Abstract: Provides is low scattering silica glass suitable as a material of an optical communication fiber. Silica glass has a fictive temperature of at least 1,000° C. and a void radius of at most 0.240 nm, as measured by positron annihilation lifetime spectroscopy. A method for heat-treating silica glass is also provided, which comprises holding silica glass to be heat-treated in an atmosphere at a temperature of at least 1,200° C. and at most 2,000° C. under a pressure of at least 30 MPa, and cooling the silica glass at an average temperature-decreasing rate of at least 40° C./min during cooling within a temperature range of from 1,200° C. to 900° C. A method for heat-treating silica glass also comprises holding silica glass to be heat-treated in an atmosphere at a temperature of at least 1,200° C. and at most 2,000° C. under a pressure of at least 140 MPa, and cooling the silica glass in an atmosphere under a pressure of at least 140 MPa during cooling within a temperature range of from 1,200° C. to 900° C.

Abstract: To provide low scattering silica glass suitable as a material of an optical communication fiber. Silica glass, which has a fictive temperature of at least 1,000° C., and which has a void radius of at most 0.240 nm as measured by positron annihilation lifetime spectroscopy. A method for heat-treating silica glass, which comprises holding silica glass to be heat-treated in an atmosphere at a temperature of at least 1,200° C. and at most 2,000° C. under a pressure of at least 30 MPa, and cooling the silica glass at an average temperature-decreasing rate of at least 40° C./min during cooling within a temperature range of from 1,200° C. to 900° C. A method for heat-treating silica glass, which comprises holding silica glass to be heat-treated in an atmosphere at a temperature of at least 1,200° C. and at most 2,000° C. under a pressure of at least 140 MPa, and cooling the silica glass in an atmosphere under a pressure of at least 140 MPa during cooling within a temperature range of from 1,200° C. to 900° C.

Abstract: A reduction in the air tightness of a reaction chamber of an apparatus due to thermal degradation of a vacuum seal at the connection seal between the reaction chamber and a manifold is prevented. Contamination of the inside of the chamber from particles produced from heating members of the apparatus is reduced. The wall of the reaction chamber extends to a location sufficiently spaced from the lower end of the heater surrounding the reaction chamber. An exhaust port is provided in the wall to exhaust the reaction chamber.