JPH0693037B2 - Positron beam generator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positron beam generator used in, for example, a transmission positron microscope.

[0002]

2. Description of the Related Art Conventionally, as a method for obtaining a positron beam, (1) a method using a radioisotope (RI) that decays β ⁺ (for example, Physical Review Let-ters vol 6
0.No3, pp169 (1988)). Alternatively, (2) there is a method of irradiating an electron beam from an electron linac to a target and utilizing electron pair generation by bremsstrahlung γ rays.

[0003]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The positron obtained by the method (1) has a current amount of about several tens of femtoamps at most, and for use as a positron microscope, for example, it is necessary to increase the current and simultaneously increase the brightness. On the other hand, according to the method (2), although a positron beam with a current about 10 ⁴ times that of the method (1) can be obtained, the length of the entire apparatus is very long, about several tens of meters, and the inside of the apparatus is very long. Therefore, there is a problem that it is too large to be used as a positron beam generator only, and requires advanced knowledge for operation, such as the need to shield incidentally generated neutrons.

It is an object of the present invention to provide an apparatus capable of generating a positron beam with high brightness with a simple and compact apparatus structure.

[0005]

Means for Solving the Problems A configuration for achieving the above object will be described with reference to FIG. 1 corresponding to an embodiment. The present invention is directed to an isotope (R) that generates a positron.
I layer) 1a, and a positron emission surface 1 obtained by uniformly forming a moderator 1b on the surface of the spherical concave surface, and the emission surface provided at a position facing the positron emission surface 1. The positron from 1 is the central axis CL of the spherical concave surface
A lead-out electrode 3 for drawing out in a direction along with, a power source 4 for providing a predetermined potential difference between the electrode 3 and the positron-emitting surface 1, and a positron-emitting surface 1 arranged around the outside of the positron-emitting surface 1. It is characterized by including the correction electrode 2 for correcting the electric field formed between the extraction electrode 3 and the extraction electrode 3.

[0006]

The positron emitted from RI exhibits not a single spectrum (single energy) but a continuous spectrum having the maximum energy determined by the nuclide. To convert such a continuous spectrum of positrons into a single spectrum, the positrons should be decelerated to about thermionic velocity by a moderator, and the decelerated positrons should be more vacuum than those in the solid. It is known that the moderator is more stable in the inside, and is emitted from the moderator in a direction approximately perpendicular to the surface of the moderator at a speed of the order of thermoelectrons.

Therefore, in the present invention, the positron emitting surface 1 having a spherical (concave) shape is formed by the RI layer 1a and the moderator 1b.
By forming the, positrons having a speed of the order of thermoelectrons are generated in the direction toward one point, that is, the center C of the spherical surface. As a result, the emitted positrons are aligned in the direction in which they are initially focused in the direction in which they should be focused, so that the extraction electrode 3 can efficiently focus them. Moreover, even if the area of the positron emission surface 1 is increased, the beam divergence is small, which makes it possible to obtain a large current beam.

Here, the positron emitting surface 1 and the extraction electrode 3
As shown in FIG. 2 (a), the electric field lines L at the peripheral edge of the positron emission surface 1 bulge outwards if a potential difference is simply applied to the positrons, and the focusing efficiency of positrons is not so good.
Therefore, in the present invention, the auxiliary electrode 2 is provided so that the electric lines of force L around the positron emission surface 1 are extended in a substantially linear shape toward the extraction electrode 3 as shown in FIG. 2 (b). to correct.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the configuration of an embodiment of the present invention.
An RI layer 1a made of ²² Na or the like is uniformly laminated on the spherical surface (concave surface) formed on the base 1c, and further, a moderator 1b made of W or Ti is uniformly laminated on the inner surface of the RI layer 1b. Thus, the spherical positron emission surface 1 is formed. In addition, the extraction electrode 3 faces the positron emission surface 1.
Are arranged, and the opposing surface thereof has a spherical shape concentric with the center C of the positron emitting surface 1. Further, in the center of the extraction electrode 3, a hole 3 centered on the central axis CL of the spherical surface is formed.
a is formed.

A power source 4 is connected between the positron emission surface 1 and the extraction electrode 3, and a potential difference is applied between them so that the extraction electrode 3 has a negative potential. Also,
Auxiliary electrodes 2 are arranged along the lateral periphery of the positron emission surface 1. The auxiliary electrode 2 corrects the electric lines of force of the electric field formed between the positron emitting surface 1 and the extraction electrode 3 to the shape described above.

With the above-described structure, positrons having a single spectrum and decelerated at a speed of about thermionics from the positron emission surface 1 (inner surface of the moderator 1b) are applied to the surface. It occurs almost vertically. The generated positrons are formed by the extraction power source 4 between the positron emission surface 1 and the extraction electrode 3 and are corrected by the auxiliary electrode 2, that is, the electric force lines L as shown in FIG. 2 (b). In the electric field having the above, the positron beam PB of high brightness is accelerated while being efficiently focused and travels.

Here, the positron emission surface 1 and the extraction electrode 3
The shape of the electric lines of force L of the electric field formed between
The shape / potential of the auxiliary electrode 2 to have the shape shown in (b) is similar to the method of determining the shape / potential of the Wehnelt electrode in the high-focus electron gun, with the potential at the boundary surface of the positron beam as the initial condition. It can be calculated by various calculations.

Since the hole 3a of the extraction electrode 3 is usually small, the electric field formed between the extraction electrode 3 and the positron emission surface 1 is hardly disturbed. When it is desired to increase the size of 3a, a spherical metal mesh electrode made of metal is put in the hole 3a, or
Alternatively, the extraction electrode 3 itself may be a spherical mesh electrode.

Further, in the above embodiments, the extraction electrode 3
The surface facing the positron emission surface 1 is spherical, but the shape is not limited to this, and the same effect can be achieved even if the shape of the extraction electrode is conical, for example.

[0015]

As described above, according to the present invention, the positron emitting surface 1 having a spherical (concave) shape is formed by the RI and the moderator that emit the positron, and the positron is extracted from this emitting surface. Since it is configured to extract along the central axis of the emission surface with the use of the auxiliary electrode, it is possible to obtain a high-intensity positron beam applicable to, for example, a transmission positron microscope with a small and simple device configuration. Becomes

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation of the auxiliary electrode 3 according to the embodiment of the present invention.

[Explanation of symbols]

 1 ...- Positron emitting surface 1a ... RI layer 1b ... Moderator 1c ... Base 2 ... Auxiliary electrode 3 ... Extraction electrode 3a ... Hole 4・ ・ ・ ・ Power source C ・ ・ ・ ・ Center of spherical surface CL ・ ・ ・ ・ ・ ・ Center axis of spherical surface PB ・ ・ ・ ・ Positron beam

Claims (1)

[Claims] 1. A positron emitting surface obtained by uniformly forming a moderator on a surface of a spherical concave surface formed by a positron generating isotope, and a positron emitting surface disposed at a position facing the positron emitting surface. An extraction electrode for extracting positrons from the emission surface in a direction along the central axis of the spherical concave surface, a power source for providing a predetermined potential difference between the electrode and the positron emission surface, and an outside of the positron emission surface. A positron beam generator provided with an auxiliary electrode which is arranged in the periphery and corrects an electric field formed between the positron emitting surface and the extraction electrode.