JPH06148395A - Generating apparatus of slow positron - Google Patents

Abstract

PURPOSE:To enable improvement of the efficiency of converting fast positrons into slow positrons and also generation of the slow positrons in a state of uniform energy. CONSTITUTION:Since a metal thin film 12 is provided between a radioactive isotope <22>Na and a moderator 13, fast positrons e<+> radiated from the radioactive isotope <22>Na are scattered and decelerated when they pass through the metal thin film 12, a peak value of an energy distribution shifts onto the low energy side, the depth of implantation into the moderator 13 becomes small and slow positrons e<+> emitted from the moderator 13 increase in the amount. Therefore the efficiency of turning the fast positrons e<+> into the slow positrons e<+> is improved and also the slow positrons e<+> in a state of uniform energy are generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-speed positron generator which improves the efficiency of converting a high-speed positron into a low-speed positron.

[0002]

2. Description of the Related Art Positrons as antiparticles of electrons can be obtained from RI (radioisotope) such as β ⁺ -decaying ²² Na, ⁵⁵ Co and ⁶⁴ Cu.

The positrons incident on the material lose their energy by inelastically colliding with surrounding electrons and are decelerated to the thermal energy (kT) in a short time of about 10 ^-12 seconds. The thermalized positron annihilates with the electron with a lifetime of 10 ^-10 to 10 ^-7 seconds and emits γ-rays of about 511 [keV]. At this time, the energy and the momentum are preserved, so by observing the emitted γ-rays in detail, the state of the electron at the position where the positron disappears, the lattice defect of the substance, and the like can be known.

By the way, the penetration depth of high energy positrons into, for example, aluminum due to β ⁺ decay is 100 to 200.
Since it reaches [μm], in order to utilize positrons for surface and interface studies, it is necessary to slow them down and implant them into the sample in the state of uniform energy.

FIG. 5 is a conceptual diagram of a conventional slow positron generator used in a device for analyzing the surface of a substance using slow positrons.

In the figure, the slow positron generator 1 is
The vacuum container 2 and ²² Na serving as RI that emits high-speed positrons inside the vacuum container 2 and the front face of the ²² Na (upper side of the figure), and the high energy emitted by β ⁺ decay (high-speed ) Is placed in front of the moderator 3 and a moderator 3 composed of multiple ribbons of tungsten to make the positron of low energy (slow) positron, and it is decelerated and almost stopped (about 1 [eV]). ) An electrode (grid) 4 for accelerating (drawing) positrons to a speed required for transportation and a power supply 5 for applying a predetermined voltage V to the grid 4.

The positron e ^{+ that} has passed through the grid 4 is transported to a sample (not shown) via a passage.
On the way, the speed required for surface analysis (100 eV to 50 ke
It accelerates to V), and injects into a sample.

Here, in ²² Na used in the above-mentioned conventional slow positron generator, the generated positron e ⁺ energy has a positron generation frequency peak value of about 2 as shown in FIG.
It has a very wide distribution from 0 to 540 [keV], which is 10 [keV]. Note that FIG. 6 is a graph showing the relationship between the energy of positrons obtained from ²² Na and the frequency of their generation. In the figure, the horizontal axis represents the energy of positrons and the vertical axis represents the frequency of positron generation.

[0009]

However, although positrons having such a wide energy distribution can be driven into the moderator as they are to decelerate and slow them down, positrons around 210 keV, where the frequency of occurrence is a peak, are driven into the depth of implantation. Is deep and the rate of returning to the surface by thermal diffusion is small. That is, the structure has a low efficiency of slowing down positrons, which are most frequently generated.

Therefore, an object of the present invention is to provide a low-speed positron generator capable of improving the efficiency of converting a positron having a high frequency of occurrence into a slow positron, and improving the efficiency of slowing down positrons from RI as a whole. Especially.

[0011]

To achieve the above object, the present invention provides a slow positron generator in which a moderator converts a high-speed positron emitted from a radioisotope into a low-speed positron. Between them, a metal thin film that slows down the velocity of high-frequency positrons that are frequently generated is provided.

[0012]

According to the above structure, since the metal thin film is provided between the radioisotope and the moderator, the fast positrons emitted from the radioisotope are scattered and decelerated when passing through the metal thin film, and the positron is shallowly driven into the moderator. , The number of slow positrons emanating from the moderator increases.

[0013]

An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a conceptual diagram of one embodiment of the slow positron generator of the present invention.

In the figure, the slow positron generator 10 is shown.
Is a vacuum container 11 and ²² Na as a radioactive isotope 16 which is placed in the vacuum container 11 and emits high-speed positrons.
And a thin metal film 12 as a moderator, which is disposed on the front surface of ²² Na (upper side in the figure) and scatters and decelerates high-speed positrons.
When this is placed in front of the metal thin film 12, a moderator 13 to the high speed of the positron e ⁺ slow positron e ^+, is disposed in front of the moderator 13, it requires a positron e ⁺ close to almost stop state to the beam transport It is composed of a grid 14 for accelerating to various speeds and a power supply 15 for applying a voltage V to this grid 14.

Although ²² Na was used for RI, it is not limited to this, and ⁵⁵ Co and ⁶⁴ can be used if a high-speed positron is emitted.
Cu or the like may be used.

Here, it is generally known that the average penetration depth Z _1/2 of positron e ⁺ implanted in a metal thin film is proportional to the power of the energy of the positron e ⁺ implanted as shown in FIG. Figure 2 shows positron e ⁺ in copper and aluminum.
2 is a graph showing the energy dependence of the average penetration depth of. In the figure, the horizontal axis represents the energy of positrons, the left vertical axis represents the average penetration depth, and the right vertical axis represents the average penetration depth of aluminum and copper. From the figure, the average permeation depth Z _1/2 of copper is expressed by the formula 1, and the average permeation depth Z _1/2 of aluminum is expressed by the formula 2. Further, it is considered that the average penetration depth Z _1/2 of tungsten is Equation 3.

