Mattias Klintenberg

The Quest for the Ideal Inorganic Scintillator

The past half century has witnessed the discovery of many new inorganic scintillator materials and numerous advances in our understanding of the basic physical processes governing the transformation of ionizing radiation into scintillation light. Whereas scintillators are available with a good combination of physical properties, none provides the desired combination of stopping power, light output, and decay time. A review of the numerous scintillation mechanisms of known inorganic scintillators reveals why none of them is both bright and fast. The mechanisms of radiative recombination in wide-band gap direct semiconductors, however, remain relatively unexploited for scintillators. We describe how suitably doped semiconductor scintillators could provide a combination of high light output, short decay time, and linearity of response that approach fundamental limits.

Lu2SiO5 by single-crystal X-ray and neutron diffraction.

The structure of dilutetium silicon pentaoxide, Lu2SiO5, has isolated ionic SiO4 tetrahedral units and non-Si-bonded O atoms in distorted OLu4 tetrahedra. The OLu4 tetrahedra form edge-sharing infinite chains and double O2Lu6 tetrahedra along the c axis. The edge-sharing chains are connected to the O2Lu6 double tetrahedra by isolated SiO4 units. The structure has been determined by neutron diffraction.

Determining point charge arrays that produce accurate ionic crystal fields for atomic cluster calculations

In performing atomic cluster calculations of local electronic structure defects in ionic crystals, the crystal is often modeled as a central cluster of 5–50 ions embedded in an array of point charges. For most crystals, however, a finite three-dimensional repeated array of unit cells generates electrostatic potentials that are in significant disagreement with the Madelung (infinite crystal) potentials computed by the Ewald method. This is illustrated for the cubic crystal CaF2. We present a novel algorithm for solving this problem for any crystal whose unit cell information is known: (1) the unit cell is used to generate a neutral array containing typically 10 000 point charges at their normal crystallographic positions; (2) the array is divided into zone 1 (a volume defined by the atomic cluster of interest), zone 2 (several hundred additional point charges that together with zone 1 fill a spherical volume), and zone 3 (all other point charges); (3) the Ewald formula is used to compute the site potential...

Accurate crystal fields for embedded cluster calculations

Abstract Local electronic structure defects in ionic crystals is commonly modeled using embedded cluster calculations. In this context we describe how to embed the quantum cluster (QC) in an array of point charges. Specifically, the method calculates an array of point charges that reproduces the electrostatic potential of the infinite crystal within an accuracy usually μ V in the interior of the QC.

Band structure calculations of cerium activated inorganic scintillators

Abstract Trivalent cerium has been used extensively in recent years as an activator in crystalline hosts for fast scintillators. The scintillation efficiencies have been found to vary greatly, however. For example, no scintillation is observed from Ce-doped Y2O3 and Lu2O3, the efficiency is low for CeF3, whereas the efficiency is very high for Lu2SiO5:Ce. This behavior has been attributed in large part to the location of the ground 4f and excited 5d levels of Ce3+ with respect to the fundamental band gap of the host. We have performed band structure calculations to predict the band gaps and the location of the Ce and Lu 4f and 5d levels in the fluoride host CeF3 and in the oxide host LuAlO3. Calculations were made using a full-potential linear muffintin orbital (FP-LMTO) approach. We find that Ce 4f levels in CeF3 are several eV above the top of the valence band (VB), thus reducing the probability of hole capture. The Lu 4f levels in LuAlO3 are predicted to be several eV below the top of the VB. Both of these results are in agreement with experiment. Preliminary calculations on LuAlO3:Ce locate the Ce 4f energy levels approximately 3 eV above the VB (O2p) which is not consistent with the high scintillation efficiency of LuAlO3:Ce.

First-principles calculations of hole trapping and transport: Effects on scintillator luminescence

Abstract An ab initio molecular orbital program is applied to clusters to compute energies and electron densities that describe the formation and transport of holes produced by ionizing radiation and their effects on luminescence of inorganic scintillator crystals. The resulting electron density plots for CsI show that a relaxed hole is shared equally by two bound I atoms (the classic V k center). The calculated energy barrier for motion of the self-trapped hole is in excellent agreement with that expected from experimental measurements of the temperature dependence of the slow scintillation rise time of Tl-activated CsI. For CaF 2 initial V k center formation is again predicted. The calculated energy barrier against hole migration is in reasonable agreement with the experimental value, however the scintillation rise time of Eu 2+ -activated CaF 2 is very fast. In this case the Eu 2+ activator promptly captures a hole before it can become self-trapped.

Ab-initio cluster calculations of hole formation and trapping in PbF/sub 2/ and PbF/sub 4/ [scintillation]

We have used ab-initio quantum chemistry computer codes to model the formation of holes and the energy barriers for their diffusion in two lead fluoride hosts of potential interest for scintillation-PbF/sub 2/ and PbF/sub 4/. The crystals were modeled by Pb/sub 24/F/sub 48/ and Pb/sub 14/F/sub 56/ atomic clusters embedded in arrays of several thousand point charges to reproduce the Madelung potential to an accuracy of several mV throughout the cluster. Cubic PbF/sub 2/ has the same crystal structure as CaF/sub 2/, however their electronic structures are different. It is known experimentally that in CaF/sub 2/ holes travel easily along rows of F atoms which accounts for the high luminous efficiency of the scintillator CaF/sub 2/:Eu. In contrast, these calculations show that in PbF/sub 2/ holes are trapped on the Pb atoms by an energy barrier of /spl sim/1 eV. This result is consistent with the failure of PbF/sub 2/ as an activated scintillator. Similar calculations on the experimentally unexplored crystal PbF/sub 4/ predict that the holes are trapped on F atoms with an energy barrier of /spl sim/1 eV and is therefore not a promising host for an activated scintillator. These computational techniques can be applied to other crystals to find those with mobile holes for new heavy-atom scintillators and solid-state detectors.

On the Coulomb operator for embedded cluster calculations in periodic systems

Abstract In performing ab-initio or DFT embedded cluster calculations in infinite periodic systems one unsolved problem is how to embed the quantum cluster in the general case. We show that the Ewald real space sum can be implemented as an operator in the Hamiltonian when the reciprocal space sum is made small.