An implanted diode X-ray detector in silicon optical elements

Abstract

The alignment of double crystal X-ray monochromators in X-ray beams is a task that requires a significant amount of time and effort. If solid state X-ray detectors were to be built in to the second crystal of the pair, and could produce a signal related to the reflected X-ray flux, the improvement in controlling the second crystal pitch angle would be great. We therefore propose to implant a thin photodiode in the diffracting surface of the second crystal of a double crystal monochromator. Once the crystal is bathed in low divergence monochromatic radiation and aligned away from the Bragg condition X-rays penetrating the bulk are absorbed due to photo ionisation. This will produce a current in the photodiode depletion region proportional to the absorbed energy. As the angle is changed and the Bragg condition approached this signal will decrease. From this effect an error signal is derived for use in the servo loop. The goal is to produce a position sensitive detector structure on the silicon optical surface. This would allow more sophisticated motion control to trim and maintain the other angles: yaw, roll, and curvatures. Such a system would dramatically reduce the set up time for focussing optics on synchrotron X-ray stations. An implanted position sensitive detector would also be useful in analyser based X-ray imaging systems. In this application the diode array fabricated in the analyzer would act as an imaging detector. The image from the analyzer detector and the diffracted image can be used to produce a phase contrast image using only one angle of the analyzer. This is in contrast to the requirement for two images in the standard ABI systems using the basic DEI algorithms. This simplification to the protocol will be of great benefit to the application of this technique to medical imaging. We present initial reflectivity tests of a silicon wafer implanted with diode test structures and show that the changes in the optical characteristics of the silicon still allow the crystal to be used in a precision monochromator / analyser.