CHAPTER 9 : POSITION SENSITIVE PHOTOMULTIPLIER TUBES

CHAPTER 9
POSITION SENSITIVE
PHOTOMULTIPLIER TUBES
The current multiplication mechanism offered by dynodes makes
photomultiplier tubes ideal for low-light-level measurement. As explained earlier, there are various types of dynode structures available
for different photometric purposes. Popular conventional dynode structures are the box-and-grid type, linear-focused type, circular-cage type
and venetian-blind types. Furthermore, the MCP (microchannel plate)
has recently been utilized as a dynode structure.
Two unique dynode structures are introduced in this chapter: the
"metal channel dynode" and "grid type dynode". These dynode structures provide wide dynamic range, high gain, high position resolution,
and are currently used in position-sensitive photomultiplier tubes.
Common methods for reading out the output signal from a positionsensitive photomultiplier tube are illustrated in Figure 9-1. In a
multianode device, the output signal is read using independent multiple anodes. The cross-plate (wire) anode signal is read out by means
of current or charge-dividing center-of-gravity detection.
© 2007 HAMAMATSU PHOTONICS K. K.
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MULTI ANODE
Y2
RESISTORS
CHARGE-DIVISION
READOUT CIRCUIT
X1
X2
SUM
DIV.
X=
Y1
X2
X1+X2
CROSS-WIRE (PLATE) ANODE
THBV3_0901EA
Figure 9-1: Anode output readout methods for position sensitive photomultiplier tubes
The following sections describe "metal channel dynode structures
combined with multianode readout", "metal channel dynode structures
combined with a cross-plate anode" and "grid type dynode structures
combined with a cross-wire anode" for position sensitive photomultiplier tubes.
© 2007 HAMAMATSU PHOTONICS K. K.
9.1
9.1
Multianode Photomultiplier Tubes
169
Multianode Photomultiplier Tubes
9.1.1 Metal channel dynode type multianode photomultiplier tubes
(1) Structure
Figure 9-2 shows the electrode structure for metal channel dynodes and the associated electron trajectories. Compared to the other types of dynodes, metal channel dynode type multianode photomultiplier tubes
feature very low crosstalk during secondary electron multiplication. This is because the photoelectrons
emitted from the photocathode are directed onto the first dynode by the focusing mesh and then flow to the
second dynode, third dynode, . . . last dynode and finally to the anode, while being multiplied with a
minimum spatial spread in the secondary electron flow.
The overall tube length can be kept short because the metal channel dynodes are very thin and assembled in close-proximity to each other.
PHOTOCATHODE
FOCUSING MESH
METAL CHANNEL
DYNODES
MULTIANODE
THBV3_0902EA
Figure 9-2: Electrode structure and electron trajectories
Multianode photomultiplier tubes using metal channel dynodes can be roughly classified into two groups.
One group uses a matrix type multianode and the other group uses a linear type multianode.
© 2007 HAMAMATSU PHOTONICS K. K.
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Metal Channel Dynode Multianode Photomultiplier Tubes
Type
Linear
Matrix
Matrix
M4
M16
M64
L16
L32
M64
Number of Anodes
4
16
64
16
32
64
Pixel Size
(mm)
9×9
4×4
2×2
0.8 × 16
0.8 × 7
5.8 × 5.8
Anode Shape
THBV3_0903EA
Figure 9-3: Anode patterns for metal channel dynode type multianode photomultiplier tubes
(2) Characteristics
In this section, we first describe basic characteristics of matrix type multianode photomultiplier tubes
by discussing "crosstalk", "magnetic immunity" and "uniformity" in 64 channel matrix type multianodes.
"Crosstalk" is a measure to indicate how accurately the light (signal) incident on a certain position of the
photocathode is detected while still retaining the position information. In photomultiplier tube operation,
crosstalk is mainly caused by the broadening of the electron flow when light is converted into electrons and
those electrons are multiplied by the dynode section. The incident light spread within the faceplate is
another probable cause of crosstalk.
A typical setup for measuring crosstalk is shown in Figure 9-4 and an example of measurement data in
Figure 9-5.
