arXiv subjects

B. A. Brown

Publications and source records attributed to B. A. Brown.

At least 37 records · Page 2Linked to original sources

arXiv2024

Proton-unbound states in $^{24}{\rm Al}$ relevant for the $^{23}{\rm Mg}(p,γ)$ reaction in novae

Background: The nucleosynthesis of several proton-rich nuclei is determined by radiative proton-capture reactions on unstable nuclei in nova explosions. One such reaction is $^{23}{\rm Mg}(p,γ)^{24}{\rm Al}$, which links the NeNa and MgAl cycles in oxygen-neon (ONe) novae. Purpose: To extract $^{23}{\rm Mg}(p,γ)$ resonance strengths from a study of proton-unbound states in $^{24}{\rm Al}$, produced via the $^{24}$Mg($^{3}$He,$t$) reaction. Methods: A beam of $^3 {\rm He}^{2+}$ ions at 50.7 MeV was used to produce the states of interest in $^{24}$Al. Proton-triton angular correlations were measured with a $K=600$ QDD magnetic spectrometer and a silicon detector array, located at iThemba LABS, South Africa. Results: We measured the excitation energies of the four lowest proton-unbound states in $^{24}$Al and place lower-limits on $Γ_p/Γ$ values for these four states. Together with shell-model calculations of partial gamma widths, the experimental data are also used to determine resonance strengths for the three lowest $^{23}{\rm Mg}(p,γ)^{24}{\rm Al}$ resonances. Conclusions: The energy of the dominant first $^{23}{\rm Mg}(p,γ)$ resonance is determined to be $E_{r} = 478 \pm 4$ keV, with a resonance strength $ωγ= 19 \pm 9$ meV.

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arXiv2024

Probing proton cross-shell excitations through the two-neutron removal from 38Ca

Bound states of the neutron-deficient, near-dripline nucleus $^{36}$Ca were populated in two-neutron removal from the ground state of $^{38}$Ca, a direct reaction sensitive to the single-particle configurations and couplings of the removed neutrons in the projectile wave function. Final-state exclusive cross sections for the formation of $^{36}$Ca and the corresponding longitudinal momentum distributions, both determined through the combination of particle and $γ$-ray spectroscopy, are compared to predictions combining eikonal reaction theory and shell-model two-nucleon amplitudes from the USDB, USDC, and ZBM2 effective interactions. The final-state cross-section ratio $σ(2^+_1)/σ(0^+)$ shows particular sensitivity and is approximately reproduced only with the two-nucleon amplitudes from the ZBM2 effective interaction that includes proton cross-shell excitations into the $pf$ shell. Characterizing the proton $pf$-shell occupancy locally and schematically, an increase of the $sd$-$pf$ shell gap by $250$ keV yields an improved description of this cross-section ratio and simultaneously enables a reproduction of the $B(E2;0^+_1\to2^+_1)$ excitation strength of $^{36}$Ca. This highlights an important aspect if a new shell-model effective interaction for the region was to be developed on the quest to model the neutron-deficient Ca isotopes and surrounding nuclei whose structure is impacted by proton cross-shell excitations.

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arXiv2023

Measurement of the Isolated Nuclear Two-Photon Decay in $^{72}\mathrm{Ge}$

The nuclear two-photon or double-gamma ($2γ$) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the $2γ$ decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to $\sim100$\,keV and half-lives as short as $\sim10$\,ms. The half-life for the $2γ$ decay of the first-excited $0^+$ state in bare $^{72}\mathrm{Ge}$ ions was determined to be $23.9\left(6\right)$\,ms, which strongly deviates from expectations.

