$-delayed neutron branching ratio for $^{73}\mathrm{Co}$ was obtained. The experimental results are discussed in comparison with shell-model calculations.

An experiment on 235U was performed with a white-spectrum neutron beam at the Los Alamos Neutron Science Center. Gamma rays from the (n, γ) and (n, f) reactions were measured with the DANCE array. We present spectra of γ rays emitted shortly after any of the reactions occurs. A novel method for background estimation was applied. The spectrum of the late fission γ rays is compared with CGMF predictions.

Two measurements of fission γ rays were performed with the DANCE and NEUANCE arrays using the reactions 235 U(n, f) and 252 Cf(sf). Utilizing the fast time response of the detectors and a method for estimating the accidental background, we obtained the energy spectrum of the late prompt fission γ rays as a function of the time since fission. The experimental results are compared with predictions of the code CGMF folded with GEANT4 simulations of the detector response.

This paper reports on the first β-decay study of 74,75Cu isotopes using the technique of total absorption spectroscopy (TAS). The experiment was performed at the National Superconducting Cyclotron Laboratory at Michigan State University using the Summing NaI(Tl) (SuN) detector. The Cu isotopes are good candidates to probe the single-particle structure in the region because they have one proton outside the Z = 28 shell. Comparing the β-decay intensity distributions in the daughter Zn isotopes to the theoretical predictions provides a stringent test of the calculations. The nuclei in this region are also identified as playing an important role in the astrophysical r-process. The measured β-decay intensity distributions provide essential nuclear physics inputs required to better understand heavy element nucleosynthesis.

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 new experimental setup for low-energy Coulomb excitation experiments was constructed in a collaboration between the National Superconducting Cyclotron Laboratory (NSCL), Lawrence Livermore National Laboratory (LLNL), and the University of Rochester and was commissioned at the general purpose beam line of NSCL’s ReA3 reaccelerator facility. The so-called JANUS setup combines γ -ray detection with the Se gmented G e A rray (SeGA) and scattered particle detection using a pair of segmented double-sided Si detectors (Bambino 2). The low-energy Coulomb excitation program that JANUS enables will complement intermediate-energy Coulomb excitation studies that have long been performed at NSCL by providing access to observables that quantify collectivity beyond the first excited state, including the sign and magnitude of excited-state quadrupole moments. In this work, the setup and its performance will be described based on the commissioning run that used stable 78Kr impinging onto a 1.09 mg/cm 2 208Pb target at a beam energy of 3.9 MeV/u.

Author(s): Elman, B; Gade, A; Janssens, RVF; Ayangeakaa, AD; Bazin, D; Belarge, J; Bender, PC; Brown, BA; Campbell, CM; Carpenter, MP; Crawford, HL; Crider, BP; Fallon, P; Forney, AM; Harker, J; Liddick, SN; Longfellow, B; Lunderberg, E; Prokop, CJ; Sethi, J; Taniuchi, R; Walters, WB; Weisshaar, D; Zhu, S | Abstract: The neutron-rich Ni isotopes have attracted attention in recent years because of the occurrence of shape or configuration coexistence. We report on the difference in population of excited final states in Ni70 following γ-ray tagged one-proton, one-neutron, and two-proton knockout from Cu71, Ni71, and Zn72 rare-isotope beams, respectively. Using variations observed in the relative transition intensities, signaling the changed population of specific final states in the different reactions, the role of neutron and proton configurations in excited states of Ni70 is probed schematically, with the goal of identifying those that carry, as leading configuration, proton excitations across the Z=28 shell closure. Such states are suggested in the literature to form a collective structure associated with prolate deformation. Adding to the body of knowledge for Ni70, 29 new transitions are reported, of which 15 are placed in its level scheme.

