SRS Labs SR430, Manual

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Using Photomultiplier Tubes
PMT's which are specifically designed for photon
counting will specify their noise in terms of the rate
of output pulses whose amplitudes exceed some
fraction of a pulse from a single photon. More of-
ten, the noise is specified as an anode dark cur-
rent. Assuming the primary source of dark current
is thermionic emission from the photocathode, the
dark count rate is given by :
Dark Count (in kHz) = 6 x Dark Current (in nA) /
Gain (in millions)
PMT BASE DESIGN
PMT bases which are designed for general pur-
pose applications are not appropriate for photon
counting. General purpose bases will not allow
high count rates, and often cause problems such
as double counting and poor plateau characteris-
tics. A PMT base with the proper high voltage ta-
per, bypassing, snubbing, and shielding is re-
quired for premium photon counting performance.
CAUTION: Lethal High Voltages are used in PMT
applications. Use extreme caution when working
with these devices. Only those experienced with
high voltage circuits should attempt any of these
procedures. Never work alone.
DYNODE BIASING
A PMT base provides bias voltages to the PMT's
photocathode and dynodes from a single negative
high voltage power supply. The simplest design
consists of a resistive voltage divider, as shown in
Figure A.
In this configuration the voltage between each dy-
node, and so the current gain at each dynode, is
the same. Typical current gains are 3 to 4, and so
there will typically be 3 to 4 electrons leaving the
first dynode, with a variance of about 2 electrons.
This large relative variance (due to the small num-
Figure A. Resistive Divider PMT Base
ber of ejected electrons) gives rise to a large varia-
tions in the pulse height of the detected signal.
Since statistical fluctuations in pulse height are
caused by the low gain of the first few stages of
the multiplier chain, increasing the gain of these
stages will reduce pulse height variations and so
improve the plateau characteristics of the PMT. To
increase the gain of the first few stages, the resis-
tor values in the bias chain are tapered to increase
the voltage in the front end of the multiplier chain.
The resistor values are slowly tapered so that the
electrostatic focusing of electrons in the multiplier
chain is not adversely affected.
Current for the electron multiplier is provided by
the bias network. Current drawn from the bias net-
work will cause the dynode potentials to change,
and so change the tube gain. This problem is of
special concern in lifetime measurements. The
shape of exponential decay curves will be
changed if the tube gain varies with count rate. To
be certain that this is not a problem, repeat the
measurement at half the original intensity.
The problem of gain variation with count rate is
avoided if the current in the bias network is about
20 times the output current from the PMT's anode.
Example: If a PMT is operated so that it gives 20
mV pulses of 5 ns duration into a 50 Ohm cable,
then the average current at 50 MHz count rate will
be 0.1 mA. If the bias resistors are chosen so that
the chain current is 20 x 0.1 mA = 2 mA, then the
PMT's gain will remain constant vs. count rate. If
this PMT is operated at 2500 vdc, then the power
dissipated in this base is 5 Watts.
There are a few other methods to avoid this prob-
lem which do not require high bias currents. These
methods depend on the fact that the majority of
the output current is drawn from the last few dy-
nodes of the multiplier.
(1) Replace the last few resistors in the bias chain
PHOTOCATHODE
e
PHOTON
8 DYNODES
ANODE
OUTPUT
R R R R R R R R R
_
- HIGH
VOLTAGE