Excitonic and recombination processes in CaWO4 and CdWO4 scintillators under synchrotron irradiation

Excitation spectra of intrinsic (IL) and extrinsic luminescence (EL) from CaWO4 and CdWO4 crystals, measured using synchrotron radiation (4-30 eV) at 8 K, have been analysed. In CaWO4 the IL (2.9 eV) is efficiently excited by photons of 5.5-7.0 eV at the creation of molecular excitons. The EL is efficiently excited in interband transitions (hv > 6.8 eV). The energy threshold for the multiplication of electronic excitations is E-t approximate to 15 eV in CaWO4 and E-t approximate to 10 eV CdWO4. In CdWO4, a fraction of the molecular excitons undergoes autoionization due to an overlap with the continuum of the interband transitions. Fast emission (<2 ns) is found at 80 K under the pulse excitation by electrons with an energy of 300 keV. The continuous temperature-independent emission of CaWO4 crystals in the region of 2-5 eV is interpreted as an intraband luminescence. The emission at 5.5 eV is ascribed to a metastable molecular excitons. (C) 1998 Elsevier Science Ltd. All rights reserved.