Radiation detection

Vince Lordi, Keith Ray, Kyoung Kweon, Joel Varley, Daniel Åberg

Collaborators: Nicole Adelstein (San Francisco Statue University), Paul Erhart (Chalmers, Sweden), Cedric Rocha Leão (Univ. Fed. ABC, Brazil), Kuang-Jen Wu, Adam Conway, Lars Voss, Steve Payne (LLNL)

Efficient and accurate detection of high-energy radiation, particularly gamma and X rays, is important for national security, space imaging, medical imaging, scientific diagnostics, and other applications. Particularly for national security applications, where a large number of moderately inexpensive high-performance detectors are required, high energy-resolution is desired along with operation at room temperature. To achieve high performance in detectors using a semiconductor as the active material, the semiconductor must possess high carrier mobilities, long carrier lifetimes, high resistivity, a moderate band gap (approximately 1.4 to 2.2 eV), and high average atomic number. These requirements often conflict and also depend heavily on extrinsic properties of the crystals, such as the presence of various defects. Our work focuses on using ab initio methods to model the properties of defects in semiconductors at multiple scales to understand both fundamental and process limitations of materials for room-temperature gamma detection. We have applied these methods to a number of materials—including AlSb, GaTe, CdTe, CdZnTe, and TlBr—to assess process options for optimizing performance and achieving required specifications, and we work with experimental teams to validate and test predictions of optimal growth and process conditions.

Following is a summary of some recent results. For AlSb, the crystal growth methodology was not yet fully developed and crystals were plagued by a number of uncontrolled impurities that reduced mobility and resistivity. Our analysis of native point defects and dopants in the material as a function of growth and annealing conditions allowed us to develop a process which reduced the concentration of detrimental  oxygen impurities while compensating background carbon impurities with tellurium dopants and led to the realization of improved detector-quality crystals with increased mobility and resistivity. For Cd(Zn)Te, we have investigated the electronic properties of point defects and dislocations and also found a point defect complexing mechanism that leads to Te clustering. We further quantified the effects of Zn alloying on point defects in CdZnTe. For TlBr, which exhibits high energy-resolution at room temperature but degrades over time, we have examined the fundamental vacancy-related properties associated with mixed electronic and ionic conductivity. We also investigated the optimization of device performance by tailoring dopants and metal contact materials. Additional current work includes determination of the electronic properties of glassy materials, as hosts for scintillator detectors.
 
Figure: Crystal defects in semiconductors can sensitively affect the electronic properties, which impact the performance of high-energy radiation detectors.
 
Funding from the NNSA Office of Nonproliferation and Verification Research and Development (NA-22) is acknowledged.
 
Contact:  Vince Lordi