The unexpected benefits of impurities

State-of-the-art scintillators such as LaBr3 and SrI2 offer much better energy resolution than “classical” materials such as NaI and CsI. Further improvements are, however, needed to meet the requirements set for reliable identification of different radioactive isotopes. It was recently experimentally found that the energy of resolution Ce-doped LaBr3 can be substantially improved from 2.6 to 2.0% by co-doping with Sr, Ca, or Ba. We have published a short account in Applied Physics Letters as well as an extensive analysis in Physical Review B in which we propose a mechanism that can explain this at first sight peculiar observation.

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Using a combination of first-principles calculations and thermodynamic modeling it is shown that (i) Br vacancies are the primary electron traps during the initial stage of thermalization of hot carriers, prior to hole capture by Ce dopants; (ii) isolated Br vacancies are associated with deep levels; (iii) Sr doping increases the Br vacancy concentration by several orders of magnitude; (iv) SrLa binds to VBr resulting in a stable neutral complex; and (v) association with Sr causes the deep vacancy level to move toward the conduction band edge. The latter is essential for reducing the effective carrier density available for Auger quenching during thermalization of hot carriers. Subsequent de-trapping of electrons from SrLa-VBr complexes then can activate Ce dopants that have previously captured a hole leading to luminescence. This mechanism implies an overall reduction of Auger quenching of free carriers, which is expected to improve the linearity of the photon light yield with respect to the energy of incident electron or photon.

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