Flourescence x-rays are registered through the use of a liquid nitrogen cooled Kevex Si(Li) detector, which is essentially a large reverse biased p-i-n diode that is lithium doped to compensate for p-type impurities.  Impinging photons in the intrinsic region generate electron-hole pairs which are swept out by the large electric field.  Since the number of carriers generated is proportional to incident photon energy, it is possible to count the photons in an energy-dispersive manner displaying the number of photons versus energy.  It should be noted that the detector is cooled to 77 K in order to reduce the thermal leakage current.

    For the TXRF setup at beamline 6-2, we used a detector which has an active area of 14mm² and generally a maximum count rate of 15000 cps is possible.  It was also necessary to remanufacture the detector twice to remove spurious fluorescence signals that were generated from the aluminum alloy used in the detector.  These differences in detector design are illustrated below where the blue spectrum represents the unaltered detector, the green spectrum corresponds to the version where the Ni and Cu components of the detector were replaced, and the red spectrum corresponds to the final version where Al alloy was replace with 99.999% pure aluminum.

    It should be indicated that the detector geometry was chosen so as to minimize the scattering produced by installing the detector parallel to the polarization vector of the incident beam (see theory section).

Send mail to singhace@stanford.edu with questions or comments about this web site.
Last modified: June 26, 2000