JPS5923608B2 - photomultiplier tube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photomultiplier tube having a rectangular photocathode and a rectangular outer shape when viewed from the front, which is suitable for cases where a large number of phototubes are used closely, such as in a positron CT device. .

When a positron and an electron collide, gamma rays are emitted in directions that are 180 degrees from each other.

A positron CT apparatus has been developed that utilizes this phenomenon to examine the tomographic structure of the human body, particularly lesions.

This positron CT apparatus uses a large number of photomultiplier tubes to detect the gamma rays.

By calculating the outputs of a large number of photomultiplier tubes using a computer, the location where gamma rays are generated can be determined.

At this time, in order to confirm that the gamma rays are generated from the same event, it is necessary to detect the temporal coincidence of the outputs of two or more photomultiplier tubes.

Furthermore, in order to detect gamma rays whose emitted direction is not known in advance, the photomultiplier tubes must be closely arranged.

It is necessary to determine whether gamma rays generated from the same event were detected by two or more photomultiplier tubes, or whether gamma rays generated from different events were detected individually.

The most common conventional photomultiplier tube has a cylindrical outer shape, and a circular photocathode is formed on the inner wall of one end surface.

In such a photomultiplier tube, the electric field is formed rotationally symmetrically about the tube axis, so it is easy to design an electrode (electron lens) that satisfies the conditions for focusing photoelectrons.

Further, by making the photocathode a concave surface while maintaining the rotational symmetry of the electric field, it is possible to obtain a device with less variation in electron transit time.

Therefore, in the positron CT device, the output of two or more photomultiplier tubes due to gamma rays generated from the same event is
This is suitable for determining whether the outputs are caused by different events.

However, it cannot satisfy the basic requirement of the positron CT apparatus, which is that the photocathode must be arranged closely, and is therefore unsuitable for gamma ray detection in positron CT.

A photomultiplier tube has also been developed in which a photocathode is formed inside the front end surface of an airtight container whose front surface is approximately square.

By using such a photomultiplier tube, it is possible to satisfy the basic requirement that the photocathode of the positron CT device must be closely arranged.

However, since the distance to the center of the focusing electrode is large at the corner portions of the rectangular photocathode, the travel time of photoelectrons becomes long.

Therefore, the travel time of photoelectrons varies depending on the emission location of photoelectrons on the photocathode, and the gamma rays generated from the same event are detected by two or more photomultiplier tubes.
It becomes difficult to determine whether gamma rays generated from different events have been detected.

An object of the present invention is to provide a photomultiplier tube that can solve the above-mentioned problems and can be suitably used in positron CT devices and the like.

In order to achieve the above object, the photomultiplier tube according to the present invention has a substantially rectangular entrance surface and a photocathode is provided on the inner wall corresponding to the entrance surface, in which the photocathode is placed on the entrance side. A rectangular focusing electrode is formed into a convex curved surface and has an opening in the center, and peripheral electrodes are integrally provided on each side of the square focusing electrode, which is parallel to the optical axis and whose abdomen bulges in the direction of the photocathode. It is configured.

According to the above configuration, the electric field acting on the photocathode becomes uniform and the path of photoelectrons is also averaged.
The time taken as an output after the line reaches the photoelectron is uniform, and variations in the travel time of photoelectrons are reduced.

Therefore, the above-mentioned discrimination problem can be completely solved and the object of the present invention can be completely achieved.

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

FIG. 1 is a plan view showing the entrance surface of a phototube according to an embodiment of the present invention, and FIG. 2 is a front view of the phototube, with a portion cut away to show the internal structure of the phototube.

A sealed container 1 made of transparent glass is composed of a substantially square entrance surface 1a facing the light source, a head 1b connected to the entrance surface 1a, a cylindrical base 1c, and a bottom in which a pin 6 is implanted. has been done.

The photocathode 2 is formed into a curved surface that is convex toward the incident side and is formed on the inner wall of the incident surface.

