Nanosecond Light Source, XP-20

The characteristics of the XP-20 miniature Gallium-Phosphide diode1 were measured; it was found suitable as a nanosecond light source in counter techniques, such as testing of fast photomultipliers and hodoscope arrays in high-energy physics applications. 1. Relative light output Data were obtained with 5.6 nsec pulses of 20-volt amplitude (into 50 ohms) applied to the photodiodes, which were epoxy-potted at the end of SO-ohm subminiature cables. The light sources were mounted flush against the cathode of a photomulitplier , RCA type 85 75. The output from the P. M. was applied to a pulse integrator’ followed by a multichannel pulse-height analyzer. Results from 63 samples are shown in Fig. 1. FQrty-eight of these, or 75%, fall within about a factor of two. The average output corresponds to channel 83.5. Light from minimum ionizing particles passing through I/4-inchscintillator was found to correspond approximately to channel 100. When one XP-20 was allowed to run continuously for 72 hours, its light output remained constant to within a percent. 2. Resolution Figure 2 shows the distribution in resolution of the 63 spectra obtained as described above. The average was found to be 18.2% for 20 volts applied to the XP-20 junction, improving with increased input signal. The corresponding average number of electrons emitted by the photocathode of the RCA 8575 was estimated to be 45: a logarithmic plot of resolution versus relative output yields a straight line of slope -1/2, indicating that the resolution is mainly dependent on electron