photomultiplier tubes

photomultiplier tubes
CHAPTER 8
PHOTOMULTIPLIER TUBE
MODULES
This chapter describes the structure, usage, and characteristics of
photomultiplier tube (PMT) modules. These PMT modules consist of a
photomultiplier tube, a voltage-divider circuit and a high-voltage power
supply circuit carefully assembled into the same package.
© 2007 HAMAMATSU PHOTONICS K. K.
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8.1
CHAPTER 8
PHOTOMULTIPLIER TUBE MODULES
What Are Photomultiplier Tube Modules?
Photomultiplier tube (PMT) modules are basically comprised of a photomultiplier tube, a high-voltage
power supply circuit, and a voltage-divider circuit to distribute a voltage to each dynode. In addition to this
basic configuration, various functions such as a signal conversion circuit, photon counting circuit, interface to
the PC and cooling device are integrated into a single package. PMT modules eliminate troublesome wiring
for high voltages and allow easy handling since they operate from a low external voltage. Figure 8-1 shows the
functions of PMT modules.
PMT Module
Photosensor Module
Cooler
Current-Voltage
Conversion
Amp
CPU
+
Interface
Charge Amp
+
ADC
PMT
+
Voltage-Divider Circuit
+
High-Voltage
Power Supply Circuit
Photon Counting Head
Photon
Counting
Circuit
Gate Circuit
Cooler
CPU
+
Interface
PMT Module with Added
Function
Gate Circuit
THBV3_0801EA
Figure 8-1: PMT module functions
8.2
Characteristics of Power Supply Circuits
(1) Power supply circuits
There are two major types of power supply circuits used in PMT modules. One is a Cockcroft-Walton
(CW) circuit and the other is a combination of a Cockcroft-Walton circuit and active divider circuit.
The Cockcroft-Walton circuit is a voltage multiplier circuit using only capacitors and diodes. As shown
in Figure 8-2, capacitors are arranged along each side of the alternate connection points of the serially
connected diodes. The reference voltage supplied to this circuit are doubled, tripled ... and the boosted
voltage is applied to each dynode. This circuit features low power consumption and good linearity for both
DC and pulsed currents and is designed to be compact.
© 2007 HAMAMATSU PHOTONICS K. K.
8.2 Characteristics of Power Supply Circuits
155
P
SIGNAL
PMT
CW CIRCUIT
Vcontrol
OSC
+
–
THBV3_0802EA
Figure 8-2: Cockcroft-Walton power supply circuit
Figure 8-3 shows a power supply circuit using a Cockcroft-Walton circuit combined with an active
divider circuit. The Cockcroft-Walton circuit generates a voltage that is applied to the entire photomultiplier tube and the active divider circuit applies a voltage to each dynode. In this active divider circuit,
several voltage-divider resistors near the last dynode stages are replaced with transistors. This eliminates
the effect of the photomultiplier tube signal current on the interdynode voltage, achieving good linearity up
to 60 % to 70 % of the divider circuit current. This circuit also features lower ripple and shorter settling
time compared to power supply circuits using only a Cockcroft-Walton circuit.
P
SIGNAL
PMT
ACTIVE
CIRCUIT
CW CIRCUIT
Vcontrol
+
–
OSC
THBV3_0803EA
Figure 8-3: Power supply circuit using Cockcroft-Walton circuit combined with active divider circuit
© 2007 HAMAMATSU PHOTONICS K. K.
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CHAPTER 8
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(2) Ripple noise
Since high-voltage power supplies in PMT modules use an oscillating circuit, the unwanted oscillation
noise is usually coupled into the signal output by induction. This induction noise is called "ripple". This
ripple can be observed on an oscilloscope by connecting the signal cable of a PMT module to the input of
the oscilloscope while no light is incident on the PMT module. For example, under the conditions that the
load resistance is 1 MΩ, load capacitance is 22 pF and the coaxial cable length is 45 cm, you will see a
signal output along the baseline in a low voltage range. This signal output has an amplitude from a few
hundred µV to about 3 mV and a frequency bandwidth of about 300 kHz. Figure 8-4 shows an example of
this ripple noise.
Hamamatsu PMT modules are designed to minimize this ripple noise. However, it is not possible to
completely eliminate this noise. Use the following methods to further reduce ripple noise.
