This module can send a photon count directly to our computer
interface via the digital data bus on the system backplane. Up to four modules
can be in use at any one time. The output count is normally 16-bit, but the
internal counter is actually 24 bits wide, and the 8 high-order bits can also
be read out if required. A switchable prescaler allows the incoming count to be
divided by up to 256 in order to extend the operating range still further or
(more usefully) to ensure that a 16-bit data transfer will be adequate under
all practical conditions. The photon counter also has an analogue output, which
can be connected to our analogue input amplifier (C301). This facility allows
the all-or-none characteristics of photon counting to be retained in an
analogue system. The analogue output is corrected for the discriminator
headstage dead time (see below). When the photon counter is switched off, its
output is connected directly to the input, and the discriminator headstage is
also isolated, thereby connecting the input amplifier directly to the
photomultiplier output and allowing normal analogue operation.