Background: The rapid neutron capture process is one of the main nucleosynthesis processes of elements heavier than Fe. Uncertainties in nuclear properties, such as masses, half-lives, and $\ensuremath{\beta}$-delayed neutron probabilities can cause orders of magnitude of variation within astrophysical $r$-process simulations. Presently, theoretical models are used to make global predictions of various nuclear properties for the thousands of nuclei required for these simulations, and measurements are required to benchmark these models, especially far from stability.Purpose: $\ensuremath{\beta}$-decay strength distributions can be used to not only inform astrophysical $r$-process simulations, but also to provide a stringent test for theoretical calculations. The aim of this work is to provide accurate strength distributions for $^{69,71}\mathrm{Co}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay.Method: The technique of total absorption spectroscopy was used to measure the $\ensuremath{\beta}$ decay of $^{69,71}\mathrm{Co}$ for the first time at the National Superconducting Cyclotron Laboratory. The ions were implanted in a double-sided silicon strip detector at the center of the Summing NaI(Tl) detector and identified using standard particle identification methods. The response of the detection system to the $\ensuremath{\beta}$-decay electron and subsequent $\ensuremath{\gamma}$-ray radiation was fit to the observed experimental data using a ${\ensuremath{\chi}}^{2}$-minimization technique.Results: $\ensuremath{\beta}$-feeding intensities and Gamow-Teller strength distributions were extracted from the fits of the experimental data. The $\ensuremath{\beta}$-decay intensities show that there is a large percentage of feeding to levels above 2 MeV, which have not been observed in previous studies. The resultant $\ensuremath{\beta}$-feeding intensities and Gamow-Teller strength distributions were compared to shell model and quasiparticle random phase approximation (QRPA) calculations.Conclusions: Comparing experimentally determined $\ensuremath{\beta}$-decay strength distributions provides a test of models, which are commonly used for global $\ensuremath{\beta}$-decay properties for astrophysical calculations. This work highlights the importance of performing detailed comparisons of models to experimental data, particularly far from stability and as close to the $r$-process path as possible.

This corrects the article DOI: 10.1103/PhysRevLett.116.242502.

Background: The recent observation of a neutron-star merger finally confirmed one astrophysical location of the rapid neutron-capture process (r-process). Evidence of the production of $Al140$ nuclei was seen, but there is still little detailed information about how those lighter elements are produced in such an environment. Many of the questions surrounding the $A\ensuremath{\approx}80$ nuclei are likely to be answered only when the nuclear physics involved in the production of r-process nuclei is well understood. Neutron-capture reactions are an important component of the r-process, and neutron-capture cross sections of r-process nuclei, which are very neutron rich, have large uncertainties.Purpose: Indirectly determine the neutron-capture cross section and reaction rate of $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$.Methods: The nuclear level density (NLD) and $\ensuremath{\gamma}$-ray strength function ($\ensuremath{\gamma}\mathrm{SF}$) of $^{74}\mathrm{Zn}$ were determined following a total absorption spectroscopy (TAS) experiment focused on the $\ensuremath{\beta}$ decay of $^{74}\mathrm{Cu}$ into $^{74}\mathrm{Zn}$ performed at the National Superconducting Cyclotron Laboratory. The NLD and $\ensuremath{\gamma}\mathrm{SF}$ were used as inputs in a Hauser-Feshbach statistical model to calculate the neutron-capture cross section and reaction rate.Results: The NLD and $\ensuremath{\gamma}\mathrm{SF}$ of $^{74}\mathrm{Zn}$ were experimentally constrained for the first time using $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays measured with TAS and the $\ensuremath{\beta}$-Oslo method. The NLD and $\ensuremath{\gamma}\mathrm{SF}$ were then used to constrain the neutron-capture cross section and reaction rate for the $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$ reaction.Conclusions: The uncertainty in the neutron-capture cross section and reaction rate of $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$ calculated in TALYS was reduced to under a factor of 2 from a factor of 5 in the cross section and a factor of 11 in the reaction rate using the experimentally obtained NLD and $\ensuremath{\gamma}\mathrm{SF}$.

