Single-crystal rocking curve measurements of several large-area CVD diamonds are proposed to be carried out at the Canadian Light Source BMIT facility. Thin large-area diamond targets with narrow whole-crystal rocking curves are needed to produce secondary beams of linearly polarized photons in the 8-10 GeV energy range at Jefferson Lab for the GlueX experiment, in which the authors of this proposal are active participants. The challenging part of producing these targets is machining them down to the desired thickness without too much degradation of the rocking curve width. Such measurements were first carried out at BMIT in 2016 and 2017 on a set of diamonds that had a thin window machined out of the center using UV laser ablation. That technique was successful in achieving the optimum target thickness of 20 microns, but the rocking curves measured at CLS showed large plastic deformation over the thin region of the target, explaining why the polarization observed in the GlueX photon source was sub-optimal. In 2019, the GlueX group returned to CLS with a new set of CVD samples which had been machined down to 50 microns using a IR laser cutting technique followed by mechanical polishing. Once again, the rocking curve results showed extensive broadening from plastic deformation, with just one of the six samples tested being sufficiently narrow to support running of the GlueX experiment in 2020.
This proposal requests a brief run to test 4 new samples. One is a different type of CVD diamond with very low nitrogen impurity, already thinned to 50 microns and polished by the manufacturer. The other three samples are standard type-II diamonds that are still 1.3mm thick. The rocking curves of the thick samples will show whether the plastic deformation is intrinsic to the material or whether it is introduced during the machining process, while the rocking curve of the low-impurity (electronic-grade) sample will show whether this material has any advantage in reduced plastic deformation seen in the earlier type-II samples studied. These results will determine whether there is a significant advantage in the use of electronic grade diamond, and whether the degradation seen in the earlier results is intrinsic to the CVD raw material or if it was induced by the thinning process.
