arXiv subjects

B. Alex Brown

Publications and source records attributed to B. Alex Brown.

At least 19 recordsLinked to original sources

arXiv2025

$\beta$-delayed proton pandemonium: A first detailed $^{31}$Cl($\beta p \gamma$)$^{30}$P decay scheme

Positron decays of proton-rich nuclides exhibit large $Q$ values, producing complex cascades which frequently involve various radiations, including protons and $\gamma$ rays. Often, only one of the two is measured in a single experiment, limiting the accuracy and completeness of the decay scheme. An example is $^{31}$Cl, for which protons and $\gamma$ rays have been measured separately in detail but never with substantial sensitivity to proton-$\gamma$ coincidences. We provide detailed measurements of $^{31}$Cl $\beta$-delayed proton decay including $\beta$-$p$-$\gamma$ sequences, extract spectroscopic information on $^{31}$S excited states as well as their $\beta^+$ feedings, and compare to shell-model calculations. A fast fragmented beam of $^{31}$Cl provided by the National Superconducting Cyclotron Laboratory (NSCL) was deposited in the Gaseous Detector with Germanium Tagging (GADGET) system. GADGET's gas-filled Proton Detector was used to detect $\beta$-delayed protons, and the Segmented Germanium Array (SeGA) was used to detect $\beta$-delayed $\gamma$ rays. As many as 20 previously unobserved $\beta$-delayed proton transitions are reported, most of which populate excited states of $^{30}$P. The first detailed $^{31}$Cl($\beta p \gamma$)$^{30}$P decay scheme is presented, including updated $\beta$-delayed proton energies and intensities, as well as several new $^{31}$S levels. Improved agreement is found with theoretical calculations of the Gamow-Teller strengths $B(\text{GT})$ for $^{31}$S excitation energies $7.5 < E_x < 9.5$ MeV. The present work demonstrates that the ability to detect $\beta$-delayed protons and $\gamma$ rays in coincidence is essential for accurate positron decay schemes to compare with nuclear structure theory. This phenomenon for $\beta$-delayed protons resembles the pandemonium effect originally introduced for $\beta$-delayed $\gamma$ rays.

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arXiv2025

Extreme Shape Coexistence Observed in $^{70}$Co

The shape of the atomic nucleus is a property which underpins our understanding of nuclear systems, impacts the limits of nuclear existence, and enables probes of physics beyond the Standard Model. Nuclei can adopt a variety of shapes, including spheres, axially deformed spheroids, and pear shapes. In some regions of the nuclear chart where a spherical nucleus would naively be expected, deformed nuclear states can result from collective action of constituent protons and neutrons. In a small subset of nuclei both spherical and deformed nuclear states have been experimentally observed, a phenomenon termed shape coexistence. We present spectroscopic evidence for the coexistence of $J^{\pi}=1+$ spherical and deformed states in $^{70}$Co, separated by less than 275~keV. This close degeneracy of levels with the same $J^{\pi}$ and different shapes demonstrates an extreme example of shape coexistence resulting from the interplay of independent particle motion and collective behavior in highly unstable nuclear systems and identifies the Co isotopes as a transition point between deformed ground states observed in the Cr isotopes and spherical configurations observed in the closed-shell Ni isotopes.

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arXiv2024

Motivations for Early High-Profile FRIB Experiments

This white paper is the result of a collaboration by those that attended a workshop at the Facility for Rare Isotope Beams (FRIB), organized by the FRIB Theory Alliance (FRIB-TA), on Theoretical Justifications and Motivations for Early High-Profile FRIB Experiments. It covers a wide range of topics related to the science that will be explored at FRIB. After a brief introduction, the sections address: (II) Overview of theoretical methods, (III) Experimental capabilities, (IV) Structure, (V) Near-threshold Physics, (VI) Reaction mechanisms, (VII) Nuclear equations of state, (VIII) Nuclear astrophysics, (IX) Fundamental symmetries, and (X) Experimental design and uncertainty quantification.