[0018]

[Equation 1] Z _1/2 = 0.0026E ^1.43

[0019]

[Formula 2] Z _1/2 = 0.0020E ^1.60

[0020]

[Equation 3] Z _1/2 = 0.0012E ^1.43 50 [ke for the energy E of these Equations 1, 2 and 3]
Substituting the values increased by 50 [keV] from V] to 500 [keV], the average penetration depth Z _1/2 is as shown in Table 1.

[0021]

[Table 1]

From Table 1, it can be seen that although the thickness of the metal thin film 12 is different, the characteristics are not so different.
Therefore, the material of the metal thin film 12 is not particularly limited.

A tungsten ribbon is used for the moderator 13, and a plurality of tungsten ribbons are arranged in parallel and at equal intervals perpendicular to the paper surface, and these ribbons have a voltage V (for example, 100 [V]). It is connected to the anode of the power supply 15. Although tungsten is used for the moderator 13, a metal such as iridium may be used as long as the slow positron e ⁺ can be extracted.

A mesh metal is used for the grid 14,
It is connected to the cathode of the power supply 15.

The low-speed positron e ⁺ generated by the low-speed electron generator 10 is transported to the sample (not shown) via the passage.

Next, the operation of the embodiment will be described.

In FIG. 1, high-speed positron e ⁺ diverges from ²² Na (in the forward direction), and this high-speed positron e ⁺ is ²²
When passing through the metal thin film 12 provided between the Na and the moderator 13, the light is scattered and decelerated, so that the positron near the peak value of the energy distribution of the positron e ⁺ shown in FIG.
It can be shifted to the low level side as shown in FIG. Note that FIG. 3 is a graph showing the relationship between the positron energy when the metal thin film 12 is arranged and the generation frequency thereof. In the figure, the horizontal axis represents the energy of positrons and the vertical axis represents the frequency of positron generation.

Implanting the positron e ⁺ with such energy shifted to the low level side into the moderator 13
Since the positron e ⁺ is shallowly driven into the moderator 13, the number of low-speed positrons e ⁺ of about 1 [eV] emitted from the moderator 13 is increased, so that the high-speed positron e ^{+ is changed} to the low-speed positron e ^{+. The} efficiency of making ⁺ increases.

The energy of the slow positron e ⁺ is accelerated by the voltage V applied to the grid 14 to the velocity required for beam transportation, and is transported to the sample (not shown).
Energy (100 [eV] to 50 [keV]) required for analysis as shown in Fig. 4 by accelerating again on the way.
It is incident on the sample. FIG. 4 is a graph showing the relationship between the energy of positrons generated by the slow positron generator and the frequency of positron generation. In the figure, the horizontal axis represents the energy of positrons and the vertical axis represents the frequency of positron generation.

A specific example will be described below.

(Concrete example) Considering the energy distribution of positron e ⁺ (FIG. 6), it is necessary to pass a positron e ⁺ of 100 [keV] or more, that is, a high-speed positron e ⁺ has an average of 20.
It is desirable to reduce the speed by about 0 [keV]. For that purpose, it is desirable to use a metal thin film having the following thickness.

Copper: about 3.7 [μm] Aluminum: about 20 [μm] Tungsten: about 1.7 [μm] As described above, in this embodiment, the radioactive isotope ²² N is used.
Since the metal thin film 12 is provided between the a and the moderator 13, the fast positron e ⁺ emitted from the radioisotope ²² Na
Are decelerated by passing through the metal thin film 12 and are decelerated, and by shifting the peak value of the energy distribution of the positron e ⁺ to the low level side, the implantation depth of these positrons becomes shallow and the positron e ⁺ is emitted from the moderator 13. Since the amount of slow positron e ⁺ increases, the fast positron e ⁺ is replaced by the slow positron e ^+.
+ Improves the efficiency of the, uniform state of the slow positron e ⁺ energy can be generated.

[0033]

In summary, according to the present invention, in the slow positron generator in which the high speed positron emitted from the radioisotope is converted into the low speed positron by the moderator, the energy of the positron is between the radioisotope and the moderator. Since a metal thin film that slows down the velocity of high-speed positrons with a distribution peak value is provided, the depth of implantation into these moderators can be made shallow, improving the efficiency of making high-speed positrons into slow positrons. , Slow positrons with uniform energy can be generated.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of an embodiment of a slow positron generator of the present invention.

FIG. 2 is a graph showing the energy dependence of the average penetration depth of positrons in copper and aluminum.

FIG. 3 is a graph showing the relationship between positron energy and the frequency of occurrence when a metal thin film is arranged.

FIG. 4 is a graph showing the relationship between the energy of positrons generated by the slow positron generator of this example and the frequency of positron generation.

FIG. 5 is a conceptual diagram of a conventional slow positron generator.

FIG. 6 is a graph showing the relationship between the energy of positrons obtained from radioisotopes and their frequency of occurrence.

[Explanation of symbols]

10 Low-speed positron generator 11 Vacuum container 12 Metal thin film 13 Moderator 14 Grid 15 Power supply 16 Radioisotope ( ²² Na)

Claims (1)

[Claims] 1. A slow positron generator in which a moderator converts a high-speed positron emitted from a radioisotope into a low-speed positron, and a high-speed positron with a high frequency is generated between the radioisotope and the moderator. A low-speed positron generator characterized in that a metal thin film for reducing the speed is provided.