JPH0354427B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photomultiplier tube that multiplies electrons emitted from a photocathode using multiple stages of dynodes.

Photomultiplier tubes are widely used for various measurements in the fields of nuclear and high energy physics. That is, it is considered indispensable as a photoelectric conversion device such as a scintillation counter or a Thielenkoff counter.

The Thierenkov counter uses a plurality of photomultiplier tubes to accurately capture the position of incident light, the energy of the incident light, and the time of incidence to analyze radiation phenomena.

The photomultiplier tubes used in such Thierenkov counters each have the same characteristics, and not only must each tube operate stably, but it is also strongly required that the characteristics do not change due to external disturbances. ing.

The characteristics of photomultiplier tubes change when they are affected by the accidental generation of magnetic fields or leakage magnetic fields.

Conventional photomultiplier tubes do not give much consideration to the change in characteristics due to the generation of the magnetic field, and do not guarantee measurement at several Gauss or higher.

Therefore, in order to use conventional photomultiplier tubes in places where disturbances due to magnetic fields are expected, the following considerations have been necessary.

The first measure is to use a long light guide and place the photomultiplier tube in a location where it is less affected by the magnetic field, but the problem remains that the energy resolution deteriorates. The second measure is to shield the outer periphery of the tube, but shielding in the axial direction of the tube is difficult and complete shielding is impossible.

An object of the present invention is to provide a photomultiplier tube that can provide stable operation even in relatively large magnetic fields.

To achieve the above object, the photomultiplier tube according to the present invention includes a photocathode, a plurality of mesh dynodes, an anode, and a plate-shaped final stage dynode, which are arranged parallel to each other in this order in a vacuum vessel. The mesh dynodes forming the plurality of mesh dynodes are arranged in a plane parallel to each other with intervals maintained, and two mesh dynodes are arranged on the surface of the doubler.
The anode is formed from a net-like electrode, and the plate-shaped The final stage dynode has a secondary electron emitting surface formed on the anode side, and the distance from the photocathode to the first stage of the plurality of mesh dynodes is the diameter of the photocathode or the square photocathode. 1/15 to 1/ of the diameter of the inscribed circle
5, and the spacing between each mesh dynode, the spacing between the final stage mesh dynode and the anode, and the spacing between the anode and the plate-shaped final stage dynode are within the range of the mesh dynode of the plurality of stages from the photocathode. The distance is smaller than the distance to the first stage of the dynode.

The present invention will be described in more detail below with reference to the drawings and the like.

FIG. 1 is a longitudinal section showing an embodiment of a photomultiplier tube according to the present invention.

This photomultiplier tube consists of a bottomed cylindrical vacuum container 1
Photocathode PC inside, 8-stage mesh dynode Dy1
~Dy8, anode A, and plate-shaped final stage dynode Dy9 are arranged in this order.The photocathode PC is formed across the bottom inner wall of the vacuum vessel 1. Also, mesh dynode Dy1
~Dy9 extend over the entire inner diameter of the vacuum vessel 1 on a plane perpendicular to the tube axis.

Each electrode is connected to a pin 2 formed on the surface opposite to the surface on which the photocathode PC is formed. These connection structures are not particularly unusual structures and are not related to the gist of the present invention, so illustrations are omitted.

Borosilicate glass with an outer diameter of 3 inches (2.54 x 3 = 76.2 mm) was used as the bottomed cylindrical vacuum container 1,
A bialkali photocathode was used as the photocathode PC. The bialkali photocathode matches the spectrum of light emitted by the scintillator.

Figure 2 shows part of a multi-stage mesh dynode.
FIG. 2 is an enlarged perspective view of an anode and a plate-shaped final stage dynode. FIG. 3 is an enlarged plan view showing an electric field forming electrode which is a part of the mesh dynode.

In this example, there are eight mesh dynodes.
Dy1 to Dy8 have substantially the same shape.

The distance of the mesh dynode Dy1 from the photocathode PC is 7 mm, which corresponds to about 1/11 of the diameter of the photocathode.

Although it is preferable that this distance be small, the manufacturing limit is approximately 1/15 of the diameter of the photocathode.

Note that the effects of the present invention can be obtained even with a diameter of about 1/5.

As shown in FIG. 2, the mesh dynode Dy7 is formed based on a large number of rods B7 having a pitch 1 of 0.72 mm and having an isosceles trapezoid cross section arranged in parallel within one plane.

