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Bennett, M. B.

Publications and source records attributed to Bennett, M. B..

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Doppler broadening in Mg 20 ( β p γ ) Ne 19 decay

The $^{15}$$\textbf{O}$$\Big({α,γ}\Big)$$^{19}$$\textbf{Ne}$ bottleneck reaction in Type I x-ray bursts is the most important thermonuclear reaction rate to constrain experimentally, to improve the accuracy of burst light-curve simulations. A proposed technique to determine the thermonuclear rate of this reaction employs the $^{20}$$\textbf{Mg}$$\Big({βpα}\Big)$$^{15}$$\textbf{O}$ decay sequence. The key $^{15}$$\textbf{O}$$\Big({α,γ}\Big)$$^{19}$$\textbf{Ne}$ resonance at an excitation of 4.03 MeV is now known to be fed in $^{20}$$\textbf{Mg}$$\Big({βpγ}\Big)$$^{19}$$\textbf{Ne}$ however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future $^{20}$$\textbf{Mg}$$\Big({βpα}\Big)$$^{15}$$\textbf{O}$ measurements. To determine the energy of the proton transition feeding the 4.03 MeV state in $^{19}$$\textbf{Ne}$. A fast beam of $^{20}$$\textbf{Mg}$ was implanted into a plastic scintillator, which was used to detect $β$ particles. 16 high purity germanium detectors were used to detect $γ$ rays emitted following $βp$ decay. A Monte Carlo method was used to simulate the Doppler broadening of $^{19}$$\textbf{Ne}γ$-ray lines and compare to the experimental data. The center of mass energy between the proton and $^{19}$$\textbf{Ne}$, feeding the 4.03 MeV state, is measured to be $1.21^{+0.25}_{-0.22}$ $\textbf{MeV}$, corresponding to a $^{20}$$\textbf{Na}$ excitation energy of $7.44^{+0.25}_{-0.22}$$\textbf{MeV}$. Absolute feeding intensities and $γ$-decay branching ratios of $^{19}$$\textbf{Ne}$ states were determined including the 1615 keV state, which has not been observed before in this decay. A new $γ$ decay branch from the 1536 keV state in $^{19}$$\textbf{Ne}$ to the ground state is reported. The lifetime of the 1507 keV state in $^{19}$$\textbf{Ne}$ is measured to be $4.3^{+1.3}_{-1.1}$ ps resolving discrepancies in the literature. Conflicting $^{20}$$\textbf{Mg}$$\Big({βp}\Big)$ decay schemes in published literature are clarified.The utility of this Doppler broadening technique to provide information on $β$-delayed nucleon emission and excited-state lifetimes has been further demonstrated. In particular, knowledge of the proton energies feeding the 4.03 MeV $^{19}$$\textbf{Ne}$ state in $^{20}$$\textbf{Mg}$ $β$ decay will facilitate future measurements of the $α$-particle branching ratio.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Toward complete spectroscopy using β decay: The example of Cl 32 ( β γ ) S 32

$^{32}\textbf{Cl}$ is a neutron-deficient isotope with a $β$-decay half-life of 298 ms and a spin and parity of $J^π=1^+$. Previous measurements of $^{32}\textbf{Cl}$ $β$-delayed $γ$ rays have yielded a $β$-decay scheme with twelve $β$-decay transitions, contributing to studies of nuclear structure and fundamental symmetries. Those experiments have been limited to the observation of $^{32}\textbf{S}$ states with $J^π = 0^+,1^+, 2^+$. Our goal is to search for new $β$-delayed $γ$ rays and $β$-decay transitions of $^{32}\textbf{Cl}$ to $^{32}\textbf{S}$. A measurement of $^{32}\textbf{Cl}$ $β$-delayed $γ$ decay has been performed using the Clovershare array of high-purity germanium detectors at the National Superconducting Cyclotron Laboratory. By acquiring the highest-statistics $^{32}\textbf{Cl}$ $β$-delayed $γ$-ray spectrum to date and exploiting a new sensitivity to $γ-γ$ coincidences, this experiment has enabled the observation of nine previously unobserved $β$-delayed $γ$-ray transitions, leading to the inference of five $β$-decay transitions never before observed in $^{32}\textbf{Cl}$ $β$-delayed $γ$ decay. The set of observed states includes negative-parity states for the first time. By combining the new information with data from previous work, the lifetimes and partial widths of the 8861- and 9650-keV states of $^{32}\textbf{S}$ have been determined. In addition, the $^{31}\textbf{P}$($p,α$)$^{28}\textbf{Si}$ resonance strength of the 9650-keV state has been limited to $ωγ<9.8$ meV, which is an improvement over direct measurements. An enhanced decay scheme has been constructed. Most of the excited bound $^{32}\textbf{S}$ states that would correspond to allowed and first-forbidden $β$-decay transitions have been observed, demonstrating the potential of $β$-decay experiments to approach complete spectroscopy measurements at the next generation of radioactive beam facilities. The observed positive-parity levels are well matched by $sd$ shell-model calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Detailed study of the decay Cl 31 ( β γ ) S 31