1. Place a low-pass filter downstream from the PMT module signal output.
5 (mV/div)
2. Raise the control voltage to increase the photomultiplier tube gain and lower the amplifier gain.
1 (µs/div)
THBV3_0804EA
Figure 8-4: Ripple noise
(3) Settling time
The high voltage applied to the photomultiplier tube changes as the input voltage for the PMT module
control voltage is changed. However, this response has a slight delay versus changes in the control voltage.
The time required for the high voltage to reach the target voltage is called the "settling time". This settling
time is usually defined as the time required to reach the target high voltage when the control voltage is
changed from +1.0 V to +0.5 V. Figure 8-5 shows a change in the high voltage applied to the cathode.
© 2007 HAMAMATSU PHOTONICS K. K.
8.3 Current Output Type and Voltage Output Type
157
0V
1000 V
50 (ms/div)
THBV3_0805EA
Figure 8-5: Changes in cathode voltage when control voltage is changed from +1.0 V to +0.5 V
8.3
Current Output Type and Voltage Output Type
(1) Connection method
Since PMT modules have an internal high-voltage power supply and voltage divider circuit in their
packages, there is no need to apply a high voltage from an external power supply. All that is needed is
simple wiring and low voltage input as shown in the connection diagram. When using a typical PMT
module, supply approximately 15 V to the low voltage input, ground the GND terminal, and connect the
control voltage and reference voltage input according to the gain adjustment method.
When the low voltage input is within the range specified in our catalog, the high voltage applied to the
photomultiplier tube from the power supply circuit in the PMT module is kept stable. This holds true even
if the output of the low-voltage power supply fluctuates somewhat. However, if high noise pulses are
generated from the low-voltage power supply, they may cause erroneous operation or a breakdown in the
PMT module.
(2) Gain adjustment
The photomultiplier tube gain can be adjusted by changing the control voltage. There are two methods
for adjusting the control voltage.
When directly inputting the control voltage as shown in Figure 8-6, the control voltage input range must
always be below the maximum rating. The output terminal of the reference voltage must be left unconnected. Be careful not to connect it to ground.
© 2007 HAMAMATSU PHOTONICS K. K.
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CHAPTER 8
PHOTOMULTIPLIER TUBE MODULES
SIGNAL
OUTPUT
PHOTOSENSOR MODULE
LOW VOLTAGE INPUT
GND
Vref OUTPUT
Vcontrol INPUT
+15 V
GND
• Adjust Vcontrol (control voltage) to adjust sensitivity.
• Electrically isolate Vref (referene voltage) output.
+0.3 V to
+1.1 V
GND
THBV3_0806EA
Figure 8-6: Sensitivity adjustment by changing voltage
Figure 8-7 shows a gain adjustment method using a trimmer potentiometer which is connected between
the control voltage and reference voltage outputs. When adjusting the trimmer potentiometer, do so carefully and correctly while monitoring the control voltage with a voltmeter or tester.
SIGNAL
OUTPUT
+15 V
GND
PHOTOSENSOR MODULE
LOW VOLTAGE INPUT
GND
Vref OUTPUT
Vcontrol INPUT
MONITOR
TRIMMER POTENTIOMETER (10 kΩ)
When using a trimmer potentiometer, adjust sensitivity
while monitoring Vcontrol (control voltage)
THBV3_0807EA
Figure 8-7: Sensitivity adjustment using trimmer potentiometer
(3) Current output type module
In current output type PMT modules, the anode current of the photomultiplier tube is directly available
as the output from the module. This current output from the photomultiplier tube must be converted to a
voltage by an external signal processing circuit. An optimal current-to-voltage conversion method must be
selected according to the application and measurement purpose.
(4) Voltage output type module
In voltage output type PMT modules, an op-amp is connected near the photomultiplier tube anode to
convert the current to a voltage. This is more resistant to external noise than when extracting the current
output of a photomultiplier tube by using a signal cable. Using an internal amplifier is especially effective
in measurement frequencies ranging from several tens of kilohertz to a few megahertz where external
noise effects first become noticeable. However, amplifier power consumption tends to increase in frequency bands higher than 10 MHz. Using an external amplifier connected to a current output type PMT
module might be better in this case.
Voltage output type PMT modules incorporate an op-amp for current-to-voltage conversion. The amp's
feedback resistor and capacitor also function as a charge amplifier, making it possible to perform pulse
measurement such as scintillation counting.
© 2007 HAMAMATSU PHOTONICS K. K.
