Calculation of Energy Levels of Cerium in Inorganic Scintillator Crystals
- PDF / 674,978 Bytes
- 11 Pages / 414.72 x 648 pts Page_size
- 42 Downloads / 219 Views
ABSTRACT Fully relativistic ab initio calculations have been performed on energy levels of cerium in BaF2 , LaF 3, YAP and YAG. Also nonrelativistic calculations were done on cerium in LSO. The results are in fair agreement with experiment. Both the splitting of the 4f and 5d levels and the 4f-5d energy gap can be explained in the high symmetry crystal BaF 2 as well as in the low symmetry crystals LaF 3, YAP, YAG and LSO. The more traditional nonrelativistic approach is not capable to handle 4f-5d energy differences but can be used for the description of the 5d state and other properties which do not critically depend on the 4f state. We have estimated the position of the cerium levels in the gap of the host materials. So important information for luminescence of cerium centres is obtained. INTRODUCTION The doping of ionic crystals with cerium can provide crystals with good scintillation properties on X-ray or gamma-ray irradiation. These properties are: large light output in photons/MeV, a short decay time, and a wavelength of the emitted light lying in a range suitable for efficient detection. Other important properties are radiation hardness, high density and atomic number and last but not least it must be possible to grow large single crystals.
The process of scintillation is very complex, but three major stages can be defined, in each of which several mechanisms can play a role. In the first stage the radiation produces primary very hot electrons which in turn produce a cloud of secondary hot electrons thermalizing to electron hole pairs of low energy. Then mostly already 90 percent of the radiated energy is dissipated. In the second stage electron hole pairs very likely form excitonic centres of many forms in the scintillation crystals. We have the process of energy transfer from these centres to the scintillation centres, for instance the cerium defects studied in this paper. In the third stage the scintillation centres radiate the energy. These scintillation centres are localized electronic states in the crystal and ionic centres are the best. Some rare earth ions provide fast scintillation centres with narrow lines and particularly cerium provides scintillation light with a favourable wavelength in the 300 to 600 nm range. In figure 1 a schematic overview is given of the scintillation process.
IThis study in the program of the Foundation for Fundamental Research on Matter (FOM) has been supported by The Netherlands Technology Foundation (STW) and by The National Computer Facility (NCF)
355 Mat. Res. Soc. Symp. Proc. Vol. 348. @1994 Materials Research Society
The actual subprocesses or mechanisms are quantitatively unknown and a good theory must still be developed. Also experimentally little has been measured of properties directly related to the mechanisms involved. However this is not true for the last stage, the luminescence of the scintillation centres. Good theoretical models exist for ionic centres and accurate experimental facts are available about these centres, particularly for cerium. In appe
Data Loading...
