Secondary Electrons from X-Ray Photocathodes

Streak cameras are used to study the x-rays that are emitted from Inertial Confinement Fusion (ICF) targets as they implode. These cameras operate by using a photocathode to convert the x-ray flux to a secondary electron current. Secondary electrons are then focused onto a phosphor screen using the streak tube electron optics, and finally recorded using a charged coupled device (CCD) camera. The pulse height distributions for the number of secondary electrons per each absorbed x-ray event in the photocathode can be traced from the CCD data. Several photocathode materials were tested for their secondary electron number distributions by using a DC x-ray source and a slow ramp to uniformly illuminate the streak camera. The data was analyzed by generating histograms of the values of CCD superpixels that integrated the recorded signal in the pixels surrounding an x-ray event. The distribution for the number of secondary electrons produced per each absorbed x-ray event can then be derived by using the distribution for the number of CCD electrons recorded per single streak tube electron.