The use of argon as a detection and shielding medium for neutrino and dark matter experiments has made the precise knowledge of the cross section for neutron capture on argon an important design and operational parameter. Since previous measurements were averaged over thermal spectra and have significant disagreements, a differential measurement has been performed using a Time-Of-Flight neutron beam and a $\sim$4$\pi$ gamma spectrometer. A fit to the differential cross section from $0.015-0.15$\,eV, assuming a $1/v$ energy dependence, yields $\sigma^{2200} = 673 \pm 26 \text{ (stat.)} \pm 59 \text{ (sys.)}$\,mb., Comment: 5 pages, 4 figures; final version of the paper; added details and fix references

$\ensuremath{\beta}$-decay half-lives of neutron-rich nuclides in the $A=100--110$ mass region have been measured using an implantation station installed inside of the Summing NaI(Tl) (SuN) detector at the National Superconducting Cyclotron Laboratory. Accurate half-lives for these nuclides are important for nuclear astrophysics, nuclear structure, and nuclear technology. The half-lives from the present work are compared with previous measurements, showing overall good agreement.

Numerous scientific fields including astrophysics, nuclear power, and nuclear forensics require a knowledge of basic nuclear properties for large numbers of short-lived, radioactive isotopes far removed from stable nuclei. Neutron-capture cross sections are one such piece of nuclear data where direct measurements are not possible and theoretical predictions can vary by orders of magnitude. A recently developed indirect technique for inferring neutron capture rates, the β -Oslo method, has been introduced but not compared against a known, directly measured neutron capture cross section. To provide this benchmark, two indirect methods based on β decay and charged-particle reactions were used to extract the nuclear level density and γ -ray strength function of 51 Ti . The nuclear level density and γ -ray strength function from the two data sets were found to be equivalent and were used to extract the neutron capture cross section of 50 Ti which agrees with previous direct measurements at high neutron energies. The results demonstrate the validity of the β -Oslo method for extracting neutron capture cross sections of short-lived nuclei and provide a sufficiently small uncertainty to be used in various applications.

A digital data acquisition system (DDAS) has been implemented for the Low Energy Neutron Detector Array (LENDA). LENDA is an array of 24 BC-408 plastic-scintillator bars designed to measure low-energy neutrons with kinetic energies in the range of 100 keV to 10 MeV from (p,n)-type charge-exchange reactions. Compared to the previous data acquisition (DAQ) system for LENDA, DDAS offers the possibility to lower the neutron detection threshold, increase the overall neutron-detection efficiency, decrease the dead time of the system, and allow for easy expansion of the array. The system utilized in this work was XIA's Digital Gamma Finder Pixie-16 250 MHz digitizers. A detector-limited timing resolution of 400 ps was achieved for a single LENDA bar. Using DDAS, the neutron detection threshold of the system was reduced compared to the previous analog system, now reaching below 100 keV. The new DAQ system was successfully used in a recent charge-exchange experiment using the $^{16}$C(p,n) reaction at the National Superconducting Cyclotron Laboratory (NSCL)., Typo fixed in Abstract

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.

Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their $\ensuremath{\gamma}$-emission probability at very low $\ensuremath{\gamma}$ energies. In this work, we present measurements of the $\ensuremath{\gamma}\text{-decay}$ strength of $^{70}\mathrm{Ni}$ over the wide range $1.3\ensuremath{\le}{E}_{\ensuremath{\gamma}}\ensuremath{\le}8$ MeV. A significant enhancement is found in the $\ensuremath{\gamma}\text{-decay}$ strength for transitions with ${E}_{\ensuremath{\gamma}}l3$ MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed $E1$-strength calculations within the quasiparticle time-blocking approximation, which describe our data above ${E}_{\ensuremath{\gamma}}\ensuremath{\simeq}5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $\ensuremath{\gamma}$ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation, and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich $^{72,74,76}\mathrm{Ni}$.

