Nanocoulomb Proton Burst

At the UMASS Lowell Van-de-Graph accelerator, we are required to provide proton irradiation doses which corresponds to proton charges much less than one $\mu $C. We found that machine regulation requires a proton beam of about 1 $\mu $A. We designed a shutter provided with a slit which allows us to irradiate a sample for a fraction of 1 second with a 1 $\mu $A beam. To determine the time, $\Delta $t, during which the shutter is open, we use a light beam. Our equipment consists of the following: a photomultiplier tube (BURLE S83010E) set up as a photodiode, an electrometer (KEITHLEY 6514), a HeNe laser and a shutter that interrupts the laser beam. We measure the photodiode current, I$_{0}$, with the shutter open and we measure the charge with the electrometer, Q=I$_{0}\Delta $t, delivered to the target which the shutter is operated. From this data we obtain $\Delta $t, the time during which the shutter permits the light beam to pass, and during which we would pass the proton beam.