arXiv subjects

S. N. Liddick

Publications and source records attributed to S. N. Liddick.

At least 19 recordsLinked to original sources

arXiv2025

Constraining the Synthesis of the Lightest p Nucleus 74Se

We provide the first experimental cross section of the $^{73}\text{As}(p,γ)^{74}\text{Se}$ reaction to constrain one of the main destruction mechanisms of the p nucleus $^{74}\text{Se}$ in explosive stellar environments. The measurement was done using a radioactive $^{73}\text{As}$ beam at effective center-of-mass energies of 2.9 and 2.3 MeV/nucleon. Along with the total cross-section measurement, statistical properties of the $^{74}\text{Se}$ compound nucleus were extracted, constraining the reaction cross section in the upper Gamow window of the $γ$ process. The impact of the experimentally constrained reaction rate on $^{74}\text{Se}$ production in Type II supernovae was investigated through Monte Carlo one-zone network simulations. The results indicate that the overproduction of $^{74}$Se by Type II supernova models cannot be resolved by nuclear physics alone and point toward the need for a more detailed understanding of the astrophysical conditions of relevance for the $γ$ process.

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arXiv2025

Beta-decay Half Lives beyond $^{54}$Ca: A Systematic Survey of Decay Properties approaching the Neutron Dripline

In an experiment performed at the Facility for Rare Isotope Beams (FRIB) using the FRIB Decay Station initiator (FDSi), 15 new half lives of isotopes near $^{54}$Ca were measured. A new method of extracting lifetimes from experimental data, taking into account the unknown $β$-delayed neutron emission branches of very neutron-rich nuclei, was developed to enable systematic uncertainty analysis. The experiment observed a dramatic change in the half-life systematics for the isotopes with neutron number N =34. Beyond N =34, the decline of nuclear lifetime is much slower, leading to longer than anticipated lifetimes for near-dripline nuclei. State-of-the-art shell-model calculations can explain the experimental results for Z$>$19 nuclei, revealing the imprint of shell effects and the need for modification of single-particle neutron states. The results from a newly developed QRPA model with potential for making global predictions were also tested against the experimental results and good agreement was found.

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arXiv2025

Exploring $β$ decay and $β$-delayed neutron emission in exotic $^{46,47}$Cl isotopes

In this paper, $β^-$ and $β$-delayed neutron decays of $^{46,47}$Cl are reported from an experiment carried out at the National Superconducting Cyclotron Laboratory using the Beta Counting System. The half-lives of both $^{46}$Cl and $^{47}$Cl were extracted. Based on the delayed $γ$-ray transitions observed, the level structure of $N = 28$ $^{46}$Ar was determined. Completely different sets of excited states above the first $2^+$ state in $^{46}$Ar were populated in the $^{46}$Cl $\beta0n$ and $^{47}$Cl $\beta1n$ decay channels. Two new $γ$-ray transitions in $^{47}$Ar were identified from the very weak $^{47}$Cl $\beta0n$ decay. Furthermore, $^{46}$Cl $\beta1n$ and $^{47}$Cl $\beta2n$ were also observed to yield different population patterns for levels in $^{45}$Ar, including states of different parities. The experimental results allow us to address some of the open questions related to the delayed neutron emission process. For isotopes with large neutron excess and high $Q_β$ values, delayed neutron emission remains an important decay mode and can be utilized as a powerful spectroscopic tool. Experimental results were compared with shell-model calculations using the FSU and $V_{MU}$ effective interactions.

