Advanced Scintillators for Gamma-ray Detection

Stephen E. Derenzo
Lawrence Berkeley National Laboratory and
University of California, Berkeley
January 28, 2003

Abstract:

We review currently available scintillators to understand why their energy and timing resolutions are so inferior to fundamental limits, and the advantages of an unexploited scintillation mechanism- fast radiative electron-hole recombination in direct-gap semiconductors. Previously we showed that many direct-gap semiconductors such as HgI2, PbI2, CuI, ZnO. and CdS at cryogenic temperatures have luminosities comparable to room-temperature Bi4Ge3O12, and that their scintillation light is emitted in a few ns or less. We believe that fast, efficient, room-temperature scintillation from direct-gap semiconductors should be possible by suitable doping and describe our preliminary results with a new scintillator CdS(In,Te), which has a monoexponential decay time of 3.5 ns. This is the first example of a new class of fast room-temperature semiconductor scintillators in which ionization holes are trapped by impurity atoms and then recombine radiatively with electrons provided by an impurity donor band. Because the donor band provides electrons of both spin states, the radiative transition is spin allowed and fast. Applications of luminous, fast, dense, heavy-atom scintillators in medical imaging and homeland security will be described.