An active beamstop for accurate measurement of high intensity X-ray beams

In order to accurately measure the X-ray flux and determine the dose absorbed by the sample, we have designed an active beamstop device. Based on the photoelectric effect, the device consists of a beamstop, an insulator, and a concentric photoelectron collector. Tests of this device on the macromolecular crystallography beamline (BL17U1) at the Shanghai Synchrotron Research Facility (SSRF) show that the active beamstop is sufficient for determining the X-ray flux with the relative error smaller than 0.2%. The device can be used as a non-invasive, real-time intensity monitor at the sample during data collection. It can also be used with standard collimated beams or slits and served as a warning when the beam alignment drifts.