Gamma-rays enter new dimension
Researchers in the US have created new gamma-ray detector materials that may lead to the development of handheld devices for nuclear, national security and biomedical applications.
The semiconductor crystal materials, caesium mercury sulphide and caesium mercury selenide, operate at room temperature, unlike some materials such as high-purity germanium (HPGe). They are also stoichiometric compounds, so in principle they are easier to grow as large crystals by avoiding the undesirable concentration gradients generally seen in materials such cadmium zinc telluride (CZT).
The team at Northwestern University in Evanston, Illinois, led by Professor Mercouri Kanatzidis, first looked for candidate detector materials with the right energy band gaps, by carrying out band structure calculations for their ground and excited states. Based on the density size of the band gaps and the well-dispersed conduction bands, caesium mercury sulphide and caesium mercury selenide emerged as viable candidates.
The team produced the materials using a technique called dimensional reduction (DR), which is the breaking down of an initial 3D structure that has high density, but is of no use due to its very small band gap, into lower dimensional arrangements.
Kanatzidis explains, ‘These lower dimensional structures have larger band gaps more suitable for the application of gamma-ray detection. The breaking down of the initial structure is done by adding another chemical component. Because the chemical component has high density, so too does the product.’
The next phase involved preparing centimetre-long crystalline ‘ingot’ or boules of the materials using the Bridgman- Stockbarger technique. They then characterised them using standard electrical resistivity and photoconductivity methods. The crystals were then cut, polished and used to determine mobility-carrier lifetime product (μτ) values – the critical performance parameter of semiconductor detectors, as it determines their charge collection efficiency.
These (μτ) values – the figures of merit – are comparable to those of existing materials, says Kanatzidis. He adds that with one to two years’ more work on optimising the new materials, with regard to purity and electrical resistivity, they should outperform them. Over the same timescale the Northwestern team will also work on increasing the crystal size achievable by the process.
The team is now collaborating with institutions, such as the Argonne National Laboratory, on developing a working detector based around the new materials, while continuing to use DR to see if even better materials can be identified.
Professor Neil Hyatt of the University of Sheffield and Royal Academy of Engineering, UK, and NDA Chair in Radioactive Waste Management, says, ‘This is very carefully planned and executed research of the highest quality. It blends an understanding of crystal chemistry, electron scattering theory and electronic structure to identify a family of new candidate detector materials. It provides enormous scope for the development of new materials with improved properties, so this approach has enormous potential in that regard. The key challenge though will be to manufacture these materials at competitive prices and demonstrate that the long-term performance exceeds that of existing detector materials.’