arXiv subjects

M. P. Carpenter

Publications and source records attributed to M. P. Carpenter.

At least 19 recordsLinked to original sources

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 $\beta$-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

Triaxiality and shape dynamics in $^{70}$Ge

The electromagnetic properties of low-lying states in $^{70}$Ge were investigated via multi-step Coulomb excitation of a $^{70}$Ge beam impinging on a $^{208}$Pb target at the ATLAS facility of the Argonne National Laboratory. A total of 27 transitional elements and six diagonal matrix elements coupling 11 low-lying states, were extracted from the measured cross sections. These were used to calculate reduced transition probabilities, spectroscopic quadrupole moments, and rotational invariant shape parameters, providing enhanced precision and expanding on previous studies. The experimental data were compared within several theoretical frameworks, including the generalized triaxial rotor model, configuration interaction shell-model calculations, and computations within the combined frameworks of relativistic density functional theory and the five-dimensional collective Hamiltonian. The results demonstrate a good agreement with the experimental data and, in conjunction with calculations using a two-state mixing model, support significant triaxiality and strong mixing between the $0^+_1$ and $0^+_2$ states. This results in the magnitudes of their respective quadrupole deformations $[\beta_\text{rms}(0^+_1) = 0.228\,(3),\,\beta_\text{rms}(0^+_2) = 0.273\,(1)]$ being more similar than previously observed. The implications of these results for understanding the complex shape coexistence phenomena, the role of triaxiality, and shape evolution along the Ge isotopic chain are discussed.

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arXiv2025

High-spin spectroscopy and the onset of quasicollective structures in $^{69}$Ga

The intermediate- and high-spin level structure of the odd-$A$ $^{69}$Ga nucleus was investigated via the $^{26}$Mg($^{48}$Ca, $p4n\gamma$) fusion evaporation reaction at a beam energy of 195 MeV. The experiment was performed using the Gammasphere multidetector array in conjunction with the Fragment Mass Analyzer (FMA), with mass and charge identification achieved via an ionization chamber placed at the focal plane of the spectrometer. Coincidence relationships between the $^{69}$Ga reaction products and emitted $\gamma$ rays were analyzed to establish the level sequences, while angular distribution and angular correlation measurements were used to propose spin and parity assignments. As a result, the level scheme of the nucleus has been considerably extended. Near the ground state, the structure of $^{69}$Ga is well described by single-particle excitations, with shell-model calculations using the JUN45 and jj44b effective interactions providing a satisfactory interpretation of the observed levels. At spins in excess of 21/2 $\hbar$, three sequences of $E2$ transitions have been delineated, suggesting the onset of collectivity. An interpretation within the framework of the tilted-axis-cranking covariant density functional theory is proposed which reveals the role of $g_{9/2}$ protons and neutrons in this angular momentum regime.

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arXiv2025

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

The 247-keV state in $^{54}$Sc, populated in the $\beta$ 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 $\pi f_{7/2}\otimes\nu f_{5/2}$ spin-coupled multiplet, which decays to the $3^+,\pi f_{7/2} \otimes \nu p_{1/2}$ 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, $e_\pi$ = 1.30(8)$e$ and $e_\nu$ = 0.452(7)$e$, respectively, that are in contrast to reports of $e_\pi \approx 1.1-1.15e$ and $e_\nu \approx 0.6-0.8e$ 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

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

In this paper, $\beta^-$ and $\beta$-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 $\gamma$-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 $\gamma$-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 $Q_{\beta}$ 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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arXiv2024

Meaurement of spin vs. TKE of $^{144}$Ba produced in spontaneous fission of $^{252}$Cf

We measure the average spin of $^{144}$Ba, a common fragment produced in $^{252}$Cf(sf), as a function of the total kinetic energy (TKE). We combined for the first time a twin Frisch-gridded ionization chamber with a world-class $\gamma$-ray spectrometer that was designed to measure high-multiplicity $\gamma$-ray events, Gammasphere. The chamber, loaded with a $^{252}$Cf(sf) source, provides a fission trigger, the TKE of the fragments, the approximate fragment masses, and the polar angle of the fission axis. Gammasphere provides the total $\gamma$-ray yield, fragment identification through the tagging of decay $\gamma$ rays, and the feeding of rotational bands in the fragments. We determine the dependence of the average spin of $^{144}$Ba on the fragments' TKE by correlating the fragment properties with the distribution of discrete levels that are fed. We find that the average spin only changes by about $0.5$ $\hbar$ across the TKE range of 158-203 MeV. The virtual independence of the spin on TKE suggests that spin is not solely generated through the statistical excitation of rotational modes, and more complex mechanisms are required.