Background: 31 Cl is a neutron-deficient isotope with a half-life of T 1/2 = 190 (1) ms. The nuclear structure of its daughter, 31 S, is important for the determination of the thermonuclear 30 P(p,γ) 31 S reaction rate, which affects the final isotopic abundances of the ejecta from classical oxygen-neon novae. Purpose: Here, we aim to determine the β feedings, γ-decay branchings, and excitation energies of states populated in 31 S and create a comprehensive decay scheme for comparison with predictions based on the shell model. Methods: Using a 31 Cl rare istope beam implanted into a plastic scintillator and an array of high-purity Ge detectors, γ rays from the 31 Cl(βγ) 31 S decay sequence were measured. Shell-model calculations using the USDB and the recently-developed USDE interactions were performed for comparison. Results: A 31 Cl β-decay scheme was constructed from the experimental data and compared to the USDB and USDE shell-model calculations based on the β feeding and γ-decay branches of each observed state. 33 new γ-ray transitions and ten new β decay branches were observed. The β feeding and γ-decay branches of each observed state were compared to those from the USDB and USDE shell-model calculations. For every allowed transition predicted by the USD calculations up to an excitation energy of 6.4 MeV in 31 S, an analogous transition was found in the experimental data, enabling a one-to-one comparison with the shell model. Using these identifications, spin and parity arguments were made for observed states. Conclusions: The new 31 Cl γ-decay scheme presented in this work is the most complete and precise one for this nucleus constructed to date, incorporating statistics over an order of magnitude higher than previous work on 31 Cl β-delayed γ decay. Of particular interest is the discovery of a 6390-keV state that mixes with the isobaric analog state and affects the 30 P(p,γ) 31 S reaction rate. Other states observed in the decay are not expected to strongly affect the 30 P(p,γ) 31 S reaction rate, but the comprehensive comparison to the shell model helps to clarify spin and parity assignments of resonances that might affect the rate.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

New portal to the O 15 ( α , γ ) Ne 19 resonance triggering CNO-cycle breakout

The 15 O(α,γ) 19 Ne reaction is expected to trigger the initial path for breakout from the CNO hydrogen-burning cycles to the rapid proton capture (rp) process in type I x-ray bursts on accreting neutron stars. The thermonuclear reaction rate has a major impact on models of type I x-ray burst observables and it depends on the small α-particle branching ratio, Γ α /Γ, of the 4.03 MeV state in 19 Ne. Attempts to measure Γ α /Γ by populating the 4.03 MeV state using nuclear reactions have only led to strong upper limits. In the present work, we report the first experimental evidence that the 4.03 MeV 19 Ne state is populated in 20 Mg β-delayed proton emission. Furthermore this new channel has the potential to provide the necessary sensitivity to detect a finite value of Γ α /Γ.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Confirmation of the isomeric state in P 26

Here, we report the independent experimental confirmation of an isomeric state in the proton drip-line nucleus 26 P. The γ-ray energy and half-life determined are 164.4 ± 0.3(sys) ± 0.2(stat) keV and 104 ± 14 ns, respectively, which are in agreement with the previously reported values. These values are used to set a semiempirical limit on the proton separation energy of 26 P, with the conclusion that it can be bound or unbound.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

β -delayed γ decay of P 26 : Possible evidence of a proton halo

In this study, measurements of β decay provide important nuclear structure information that can be used to probe isospin asymmetries and inform nuclear astrophysics studies. Purpose: To measure the β-delayed γ decay of 26 P and compare the results with previous experimental results and shell-model calculations. A 26 P fast beam produced using nuclear fragmentation was implanted into a planar germanium detector. Its β-delayed γ-ray emission was measured with an array of 16 high-purity germanium detectors. Positrons emitted in the decay were detected in coincidence to reduce the background. As a result, the absolute intensities of 26 P β-delayed γ-rays were determined. A total of six new β-decay branches and 15 new γ-ray lines have been observed for the first time in 26 P β-decay. A complete β-decay scheme was built for the allowed transitions to bound excited states of 26 Si. ft values and Gamow-Teller strengths were also determined for these transitions and compared with shell model calculations and the mirror β-decay of 26 Na, revealing significant mirror asymmetries. A very good agreement with theoretical predictions based on the USDB shell model is observed. The significant mirror asymmetry observed for the transition to the first excited state (δ = 51(10)%) may be evidence for a proton halo in 26 P.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Isospin mixing reveals 30 P(p, γ) 31 S resonance influencing nova nucleosynthesis

