First consistent determination of the hydrogen content for the neutron fluence reference

Tristearin deposit on a tantalum disc © EU, 2015

Scientists from the JRC-Institute for Reference Materials and Measurements prepared tristearin (C57H110O6) layers with an areal density up to 10 mg/cm2 on tantalum backings by physical vapour deposition (PVD) and determined for the first time the hydrogen content of the initial powder, the evaporated material and the residue in the crucible by combustion analysis. It was demonstrated convincingly that the hydrogen content remains unchanged under thermal treatment.

Recoil proton telescopes are the neutron-fluence reference instruments and cross-section measurements based on them are considered ‘absolute’ and serve as an ultimate reference.  The telescopes rely on tristearin targets for which the knowledge of the hydrogen content is critical. For the manufactured targets the combustion analysis found a hydrogen content of 12.35 (10) wt%. The relative uncertainty of 0.8 % is an important component of the total relative uncertainty (1.6–2 %) for the neutron fluence determination with a recoil proton telescope.

For neutron energies from 5 to 20 MeV thicker tristearin layers of 2.5 to 25 mg/cm2 are required to compensate a drop in cross section for the H(n,n)H scattering.  To be able to produce such layers by PVD the setup for thermal evaporation was be modified at the JRC-IRMM to melt larger quantities of starting material, to prevent sputtering of the molten tristearin onto the substrates, and to protect the crystal of the thickness monitor from saturation during a single evaporation run. The mass of the deposited layers was determined accurately by substitution weighing.  The hydrogen content of tristearin was determined by combustion analysis using the Elementar Vario EL III CHNOS elemental analyser at the JRC-IRMM. Samples from strategic points of the whole preparation and deposition process were taken and the hydrogen content analysed in an interlaboratory exercise involving two more expert laboratories in this field. As a result it was shown that the evaporation of tristearin has no influence on the hydrogen content. The stability of thick tristearin layers was successfully demonstrated even for long evaporation runs.

Read more in: D. Vanleeuw,et al.: “Determination of the hydrogen content of thick tristearin layers prepared by physical vapour deposition“,  J Radioanal Nucl Chem (2015) 305-957-962, DOI 10.1007/s10967-015-4156-8