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arXiv2024

First Exploration of Monopole-Driven Shell Evolution above the N = 126 shell closure: new Millisecond Isomers in 213Tl and 215Tl

Isomer spectroscopy of heavy neutron-rich nuclei beyond the N=126 closed shell has been performed for the first time at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. New millisecond isomers have been identified at low excitation energies, 985.3(19) keV in 213Tl and 874(5) keV in 215Tl. The measured half-lives of 1.34(5) ms in 213Tl and 3.0(3) ms in 215Tl suggest spins and parities 11/2- with the single proton-hole configuration h11/2 as leading component. They are populated via E1 transitions by the decay of higher-lying isomeric states with proposed spin and parity 17/2+, interpreted as arising from a single s1/2 proton hole coupled to the 8+ seniority isomer in the (A+1)Pb cores. The lowering of the 11/2- states is ascribed to an increase of the h11/2 proton effective single-particle energy as the second g9/2 orbital is filled by neutrons, owing to a significant reduction of the proton-neutron monopole interaction between the h11/2 and g9/2 orbitals. The new ms-isomers provide the first experimental observation of shell evolution in the almost unexplored N>126 nuclear region below doubly-magic 208Pb.

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arXiv2024

Evolution of chirality from transverse wobbling in $^{135}$Pr

Chirality is a distinct signature that characterizes triaxial shapes in nuclei. We report the first observation of chirality in the nucleus $^{135}$Pr using a high-statistics Gammasphere experiment with the $^{123}$Sb($^{16}$O,4n)$^{135}$Pr reaction. Two chiral-partner bands with the configuration $\pi(1h_{11/2})^1\otimes\nu(1h_{11/2})^{-2}$ have been identified in this nucleus. Angular distribution analyses of the $\Delta I = 1$ transitions connecting the two bands reveal a dominant dipole character, and quasiparticle triaxial rotor model calculations show good agreement with the data. Since the simultaneous observation of chirality and transverse wobbling in $^{135}$Pr relies critically on these angular distribution results, we also address and refute the experimental and theoretical criticisms raised in a recent work by Lv et al., presenting additional evidence that further strengthens our interpretation. This marks the first observation of both hallmarks of triaxiality-chirality and wobbling-in the same nucleus.

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arXiv2024

Determination of the spins and parities for the 0$_{4}^{+}$ and 0$_{5}^{+}$ states in $^{100}$Zr

Two 0$^{+}$ states at 1294.5 and 1774.0 keV, together with three 2$^{+}$ and one 4$^{+}$ levels, were identified or unambiguously spin-parity assigned for the first time in $^{100}$Zr utilizing $γ$-ray spectroscopy and $γ$-$γ$ angular correlation techniques with the Gammasphere spectrometer, following the $β^{-}$ decay of neutron-rich, mass separated $^{100,100m}$Y isotopes. Comparisons with recent Monte Carlo Shell-Model (MCSM) calculations indicate that these two states are candidates for the bandhead of a sequence in a shape-coexisting spherical minimum predicted to be located around $\approx$1500 keV. According to the measured relative B(E2)$_{relative}$ transition probabilities, the 0$_{5}^{+}$ state exhibits decay properties which more closely align with those predicted for a spherical shape, while the 0$_{4}^{+}$ level is suggested to be associated with a weakly-deformed shape similar to one related to the 0$_{2}^{+}$ state.

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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

Experimental Study of the $^{\textbf{38}}$S Excited Level Scheme

Information on the $^{38}$S level scheme was expanded through experimental work utilizing a fusion-evaporation reaction and in-beam $γ$-ray spectroscopy. Prompt $γ$-ray transitions were detected by the Gamma-Ray Energy Tracking Array (GRETINA) and recoiling $^{38}$S residues were selected by the Fragment Mass Analayzer (FMA). Tools based on machine-learning techniques were developed and deployed for the first time in order to enhance the unique selection of $^{38}$S residues and identify any associated $γ$-ray transitions. The new level information, including the extension of the even-spin yrast sequence through $J^π = 8^{(+)}$, was interpreted in terms of a basic single-particle picture as well shell-model calculations which incorporated the empirically derived FSU interaction. A comparison between the properties of the yrast states in the even-$Z$ $N=22$ isotones from $Z=14$ to $20$, and for $^{36}$Si-$^{38}$S in particular, was also presented with an emphasis on the role and influence of the neutron $1p_{3/2}$ orbital on the structure in the region.