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arXiv2023

Precision Mass Measurement of Proton-Dripline Halo Candidate $^{22}$Al

We report the first mass measurement of the proton-halo candidate $^{22}$Al performed with the LEBIT facility's 9.4~T Penning trap mass spectrometer at FRIB. This measurement completes the mass information for the lightest remaining proton-dripline nucleus achievable with Penning traps. $^{22}$Al has been the subject of recent interest regarding a possible halo structure from the observation of an exceptionally large isospin asymmetry [Phys. Rev. Lett. \textbf{125} 192503 (2020)]. The measured mass excess value of $\text{ME}=18\;093.6(7)$~keV, corresponding to an exceptionally small proton separation energy of $S_p = 99.2(1.0)$~keV, is compatible with the suggested halo structure. Our result agrees well with predictions from \textit{sd}-shell USD Hamiltonians. While USD Hamiltonians predict deformation in $^{22}$Al ground-state with minimal $1s_{1/2}$ occupation in the proton shell, a particle-plus-rotor model in the continuum suggests that a proton halo could form at large quadrupole deformation. These results emphasize the need for a charge radius measurement to conclusively determine the halo nature.

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arXiv2023

$^{138}{\rm Ba}(d,α)$ study of states in $^{136}{\rm Cs}$: Implications for new physics searches with xenon detectors

We used the $^{138}$Ba$(d,α)$ reaction to carry out an in-depth study of states in $^{136}$Cs, up to around 2.5~MeV. In this work, we place emphasis on hitherto unobserved states below the first $1^+$ level, which are important in the context of solar neutrino and fermionic dark matter (FDM) detection in large-scale xenon experiments. We identify for the first time candidate metastable states in $^{136}$Cs, which would allow a real-time detection of solar neutrino and FDM events in xenon detectors, with high background suppression. Our results are also compared with shell-model calculations performed with three Hamiltonians that were previously used to evaluate the nuclear matrix element (NME) for $^{136}$Xe neutrinoless double beta decay. We find that one of these Hamiltonians, which also systematically underestimates the NME compared to the others, dramatically fails to describe the observed low-energy $^{136}$Cs spectrum, while the other two show reasonably good agreement.

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arXiv2023

Multiple Mechanisms in Proton-Induced Nucleon Removal at $\sim$100 MeV/Nucleon

We report on the first proton-induced single proton- and neutron-removal reactions from the neutron-deficient $^{14}$O nucleus with large Fermi-surface asymmetry $S_n-S_p$ = 18.6 MeV at $\sim$100 MeV/nucleon, a widely used energy regime for rare-isotope studies. The measured inclusive cross sections and parallel momentum distributions of the $^{13}$N and $^{13}$O residues are compared to the state-of-the-art reaction models, with nuclear structure inputs from many-body shell-model calculations. Our results provide the first quantitative contributions of multiple reaction mechanisms including the quasifree knockout, inelastic scattering and nucleon transfer processes. It is shown that the inelastic scattering and nucleon transfer, usually neglected at such energy regime, contribute about 50% and 30% to the loosely bound proton and deeply bound neutron removal, respectively. These multiple reaction mechanisms should be considered in analyses of inclusive one-nucleon removal cross sections measured at intermediate energies for quantitative investigation of single-particle strengths and correlations in atomic nuclei.

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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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arXiv2023

N=16 magicity revealed at the proton drip-line through the study of 35Ca

The last proton bound calcium isotope $^{35}$Ca has been studied for the first time, using the $^{37}$Ca($p, t$)$^{35}$Ca two neutron transfer reaction. The radioactive $^{37}$Ca nuclei, produced by the LISE spectrometer at GANIL, interacted with the protons of the liquid hydrogen target CRYPTA, to produce tritons $t$ that were detected in the MUST2 detector array, in coincidence with the heavy residues Ca or Ar. The atomic mass of $^{35}$Ca and the energy of its first 3/2$^+$ state are reported. A large $N=16$ gap of 4.61(11) MeV is deduced from the mass measurement, which together with other measured properties, makes $^{36}$Ca a doubly-magic nucleus. The $N = 16$ shell gaps in $^{36}$Ca and $^{24}$O are of similar amplitude, at both edges of the valley of stability. This feature is discussed in terms of nuclear forces involved, within state-of-the-art shell model calculations. Even though the global agreement with data is quite convincing, the calculations underestimate the size of the $N = 16$ gap in 36Ca by 840(110) keV.

