Engineering topics

David, H. M.

Publications and source records attributed to David, H. M..

6 records

DOE OSTI2021

The polarization sensitivity of GRETINA

Compton polarimeters have played an important role in the study of nuclear structure physics, but have often been limited in their applications because of relatively low -ray detection efficiency. With the advent of -ray tracking detector arrays, which feature nearly solid angle coverage and the ability to identify the location of Compton-scattering events to within a few millimeters, this limitation can be overcome. Here we present a characterization of the performance of the Gamma Ray Energy Tracking In-beam Nuclear Array (GRETINA) as a Compton polarimeter using the 24 Mg(p,p') reaction at 2.45 MeV proton energy. We also discuss a new capability added to the simulation package UCGretina to simulate the emission of polarized photons, and compare it to the measured data. Lastly, we use these simulations to predict the performance of the Gamma Ray Energy Tracking Array (GRETA).

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2019

Identification of $\gamma$-decaying resonant states in 26 Mg and their importance for the astrophysical s process

The 22 Ne(α, n) reaction is expected to provide the dominant neutron source for the weak s process in massive stars and intermediate-mass (IM) Asymptotic Giant Branch (AGB) stars. However, the production of neutrons in such environments is hindered by the competing 22 Ne(α,γ) 26 Mg reaction. Here, the 11 B( 16 O,p) fusion-evaporation reaction was used to identify γ-decay transitions from 22 Ne + α resonant states in 26 Mg. Spin-parity restrictions have been placed on a number of α-unbound excited states in 26 Mg and their role in the 22 Ne(α,γ)26Mg reaction has been investigated. In particular, a suspected natural-parity resonance at E c.m. = 557(3) keV, that lies above the neutron threshold in 26 Mg, and is known to exhibit a strong α-cluster character, was observed to γ decay. Furthermore, a known resonance at E c.m. = 466(4) keV has been definitively assigned 2 + spin and parity. Consequently, uncertainties in the 22 Ne(α,γ) stellar reaction rate have been reduced by a factor of ~20 for temperatures ~0.2 GK.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2019

Proton decay of 108 I and its significance for the termination of the astrophysical rp -process

Employing the Argonne Fragment Mass Analyzer and the implantation-decay-decay correlation technique, a weak 0.50(21)% proton decay branch was identified in 108 I for the first time. Here the 108 I proton-decay width is consistent with a hindered l = 2 emission, suggesting a d 5/2 origin. Using the extracted 108 I proton-decay Q value of 597(13) keV, and the Q α values of the 108 I and 107 Te isotopes, a proton-decay Q value of 510(20) keV for 104 Sb was deduced. Similarly to the 112,113 Cs proton-emitter pair, the value is lower than that for the less-exotic neighbor 109 I, possibly due to enhanced proton-neutron interactions in N ≈ 2 nuclei. In contrast, the present is higher than that of 105 Sb, suggesting a weaker interaction energy.

104Sb↗

DOE OSTI2018

Superallowed α Decay to Doubly Magic Sn 100

In this paper, we report the first observation of the 108 Xe → 104 Te → 100 Sn α-decay chain. The α emitters, 108 Xe [E α = 4.4(2) MeV, T 1/2 = 58 +106 -23 μs] and 104 Te[E α = 4.9(2) MeV, T 1/2 < 18 ns], decaying into doubly magic 100 Sn were produced using a fusion-evaporation reaction 54 Fe( 58 Ni, 4n) 108 Xe, and identified with a recoil mass separator and an implantation-decay correlation technique. This is the first time alpha radioactivity has been observed to a heavy self-conjugate nucleus. A previous benchmark for study of this fundamental decay mode has been the decay of 212 Po into doubly magic 208 Pb. Enhanced proton-neutron interactions in the N = Z parent nuclei may result in superallowed α decays with reduced α-decay widths significantly greater than that for 212 Po. Finally, from the decay chain, we deduce that the α-reduced width for 108 Xe or 104 Te is more than a factor of 5 larger than that for 212 Po.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2018

Sub-shell closure and shape coexistence in the transitional nucleus Zr 98

In the rapid shape change from spherical to deformed nuclei in the Z=40 Zr isotopic chain, recent work has identified shape coexistence in 96 Zr. Between 96 Zr and the strongly deformed 100 Zr, 98 Zr is expected to also exhibit coexistence of nuclear shapes. The degree of mixing between different configurations is mainly determined by the nucleon-nucleon interactions. For nuclear model predictions, experimental constraints are needed, but they are barely available for 98 Zr. To study low-lying transitions in 98 Zr, a Coulomb excitation experiment was conducted at the Argonne Tandem-Linac Accelerator System (ATLAS) facility using a 98 Zr beam extracted from the Californium Rare Ion Breeder Upgrade (CARIBU) ion source and Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) for γ-ray spectroscopy coupled to the compact heavy ion counter (CHICO2) for ion detection. Furthermore, this paper reports on the first decisive deduction of the B(E2;2 + 1 →0 + 1 ) transition strength in 98 Zr and on its interpretation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2018

Low-spin structure of the N = 82 nucleus Cs 137

Excited states in 137 Cs have been studied by using the near-barrier, single-proton transfer reaction 10 B( 136 Xe, 9 Beγ) 137 Cs in inverse kinematics and performing particle-γ and particle-γγ coincidence measurements. The low-spin level scheme of 137 Cs has been augmented: (i) The expected set of single-proton, seniority ν = 1 states has been delineated in its entirety; (ii) Another member of the v = 3 proton 0g 7/2 multiplet, a low-lying 9/2 + level, has been identified and this set of states now approaches completion; (iii) A second 9/2 + and a candidate 13/2 + level have been observed. Furthermore, the results are discussed in the framework of shell-model calculations in the proton 2s1 d 0 g 0 h shell.

74 ATOMIC AND MOLECULAR PHYSICS↗