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arXiv2025

Universal Effective Charges in the $sd$ and $fp$ Shells

The 247-keV state in $^{54}$Sc, populated in the $β$ decay of $^{54}$Ca, is reported here as a nanosecond isomer with a half-life of 26.0(22) ns. The state is interpreted as the $1^+$ member of the $πf_{7/2}\otimesνf_{5/2}$ spin-coupled multiplet, which decays to the $3^+,πf_{7/2} \otimes νp_{1/2}$ ground state. The new half-life corresponds to a pure $E2$ transition with a strength of 1.93(16) W.u., providing the most precise, unambiguous $B(E2)$ value in the neutron-rich $fp$ region to date for a nucleus with valence protons above $Z=20$. Notably, it is roughly four times larger than the $B(E2; 1/2^{-} \rightarrow 5/2^{-})$ value in $^{55}$Ca. The results, as compared to semi-empirical and ab initio shell-model calculations, indicate (1) a weak $N=34$ sub-shell gap relative to $N = 32$, (2) a large $E2$ enhancement in Sc as compared to Ca due to $1p-1h$ proton excitations across $Z=28$, and (3) empirical effective proton and neutron charges, $e_π$ = 1.30(8)$e$ and $e_ν$ = 0.452(7)$e$, respectively, that are in contrast to reports of $e_π\approx 1.1-1.15e$ and $e_ν\approx 0.6-0.8e$ for $fp$-shell nuclei near $N = Z$. We demonstrate that these reports are erroneous and that, in fact, a universal set of effective charges can be used across the $sd$ and $fp$ shells.

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arXiv2025

White Paper on Software Infrastructure for Advanced Nuclear Physics Computing

This White Paper documents the discussion and consensus conclusions of the workshop "Software Infrastructure for Advanced Nuclear Physics Computing" (SANPC 24), which was held at Jefferson Lab on June 20-22, 2024. The workshop brought together members of the US Nuclear Physics community with data scientists and funding agency representatives, to discuss the challenges and opportunities in advanced computing for Nuclear Physics in the coming decade. Opportunities for sustainable support and growth are identified, within the context of existing and currently planned DOE and NSF programs.

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arXiv2024

The evidence of $N=16$ shell closure and $β$-delayed neutron emission from $^{25}$F

We measured the $β$-delayed neutron emission from $^{25}$F for the first time at the Facility for Rare Isotope Beams (FRIB). Using combined neutron and $γ$-ray detector systems of the FRIB Decay Station Initiator (FDSi), we observed $β$-decay transitions populating neutron unbound states between 4.2 and 8 MeV in $^{25}$Ne. The experimental results led to the revision of the $β$-decay half-life and $β$-delayed neutron-emission probability of $^{25}$F. The $β$-decay strength distribution of $^{25}$F extracted from the data agrees with the shell-model predictions using the USDB and SDPF-M effective interactions. This result indicates that the spherical neutron $N = 16$ shell gap persists in $^{25}$F and $^{25}$Ne.

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arXiv2024

Enhanced production of 60Fe in massive stars

Massive stars are a major source of chemical elements in the cosmos, ejecting freshly produced nuclei through winds and core-collapse supernova explosions into the interstellar medium. Among the material ejected, long lived radioisotopes, such as 60Fe (iron) and 26Al (aluminum), offer unique signs of active nucleosynthesis in our galaxy. There is a long-standing discrepancy between the observed 60Fe/26Al ratio by γ-ray telescopes and predictions from supernova models. This discrepancy has been attributed to uncertainties in the nuclear reaction networks producing 60Fe, and one reaction in particular, the neutron-capture on 59Fe. Here we present experimental results that provide a strong constraint on this reaction. We use these results to show that the production of 60Fe in massive stars is higher than previously thought, further increasing the discrepancy between observed and predicted 60Fe/26Al ratios. The persisting discrepancy can therefore not be attributed to nuclear uncertainties, and points to issues in massive-star models.

