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Brown, B. A.

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

54 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Direct observation of the exotic β – γ – α decay mode in the T z = – 1 nucleus Na 20

The exotic β – γ – α decay mode of 20 Na has been directly observed for the first time in the Day-one experiment at the Beijing Radioactive Ion-beam Facility. The 20 Na source was produced by using a 100-MeV proton beam bombarding a stack of microporous MgO thick target and delivered as an intense mass separated beam after online ionization. A high-efficiency simultaneous measurement of β, γ, and α transitions enables the β-delayed γ – γ and α – γ coincidence spectroscopy. Here three β – γ – α exotic decay sequences in 20 Na are discovered, which expands the rare decay modes observed in β decay. Moreover, a β – α –decay sequence to the 6130-keV 3 – state of 16 O is observed, which is likely through the 12367-keV 1 + state in 20 Ne. The experimentally deduced B(F) and B(GT) are compared to the shell-model calculation, the allowed β transition strengths can be well accounted for by using sd shell-model space interactions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Exploiting Isospin Symmetry to Study the Role of Isomers in Stellar Environments

Proton capture on the excited isomeric state of 26 Al strongly influences the abundance of 26 Mg ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic 26 Al in presolar grains. This reaction also affects the temperature range for thermal equilibrium between the ground and isomeric levels. We present a novel technique, that exploits the isospin symmetry of the nuclear force, to address the long-standing challenge of determining proton-capture rates on excited nuclear levels. Such a technique has in-built tests that strongly support its veracity and, for the first time, we have experimentally constrained the strengths of resonances that dominate the astrophysical 26m Al(p, γ) 27 Si reaction. Furthermore, these constraints demonstrate that the rate is at least a factor ~ 8 lower than previously expected, indicating an increase in the stellar production of 26 Mg and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of 26 Al.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

25 Siβ + -decay spectroscopy

Background: β-decay spectroscopy provides valuable information on exotic nuclei and a stringent test for nuclear theories beyond the stability line. Purpose: To search for new β-delayed protons and γ rays of 25 Si to investigate the properties of 25 Al excited states. Method: 25 Si β decays were measured by using the Gaseous Detector with Germanium Tagging system at the National Superconducting Cyclotron Laboratory. The protons and γ rays emitted in the decay were detected simultaneously. Here, a Monte Carlo method was used to model the Doppler broadening of 24 Mg γ-ray lines caused by nuclear recoil from proton emission. Shell-model calculations using two newly developed universal sd-shell Hamiltonians were performed. Results: The most precise 25Si half-life to date has been determined. A new proton branch at 724(4) keV and new proton-γ-ray coincidences have been identified. Three 24 Mg γ-ray lines and eight 25 Al γ-ray lines are observed for the first time in 25 Si decay. The first measurement of the 25 Si β-delayed γ-ray intensities through the 25 Al unbound states is reported. All the bound states of 25 Al are observed to be populated in the β decay of 25 Si. Several inconsistencies between the previous measurements have been resolved, and new information on the 25 Al level scheme is provided. An enhanced decay scheme has been constructed and compared to the mirror decay of 25 Na and the shell-model calculations. Conclusions: The measured excitation energies, γ-ray and proton branchings, log $ft$ values, and Gamow-Teller transition strengths for the states of 25 Al populated in the β decay of 25 Si are in good agreement with the shell model calculations, offering gratifyingly consistent insights into the fine nuclear structure of 25 Al.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Precision Mass Measurements of Neutron-Rich Scandium Isotopes Refine the Evolution of N = 32 and N = 34 Shell Closures

In this work, we report high-precision mass measurements of 50–55 Sc isotopes performed at the LEBIT facility at NSCL and at the TITAN facility at TRIUMF. Our results provide a substantial reduction of their uncertainties and indicate significant deviations, up to 0.7 MeV, from the previously recommended mass values for 53–55 Sc. The results of this work provide an important update to the description of emerging closed-shell phenomena at neutron numbers N = 32 and N = 34 above proton-magic Z = 20. In particular, they finally enable a complete and precise characterization of the trends in ground state binding energies along the N = 32 isotone, confirming that the empirical neutron shell gap energies peak at the doubly magic 52 Ca. Moreover, our data, combined with other recent measurements, do not support the existence of a closed neutron shell in 55 Sc at N = 34. The results were compared to predictions from both ab initio and phenomenological nuclear theories, which all had success describing N = 32 neutron shell gap energies but were highly disparate in the description of the N = 34 isotone.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Determination of the 60 Zn level density from neutron evaporation spectra

