Search

We discuss the physics design of the next-generation spallation neutron target-moderator-reflector-shield (TMRS) assembly for the Manuel Lujan Jr. Neutron Scattering Center (Lujan Center) at the Los Alamos Neutron Science Center (LANSCE). We developed this TMRS to improve the neutronic performance of the Lujan Center in the intermediate energy range (keV–MeV) enabling a variety of new nuclear physics experiments, as well as increasing the quality of the ongoing experiments. The results of our Monte Carlo optimization study indicate that the new TMRS will generate keV–MeV neutrons with higher intensity and improved time resolution. At the same time, the new design largely maintains its current performance in the cold and thermal ranges supporting materials research. In this article we show that the new TMRS has the potential to enhance the Lujan Center’s experimental capability in a wider interval of energies – cold to intermediate – after its scheduled installation in 2020. [ABSTRACT FROM AUTHOR]

We have developed a hybrid Unstructured Mesh (UM)/Constructive Solid Geometry (CSG) model of the next-generation spallation neutron target-moderator-reflector-shield (TMRS) assembly Mark-IV at the Los Alamos Neutron Science Center (LANSCE) to perform MCNP neutronics calculations in support of the engineering design process. The hybrid model consists of an essential component of the new TMRS—the upper target—built as UM and the rest of the model that resembles the original TMRS Mark-III built as CSG. The UM model for the neutronics analysis as well as the model for the subsequent engineering analyses such as structural stress and fluid dynamics are automatically converted from the identical computer aided design (CAD) model. Such an automatic conversion is highly time-efficient and preserves a great level of detail of the original model. In addition, only a relatively small UM portion of our model (0.003 m3) is embedded into the surrounding CSG universe (314 m3), enabling a more efficient MCNP calculation due to the reduced requirements on computer memory and shorter computation time in comparison with a fully extended UM model. Our MCNP calculations focus on the neutronics performance and energy deposition in the upper target. They employ high-energy nuclear physics models, tabular data, and scattering kernels, use an 800 MeV proton beam as a source, tally secondary fast and thermal neutrons with point detectors, and apply mesh tallies and elemental edit outputs to score energy deposition from all particles, i.e., neutrons, photons, protons, and charged pions. Because the use of UM marks a major departure from our original method based entirely on the traditional CSG modeling with MCNPX 2.7.0, we validate the new hybrid UM/CSG model against the legacy CSG model. We found a good agreement between the two models, which demonstrates that MCNP6.2 with UM/CSG hybrids can solve complex neutronics problems at large accelerator facilities, including spallation neutron sources. [ABSTRACT FROM AUTHOR]

That dimension is so large that dynamically loading the sample to 100 GPa pressures (1 MB) is not plausible using a laboratory-scale laser system because the required laser energy is much larger than that of any existing system. Therefore, it is proper to estimate the ablation pressure from a laser pulse with a vacuum wavelength = 0.35 I µ i m using the empirical formula I p i [GPa] = 42 ( I I i [TW/cm SP 2 sp ]) SP 0.71 sp , where I I i and p are the laser intensity and ablation pressure in the units indicated.[6] Thus, to drive the 3.6 cm doped sample considered above to 0.68 MB for 80 ns would require a 1.6 MJ laser. In that spirit, we kept the same moderator, sample, and detector when considering the laser neutron source. We show that sample-dominated experiment designs using laser neutron sources are feasible albeit with larger detectors. [Extracted from the article]

Neutron resonance spectroscopy (NRS) has been used extensively to make temperature measurements that are accurate, absolute, and nonperturbative within the interior of material samples under extreme conditions applied quasistatically. Yet NRS has seldom been used in dynamic experiments. There is a compelling incentive to do so because of the significant shortcomings of alternative techniques. An important barrier to adopting dynamic NRS thermometry is the difficulty in fielding it with conventional spallation neutron sources. To enable time-dependent and spatially resolved temperature measurements in dynamic environments, more compact neutron sources that can be used at user facilities in conjunction with other diagnostic probes (such as x-ray light sources) are required. Such sources may be available using ultrafast high-intensity optical lasers. We evaluate such possibilities by determining the sensitivities of the temperature estimate on neutron-beam and diagnostic parameters. Based on that evaluation, requirements are set on a pulsed neutron-source and diagnostics to make a meaningful dynamic temperature measurement. Dynamic thermometry measurements are examined in this context when driven by two alternative fast-neutron sources: the Los Alamos Neutron Scattering Center (LANSCE) proton accelerator driving isotropic spallation neutrons as a baseline and a laser-plasma ion accelerator driving a neutron beam from deuterium breakup. Strategies to close the gap between the required and demonstrated performance of laser-based fast-neutron sources are presented. A short-pulse high-intensity laser with state-of-the-art pulse contrast and an energy of a few hundred Joules would drive a compact neutron source suitable for NRS thermometry that could transform the dynamic study of materials. [ABSTRACT FROM AUTHOR]

Because of their potential low cost, bright light, and fast decay time, LuAG:Ce ceramic scintillators have attracted a broad interest in the high-energy physics community. One crucial issue for their application in future high-energy physics experiments is their radiation hardness against neutrons and protons expected at future hadron colliders. We report optical and scintillation performance of 1-mm LuAG:Ce ceramic samples doped with Mg2+ (and Ca2+) and their radiation damage induced by hadrons. While Mg2+ co-doping improves their light output, Ca2+ co-doping improves their fast to total (F/T) ratio. LuAG:Ce ceramic samples were irradiated at the Los Alamos Neutron Science Center (LANSCE), Los Alamos, NM, USA, by neutrons up to $6.7\times 10^{15}\,\,\text{n}_{\mathrm {eq}}$ /cm2 and by 24-GeV and 800-MeV protons at CERN PS-IRRAD up to $1.2\times 10^{15}$ p/cm2 and at LANSCE up to $2.3\times 10^{14}$ p/cm2, respectively. All samples show excellent radiation hardness with more than 90% of light after irradiation. The RIAC values induced by neutrons are found to be a factor of 2 smaller than lutetium–yttrium oxyorthosilicate (LYSO:Ce) crystals. The RIAC values induced by protons are also found a factor of 2 smaller than LYSO:Ce crystals in LuAG:Ce ceramic samples with good optical quality. Research and development will continue to develop LuAG:Ce scintillating ceramics with improved optical quality for future investigation. [ABSTRACT FROM AUTHOR]

One crucial issue for the application of scintillation crystals in future high-energy physics experiments is radiation damage in a severe radiation environment, such as the high luminosity large hadron collider. While radiation damage induced by ionization dose in inorganic scintillators is well understood, investigations are ongoing to understand radiation damage induced by hadrons, including both charged hadrons and neutrons. Aiming at understanding neutron-induced radiation damage in fast inorganic scintillators, lutetium yttrium oxyorthosilicate (LYSO)/lutetium fine silicate (LFS), BaF2, and PWO crystals were irradiated at Los Alamos Neutron Science Center by a combination of particles, including neutrons, protons, and γ-rays. The results indicate that LYSO/LFS and BaF2 crystal plates are radiation hard up to a 1-MeV equivalent neutrons fluence of 9 × 1015 equivalent neutron (neq)/cm2, but not PWO, and the neutron-induced radiation damage in LYSO crystals is a factor of ten less than protons. [ABSTRACT FROM AUTHOR]