JPH04321600A - Heat treatment of single crystal - Google Patents

Abstract

PURPOSE:To provide a heat treatment of single crystal causing no discoloration to yellowish green color and/or no decline in fluorescence output, thus excellent in scintillator characteristics. CONSTITUTION:The objective heat treatment of single crystal consisting of a cerium-activated gadolinium silicate compound, characterized by heating in a low-oxygen atmosphere a single crystal of the cerium-activated gadolinium silicate compound of the generic formula Gd2(x+y)LnxCeySiO5 (Ln is at least one element selected from the rare earth elements; x is 0-2; y is greater than zero but >=2).

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for heat treating a single crystal of a cerium-activated gadolinium silicate compound.

0002

[Prior Art] A scintillator made of a gadolinium silicate compound Gd2SiO5 (hereinafter referred to as GSO) activated with cerium has a short fluorescence decay time of 60 ns and a large radiation absorption coefficient, so it has been put into practical use as a radiation detector for positron CT and other devices. There is. However, although the fluorescence output is higher than that of the BGO scintillator, it is only about 20% that of the NaI (Tl) scintillator, and improvement has been desired.

Heat treatment can be considered as a method of improving the fluorescence output of a scintillator. A heat treatment method for improving the fluorescence output of an oxide scintillator such as a GSO scintillator is a heat treatment method in which a single crystal of a tungstic acid compound is heated at a temperature below the melting point of the single crystal or 200° C. lower than the melting point of the single crystal in an oxygen-containing atmosphere. (Tokuko Showa 64-6
(described in Publication No. 160).

0004

[Problems to be Solved by the Invention] A GSO scintillator is an acid compound single crystal scintillator similar to a tungstic acid compound single crystal, but when the above-mentioned conventional heat treatment in an oxygen-containing atmosphere is applied, the single crystal becomes yellowish-green. It was found that this conventional technique could not be applied because the fluorescent output was reduced due to coloration.

The present invention provides a single crystal heat treatment method for obtaining a scintillator with high fluorescence output.

[0006]

[Means for Solving the Problems] The present inventors have discovered that scintillator properties such as fluorescence output can be improved by heat-treating a single crystal made of a cerium-activated gadolinium silicate compound in an oxygen-poor atmosphere. Ta.

[0007] The present invention has the general formula Gd2-(x+y)
LnxCeySiO5 (here, Ln represents at least one element selected from rare earth elements, x is 0 to 2
and y is a value greater than 0 and less than or equal to 2. The present invention relates to a heat treatment method for a single crystal made of a cerium-activated gadolinium silicate compound, in which the single crystal made of a cerium-activated gadolinium silicate compound shown in ) is heated in an oxygen-poor atmosphere.

[0008] In the present invention, the values of x and y in claim 1 define the limit numerical values constituting the cerium-activated gadolinium silicate compound. x is 0 and y is 0.0
A value of 0.01 to 0.2 is preferable because the fluorescent output is significantly improved by heat treatment. Further, in the formula, Ln is at least one element selected from rare earth elements, but Ln is Sc
, Tb, Dy, Ho, Er, Tm, Yb and Lu, and x is 0
．． Values of 1 to 1.999 and y of 0.001 to 0.2 are also preferred since they exhibit good scintillator properties.

In the present invention, heat treatment is performed in an oxygen-poor atmosphere to prevent coloring of the single crystal and improve scintillator characteristics such as fluorescent output. The oxygen-poor atmosphere is preferably an atmosphere in which the amount of oxygen is less than the content in air, and is not particularly limited, but for example, a nitrogen atmosphere may be used. This heat treatment is normally performed on the single crystal after growth, but the heat treatment may be incorporated into the growth process and the single crystal growth and heat treatment may be performed consecutively or simultaneously.

[0010] The temperature of the heat treatment is determined by the type of scintillator, the structure of the heat treatment furnace, the time of the heat treatment, etc. At high temperatures close to the melting point of the scintillator crystal, the crystal may melt;
It is preferable to perform the heat treatment at a temperature 50° C. lower because the scintillator properties can be significantly improved. The heat treatment time is also determined by the type of scintillator, the structure of the heat treatment furnace, the heat treatment time, etc., but if the heat treatment is performed for more than 1 hour, the performance will be significantly improved, and if it is performed for less than 15 hours, it will be economical. . Therefore, the preferred time is 1 to 15 hours.

