Specification for diamond crystal radiator
|Rocking curve width||20 μr r.m.s. maximum (integrated over the whole crystal active area)|
|Thickness||20 μm in the active area|
|Size||5.6 mm x 5.6 mm minimum active area|
|Plate shape||rectangular or square|
|Orientation error||5° maximum|
|Variation in thickness||±5μm maximum|
The quality requirements for the Gluex diamond radiator are mainly driven by the requirement for the polarization of the photons by the GlueX project. The two things which affect the photon polarization are the mosaic spread and crystal thickness. In order to get the necessary photon polarization, the mosaic spread (integrated over the whole crystal) must be less than 20 micro radians (r.m.s.) and the thickness must be less than 20 microns. The mosaic spread can be estimated from the rocking curve width. The size of the diamond crystal is determined by the electron beam spot size, since the electron beam has an size of 1.0 mm r.m.s. by 0.7 mm r.m.s., the size of the crystal must be no smaller than 5.6 mm by 5.6 mm, so that less than 5% of the total photon beam flux originates in the frame around the active region of the crystal, assumed to be 300 microns thick.
Variation of the thickness over the whole crystal should be less than ±5 microns. The crystal plate should have either rectangular or square shape. The orientation of the crystal should be <001>, the error of the orientation should be less than 5 degrees. The crystal plate should be free from cracks.