Speaker
Description
Early studies of the photoconductive properties of materials eventually led to an understanding of the solid-state and particularly of a new class of materials – the semiconductors. Their sheer number and diversity of properties give rise to a wide range of sensitivities covering almost the entire electromagnetic spectrum − from terahertz to gamma-ray wavelengths. The application of semiconductors to radiation detection and measurement through an understanding of the electronic properties of matter has come to dominate radiation physics – a field that did not exist 150 years ago. Of special interest has been the rise of compound semiconductors and more recently the organic semiconductors. Compound semiconductors have a number of distinct advantages over their elemental counterparts, Si and Ge. For example, wide-gap materials offer the ability to operate in a range of hostile thermal and radiation environments while still maintaining spectral resolutions <2% FWHM at X and gamma-ray wavelengths. Narrow-gap materials offer the potential of exceeding the spectral resolution of Ge by a factor of three. However, while compounds are routinely used as detection media at infra-red, optical and UV wavelengths, for hard X- and gamma-ray applications their development has been plagued by material and fabrication problems. So far only a few have evolved sufficiently to produce commercial detection systems. Recently there have been considerable advances in the development of organic semiconductors largely spurred on by the smart phone and tablet industries. While not destined to replace silicon-based technologies, they offer the promise of low cost, fully flexible devices for large-area displays and solid-state lighting. Much research currently focuses on solar cell and energy harvesting applications, although the hybrid organic-inorganic perovskites (HOIPs), such as formamidinium lead triiodide (CH(NH2)2PbI3), are now being explored as X- and gamma-ray detection media.
As well as on-going activities in semiconductors, scintillator research has undergone a renaissance since the discovery of the cerium doped lanthanum halides in the early 1990’s − the most notable outcome of which, has been the recent availability of large volume (>350 cm3) LaBr3(Ce3+), LaCl3(Ce3+) and CeBr3 high resolution scintillation detectors, with FWHM energy resolutions of 3−4% being achieved at 662 keV. Recent experiments with co-doping, using activators such as Eu and Pr, suggest that a further improvement to 2% is possible. These results are comparable with the best results obtained with a compound semiconductor such as CdZnTe. At present, the outstanding issue in scintillation physics is understanding and mitigating the effects of energy non-proportionality which currently limits the attainable energy resolution in all scintillators.
In this talk we summarize recent developments in scintillator and semiconductor research from a materials perspective. To improve performance, underlying efforts in both should continue on achieving material perfection. In the near and intermediate terms, performance improvement for semiconductors lies in the controlled and directed manipulation of charge, for example employing single carrier sensing techniques to neutralize the degrading effects of the poorest carrier, while for scintillators, it lies with the understanding and reduction of non-proportionality effects. In the longer term, it is clear that for semiconductors conventional detection techniques based on the manipulating the electrons charge using electric fields will reach an impasse in terms of sensitivity. A major step forward should be possible, by exploiting other obscure internal degrees of freedom of the electron in addition to its charge for nonvolatile information processing. For example, utilizing its spin or alternately the valley degree of freedom implicit in the band structure of some semiconductors.
