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Advanced Cold Moderators for Intense Cold Neutron Beams in Materials Research

Success story

Image of pellets made at JINR-Dubna and used to moderate fast neutrons (left), and the schematic representation of their molecular structure. (Photo: JINR Dubna)

Neutron scattering is a set of powerful techniques for analyzing materials, ranging from biomaterials, through turbine engines, through to hard magnets. Intense sources of neutrons for this purpose are provided by research reactors and accelerators. However, neutrons are created at very high energies and must be slowed down in a moderator, which in a reactor is traditionally water or heavy water that is at a few tens to hundreds of degrees Celsius. Moderated neutrons then emerge with energies characteristic of the kinetic energy of the moderator (water). These “thermal neutrons” are useful for many experiments, but more useful still are “cold neutrons” with even lower energies that are moderated by substances held at cryogenic temperatures: traditionally a tank of liquid hydrogen or solid methane.

The main objective of this CRP was to advance intense cold neutron beams for materials research.

The specific objectives were to:

  • Increase the efficiency in the production of cold neutron fluxes at existing and planned cold neutron facilities by developing new moderators;
  • Improve neutron transport codes and data libraries by implementing mesoscale neutron scattering for new moderators.

Solid methane suffers from radiolysis under intense radiation fields and can swell dramatically. This means its service lifetime is short. Both hydrogen and methane are potentially flammable and explosive, which limits their use in many cases to larger laboratories.

There were many potential cold moderator materials and concepts that could be developed. For this to happen they needed experimental characterization, a fundamental understanding of the processes of moderation in each case, and the ability to model and predict performance of the moderator when in use in different scenarios. Finally, moderators need to be tested and installed at facilities.

At the end of the CRP, the understanding of the performance of mesitylene, m-xylene and triphenylmethane had been considerably improved. These are potential “replacements” for methane that are much more resistant to radiolysis with much lower risks concerning flammability and explosion. A new concept of “pelletized” moderator has been put into service at the Dubna reactor in Russia. Pellets of the solid moderator are blown into position in the same manner grain is blown into a silo. At the end of their useful service, the grains are melted, and then replaced.

Liquid-hydrogen moderators have also been the subject of much development. The traditional bulk “tanks” of liquid hydrogen are being replaced by low-dimensional forms: i.e., 1-d tubes, or 2-d shapes like discs. This is possible when liquid-hydrogen can be stabilized at high purity as one of its two nuclear spin states: para-hydrogen. At very low neutron energies, para-hydrogen becomes far more transparent, allowing some dimensions to be extended and neutron scattering instruments to be placed more effectively around it. There can also be considerable gains in brightness for the same reactor or accelerator power: the European Spallation Source expects up to a 5-fold increase due to this effect. The first moderator of this kind is being built at the research reactor in Budapest.

Other geometric effects were examined, and work was performed to characterize the high-albedo material, nanodiamonds, that could be used as liners to limit the “leakage” of “very cold” neutrons by reflecting them back into the experimental apparatus.

Finally, several developments were made in the Monte Carlo codes that are used to optimize moderator design. There are many codes, some of which are better optimized to some problems than others. A new format called Monte Carlo Particle List was developed that acts as a kind of lingua franca, allowing the porting of the results from one code directly into another.

For further information related to this CRP, please see the CRP page.



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