Nuclear reactions of interest for astrophysics and applications often rely on statistical model calculations for nuclear reaction rates, particularly for nuclei far from β stability. However, statistical model parameters are often poorly constrained, where experimental constraints are particularly sparse for exotic nuclides. For example, our understanding of the breakout from the NiCu cycle in the astrophysical rp-process is currently limited by uncertainties in the statistical properties of the proton-rich nucleus 60 Zn. We have determined the nuclear level density of 60 Zn using neutron evaporation spectra from 58 Ni( 3 He,n) measured at the Edwards Accelerator Laboratory. We compare our results to a number of theoretical predictions, including phenomenological, microscopic, and shell-model-based approaches. Notably, we find the 60 Zn level density is somewhat lower than expected for excitation energies populated in the 59 Cu(p,γ) 60 Zn reaction under rp-process conditions. This includes a level density plateau from roughly 5 to 6 MeV excitation energy, which is counter to the usual expectation of exponential growth and all theoretical predictions that we explore. Here, a determination of the spin distribution at the relevant excitation energies in 60 Zn is needed to confirm that the Hauser-Feshbach formalism is appropriate for the 59 Cu(p,γ) 60 Zn reaction rate at x-ray burst temperatures

59 ≤ A ≤ 89↗
DOE OSTI2020

Shape Changes in the N = 28 Island of Inversion: Collective Structures Built on Configuration-Coexisting States in 43 S

The neutron-rich nuclei in the N = 28 island of inversion have attracted considerable experimental and theoretical attention, providing great insight into the evolution of shell structure and nuclear shape in exotic nuclei. In this study, for the first time, quadrupole collectivity is assessed simultaneously on top of the 3/2 - ground state and the 7/2 - shape-coexisting isomer of 43 S, putting the unique interpretation of shape and configuration coexistence at N = 27 and 28 in the sulfur isotopic chain to the test. From an analysis of the electromagnetic transition strengths and quadrupole moments predicted within the shell model, it is shown that the onset of shape coexistence and the emergence of a simple collective structure appear suddenly in 43 S with no indication of such patterns in the N = 27 isotone 45 Ar.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Accessing the Single-Particle Structure of the Pygmy Dipole Resonance in 208 Pb

New experimental data on the neutron single-particle character of the Pygmy Dipole Resonance (PDR) in Pb 208 are presented. They were obtained from ( d , p ) and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany. The new data are compared to the large suite of complementary, experimental data available for Pb 208 and establish ( d , p ) as an additional, valuable, experimental probe to study the PDR and its collectivity. Furthermore, besides the single-particle character of the states, different features of the strength distributions are discussed and compared to large-scale shell model (LSSM) and energy-density functional plus quasiparticle-phonon model theoretical approaches to elucidate the microscopic structure of the PDR in Pb 208 .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Shell-model studies of the astrophysical r p -process reactions S 34 ( p , γ ) Cl 35 and Cl 34 g , m ( p , γ ) Ar 35

Background: Dust grains condensed in the outflows of presolar classical novae should have been present in the protosolar nebula. Candidates for such presolar nova grains have been found in primitive meteorites and can in principle be identified by their isotopic ratios, but the ratios predicted by state-of-the-art one-dimensional hydrodynamic models are uncertain due to nuclear-physics uncertainties. Purpose: To theoretically calculate the thermonuclear rates and uncertainties of the S 34 ( p , γ ) Cl 35 and Cl 34 g , m ( p , γ ) Ar 35 reactions and investigate their impacts on the predicted 34 S/ 32 S isotopic ratio for presolar nova grains. Method: A shell-model approach in a (0 +1 ) ℏ ω model space was used to calculate the properties of resonances in the 34 S(p ,γ ) 35 Cl and 34 g,m Cl (p ,γ ) 35 Ar reactions and their thermonuclear rates. Uncertainties were estimated using a Monte Carlo method. The implications of these rates and their uncertainties on sulfur isotopic nova yields were investigated using a postprocessing nucleosynthesis code. The rates for transitions from the ground state of 34 Cl as well as from the isomeric first excited state of 34 Cl were explicitly calculated. Results: At energies in the resonance region near the proton-emission threshold, many negative-parity states appear. Energies, spectroscopic factors, and proton-decay widths are reported. The resulting thermonuclear rates are compared with previous determinations. Conclusions: The shell-model calculations alone are sufficient to constrain the variation of the 34 S/ 32 S ratios to within about 30%. Uncertainties associated with other reactions must also be considered, but in general we find that the 34 S/ 32 S ratios are not a robust diagnostic to clearly identify presolar grains made from nova ejecta.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Microscopic calculations of nuclear level densities with the Lanczos method

