Rotation method flux monitoring with SIS3801/3806 Multiscalers

The rotation method is one way to obtain macromolecular crystallography data. The crystal is mounted on a goniostat and rptated through a small angle while its exposed to the Synchrotron Radiation beam. Diffraction data are recorded using a CCD. As the flux of the incoming beam may vary over time, flux monitoring is required to get an accurate measure for the intensity of the diffraction peaks. The incident flux measured with a PIN-diode or ionisation chamber can be read out with a scaler after a voltage to frequency conversion. The quantity of interest is the number of incident photons per angular interval. This observable can be readily measured with the SIS3801 or the SIS3806 VME multiscalers by using an angular encoder synchronised to the goniostat rotation. The generation of a counter interval can be initiated by an external signal on both multiscalers, in the firmware revisions 5 and 6 the user has the possibility to run the external next time slice signal (clock ticks from the angular encoder) through an internal prescaler. The actual flux V/F converter output signal is connected to one of the inputs of the scaler board. The internal prescaler is 24-bits deep and can be programmed in steps of 1 from 2 to 0xFFFFFF (the unprescaled signal can be used to generate the next counter slice by switching the external prescaler off).

Last update 8.9.99 by Matthias Kirsch