Stephen E. Derenzo, Marvin J. Weber, and Mattias K. Klintenberg,
Lawrence Berkeley National Laboratory

Research Objectives

The purpose of this research is to discover improved inorganic scintillator crystals for the detection of gamma rays in medical imaging, guided by computer simulations of critical processes.

Computational Approach

The crystal is modeled as an atomic cluster of 30 to 50 atoms embedded in an array of thousands of point charges optimized to reproduce the electrostatic field of the infinite crystal. The Schrödinger equation is solved for the embedded atomic cluster using the Jaguar quantum chemistry package. This provides the molecular orbitals and energies for each of the typically 500 electrons in the system and the total energy of the system. The energies for ground, hole, electron and excited states are computed for various atomic geometries to determine the relaxed configuration, the barrier configuration for hole transport, and the configurational overlap that causes non-radiative quenching. We also compare electron, hole, and excited state energies with and without an impurity atom to determine whether the initial reaction is electron capture or hole capture, and to model the subsequent capture of the other carrier to form the excited state.

Images of the spatial distributions of the two electrons that describe an exciton (excited state) in the cesium iodide crystal, determined by solving the Schrödinger equation for a system of 520 electrons, 65 nuclei, and 8127 point charges. Left panel shows the charge distribution of the lower energy electron, which appears as two dumbbells, each concentrated on an iodine ion. Right panel shows the charge distribution of the higher energy electron, which is spread out over all 65 ions. Horizontal and vertical coordinates are in Ångstroms, and the color scale is the power of ten of the electron density.

Accomplishments

(1) The first development of a general method for determining optimized point charge arrays that accurately reproduce the electrostatic field of the infinite crystal for any crystal whose structure is known. (2) Development of methods for computing the energy barrier for hole transport and their application to CsI, PbF2, PbF4, and CaF2. (3) Modeling of the ultra-fast (<100 ps) hole transport that occurs in CaF2:Eu and CdS:Te. (4) Modeling the impurity conduction bands in ZnO:Ga and CdS:In in support of the valence-conduction recombination theory that explains their fast, bright optical emissions and their electrical conductivity.

Significance

The fields of medical imaging, high energy physics, nuclear physics, and astrophysics would greatly benefit from a scintillator with high density and improved light output and response time.

Publications

S. E. Derenzo and M. J. Weber, "Prospects for first principle calculations of scintillator properties," Nucl. Instr. Meth. A 422, 111 (1999).

W. W. Moses, S. E. Derenzo, and M. J. Weber, "Prospects for dense, infrared emitting scintillators," IEEE Trans. Nucl. Sci. NS-45, 462 (1998).

S. E. Derenzo, M. Klintenberg, and M. J. Weber, "Ab initio computations of hole transport and excitonic processes in inorganic scintillators," in Proceedings of the Third International Conference on Excitonic Processes in Condensed Matter (EXCON '98), edited by R. T. Williams and W. M. Yen (Boston, 1998), pp. 391-402.

http://cfi.lbl.gov/instrumentation/