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arXiv2023

Direct measurement of hexacontatetrapole, $\textbf{E6}$ γ decay from $^{\textbf{53m}}$Fe

The only proposed observation of a discrete, hexacontatetrapole ($E6$) transition in nature occurs from the T$_{1/2}$ = 2.54(2)-minute decay of $^{53m}$Fe. However, there are conflicting claims concerning its $γ$-decay branching ratio, and a rigorous interrogation of $γ$-ray sum contributions is lacking. Experiments performed at the Australian Heavy Ion Accelerator Facility were used to study the decay of $^{53m}$Fe. For the first time, sum-coincidence contributions to the weak $E6$ and $M5$ decay branches have been firmly quantified using complementary experimental and computational methods. Agreement across the different approaches confirms the existence of the real $E6$ transition; the $M5$ branching ratio and transition rate have also been revised. Shell model calculations performed in the full $pf$ model space suggest that the effective proton charge for high-multipole, $E4$ and $E6$, transitions is quenched to approximately two-thirds of the collective $E2$ value. Correlations between nucleons may offer an explanation of this unexpected phenomenon, which is in stark contrast to the collective nature of lower-multipole, electric transitions observed in atomic nuclei.

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arXiv2022

Dissipative reactions with intermediate-energy beams -- a novel approach to populate complex-structure states in rare isotopes

A novel pathway for the formation of multi-particle-multi-hole (np-mh) excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of 38Ca on a Be target. The negative-parity,complex-structure final states in 38Ca were observed following the in-beam gamma-ray spectroscopy of events in the 9Be(38Ca,38Ca+gamma)X reaction in which the scattered projectile lost longitudinal momentum of order p = 700 MeV/c. The characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly-energetic projectile-target collision. Unlike the far-less dissipative, surface-grazing reactions usually exploited for the in-beam gamma-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multi-particle configurations in rare isotopes - final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.

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arXiv2022

First application of Markov Chain Monte Carlo-based Bayesian data analysis to the Doppler-Shift Attenuation Method

Motivated primarily by the large uncertainties in the thermonuclear rate of the $^{30}$P$(p,γ)^{31}$S reaction that limit our understanding of classical novae, we carried out lifetime measurements of $^{31}$S excited states using the Doppler Shift Lifetimes (DSL) facility at the TRIUMF Isotope Separator and Accelerator (ISAC-II) facility. The $^{31}$S excited states were populated by the $^{3}$He$(^{32}$S$,α)^{31}$S reaction. The deexcitation $γ$ rays were detected by a clover-type high-purity germanium detector in coincidence with the $α$ particles detected by a silicon detector telescope. We have applied modern Markov chain Monte Carlo-based Bayesian methods to perform lineshape analyses of Doppler-shift attenuation method $γ$-ray data for the first time. We have determined the lifetimes of the two lowest-lying $^{31}$S excited states. First experimental upper limits on the lifetimes of four higher-lying states have been obtained. The experimental results were compared to shell-model calculations using five universal $sd$-shell Hamiltonians. Evidence for $γ$ rays originating from the astrophysically important $J^π=3/2^+$, 260-keV $^{30}$P$(p,γ)^{31}$S resonance has also been observed, although strong constraints on the lifetime will require better statistics.

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arXiv2022

$β^{+}$ Gamow-Teller strengths from unstable $^{14}$O via the $(d,{}^2\text{He})$ reaction in inverse kinematics

For the first time, the $(d,{}^2\text{He})$ reaction was successfully used in inverse kinematics to extract the Gamow-Teller transition strength in the $β^{+}$ direction from an unstable nucleus. The nucleus studied was $^{14}$O, and the Gamow-Teller transition strength to $^{14}$N was extracted up to an excitation energy of 22 MeV. The measurement of the $(d,{}^2\text{He})$ reaction in inverse kinematics was made possible by the combination of an active target time projection chamber and a magnetic spectrometer. The data were used to test shell-model and state-of-the-art coupled cluster calculations. Shell-model calculations reproduce the measured Gamow-Teller strength distribution up to about 15 MeV reasonably well, after the application of a phenomenological quenching factor. Coupled-cluster calculation reproduces the full strength distribution well without such quenching, owing to the large model space, the inclusion of strong correlations, and the coupling of the weak interaction to two nucleons through two-body currents. This indicates that such calculations provide a very promising path for answering long-standing questions about the observed quenching of Gamow-Teller strengths in nuclei.