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arXiv2023

Low spin spectroscopy of neutron-rich 43,44,45Cl via β and (β}n decay

β decay of neutron-rich isotopes 43,45 S,studied at the National Superconducting Cyclotron Laboratory is reported here. β delayed γ transitions were detected by an array of 16 clover detectors surrounding the Beta Counting Station which consists of a 40x40 Double Sided Silicon Strip Detector followed by a Single Sided Silicon Strip Detector. β decay half-lives have been extracted for 43,45 S by correlating implants and decays in the pixelated implant detector with further coincidence with γ transitions in the daughter nucleus. The level structure of 43,45 Cl is expanded by the addition of 20 new γ transitions in 43Cl and 8 in 45 Cl with the observation of core excited negative-parity states for the first time. For 45 S decay, a large fraction of the β decay strength goes to delayed neutron emission populating states in 44 Cl which are also presented. Comparison of experimental observations is made to detailed shell-model calculations using the SDPFSDG-MU interaction to highlight the role of the diminished N = 28 neutron shell gap and the near degeneracy of the proton s 1/2 and d 3/2 orbitals on the structure of the neutron-rich Cl isotopes. The current work also provides further support to a ground state spin-parity assignment of 3/2 + in 45 Cl.

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arXiv2023

$β^-$ decay of neutron-rich $^{45}$Cl at magic number N=28

Results from the study of $β^-$-decay of $^{45}$Cl, produced in the fragmentation of a 140-MeV/u $^{48}$Ca beam, are presented. The half-life for $^{45}$Cl $β$-decay is measured to be 513(36) ms. The $β^-$ and $β^- 1n$ decay of $^{45}$Cl populated excited states in $^{45,44}$Ar, respectively. On the basis of $γ$-ray singles and $γ$-$γ$ coincidence data, decay schemes for the two daughter nuclei have been established. They are compared with shell model calculations using the FSU interaction. The low-lying negative parity states for $^{45}$Ar are well described by a single particle (neutron) occupying orbitals near the Fermi surface, whereas neutron excitations across the $N = 20$ shell gap are needed to explain the positive-parity states which are expected to be populated in allowed Gamow-Teller $β$-decay of $^{45}$Cl. The highest $β$-feeding to the 5/2$^+$ state in $^{45}$Ar from the ground state of $^{45}$Cl points towards a 3/2$^+$ spin-parity assignment of the ground state of the parent over the other possibility of 1/2$^+$. The high Q$_{β^-}$ value of $^{45}$Cl decay allows for the population of $1p1h$ states above the neutron separation energy in $^{45}$Ar leading to positive parity states of $^{44}$Ar being populated by removal of one neutron from the $sd$ shell. The spin-parities of the excited levels in $^{44}$Ar are tentatively assigned for the first time by comparison with the shell model calculations. The 2978~keV level of $^{44}$Ar is identified as the excited 0$^+$ level which could correspond to a different configuration from the ground state.

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arXiv2023

Cross Section Measurement of the $^{82}$Kr(p,$γ$)$^{83}$Rb Reaction in Inverse Kinematics

The total cross section of the $^{82}$Kr(p,$γ$)$^{83}$Rb reaction was measured for the first time at effective center-of-mass energies between 2.4 and 3.0 MeV, within the relevant Gamow window for the astrophysical $γ$ process. The experiment took place at the National Superconducting Cyclotron Laboratory at Michigan State University using the ReA facility. A $^{82}$Kr beam was directed onto a hydrogen gas cell located at the center of the Summing NaI(Tl) (SuN) detector. The obtained spectra were analyzed using the $γ$-summing technique and the extracted cross section was compared to standard statistical model calculations using the \textsc{non-smoker} and \textsc{talys} codes. The comparison indicates that standard statistical model calculations tend to overproduce the cross section of the $^{82}$Kr(p,$γ$)$^{83}$Rb reaction relative to the experimentally measured values. Furthermore, the experimental data was used to provide additional constraints on the nuclear level density and $γ$-ray strength function used in the statistical model calculations.