A new method for computing the density of states in nuclei making use of an extrapolated form of the tridiagonal matrix obtained from the Lanczos method is presented. It will be shown that the global, average properties of the entire Lanczos matrix can be predicted from just four Lanczos iterations. The extrapolated Lanczos matrix (ELM) approach provides for an accurate computation of the density of states described within the configuration space, which, in some cases, is sufficient to accurately calculate the density of states at, or near, the neutron separation energy. Comparisons between theory and experiment are shown for $^{57}Fe$, $^{74}Ge$, and $^6Ge$. In addition, we show results for the J -dependence of moments and the level density for these three nuclei.

Shell model↗
DOE OSTI2020

Low-energy 23 Al β-delayed proton decay and 22 Na destruction in novae

The radionuclide 22 Na is a target of γ-ray astronomy searches, predicted to be produced during thermonuclear runaways driving classical novae. The 22 Na (p, γ) 23 Mg reaction is the main destruction channel of 22 Na during a nova, hence, its rate is needed to accurately predict the 22 Na yield. However, experimental determinations of the resonance strengths have led to inconsistent results. In this Rapid Communication, we report a measurement of the branching ratios of the 23 Al β-delayed protons as a probe of the key 204-keV (center-of-mass) 22 Na (p, γ) 23 Mg resonance strength. We report a factor of 5 lower branching ratio compared to the most recent literature value. The variation in 22 Na yield due to nuclear data inconsistencies was assessed using a series of hydrodynamic nova outburst simulations and has increased to a factor of 3.8, corresponding to a factor of ≈ 2 uncertainty in the maximum detectability distance. Finally, this is the first reported scientific measurement using the Gaseous Detector with Germanium Tagging system.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Nuclear mass measurements map the structure of atomic nuclei and accreting neutron stars

We introduce here mass excesses (ME) of neutron-rich isotopes of Ar through Fe, obtained via time of flight B ρ mass spectrometry at the National Superconducting Cyclotron Laboratory. Our new results have significantly reduced systematic uncertainties relative to a prior analysis, enabling the first determination of ME for Ti 58 , 59 , V 62 , Cr 65 , Mn 67 , 68 , and Fe 69 , 70 . Our results show the N = 34 subshell weaken at Sc and vanish at Ti, along with the absence of an N = 40 subshell at Mn. This further leads to a cooler accreted neutron star crust, highlighting the connection between the structure of nuclei and neutron stars.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Two-neutron knockout as a probe of the composition of states in Mg 22 , Al 23 , and Si 24

Simpson and Tostevin proposed that the width and shape of exclusive parallel momentum distributions of the A - 2 residue in direct two-nucleon knockout reactions carry a measurable sensitivity to the nucleon single-particle configurations and their couplings within the wave functions of exotic nuclei. We further report here on the first benchmarks and use of this new spectroscopic tool. Exclusive parallel momentum distributions for states in the neutron-deficient nuclei Mg 22 , Al 23 , and Si 24 populated in such direct two-neutron removal reactions were extracted and compared to predictions combining eikonal reaction theory and shell-model calculations. For the well-known Mg 22 and Al 23 nuclei, measurements and calculations were found to agree, supporting the dependence of the parallel momentum distribution width on the angular momentum composition of the shell-model two-neutron amplitudes. In Si 24 , a level at 3439(9) keV, of relevance for the important Al 23 ( p , γ ) Si 24 astrophysical reaction rate, was confirmed to be the 2 2 + state, whereas the 4 1 + state, expected to be strongly populated in two-neutron knockout, was not observed. This puzzle is resolved by theoretical considerations of the Thomas-Ehrman shift, which also indicates that a previously reported 3471-keV state in Si 24 is, in fact, the ( 0 2 + ) level with one of the largest experimental mirror-energy shifts ever observed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS2005

A Search for Ram-pressure Stripping in the Hydra I Cluster

Ram-pressure stripping is a method by which hot interstellar gas can be removed from a galaxy moving through a group or cluster of galaxies. Indirect evidence of ram-pressure stripping includes lowered X- ray brightness in a galaxy due to less X-ray emitting gas remaining in the galaxy. Here we present the initial results of our program to determine whether cluster elliptical galaxies have lower hot gas masses than their counterparts in less rich environments. This test requires the use of the high-resolution imaging of the Chundru Observatory and we present our analysis of the galaxies in the nearby cluster Hydra I.

Brown, B. A.↗