Specifications

Using Photomultiplier Tubes
67
2 ns
RINGING
WITHOUT
SNUBBER
SNUBBER
CANCELS
RINGING
ANODE RINGING CANCELED WITH
SNUBBER CABLE
PMT BASE CONCLUSIONS:
(1) Taper voltage divider for higher gain in first
stages.
(2) Bypass last few dynodes in pulsed
applications.
(3) Use a snubber circuit to shape the output
pulse.
CATHODE SHIELDING
Head-on PMT's have a semitransparent
photocathode which is operated at negative high
voltage. Use care so that no objects near ground
potential contact the PMT near the photocathode.
Electron trajectories inside the PMT will be
affected by magnetic fields. A field strength of a
few Gauss can dramatically reduce the gain of a
PMT. A magnetic shield made of a high
permeability material should be used to shield the
PMT.
PREAMPLIFIERS
The output of a PMT is a current pulse. This
current is converted to a voltage by a load resistor.
One would like to use a large resistor to get a large
voltage pulse, however in photon counting it is
important to maintain a high bandwidth for the
output signal. Since charge on the anode is
removed by the load resistance, smaller load
resistances increase the bandwidth. The bandwidth
of a 10 pF anode with a 100 Ohm load is 300
MHz.
For convenience, 50 Ohm systems are usually
used. The current pulse from the PMT travels
down a 50 Ohm cable which is terminated by the
50 Ohm input impedance of a preamplifier. The
attenuation of RG-58 coax cable at 300 MHz is
about 1 dB/ 10 ft. and so it does not significantly
degrade performance in this application.
To allow counting to 200 MHz, a preamplifier
with a bandwidth which is somewhat larger than
200 MHz is required. The SR445A preamplifier
has four gain of 5 amplifiers, each with 50 Ohm
input impedance and a 350 MHz bandwidth. The
amplifiers may be cascaded for gains of 5, 25, 125,
or 625.
GAIN REQUIREMENT
The SR400 Photon Counter can detect pulses as
low as 2 mV. To allow for some adjustment of the
discriminator threshold and to provide better noise
immunity, a more practical lower limit on pulse
size is about 10 mV. The highest discriminator
level which may be set is 300 mV. The
preamplifier should have enough gain to amplify
anode pulses to between 10 mV and 300 mV ( 100
mV is a good target value ).
Using the result that pulse height ( in mV ) is
about 4x the tube gain ( in millions ) divided by
the risetime ( in ns ), a PMT with a gain of 4
million and a risetime of 2 ns will provide 8
mVoutput pulses. Half of the pulse amplitude will
be lost in the anode snubber, so a gain of 25 is
required to boost the output pulses to 100 mV
amplitude.
PLATEAUING A PMT
Due to the statistical nature of the secondary
emission process, there is a distribution of signal
pulse heights coming from the PMT. There is
another distribution of noise pulse heights. Noise
which results from thermionic emission from the
photocathode can not be distinguished from signal,
however, noise pulses from dynode thermionic
emission will have a lower mean pulse height. The
PMT should be operated at sufficient high voltage
that the mean signal pulse height is well above the
pulse height of other noise sources such as preamp
noise and EMI pickup.