[en] We report electron spin resonance (ESR) measurements of E'-center (a ''trivalent silicon'' center in SiO₂) density as well as capacitance versus voltage (C-V) measurements on γ-irradiated metal/oxide/silicon (MOS)= structures. We also report a considerable refinement of earlier ESR measurements of the dependence of radiation-induced P/sub b/ -center (a ''trivalent silicon'' center at the Si/SiO₂ interface) occupation as a function of the Fermi level at the Si/SiO₂ interface. These measurements indicate that the P/sub b/ centers are neutral when the Fermi level is at mid-gap. Since the P/sub b/ centers are largely responsible for the radiation-induced interface states, one may take ΔV/sub mg/ C/sub ox//e (where ΔV/sub mg/ is the ''mid-gap'' C-V shift, C/sub ox/ is the oxide capacitance, and e is the electronic charge) as the density of holes trapped in the oxide. We find that radiation-induced E' density equals ΔV/sub mg/ C/sub ox//e in oxides grown in both stream and dry oxygen. Etch-back experiments demonstrate that the E' centers are concentrated very near the Si/SiO₂ interface (as are the trapped holes). Furthermore, we have subjected irradiated oxide structures to a sequence of isochronal anneals and find that the E' density and ΔV/sub mg/ annealing characteristics are virtually identical. We conclude that the E' centers are largely responsible for the deep hole traps in thermal SiO₂ on silicon