Semiconductor Nuclear and X-Ray Detectors
Since RMD’s inception in 1974, the company has continuously engaged in developing semiconductor materials for applications in ionizing radiation detection. Similar to its initial work with CdTe detectors, this endeavor primarily focuses upon ‘room-temperature’ operable semiconductor detectors. These are materials that notably possess large electronic bandgaps, which intrinsically helps to minimize electronic noise without the need for cryogenic cooling. This latter characteristic, along with many of these materials’ possessing high efficiency, makes them attractive for portable and handheld applications where instrument size and weight are key characteristics. Additionally, these materials can offer greater energy resolution, or scatter rejection, because of higher conversion gains as compared to scintillation detectors.
Research encompasses identifying materials, developing methods to both purify and grow crystalline ingots, processing materials into operable detectors, and investigating electronics and algorithms to best extract signal data.
Material selection can originate from many sources – variants of more established (or earlier studied) compounds, theoretical modeling or literature searches. A desirable attribute is to utilize high atomic number materials that can provide efficient x- and γ-ray detection. Analogous neutron detection schemes require materials possessing high interaction cross-sections. Electronic properties such as bandgap and charge mobility are also key factors. Motivation can also extend to pragmatic aspects of production, such as finding materials that are less costly to prepare or less susceptible to defects.
RMD generally utilizes melt-grown methods for producing ingots of semiconductors. Sizes may range from a few cm3 when first exploring electronic properties, to over a kg (in mass) for materials where large substrates are needed for imaging applications. It has also been a customary aspect of RMD’s study to employ extensive purification methods (e.g. zone refining) to improve electronic behavior.
In recent years, the semiconductor-material thallium bromide (TlBr) has been the focus of considerable research due to its promising electronic properties and extraordinarily high detection efficiency. The high detection efficiency arises from its very high density and effective atomic number. The material had been studied by RMD and others in the late 1980’s / early 1990’s where the initial promise was demonstrate. Subsequently, in 2000’s it was discovered that considerably greater purification was essential to demonstrate the full potential of TlBr. Since that point, it has become sought as an alternative material that may provide lesser costs, and function in suitable roles for homeland security and nuclear medicine.
RMD produces TlBr detectors in a range of sizes and types – single element devices generally vary from 5 mm3 cubes to ~ 1 cm3 planar devices, and multi-element pixelated arrays can reach as large as 4 cm2. Devices can be fabricated with differing electrode materials and patterns, and high density polymer bump bonds can also be applied.
Depending upon device design, spectroscopic TlBr detectors provide energy resolutions typically within a range of 1.5% to 3% FWHM (at 662 keV), situating their performance between high performing inorganic scintillators such as LaBr3 and very high resolution semiconductor CZT detectors. RMD and its collaborators have also been active in addressing stability issues associated to TlBr.
RMD’s current work with TlBr extends beyond detector fabrication and now includes exploring instrument designs for security applications such as personal radiation detectors (PRDs) and radioisotope identifiers (RIIDs).
Detecting thermal neutrons is a specialized area of instrument design because only a few, limited chemical isotopes have sufficient sensitivity (via neutron absorption cross-section). By-products such as charged particles and ions provide the true means of detection, for example, typical reactions such as 10B(n, α)7Li or 6Li(n, α)3H. Finding materials that can include such key isotopes is vital to developing new neutron detectors – an urgent need in light of diminishing supplies of 3He.
RMD is developing a technology based on the lithium selenoindate (LiInSe2) to produce solid-state neutron detectors. Similar to other semiconductors studied by RMD, this a wide bandgap material capable of room temperature operation which produces signals induced through ionization. Separating neutron signals from those created by gamma-rays is easily accomplished through pulse height discrimination, as the neutron events yield over 4 MeV energy. RMD has grown ingots of LiInSe2 to 1” diameter and utilized both natural and enriched forms of Li (the enriched form can provide 90% neutron efficiency in only a 2 mm thickness).
LiInSe2 based semiconductors
Wide Band Gap – Photoconductors
Wide band gap semiconductors offer the potential for low noise photodetection with high quantum efficiency (QE). The wide variety of scintillators now available – bringing options in efficiency, light output and speed – also produce a range of spectral outputs. Matching new scintillators with photoconductors of similar QE response is necessary towards extracting their full potential. Additionally, there are many applications where the compactness of a solid-state device is desirable, but the lesser QE of a silicon photoconductor compromises detector energy resolution. By varying the composition of wide bandgap ternary compounds, or simply better exploiting binary materials, the band gap can be tuned and QE can be optimized to efficiently detect light from a scintillator.
RMD’s experience with semiconductor growth allows development of customize materials for desired wavelength response. Materials such as TlBr can be alloyed with related compounds (e.g. other thallium halides, TlBr-TlI-TlCl) to alter absorption edge and responsivity (see color change below).
RMD’s expertise in developing crystalline semiconductor detectors has lead to the study of alternative uses in digital radiography. Several of these primarily high-resistivity materials can also be fabricated into polycrystalline layers, while retaining sufficient key electrical properties to operate as integration-mode image sensors. Commonly known as ‘direct converter’ films, semiconductors offer the potential of high conversion efficiency without the image blur that normally accompanies thick scintillator screens. The intrinsic benefit to these materials – TlBr, PbI2 and HgI2 – high stopping efficiency at radiographic energies also enables use of thinner films, requiring only a few hundred µm. These materials can also be fabricated into large area films using low temperatures processes that are compatible with multiple technologies, most notably being a-Si thin film transistor arrays. Specific methods that RMD has studied included physical vapor deposition of PbI2 and TlBr, and particle-in-binder formulations of HgI2.