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Counting gamma rays with perovskites


Since their discovery at the turn of the 20th century, gamma rays have been used to probe everything from distant stars to cancerous tumours.

Gamma-ray spectroscopy in particular, has become a critical technology that underpins multiple applications in the modern world. Germanium (Ge) or Cadmium Zinc Telluride (CZT) single crystals are the current industry standard for detectors, but their high fabrication costs mean that there may be space in the market for lower cost alternatives, especially for room-temperature applications.

When a gamma-ray photon hits one of these detectors, its energy is either entirely absorbed by the material’s atoms (photoelectric effect), or inelastically scattered from them (Compton scattering). Both processes ionize a number of electron-hole pairs, producing an electrical pulse that can be collected. Resolving these pulses produces a characteristic spectrum that can be used to examine the gamma-ray source. This is what researchers from the Los Alamos National Laboratory have attempted to reproduce, using a novel perovskite single crystal detector.

Lead halide perovskite semiconductors have shown promise as materials for both X-ray and gamma-ray detection, largely thanks to the heavy elements they contain. The Los Alamos team – writing in an upcoming issue of Materials Today[DOI: 10.1016/j.mattod.2020.02.022] – opted to use a chlorine-doped methylammonium lead tribromide (MAPbBr3-x Clx) for their detector. The material has a high absorption cross-section over a wide energy range, comparable to CZT semiconductors. In addition, Monte Carlo simulation also suggested that, in the presence of a gamma source, it could produce a sharp photoelectric peak along with a low ‘Compton shoulder’ – key features in the energy spectrum.

Once grown, the researchers incorporated their perovskite crystals into a p-i-n junction, with high conductivity contacts on either side. While this device responded to gamma-ray radiation from a range of sources, it worked only for a few minutes before being saturated by a large dark current. To investigate the source of this noise, they fabricated symmetric devices with p-type (Au) and n-type (Ag) contacts on both sides. This allowed them to separate out the electron and hole currents from one another. They found that this dark current density was two orders of magnitude higher in the electron-only devices than in the hole-only ones. The hole-only devices also provided better performance as the temperature was increased from 175 K to room temperature (290 K), prompting them to focus on hole-only devices.

To test these p-i-p perovskite devices as photon pulse counters, different biases were applied at varying temperatures. Decreasing the temperature was found to have a more significant impact on performance than increasing the voltage, so to achieve the cleanest, sharpest signals, they’d need to cool the device slightly. Combining these findings, they exposed their device to a gamma source (137Cs) and, at 240 K and under a bias of 45 V, they began to accumulate counts and construct an energy spectrum.

The authors say that the resulting spectrum “…. resembles the Compton scattering edge and counts from photo-electric peaks”, though these features are not as clear as the comparable spectrum from a CZT detector. This analysis sets out some of the fundamental mechanisms that define these devices, and as such, marks a major step forward on the path toward perovskite-based gamma-ray spectroscopy.

This story was written for Materials Today, and was published on 24 / 3/ 20:

Original research paper ($): Fangze Liu, Michael Yoho, Hsinhan Tsai, KasunFernando, Jeremy Tisdale, Shreetu Shrestha, Jon K. Baldwin, Aditya D. Mohite,Sergei Tretiak, Duc T. Vo, Wanyi Nie. “The working principle of hybridperovskite gamma-ray photon counter”, Materials Today, Article in Press. DOI:10.1016/j.mattod.2020.02.022