8.4 Photon Counting Head
8.4
159
Photon Counting Head
Photon counting heads contain a low level discriminator and pulse shaper along with a photomultiplier
tube and a high-voltage power supply. Figure 8-8 shows the block diagram of a typical photon counting head.
The current pulses from the photomultiplier tube are amplified by the amplifier, and then only those pulses
higher than a certain threshold are discriminated by the comparator and converted to voltage pulses by the
pulse shaper for output. In photon counting heads, the high voltage to be applied to the photomultiplier tube is
preadjusted based on the plateau voltage measured prior to shipment. Supplying a low voltage from an external power supply is all that is needed for photon counting.
PMT
AMP
COMPARATOR
+
PULSE
SHAPER
OUTPUT
TO PULSE
COUNTER
HIGH-VOLTAGE
POWER SUPPLY
LLD.
POSITIVE
LOGIC
VOLTAGE-DIVIDER
CIRCUIT
GND
RL
50 Ω
+5 V
POWER INPUT
THBV3_0808EA
Figure 8-8: Block diagram of photon counting head
(1) Output characteristics
Each type of photon counting head is slightly different so that the internal circuit constants match the
time characteristics and pulse waveforms of the photomultiplier tube being used. Because of this, output
characteristics such as the pulse voltage and pulse width differ depending on individual photon counting
heads, though their output is a positive logic signal.
The output impedance of photon counting heads is designed to be approximately 50 ohms in order to
handle high-speed signals. When connecting a photon counting head to a measurement device with a
cable, a 50-ohm impedance cable is preferable and the input impedance of the measurement device should
be set to 50 ohms. If the input impedance of the external circuit is not around 50 ohms and an impedance
mismatch occurs, the pulses reflected from the input end of the external circuit return to the photon counting head and then reflect back from there. This might result in erroneous counts. When the input impedance of the external circuit is 50 ohms, the amplitude of the signal voltage will be one-half that at the input
end. So it is necessary to select an external circuit that matches the minimum input voltage specifications.
(2) Counting sensitivity
Counting sensitivity indicates a count value obtained from a photon counting head when an absolute
amount of light (pW) at a certain wavelength enters the photon counting head. Counting sensitivity is
directly related to quantum efficiency and collection efficiency.
© 2007 HAMAMATSU PHOTONICS K. K.
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(3) Count linearity
When individual photons enter at constant intervals within the time resolution of a photon counting
head, it is theoretically possible to measure the photons up to the reciprocal of pulse-pair resolution. Photon counting is usually used in low-light-level measurements of chemiluminescence and bioluminescence,
so the light input is a random event. In this case, when the light level is increased and exceeds a certain
level, the count value becomes saturated and is no longer proportional to the light level. Count linearity is
a measure for indicating the loss in the counted value compared to the theoretical value. This is defined as
the count value at 10 % loss. The pulse-pair resolution of the internal circuit determines the count linearity
characteristics of the photon counting head. At a higher count rate, however, time characteristics of the
photomultiplier tube also become an important factor.
Figure 8-9 shows typical count linearity characteristics of a photon counting head with a pulse pair
resolution of 18 ns. The count value at 10 % loss is 6×106 s-1.
108
THEORETICAL VALUE
OUTPUT COUNT (s-1)
Correction Formula
N=
107
M
1-M · t
MEASURED
VALUE
N: Count after correction
M: Actual count
t: Pulse pair resolution (18 ns)
10 % LOSS (at 6×106)
106
105
RELATIVE LIGHT LEVEL
THBV3_0809EA
Figure 8-9: Count linearity characteristics
(4) Improving the count linearity
When the count measured during photon counting exceeds 106 s-1, the pulses begin to overlap causing
counting errors. To increase the count linearity:
1. Increase the pulse-pair resolution of the circuit.
2. Use a prescaler to divide the frequency.
3. Approximate the output by using a correction formula.
Figure 8-10 shows the improvement in count linearity when the output is approximated by a correction
formula.
© 2007 HAMAMATSU PHOTONICS K. K.
8.4 Photon Counting Head
161
108
OUTPUT COUNT (s-1)
AFTER CORRECTION
107
BEFORE
CORRECTION
106
105
RELATIVE LIGHT LEVEL
THBV3_0810EA
Figure 8-10: Count linearity before and after correction
(5) Temperature characteristics
Since the photon counting method uses a technique that measures pulses higher than a certain threshold
value, it is less affected by gain variations in the photomultiplier tube caused by output instability of the
power supply and changes in ambient temperature. Changes in the count value versus temperature variations are plotted in Figure 8-11. The rate of these changes is about one-half the anode output temperature
coefficient of photomultiplier tubes.
