A NEW BEAM INTENSITY MONITOR, BASED ON AVALANCHE PHOTO DIODE

这个子项目是许多研究子项目中的一个 由NIH/NCRR资助的中心赠款提供的资源。子项目和 研究者（PI）可能从另一个NIH来源获得了主要资金， 因此可以在其他CRISP条目中表示。所列机构为 研究中心，而研究中心不一定是研究者所在的机构。 我们已经设计并建造了一个新的强度监测器的原型，以取代现有的标准微型离子室放置在上游的标本位置。新的监测器结合了雪崩光电二极管（APD）探测器，我们已经开发了超小角X射线散射（USAXS）和12 μ m厚的Kapton箔放置在光束。与光束强度成比例的低水平Kapton x射线散射由APD进行光子计数。新的监测器旨在消除由于云母窗和短的氮气路径引起的束流吸收和来自离子室的低水平背景散射。作为该开发项目的一部分，我们改进了APD中电子元件的屏蔽和接地方案，成功地消除了假光子计数。由于原型强度监测器直接安装在光束定义狭缝的下游，因此两组保护狭缝实际上消除了非常弱的Kapton箔散射水平。新的光束强度监测器在BL 4 -2的日常使用中，并在光束质量方面取得了令人满意的改善，即，基本上减少了散射背景，并增加了样品位置处的光子通量。百分之二十鉴于这一成功，我们最近获得了一种新的APD元件，并正在开发一种更紧凑的专用强度监测APD探测器，以便它可以放置在更靠近Kapton箔的位置，以提高光子收集效率，从而提高光束强度测量的统计意义。Kapton箔专用的真空外壳也正在设计中，以进一步优化。

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.