The Digital Data Acquisition System (DDAS) at the National Superconducting Cyclotron Laboratory (NSCL) has expanded to instrument arrays composed of fast-scintillator detectors. The expansion has motivated the development of software designed to optimize the time- and energy-resolving capabilities of the system, which is a collection of 16-channel FPGA-programmable modules running 12- and 14-bit ADCs with sampling frequencies of 100 and 250 MSPS, respectively. Using the techniques described herein, the time resolution of the DDAS electronics has been substantially improved. For signal amplitudes occupying 10 % the full range of the ADC, the time resolution of the DDAS electronics, measured online, has been reduced to 100 ps and 40 ps for 100 MSPS and 250 MSPS modules, respectively. A time resolution of ≈ 350 ps, at 511 keV, between two 38 mm×38 mm lanthanum bromide (LaBr3) detectors, equipped with Hamamatsu R6231 photomultiplier tubes (PMTs), has also been realized. Similar optimization techniques applied to the DDAS energy-extraction algorithms have yielded energy resolutions below 2% at 1.33 MeV for both the 100 and 250 MSPS digitizers using the same LaBr3 detectors. The techniques described in this work are broadly applicable to other digital acquisition systems that are capable of recording the digitized raw detector signals.

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.

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

International audience; Neutron-rich isotopes are an important source of new information on nuclear physics. Specifically, the spin-isospin components in the nucleon-nucleon (NN) interaction, e.g., the proton-neutron tensor force, are expected to modify shell structure in exotic nuclei. These potential changes in the intrinsic shell structure are of fundamental interest. The study of the excitation energy of states corresponding to specific configurations in even-even isotopes, together with the single-particle character of the first excited states of odd-A, neutron-rich Ni isotopes, probes the evolution of the neutron orbitals around the Fermi surface as a function of the neutron number a step forward in the understanding of the region and the nature of the NN interaction at large N/Z ratios. In an experiment carried out at the National Superconducting Cyclotron Laboratory [1], new spectroscopic information was obtained for 68Ni and the distribution of single-particle strengths in 67,69Ni was characterized by means of single-neutron knockout from 68,70Ni secondary beams. The spectroscopic strengths, deduced from the measured partial cross sections to the individual states tagged by their de-exciting gamma rays, is used to identify and quantify configurations that involve neutron excitations across the N = 40 harmonic oscillator shell closure. The de-excitation γ rays were measured with the GRETINA tracking array [2]. The results challenge the validity of the most current shell-model Hamiltonians and effective interactions, highlighting shortcomings that cannot yet be explained. These results suggest that our understanding of the low-energy states in such nuclei is not complete and requires further investigation.

Low-lying excited states in $^{38,40}\mathrm{P}$ have been identified in the $\ensuremath{\beta}$ decay of ${T}_{z}=+5,\phantom{\rule{0.16em}{0ex}}+6, ^{38,40}\mathrm{Si}$. Based on the allowed nature of the Gamow-Teller (GT) decay observed, these states are assigned spin and parity of ${1}^{+}$ and are core-excited 1p1h intruder states with a parity opposite to the ground state. The occurrence of intruder states at low energies highlights the importance of pairing and quadrupole correlation energies in lowering the intruder states despite the $N=20$ shell gap. Configuration interaction shell model calculations with the state-of-art SDPF-MU effective interaction were performed to understand the structure of these 1p1h states in the even-$A$ phosphorus isotopes. States in $^{40}\mathrm{P}$ with $N=25$ were found to have very complex configurations involving all the $fp$ orbitals leading to deformed states as seen in neutron-rich nuclei with $N\ensuremath{\approx}28$. The calculated GT matrix elements for the $\ensuremath{\beta}$ decay highlight the dominance of the decay of the core neutrons rather than the valence neutrons.

Neutron-capture reactions play an important role in heavy element nucleosynthesis, since they are the driving force for the two processes that create the vast majority of the heavy elements. When a neutron capture occurs on a short-lived nucleus, it is extremely challenging to study the reaction directly and therefore the use of indirect techniques is essential. The present work reports on such an indirect measurement that provides strong constraints on the 68Ni(n, γ)69Ni reaction rate. This is done by populating the compound nucleus 69Ni via the β decay of 69Co and measuring the γ-ray deexcitation of excited states in 69Ni. The β-Oslo method was used to extract the γ-ray strength function and the nuclear level density. In addition the half-life of 69Co was extracted and found to be in agreement with previous literature values. Before the present results, the 68Ni(n, γ)69Ni reaction was unconstrained and the purely theoretical reaction rate was highly uncertain. The new uncertainty on the reaction rate based on the present experiment (variation between upper and lower limit) is approximately a factor of 3. The commonly used reaction libraries JINA-REACLIB and BRUSLIB are in relatively good agreement with the experimental rate. The impact of the new rate on weak r-process calculations is discussed. This research was first published in Journal of Physics G: Nuclear and Particle Physics. © IOP Publishing.

