CHAPTER 3 : BASIC OPERATING METHODS OF PHOTOMULTIPLIER TUBES

CHAPTER 3
BASIC OPERATING METHODS
OF PHOTOMULTIPLIER TUBES
This section provides the first-time photomultiplier tube users with
general information on how to choose the ideal photomultiplier tube
(often abbreviated as PMT), how to operate them correctly and how to
process the output signals. This section should be referred to as a quick
guide. For more details, refer to the following chapters.
© 2007 HAMAMATSU PHOTONICS K. K.
22
3.1
CHAPTER 3 BASIC OPERATING METHODS OF PHOTOMULTIPLIER TUBES
Using Photomultiplier Tubes
3.1.1 How to make the proper selection
LIGHT SOURCE
SAMPLE
MONOCHROMATOR
PMT
THBV3_0301EA
Figure 3-1: Atomic absorption application
Figure 3-1 shows an application example in which a photomultiplier tube is used in absorption spectroscopy. The following parameters should be taken into account when making a selection.
Incident light conditions
Selection reference
<Photomultiplier tubes>
<Circuit Conditions>
Light wavelength
Window material
Photocathode spectral response
Light intensity
Number of dynodes
Dynode type
Voltage applied to dynodes
Light beam size
Effective diameter (size)
Viewing configuration (side-on or head-on)
Speed of optical phenomenon
Time response
Signal processing method
(analog or digital method)
Bandwidth of associated circuit
It is important to know beforehand the conditions of the incident light to be measured. Then, choose a
photomultiplier tube that is best suited to detect the incident light and also select the optimum circuit conditions that match the application. Referring to the table above, select the optimum photomultiplier tubes, operating conditions and circuit configurations according to the incident light wavelength, intensity, beam size and
the speed of optical phenomenon. More specific information on these parameters and conditions are detailed
in Chapter 2 and later chapters.
© 2007 HAMAMATSU PHOTONICS K. K.
3.1
Using Photomultiplier Tubes
23
3.1.2 Peripheral devices
As shown in Figure 3-2, operating a photomultiplier tube requires a stable source of high voltage (normally
1 to 2 kilovolts), voltage-divider circuit for distributing an optimum voltage to each dynode, a housing for
external light shielding, and sometimes a shield case for protecting the photomultiplier tube from magnetic or
electric fields.
VOLTAGE-DIVIDER
CIRCUIT
HV POWER
SUPPLY
PMT
HOUSING
LIGHT
SIGNAL
DETECTION
CIRCUIT
SHIELD CASE
THBV3_0302EA
Figure 3-2: Basic operating method
High-voltage power supply
A negative or positive high-voltage power supply of one to two kilovolts is usually required to operate
a photomultiplier tube. There are two types of power supplies available: modular power supplies like that
shown in Figure 3-3 and bench-top power supplies like that shown in Figure 3-4.
C4900
High voltage output: -1250 V
Current output: 600 µA
Figure 3-3: Modular high-voltage power supply
C9525
High voltage output: -2000 V
Current output: 2 mA
Figure 3-4: Bench-top high-voltage power supply
© 2007 HAMAMATSU PHOTONICS K. K.
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CHAPTER 3 BASIC OPERATING METHODS OF PHOTOMULTIPLIER TUBES
Since the gain of photomultiplier tubes is extremely high, they are very susceptible to variations in the
high-voltage power supply. If the output stability of a photomultiplier tube should be maintained within
one percent, the power supply stability must be held within 0.1 percent.
Voltage-divider circuit
Supply voltage must be distributed to each dynode. For this purpose, a voltage-divider circuit is usually
used to divide the high voltage and provide a proper voltage gradient between each dynode. To allow easy
operation of photomultiplier tubes, Hamamatsu provides socket assemblies that incorporate a photomultiplier tube socket and a matched divider circuit as shown in Figures 3-5 to 3-8.
(1) D-type socket assembly with built-in divider circuit
SOCKET
SIGNAL OUTPUT
SIGNAL GND
PMT
POWER SUPPLY GND
HIGH VOLTAGE INPUT
VOLTAGE-DIVIDER CIRCUIT
THBV3_0305EA
Figure 3-5: D-type socket assembly
(2) DA-type socket assembly with built-in divider circuit and amplifier
SOCKET
AMP
LOW VOLTAGE INPUT
SIGNAL OUTPUT
PMT
SIGNAL GND
HIGH VOLTAGE INPUT
VOLTAGE-DIVIDER CIRCUIT
THBV3_0306EA
Figure 3-6: DA-type socket assembly
© 2007 HAMAMATSU PHOTONICS K. K.
3.1
Using Photomultiplier Tubes
25
(3) DP-type socket assembly with built-in voltage divider and power supply
SOCKET
SIGNAL OUTPUT
SIGNAL GND
PMT
LOW VOLTAGE INPUT
HV
POWER
SUPPLY
VOLTAGE PROGRAMMING
VOLTAGE-DIVIDER
CIRCUIT
POWER SUPPLY GND
THBV3_0307EA
Figure 3-7: DP-type socket assembly
(4) DAP-type socket assembly with built-in voltage divider, amplifier and power supply
SOCKET
AMP
SIGNAL OUTPUT
SIGNAL GND
PMT
HV
POWER
SUPPLY
LOW VOLTAGE INPUT
VOLTAGE PROGRAMMING
POWER SUPPLY GND
VOLTAGE-DIVIDER
CIRCUIT
THBV3_0308EA
Figure 3-8: DAP-type socket assembly
© 2007 HAMAMATSU PHOTONICS K. K.
