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Characterization of GaN:Si and ZnO:Ga for position-resolved fast timing applications

This paper characterizes single-crystal GaN:Si and ZnO:Ga as high-performance, fast-timing scintillators that significantly outperform traditional YAP:Ce in timing and position resolution, proposing them as superior drop-in replacements for alpha detectors in Associated Particle Imaging systems.

Original authors: Julius Meyer, Joshua W. Cates, Woon-Seng Choong, Juan Cristhian Luque Gutierrez, Federico Moretti, Ryan Pavlovsky, Mauricio Ayllon Unzueta, Weronika W. Wolszczak, Markus Roth, Arun Persaud

Published 2026-06-23

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Julius Meyer, Joshua W. Cates, Woon-Seng Choong, Juan Cristhian Luque Gutierrez, Federico Moretti, Ryan Pavlovsky, Mauricio Ayllon Unzueta, Weronika W. Wolszczak, Markus Roth, Arun Persaud

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to catch a speeding bullet with a camera. To get a clear picture, your camera needs two things: it must be incredibly fast (to freeze the motion) and incredibly bright (to see the bullet clearly). In the world of nuclear physics, scientists use special crystals called scintillators to act as these cameras. When a particle hits the crystal, it flashes light. The faster and brighter the flash, the better the "picture" of where the particle came from and what it was.

For years, the gold standard for this job has been a crystal called YAP:Ce. It's a reliable workhorse, but it's a bit like an old sports car: it gets the job done, but it's not the fastest thing on the track.

This paper introduces two new, high-tech "race cars": GaN:Si (Gallium Nitride doped with Silicon) and ZnO:Ga (Zinc Oxide doped with Gallium). The researchers tested these new materials to see if they could outperform the old champion, specifically for a technique called Associated Particle Imaging (API). Think of API as a way to map the chemical makeup of soil, check for hidden landmines, or scan cargo for security. It works by firing particles at a target and catching the "associated" particles that bounce back. To get a sharp 3D map, you need to catch those bouncing particles with extreme speed and precision.

Here is what the researchers found, broken down simply:

1. The Speed Test (Timing Resolution)

Imagine a stopwatch. The old YAP:Ce crystal takes about 144 picoseconds (a picosecond is one-trillionth of a second) to register a hit. That's fast, but in the world of particle physics, it's like running a race in slow motion.

The new crystals are lightning fast:

GaN:Si registered hits in just 35 picoseconds.
ZnO:Ga registered hits in 49 picoseconds.

The Analogy: If the old crystal was a human blinking, these new crystals are like a camera shutter snapping in a fraction of a blink. They are roughly 3 to 4 times faster than the old standard. This speed is crucial because it reduces "pile-up," which is like trying to hear two people talking at once; if they talk too close together, you can't tell who said what. These new crystals let the "talkers" speak so fast that the system can hear every single word clearly.

2. The Brightness Test (Light Yield)

Speed is great, but you also need light. If the flash is too dim, your camera sensor can't see it.

YAP:Ce is very bright (about 6,090 photons per unit of energy).
ZnO:Ga is moderately bright (about 2,600 photons).
GaN:Si is the dimmest of the three (about 1,060 photons).

The Catch: While GaN:Si is the fastest, it's not as bright as the others. However, the researchers found that even with less light, its incredible speed makes it a very strong contender. It's like a sprinter who is slightly shorter than the others but runs so fast they still win the race.

3. The "Where" Test (Position Resolution)

In these imaging systems, knowing exactly where the particle hit is just as important as knowing when it hit.

YAP:Ce could pinpoint a location within 0.2 millimeters (about the width of a human hair).
GaN:Si was a bit fuzzier, pinpointing within about 1 millimeter.
ZnO:Ga was estimated to be around 0.3 millimeters.

The Analogy: Imagine trying to draw a dot on a piece of paper. YAP:Ce draws a tiny, precise dot. GaN:Si draws a slightly larger, fuzzier dot. But for many security or mapping tasks, that "fuzzier" dot is still precise enough to be incredibly useful.

4. The "Self-Absorption" Problem

The paper notes a tricky issue with these new materials. Because they are so efficient at making light, they are also very good at eating their own light before it can escape the crystal. It's like shouting in a room lined with thick velvet; your voice is loud, but the walls swallow the sound before it reaches the door.

The researchers found that ZnO:Ga and GaN:Si suffer from this "self-absorption" because the light they emit is very close to the color the material naturally absorbs.
The old YAP:Ce doesn't have this problem because its light is a different color that passes right through.

However, the researchers showed that by making the crystals thinner or adding a mirror coating (to bounce the light back out), they could fix this issue and get even better results.

The Bottom Line

The paper concludes that GaN:Si and ZnO:Ga are excellent, high-performance replacements for the older YAP:Ce crystal in specific applications like Associated Particle Imaging (API).

GaN:Si is the "speed demon" of the group, offering the fastest timing resolution ever measured for this type of application.
ZnO:Ga offers a great balance of speed and brightness.
Both are proposed as "drop-in" replacements, meaning they could potentially swap into existing systems to make them much sharper and faster, helping to create better 3D maps of the world, detect hidden threats, or explore other planets.