UV SPOT LIGHT SOURCE
d
HIGH VOLTAGE
POWER SUPPLY
QUARTZ FIBER
SCINTILLATING FIBER
(KURARAY, L=3 m)
PMT
AMMETER
THBV3_0904EA
Figure 9-4: Crosstalk measurement method
© 2007 HAMAMATSU PHOTONICS K. K.
9.1
Multianode Photomultiplier Tubes
d=0 mm
171
d=0.5 mm
0.3
1.4
0.4
0.4
2.6
0.6
0.8
100
1.2
1.5
100
1.9
0.2
1.1
0.3
0.3
1.8
0.4
SCINTILLATING FIBER (1.0 mm dia.)
SUPPLY VOLTAGE: 800 (V)
d: DISTANCE
SCINTILLATING FIBER (1.0 mm dia.)
SUPPLY VOLTAGE: 800 (V)
d: DISTANCE
THBV3_0905EA
Figure 9-5: Crosstalk measurement example
Data shown in Figure 9-5 is measured by irradiating a light spot (signal) on the photomultiplier tube
faceplate, through a 1 mm diameter optical fiber placed in close contact with the faceplate. The output of
each anode is expressed as a relative value, with 100 % being equal to the peak anode output produced
from the incident light spot. Results show that crosstalk is 0.2 % to 1.4 % when the 1 mm diameter
scintillating fiber is positioned in tight contact with the photomultiplier tube faceplate (d=0 mm). However, the crosstalk becomes 0.3 % to 2.6 % worse when the scintillating fiber is moved 0.5 millimeters
away from the faceplate. This is of course due to light spread at the scintillating fiber exit. Bringing the
optical fiber into tight contact with the photomultiplier tube faceplate is therefore recommended in order to
make accurate measurements using scintillating fibers.
Next, let's discuss magnetic characteristics. Matrix type multianode photomultiplier tubes have excellent immunity to magnetic fields. This is because all parts except the photocathode are housed in a metal
package and also because the distance between dynode electrodes is very short. Magnetic characteristics
of a 64-channel multianode photomultiplier tube are explained below.
Figure 9-6 shows how the anode output is adversely affected by external magnetic fields applied along
the three axes (X, Y, Z). Each data is plotted as a relative output value, with 100 % corresponding to an
output with no magnetic field applied. Output is still maintained as high as 60 % versus 13 mT of the
magnetic field in the X direction.
110
+
X axis
+
−
X axis
90
RELATIVE OUTPUT (%)
Y
Z
100
+
80
−
70
Z axis
60
Z axis
−
50
Y axis
Y axis
40
30
20
10
0
−10
−5
-0
5
Z axis
Y axis
X axis
10
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
17 18 19 20 21 22 23 24
25 26 27 28 29 30 31 32
33 34 35 36 37 38 39 40
41 42 43 44 45 46 47 48
49 50 51 52 53 54 55 56
57 58 59 60 61 62 63 64
MAGNETIC FIELD (mT)
THBV3_0906EA
Figure 9-6: Effects of external magnetic fields on anode output (anode channel No. 29)
© 2007 HAMAMATSU PHOTONICS K. K.
X
172
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POSITION-SENSITIVE PHOTOMULTIPLIER TUBES
100
90
80
70
60
50
40
30
20
10
0
64
1
3
RELATIVE ANODE OUTPUT
Figure 9-7 shows typical uniformity data obtained from each anode when uniform light is illuminated
over the entire photocathode of a 64-channel multianode photomultiplier tube. The non-uniformity observed here probably originates from gain variations in the secondary electron multiplier because the photocathode itself has good uniformity. Currently, non-uniformity between each anode is about "1:3" on
average.
48
5
32
PIXEL NUMBER
7
PIXEL NUMBER
16
THBV3_0907EA
Figure 9-7: 64-channel multianode output uniformity
Uniformity of one pixel (one anode) is shown in Figure 9-8. This data is measured by input of weak DC
light of 50 µm diameter to an anode of 2 square millimeters per pixel, while scanning the light every 0.1
millimeters on the photocathode.
