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 recently designed and built a new intensity monitor to replace the existing standard miniature ion chamber placed immediately upstream of the sample position. The new monitor is based on an earlier prototype version using an avalanche photodiode (APD) detector and a 12 micron thick Kapton foil placed in the beam. The low level Kapton x-ray scattering, which is proportional to the beam intensity, is photon-counted by the APD. The new monitor eliminates both the beam absorption and the low-level background scattering from the ion chamber originally used at the station. As a part of this development project we have improved the shielding and grounding scheme of the electronic components in the APD and successfully eliminated false photon counts. In the new, more compact design developed this year we were able to considerably reduced the distance between the APD and the kapton foil, which resulted in a substantial increase of photon collection efficiency lowering our statistical error of the incoming beam intensity measurement by more than a factor of five.