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arXiv2023

Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB

Excited-state spectroscopy from the first Facility for Rare Isotope Beams (FRIB) experiment is reported. A 24(2)-$μ$s isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV $γ$ rays in coincidence with $^{32}\textrm{Na}$ nuclei. This is the only known microsecond isomer ($1{\text{ }μ\text{s}}\leq T_{1/2} < 1\text{ ms}$) in the region. This nucleus is at the heart of the $N=20$ island of shape inversion and is at the crossroads of 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 $^{32}\textrm{Mg}$, $^{32}\textrm{Mg}+π^{-1} + ν^{+1}$. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of $^{32}\textrm{Mg}$, 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 $0_2^+$ state at 1058 keV. We suggest two possible explanations for the 625-keV isomer in $^{32}$Na: 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.

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arXiv2022

$^{57}$Zn $β$-delayed proton emission establishes the $^{56}$Ni $rp$-process waiting point bypass

We measured the $^{57}$Zn $β$-delayed proton ($β$p) and $γ$ emission at the National Superconducting Cyclotron Laboratory. We find a $^{57}$Zn half-life of 43.6 $\pm$ 0.2 ms, $β$p branching ratio of (84.7 $\pm$ 1.4)%, and identify four transitions corresponding to the exotic $β$-$γ$-$p$ decay mode, the second such identification in the $f p$-shell. The $p/γ$ ratio was used to correct for isospin mixing while determining the $^{57}$Zn mass via the isobaric multiplet mass equation. Previously, it was uncertain as to whether the rp-process flow could bypass the textbook waiting point $^{56}$Ni for astrophysical conditions relevant to Type-I X-ray bursts. Our results definitively establish the existence of the $^{56}$Ni bypass, with 14-17% of the $rp$-process flow taking this route.

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arXiv2021

$β$-decay of $^{61}$V and its Role in Cooling Accreted Neutron Star Crusts

The interpretation of observations of cooling neutron star crusts in quasi-persistent 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 $β$-decay ground-state to ground-state transition strengths of neutron-rich rare isotopes. Nuclei with mass number $A=61$ 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 $β$-decay of $^{61}$V. This work reports the first experimental determination of this transition strength, a ground-state branching of 8.1$^{+2.2}_{-2.0} \%$, corresponding to a log $ft$ value of 5.5$^{+0.2}_{-0.2}$. This result was achieved through the measurement of the $β$-delayed $γ$ rays using the total absorption spectrometer SuN and the measurement of the $β$-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.

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arXiv2020

A novel approach for extracting model-independent nuclear level densities far from stability

The level density of quantum states in statistical mesoscopic systems is a critical input for various fields of physics, including nuclear physics, nuclear astrophysics, atomic physics and their applications. In atomic nuclei, the level density is a fundamental measure of their complex structure at relatively high energies. Here we present the first model-independent measurement of the absolute partial nuclear level density for a short-lived unstable nucleus. For this purpose, we introduce the ``Shape method'' to extract the shape of the $γ$-ray strength function. Combining the Shape method with the existing $β$-Oslo technique allows the extraction of the nuclear level density without the need for theoretical input. We benchmark the Shape method using results for the stable $^{76}$Ge nucleus, finding an excellent agreement to previous experimental results. We apply the Shape method to new experimental data on the short-lived $^{88}$Kr nucleus. Our method opens the door for measurements of the nuclear level density and $γ$-ray strength function far away from stability, a pivotal input required to understand the role of exotic nuclei in forming the cosmos.

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arXiv2020

Independent Normalization for $γ$-ray Strength Functions: The Shape Method

The Shape method, a novel approach to obtain the functional form of the $γ$-ray strength function ($γ$SF) in the absence of neutron resonance spacing data, is introduced. When used in connection with the Oslo method the slope of the Nuclear Level Density (NLD) is obtained simultaneously. The foundation of the Shape method lies in the primary $γ$-ray transitions which preserve information on the functional form of the $γ$SF. The Shape method has been applied to $^{56}$Fe, $^{92}$Zr, $^{164}$Dy, and $^{240}$Pu, which are representative cases for the variety of situations encountered in typical NLD and $γ$SF studies. The comparisons of results from the Shape method to those from the Oslo method demonstrate that the functional form of the $γ$SF is retained regardless of nuclear structure details or $J^π$ values of the states fed by the primary transitions.

