JPS5923609B2 - Secondary electron multiplier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dynode of a secondary electron multiplier tube, and more particularly, to a dynode of a secondary electron multiplier tube in which the arrangement of thin plates constituting the dynode is improved.

Hereinafter, in this specification, 1. The thin plate constituting a dynode refers to a single thin plate or a thin plate-like material having at least one surface capable of emitting secondary electrons, and is used in such a way that a plurality of these thin plates form the basic unit of the dynode.

In addition, the secondary electron multiplier (dynode assembly) of the secondary electron multiplier is formed by one or more dynodes arranged perpendicularly to the main axis of the tube of the secondary electron multiplier. I decided to do it.

A secondary electron multiplier tube is generally placed in a vacuum tubular container.
It contains an electron source such as a photocathode, followed by a secondary electron multiplier consisting of a series of dynodes, and finally an electron collecting electrode (collector).

These electrodes are arranged in the above order along the main axis of the tube.

During operation of the secondary electron multiplier, appropriate voltages are applied to these electrodes in order to generate an electric field that accelerates electrons along the principal axis.

At this time, the electrons emitted from the electron source collide with each dynode in order along the tube axis, are multiplied each time, and collected by the collector.

First, a specific example of a so-called inesian type dynode assembly used in a conventional secondary electron multiplier will be described.

FIG. 3 is a partially enlarged sectional view of a conventional inesian type dynode assembly.

In the figure, reference numerals 101 and 102 indicate thin plates forming the dynode 100, and the end faces in the transverse direction are shown in the paper.

Similarly, 201 and 202 are thin plates constituting the next stage dynode 200.

In a conventional inesian type dynode, the thin plates are arranged at an angle of 45 degrees with respect to the main axis, and the arrangement pitch in the direction perpendicular to the main axis of the thin plate secondary electron multiplier is d, and the thickness of the dynode in the main axis direction is t. , d/l was approximately 1.

The basic configuration appears to be to allow all electrons to collide with one of the thin plates of the dynode, provided that the electrons are incident on the dynode parallel to the principal axis.

Hereinafter, in this specification, the shape of such a dynode will be referred to as geometrically optically opaque or simply opaque.

The inventor of the present invention has discovered that such a conventional dynode does not necessarily have an ideal shape from the point of view of sending secondary electrons to the next stage.

That is, (i) the secondary electrons emitted from the part of the thin plate forming the dynode near the front dynode are returned to the dynode as shown by the arrow a1a2 in FIG. 1 due to the electric field generated by the front dynode.

(11) Some of the secondary electrons emitted at the center of the thin plate are b
As shown in 1b2, it collides with the back surface of the adjacent thin plate.

Since the energy of these secondary electrons is extremely low, further secondary electrons cannot be emitted.

Therefore, the present inventors set the ratio a/l (d/1=1 in the conventional case) of the pitch d of the thin plates constituting the dynode to the thickness t of the dynode to a range of 1.50 to 1.75. We are proposing to increase the probability that the electrons emitted from the dynode will gradually reach the dynode.

The dynode related to this proposal has a geometrically optically transparent part between the thin plates, but when there is an input of signal electrons with the same spatial frequency as this pitch, or when the dynode assembly is
When used after the stage, there is no problem and the multiplication factor can be significantly improved.

FIG. 4 is a graph showing the results of experiments related to the above proposal.

When d/l is from 1.0 to 1.5, the arrival rate η to the next stage dynode (let η = 1 when d/l = 1) increases monotonically, and when d/l is from 1.5 1.9 between 1.75 and 1.75
Among them, it is the highest at 1.64.

When d/l is greater than 1.7, the above ratio decreases.

This is because the effect of the electric field mentioned above is d/l from 1.5 to 1.
It saturates between 75 and d/l is 1.5 to 1.7.
This is considered to be because when the value exceeds 5, the transparent portion of the thin plate of the dynode becomes larger when viewed from the incident electrons, and the rate at which emitted electrons collide with the thin plate of the next stage dynode decreases.