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CHAPTER 3 BASIC OPERATING METHODS OF PHOTOMULTIPLIER TUBES
Housing
Since photomultiplier tubes have very high sensitivity, they may detect extraneous light other than the
light to be measured. This decreases the signal-to-noise ratio, so a housing is required for external light
shielding.
Photomultiplier tube characteristics may vary with external electromagnetic fields, ambient temperature, humidity, or mechanical stress applied to the photomultiplier tube. For this reason, a magnetic or
electric shield is also required to protect the photomultiplier tube from such adverse environmental factors.
Moreover, a cooled housing is sometimes used to maintain the photomultiplier tube at a constant temperature or at a low temperature for more stable operation.
(3)
(4)
(5)
(6)
54
(7)
(8)
80 ± 2
5
60
(2)
35.2 ± 1
(1)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
Insulator
PMT (1-1/8" Side-on PMT)
E989 Shield Case
Clamp
2-M3, L=5
Flange
Socket Assembly
O-ring
Installation Base
2-M3, L=5
(9) (10)
[Flange Fastening Positions to Installation Base]
48
3-M3
54.0 ±
0.1
12
0°
12
0°
Direction of Incident Light
THBV3_0309EA
Figure 3-9: Housing example (with built-in magnetic shield case)
© 2007 HAMAMATSU PHOTONICS K. K.
3.1
Using Photomultiplier Tubes
27
Integral power supply module
To make the use of photomultiplier tubes as easy as possible, Hamamatsu Photonics provides PMT
modules which incorporate a photomultiplier tube in a compact case, along with all the necessary components such as a high-voltage power supply and operating circuit. (Figure 3-10)
PMT modules are easy to handle since they operate by supplying only low voltage, making the equipment compact and simple to use.
METAL PACKAGE PMT
AMP
LIGHT
H.V.
CIRCUIT
STABILIZED
H.V. CIRCUIT
Vee LOW VOLTAGE INPUT (-11.5 to -15.5V)
SIGNAL OUTPUT (VOLTAGE OUTPUT)
SIGNAL OUTPUT (CURRENT OUTPUT)
Vcc LOW VOLTAGE INPUT (+11.5 to +15.5V)
H5784 SERIES
H5773/H5783 SERIES
GND
REF. VOLTAGE OUTPUT (+12V)
CONTROL VOLTAGE INPUT (0 to +1.0V)
THBV3_0310EA
Figure 3-10: Structure of an integral power supply module
Various types of PMT modules are available, including those that have internal gate circuits, photon
counting circuits or modulation circuits. Refer to Chapter 8 for detailed information.
© 2007 HAMAMATSU PHOTONICS K. K.
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CHAPTER 3 BASIC OPERATING METHODS OF PHOTOMULTIPLIER TUBES
3.1.3 Operating methods (connection circuits)
The output from a photomultiplier tube can be processed electrically as a constant current source. It is
best, however, to connect it to an optimum circuit depending on the incident light and frequency characteristics required. Figure 3-11 shows typical light measurement circuits which are commonly used. The DC
method and AC method (analog method) are mainly used in rather high light levels to moderate light
levels. At very low light levels, the photon counting method is most effective. In this method, light is
measured by counting individual photons which are the smallest unit of light.
The DC method shown in Figure 3-11 (a) detects DC components in the photomultiplier tube output by
means of an amplifier and a lowpass filter. This method is suited for detection of relatively high light levels
and has been widely used. The AC method shown in (b) extracts only AC components from the photomultiplier tube output via a capacitor and converts them into digital signals by using an AD converter. This
method is used in regions where modulated light or light intensity is low and the AC components are
predominant in the output signal over the DC components. In the photon counting method shown in (c),
the output pulses from the photomultiplier tube are amplified and only the pulses with an amplitude higher
than the preset discrimination pulse height are counted as photon signals. This method allows observation
of discrete output pulses from the photomultiplier tube, and is the most effective technique in detecting
very low light levels. Other measurement methods include a lock-in detection technique using an optical
chopper, which features low noise and is used for detecting low-light-level signals.
INPUT LIGHT
DC AMP
PMT
ADC
RL
PC
LOWPASS
FILTER
a) DC measurement
DC AMP
INPUT LIGHT
PMT
PC
RL
HIGH-SPEED
ADC
b) AC Measurement
PULSE
AMP
INPUT LIGHT
PMT
PC
RL
COMPARATOR
PULSE
COUNTER
c) Photon Counting
THBV3_0311EA
Figure 3-11: Light measurement methods using PMT
These light measurement methods using a photomultiplier tube and the connection circuit must be
optimized according to the intensity of incident light and the speed of the event to be detected. In particular, when the incident light is very low and the resultant signal is small, consideration must be given to
minimize the influence of noise in the succeeding circuits. As stated, the lock-in detection technique and
photon counting method are more effective than the DC method in detecting low level light. When the
incident light to be detected changes in a very short period, the connected circuit should be designed for a
wider frequency bandwidth as well as using a fast response photomultiplier tube. Additionally, impedance
matching at high frequencies must also be taken into account. Refer to Chapters 5 and 6 for more details on
these precautions.
© 2007 HAMAMATSU PHOTONICS K. K.
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