Colour centres in plasma-sprayed hydroxyapatite.
Very pure hydroxyapatite is diamagnetic and shows no electron paramagnetic resonance (EPR) spectrum. Plasma-sprayed coatings made with such a pure hydroxyapatite contain paramagnetic point-defects. Irradiated deposits show a strong EPR absorption assigned to an O2- anion adjacent to a calcium vacancy, with anisotropic parameters, g1 = 2.004, g2 = 2.013 and g3 = 2.038. The EPR spectra and colour fade away following thermal annealing. These features are discussed in terms of point-defects in the hydroxyapatite crystal structure.
Electron irradjation damage in magnesium oxide
Abstract High densities of small defect clusters have been observed in MgO irradiated in the high voltage electron microscope. The threshold voltage at nominal room temperature has been determined as 480 ± 10 kV, corresponding to 64 ± 2 eV for the displacement energy of the magnesium ions. This energy is very similar to the value for oxygen ion displacement from optical absorption studies on accelerator irradiated specimens.1 The similarity of the displacement energy for the two ions is discussed and the values compared with those for other ceramic materials. Calculations of the displacement cross sections show that at 600 kV the rate of production of oxygen ions is a factor ∼1.8 higher than the rate foi magnesium ions, whereas at 1 MV the rates are very similar. The effect of increasing dose and irradiation temperature on the defect clusters has been investigated and the results are compared with neutron irradiation damage in MgO and electron irradiation damage in other materials.
Growth of magnesium oxide during neutron irradiation
Abstract The macroscopic and x-ray growth of single crystal and polycrystalline magnesium oxide after neutron irradiation at 75–100°C has been investigated. At doses below 1020 nvt the x-ray growth was found to exceed the macroscopic growth by a small but significant amount. At doses above 1020nvt the macroscopic growth continued to increase at a reduced rate but the x-ray growth decreased and rapidly became much less than the macroscopic growth. Annealing studies show that the recovery was dependent on dose and that the lattice parameter changes became negative before returning to the pre-irradiation value. An interpretation of the data is presented involving the formation of interstitial loops at doses above 1020 nvt and during annealing. Mean values of the distortion per defect are derived which correspond to the distortion averaged over all unannealed defects, whether isolated or clustered. For vacancies the distortion is a contraction having a maximum value of 0·4 atomic volumes and decreasing with i...