However, regarding the first stage dynode, the geometrically optically transparent portion poses a problem.

The emission source of electrons incident on the first stage dynode is often a uniform emission surface such as a photocathode.

When a secondary electron multiplier is still used for material analysis, the particle beam is incident through a slit or an aperture.

In such a case, it is practically difficult to require the slit or aperture to have a specific structure in relation to the first stage dynode.

Generally speaking, in a secondary electron multiplier, an image formed by an incident particle beam such as an incident electron or an incident ion is independent of the spatial frequency of the thin plate of the first stage dynode.

Therefore, the dynode according to the above proposal is inappropriate as a first stage dynode.

An object of the present invention is to provide a secondary electron multiplier whose performance as a whole is improved by improving the entire dynode assembly.

To achieve the above object, the present invention provides a secondary electron multiplier having a multi-stage inesian dynode assembly, in which the first stage dynode is geometrically opaque when viewed from the incident particle beam source. and two or more next-stage dynodes in which the pitch d of the thin plate in a plane perpendicular to the main axis of the secondary electron multiplier and the thickness t in the direction of the main axis satisfy a relationship of 1.5≦d/l≦1·75. It is composed of:

According to the configuration, if the incident particle beam source is photoelectrons, the first stage dynode is opaque when viewed from the incident photoelectrons, so that there are no electrons that do not collide with the first stage dynode.

Two or more dynodes according to the above-mentioned proposal are used for assembling the dynodes in the second and subsequent stages, and the efficiency of transfer to the subsequent stage is improved, so that the multiplication factor can be improved as a whole.

The secondary electron multiplier according to the present invention will be explained in more detail below with reference to the drawings and the like.

FIG. 1 is a longitudinal sectional view showing an embodiment in which the present invention is applied to a photomultiplier tube.

A photocathode 2 is formed on one bottom surface of a cylindrical glass airtight container 1.

Inside this cylinder, there is a dynode 3 of the first stage (first stage), and dynodes 4.5, 6, 7, 8, 9, 1 from the second stage to the ninth stage.
0. and 11 8-stage dynodes, followed by a 10th-stage dynode.

The first stage dynode is a so-called opaque dynode with d/l=1, and the second to ninth stage dynodes are dynodes with d/l=1.6 according to the previous proposal by the present inventors.
The 10th stage dynode is a dish-shaped dynode with an opening facing the 9th stage dynode 11.

A net-like collection electrode 13 is arranged between the ninth stage dynode 11 and the tenth stage dynode.

FIG. 2 shows an enlarged view of the first stage dynode and the second stage dynode.

The nine thin plates forming the first stage dynode 3 have the same shape, each long side having a length of 19 mm, and each short side having a length of 3.11 mm.

The nine thin plates forming the first stage dynode 3 are arranged at an angle of 45 degrees with respect to the main axis of the photomultiplier tube and with a pitch d = 2.4 rran in a plane parallel to the main axis. .

The ratio between the pitch d and the thickness t of the first stage dynode 3 is d/l
=2.4/(3,IXo,7)=1.08, which is approximately 1 and is opaque.

The second stage dynode 4 is similarly formed from nine thin plates.

These nine thin plates have the same shape and the length of each long side is 19
mm, the length of the short side is 241 mm.

The nine thin plates are arranged in the first direction with respect to the main axis of the photomultiplier tube.
They are arranged on the opposite side from the dynode at an angle of 45 degrees and at a pitch d=2-4 m in a plane parallel to the main axis.

The ratio between the pitch d and the thickness t of the second stage dynode 4 is d/22.4/(2, I x O,7) = 1.6.

Note that the upper side of each thin plate of the second stage dynode 4 is shifted by 0.23 mm from the lower side of each thin plate of the first stage dynode 3 in the arrangement direction of the thin plates (toward the right in the figure).

The net-like electrodes 31 and 41 shown in FIG. 2 are given a potential equal to that of the first dynode 3 and the second dynode 4, respectively.

