A NEW BEAM INTENSITY MONITOR, BASED ON AVALANCHE PHOTO DIODE

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 我们已经设计和建造了一种新的强度监测器的原型，以取代放置在样品位置上游的现有标准微型电离室。新的监视器结合了我们开发的用于超小角X射线散射(USAXS)的雪崩光电二极管(APD)探测器和放置在光束中的12？m厚的Kapton箔。低能级的Kapton x射线散射与束流强度成正比，由雪崩探测器进行光子计数。新的监视器旨在消除由于云母窗口和短氮气路径而从电离室产生的束流吸收和低水平背景散射。作为该开发项目的一部分，我们改进了雪崩光电二极管电子元件的屏蔽和接地方案，并成功地消除了虚假光子计数。由于原型强度监测器安装在定义狭缝的束流的正下游，因此两组保护狭缝实际上消除了非常弱的Kapton箔散射。新的光束强度监测器已在BL4-2上常规使用，并在光束质量方面取得了令人满意的改善，即显著减少了散射本底，并使样品位置的光子通量增加了约20%。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器apd探测器，以便它可以放置在离Kapton箔更近的地方，以获得更高的光子收集效率，以提高光束强度测量的统计意义。Kapton箔的专用真空外壳也正在设计中，以进行进一步的优化。

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.