SRS Labs SR430, Manual

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109 109
Using Photomultiplier Tubes
with Zener Diodes. As long as there is some re-
verse current through a Zener, the voltage across
the diodes is nearly constant. This will prevent the
voltage on these stages from dropping as the out-
put current is increased.
(2) Use external power supplies for the last few dy-
nodes in the multiplier chain. This approach dissi-
pates the least amount of electrical power since
the majority of the output current comes from low-
er 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 dy-
node voltage from dropping. ( Use 20x the aver-
age 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 ex-
ample, 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
µC. 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 0.1µC / 200V = 0.05 µF
The current from higher dynodes is smaller so the
capacitors bypassing these stages may be small-
er. 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
are usually connected in series. ( See Figure B )
By passing the dynodes of a PMT may cause high
frequency ringing of the anode output signal. This
can cause multiple counts for a single photon. The
problem is significantly reduced by using small re-
sistors between the dynodes and the bypass ca-
pacitors, as shown in Figure B.
SNUBBING
Snubbing refers to the practice of adding a net-
work to the anode of the PMT to improve the
shape of the output pulse for photon counting ap-
plications. This 'network' is usually a short piece of
50 Ohm coax cable which is terminated into a re-
sistor of less than 50 Ohms. There are four impor-
tant reasons for using a snubber network:
(1) Without some dc resistive path between the
anode and ground, anode current will charge the
signal cable to a few hundred volts. When the sig-
nal cable is connected to a preamplifer or to a sig-
nal input, the stored charge on the cable may
damage the front-end of the instrument. If you de-
cide not to use a snubber network, please install a
100 kOhm resistor between the anode and ground
to protect your instruments.
(2) The risetime of the output current pulse is often
much faster than the falltime. A snubber network
may be used to sharply reduce the falltime, greatly
improving the pulse pair resolution of the PMT.
(3) Ringing ( with a few nanosecond period ) is
very common on PMT outputs ( especially if the fi-
nal dynode stages are bypassed with capacitors ).
A snubber network may be used to cancel these
rings which can cause multiple counts from a sin-
gle photon.
(4) The snubber network will help to terminate re-
Figure B. PMT Base For Photon Counting
OUTPUT PHOTOCATHODE
- HIGH
VOLTAGE
8 DYNODES ANODE
1.5R R R R
100 100 100 100
Vz Vz Vz Vz
C/8 C/4 C/2
50
Ω
C
SNUBBER
10 INCHES
RG174
NOTE: ZENER DIODES MAY BE REPLACED WITH RESISTORS
IN LOW AVERAGE COUNT RATE APPLICATIONS
2R