TEMPERATURE COEFFICIENT (%/°C)
1.0
0.5
PHOTON COUNTING HEAD
COUNT SENSITIVITY
0
PMT USED IN OTHER THAN
PHOTON COUNTING
-0.5
-1.0
200
300
400
500
600
700
800
WAVELENGTH (nm)
THBV3_0811EA
Figure 8-11: Temperature coefficient comparison
© 2007 HAMAMATSU PHOTONICS K. K.
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(6) Photon counting ASIC (Application Specific Integrated Circuit)
A photon counting circuit is fabricated using many components such as ICs and resistors. The capacitance and inductance of those components and wiring impose limits on the frequency band and power
consumption of the circuit. The circuit board of course requires a space for mounting component. The
photon counting ASIC is an integrated circuit consisting of 16 amplifiers, 16 discriminators and 16 pulse
shaping circuits, which are the basic elements for photon counting circuits. This ASIC simultaneously
performs parallel processing of input signals from a maximum of 16 photomultiplier tubes or from a 16channel multianode photomultiplier tube, and outputs a LVDC voltage pulse according to each input. The
block diagram of a photon counting ASIC is shown below in Figure 8-12. Integrating the circuit gives the
ASIC a counting efficiency of 1.0×108 s-1 or more per channel, low power consumption and a compact
size. This ASIC is also designed to allow LLD and ULD adjustments by 8-bit DAC from external control,
so that the gain difference between photomultiplier tubes and the gain fluctuation between the anodes of a
multianode photomultiplier tube can be corrected. Furthermore, accurate measurement can be performed
not only by single photon counting but also in multi photon events, by matching the photomultiplier tube
gain with the input charge range of the ASIC. In this case, one voltage pulse of positive logic is output in
response only to a pulse signal that enters within the LLD to ULD input range or a pulse signal higher than
the LLD threshold level. This allows measurement for taking timings. However, the output does not contain pulse height information.
16-Channel Amp / Discriminator
PMT
IN1
–
Amp
+
0 to 1600 fC
8-bit DAC
0 to 400 fC
8-bit DAC
Upper
Discriminator
LVDS Buffer
–
–
+
+
Lower
Discriminator
LVDS Buffer
–
–
+
+
OUT1HA
OUT1HB
OUT1LA
OUT1LB
DAC Control
Multichannel Counter
/ Readout Circuit
PMT
IN16
–
Amp
+
0 to 1600 fC
8-bit DAC
0 to 400 fC
8-bit DAC
Upper
Discriminator
LVDS Buffer
–
–
+
+
Lower
Discriminator
LVDS Buffer
–
–
+
+
OUT16HA
OUT16HB
OUT16LA
OUT16LB
DAC Control
Computer
or
Main System
THBV3_0812EA
Figure 8-12: Block diagram of photon counting ASIC
© 2007 HAMAMATSU PHOTONICS K. K.
8.5
8.5
Gate Function
163
Gate Function
When excitation light such as from a laser or xenon flash lamp enters a photomultiplier tube, the signal
processing circuit may become saturated causing adverse effects on the measurement. There is a method to
block such excessive light by using a mechanical shutter but this method has problems such a limited mechanical shutter speed and service life. On the other hand, electronic gating, which is controlled by changing
the electrical potential on a dynode in the photomultiplier tube, offers much higher speeds and higher extinction ratio. The H7680 is a gated PMT module using a linear focused type photomultiplier tube that features
fast time response. The H7680 delivers a high extinction ratio and high-speed gating since it controls the bias
voltage applied to multiple dynodes.
There are two modes of gating: a normally-off mode that turns on the gate of the photomultiplier tube when
a gate signal is input and a normally-on mode that turns off the gate when a gate signal is input. Select the
desired mode according to the application.
(1) Gate noise
Performing high-speed gate operation requires high-speed gate pulses. When a gate pulse is input,
induction noise is induced in the anode signal through the electrostatic capacitance present between the
electrodes of the photomultiplier tube as shown in Figure 8-13. This is referred to as "gate noise". This gate
noise can be reduced by reducing the gate pulse voltage or by using a noise-canceling technique. However,
completely eliminating this noise is difficult. So increasing the photomultiplier tube gain or using a photomultiplier tube with a higher gain is required so that the signal output becomes larger than the gate noise.
