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  1. β -delayed neutron emissions from N > 50 gallium isotopes

    β-delayed γ-neutron spectroscopy has been performed on the decay of A = 84 to 87 gallium isotopes at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of 3He neutron counters (BRIKEN). β-2n-γ events were measured in the decays of all of the four isotopes for the first time, which is direct evidence for populating the excited states of two-neutron daughter nuclei. Detailed decay schemes with the γ branching ratios were obtained for these isotopes, and the neutron emission probabilities (Pxn) were updated from the previous study. Hauser-Feshbach statistical model calculations were performed to understand the experimentalmore » branching ratios. We found that the P1n and P2n values are sensitive to the nuclear level densities of 1n daughter nuclei and showed that the statistical model reproduced the P2n/P1n ratio better when experimental levels plus shell-model level densities fit by the Gilbert-Cameron formula were used as the level-density input. We also showed the neutron and γ branching ratios are sensitive to the ground-state spin of the parent nucleus. Our statistical model analysis suggested J ≤ 3 for the unknown ground-state spin of the odd-odd nucleus 86Ga, from the Iγ(4+ → 2+)/Iγ(2+ → 0+) ratio of 84Ga and the P2n/P1n ratio. In conclusion, these results show the necessity of detailed understanding of the decay scheme, including data from neutron spectroscopy, in addition to γ measurements of the multineutron emitters.« less
  2. Evidence of nonstatistical neutron emission following $$β$$ decay near doubly magic 132Sn

    Models of the β-delayed neutron emission (βn) assume that neutrons are emitted statistically via an intermediate compound nucleus post β decay. Evidence to the contrary was found in an 134In β-decay experiment carried out at ISOLDE CERN. Neutron emission probabilities from the unbound states in 134Sn to known low-lying, single-particle states in 133Sn were measured. The neutron energies were determined using the time-of-flight technique, and the subsequent decay of excited states in 133Sn was studied using γ-ray detectors. Individual βn probabilities were determined by correlating the relative intensities and energies of neutrons and γ rays. The experimental data disagree withmore » the predictions of representative statistical models which are based upon the compound nucleus postulate. Our results suggest that violation of the compound nucleus assumption may occur in β-delayed neutron emission. This impacts the neutron-emission probabilities and other properties of nuclei participating in the r-process. A model of neutron emission, which links the observed neutron emission probabilities to nuclear shell effects, is proposed.« less
  3. The Pixel Luminosity Telescope: a detector for luminosity measurement at CMS using silicon pixel sensors

    The Pixel Luminosity Telescope is a silicon pixel detector dedicated to luminosity measurement at the CMS experiment at the LHC. It is located approximately 1.75 m from the interaction point and arranged into 16 “telescopes”, with eight telescopes installed around the beam pipe at either end of the detector and each telescope composed of three individual silicon sensor planes. The per-bunch instantaneous luminosity is measured by counting events where all three planes in the telescope register a hit, using a special readout at the full LHC bunch-crossing rate of 40 MHz. The full pixel information is read out at amore » lower rate and can be used to determine calibrations, corrections, and systematic uncertainties for the online and offline measurements. This paper details the commissioning, operational history, and performance of the detector during Run 2 (2015–18) of the LHC, as well as preparations for Run 3, which will begin in 2022.« less
  4. $$β$$-delayed neutron spectroscopy of 133In

    The decay properties of 133In were studied in detail at the ISOLDE Decay Station. The implementation of the Resonance Ionization Laser Ion Source allowed separate measurements of its 9/2+ ground state (133more » $$g$$In) and 1/2- isomer (133$$m$$In). With the use of $$β$$-delayed neutron and $$γ$$ spectroscopy, the decay strengths above the neutron separation energy were quantified in this neutron-rich nucleus for the first time. The allowed Gamow-Teller transition 9/2+→7/2+ was located at 5.93 MeV in the 133$$g$$In decay with a log $ft$ = 4.7(1). In addition, several neutron-unbound states were populated at lower excitation energies by the first-forbidden decays of 133$g,m$In. We assigned spins and parities to those neutron-unbound states based on the $$β$$-decay selection rules, the log $ft$ values, and systematics.« less
  5. Microsecond Isomer at the N = 20 Island of Shape Inversion Observed at FRIB

    Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)-μs isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV γ rays in coincidence with 32Na nuclei. This is the only known microsecond isomer (1 μs ≤T1/2<1 ms) in the region. This nucleus is at the heart of the N=20 island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories. It can be represented as the coupling of a proton hole and neutron particle to 32Mg, 32Mg+π-1+1. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of 32Mg, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed 2+ state at 885 keV and a low-lying shape-coexisting 0more » $$^+_2$$ state at 1058 keV. Here, we suggest two possible explanations for the 625-keV isomer in 32Na: a 6- spherical shape isomer that decays by E2 or a 0+ deformed spin isomer that decays by M2. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.« less
  6. Demonstration of the neutron tracking capability of NEXT array in time-of-flight measurements to improve energy resolution