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arXiv2024

Nuclear shell model study of neutrinoless double beta decay under Left-Right symmetric model

We use the large scale nuclear shell model to calculate the nuclear matrix elements for the neutrino mediated neutrinoless double beta decay within the Left-Right symmetric model for four nuclei: $^{76}$Ge, $^{82}$Se, $^{130}$Te and $^{136}$Xe. We perform a systematic analysis on the general magnitude of different terms for related mechanisms. For the $η$ mechanism, we find that the weak magnetism $R$ term dominates the decay rate while the $p$-wave effect is suppressed. While for the $λ$ mechanism, the $ω$ and the $q$ terms are with equal importance. For the latter $q$ term, important contributions from weak-magnetism MM part are observed. Finally, we give the constraints on the new physics parameters $m_{ββ}$, $λ$ and $η$ from current experiments.

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arXiv2024

Dense Nuclear Matter Equation of State from Heavy-Ion Collisions

The nuclear equation of state (EOS) is at the center of numerous theoretical and experimental efforts in nuclear physics. With advances in microscopic theories for nuclear interactions, the availability of experiments probing nuclear matter under conditions not reached before, endeavors to develop sophisticated and reliable transport simulations to interpret these experiments, and the advent of multi-messenger astronomy, the next decade will bring new opportunities for determining the nuclear matter EOS, elucidating its dependence on density, temperature, and isospin asymmetry. Among controlled terrestrial experiments, collisions of heavy nuclei at intermediate beam energies (from a few tens of MeV/nucleon to about 25 GeV/nucleon in the fixed-target frame) probe the widest ranges of baryon density and temperature, enabling studies of nuclear matter from a few tenths to about 5 times the nuclear saturation density and for temperatures from a few to well above a hundred MeV, respectively. Collisions of neutron-rich isotopes further bring the opportunity to probe effects due to the isospin asymmetry. However, capitalizing on the enormous scientific effort aimed at uncovering the dense nuclear matter EOS, both at RHIC and at FRIB as well as at other international facilities, depends on the continued development of state-of-the-art hadronic transport simulations. This white paper highlights the essential role that heavy-ion collision experiments and hadronic transport simulations play in understanding strong interactions in dense nuclear matter, with an emphasis on how these efforts can be used together with microscopic approaches and neutron star studies to uncover the nuclear EOS.

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arXiv2023

Global trends of the electric dipole polarizability from shell-model calculations

Shell-model calculations of the electric dipole (E1) polarizability have been performed for the ground state of selected p- and sd-shell nuclei, substantially advancing previous knowledge. Our results are slightly larger compared with the somewhat more scattered photo-absorption cross-section data, albeit agreeing with ab initio calculations at shell closures and presenting a smooth trend that follows the leptodermus approximation provided by the finite-range droplet model (FRDM). The total E1 strengths also show an increasing trend proportional to the mass number which follows from the classical oscillator strength (TRK) sum rule for the E1 operator. The enhancement of the energy-weighted sum over E1 excitations with respect to the TRK sum rule arises from the use of experimental single-particle energies and the residual particle-hole interaction.

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arXiv2023

Nuclear charge radii of silicon isotopes

The nuclear charge radius of $^{32}$Si was determined using collinear laser spectroscopy. The experimental result was confronted with ab initio nuclear lattice effective field theory, valence-space in-medium similarity renormalization group, and mean field calculations, highlighting important achievements and challenges of modern many-body methods. The charge radius of $^{32}$Si completes the radii of the mirror pair $^{32}$Ar - $^{32}$Si, whose difference was correlated to the slope $L$ of the symmetry energy in the nuclear equation of state. Our result suggests $L \leq 60$\,MeV, which agrees with complementary observables.

