Th. Pawlik

X-ray storage phosphors

Abstract In X-ray storage phosphors an image is formed and stored by generation of room temperature stable radiation-induced electron and hole trap centres. The image is read-out by recording a photo-stimulated luminescence (PSL) from a doped activator, generally stimulating the electron trap centres. The best-known and hitherto most efficient X-ray storage phosphor is BaFBr:EuZ+. However, the exact mechanism of its functioning is not yet understood. The present discussion of the storage and read-out mechanisms is critically reviewed. New results about the role of oxygen Contamination of BaFBr:Eu2+ are presented: if there is much less oxygen present than Edc, then the PSL efficiency decreases and the stimulation energy increases. A new efficient X-ray storage phosphor is presented: Cs2NaYF6 doped with trivalent rare earth activators. Its properties are described and preliminary results on X-radiation-induced radiation damage centres in undoped Cs2NaYF6 are presented.

Electron paramagnetic resonance investigations of nickel defects in natural diamonds

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectra of natural blue diamonds from the Argyle mine in Western Australia are reported for the first time. These diamonds are shown to contain the NE2 centre which has been observed primarily in synthetic diamonds and has been proposed to be with three neighbouring nitrogens. A previously unreported EPR centre is observed and is found to have S = 3/2, g = 2.02 and superhyperfine interaction from one nitrogen. The results are found to be consistent with an on a substitutional site with an on a fourth nearest neighbour site. This defect is found exclusively in Argyle blue diamonds.

Hole‐trapping sites and the mechanism of the photostimulated luminescence of the x‐ray storage phosphor RbI:Tl+

Radiation damage centers generated by x irradiation at 80 and 300 K in the storage phosphor RbI:Tl+ were investigated with optically detected magnetic resonance and optical measurements. It is shown that the intensity of the photostimulated luminescence depends on the concentration of Tl2+ centers. Bleaching into any of the seven identified Tl2+ absorption bands destroys the Tl2+ centers and leads to a Tl+ emission as well as to VK centers. Proportionally to photodestruction of Tl2+ centers, the photostimulated luminescence is decreased. The traps for information storage in RbI:Tl+ are F centers and Tl2+ centers.

Electron and hole centres in the x-irradiated elpasolite crystal studied by means of electron paramagnetic resonance and electron nuclear double resonance

We report on the first investigation of radiation-induced defects in the cubic elpasolite produced by low-temperature x-irradiation. Upon x-irradiation at 30 K a -type hole centre and an electron centre identified as are formed. The centre is stable up to 70 K. At this temperature it recombines with a proportion of the centres. The remaining centres decay at approximately 150 K. The properties of the centre in are very similar to those of the centre in NaCl. The centre can be described by a Jahn - Teller distorted ground state. In ENDOR experiments, evidence was found for a reorientation of the centre complex.

Investigation of radiation-induced defects in Cs2NaYF6

We present a detailed study of the radiation damage centres produced in Cs 2 NaYF 6 upon X-irradiation at 10 and 300 K. The defects were investigated using magnetooptical and magnetic resonance spectroscopy. As hole trap centres at low temperatures VK centres and HA-type centres were identified by EPR and their optical transitions by optical absorption and the magnetic circular dichroism of the absorption. As electron trap centres F centres were investigated in detail by ENDOR.

Investigation of the x-ray storage phosphors Cs2NaYF6:Pr3+ or Ce3+

We present Cs2NaYF6:Ce3+ or Pr3+ as new x-ray storage phosphors. The radiation damage centers upon x irradiation were investigated using magneto-optical and magnetic resonance spectroscopy. F centers are the photostimulable electron trap centers. For the activator Pr3+ it could be shown that upon room temperature x irradiation only F centers and Pr4+ hole trap centers are involved in the information storage and readout process. There are indications that the process is the same for Ce3+-doped Cs2NaYF6.

Optically detected electron paramagnetic resonance of Ni-related defects in synthetic diamond crystals

Synthetic diamond crystals grown using a solvent catalyst that contains Ni were studied by optical detection of electron paramagnetic resonance (ODEPR) using the magnetic circular dichroism of the optical absorption (MCDA). The MCDA spectra in the infrared spectral region consist of a derivative-like line at 1.06 eV, single sharp lines at 1.29 and 1.40 eV, a complex phonon-split band with the zero-phonon line (ZPL) at 1.69 eV and a derivative-like doublet structure superimposed at 1.72 eV. All of these optical absorptions were shown to originate from paramagnetic defects. Using ODEPR it was possible to unambiguously assign the 1.72 eV doublet to the NE4 centre and the 1.40 eV line to the NIRIM-2 centre. The defect responsible for the ZPL at 1.06 eV has the same symmetry as the NIRIM-2 centre with a slightly larger -value. The NE4 centre was assigned to substitutional associated with a carbon vacancy in a nearest-neighbour position whereas the NIRIM-2 centre was assigned to interstitial with a distortion along a [111] axis.

Mn2+ as a probe in RbCaF3: local order parameter of the structural phase transition measured by ENDOR

The 195 K cubic-to-tetragonal structural phase transition (SPT) in RbCaF3 crystals has been investigated by electron nuclear double resonance (ENDOR) using Mn2+ as a probe. The rotational angle of the fluorine octahedra, which is the local order parameter of the transition, has been determined at 40 K for the first and second shells of F- ions surrounding the Mn2+ impurities. The values obtained are 5.1 degrees +or-0.2 degrees for the first shell and 7.4 degrees +or-1 degrees for the second shell. The thermal evolution of the first shell angle at temperatures close to that of the SPT has also been studied. The results have been compared with those in RbCdF3:Mn2+. The first shell angle measured at 40 K is similar to the one in RbCdF3:Mn2+. The second shell angle is almost double that in RbCdF3:Mn2+. Covalency effects through Cd2+ ions are suggested to account for this difference.

An electron - nuclear double-resonance study of the F centre in CsBr

Electron - nuclear double-resonance (ENDOR) measurements were performed on the F centre in CsBr using the stationary ENDOR method. Five Cs and three Br shells could be resolved. The observed superhyperfine interactions cannot be explained by a spherically symmetric F-centre wavefunction. Lattice-ion overlaps and an admixture of -type functions (L = 4) have to be included in the description of the F-centre wavefunction.

ENDOR investigation of cubic Fe3+ centres in NaCl single crystals

Cubic Fe3+ centres in NaCl single crystals have been studied with electron nuclear double resonance (ENDOR) at T=17 K. The superhyperfine (SHF) and quadrupole interactions of the paramagnetic ion with the nearest set of four Cl- anions and the second-nearest set of twelve Na+. Cations have been determined. The results confirm the structural model of an Fe3+ ion at a tetrahedrally symmetric interstitial position with the nearest four Na+ cations missing, as suggested previously from the analysis of the SHF structure of their EPR spectra.