0
0.5
1
RELATIVE OUTPUT
100
90
80
70
60
50
40
30
20
10
0 4
3.5
3
1.5
2.5
2
2
mm
1.5
2.5
1
3
mm
0.5
3.5
4
0
THBV3_0908EA
Figure 9-8: Anode output uniformity per pixel
© 2007 HAMAMATSU PHOTONICS K. K.
9.1
Multianode Photomultiplier Tubes
173
We next describe basic "crosstalk" and "uniformity" characteristics of linear multianode photomultiplier tubes.
A typical setup for measuring crosstalk of a 16-channel linear multianode photomultiplier tube is shown
in Figure 9-9 and the typical measurement data in Figure 9-10. In this measurement, a light spot emitted
through the 100 µm aperture in the X-Y stage was scanned along the photocathode. Typical crosstalk
obtained from the 16-channel linear multianode was approximately 3 %.
DARK BOX
X-Y STAGE
CONTROLLER
CHANNEL
CHANGER
X-Y
STAGE
GP-IB
LAMP
BOX
LAMP POWER
SUPPLY
HIGH VOLTAGE
POWER SUPPLY
AMMETER
PC
THBV3_0909EA
Figure 9-9: Crosstalk measurement method
OUTPUT DEVIATION (%)
SPATIAL RESOLUTION
AND CROSSTALK SCAN
SCAN POSITION
100
80
DEAD SPACE
EFFECTIVE SPACE
60
1 CH
SIGNAL
B
A
20
0
16 CH
(TOP VIEW)
40
1
2
3
4
5
6
7
SUPPLY VOL.: -800 V
LIGHT SOURCE: TUNGSTEN LAMP
SPOT DIA.: 100 µm
SCAN PITCH: 50 µm
POSITION (channelsd)
CH
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
CROSS-TALK RATIO (%)
1
2
3
4
5
6
7
8
9 10 11
100 2.9 0.6 0.2 0.1 — — — — — —
2.9 100 3.1 0.5 0.2 0.1 — — — — —
0.8 2.8 100 2.8 0.6 0.2 0.1 — — — —
0.3 0.8 2.7 100 3.2 0.6 0.2 0.1 — — —
0.1 0.3 0.8 2.9 100 3.1 0.6 0.2 0.1 — —
— 0.1 0.3 0.8 2.7 100 3.0 0.6 0.2 0.1 —
— — 0.1 0.3 0.8 2.7 100 3.0 0.6 0.2 0.1
— — — 0.1 0.3 0.8 2.9 100 2.9 0.6 0.2
— — — — 0.1 0.3 0.8 2.9 100 2.9 0.6
— — — — — 0.1 0.3 0.8 3.1 100 2.7
— — — — — — 0.1 0.4 0.8 3.3 100
— — CROSSTALK
— — — — — 0.1 0.4 0.9 3.2
— AREA
— B—/ AREA
— A—× 100
— — — 0.1 0.4 0.8
— — — — — — — — — 0.1 0.4
— — — — — — — — — — 0.1
— — — — — — — — — — —
12
—
—
—
—
—
—
—
0.1
0.2
0.6
3.8
100
3.1
0.8
0.4
0.1
13
—
—
—
—
—
—
—
—
0.1
0.2
0.6
2.8
100
3.1
0.9
0.4
14
—
—
—
—
—
—
—
—
—
0.1
0.2
0.6
2.8
100
3.2
0.9
15
—
—
—
—
—
—
—
—
—
—
0.1
0.2
0.6
2.7
100
3.1
16
—
—
—
—
—
—
—
—
—
—
—
0.1
0.3
0.6
2.9
100
THBV3_0910EA
Figure 9-10: Crosstalk of 16-channel linear anode
© 2007 HAMAMATSU PHOTONICS K. K.
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Some 16-channel and 32-channel linear multianode photomultiplier tubes are low crosstalk types. Some
use a special faceplate containing black glass partitions or an electrode structure having shielding walls
between the anodes of each channel. Typical crosstalk values measured with a low crosstalk type are
shown in Figure 9-11.
