JPH0990097A - Slow positron beam source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to generate good-quality slow positron beams by increasing the efficiency in the extraction of them. SOLUTION: White positrons generated from a positron source 1 are incident on a positron moderator 2, which emits slow positrons. The slow positrons at once emitted again and generated around the center of a positron moderator 2 are accelerated in the direction of the positron source 1 by the potential difference applied between the positron source 1 and the positron moderator 2, focusing on the center axis by the effect of a magnetic field formed by a coil 3. Then, these slow positrons pass through a through hole 1e at the center of the positron source 1 and are outputted as positron beams with a prescribed energy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slow positron beam generator used for slow positron diffraction, positron annihilation Auger electron spectroscopy, positron microscopy, depth direction analysis of vacancy defects due to positron annihilation, and the like.

[0002]

2. Description of the Related Art Positrons, which are antiparticles of electrons, can be obtained from RI (radioisotope) of β Na ^{+ 22} Na, ⁵⁵ Cu, ⁶⁴ Co, etc. However, the positron energy generated from RI is distributed over a very wide range from around 0 eV to a predetermined maximum energy determined by the type of RI (for example, 540 Kev for ²² Na). It cannot be used in the technical field related to slow positrons as described above.

In order to convert positrons having a continuous energy distribution into positrons having uniform energy, positrons are injected into a positron moderator, the positrons are decelerated in the moderator, and then some of the positrons are decelerated by diffusion. The property of reaching the surface of the material and being spontaneously released from the surface with energy equivalent to the work function is used.

A reflection type slow positron beam generator using such a conventional technique will be described with reference to FIG. ²² Na, ⁵⁵
High-speed, white positrons generated from a positron beam source 31 such as Cu reach the positron moderator 32 made of Cu (111) or the like. The positrons diffuse near the surface of the positron moderator 32 and are re-emitted with energy of about work function. The re-emitted positrons become monochromatic, and the positrons re-emitted by the accelerating electrode 33 are accelerated and used for the above purpose.

[0005]

However, in the above-mentioned conventional structure, the moderator has a conversion efficiency of 10 ⁻⁴ to slow positrons.
In addition to being extremely low at 10 ⁻³ , most of the re-emitted positrons are shielded by the positron beam source 31 and do not move toward the accelerating electrode, so the slow positrons output from the entire device are extremely low. The beam was also of poor quality, with the central part missing.

Although not shown, the positron beam source 31 is installed in a vacuum chamber or the like, and the holder is used to fix the positron beam source to the wall of the chamber by using a holder or the like. Since the tool interferes with the output of the slow positron toward the accelerating electrode, the extraction efficiency of the slow positron was further reduced.

The present invention was devised to solve the above problems, and provides a slow positron beam generator capable of increasing the extraction efficiency of slow positrons and generating a high-quality positron beam. .

[0008]

In order to achieve the above object, a slow positron beam generator of the present invention converts a white positron into a slow positron and a positron source having a through hole for generating a white positron. Install it facing the moderator,
It is characterized in that white positrons from the positron source are made incident on the moderator, and low-speed positrons emitted from the moderator are output toward the positron source side through the through holes.

White positrons are generated from the positron source and enter the moderator. The incident white positron is decelerated in the moderator, and a part of what reaches the surface of the moderator by diffusion is re-emitted from the surface with energy of about work function and becomes a positron of low speed and monochromatic energy.

The slow positrons emitted from the moderator surface on the positron source side are output in the direction of the positron source side. Since the positron source has a through hole, the slow positron is extracted through this through hole. The positron beam having a good shape can be efficiently extracted.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the structure of a slow positron beam generator according to the present invention.

Reference numeral 1 is a ring-shaped positron source, 2 is a positron moderator made of tungsten or the like whose surface is cleaned, 3 is a coil for generating a magnetic field on the central axis, 4 is a positron source 1 and positron moderator 2. Power supply for applying a potential between
Reference numeral 5 is a cylindrical vacuum chamber, and 6 is a support.

A positron source 1 and a positron moderator 2 are installed in a vacuum chamber 5, which are attached to the walls of the vacuum chamber by supports 6 and 7, respectively, and a coil 3 is installed around the outside of the vacuum chamber 5. Has been done. The power source 4 is usually installed outside the vacuum chamber 5.

By the way, the positron source 1 is circular and has a through hole formed in the center thereof. The details are shown in FIG. 2, the upper part is a plan view and the lower part is a cross-sectional view.

1a is made of, for example, SUS, and the main part of the positron source 1 having a thick circular shape (thick five-yen coin shape with a circular through hole 1e formed in the center) and 1b has a ring shape. Groove, 1c generates positron by β ⁺ decay ²²
Radioisotopes such as Na, ⁵⁵ Cu, and ⁶⁴ Co (radioisotopes), 1d is a thin plate such as titanium, and 1f is a shielding plate such as lead.

The groove 1b contains a radioactive isotope 1c, and a thin plate 1d is welded to the main portion 1a. Further, the thickness of the thin plate 1d is made thin enough to allow most of the positrons generated from the radioisotope in the groove 1b to pass therethrough, and the opposite side of the thin plate 1c is a high-speed positron generated from the radioisotope. In order to prevent it from penetrating, it is shielded with a thick metal plate 1f such as lead.

White positrons having a wide energy distribution generated from the radioisotope 1c of the positron source 1 enter the positron moderator 2 and lose energy while colliding with electrons in the moderator 10 inelastically. ^-It is decelerated to about thermal energy (KT) in about ¹² seconds. Here, the decelerated positron annihilates with the electron in a time of 10 ⁻¹⁰ to 10 ⁻⁷ seconds while drifting in the non-direction near the surface of the moderator.
Only a part of what has reached the clean surface of the positron moderator 2 is re-emitted into the vacuum before the annihilation of positrons in this very short time.