physics.data-an↗

arXiv2020

A technique for the study of (p,n) reactions with unstable isotopes at energies relevant to astrophysics

We have developed and tested an experimental technique for the measurement of low-energy (p,n) reactions in inverse kinematics relevant to nuclear astrophysics. The proposed setup is located at the ReA3 facility at the National Superconducting Cyclotron Laboratory. In the current approach, we operate the beam-transport line in ReA3 as a recoil separator while tagging the outgoing neutrons from the (p,n) reactions with the low-energy neutron detector array (LENDA). The developed technique was verified by using the $^{40}$Ar(p,n)$^{40}$K reaction as a probe. The results of the proof-of-principle experiment with the $^{40}$Ar beam show that cross-section measurements within an uncertainty of $\sim$25\% are feasible with count rates up to 7 counts/mb/pnA/s. In this article, we give a detailed description of the experimental setup, and present the analysis method and results from the test experiment. Future plans on using the technique in experiments with the separator for capture reactions (SECAR) that is currently being commissioned are also discussed.

physics.ins-det↗

arXiv2019

Superallowed $0^+ \rightarrow 0^+$ $β$ decay of $T =2$ $^{20}$Mg: $Q_{\textrm{EC}}$ value and $βγ$ branching

\textbf{Background}: Superallowed $0^+ \rightarrow 0^+$ $β$ decays of isospin $T=2$ nuclides can be used to test theoretical isospin symmetry breaking corrections applied to extract the CKM matrix element $V_{ud}$ from $T = 0,1$ decays by measuring precise $ft$ values and also to search for scalar currents using the $β-ν$ angular correlation. Key ingredients include the $Q_{\textrm{EC}}$ value and branching of the superallowed transition and the half life of the parent. \textbf{Purpose}: To determine a precise experimental $Q_{\textrm{EC}}$ value for the superallowed $0^+ \rightarrow 0^+$ $β$ decay of $T=2$ $^{20}$Mg and the intensity of $^{20}$Mg $β$-delayed $γ$ rays through the isobaric analog state in $^{20}$Na. \textbf{Method}: A beam of $^{20}$Mg was produced using the in-flight method and implanted into a plastic scintillator surrounded by an array of high-purity germanium detectors used to detect $β$-delayed $γ$ rays. The high-resolution $γ$-ray spectrum was analyzed to measure the $γ$-ray energies and intensities. \textbf{Results}: The intensity of $^{20}$Mg $β$-delayed $γ$ rays through the isobaric analog state in $^{20}$Na was measured to be $(1.60 \pm 0.04_{\textrm{stat}} \pm 0.15_{\textrm{syst}} \pm 0.15_{\textrm{theo}}) \times 10^{-4}$, where the uncertainties are statistical, systematic, and theoretical, respectively. The $Q_{\textrm{EC}}$ value for the superallowed transition was determined to be $4128.7 \pm 2.2$ keV based on the measured excitation energy of $6498.4 \pm 0.2_{\textrm{stat}} \pm 0.4_{\textrm{syst}}$ keV and literature values for the ground-state masses of $^{20}$Na and $^{20}$Mg. \textbf{Conclusions}: The $β$-delayed $γ$-decay branch and $Q_{\textrm{EC}}$ value are now sufficiently precise to match or exceed the sensitivity required for current low-energy tests of the standard model.

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arXiv2019

Probing the role of proton cross-shell excitations in 70Ni using nucleon knockout reactions

The neutron-rich Ni isotopes have attracted attention in recent years due to the occurrence of shape or configuration coexistence. We report on the difference in population of excited final states in 70Ni following gamma-ray tagged one-proton, one-neutron, and two-proton knockout from 71Cu, 71Ni, and 72Zn 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 70Ni 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 70Ni, 29 new transitions are reported, of which 15 are placed in its level scheme.

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