In this embodiment, the curved surface is a spherical surface centered on the central axis (optical axis) of the phototube.

FIG. 3 is a perspective view showing a rectangular focusing electrode taken out.

The rectangular focusing electrode 3 has an opening 3a in the center, and peripheral electrodes 3b, 3c, 3d and 3e are fixed on the four sides.

Each peripheral electrode is a plate-shaped body parallel to the optical axis of the phototube, and has a shape in which the abdomen bulges in the direction of the photocathode 2 .

The photocathode side of this peripheral portion has a shape that is concentric with the center of the spherical surface that defines the photocathode 2 and is in contact with an imaginary spherical surface whose diameter is smaller than the spherical surface.

Photoelectrons from the photocathode 2 are guided through the opening 3a to the dynode assembly, forming a dynode 4a.

4b...-4n are multiplied, collected by the collector 5, and outputted via a predetermined pin 6.

FIG. 4 is an explanatory diagram showing another modification of the photocathode.

In this modification, the entrance surface 1a of the sealed container is rectangular.

The photocathode 2 has an axis X that is perpendicular to the central axis 2 of the phototube and parallel to the long side of the entrance surface 1a as its long axis, and is aligned with an ellipsoid of revolution P whose rotation center is the long axis.

In this case, the shape of the tip of the peripheral edge of the focusing electrode is such that it is concentric with the center of rotation of the ellipsoid of revolution and is in contact with another ellipsoid of revolution that is similar and smaller in shape.

Therefore, the tip of the peripheral electrode corresponding to the short side of the photocathode has an arcuate shape, and the tip of the peripheral electrode corresponding to the long side of the photocathode has an elliptical shape.

However, the shapes of the peripheral edges of the photocathode and the focusing electrode do not necessarily have to be defined by an ellipsoid of revolution, and may be similar to this shape.

In short, a shape is selected that allows photoelectrons from the photocathode to reach the opening 3a of the focusing electrode 3 in a constant travel time regardless of where they are generated.

If the opening 3 is provided offset from the center of the focusing electrode 3, special consideration must be taken.

However, there is no difference in that the photocathode is formed into a curved surface that is convex toward the incident side, and the peripheral edge of the focusing electrode has a shape in which the abdomen bulges in the direction of the photocathode.

[Brief explanation of drawings]

FIG. 1 is a plan view of a phototube according to the present invention viewed from the incident side;
FIG. 2 is a front view, partially cut away to show the internal structure. FIG. 3 is a perspective view showing the focusing electrode of the apparatus according to the embodiment. FIG. 4 is an explanatory diagram showing another modification of the photocathode. 1-... Sealed container, 1a... Incident surface of sealed container, 1b
...Head of the sealed container, 1c...Base of the sealed container, 2
... Photocathode, 3... Focusing electrode, 3a... Opening of focusing electrode, 3b... Periphery of focusing electrode, 4a... Dynode, 5... Collector, 6... Pin .

Claims (1)

[Scope of Claims] 1. In a photomultiplier tube having a substantially rectangular entrance surface and a photocathode provided on an inner wall corresponding to the entrance surface, the photocathode is formed into a curved surface convex toward the entrance side, and a center A photoelectron multiplier characterized in that a peripheral electrode is integrally provided on each side of a rectangular focusing electrode with an opening provided therein, parallel to the optical axis, and having an abdomen bulging in the direction of the photocathode. tube. 2. The photomultiplier tube according to claim 1, wherein when the entrance surface of the phototube is approximately square, the photocathode is a spherical surface whose center coincides with the central axis of the tube. -3 When the entrance surface of the phototube is approximately rectangular, the photocathode is approximately an ellipsoid of revolution or an ellipsoid thereof whose center of rotation is an axis perpendicular to the central axis of the tube and parallel to the long side of the rectangle. The photomultiplier tube according to claim 1, which has a curved surface.