Monte Carlo Modeling and Design of Photon Energy Attenuation Layers for >10× Quantum Yield Enhancement in Si-Based Hard X-ray Detectors

Abstract

High-energy (>20 keV) 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 >10 keV 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 attenuation 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 ≤10 keV via inelastic scattering suitable for efficient photoelectric absorption by Si. Our Monte Carlo simulation results demonstrate that a 10–30× 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.

Ultrafast CMOS image sensors and data-enabled super-resolution for multimodal radiographic imaging and tomography

“…Three CMOS image sensors were taped out towards this goal, as shown in Figure 2. During the phase one of this research [4], theoretical modeling and preliminary tests demonstrated more than 10× quantum efficiency improvement for high-energy X-ray photons (>10 keV) by depositing a photon-attenuation-layer (PAL) on a CMOS image sensor [5,6]. In the phase two of this research, a block-wise compact readout architecture based on unit-length-capacitor and asynchronous successive-approximation (SAR) analog to digital converter (ADC) [7] was proposed and implemented, which enabled the image sensor fabricated using a standard 180-nm process to run at 76 thousand-frames-per-second (kfps).…”

Simulating 50 keV X-ray Photon Detection in Silicon with a Down-Conversion Layer

“…Additionally, the number of photoelectrons generated before impact ionization divided by the number of incident photons, called the quantum yield (QY), is a useful parameter to compare to existing technologies. The QY of 1 µm PAL, 20 keV X-rays, and 50 µm Si is ~20%, much greater than the <5% using the silicon direct detection method [3,12]. A similar technology using a photocathode only obtained 5% photoelectron generation with 7.5-keV photons [13].…”

“…As previously mentioned, MCNP does not specify the number of photons exiting the PAL orthogonally, which may lead to an overestimation of the QY. On the other hand, MCNP's default cutoff photon energy value is below roughly 1 keV, which may underestimate the QY [3]. However, recent experimental results awaiting publication demonstrate significant relative signal enhancement from the Si reference.…”

“…This approach uses a Photon Attenuation Layer (PAL) made of high-Z material that inelastically scatters the high energy X-rays into lower energy X-rays. These lower energy X-rays then pass through the PAL to the semiconductor (Si) where their lower energy permits higher absorption rates and more efficient conversion of X-ray photons into EHPs and/or thinner detector thickness [3]. In the first part of the paper, this novel X-ray detector concept will be described, and the PAL modeled.…”

“…Thicker scintillators absorb a larger fraction of the incident radiation than thinner ones-they have higher absorption efficiency-but at the cost of more charge-sharing [5]. To combat these shortcomings, an innovative design was proposed in [3], consisting of a photon-attenuation layer (PAL) above a silicon detector. The proximity of the PAL to the silicon layer may yield enhanced absorption efficiency without or significantly mitigate additional charge sharing.…”