Specifications

Using Photomultiplier Tubes
65
Current for the electron multiplier is provided by
the bias network. Current drawn from the bias
network 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
problem 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 dynodes of the multiplier.
(1) Replace the last few resistors in the bias chain
with Zener Diodes. As long as there is some
reverse current through a Zener, the voltage across
the diodes is nearly constant. This will prevent the
voltage on these stages from dropping as the
output current is increased.
(2) Use external power supplies for the last few
dynodes in the multiplier chain. This approach
dissipates the least amount of electrical power
since the majority of the output current comes
from lower voltage power supplies, however it is
the most difficult to implement.
(3) If the average count rate is low, but the peak
count rate is high, then bypass capacitors on the
last few stages may be used to prevent the dynode
voltage from dropping. ( Use 20x the average
output current for the chain current.) For a voltage
drop of less than 1%, the stored charge on the last
bypass capacitor should be 100x the charge output
during the peak count rate. For example, the
charge output during a 1 ms burst of a 100 MHz
count rate, each with an amplitude of 10 mV into
50 Ohms and a pulse width of 5 ns, is 0.1 uC. If
the voltage on the last dynode is 200 Vdc, then the
bypass capacitor for the last dynode should have a
value given by:
C = 100 Q/V = 100 x 0.1uC / 200V = 0.05 uF
The current from higher dynodes is smaller so the
capacitors bypassing these stages may be smaller.
Only the final four or five dynodes need to be
bypassed, usually with a capacitor which has half
the capacitance of the preceding stage. To reduce
the voltage requirement for these capacitors, they
PHOTOCATHODE
- HIGH
VOLTAGE
8 DYNODES ANODE
2R 1.5R R R R
100 100 100100
Vz Vz Vz Vz
C/8 C/4 C/2
50
C
OUTPUT
SNUBBER
10 INCHES
RG174
NOTE: ZENER DIODES MAY BE REPLACED WITH RESISTORS
IN LOW AVERAGE COUNT RATE APPLICATIONS
FIGURE B: PMT BASE FOR PHOTON COUNTING