Specifications

Using Photomultiplier Tubes
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USING PHOTOMULTIPLIER TUBES
PHOTOMULTIPLIER SELECTION FOR
PHOTON COUNTING
Photomultiplier Tubes (PMT's) are high-gain, low
noise light detectors. They can detect single
photons over a spectral range of 180 to 900 nm.
Windowless PMT's can be used from the near UV
through the X ray region, and may also be used as
particle detectors.
Photons which strike the PMT's photocathode
eject an electron by the photoelectric effect. This
electron is accelerated toward the first dynode by a
potential of 100 to 400 Vdc. Secondary electrons
are ejected when the electron strikes the first
dynode, and these electrons are accelerated toward
the second dynode. The process continues,
typically for 10 dynodes, each providing an
electron gain of about 4, to produce 1,000,000
electrons which are collected by the anode. If
these electrons arrive in a 5 ns pulse into a 50
Ohm load, they will produce a 1.6 mV pulse.
These pulses may be amplified and counted.
GEOMETRY
There are two basic geometries for photomultiplier
tubes: head-on and side-on types. The head-on
type has a semitransparent photocathode, and a
linear array of dynodes. The head-on types offer
large photocathodes with uniform sensitivity, and
lower noise. These PMT's must be operated at a
higher voltage, and are usually larger and more
expensive than the side-on types. Side-on types
have an opaque photocathode and a circular cage
of dynodes.
SPECTRAL RESPONSE
There are a variety of materials which are used as
photocathodes: the workfunction of the
photocathode will determine the spectral response
(and will influence the dark count rate) of the
PMT. For photon counting, the figure of merit is
the "quantum efficiency" of the PMT. A 10%
quantum efficiency indicates that 1 in 10 photons
which strike the photocathode will produce a
photoelectron -- the rest of the incident photons
will not be detected. The quantum efficiency is a
function of wavelength, so select the PMT for the
best quantum efficiency over the wavelength
region of interest.
GAIN AND RISETIME
It is important to select a PMT with sufficient
gain, and short enough risetime, to produce a
detectable pulse for counting. In addition, the
risetime is an important figure of merit to
determine the maximum count rate for the tube.
The criteria for a "detectable pulse" depends on
the electrical noise environment of your
laboratory, and the noise your preamplifier. In
laboratories with Q-switched lasers or pulsed
discharges, it is difficult to reduce the noise on any
coax cable below a few millivolts. A good, wide
bandwidth preamplifier (such as the SR445A) will
have about 6.4 nV per root Hertz noise over its
350 MHz bandwidth. Peak noise will be about 2.5
times the rms noise, and so it is important that the
PMT provide pulses of at least 100 uV amplitude.
Use manufacturer's specifications for the current
gain and risetime to estimate the pulse amplitude
from the PMT:
Amplitude (mV) = 4 x Gain (in millions)/
Risetime (in ns)
This formula assumes that the electrons will enter
a 50 Ohm load in a square pulse whose duration is
twice the risetime. (Since the risetime will be
limited to 1.0 ns by the 350 MHz bandwidth of the
preamplifier, do not use risetimes less than 1.5 ns
in this formula.)
The current gain of a PMT is a strong function of
the high voltage applied to the PMT. Very often,
PMT's will be operated well above the high
voltage recommended by the manufacturer, and so
at substantially higher current gains (10x to 100x
above specs). There are usually no detrimental
affects to the PMT so long as the anode currents
are kept well below their rated values.