-
Constraining the Synthesis of the Lightest p Nucleus 74Se
Authors:
A. Tsantiri,
A. Spyrou,
E. C. Good,
K. Bosmpotinis,
P. Giuliani,
H. Arora,
G. Balk,
L. Balliet,
H. C. Berg,
J. M. Berkman,
C. Dembski,
P. DeYoung,
P. A. Denissenkov,
N. Dimitrakopoulos,
A. Doetsch,
T. Gaballah,
R. Garg,
A. Henriques,
R. Jain,
S. N. Liddick,
S. Lyons,
R. S. Lubna,
B. Monteagudo Godoy,
F. Montes,
S. Nash
, et al. (20 additional authors not shown)
Abstract:
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,…
▽ More
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.
△ Less
Submitted 25 November, 2025;
originally announced November 2025.
-
Beta-decay Half Lives beyond $^{54}$Ca: A Systematic Survey of Decay Properties approaching the Neutron Dripline
Authors:
W. -J. Ong,
Z. Y. Xu,
R. Grzywacz,
A. Ravlić,
I. Cox,
J. M. Allmond,
T. T. King,
B. C. Rasco,
K. P. Rykaczewski,
H. Schatz,
B. M. Sherrill,
B. Tarasov,
B. A. Brown,
S. Ajayi,
H. Arora,
A. D. Ayangeakaa,
H. C. Berg,
J. M. Berkman,
D. L. Bleuel,
K. Bosmpotinis,
M. P. Carpenter,
G. Cerizza,
A. Chester,
J. M. Christie,
H. L. Crawford
, et al. (61 additional authors not shown)
Abstract:
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…
▽ More
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.
△ Less
Submitted 22 October, 2025;
originally announced October 2025.
-
Universal Effective Charges in the $sd$ and $fp$ Shells
Authors:
T. H. Ogunbeku,
J. M. Allmond,
T. J. Gray,
W. -J. Ong,
B. A. Brown,
A. Gargano,
R. Grzywacz,
J. D. Holt,
A. O. Macchiavelli,
T. Miyagi,
S. Neupane,
B. C. Rasco,
H. Schatz,
B. M. Sherrill,
O. B. Tarasov,
H. Arora,
A. D. Ayangeakaa,
H. C. Berg,
J. M. Berkman,
D. L. Bleuel,
K. Bosmpotinis,
M. P. Carpenter,
G. Cerizza,
A. Chester,
J. M. Christie
, et al. (57 additional authors not shown)
Abstract:
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.…
▽ More
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.
△ Less
Submitted 26 June, 2025;
originally announced June 2025.
-
The evidence of $N=16$ shell closure and $β$-delayed neutron emission from $^{25}$F
Authors:
J. F. Peltier,
Z. Y. Xu,
I. Cox,
R. Grzywacz,
R. S. Lubna,
N. Kitamura,
S. Neupane,
J. M. Allmond,
J. Christie,
A. A. Doetsch,
P. Dyszel,
T. Gaballah,
T. T. King,
K. Kolos,
S. N. Liddick,
M. Madurga,
T. H. Ogunbeku,
B. M. Sherrill,
K. Siegl
Abstract:
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…
▽ More
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.
△ Less
Submitted 16 December, 2024;
originally announced December 2024.
-
Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB
Authors:
T. J. Gray,
J. M. Allmond,
Z. Xu,
T. T. King,
R. S. Lubna,
H. L. Crawford,
V. Tripathi,
B. P. Crider,
R. Grzywacz,
S. N. Liddick,
A. O. Macchiavelli,
T. Miyagi,
A. Poves,
A. Andalib,
E. Argo,
C. Benetti,
S. Bhattacharya,
C. M. Campbell,
M. P. Carpenter,
J. Chan,
A. Chester,
J. Christie,
B. R. Clark,
I. Cox,
A. A. Doetsch
, et al. (41 additional authors not shown)
Abstract:
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…
▽ More
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.
△ Less
Submitted 26 April, 2023; v1 submitted 22 February, 2023;
originally announced February 2023.