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arXiv2022

Exploiting dissipative reactions to perform in-beam $γ$-ray spectroscopy of the neutron-deficient isotopes \nuc{38,39}{Ca}

The neutron-deficient Ca isotopes continue to attract attention due to their importance for testing isospin symmetry and their relevance in capture reactions of interest for nova nucleosynthesis and the shape of light curves in Type I X-ray bursts. To date, spectroscopic information on 38,39 Ca is largely limited to data on lower-spin excited states. Here, we report in-beam γ-ray spectroscopy of complementary higher-spin, complex-structure states in 39 Ca populated in fast-beam-induced, momentum-dissipative processes leading to neutron pickup onto excited configurations of the projectile, 9 Be(38 Ca , 39 Ca + γ)X. Such a dissipative reaction was recently characterized for the case of inelastic scattering of 38 Ca off 9 Be, 9 Be(38 Ca, 38 Ca + γ)X. Additional data and discussion on the nuclear structure of 38 Ca is also presented. An explanation for the more-complex-structure states, populated with small cross sections in one-nucleon knockout reactions, and observed in the tails of their longitudinal momentum distributions, is also offered.

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arXiv2022

The Core of $^{25}$F studied by the $^{25}$F(-1p)$^{24}$O reaction

The $^{25}$F($5/2^+) (-1p) ^{24}$O reaction was studied at the NSCL using the S800 spectrometer. The experimental spectroscopic factor for the ground-state to ground-state transition indicates a substantial depletion of the proton $d_{5/2}$ strength compared to shell-model expectations. Our result supports the findings reported by Tang \textit{et al.}, from their study of the $(p,2p)$ reaction at RIBF. The overlap between the $^{25}$F and $^{24}$O ground-states is considerably less than anticipated if $^{24}$O acted as a robust and rigid doubly-magic core in $^{25}$F. We interpret the results within the framework of the Particle-Vibration Coupling (PVC) of a $d_{5/2}$ proton coupled to a quadrupole phonon of an effective core. This approach provides a good description of the experimental data by requiring an effective $^{24}$O* core with a phonon energy of $\hbarω_2$= 3.2 MeV, and a $B(E2) ~ 2.7$ W.u., softer and more collective than a bare $^{24}$O. Both the Nilsson deformed mean field and the PVC models appear to capture the properties of the effective core of $^{25}$F, suggesting that the additional proton tends to polarize the free, doubly magic $^{24}$O in such a way that it becomes either slightly deformed or a quadrupole vibrator.

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arXiv2022

The structure of $^{36}$Ca under the Coulomb magnifying glass

Detailed spectroscopy of the neutron-deficient nucleus $^{36}$Ca was obtained up to 9 MeV using the $^{37}$Ca($p$,$d$)$^{36}$Ca and the $^{38}$Ca($p$,$t$)$^{36}$Ca transfer reactions. The radioactive nuclei, produced by the LISE spectrometer at GANIL, interacted with the protons of the liquid Hydrogen target CRYPTA, to produce light ejectiles (the deuteron $d$ or triton $t$) that were detected in the MUST2 detector array, in coincidence with the heavy residues %identified by a zero degree detection system. %States have been measured up to 9 MeV. Our main findings are: i) a similar shift in energy for the 1$^+_1$ and 2$^+_1$ states by about -250 keV, as compared to the mirror nucleus $^{36}$S, ii) the discovery of an intruder 0$^+_2$ state at 2.83(13) MeV, which appears below the first 2$^+$ state, in contradiction with the situation in $^{36}$S, and iii) a tentative 0$^+_3$ state at 4.83(17) MeV, proposed to exhibit a bubble structure with two neutron vacancies in the 2s$_{1/2}$ orbit. The inversion between the 0$^+_2$ and 2$^+_1$ states is due to the large mirror energy difference (MED) of -516(130) keV for the former. This feature is reproduced by Shell Model (SM) calculations, using the $sd$-$pf$ valence space, predicting an almost pure intruder nature for the 0$^+_2$ state, with two protons (neutrons) being excited across the $Z$=20 magic closure in $^{36}$Ca ($^{36}$S). This mirror system has the largest MEDs ever observed, if one excludes the few cases induced by the effect of the continuum.