5.0
LOW CROSSTALK TYPE
4.0
CONVENTIONAL TYPE
3.0
2.0
5
ch
6
ch
7
ch
8
ch
9
ch
ch
ch
16
ch
4
15
ch
ch
3
13
ch
14
ch
ch
ch
2
12
ch
11
1
10
0.0
ch
1.0
THBV3_0911EA
Figure 9-11: Crosstalk values of 16-channel low-crosstalk type
Figure 9-12 shows typical uniformity data of a linear multianode photomultiplier tube. This data was
obtained from each anode when uniform light was illuminated over the entire photocathode of a 32-channel linear multianode photomultiplier tube. As with the matrix type, non-uniformity mainly originates
from gain variations in the secondary electron multiplier. Currently, non-uniformity between each anode is
about "1:1.7" on average.
OUTPUT DEVIATION (%)
100
80
60
40
20
0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31
CHANNEL
THBV3_0912EA
Figure 9-12: 32-channel linear multianode output uniformity
© 2007 HAMAMATSU PHOTONICS K. K.
9.1
Multianode Photomultiplier Tubes
175
Since 16-channel and 32-channel linear multianode photomultiplier tubes have a one-dimensional array
of anodes, they are mainly used as detectors for multichannel spectrophotometry. Due to its shape, the 32channel type is often used in combination with a grating or prism, and recent applications include laser
scanning microscopes.
Linear multianode photomultiplier tubes are also available with a band-pass filter attached to the faceplate. This allows detecting light only in the wavelength range of interest, just like using a grating or prism.
There is no loss of light caused by the entrance slit which is used with the grating for separating the light
into different wavelengths. Since light must uniformly strike the entire surface of the band-pass filter,
Hamamatsu also provides a dedicated mixing fiber combined with a lens for this purpose. Figure 9-13
shows a photomultiplier tube with a band-pass filter and a dedicated mixing fiber combined with a lens.
Figure 9-13: Photomultiplier tube with band-pass filter
Mixing fiber + lens
Dichroic mirrors can also be used for dispersing light into a spectrum. One example is illustrated in
Figure 9-14 showing a very compact device containing an optical system and a detector.
DICHROIC MIRROR
MIRROR
LIGHT
BPF
BPF
BPF
BPF
PHOTOCATHODE
LINEAR MULTIANODE
PMT
A ch
B ch
C ch
D ch
ANODE
THBV3_0914EA
Figure 9-14: Multianode photomultiplier tube assembled with dichroic mirrors
© 2007 HAMAMATSU PHOTONICS K. K.
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9.1.2 Multianode MCP-PMT
The multianode MCP-PMT is explained in detail in section 10.4 of Chapter 10.
9.1.3 Flat panel type multianode photomultiplier tubes
(1) Characteristics
Metal channel dynodes are mainly used in 1-inch square metal package photomultiplier tubes and flat
panel type (2 square inches) photomultiplier tubes, which can be selected according to the particular application.
This section introduces a flat panel type photomultiplier tube with an overall height as short as 15
millimeters. As shown in Figure 9-15, this photomultiplier tube features a large effective area and minimal
dead area (insensitive area).
3/4-inch circular type
1-inch square type
(with no flange)
25.7 mm
25.7 mm
Effective Area
22 mm
Effective Area
24 mm
73 %
87 %
Flat panel type
52.0 mm
32.2 mm
32.2 mm
1-inch square type
(with flange)
Effective Area
15 mm
59 %
49 mm
89 %
THBV3_0915EA
Figure 9-15: Comparison of effective area ratio
Typical spatial resolution obtained with a flat panel type 64-channel photomultiplier tube is shown in
Figure 9-16. This spatial resolution data (output distribution of each anode) was measured by scanning the
photocathode surface with a 1-millimeter collimated light beam emitted from a tungsten lamp through a
blue filter.
© 2007 HAMAMATSU PHOTONICS K. K.
