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Nanostructures enhance the efficiency of a scintillator

24 August 2020

Calculations show that when struck by an energetic particle, the layered device can direct the resulting low-energy photons toward a photodetector.

Many medical devices, security scanners, and scientific instruments use scintillators to detect high-energy radiation. The scintillating material absorbs high-energy photons and converts each one’s energy into hundreds or even thousands of electron–hole pairs. Those pairs migrate to luminescence centers, typically the sites of dopant molecules, where they recombine and emit lower-energy visible photons that are more easily detectable. The modern setup—a scintillator coupled with a photodetector—was introduced in the mid 20th century. Since then, researchers have improved its efficiency by using better scintillation materials and electronics and enhanced coupling between the two.

A remaining source of inefficiency is the isotropic emission of visible photons from luminescence centers; only a fraction of those photons actually reach the photodetector. But a new scintillator design from Yaniv Kurman, of Technion–Israel Institute of Technology, and his colleagues promises to increase that fraction. According to the researchers’ calculations, their proposed scintillator, shown in the figure, could increase the number of photons that reach the photodetector by a factor of five compared with an unstructured scintillator.

Diagram of proposed scintillator
Credit: Y. Kurman et al., Phys. Rev. Lett. 125, 040801 (2020).

The design features alternating layers of scintillating (green) and dielectric (purple) materials, each a few hundred nanometers wide—comparable to the wavelength of the emitted visible light. Because the luminescence centers are sandwiched between dielectric layers, they’re effectively in optical cavities, which restrict the states available to emitted photons. (See the article by Serge Haroche and Daniel Kleppner, Physics Today, January 1989, page 24.)  In their calculations, the researchers used refractive index and emission wavelength values from standard scintillation materials. Then they optimized the thicknesses of the layers such that emission was enhanced in detectable directions and suppressed in undetectable ones.

The periodic structure would reduce the average delay time between a high-energy photon’s interaction with the scintillator and the first visible photon reaching the photodetector, thereby increasing the temporal resolution in time-of-flight measurements. But it may be some time before the new detectors are built and incorporated in experiments: Precisely assembling thousands of submicron layers is a challenge for existing fabrication techniques. Early applications will likely be limited to detecting lower-energy particles such as soft x rays, which are easier to capture and would only require a few hundred layers. (Y. Kurman et al., Phys. Rev. Lett. 125, 040801, 2020.)

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