13 research outputs found
Orbital dependent nucleonic pairing in the lightest known isotopes of tin
By studying the 109Xe-->105Te-->101Sn superallowed alpha-decay chain, we
observe low-lying states in 101Sn, the one-neutron system outside doubly magic
100Sn. We find that the spins of the ground state (J = 7=2) and first excited
state (J = 5=2) in 101Sn are reversed with respect to the traditional level
ordering postulated for 103Sn and the heavier tin isotopes. Through simple
arguments and state-of-the-art shell model calculations we explain this
unexpected switch in terms of a transition from the single-particle regime to
the collective mode in which orbital-dependent pairing correlations, dominate.Comment: 5 pages 3 figure
Investigating High-Energy Proton-Induced Reactions on Spherical Nuclei: Implications for the Pre-Equilibrium Exciton Model
A number of accelerator-based isotope production facilities utilize 100- to
200-MeV proton beams due to the high production rates enabled by high-intensity
beam capabilities and the greater diversity of isotope production brought on by
the long range of high-energy protons. However, nuclear reaction modeling at
these energies can be challenging because of the interplay between different
reaction modes and a lack of existing guiding cross section data. A Tri-lab
collaboration has been formed among the Lawrence Berkeley, Los Alamos, and
Brookhaven National Laboratories to address these complexities by
characterizing charged-particle nuclear reactions relevant to the production of
established and novel radioisotopes. In the inaugural collaboration
experiments, stacked-targets of niobium foils were irradiated at the Brookhaven
Linac Isotope Producer (E=200 MeV) and the Los Alamos Isotope Production
Facility (E=100 MeV) to measure Nb(p,x) cross sections between 50
and 200 MeV. The measured cross-section results were compared with literature
data as well as the default calculations of the nuclear model codes TALYS, CoH,
EMPIRE, and ALICE. We developed a standardized procedure that determines the
reaction model parameters that best reproduce the most prominent reaction
channels in a physically justifiable manner. The primary focus of the procedure
was to determine the best parametrization for the pre-equilibrium two-component
exciton model. This modeling study revealed a trend toward a relative decrease
for internal transition rates at intermediate proton energies (E=20-60 MeV)
in the current exciton model as compared to the default values. The results of
this work are instrumental for the planning, execution, and analysis essential
to isotope production.Comment: 37 pages, 62 figures. Revised version, published in Physical Review
Design, construction, and characterization of a compact DD neutron generator designed for 40Ar/39Ar geochronology
A next-generation, high-flux DD neutron generator has been designed,
commissioned, and characterized, and is now operational in a new facility at
the University of California Berkeley. The generator, originally designed for
40Ar/39Ar dating of geological materials, has since served numerous additional
applications, including medical isotope production studies, with others planned
for the near future. In this work, we present an overview of the High Flux
Neutron Generator (HFNG) which includes a variety of simulations, analytical
models, and experimental validation of results. Extensive analysis was
performed in order to characterize the neutron yield, flux, and energy
distribution at specific locations where samples may be loaded for irradiation.