The distribution of single-particle strength in 67,69Ni was characterized with one-neutron knockout reactions from intermediate-energy 68,70Ni secondary beams, selectively populating neutron-hole configurations at N = 39 and 41, respectively. The spectroscopic strengths deduced from the measured partial cross sections to the individual final states, as tagged by their γ-ray decays, are used to identify and quantify neutron configurations in the wave functions. While 69Ni compares well to shell-model predictions, the results for 67Ni challenge the validity of current effective shell-model Hamiltonians by revealing discrepancies that cannot be explained so far. These results suggest that our understanding of the low-lying states in the neutron-rich, semi-magic Ni isotopes may be incomplete and requires further investigation on both the experimental and theoretical sides.

The $\ensuremath{\beta}$-decay intensity of $^{70}\mathrm{Co}$ was measured for the first time using the technique of total absorption spectroscopy. The large $\ensuremath{\beta}$-decay $Q$ value [12.3(3) MeV] offers a rare opportunity to study $\ensuremath{\beta}$-decay properties in a broad energy range. Two surprising features were observed in the experimental results, namely, the large fragmentation of the $\ensuremath{\beta}$ intensity at high energies, as well as the strong competition between $\ensuremath{\gamma}$ rays and neutrons, up to more than 2 MeV above the neutron-separation energy. The data are compared to two theoretical calculations: the shell model and the quasiparticle random phase approximation (QRPA). Both models seem to be missing a significant strength at high excitation energies. Possible interpretations of this discrepancy are discussed. The shell model is used for a detailed nuclear structure interpretation and helps to explain the observed $\ensuremath{\gamma}$-neutron competition. The comparison to the QRPA calculations is done as a means to test a model that provides global $\ensuremath{\beta}$-decay properties for astrophysical calculations. Our work demonstrates the importance of performing detailed comparisons to experimental results, beyond the simple half-life comparisons. A realistic and robust description of the $\ensuremath{\beta}$-decay intensity is crucial for our understanding of nuclear structure as well as of $r$-process nucleosynthesis.

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 $^{26}\text{P}$.

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.

Nuclear reactions where an exotic nucleus captures a neutron are critical for a wide variety of applications, from energy production and national security, to astrophysical processes, and nucleosynthesis. Neutron capture rates are well constrained near stable isotopes where experimental data are available; however, moving far from the valley of stability, uncertainties grow by orders of magnitude. This is due to the complete lack of experimental constraints, as the direct measurement of a neutron-capture reaction on a short-lived nucleus is extremely challenging. Here, we report on the first experimental extraction of a neutron capture reaction rate on ^{69}Ni, a nucleus that is five neutrons away from the last stable isotope of Ni. The implications of this measurement on nucleosynthesis around mass 70 are discussed, and the impact of similar future measurements on the understanding of the origin of the heavy elements in the cosmos is presented.

Author(s): Crider, BP; Prokop, CJ; Liddick, SN; Al-Shudifat, M; Ayangeakaa, AD; Carpenter, MP; Carroll, JJ; Chen, J; Chiara, CJ; David, HM; Dombos, AC; Go, S; Grzywacz, R; Harker, J; Janssens, RVF; Larson, N; Lauritsen, T; Lewis, R; Quinn, SJ; Recchia, F; Spyrou, A; Suchyta, S; Walters, WB; Zhu, S | Abstract: Shape coexistence near closed-shell nuclei, whereby states associated with deformed shapes appear at relatively low excitation energy alongside spherical ones, is indicative of the rapid change in structure that can occur with the addition or removal of a few protons or neutrons. Near 68Ni (Z=28, N=40), the identification of shape coexistence hinges on hitherto undetermined transition rates to and from low-energy 0+ states. In 68,70Ni, new lifetimes and branching ratios have been measured. These data enable quantitative descriptions of the 0+ states through the deduced transition rates and serve as sensitive probes for characterizing their nuclear wave functions. The results are compared to, and consistent with, large-scale shell-model calculations which predict shape coexistence. With the firm identification of this phenomenon near 68Ni, shape coexistence is now observed in all currently accessible regions of the nuclear chart with closed proton shells and mid-shell neutrons.