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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

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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arXiv2023

Executive Summary of the Topical Program: Nuclear Isomers in the Era of FRIB

We report on the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program "Nuclear Isomers in the Era of FRIB". We outline the many ways isomers influence and contribute to nuclear science and technology, especially in the four FRIB pillars: properties of rare isotopes, nuclear astrophysics, fundamental symmetries, and applications for the nation and society. We conclude with a resolution stating our recommendation that the nuclear physics community actively pursue isomer research. A white paper is forthcoming.

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arXiv2022

E2 Rotational Invariants of $0^+_1$ and $2^+_1$ states for $^{106}$Cd: the Emergence of Collective Rotation

The collective structure of $^{106}$Cd is elucidated by multi-step Coulomb excitation of a 3.849 MeV/$A$ beam of $^{106}$Cd on a 1.1 mg/cm$^2$ $^{208}$Pb target using GRETINA-CHICO2 at ATLAS. Fourteen $E2$ matrix elements were obtained. The nucleus $^{106}$Cd is a prime example of emergent collectivity that possesses a simple structure: it is free of complexity caused by shape coexistence and has a small, but collectively active number of valence nucleons. This work follows in a long and currently active quest to answer the fundamental question of the origin of nuclear collectivity and deformation, notably in the cadmium isotopes. The results are discussed in terms of phenomenological models, the shell model, and Kumar-Cline sums of $E2$ matrix elements. The ${\langle 0_2^+ ||E2||2_1^+ \rangle}$ matrix element is determined for the first time, providing a total, converged measure of the electric quadrupole strength, $\langle Q^2 \rangle$, of the first-excited $2_1^+$ level relative to the $0_1^+$ ground state, which does not show an increase as expected of harmonic and anharmonic vibrations. Strong evidence for triaxial shapes in weakly collective nuclei is indicated; collective vibrations are excluded. This is contrary to the only other cadmium result of this kind in $^{114}$Cd by C. Fahlander et al., Nucl. Phys. A485, 327 (1988), which is complicated by low-lying shape coexistence near midshell.

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arXiv2022

Emergence of an island of extreme nuclear isomerism at high excitation near $^{208}$Pb

Metastable states with T$_{1/2}$ = 8(2) ms in $^{205}$Bi and T$_{1/2}$ = 0.22(2) ms in $^{204}$Pb, with $\approx $ 8 MeV excitation energy and angular momentum $\ge $ 22 $\hbar $, have been established. These represent, by up to two orders of magnitude, the longest-lived nuclear states above an excitation energy of 7 MeV, ever identified in the nuclear chart. Additionally, the half-life of the 10.17 MeV state in $^{206}$Bi has been determined to be 0.027(2) ms, the next highest value in this highly excited regime. These observations indicate the emergence of an island of extreme nuclear isomerism arising from core-excited configurations at high excitation in the vicinity of the doubly closed-shell nucleus $^{208}$Pb. These results are expected to provide discriminating tests of the effective interactions used in current large-scale shell-model calculations.

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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

Shapes, Softness and Non-Yrast Collectivity in 186W

Non-yrast, excited states in neutron-rich $^{186}$W were populated via inelastic-scattering reactions using beams of $^{136}$Xe nuclei accelerated to 725 and 800 MeV. Levels populated in the reactions were investigated via particle-$ γ$ coincidence techniques using the Gammasphere array of High-Purity Germanium detectors and the compact heavy-ion counter, CHICO2. The $ K^π = 2 ^{+} $ ($γ$), $ K^π = 0^{+}$ and $ K^π = 2^{-} $ (octupole) rotational side bands were extended to spins $ 14\hbar $, $ 12\hbar $, and $ 13\hbar $, respectively. A staggering pattern observed in the energies of levels in the $ K^π = 2^{+} $ band was found to be consistent with a potential that gets softer to vibration in the $ γ$ degree of freedom with increasing spin. The odd-even staggering of states in the $ K^π = 2^{-}$ band was found to exhibit a phase opposite to that seen in the $ γ$ band; an effect most probably associated with Coriolis coupling to other, unobserved octupole vibrational bands in $^{186}$W.

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