This isosceles trapezoid has a base of 0.36mm, a top of 0.13mm, and a height of
The apex side, which is 0.18 mm and is sandwiched between two equal sides, is the electron incident surface.These rods B7 and B8 are made of stainless steel, and an antimony-potassium-cesium secondary electron surface is formed on their surfaces. Electrodes M1 to M8 for forming an electric field are integrally provided on the electron incident surface of each dynode. Furthermore, the second
Only M7 and M8 are shown in the figure. FIG. 3 shows an enlarged view of a part of the electrode for forming the electric field. As shown in FIG. 3, this electrode for forming an electric field is made of thin stainless steel in which a plurality of closely spaced regular hexagonal holes are formed by etching. The length of one side of this regular hexagonal hole is 0.42 mm, which is considerably smaller than the spacing between the bars.

These nets are arranged to form equipotential surfaces perpendicular to the electron incident direction.

The interval between adjacent dynodes of mesh dynodes Dy1 to Dy8 is 1.08 mm. The rod of each subsequent dynode is assembled so as to be located in the gap between the preceding dynodes and the preceding dynode.

A net-like anode A is placed next to the final mesh dynode Dy8. In this example, an anode A having the same shape as the electric field forming electrode was used.

Next to this anode A, a flat final stage dynode Dy9 is arranged. Final stage dynode Dy
The surface of 9 facing the anode A is a secondary electron emitting surface.

Each of the mesh dynodes Dy1 to Dy8, the anode A, and the final stage dynode Dy9 are each fixed to a frame (not shown), and the frame is connected to the first
It is fixed to the pillar as shown in the figure.

FIG. 4 is a circuit diagram showing a connection example of the photomultiplier tube.

In the figure, PC is a photocathode, Dy1 to Dy8 are the mesh dynodes mentioned above, and Dy9 is a plate-shaped dynode at the final stage.

Resistor R1 is 300KΩ, resistor R2 is 100KΩ, resistor R3 is 10 to 100KΩ.Capacitor C1 is 0.01μF, capacitor C2 is
It is 0.047μF.

Apply 1000 volts to the HV terminal. The measurements described below were performed based on the above conditions.

Next, various characteristics of the photomultiplier tube with the above configuration will be explained.

(Quantum Efficiency) FIG. 5 is a graph showing the spectral characteristics of the photocathode PC. Maximum quantum efficiency is 22 at wavelength 350 nanom
%.

(Gain) The gain of a photomultiplier tube is defined by the ratio of the anode output current to the photocathode emission current. FIG. 6 shows the relationship between gain and applied voltage. Applied voltage
The gain is 3×10 ⁴ at 1000V.

(Performance with respect to time) Although the photomultiplier tube according to the present invention has a relatively large photocathode of 3 inches, excellent performance with respect to time is obtained.

This is due to the photoelectrons of the photomultiplier tube according to the present invention.
This is thought to be due to the fact that the orbit of the secondary electron is short and parallel to the tube axis.

(Transition time) 800 nano whose half width of time is 50 pico sec or less
The measurement was performed using a laser diode light pulse with a wavelength of m.

The output waveform of this photomultiplier tube is shown in FIG. Transition time when applied voltage is 1000V
15 nano sec and a rise time of 5.3 nano sec were obtained.

(Dispersion of transit time) 550 nano whose time half-width is 50 pico sec or less
Measurements were made using light pulses from a light emitting diode with a wavelength of m. To set the most severe conditions, the intensity of the light pulses was set such that a single photoelectron was generated by each light pulse and the photocathode PC was uniformly irradiated. Half width 0.727nano under this condition
A transit time variance of sec was obtained. This is shown in FIG.

(Effect of magnetic field) Conventional photomultiplier tubes (box-and-grit type) are easily affected by magnetic fields, and their characteristics change depending on the presence of a magnetic field.

This is considered to be because the distance from the photocathode to the first stage dynode is large, the entrance aperture of the first stage dynode is small, and the travel distance of electrons between each dynode is large.

The photomultiplier tube according to the present invention has photoelectrons and two
Since the traveling distance of the secondary electrons is minimized, resistance to magnetic fields is significantly improved.

FIG. 9 shows the change in gain with respect to the change in the magnetic field of the photomultiplier tube when the applied voltage is 1000V. Note that the energy resolution is optimal when 1000 V is applied to this photomultiplier tube, but resistance to magnetic fields can be increased by increasing this voltage.

There is no practical problem even if a magnetic field of up to 200 Gauss is applied in the tube axis direction (Z) of the photomultiplier tube and up to 60 Gauss in the direction perpendicular to the tube axis (X, Y).

