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Electron-spin-resonance studies have been conducted at 9.2 and 23.1 GHz on synthetic calcium hydroxyapatite single crystals after exposure to ionizing radiation. The major paramagnetic defect is a spin-1/2 holelike center which is observable at 92 K but not at room temperature. It exhibits hyperfine interaction with a hydrogen nucleus as proved by deuterium substitution. The center is believed to be an 0 ion which results from removal of hydrogen from an OH ion. The hyper6ne interaction is w1th the hydI'ogen of an adjacent OH Ion. In first approximation the defect 1s axially symmetric with respect to the pseudohexagonal c axis; the spin-Hamiltonian parameters are g~I --2.0018~0.0002, g~= 2.0683~0.0002, A~~---5.6~0.2 G, and A j = +5.9~0.2 G. "Forbidden" transitions -+ -+ EM, =~1 cause complex unresolved spectra for orientations intermediate between H j[c nnd H i c. To Gt computer-simulated spectra to the experimental spectra, it was necessary to postulate that the hypefflne interaction is with an OH ion tilted 6 or 7 relative to c in six geometrically inequivalent directions. The isotropic hyperfine interaction was calculated using an approximate wave function constructed by a linear combination of orbitals from the 0 and OH ions. This establishes that the hole is strongly localized on the 0 and gives the absolute signs of the hyperfine components. the investigations of Shcherbakova et aE ' and of Gilinskaya et aE. ' on mineral fluorapatite crystals. The latter study is of particular interest since it reports the presence of the CO, and COS centers substituted for PO& in the apatite structure. This substitution is of interest because of its relation to carbonate impurity in biological crystals. Some similar studies on synthetic single crystals of chlorapatite, Ca, (POt), C1, have been reported. Banks et aE. '7 have studied by ESR the substitution of CrO~for PO', in chlorapatite single crystals. ESR investigations of Mna' substituted at the calcium sites were done by Piper' and Piper and I rener. " Knotterus et aE. 19 described an optical and ESR study of several electx'on-excess and bole-excess centex'8 1Q 1x'1Rd1Rtedchlorapatite. Hovrever, the models that these authors ploposed fox' two of the holellke centex'8 where disputed by Roufosse, Stapelbroek, Bartram, and Gilliam, " whose computer-simulated fit of the spectrum and agxeement between observed and calculated hyperfine parameters for one of the defects

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