The slopes of the third stage dynode 5 to the ninth stage dynode 11 only change alternately, and the other parts have the same shape as the second dynode 4.
It's no different.

The degree of amplification will be compared between an embodiment of a photomultiplier tube using a dynode assembly having the above configuration and a photomultiplier tube using a conventional dynode assembly.

A voltage of 1,200 volts was connected to the photocathode 2 of the photomultiplier tube according to the embodiment, and the other electrodes were connected to each other and grounded through an ammeter to disable the dynode assembly and measure the photoelectric conversion efficiency.

When the photocathode 2 was irradiated with 1/100 lumen of light, the output current was changed to 1 microampere.

A voltage of -1,200 volts is connected to the photocathode 2 of the photomultiplier tube, and a voltage of -1,100 volts is applied to the first stage dynode, increasing voltage by 100 volts in order from the second stage dynode to the 10th stage dynode.
The stage dynode is set to 1100 volts and the collection electrode is grounded via an ammeter.

In this state, minus 7 of 10 is applied to the photocathode 2 of the photomultiplier tube.
When irradiated with the same light, the output current was changed to 100 microamperes.

That is, the secondary electron multiplication factor of the dynode assembly of this photomultiplier tube is 5×10 to the seventh power.

Next, similarly, from the first stage dynode to the ninth stage dynode, the dynodes having the same shape as the first stage dynode (d
When the secondary electron multiplication factor of the dynode assembly was measured when using the dynode assembly (1), the secondary electron multiplication factor was 5×
It was 10 to the fifth power.

Note that all other conditions were the same.

Therefore, the secondary electron multiplication factor of the photomultiplier tube according to the present invention is 100 times higher than that of the conventional photomultiplier tube.

A simple comparison shows that the secondary electron multiplier of the present invention is compared to a conventional secondary electron multiplier (the ratio of the pitch of the arrangement of the thin plates of the dynode to the distance of the thin plates in the tube axis direction is 1). A larger electron multiplication factor can be obtained.

This result is calculated to mean that the multiplication factor of the second to ninth stage dynodes is approximately 1.5 times for each stage.

This is different from the approximately 2x multiplication factor described with reference to FIG.

The reason for this is that the electrons are not incident uniformly (they are concentrated at a specific location, and the emission efficiency of the emitted electrons from that location has been quite high in the past, so the emission efficiency has saturated).

As explained in detail above, in the present invention, an opaque dynode is used in the first stage, and two or more dynodes with partially transparent parts are used in the subsequent stages, so the multiplication factor of the secondary electron multiplier is significantly improved. can be done.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view showing an embodiment in which the present invention is applied to a photomultiplier tube, and FIG. 2 is a cross-sectional view showing an enlarged first-stage dynode and second-stage dynode of the photomultiplier tube. Third
FIG. 4 is a schematic diagram for explaining the problems of the conventional dynode, and FIG. 4 is a graph for explaining the characteristics of the dynode used in the second stage and subsequent stages in the secondary electron multiplier according to the present invention. 1... Cylindrical glass airtight container, 2... Photocathode, 3
- First stage dynode, 4 to 11 - 2nd to 9th stage dynodes with transparent parts, 12... 10th stage dynode, 13... Collection electrode.

Claims (1)

[Scope of Claims] 1. A secondary electron multiplier having a multi-stage Venetian dynode assembly, comprising: a first-stage dynode that is geometrically optically opaque when viewed from an incident particle beam source; A secondary electron multiplier tube comprising two or more first-stage dynodes in which the pitch d of the thin plates in a plane perpendicular to the main axis and the thickness t in the direction of the main axis have the following relationship. 1.5≦d/l≦1.75 2 The secondary electron multiplier tube is a photomultiplier tube, and the dynode pitch d from the second stage dynode to the front stage of the collection electrode and the thickness t in the main axis direction are A secondary electron multiplier according to claim 1, wherein all ratios are within the above range.