[0011]

[Operation] By heating a single crystal of a cerium-activated gadolinium silicate compound in an oxygen-poor atmosphere, coloration that causes light absorption is reduced and fluorescence output is improved. The reason for this is thought to be as follows. Ce, which is an activator, is
Generally, it exists as a trivalent ion in a crystal, and it is thought that this acts as a luminescent center. However, due to the influence of crystal growth conditions, some Ce exists in a tetravalent state, which becomes a factor that inhibits coloring and scintillation luminescence. When such a cerium-activated gadolinium silicate compound containing tetravalent Ce ions is heated in an atmosphere with little oxygen, the tetravalent ions in the crystal are reduced to trivalent, reducing coloration and improving scintillation properties. .

0012

[Example] Next, an example of the present invention will be described.

Example 1 The composition is of the formula Gd2-(x+y)LnxCeySiO5
x is 0, y is 0.005, and the melting point is 1900°C
GSO single crystals were grown using the Czochralski method. A scintillator of 30 mmφ×100 mm was cut out from the grown single crystal, and both end faces of the 30 mmφ were mirror-polished. Then, the light transmittance in the 100 mm direction and the scintillator characteristics were measured. Next, this is heated to 15
Heating was carried out at a temperature of 0.000C for 10 hours in a nitrogen atmosphere containing 0.5% by volume of oxygen. After heat treatment, the light transmittance and scintillator properties of GSO were measured. The results are shown in Table 1. Note that the energy resolution in Table 1 is 662 keV.
and the light transmittance is the value at 430 nm.

Comparative Example 1 A scintillator was heat-treated in the same manner as in Example 1 except that the atmosphere for the heat treatment in Example 1 was air.
Characteristics were measured in the same manner as in Example 1. The results are shown in Table 1.

Example 2 Formula in Example 1 Gd2-(x+y)LnxCe
In the composition of ySiO5, Ln is Lu element and x is 0.
GSO with a melting point of 1900°C when 5 and y are 0.005
A single crystal (hereinafter referred to as LGSO) was grown by the Czochralski method, and in the same manner as in Example 1, a scintillator was cut out, heat treated, and the characteristics before and after the heat treatment were measured. The results are shown in Table 1.

[0016]

[Table 1]

The following can be seen from Table 1. The fluorescent output after heat treatment was improved by 35% in Example 1 and by 10% in Example 2 compared to before heat treatment. Also 662keV
The energy resolution after heat treatment is smaller than before heat treatment, which is an improvement. Furthermore, it can be seen that the light transmittance of the crystals was also improved by the heat treatment, and the coloring was reduced. In the case of LGSO of Example 2, it is considered that the scintillator properties were improved due to a change in Ce from tetravalent to trivalent as described above.

[0018]

[Effects of the Invention] By carrying out the heat treatment of the present invention, scintillator characteristics such as fluorescence output and energy resolution of a single crystal of a cerium-activated gadolinium silicate compound can be improved. Further, by using such a scintillator with high fluorescence output as a radiation detector for positron CT, a diagnostic image with high spatial resolution can be obtained. Furthermore, it is possible to reduce variations in scintillator characteristics due to differences in single crystal growth conditions.

Claims (6)

[Claims] [Claim 1] General formula Gd2-(x+y)Lnx
CeySiO5 (here, Ln represents at least one element selected from rare earth elements, x is 0 to 2 and y
is a value greater than 0 and less than or equal to 2. 1. A heat treatment method for a single crystal of a cerium-activated gadolinium silicate compound, which comprises heating a single crystal of a cerium-activated gadolinium silicate compound represented by ) in an oxygen-poor atmosphere. 2. The method for heat treatment of a single crystal according to claim 1, wherein x is 0 and y is a value of 0.001 to 0.2. [Claim 3] Ln is Sc, Tb, Dy, Ho, Er
, represents at least one element selected from the group consisting of Tm, Yb and Lu, x is 0.1 to 1.999 and y
2. The method for heat treatment of a single crystal according to claim 1, wherein: is a value of 0.001 to 0.2. 4. The single crystal heat treatment method according to claim 1, wherein the oxygen-poor atmosphere is a nitrogen atmosphere. 5. The method for heat treatment of a single crystal according to claim 1, wherein the heating temperature is 50 to 550° C. lower than the melting point of the single crystal obtained. 6. The single crystal heat treatment method according to claim 1, wherein the heating time is 1 to 15 hours.