9.1
Multianode Photomultiplier Tubes
177
Y-axis
Supply Voltage: 1000 V
Spot Diameter: 1.0 mm
Scanning Pitch: 0.1 mm
X-axis
Cross Uniformity of X-Axis
Cross Uniformity of Y-Axis
100
Relative Anode output (%)
Relative Anode output (%)
100
80
60
40
20
0
0
5
10
15
20
25
30
35
80
60
40
20
0
40
0
5
10
Position (mm)
15
20
25
30
35
40
Position (mm)
THBV3_0916EA
Figure 9-16: Spatial resolution of center anodes
Figure 9-17 shows typical crosstalk characteristics measured by irradiating the center of an anode (anode pitch 6 mm) with a light beam of 5 square millimeters. Relative outputs of adjacent anodes are shown
in the figure by setting the output of this anode as 100 %,. As can be seen in the figure, this flat panel type
64-channel multianode photomultiplier tube has a crosstalk of 2 to 3 % at the center anodes.
—
—
—
—
—
—
0.2
1.8
0.2
—
—
1.5
100
2.7
—
—
0.2
2.6
0.3
—
—
—
—
—
—
Supply Voltage: 1000 V
Light Source: Tungsten Lamp
Spot Size: 5 square millimeters
THBV3_0917EA
Figure 9-17: Crosstalk characteristics of center anodes
© 2007 HAMAMATSU PHOTONICS K. K.
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POSITION-SENSITIVE PHOTOMULTIPLIER TUBES
To take full advantage of the effective area, the photoelectrons emitted from the edges of the photocathode are focused toward the dynodes. This tends to increase anode crosstalk (3 % to 6 %) particularly in the
corner areas. (See Figure 9-18.)
100
5.5
—
3.5
0.5
—
—
—
—
Supply Voltage: 1000 V
Light Source: Tungsten Lamp
Spot Size: 5 square millimeters
THBV3_0918EA
Figure 9-18: Crosstalk characteristics of anodes in corner area
9.2
Center-of-Gravity Position Sensitive Photomultiplier Tubes
9.2.1 Metal channel dynode type multianode photomultiplier tubes
(cross-plate anodes)
(1) Structure
Figure 9-19 shows the electrode structure of a metal channel dynode type multianode photomultiplier
tube using a cross-plate anode.
In this photomultiplier tube, photoelectrons emitted from the photocathode are multiplied by each dynode and the multiplied secondary electrons are then reflected back from the last dynode and read out from
the plate type anodes (cross-plate anodes) arranged in two layers intersecting with each other.
PHOTOCATHODE
FOCUSING MESH
METAL CHANNEL
DYNODES
X ANODE
Y ANODE
LAST DYNODE
CROSS-PLATE ANODE TYPE
THBV3_0919EA
Figure 9-19: Electrode structure
© 2007 HAMAMATSU PHOTONICS K. K.
9.2 Center-of-Gravity Position Sensitive Photomultiplier Tubes
179
Figure 9-20 illustrates the center-of-gravity detection method for reading out the output signal from a
position-sensitive photomultiplier tube using a cross-plate anode. The electron bunch released from the
last dynode is collected by anodes linearly arranged in the X and Y directions. Since each anode in the
same direction is connected by a resistor string, the collected electrons are divided into four signal components X1, X2, Y1 and Y2 corresponding to the anode position at which the secondary electrons arrive. By
inputting these signals to summing (SUM) and divider (DIV) circuits, the center of gravity in the X and Y
directions can be obtained from Eq. 9-1.
X=
X2
(X1+X2)
Y=
Y2 ................................................................................ (Eq. 9-1)
(Y1+Y2)
Y2
RESISTORS
RESISTORS
Y1
X1
SUM
X2
DIV
X=
X2
X1+X2
THBV3_0920EA
Figure 9-20: Center-of-gravity measurement method
(2) Characteristics
This section describes spatial resolution characteristics obtained by center-of-gravity detection using
6(X) + 6(Y) cross-plate anodes respectively arranged in the XY directions. This spatial resolution data
(output distribution of each anode) was measured by scanning the photocathode surface with a 1-millimeter collimated light beam emitted from a tungsten lamp. Results are shown in Figures 9-21 and 9-22.