    Precise neutron-energy measurements are required to probe the nuclear structure effects of neutron-rich nuclei, where β-delayed neutron emission becomes a dominant decay mode. The Neutron dEtector with Xn Tracking (NEXT) array has been designed and constructed to measure β-delayed neutrons with better energy resolution. The new design localizes the neutron interaction position by optically segmenting the detector along the direction of the neutron flight path, reducing the associated uncertainties in the neutron time-of-flight measurements. This significantly improves the energy resolution without losing the necessary detection efficiency. The proof-of-principle and efficiency measurements showed promising results. Herein, this article details the implementationmore » of the neutron tracking capability of NEXT array in time-of-flight measurements.« less
  7. Crossing N = 28 Toward the Neutron Drip Line: First Measurement of Half-Lives at FRIB

    Here, 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 48Ca 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 38Mg half-life is observed as compared with itsmore » 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.« less
  8. Neutron detection efficiency of the Neutron dEtector with Xn Tracking (NEXT)

    An efficient neutron detection system with good energy resolution is required to correctly characterize decays of neutron-rich nuclei where β-delayed neutron emission is a dominant decay mode. The Neutron dEtector with Xn Tracking (NEXT) has been designed to measure β-delayed neutron emitters. By segmenting the detector along the neutron flight path, NEXT reduces the associated uncertainties in neutron time-of-flight measurements, improving energy resolution while maintaining detection efficiency. Detector prototypes are comprised of optically separated segments of a neutron-gamma discriminating plastic scintillator coupled to position-sensitive photomultiplier tubes. In this work we discuss how the first performance studies of this detector showedmore » that high intrinsic neutron detection efficiency could be achieved while retaining good energy resolution. The results from the efficiency measurements using neutrons from direct reactions are presented« less
  9. 25Siβ+-decay spectroscopy

    Background: β-decay spectroscopy provides valuable information on exotic nuclei and a stringent test for nuclear theories beyond the stability line. Purpose: To search for new β-delayed protons and γ rays of 25Si to investigate the properties of 25Al excited states. Method: 25Si β decays were measured by using the Gaseous Detector with Germanium Tagging system at the National Superconducting Cyclotron Laboratory. The protons and γ rays emitted in the decay were detected simultaneously. Here, a Monte Carlo method was used to model the Doppler broadening of 24Mg γ-ray lines caused by nuclear recoil from proton emission. Shell-model calculations using twomore » newly developed universal sd-shell Hamiltonians were performed. Results: The most precise 25Si half-life to date has been determined. A new proton branch at 724(4) keV and new proton-γ-ray coincidences have been identified. Three 24Mg γ-ray lines and eight 25Al γ-ray lines are observed for the first time in 25Si decay. The first measurement of the 25Si β-delayed γ-ray intensities through the 25Al unbound states is reported. All the bound states of 25Al are observed to be populated in the β decay of 25Si. Several inconsistencies between the previous measurements have been resolved, and new information on the 25Al level scheme is provided. An enhanced decay scheme has been constructed and compared to the mirror decay of 25Na and the shell-model calculations. Conclusions: The measured excitation energies, γ-ray and proton branchings, log $ft$ values, and Gamow-Teller transition strengths for the states of 25Al populated in the β decay of 25Si are in good agreement with the shell model calculations, offering gratifyingly consistent insights into the fine nuclear structure of 25Al.« less
  10. Low-energy 23Al β-delayed proton decay and 22Na destruction in novae

    The radionuclide 22Na is a target of γ-ray astronomy searches, predicted to be produced during thermonuclear runaways driving classical novae. The 22Na (p, γ) 23Mg reaction is the main destruction channel of 22Na during a nova, hence, its rate is needed to accurately predict the 22Na yield. However, experimental determinations of the resonance strengths have led to inconsistent results. In this Rapid Communication, we report a measurement of the branching ratios of the 23Al β-delayed protons as a probe of the key 204-keV (center-of-mass) 22Na (p, γ) 23Mg resonance strength. We report a factor of 5 lower branching ratio comparedmore » to the most recent literature value. The variation in 22Na yield due to nuclear data inconsistencies was assessed using a series of hydrodynamic nova outburst simulations and has increased to a factor of 3.8, corresponding to a factor of ≈ 2 uncertainty in the maximum detectability distance. Finally, this is the first reported scientific measurement using the Gaseous Detector with Germanium Tagging system.« less
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