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arXiv2022

Constraining the $^{30}$P($p,γ)^{31}$S reaction rate in ONe novae via the weak, low-energy, $β$-delayed proton decay of $^{31}$Cl

The $^{30}$P$(p,γ)^{31}$S reaction plays an important role in understanding nucleosynthesis of $A\geq 30$ nuclides in oxygen-neon novae. The Gaseous Detector with Germanium Tagging was used to measure $^{31}$Cl $β$-delayed proton decay through the key $J^π=3/2^{+}$, 260-keV resonance. The intensity $I^{260}_{βp} = 8.3^{+1.2}_{-0.9} \times 10^{-6}$ represents the weakest $β$-delayed, charged-particle emission ever measured below 400 keV, resulting in a proton branching ratio of $Γ_p / Γ= 2.5^{+0.4}_{-0.3} \times 10^{-4}$. By combining this measurement with shell-model calculations for $Γ_γ$ and past work on other resonances, the total $^{30}$P$(p,γ)^{31}$S rate has been determined with reduced uncertainty. The new rate has been used in hydrodynamic simulations to model the composition of nova ejecta, leading to a concrete prediction of $^{30}$Si/$^{28}$Si excesses in presolar nova grains and the calibration of nuclear thermometers.

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arXiv2022

Isospin Mixing and the Cubic Isobaric Multiplet Mass Equation in the Lowest T = 2, A = 32 Quintet

The isobaric multiplet mass equation (IMME) is known to break down in the first T = 2, A = 32 isospin quintet. In this work we combine high-resolution experimental data with state-of-the-art shell-model calculations to investigate isospin mixing as a possible cause for this violation. The experimental data are used to validate isospin-mixing matrix elements calculated with newly developed shell-model Hamiltonians. Our analysis shows that isospin mixing with nonanalog T = 1 states contributes to the IMME breakdown, making the requirement of an anomalous cubic term inevitable for the multiplet.

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arXiv2021

Finite-temperature electron-capture rates for neutron-rich nuclei around N=50 and effects on core-collapse supernovae simulations

The temperature dependence of stellar electron-capture (EC) rates is investigated, with a focus on nuclei around $N=50$, just above $Z=28$, which play an important role during the collapse phase of core-collapse supernovae (CCSN). Two new microscopic calculations of stellar EC rates are obtained from a relativistic and a non-relativistic finite-temperature quasiparticle random-phase approximation approaches, for a conventional grid of temperatures and densities. In both approaches, EC rates due to Gamow-Teller transitions are included. In the relativistic calculation contributions from first-forbidden transitions are also included, and add strongly to the EC rates. The new EC rates are compared with large-scale shell model calculations for the specific case of $^{86}$Kr, providing insight into the finite-temperature effects on the EC rates. At relevant thermodynamic conditions for core-collapse, the discrepancies between the different calculations of this work are within about one order of magnitude. Numerical simulations of CCSN are performed with the spherically-symmetric GR1D simulation code to quantify the impact of such differences on the dynamics of the collapse. These simulations also include EC rates based on two parametrized approximations. A comparison of the neutrino luminosities and enclosed mass at core bounce shows that differences between simulations with different sets of EC rates are relatively small ($\approx 5\%$), suggesting that the EC rates used as inputs for these simulations have become well constrained.

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