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arXiv2023

$ T=1 $ Pairing Along the $ N=Z $ Line

Pairing energies for the addition of two neutrons on even-even nuclei with $ N=Z $ are studied. The $ Z $ dependence is attributed to the number and type of orbitals that are occupied in the valence shell-model space. Properties in the region from $ Z=60-100 $ depend on the location of the $ 0g_{9/2} $ orbital.

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arXiv2022

$β^{2}$ corrections to spherical EDF calculations for root-mean-square charge radii

Root-mean-square charge radii are discussed in terms of spherical Energy Density Functional (EDF) models corrected for quadrupole deformations. Comparisons between experiment and theory are made for the absolute radii of all even-even nuclei, for the isotonic shift between cadmium and tin isotopes, the isotopic shifts of the calcium isotopes and the isotonic shift for nuclei with $ N=28 $. We conclude that the data are well described in this approach, except for the sharp rise just after the neutron magic numbers.

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arXiv2022

The nuclear shell model towards the drip lines

Applications of configuration mixing methods for nuclei near the proton and neutron drip lines are discussed. A short review of magic numbers is presented. Prospects for advances in the regions of four new "outposts" are highlighted: $^{28}$O, $^{42}$Si, $^{60}$Ca and $^{78}$Ni. Topics include: shell gaps, single-particle properties, islands-of-inversion, collectivity, neutron decay, neutron halos, two-proton decay, effective charge, and quenching in knockout reactions.

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arXiv2022

Ground State Magnetic Dipole Moment of $^{40}$Sc

The hyperfine coupling constants of the proton dripline odd-odd $^{40}$Sc nucleus were deduced from the hyperfine spectrum of the $3d4s\,^3\text{D}_2$\,$\leftrightarrow$\,$3d4p\,^3\text{F}^\circ_3$ transition in Sc II, measured by the bunched beam collinear laser spectroscopy technique. The ground state magnetic dipole and electric quadrupole moments were determined for the first time as $μ$\,=\,$+5.57(4)(2)\,μ_N$ and $Q$\,=\,$+42(38)(28)\,e^2\;{\rm fm^2}$, respectively. The magnetic moment is well reproduced by the additivity rule with magnetic moments of neighboring odd-even nuclei in the vicinity of the doubly-magic $^{40}$Ca nucleus. An ab-initio multishell valence-space Hamiltonian was also employed to calculate the magnetic moment of $^{40}$Sc, which spans across the $sd$ and $fp$ nuclear shells, where we obtained good agreements.

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arXiv2021

Hidden spin-isospin exchange symmetry

The strong interactions among nucleons have an approximate spin-isospin exchange symmetry that arises from the properties of quantum chromodynamics in the limit of many colors, $N_c$. However this large-$N_c$ symmetry is well hidden and reveals itself only when averaging over intrinsic spin orientations. Furthermore, the symmetry is obscured unless the momentum resolution scale is close to an optimal scale that we call $Λ_{{\rm large-}N_c}$. We show that the large-$N_c$ derivation requires a momentum resolution scale of $Λ_{{\rm large-}N_c} \sim 500$ MeV. We derive a set of spin-isospin exchange sum rules and discuss implications for the spectrum of $^{30}$P and applications to nuclear forces, nuclear structure calculations, and three-nucleon interactions.

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arXiv2021

Charge Radius of Neutron-deficient $^{54}$Ni and Symmetry Energy Constraints Using the Difference in Mirror Pair Charge Radii

The nuclear root-mean-square charge radius of $^{54}$Ni was determined with collinear laser spectroscopy to be $R(^{54}$Ni) = 3.737\,(3)~fm. In conjunction with the known radius of the mirror nucleus $^{54}$Fe, the difference of the charge radii was extracted as $ΔR_{\rm ch}$ = 0.049\,(4)~fm. Based on the correlation between $ΔR_{\rm ch}$ and the slope of the symmetry energy at nuclear saturation density ($L$), we deduced $20 \le L \le 70$\,MeV. The present result is consistent with the $L$ from the binary neutron star merger GW170817, favoring a soft neutron matter EOS, and barely consistent with the PREX-2 result within 1$σ$ error bands. Our result indicates the neutron-skin thickness of $^{48}$Ca as 0.15\,-\,0.19\,fm.

