Search

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.

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.

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.

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 $\ensuremath{\gamma}\mathrm{SF}$.

$\ensuremath{\beta}$-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.

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

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.

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.

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 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.