Specifications

Typical Experiment
66
are usually connected in series. ( See Figure B )
Bypassing 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
resistors between the dynodes and the bypass
capacitors, as shown in Figure B.
SNUBBING
Snubbing refers to the practice of adding a
network to the anode of the PMT to improve the
shape of the output pulse for photon counting
applications. This 'network' is usually a short piece
of 50 Ohm coax cable which is terminated into a
resistor of less than 50 Ohms. There are four
important 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
signal cable is connected to a preamplifer or to a
photon counter, the stored charge on the cable may
damage the front-end of the instrument. If you
decide 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
final dynode stages are bypassed with capacitors ).
A snubber network may be used to cancel these
rings which can cause multiple counts from a
single photon.
(4) The snubber network will help to terminate
reflections from the input to the preamplifier.
A good starting point for a snubber network is a 10
inch piece of RG174/U coax cable with a small 50
Ohm pot connected to the end so that the
terminating impedance may be adjusted from 0 to
50 Ohms. ( A 10 inch cable will have a round trip
time of about 5 ns -- be sure your PMT has a
risetime less than this.) The other end of this cable
is connected to the anode of the PMT, together
with the output signal cable.
Output current pulses will split, 50% going out the
signal cable, and 50% going into the snubber. If
the snubber pot is adjusted to 50 Ohms there will
be no reflection -- the only affect the snubber has
is to attenuate the signal by a factor of two.
The reflection coefficient for a cable with a
characteristic impedance R
0
, terminated into a
resistance R
t
, is given by:
Reflection Coefficient = ( R
t
- R
0
) / ( R
t
+ R
0
)
If the pot is adjusted to a value below 50 Ohms,
then some portion of the signal will be inverted
and reflected back toward the anode. This
reflected (and inverted ) signal is delayed by the
round trip time in the snubber cable and sent out
the signal cable. The amount of the reflection is
adjusted for the best pulse shape as shown in the
figure below.
2 ns
PMT PULSE
WITHOUT
SNUBBER
PMT PULSE
WITH
SNUBBER
A SLOW FALLTIME PULSE SHAPE IS
IMPROVED WITH A SNUBBER
The round trip time in the snubber cable may be
adjusted so that the reflected signal cancels anode
signal ringing. This is done by using a cable length
with a round trip time equal to the period of the
anode ringing.