Specifications

Typical Experiment
64
Conclusions: Select a PMT with a risetime < 3 ns
and a current gain > 5 million.
DARK COUNTS
PMT's are the quietest detectors available. The
primary noise source is thermionic emission of
electrons from the photocathode and from the first
few dynodes of the electron multiplier. PMT
housings which cool the PMT to about -20° C can
dramatically reduce the dark count ( from a few
kHz to a few Hz). The residual counts arise from
radioactive decays of materials inside the PMT
and from cosmic rays.
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 often, the noise is specified as an anode dark
current. 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
purpose 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
characteristics. A PMT base with the proper high
voltage taper, bypassing, snubbing, and shielding
is required 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
dynode, 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
number of ejected electrons) gives rise to a large
variations 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
resistor 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.
PHOTOCATHODE
e
photon
- HIGH
VOLTAGE
8 DYNODES
ANODE
OUTPUT
RRRRRRRRR
_
FIGURE A: RESISTIVE DIVIDER PMT BASE