Specifications

Typical Experiment
68
There are two reasons for carefully selecting the
input discriminator level. The first is to improve
the signal-to-noise ratio by setting the
discriminator level above most of the noise pulses,
but below most of the signal pulses. The second
reason is to reduce drift: if the discriminator
threshold is set to the middle of the signal pulse
height distribution, then small changes in the tube
gain can cause a large change in the count rate.
There is no exact prescription for setting the
discriminator threshold: the procedure used will
depend somewhat on the nature of the
measurement. If dark counts are a problem then
the discriminator level should be set higher than
when drift is a concern. If the PMT is cooled (
reducing thermionic emission ) then a lower
discriminator level is probably okay. If the PMT
has a ring on the anode signal then the
discriminator level should be set high enough so
that the rings are not counted.
THE 'CORRECT' WAY
The tube should be operated at the maximum high
voltage recommended by the manufacturer. Use
enough preamplifier gain so that the single photon
pulse height is about 100 mV. Provide enough
light to the PMT for a count rate of a few
megahertz. Using a 300 MHz oscilloscope, adjust
the snubber termination for minimum ringing on
the anode signal. Take the pulse height spectrum
of the anode signal. This is very time consuming
and involves recording the count difference
between successive discriminator steps and
plotting the result Vs discriminator value. If the
PMT dark count rate is a concern, then you will
also need to take the pulse height spectrum of the
dark count signal. It will take much longer to take
the dark count spectrum because the count rate
should be much lower. The object is to find a
discriminator level which is higher than the mean
noise pulse height, and below the mean signal
pulse height.
THE 'FAST AND PRETTY GOOD' WAY
This technique works very well and is particularly
suited for those who do not want to make a career
out of plateauing their PMT's. The PMT should be
operated at (or a bit above) the recommended
maximum high voltage. Provide enough
illumination for a count rate of a few megahertz,
and enough preamp gain to get pulse heights of
about 100 mV. Using a 300 MHz oscilloscope,
adjust the snubber termination impedance for the
best pulse shape. Look carefully at the anode pulse
shape and set the discriminator to a level which is
above any ringing, but well below the mean pulse
height. If there is lots of EMI or amplifier noise
then increase the PMT's high voltage to increase
the signal pulse height.