Chris Lavelle

Chris Lavelle Education
Ph.D., Indiana University, 2007.

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Former Graduate Student

Project: Neutronics of the LENS Source

The neutron is one of the most important probes in the materials sciences. The neutral charge of the neutron allows it to penetrate deeply into a sample and scatter directly from the nuclei, giving vital information on static and dynamic structures in the fields of condensed matter physics, biology, and physical chemistry. Low energy neutrons, those neutrons whose energy is comparable to room temperature energies or below, are most often used in this work since they have long wavelengths. Even lower energy neutrons are ultra cold neutrons (UCN), neutrons whose energy is so low they may be trapped within a magnetic bottle. UCN are of great interest in studies of the weak force interaction in fundamental physics, and are produced from sources of low energy neutrons. Continued progress in these fields will ultimately depend on developing efficient high intensity sources of low energy neutrons.

My work focused on both the neutronic aspect of the LENS design and understanding the condensed matter physics behind the elementary excitations used to slow neutrons to very cold energies. The neutronic work is accomplished through advanced Monte Carlo simulation and experimental measurements of the simulation predictions. Using MCNP, a workhorse neutron Monte Carlo package developed at Los Alamos, we have optimized the target, reflector, and moderator to produce a high brilliance of cold neutrons. Experimental measurments of the constructed geometry have produced relatively close agreement with the simulations. Future neutronic work will focus on employing advanced concepts in neutron moderation, such as grooves, cavities, and reflector/filter configurations, which can lead to significant increases in cold neutron intensity.

Low energy neutrons are produced through elastic scattering of the high energy primary neutrons with hydrogen. These collisions are efficient at reducing the energy of the incident neutron because the recoiling hydrogen nucleus carries away a large fraction of the incident energy. Although the light mass and high cross-section of hydrogen make it ideal for slowing down primary neutrons, hydrogen recoil alone is not sufficient for cold neutron production. As the neutron energy becomes comparable to the binding energies of the molecules in the moderator material, the moderator nuclei are no longer free to recoil. Once this regime of moderator temperature and neutron energy is reached, the ominant energy transfer mode becomes inelastic scattering with the elementary excitations of the material. In these collisions the neutron loses energy by exciting either the moderator molecules themselves or collective excitations in the crystal lattice.

The modes of excitation available in moderators are a strong function of temperature. For instance, methane, the leading moderator material at present, has an orientational phase transition at 20 K, the phase I to phase II transition. In this transition, the almost free rotations in the high temperature phase I become strongly hindered at ¾ of the lattice sites in phase II. Forthcoming studies at LENS will investigate the affect of these modes both by maintaining careful thermodynamic control over the moderator, and by employing dopants which are known to perturb the rotational potential experienced by the molecules in the lattice. These perturbations can change the energy levels of the rotational excitations in the solid, giving important information on exactly how these energy levels affect to energy distribution of the moderated neutron flux.

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