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Age‐momentum measurements and positron lifetime measurements were carried out in order to investigate vacancy‐type defects produced in 3 MeV electron irradiated synthetic silica glasses, in which 1 ppm and 300 ppm of –OH bonds were contained. A high formation probability of positronium (Ps) was found for unirradiated specimens. These Ps atoms were considered to be formed in open‐volume defects (∼7×10-2 nm3). The formation probability of Ps was drastically decreased by the electron irradiation, especially for the specimens which contained 1 ppm of –OH bonds, even though the size of the open‐volume defects and the momentum distribution of electrons at the open‐volume defects are independent on the –OH concentration and on the electron irradiation. Since the values of second lifetime components of positron lifetime spectra were ∼300 ps and very close to the estimated value of the lifetime of the positrons which are considered to be trapped in the mono‐ or divacancies, the decrease of formation probability of Ps can be attributed to an increase of positron annihilation in vacancy‐type defects generated by the electron irradiation. Positrons were suspected to be trapped in such defects.