Christopher Lavelle

Neutronic Design and Measured Performance of the Low Energy Neutron Source (LENS) Target Moderator Reflector Assembly

Abstract The Low Energy Neutron Source (LENS) is an accelerator-based pulsed cold neutron facility under construction at the Indiana University Cyclotron Facility (IUCF). The idea behind LENS is to produce pulsed cold neutron beams starting with ∼ MeV neutrons from (p,n) reactions in Be which are moderated to meV energies and extracted from a small solid angle for use in neutron instruments which can operate efficiently with relatively broad ( ∼ 1 ms ) neutron pulse widths. Although the combination of the features and operating parameters of this source is unique at present, the neutronic design possesses several features similar to those envisioned for future neutron facilities such as long-pulsed spallation sources (LPSS) and very cold neutron (VCN) sources. We describe the underlying ideas and design details of the target/moderator/reflector system (TMR) and compare measurements of its brightness, energy spectrum, and emission time distribution under different moderator configurations with MCNP simulations. Brightness measurements using an ambient temperature water moderator agree with MCNP simulations within the 20% accuracy of the measurement. The measured neutron emission time distribution from a solid methane moderator is in agreement with simulation and the cold neutron flux is sufficient for neutron scattering studies of materials. We describe some possible modifications to the existing design which would increase the cold neutron brightness with negligible effect on the emission time distribution.

Ultracold-neutron production in a pulsed-neutron beam line

We present the results of an Ultracold neutron (UCN) production experiment in a pulsed neutron beam line at the Los Alamos Neutron Scattering Center. The experimental apparatus allows for a comprehensive set of measurements of UCN production as a function of target temperature, incident neutron energy, target volume, and applied magnetic field. However, the low counting statistics of the UCN signal expected can be overwhelmed by the large background associated with the scattering of the primary cold neutron flux that is required for UCN production. We have developed a background subtraction technique that takes advantage of the very different time-of-flight profiles between the UCN and the cold neutrons, in the pulsed beam. Using the unique timing structure, we can reliably extract the UCN signal. Solid ortho-D2 is used to calibrate UCN transmission through the apparatus, which is designed primarily for studies of UCN production in solid O2. In addition to setting the overall detection efficiency in the apparatus,UCN production data using solid D2 suggest that the UCN upscattering cross-section is smaller than previous estimates, indicating the deficiency of the incoherent approximation widely used to estimate inelastic cross-sections in the thermal and cold regimes.

Demonstration of neutron detection utilizing open cell foam and noble gas scintillation

We present results demonstrating neutron detection via a closely spaced converter structure coupled to low pressure noble gas scintillation instrumented by a single photo-multiplier tube (PMT). The converter is dispersed throughout the gas volume using a reticulated vitreous carbon foam coated with boron carbide (B4C). A calibrated cold neutron beam is used to measure the neutron detection properties, using a thin film of enriched 10B as a reference standard. Monte Carlo computations of the ion energy deposition are discussed, including treatment of the foam random network. Results from this study indicate that the foam shadows a significant portion of the scintillation light from the PMT. The high scintillation yield of Xe appears to overcome the light loss, facilitating neutron detection and presenting interesting opportunities for neutron detector design.