© 2007 HAMAMATSU PHOTONICS K. K.
CHAPTER 9
POSITION-SENSITIVE PHOTOMULTIPLIER TUBES
100
RELATIVE OUTPUT (%)
80
PX1
PX2
PX3
PX4
PX5
PX6
60
40
20
0
0
10
20
30
POSITION (mm)
SUPPLY VOLTAGE : -800 V
: TUNGSTEN LAMP
LIGHT SOURCE
SPOT DIAMETER : 1 mm
THBV3_0921EA
Figure 9-21: Spatial resolution of X anodes
100
80
RELATIVE OUTPUT (%)
180
PY1
PY2
PY3
PY4
PY5
PY6
60
40
20
0
0
10
20
30
POSITION (mm)
SUPPLY VOLTAGE : -800 V
: TUNGSTEN LAMP
LIGHT SOURCE
SPOT DIAMETER : 1 mm
THBV3_0922EA
Figure 9-22: Spatial resolution of Y anodes
© 2007 HAMAMATSU PHOTONICS K. K.
9.2 Center-of-Gravity Position Sensitive Photomultiplier Tubes
181
Figure 9-23 introduces a circuit diagram for scintillation imaging of 511 keV gamma-rays. It utilizes a
position sensitive photomultiplier tube with 6(X) + 6(Y) cross-plate anodes and a mosaic array of scintillators
(BGO of 2.2 mm×2.2 mm×15 mm arranged in a pattern of 9×9=81 pieces). An actual image obtained is
shown in Figure 9-24.
PX1
X1
INTEGRATION
A/D
PX2
PX3
X1
X1 + X2
PX4
9 × 9 BGO ARRAY
X ADDRESS
PX5
PX6
X2
PY1
Y1
INTEGRATION
A/D
INTEGRATION
A/D
PY2
Y ADDRESS
PY3
Y1
Y1 + Y2
PY4
PY5
PY6
Y2
INTEGRATION
Sum
A/D
EVENT DETECTION
ENERGY DISCRI.
EVENT SIGNAL
THBV3_0923EA
Figure 9-23: Scintillation imaging circuit using gamma-rays irradiated
on mosaic pattern scintillators (BGO)
Figure 9-24: Scintillation image obtained by gamma-rays irradiated
on mosaic pattern scintillators (BGO)
This scintillation imaging shows the mosaic pattern of 81 (9×9) BGO scintillators (2.2 mm×2.2 mm×15
mm). Off-center distortion in the image can be corrected by a lookup table.
© 2007 HAMAMATSU PHOTONICS K. K.
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9.2.2 Grid type dynode photomultiplier tubes (Cross-wire anodes)
(1) Structure
Figure 9-25 shows the electrode structure for grid type dynodes and the associated electron trajectories.
The significant difference compared to ordinary box-and-grid dynodes is that the electron multiplier is
fabricated from flat grid-like dynodes. These dynodes have a very fine structure that emits secondary
electrons while suppressing the spatial spread of secondary electrons at each dynode.
In this photomultiplier tube, photoelectrons emitted from the photocathode are multiplied by each dynode (up to a total gain of 105 or more) and then the multiplied secondary electrons are reflected back from
the last dynode (reflection type) and read out from the wire type anodes (cross-wire anodes) arranged in
two layers intersecting with each other. The first dynode is placed in close proximity to the photocathode
to minimize the spatial spread of photoelectrons.
PHOTON
PHOTOCATHODE
FOCUSING MESH
GRID TYPE
DYNODES
X ANODE
Y ANODE
Y
LAST DYNODE
0
X
THBV3_0925EA
Figure 9-25: Electrode structure and electron trajectories
(2) Characteristics
A photomultiplier tube using a 12-stage grid type dynode yields a gain of 105 or more at 1250 volts. This
type of photomultiplier tube is available in a circular envelope of 3 or 5 inches in diameter.