A notable design feature of the HFNG is the possibility for sample irradiation
internal to the cathode, just 8 mm away from the neutron production site, thus
maximizing the neutron flux (n/cm2/s). The generator's maximum neutron flux at
this irradiation position is 2.58e7 n/cm2/s +/- 5% (approximately 3e8 n/s total
yield) as measured via activation of small natural indium foils. However,
future development is aimed at achieving an order of magnitude increase in
flux. Additionally, the deuterium ion beam optics were optimized by simulations
for various extraction configurations in order to achieve a uniform neutron
flux distribution and an acceptable heat load. Finally, experiments were
performed in order to benchmark the modeling and characterization of the HFNG.Comment: 31 pages, 20 figure
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Secondary neutron production from thick target deuteron breakup
Thick target deuteron breakup is a variable-energy accelerator-based source of high-energy neutrons, with applications in fundamental and applied nuclear science and engineering. However, the breakup mechanism remains poorly understood, and data on neutron yields from thick target breakup remain relatively scarce. In this work, the double-differential neutron yields from deuteron breakup have been measured on a thick beryllium target at ϵd=33 and 40 MeV, using both time-of-flight and activation techniques. We have also introduced a simple hybrid model for the double-differential deuteron breakup cross section, applicable in the ϵd=10-100 MeV energy range on light (Z≤6) targets. This model features four empirical parameters that have been fit to reproduce experimental breakup measurements on beryllium targets, using the method of least squares. It was shown that these parameters extrapolate well to higher energies, and to other low-Z target materials. We also include optimization of the parameters that modify the Kalbach systematics for compound and preequilibrium reactions, in order to better reproduce the experimental data for beryllium targets at large angles
Measurement of
A stacked-target of natural lanthanum foils (99.9119% 139La) was irradiated using a 60 MeV proton beam at the LBNL 88-Inch Cyclotron. 139La(p,x) cross sections are reported between 35–60 MeV for nine product radionuclides. The primary motivation for this measurement was the need to quantify the production of 134Ce. As a positron-emitting analogue of the promising medical radionuclide 225Ac, 134Ce is desirable for in vivo applications of bio-distribution assays for this emerging radio-pharmaceutical. The results of this measurement were compared to the nuclear model codes TALYS, EMPIRE and ALICE (using default parameters), which showed significant deviation from the measured values
Measurement and modeling of proton-induced reactions on arsenic from 35 to 200 MeV
As72 is a promising positron emitter for diagnostic imaging that can be employed locally using a Se72 generator. However, current reaction pathways to Se72 have insufficient nuclear data for efficient production using regional 100-200 MeV high-intensity proton accelerators. In order to address this deficiency, stacked-target irradiations were performed at LBNL, LANL, and BNL to measure the production of the Se72/As72 positron emission tomography (PET) generator system via As75(p,x) between 35 and 200 MeV. This work provides the most well-characterized excitation function for As75(p,4n)Se72 starting from threshold. Additional focus was given to report the first measurements of As75(p,x)Ge68 and bolster an already robust production capability for the highly valuable Ge68/Ga68 PET generator. Thick target yield comparisons with prior established formation routes to both generators are made. In total, high-energy proton-induced cross sections are reported for 55 measured residual products from As75, Cunat, and Tinat targets, where the latter two materials were present as monitor foils. These results were compared with literature data as well as the default theoretical calculations of the nuclear model codes talys, coh, empire, and alice. Reaction modeling at these energies is typically unsatisfactory due to few prior published data and many interacting physics models. Therefore, a detailed assessment of the talys code was performed with simultaneous parameter adjustments applied according to a standardized procedure. Particular attention was paid to the formulation of the two-component exciton model in the transition between the compound and preequilibrium regions, with a linked investigation of level density models for nuclei off of stability and their impact on modeling predictive power. This paper merges experimental work and evaluation techniques for high-energy charged-particle isotope production in an extension to an earlier study of this kind
Boutique neutrons advance Ar/Ar geochronology
International audienceWe designed and tested a compact deuteron-deuteron fusion neutron generator for application to 40Ar/39Ar geochronology. The nearly monoenergetic neutrons produced for sample irradiation are anticipated to provide several advantages compared with conventional fission spectrum neutrons: Reduction of collateral nuclear reactions increases age accuracy and precision. Irradiation parameters within the neutron generator are more controllable compared with fission reactors. Confidence in the prediction of recoil energies is improved, and their likely reduction potentially broadens applicability of the dating method to fine-grained materials without vacuum encapsulation. Resolution of variation in the 39K(n,p)39Ar neutron capture cross section at 1.3 to 3.2 MeV and discovery of a strong resonance at ~2.4 MeV illuminate future pathways to improve the technique for 40Ar/39Ar dating