A NEW BEAM INTENSITY MONITOR, BASED ON AVALANCHE PHOTO DIODE

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have designed and built a prototype of a new intensity monitor to replace the existing standard miniature ion chamber placed immediately upstream of the specimen position. The new monitor combines the avalanche photodiode (APD) detector that we had developed for ultra-small-angle x-ray scattering (USAXS) and a 12 ¿m thick Kapton foil placed in the beam. The low level Kapton x-ray scattering, which is proportional to beam intensity, is photon-counted by the APD. The new monitor is intended to eliminate both the beam absorption and the low-level background scattering from the ion chamber due to the mica windows and the short nitrogen path. As a part of this development project we improved the shielding and grounding scheme of the electronic components in the APD and successfully eliminated false photon counts. The very weak level of Kapton foil scattering is practically eliminated by two sets of guard slits since the prototype intensity monitor is installed immediately downstream of the beam defining slit. The new beam intensity monitor is in routine use at BL4-2, and has resulted in satisfactory improvements in beam quality, i.e., substantially reduced scattering background and increased photon flux at the specimen position by ca. 20%. Given this success, we have recently acquired a new APD element and are developing a more compact dedicated intensity monitor APD detector, so that it can be placed much closer to the Kapton foil for higher photon collection efficiency to improve statistical significance of the beam intensity measurement. A dedicated in-vacuum housing for the Kapton foil is also being designed for further optimization.