The number of wire anodes in the X and Y directions is 16(X) + 16(Y) for the 3-inch circular type
(anode pitch: 3.75 millimeters) and 28(X) + 28(Y) for the 5-inch circular type (anode pitch: 4 millimeters).
Next, let's discuss the center-of-gravity detection method and spatial resolution characteristics. As shown
in Figure 9-25, the electron flow spreads spatially between the photocathode and the first dynode and also
between each grid dynode. When 50 µm diameter light spot scans the photocathode surface of the 3-inch
circular type photomultiplier tube, the X and Y direction spatial resolutions are obtained as shown in
Figures 9-27 and 9-28. Since the electron flow spreads in the multiplication process from the photocathode
to the anode, the width of spatial resolution measured at each anode broadens to 9.5 millimeters in the X
direction and to 8.6 millimeters in the Y direction.
Figure 9-26: Grid type dynode photomultiplier tube
© 2007 HAMAMATSU PHOTONICS K. K.
9.2 Center-of-Gravity Position Sensitive Photomultiplier Tubes
183
FWHM=9.5 (mm)
100
X9
RELATIVE OUTPUT (%)
80
X10
60
40
20
0
0
15
30
45
60
75
POSITION (mm)
THBV3_0927EA
Figure 9-27: Spatial resolution in X direction
FWHM=8.6 (mm)
100
Y8
RELATIVE OUTPUT (%)
80
Y9
60
40
20
0
0
15
30
45
60
75
POSITION (mm)
THBV3_0928EA
Figure 9-28: Spatial resolution in Y direction
© 2007 HAMAMATSU PHOTONICS K. K.
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CHAPTER 9
POSITION-SENSITIVE PHOTOMULTIPLIER TUBES
To read out the signal from this photomultiplier tube, the center-of-gravity detection method is used, as
described in the previous section 9.2.1, "Metal channel dynode type multianode photomultiplier tubes
(cross-plate anodes)".
Figure 9-29 shows plots of spatial resolution measured with light emitted from a pulsed LED while
changing the amount of light per pulse. This spatial resolution is determined by the center-of-gravity
distribution in the output signal that broadens almost in inverse proportion to the square root of the amount
of incident light according to the statistical theory. Figure 9-30 shows the center-of-gravity distribution
characteristics measured while moving a light spot on the photocathode in 1 millimeter intervals. It proves
that a resolution of 0.3 millimeters (FWHM) is obtained in the center at a light intensity of 4000 photons
per pulse. A slight distortion occurs near the off-center region because there are fewer cross-wire anodes
involved in the output signal. Figure 9-31 is a spatial linearity graph showing the electrical center-ofgravity position on the vertical axis and the light spot position on the horizontal axis.
at 560nm
SPATIAL RESOLUTION (FWHM)
(mm)
1.0
0.8
0.5
0.4
0.2
0.1
100
1000
10000
100000
NUMBER OF INCIDENT PHOTONS (Photons per event)
THBV3_0929EA
Figure 9-29: Spatial resolution vs. incident light level
COUNT PER CHANNEL
ANODE PITCH : 4 mm
LIGHT LEVEL : 4000 photons/event
0.3 mm FWHM
1 mm
CHANNEL NUMBER
THBV3_0930EA
Figure 9-30: Center-of-gravity distribution with light spot movement
© 2007 HAMAMATSU PHOTONICS K. K.
9.2 Center-of-Gravity Position Sensitive Photomultiplier Tubes
185
RELATIVE POSITION SIGNAL
100
80
60
40
20
0
10
20
30
40
50
60
70
80
90
100
110
120
X AXIS (mm)
THBV3_0931EA
Figure 9-31: Spatial linearity of grid type dynode photomultiplier tube
In the peripheral portion of the photomultiplier tube, not all electrons are focused by the cross-wire
anodes, and these electrodes cause distortion as if they are drawn toward the center. But this distortion
level is small enough to be corrected by a lookup table or similar techniques.
© 2007 HAMAMATSU PHOTONICS K. K.
MEMO
© 2007 HAMAMATSU PHOTONICS K. K.
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