SRS Labs SR430, Руководство пользователя

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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 pho-
tons over a spectral range of 180 to 900 nm. Win-
dowless 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 dy-
node, and these electrons are accelerated toward
the second dynode. The process continues, typi-
cally 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 elec-
trons 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 photoca-
thode will determine the spectral response (and
will influence the dark count rate) of the PMT. For
photon counting, the figure of merit is the "quan-
tum efficiency" of the PMT. A 10% quantum effi-
ciency indicates that 1 in 10 photons which strike
the photocathode will produce a photoelectron --
the rest of the incident photons will not be detect-
ed. The quantum efficiency is a function of wave-
length, so select the PMT for the best quantum ef-
ficiency 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 detecta-
ble pulse for counting. In addition, the risetime is
an important figure of merit to determine the maxi-
mum 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 be-
low a few millivolts. A good, wide bandwidth pre-
amplifier (such as the SR430) will have about 1.5
nV per root Hertz, or about 25 µV rms over its 250
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 µV 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 lim-
ited to 1.2 ns by the 300 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 sub-
stantially 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.
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 hous-
ings which cool the PMT to about -20° C can dra-
matically reduce the dark count ( from a few kHz
to a few Hz). The residual counts arise from radio-
active decays of materials inside the PMT and
from cosmic rays.
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