GATE PULSE
20 mV/div.
5V
Type No. : H7680-01
Vcont: 5 V
Input Gate Pulse Width: 600 ns
Repetition Rate: 100 kHz
200 ns/div.
THBV3_0813EA
Figure 8-13: Gate noise
© 2007 HAMAMATSU PHOTONICS K. K.
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CHAPTER 8
PHOTOMULTIPLIER TUBE MODULES
(2) Extinction ratio
Gating allows suppressing the anode current of the photomultiplier tube even if the anode current exceeds the maximum rating or a strong light causing the external circuit to be saturated is input to the
photomultiplier tube. The extinction ratio is the ratio of the output when the gate is "on" to the output when
the gate is "off" while a constant light level is incident on the photomultiplier tube. For example, if the
output at "gate-off" is 1 nA in normally-off mode, and the output at "gate-on" is 10 µA, then the extinction
ratio is expressed in 1 nA : 10 µA = 1 : 104.
Even if the current is being controlled by gate operation, a small amount of current equal to the percentage of the extinction ratio flows as the anode current. The anode current must be kept below the maximum
rating of the photomultiplier tube even during gate operation. If high energy light such as a laser beam
enters the photomultiplier tube, the photocathode structure itself might be damaged even if gate operation
is performed. So some measures must be taken to prevent strong light from entering the photomultiplier
tube.
© 2007 HAMAMATSU PHOTONICS K. K.
8.6 Built-in CPU+IF Type
8.6
165
Built-in CPU and IF Type
This type of PMT module has an internal CPU and interface for connection to an external unit. In this
module, the output current of the photomultiplier tube is converted to a voltage signal by a current-to-voltage
conversion amplifier. The voltage signal is then converted to digital data, or in photon counting, the output
pulses are counted within a certain time. Digital data can be easily transferred to an external processing unit,
while the PMT module is controlled by commands from the external unit. Since the signal processing circuit,
control CPU and interface for data transfer are housed in a single package, there is no need to design a digital
circuit or take noise abatement measures usually required when handling high voltage and high-speed signals.
(1) Photon counting type
This PMT module has an internal photon counting circuit followed by a 20-bit counter that counts
voltage pulses. The 20-bit counter allows a maximum count of 1,048,575 within the gate time that was set.
If the gate time is set long while the light level is relatively high, then the counter limits the measurement
count to 1,048,575 or less. In this case, shorten the gate time and acquire the data several times. After
measurements, software averaging of data acquired several times allows you to obtain the same result as
obtained using a long gate time. Figure 8-14 shows the circuit block diagram of a photon counting type
module.
AMP
PMT
Comparator
LLD.
Pulse
Shaper
+5 V
High-Voltage Power Supply
Voltage-Divider Circuit
20 bit
Counter
90 MHz
20 bit
Latch
GND
I/O
128 kbytes
ROM
4 kbyte
RAM
16-bit
CPU
16 MHz
RS-232C
Computer
RS-232C
9600 baud
THBV3_0814EA
Figure 8-14: Block diagram of photon counting type module
© 2007 HAMAMATSU PHOTONICS K. K.
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PHOTOMULTIPLIER TUBE MODULES
(2) Charge amplifier and AD converter type
Figure 8-15 shows the block diagram of a PMT module with an internal charge amplifier and AD
converter. The anode of the photomultiplier tube is connected to the charge amplifier that accumulates
charges obtained from the anode during a sampling time. The accumulated charge quantity is then converted to digital data by the AD converter.
Integration Time Setting
1000 pF
PMT
K
P
Multi-stage Voltage Rectifying Circuit
Oscillation
Circuit
High Voltage Feedback
–
+
Charge
Amp
12-bit
DAC
12-bit
ADC
ADC Data
Vcc (+5 V)
GND
ADC
Control
DAC
Control
Microcontroller
DAC data
Stabilized
Circuit Voltage Control
Rx
RS-232C
Tx
Interface
GND
External Trigger (TTL Input)
User Line (TTL Output)
THBV3_0815EA
Figure 8-15: Block diagram of charge amplifier and AD converter type module
References in Chapter 8
1) Hamamatsu Photonics Product Catalog: "Photomultiplier Tube Modules" (March, 2005)
© 2007 HAMAMATSU PHOTONICS K. K.
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