Here, the thermonuclear 30 P(p, γ) 31 S reaction rate is critical for modeling the final elemental and isotopic abundances of ONe nova nucleosynthesis, which affect the calibration of proposed nova thermometers and the identification of presolar nova grains, respectively. Unfortunately, the rate of this reaction is essentially unconstrained experimentally, because the strengths of key 31 S proton capture resonance states are not known, largely due to uncertainties in their spins and parities. Using the β decay of 31 Cl, we have observed the β-delayed γ decay of a 31 S state at E x = 6390.2(7) keV, with a 30 P(p, γ) 31 S resonance energy of E r = 259.3(8) keV, in the middle of the 30 P(p, γ) 31 S Gamow window for peak nova temperatures. This state exhibits isospin mixing with the nearby isobaric analog state at E x = 6279.0(6) keV, giving it an unambiguous spin and parity of 3/2 + and making it an important l = 0 resonance for proton capture on 30 P.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Revalidation of the isobaric multiplet mass equation for the A = 20 quintet

An unexpected breakdown of the isobaric multiplet mass equation in the A = 20, T = 2 quintet was recently reported, presenting a challenge to modern theories of nuclear structure. In the present work, the excitation energy of the lowest T = 2 state in Na-20 has been measured to be 6498.4 +/- 0.2 sta t ± 0.4 sys t keV by using the superallowed 0 + → 0 + beta decay of Mg-20 to access it and an array of high-purity germanium detectors to detect its gamma-ray deexcitation. This value differs by 27 keV (1.9 standard deviations) from the recommended value of 6525 ± 14 keV and is a factor of 28 more precise. The isobaric multiplet mass equation is shown to be revalidated when the new value is adopted.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Observation of Doppler broadening in β -delayed proton- γ decay

Background: The Doppler broadening of gamma-ray peaks is due to nuclear recoil from beta-delayed nucleon emission can be used to measure the energies of the nucleons. This method has never been tested using beta-delayed proton emission or applied to a recoil heavier than A = 10. Purpose: To test and apply this Doppler broadening method using gamma-ray peaks from the P-26(beta p gamma)Al-25 decay sequence. Methods: A fast beam of P-26 was implanted into a planar Ge detector, which was used as a P-26 beta-decay trigger. The SeGA array of high-purity Ge detectors was used to detect gamma rays from the P-26(beta p gamma)Al-25 decay sequence. Results: Radiative Doppler broadening in beta-delayed proton-gamma decay was observed for the first time. Moreover, the Doppler broadening analysis method was verified using the 1613-keV gamma-ray line for which the proton energies were previously known. The 1776-keV gamma ray de-exciting the 2720 keV Al-25 level was observed in P-26(beta p gamma)Al-25 decay for the first time and used to determine that the center-of-mass energy of the proton emission feeding the 2720-keV level is 5.1 +/- 1.0 (stat.) +/- 0.6 (syst.) MeV, corresponding to a Si-26 excitation energy of 13.3 +/- 1.0 (stat.) +/- 0.6 (syst.) MeV for the proton-emitting level. Conclusions: Finally, the Doppler broadening method has been demonstrated to provide practical measurements of the energies for beta-delayed nucleon emissions populating excited states of nuclear recoils at least as heavy as A = 25.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Shape coexistence in Ni 68

The internal-conversion and internal-pair-production decays of the first excited 0 + state in 68 Ni are studied following the β decay of 68 Co. A novel experimental technique, in which the ions of 68 Co were implanted into a planar germanium double-sided strip detector and which required digital pulse processing, is developed. The values for the energy of the first excited 0 + state and the electric monopole transition strength from the first excited 0 + state to the ground state in 68 Ni are determined to be 1605(3) keV and 7.6 (4) × 10 -3 , respectively. Comparisons of the experimental results to Monte Carlo shell-model calculations suggest the coexistence between a spherical ground state and an oblate first excited 0 + state in 68 Ni.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Classical-Nova Contribution to the Milky Way’s Al 26 Abundance: Exit Channel of the Key Al 25 ( p , γ ) Si 26 Resonance

We present that classical novae are expected to contribute to the 1809-keV Galactic γ-ray emission by producing its precursor 26 Al, but the yield depends on the thermonuclear rate of the unmeasured 25 Al(p,γ) 26 Si reaction. Using the β decay of 26 P to populate the key J π =3 + resonance in this reaction, we report the first evidence for the observation of its exit channel via a 1741.6±0.6(stat)±0.3(syst) keV primary γ ray, where the uncertainties are statistical and systematic, respectively. By combining the measured γ-ray energy and intensity with other experimental data on 26 Si, we find the center-of-mass energy and strength of the resonance to be E r =414.9±0.6(stat)±0.3(syst)±0.6(lit.) keV and ωγ=23±6(stat)$+11\atop{-10}$(lit.) meV, respectively, where the last uncertainties are from adopted literature data. Finally, we use hydrodynamic nova simulations to model 26 Al production showing that these measurements effectively eliminate the dominant experimental nuclear-physics uncertainty and we estimate that novae may contribute up to 30% of the Galactic 26 Al.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