In recent models, the neutron-rich Ni isotopes around $N=40$ are predicted to exhibit multiple low-energy excited ${0}^{+}$ states attributed to neutron and proton excitations across both the $N=40$ and $Z=28$ shell gaps. In $^{68}\mathrm{Ni}$, the three observed ${0}^{+}$ states have been interpreted in terms of triple shape coexistence between spherical, oblate, and prolate deformed shapes. In the present work a new $({0}_{2}^{+})$ state at an energy of 1567 keV has been discovered in $^{70}\mathrm{Ni}$ by using $\ensuremath{\beta}$-delayed, $\ensuremath{\gamma}$-ray spectroscopy following the decay of $^{70}\mathrm{Co}$. The precipitous drop in the energy of the prolate-deformed ${0}^{+}$ level between $^{68}\mathrm{Ni}$ and $^{70}\mathrm{Ni}$ with the addition of two neutrons compares favorably with results of Monte Carlo shell-model calculations carried out in the large $fp{g}_{9/2}{d}_{5/2}$ model space, which predict a ${0}_{2}^{+}$ state at 1525 keV in $^{70}\mathrm{Ni}$. The result extends the shape-coexistence picture in the region to $^{70}\mathrm{Ni}$ and confirms the importance of the role of the tensor component of the monopole interaction in describing the structure of neutron-rich nuclei.

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.

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

International audience; Analyses of data from both deep inelastic reactions at Argonne National Laboratory and single-and multiple-particle knockout reactions at Michigan State University revealed new gamma-ray transitions in even-even Ni-66,68,70,72(38,40,42,44) and in Co-67(40) that provide strong evidence for multiple shape coexistence at N = 38 and 40 and deep prolate minima in Ni-70(42) and isotonic Cu-71(42). A new transition at 642 keV is proposed for Ni-66 as the prolate 2(+) to 0(+) transition. Two new transitions in Ni-72 at 915 and 1225 keV were identified in the knock-out reaction study and could represent de-population of prolate states. Taken together with recent theoretical work using the Monte Carlo shell model, a well defined region of shape coexistence can be seen existing precisely between 38

The structure of semi-magic Ni-70(28)42 was investigated following complementary multinucleon-transfer and secondary fragmentation reactions. Changes to the higher-spin, presumed negative-parity states based on observed gamma-ray coincidence relationships result in better agreement with shell-model calculations using effective interactions in the neutron f(5/2)pg(9/2) model space. The second 2(+) and (4(+)) states, however, can only be successfully described when proton excitations across the Z = 28 shell gap are included. Monte Carlo shell-model calculations suggest that the latter two states are part of a prolate-deformed intruder sequence, establishing an instance of shape coexistence at low excitation energies similar to that observed recently in neighboring Ni-68.

Author(s): Chiara, CJ; Walters, WB; Janssens, RVF; Broda, R; Albers, M; Alcorta, M; Bertone, PF; Carpenter, MP; Hoffman, CR; Lauritsen, T; Rogers, AM; Seweryniak, D; Zhu, S; Kondev, FG; Fornal, B; Krolas, W; Wrzesinski, J; Larson, N; Liddick, SN; Prokop, C; Suchyta, S; David, HM; Doherty, DT | Abstract: The level schemes of 68Ni and 67Co were extended following 70Zninduced deep-inelastic reactions. No evidence for a previously reported proton intruder 0+ state at 2202 keV in 68Ni was found. In 67Co, two new states at 3216 and 3415 keV have been established; additional states associated with the intruder configuration have yet to be identified.