Electron paramagnetic resonance study of a Eu2+ related defect in CsBr:Eu needle image plates for computed radiography

The electron paramagnetic resonance (EPR) spectrum of needle image plates of CsBr doped with Eu2+, which are proposed as new X-ray storage phosphors for computed radiography, is studied at room temperature and Q-band microwave frequencies (34 GHz). X-ray diffraction analysis demonstrates that the CsBr:Eu2+ needles have an 〈0 0 1〉 out of plane (perpendicular to the plate) orientation, and contrary to expectation that the in plane orientation is not random. The room temperature EPR spectrum is attributed to a single centre which is related to Eu2+ with axial 〈0 0 1〉 symmetry. Using the spin Hamiltonian parameters extracted from the spectrum recorded with the magnetic field parallel to the needles' axes, we convincingly simulate both the spectrum of a powdered image plate and the single crystal like angular dependence of intact pieces of image plate. The knowledge of the symmetry of this centre, which appears to be related with the radiation sensitivity of the plate, presents a first step in finding its model and role in the X-ray storage process.