Cheaper Detectors for Gamma Radiation

Lead halide perovskite crystals not only can detect radiation but can also serve as a novel class of brightly fluorescent inorganic pigments (Source: Empa)

Lead halide perovskite crystals not only can detect radiation but can also serve as a novel class of brightly fluorescent inorganic pigments. (Source: Empa)

A research team at Empa and ETH Zurich has developed single crystals made of lead halide perovs­kites, which are able to gage radio­active radiation with high precision. Initial experi­ments have shown that these crystals, which can be manu­factured from aqueous solutions or low-priced solvents, work just as well as conventional cadmium telluride semi-conductors, which are consi­derably more compli­cated to produce. The discovery could slash the price of many radio-detectors – such as in scanners in the security sector, portable dosimeters in power stations and measuring devices in medical diagnostics.

Gamma photons virtually always accompany the radio­active decay of unstable isotopes. In order to identify radio­active substances, cost-effective and highly sensitive gamma detectors that work at room tempe­rature are thus in great demand. Finding suitable substances, however, is easier said than done, as Maksym V. Kovalenko, a professor at ETH Zurich and research group leader at Empa explains: the coveted semi­conductor has to be of an outstanding electronic quality, i.e. exhibit a high mobility and lifetime of carriers, along with a low density of trap states as well as of intrinsic carriers at room tempe­rature. Second, it must be composed of heavy elements that can absorb highly energetic gamma photons. Third, it should be possible to grow large single crystals of this material and these crystals must be stable, both mechani­cally and thermally.

Thus far, only cadmium telluride (CdTe) had fulfilled these requirements. However, the substance also used to produce thin-film solar cells is not water-soluble and only melts at temperatures above 1,000 degrees Celsius, which renders the production of detector crystals compli­cated and expensive. Kovalenko and his team have now succeeded in manu­facturing single crystals from another family of novel semi­conductors (lead halide perovskites) in a usual glass beaker at a cost of just a few Swiss Francs per crystal. These crystals can be used as highly sensitive detectors for gamma radiation. One possible appli­cation might be a mini Geiger counter, which can be connected to smart­phones. This might enable people in conta­minated areas, for instance, to test their food for radioactivity.

Another potential field of appli­cation for the new crystals is diagnosing metabolic problems in the brain. Dopamine receptor disorders can have numerous conse­quences: Parkinson’s, schizo­phrenia, hyper­activity (ADHD), social anxiety disorders or drug addiction and alcoholism. These disorders are diagnosed by giving patients radio­active tracer substances, which render the brain activity visible in magnetic resonance imaging (MRI). Ad­ministering radio­active substances is not without its hazards, though: if the substance is impure, it can have adverse health effects. However, its purity has to be verified swiftly as the tracer substance has a rather low half-life, which means it degrades rapidly.

In order to demonstrate the “capabi­lities” of lead halide perov­skites, Kovalenko’s team used the new single crystal detector to test the isotope purity of 18F-fallypride, a tracer substance used in clinical trials on dopamine receptors. 18F-fallypride is radio­active and has a half-life of 110 minutes, which means there is precious little time to test the substance’s radio­active purity between its production and injection. So far, purity measure­ments have been conducted in an elaborate, two-step process: first of all, the substance is separated using the high-performance liquid chroma­tography technique. In the second phase, the radio­activity is then measured with a detector. Thanks to the new crystal, the researchers were able to reduce this two-stage process success­fully to one simple step. The crystal merely needs to be held in front of the tracer substance to take the readings. (Source: Empa)

Reference: S. Yakunin et al.: Detection of gamma photons using solution-grown single crystals of hybrid lead halide perovskites, Nat. Phot. 10, 585 (2016); DOI: 10.1038/nphoton.2016.139

Link: Laboratory for Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland

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