Screening of HOPG crystals using TOF neutron transmission for VESPA and TOSCA+

The characterisation of highly oriented pyrolytic graphite (HOPG) crystals is part of an ongoing collaboration in neutron instrumentation between the European Spallation Source (ESS) and the ISIS Facility for the development and prototyping of the VESPA (ESS, in construction) and TOSCA+ (ISIS, Endeavour program) instrument projects. The screening and characterisation of HOPG crystals from various suppliers is vital to ensure that nominal performance are achieved on these instruments.

We would like to use IMAT to screen 530 HOPG crystals and characterise their mosaic spread, peak reflectivity and direction of the c-axis relative to the cut face of the crystal. All three parameters can be simultaneously obtained from time-of-flight transmission measurement of the (002) reflection of the HOPG, as demonstrated in previous experiments on ALF and IMAT. Thanks to its large 10x10 cm beam and the use of the nGEM position sensitive monitor, up to 34 crystals can be measured simultaneously on IMAT. Furthermore, thanks to IMAT high flux, only short exposure time are required (<1h per bundle of 34 crystals). IMAT is thus the only instrument that allows to screen through this many samples and provide statistically relevant information on the population of HOPG crystals.