Crystal heating on the JUMBO double crystal monochromator at SSRL

Abstract

Heating by intense synchrotron radiation causes the lattice of the first crystal in the JUMBO double crystal monochromator at SSRL to expand. Because the two crystals no longer have the same lattice spacing, they diffract X-rays of the same wavelength at different angles. This causes shifts in energy calibration and beam movement at the sample with change in photon energy or as the current in the storage ring decays. A simple thermal model that predicts the change in wavelength and angle of the monochromatic beam is described. These shifts are proportional to I sin θ tan θ, where I is the SPEAR ring current and θ is the angle of the beam incident on the first crystal. Heating experiments were conducted with Ge(111), quartz(10 10 , and InSb(111) crystals. Bragg angle shifts of up to 0.25° were measured with high precision encoders mounted directly on the crystal goniometers. Photon energy calibration shifts of up to 3.5 eV (at 1700 eV) were measured. Preliminary results show good agreement with the model for Ge and quartz crystals, but not for InSb crystals. Further study is needed to refine the model for Ge and quartz crystals and to determine a better one for InSb.