Photon Energy Attenuation Layer for Sensitivity Enhancement in Si Hard X-ray Sensors

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Arxiv.org paper "Monte Carlo Modeling and Design of Photon Energy Attenuation Layers (PALs) for 10-30x Quantum Yield Enhancement in Si-based Hard X-ray Detectors" by Eldred Lee, Michael R. James, Kaitlin M. Anagnost, Zhehui Wang, Eric R. Fossum, and Jifeng Liu from Dartmouth College and Los Alamos National Laboratory proposes photon downconversion layer:

"High-energy (>20keV) X-ray photon detection at high quantum yield, high spatial resolution and short response time has long been an important area of study in physics. Scintillation is a prevalent method but limited in various ways. Directly detecting high-energy X-ray photons has been a challenge to this day, mainly due to low photon-to-photoelectron conversion efficiencies. Commercially available state-of-the-art Si direct detection products such as the Si charge-coupled device (CCD) are inefficient for >10keV photons. Here, we present Monte Carlo simulation results and analyses to introduce a highly effective yet simple high-energy X-ray detection concept with significantly enhanced photon-to-electron conversion efficiencies composed of two layers: a top high-Z photon energy attenuator layer (PAL) and a bottom Si detector. We use the principle of photon energy down conversion, where high-energy X-ray photon energies are attenuated down to and below 10keV via inelastic scattering suitable for efficient photoelectric absorption by Si. Our Monte Carlo simulation results demonstrate that 10-30x increase in quantum yield can be achieved using PbTe PAL on Si, potentially advancing high-resolution, high-efficiency X-ray detection using PAL-enhanced Si CMOS image sensors."

"While the overall underlying principle of photon energy down conversion could be somewhat similar to scintillator-based methods, it should be emphasized that this approach is distinctive in that the attenuated photons still remain in the X-ray spectral regime as opposed to the UV and visible regime. Unlike scintillators, the down conversion primarily relies on inelastic scattering with high-Z atoms, therefore no exotic and expensive bulk crystals (as in the case of scintillators) are needed for the PAL layers. In fact, the PAL layers can be polycrystalline or even amorphous thin films, which are much easier to fabricate than bulk crystal scintillators. The response time is also no longer limited by the optical spontaneous emission lifetime in scintillators, potentially allowing for ultrafast response since X-ray photon energy down conversion time via X-ray fluorescence and/or inelastic scattering is typically much shorter than the optical fluorescence time in scintillators. This conceptual design may also offer integration capabilities to Si CIS- or QIS-based devices for high resolution X-ray imaging."

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