A shutter±photodiode combination for UV and soft X-ray beamlines

In many experiments it is important to be able to shutter the beam. In our work this is to protect radiation-sensitive samples from unnecessary exposure, or for reading out a low-noise CCD detector. Shuttering is often performed using either the beamline's safety shutter or a valve near the experiment. Both of these devices tend to be relatively slow, and frequent use can shorten their lifetime. Most beamlines also require some form of intensity monitoring. A Samson photodiode (Samson, 1967), where the photocurrent from an aluminium surface is measured, is frequently used. Using the measurements by Day et al. (1981), this photocurrent can be converted to photon ̄ux for monochromatic radiation. Of course, the photodiode must intercept the beam, as our shutter does in its closed position.