The Trapping and Release of Injected Hydrogen Isotopes in Vitreous Silica

Abstract

Abstract The behavior in vitreous silica of hydrogen isotopes, which had been introduced by 80-keV D+-ion implantation, thermal D2 doping, or recoil tritium implantation (2.7 MeV), was studied by means of FT-IR spectroscopy, ESR spectroscopy, and radioassay with a liquid scintillation counter. The hydrogen isotopes are trapped by forming hydroxyl bonds (OD, OT). In the D+-ion implantation, OD bonds are formed mainly by the rupture of the ≡Si–O–Si≡ bonds. In the thermal D2 doping, however, OD bonds are formed mainly by the isotope-exchange reaction between the preexisting OH bonds and D2 molecules. When vitreous silica implanted by recoil tritium is heated to about 1150 K, HTO and HT are released. The OD bonds produced by D+ implantation decrease upon annealing at 1150 K. The OD bond-rupture is controlled by the diffusion of hydrogen isotopes trapped as OD(OT) bonds to the surface of vitreous silica. The mechanism of the diffusion of the hydrogen isotopes through vitreous silica was explained in terms of a repetition of the chemical reaction of ≡SiOD(OT)+≡SiOSi≡→≡SiOSi≡+SiOD(OT). The diffusion coefficient (DTHTO)) of tritium according to the above mechanism was obtained as: D T(HTO)=2.3×10−4exp[−1.7×102(kJ mol−1)⁄RT]cm2s−1