High energy particle background at neutron spallation sources and possible solutions

Modern spallation neutron sources are driven by proton beams ∼ GeV energies. Whereas low energy particle background shielding is well understood for reactors sources of neutrons (∼20 MeV), for high energies (100s MeV to multiple GeV) there is potential to improve shielding solutions and reduce instrument backgrounds significantly. We present initial measured data on high energy particle backgrounds, which illustrate the results of particle showers caused by high energy particles from spallation neutron sources. We use detailed physics models of different materials to identify new shielding solutions for such neutron sources, including laminated layers of multiple materials. In addition to the steel and concrete, which are used traditionally, we introduce some other options that are new to the neutron scattering community, among which there are copper alloys as used in hadronic calorimeters in high energy physics laboratories. These concepts have very attractive energy absorption characteristics, and simulations predict that the background suppression could be improved by one or two orders of magnitude. These solutions are expected to be great benefit to the European Spallation Source, where the majority of instruments are potentially affected by high energy backgrounds, as well as to existing spallation sources.Export Date: 16 October 2014</p

Sulfur-Tolerant Molybdenum Carbide Catalysts Enabling Low-Temperature Stabilization of Fast Pyrolysis Bio-oil

Low-temperature hydrogenation of carbonyl compounds can greatly improve the thermal stability of fast pyrolysis bio-oil, thereby enabling long-term operation of upgrading reactors which generally require high temperatures to achieve deep deoxygenation. The state-of-the-art hydrogenation catalysts, precious metals such as ruthenium, although effective in carbonyl hydrogenation, deactivate due to high sulfur sensitivity. In the present work, we showed that molybdenum carbides were active and sulfur-tolerant in low-temperature conversion of carbonyl compounds. Furthermore, due to surface bifunctionality (i.e., both metallic and acid sites present), carbides catalyzed both CO bond hydrogenation and C–C coupling reactions. Combined, these reactions transformed carbonyl compounds to more stable and higher molecular weight oligomeric compounds while consuming less hydrogen than pure hydrogenation. The carbides proved to be resistant to other deactivation mechanisms including hydrothermal aging, oxidation, coking, and leaching. These properties enabled carbides to achieve and maintain good catalytic performance in both aqueous-phase furfural conversion and real bio-oil stabilization in the presence of sulfur. This finding strongly suggests that molybdenum carbides can provide a catalyst solution necessary for the development of practical bio-oil stabilization technology