Single crystal heat treatment method

1. A single crystal heat treatment method, comprising a step of heating a single crystal of a cerium-doped silicate compound represented by general formula (1) or (2) below in an atmosphere containing an oxygen concentration of 300 vol ppm or less, at a temperature T 1 (units: ° C.) that satisfies the conditions represented by formula (3) below and at a time of from 1 to 20 hours

Y 2−(x+y) Ln x Ce y SiO 5   (1)

(wherein Ln represents at least one elemental species selected from a group consisting of elements belonging to the rare earth elements, x represents a numerical value from 0 to 2, and y represents a numerical value greater than 0 but less than or equal to 0.2)

Gd 2−(z+w) Ln z Ce w SiO 5   (2)

(wherein Ln represents at least one elemental species selected from a group consisting of elements belonging to the rare earth elements, z represents a numerical value greater than or equal to 0.4 but less than or equal to 2, and w represents a numerical value greater than 0 but less than or equal to 0.2)

800≦ T 1 ≦( T m1 −580)  (3)

(wherein T m1 (units: ° C.) represents the melting point of the single crystal), and

wherein prior to said step of heating a single crystal of a cerium-doped silicate compound the single crystal is grown or cooled in an oxygen-containing atmosphere, or is heated in an oxygen-containing atmosphere.

2. A single crystal heat treatment method, comprising a step of heating a single crystal of a cerium-doped gadolinium silicate compound represented by general formula (4) below in an atmosphere containing an oxygen concentration of 300 vol ppm or less, at a temperature T 2 (units: ° C.) that satisfies the conditions represented by formula (5) below and an time of from 1 to 20 hours

Gd 2−(p+q) Ln p Ce q SiO 5   (4)

(wherein Ln represents at least one elemental species selected from a group consisting of Dy, Ho, Er, Tm, Yb, Lu, Y and Sc, which are rare earth elements having an ionic radius smaller than Tb, p represents a numerical value greater than or equal to 0.4 but less than or equal to 2, and q represents a numerical value greater than 0 but less than or equal to 0.2)

800 ≦T 2 ≦( T m2 −580)  (5)

(wherein T (units: ° C.) represents the melting point of the single crystal), and

wherein prior to said step of heating a single crystal of a cerium-doped silicate compound the single crystal is grown or cooled in an oxygen-containing atmosphere, or is heated in an oxygen-containing atmosphere.

3. A single crystal heat treatment method, comprising a step of heating a single crystal of a cerium-doped gadolinium silicate compound represented by general formula (6) below in an containing an oxygen concentration of 300 vol ppm or less, atmosphere at a temperature T 3 (units: ° C.) that satisfies the conditions represented by formula (7) below and at a time of from 1 to 20 hours

Gd 2−(r+s) Lu r Ce s SiO 5   (6)

(wherein r represents a numerical value greater than or equal to 0.4 but less than or equal to 2, and s represents a numerical value greater than 0 but less than or equal to 0.2)

800 ≦T 3 ≦( T m3 −580)  (7)

(wherein T m3 (units: ° C.) represents the melting point of the single crystal), and

wherein prior to said step of heating a single crystal of a cerium-doped silicate compound the single crystal is grown or cooled in an oxygen-containing atmosphere, or is heated in an oxygen-containing atmosphere.

4. The single crystal heat treatment method according to any one of claims 1 - 3 , wherein the oxygen-poor atmosphere contains hydrogen gas as a reducing gas in a concentration of 0.5 vol % or greater.

5. The single crystal heat treatment method of claim 1 , wherein the grown crystal is cut and processed to a predetermined dimension prior to said step of heating a single crystal of a cerium-doped silicate compound.

6. The single crystal heat treatment method according to claim 1 , wherein said temperature T 1 also satisfies the relationship 1,200° C.≦T 1 ≦1,400° C.

7. The single crystal heat treatment method according to claim 2 , wherein said temperature T 2 also satisfies the relationship 1,200° C.≦T 2 ≦1,400° C.

8. The single crystal heat treatment method according to claim 3 , wherein said temperature T 3 also satisfies the relationship 1,200° C.≦T 3 ≦1,400° C.