Preparation of 6LiF deposits and characterisation via Monte Carlo simulations and Neutron Depth Profiling

The Institute for Reference Materials and Measurements (IRMM) is measuring the 6Li(n,t)4He cross-section aiming at extending its status of standard over the MeV energy range. We developed a protocol to stretch-mount 0.75μm, 1.5μm, 8μm, and 20μm thick aluminium foils onto 0.5 mm thick tantalum rings. 6LiF samples were produced depositing, by vacuum evaporation onto aluminium backings, a layer of lithium fluoride 95.5% enriched in 6Li. We engineered dedicated tools and containers to handle and transport the resulting samples. These were characterised first at IRMM by differential weighing, then by Neutron Depth Profiling (NDP) at the TU Delft. These two measurements were found to be consistent for a selected sample, probed by a thermal neutron beam in three different regions to measure the 6LiF layer thickness and uniformity (defined as variation of the thickness relative to its average). The latter was found to be 0.8%, and the 6Li thickness to be 7.30±0.12, 7.35±0.12, and 7.29±0.12μg/cm2 in the three regions. We performed Monte Carlo simulations to estimate the uniformity of the 6LiF layer, and benchmarked the calculation against the NDP measurements. They were consistent with respect to the deposit uniformity although the simulations were found to overestimate the thickness of the layer.