Of the re-emitted slow positrons, those generated from near the center of the positron moderator 2 are focused on the central axis by the action of the magnetic field formed by the coil 3, and the positron source 1 and the positron moderator 2 Is accelerated in the direction of the positron source 1 by the potential difference applied between and, passes through the through hole 1e at the center of the positron source, and is output as a positron beam of predetermined energy.

As described above, the through hole is provided in the positron source to secure the passage for the slow positrons, and the positron source itself also serves as the extraction electrode (accelerating electrode) for the slow positrons. Slow positrons with a predetermined energy can be efficiently extracted without damaging the shape.

In the above embodiment, the positron source 1 has a ring shape, but it may have a large number of circular through holes as shown in FIG. 3, and the holes are not circular. Instead, it may be polygonal.

Further, as shown in FIG. 4, if the positron source 1 is tapered so that white positrons are concentrated in the center of the positron moderator 2 as much as possible, the slow positrons are also generated in the center of the positron moderator 2. Since the number of particles that are concentrated and pulled out from the through hole 1e increases, the conversion efficiency into slow positrons can be further increased.

Similarly, if the slow positrons are obtained by tapering the shape of the positron moderator 2 (bowl type) as shown in FIG. 5, the slow positrons will be gathered at the center of the positron source 1. , The conversion efficiency can be increased.

Next, another embodiment using a positron source having a through hole is shown in FIGS. 6 and 7.

11 is a ring-shaped positron source, 12 is a positron moderator made of tungsten or the like, the surface of which has been cleaned, 13 is a coil for generating a magnetic field on the central axis, and 15 is re-emission. An extraction electrode for extracting the generated slow positrons, 14 is a power source for applying an electric potential between the positron moderator 12 and the extraction electrode 15, 16 is a cylindrical vacuum chamber, and 17 is a support.

The positron source 11 is placed on the positron moderator 12 so as to be in contact therewith, and the positron moderator 12 is fixed to the inner wall of the vacuum chamber 16 by a support 17.

11A is, for example, a main part of a thick circular positron source 11 in which a tapered through hole 11E is formed in the center made of SUS, 11B is a ring-shaped groove, and 11B is a groove.
C is ²² Na and ⁵⁵ C which generate positrons by β ⁺ decay.
u, a radioisotope such as ⁶⁴ Co (radioisotope), 11D is a thin plate such as titanium, and 11F is a shielding plate such as lead. A radioisotope 11C is included in the groove 11B, and a thin plate 11D is welded to the main portion 11A.

Further, the thickness of the thin plate 11D is thin enough to allow most of the positrons generated from the radioisotope in the groove 11B to pass therethrough, and is generated from the radioisotope on the side opposite to the thin plate 11C. In order to prevent the high-speed positrons from penetrating, it is shielded with a thick metal plate 11F such as lead.

In the above-described embodiment, the positron source also serves as the extraction electrode. However, in this embodiment, the slow positron has the same function as in the previous embodiment except that the extraction electrode is separately required. Occurs.

FIG. 7 shows a positron source 11 used in FIG.
8A and 8B show another configuration of the positron source.

In this embodiment, since the positron source 11 has a tapered through hole 11E at the center, white positrons generated from the radioisotope 11C easily gather near the center of the positron moderator 12. Therefore, the efficiency of extracting the slow positrons is improved, and since the white positrons are obliquely incident on the surface of the positron moderator, the positrons are likely to be distributed in the shallow portion and the probability of re-emission increases.

On the other hand, in the case of the shape shown in FIG. 8B, the thin plate 11D is provided on the surface in contact with the positron moderator 12, and the white positron generated from the radioisotope 11C is directly below the positron moderator 12. As they are emitted, the white positrons are incident on the deep part of the positron moderator, and the positron moderator 12
Since the rate of diffusion and re-emission in the central portion of the is lower, the probability of re-emission is lower than that in FIG. 7, and the conversion efficiency is poor.

Further, in the case of the positron source having the shape shown in FIG. 8A, the number of white positrons incident on the positron moderator is substantially reduced, so that the conversion efficiency is lower than that in FIG.

[0034]

As described above, according to the slow positron beam generator of the present invention, the positron source is provided with a through hole,
Since the slow positrons emitted from the positron moderator through this through hole are drawn to the positron source side, a high-quality positron beam in which the beam center is formed can be efficiently obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a slow positron beam generator according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a positron source in the slow positron beam generator of FIG.

3 is a diagram showing another configuration of a positron source of the slow positron beam generator of FIG.

FIG. 4 is a diagram showing another configuration of a positron source of the slow positron beam generator of FIG.

5 is a diagram showing another configuration of the positron moderator of the low speed positron beam generator of FIG.

FIG. 6 is a view showing a slow positron beam generator according to another embodiment of the present invention.

7 is a diagram showing another configuration of a positron source of the slow positron beam generator of FIG.

8 is a diagram showing another configuration of a positron source in the slow positron beam generator of FIG.

FIG. 9 is a diagram showing a configuration of a conventional slow positron beam generator.

Claims (1)

[Claims] 1. A positron source for generating white positrons and a moderator for converting white positrons to low-speed positrons are installed to face each other, and white positrons from the positron source are made incident on the moderator.
A device for outputting slow positrons emitted from a moderator in the direction of a positron source side, wherein the positron source is provided with a through hole for pulling out slow positrons.