For reference, the characteristics of a conventional photomultiplier tube are xyz
It is shown.

Note that the X and x directions are directions perpendicular to the diode rod, and the Y and y directions are parallel directions.

The magnetic field in the direction (X, Y) perpendicular to the tube axis can be easily removed by shielding.

Figure 10 shows the characteristics when shielded with a 4 mm thick iron cylinder.

(Linearity) The linearity of conventional photomultiplier tubes (box-and-grid type) is controlled by the space charge effect generated in the subsequent diode due to large current pulses. Since each dynode is wide perpendicular to the tube axis, space charge density between the dynodes is reduced, resulting in better linearity.

As shown in FIG. 11, the linearity of the photomultiplier tube according to the present invention was extended to 10 ⁴ pico coulomb.

This is 10 times the linearity of conventional photomultiplier tubes (box-and-grid type).

(Stability) The count rate test also uses cesium-137 as a radiation source.
A light source using NaI (TI) crystal as a scintillator was used. The amount of light that makes the photon counting frequency 10 kcps is incident on this photomultiplier tube, and then the frequency increases.
A light amount of 1kcps is incident. At this time, the change in the mode of the output charge amount was only 0.3%. This data is comparable to that of a normal photomultiplier tube.

As described above, the photomultiplier tube according to the present invention has high resistance to magnetic fields and operates stably even in the presence of an external magnetic field of about 100 Gauss. It also has excellent time properties (15 nano sec transit time, 5.3 nano sec rise time and 0.727 nano sec transition time dispersion) and high linearity (up to 10 ⁴ pino coulomb).

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing an embodiment of a photomultiplier tube according to the present invention. FIG. 2 is an enlarged perspective view of a portion of a plurality of mesh diodes, an anode, and a plate-shaped final stage dynode. Third
The figure is an enlarged plan view showing an electric field forming electrode which is a part of a mesh dynode. FIG. 4 is a circuit diagram showing a connection example of the photomultiplier tube. Fifth
The figure is a graph showing the quantum efficiency of a photomultiplier tube according to the present invention. FIG. 6 is a graph showing the gain of a photomultiplier tube according to the present invention. FIG. 7 is a graph showing the time response characteristics of the photomultiplier tube according to the present invention. FIG. 8 is a graph showing the transition time dispersion of a photomultiplier tube according to the present invention. 9th
The figure is a graph showing the operating characteristics of the photomultiplier tube according to the present invention in a magnetic field. FIG. 10 is a graph showing the operating characteristics of the photomultiplier tube according to the present invention when shielded from a magnetic field perpendicular to the tube axis. FIG. 11 is a diagram showing the linearity of the output of the photomultiplier tube according to the present invention with respect to pulsed incident light. PC...Photocathode, Dy1-Dy8...Mesh dynode, Dy9...Plate-shaped dynode, A...Anode, R1, R2, R3...Resistance, C1, C2
...Capacitor.

Claims (1)

[Scope of Claims] 1. A photomultiplier tube in which a photocathode, a plurality of mesh dynodes, an anode, and a plate-shaped final stage dynode are arranged parallel to each other in this order in a vacuum vessel, wherein the plurality of stages Each mesh dynode forming a mesh dynode consists of a number of rods arranged in a plane parallel to each other at intervals and having secondary electron emission surfaces formed on their surfaces, and a planar photocathode formed by the rods. The anode is formed from a net-like electrode, and the plate-like final stage dynode has a secondary electron emitting surface formed on the anode side, and from the photocathode to the The distance to the first stage of the plurality of mesh dynodes is within the range of 1/15 to 1/5 of the diameter of the photocathode or the diameter of the inscribed circle of the square photocathode, and each mesh dynode is the interval between,
The distance between the final stage mesh dynode and the anode and the interval between the anode and the plate-shaped final stage dynode are configured to be smaller than the distance from the photocathode to the first stage of the plurality of stages of mesh dynodes. A photomultiplier tube. 2. The photomultiplier tube according to claim 1, wherein a cross-sectional view of the rod that is a component of the plurality of mesh dynodes is an isosceles trapezoid having an upper side on the electron incident side. 3. The photomultiplier tube according to claim 1, wherein the electric field forming electrode of each mesh dynode is a mesh in which a large number of regular hexagonal openings formed by etching are closely arranged. 4. The photomultiplier tube according to claim 3, wherein the mesh is formed to be smaller than the spacing between the rods that are the constituent elements of the mesh dynode.