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arXiv2018

Experimental $γ$-decay strength in $^{59, 60}$Ni compared with microscopic calculations

Nuclear level densities and $γ$-ray strength functions have been extracted for $^{59, 60}\rm{Ni}$, using the Oslo method on data sets from the $^{60}$Ni($^{3}$He,$^{3}$He$^{\prime}γ$)$^{60}$Ni and $^{60}$Ni($^{3}$He,$αγ$)$^{59}$Ni reactions. Above the neutron separation energy, S$_n$, we have measured the $γ$-ray strength functions for $^{61}$Ni and $^{60}$Ni in photoneutron experiments. The low-energy part of the $^{59,60}$Ni $γ$-ray strength functions show an increase for decreasing $γ$ energies. The experimental $γ$-ray strength functions are compared with $M1$ $γ$-ray strength functions calculated within the shell model. The $E1$ $γ$-ray strength function of $^{60}$Ni has been calculated using the QTBA framework. The QTBA calculations describe the data above $E_γ\approx$ 7 MeV, while the shell-model calculations agree qualitatively with the low energy part of the $γ$-ray strength function. Hence, we give a plausible explanation of the observed shape of the $γ$-decay strength.

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arXiv2016

White Paper on Nuclear Astrophysics

This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21-23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9- 10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12-13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade.

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arXiv2015

Effect of first forbidden decays on the shape of neutrino spectra

We examine the effect of First Forbidden (FF) decays on $β$-decay neutrino spectra by performing microscopic nuclear structure calculations. By analyzing the FF decay branches of even-even nuclei we conclude that FF decays may be responsible for part of the missing neutrinos in the so called "Reactor Neutrino Anomaly". Further calculations and more experimental data are needed for a firm conclusion.

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arXiv2014

Large low-energy $M1$ strength for $^{56,57}$Fe within the nuclear shell model

A strong enhancement at low $γ$-ray energies has recently been discovered in the $γ$-ray strength function of $^{56,57}$Fe. In this work, we have for the first time obtained theoretical $γ$ decay spectra for states up to $\approx 8$ MeV in excitation for $^{56,57}$Fe. We find large $B(M1)$ values for low $γ$-ray energies that provide an explanation for the experimental observations. The role of mixed $E2$ transitions for the low-energy enhancement is addressed theoretically for the first time, and it is found that they contribute a rather small fraction. Our calculations clearly show that the high-$\ell$ ($=f$) diagonal terms are most important for the strong low-energy $M1$ transitions. As such types of $0\hbarω$ transitions are expected for all nuclei, our results indicate that a low-energy $M1$ enhancement should be present throughout the nuclear chart. This could have far-reaching consequences for our understanding of the $M1$ strength function at high excitation energies, with profound implications for astrophysical reaction rates.

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arXiv2014

Modification of the Brink-Axel Hypothesis for High Temperature Nuclear Weak Interactions

We present shell model calculations of electron capture strength distributions in A=28 nuclei and computations of the corresponding capture rates in supernova core conditions. We find that in these nuclei the Brink-Axel hypothesis for the distribution of Gamow-Teller strength fails at low and moderate initial excitation energy, but may be a valid tool at high excitation. The redistribution of GT strength at high initial excitation may affect capture rates during collapse. If these trends which we have found in lighter nuclei also apply for the heavier nuclei which provide the principal channels for neutronization during stellar collapse, then there could be two implications for supernova core electron capture physics. First, a modified Brink-Axel hypothesis could be a valid approximation for use in collapse codes. Second, the electron capture strength may be moved down significantly in transition energy, which would likely have the effect of increasing the overall electron capture rate during stellar collapse.

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