CHESS X-ray measurements 5/2009

In April, 2009 we had about a week of beam time at CHESS in Ithaca, NY. This time, Ken had mounted a new pair of silicon crystals in the primary monochromator box. These crystals were only a couple mm thick, and about 10cm x 10cm in size. Each bounce gave an expansion factor close to 4, with a combined b in the range 16-18. We had a great deal of difficulty aligning the crystals, and were never able to get a beam spot large enough to cover the entire target. Instead, we had to repeat the rocking curve scans many times, stepping the monochromator in small increments between each scan, until the entire target surface had been exposed. These mono crystals had been salvaged from scraps that were left over from a previous project. At the end of this run, it was resolved that we needed to cut our own custom crystals, which would be significantly thicker and have b closer to 10.

The other major difficulty that was diagnosed and overcome during this run was with vibrations of the target in the mount. The target had been mounted by stretching thin tungsten wires across a square aluminum frame and gluing the diamond to the wires. This technique has been used before at Mainz and in Hall B. It was during this run that we discovered that this mounting technique is not going to be satisfactory for Hall D, from the standpoint of mechanical stability. Targets examined were:

• BNL-300 : a type-3 CVD diamond monocrystal 4mm x 4mm and 300 microns thick, on load to us from Brookhaven National Lab.
• HallB-G50 : an older HPHT diamond from the Hall B CB source, has been used as a radiator for production running since it was last examined at CHESS, in 11/2006.
• Platinum mask : a platinum mask that absorbs X-rays, with a pattern of tiny holes etched in it, useful for calibrating and focusing the camera.

All data are archived at UConn. An overview of what was learned from this run and a sampling of the results were shown at the May 2009 collaboration meeting in a talk by R. Jones.