The researchers did not claim these materials would immediately cure diseases or change medical imaging in hospitals; their focus was strictly on improving the speed and precision of particle detection for imaging and security systems.

Technical Summary: Characterization of GaN:Si and ZnO:Ga for Position-Resolved Fast Timing Applications

Problem Statement
High-performance radiation detection systems, particularly those requiring fast timing for applications such as 3D elemental mapping, medical imaging (PET/CT), and homeland security, are constrained by the intrinsic properties of current scintillator materials. While cerium-doped yttrium aluminium perovskite (YAP:Ce) is widely used in Associated Particle Imaging (API) systems, its timing precision, rate capability, and photon statistics impose fundamental limits on achievable coincidence timing resolution (CTR) and position resolution. The authors identify a need for inorganic semiconductor scintillators with near-bandgap emission and sub-nanosecond decay times to meet the demands of high-rate, high-resolution API. Specifically, the paper addresses the characterization of silicon-doped gallium nitride (GaN:Si) and gallium-doped zinc oxide (ZnO:Ga) as potential high-performance replacements for the alpha detectors in API systems, noting that prior work on ZnO:Ga has largely focused on polycrystalline forms suffering from self-absorption, and GaN:Si has not previously been proposed for API applications.

Methodology
The study characterizes three scintillator samples: a single-crystal YAP:Ce (50 mm × 50 mm × 1 mm), a bulk single-crystal ZnO:Ga (10 mm × 10 mm × 0.5 mm, 1–3% Ga doping), and an epitaxial GaN:Si layer (10 µm thick, 7 × 10¹⁸/cm³ Si concentration) grown on a sapphire wafer.

The experimental approach involved four primary measurement categories:

Scintillation Kinetics: Rise and decay times were measured using X-ray-induced time-correlated single-photon counting (TCSPC) with a 200 fs pulsed Ti:Sapphire laser driving a photo-excited X-ray tube. The system utilized a microchannel plate PMT (MCP-PMT) with a 25 ps FWHM transit time spread (TTS) and an instrument response function (IRF) of ~68 ps.
Light Yield: Absolute light yields were determined under direct alpha particle irradiation (5.47 MeV ²⁴¹Am source). The setup employed Hamamatsu R9800-100 PMTs and a DRS4 data acquisition system. Photon transport efficiency was quantified via Monte Carlo simulations to account for refractive index mismatches and absorption, allowing the conversion of detected photoelectrons to absolute photons per MeV ( $ph/MeV_\alpha$ ).
Timing Resolution: Coincidence timing resolution (CTR) and detector timing resolution (DTR) were measured using a triple-coincidence setup. Alpha particles passed through a thin EJ-214 plastic scintillator (providing a fast start signal) before stopping in the test sample (providing the stop signal). A constant fraction discriminator (CFD) was optimized for each material to extract timing parameters.
Position Resolution: Position resolution was evaluated in an experimental setup emulating an integrated API system. Samples were irradiated through coarse and fine masks inside a vacuum chamber, with light collected by a 16×16 pixel Hamamatsu H13700-03 PMT. The interaction position was reconstructed using a resistive network and center-of-mass calculation.

Key Contributions and Results

Ultra-Fast Decay Times: TCSPC measurements revealed exceedingly fast rise times ( $\le$ 15 ps) for both new materials. The primary decay components were measured at 32.4 ± 1.0 ps for GaN:Si and 805 ± 1 ps for ZnO:Ga. In contrast, YAP:Ce exhibited a primary decay of 24.3 ± 0.1 ns. The authors note that approximately 95.8% of GaN:Si's light yield is emitted within the first 32 ps, and 99.98% of ZnO:Ga's light is emitted within the first 805 ps.
Light Yield and Spectra: Ionoluminescence spectra confirmed direct, red-shifted near-bandgap emission peaks at 372 nm for GaN:Si and 401 nm for ZnO:Ga. The measured absolute light yields were:
- GaN:Si: 1060 ± 350 $ph/MeV_\alpha$
- ZnO:Ga: 2600 ± 860 $ph/MeV_\alpha$
- YAP:Ce: 6090 ± 2010 $ph/MeV_\alpha$
  The authors attribute the lower yield in GaN:Si and ZnO:Ga partly to high refractive indices causing self-absorption and total internal reflection losses, as well as the specific sample geometries (e.g., lack of reflective coating on the ZnO:Ga sample during light yield measurement).
Timing Resolution Improvements: The DTR measurements demonstrated a significant improvement over YAP:Ce:
- GaN:Si: 35 ± 9 ps
- ZnO:Ga: 49 ± 5 ps
- YAP:Ce: 144 ± 2 ps
  This represents a $\ge$ 3× improvement in timing resolution compared to the conventional YAP:Ce.
Position Resolution: Experimental position resolution for YAP:Ce was measured at $\le$ 0.2 mm. GaN:Si achieved approximately 1 mm resolution. For ZnO:Ga, experimental measurement was limited by sample size, but simulations predicted a resolution of 0.3 mm. The authors further simulated that a thinner (100 µm) ZnO:Ga sample with a reflective layer could achieve a position resolution of 0.18 mm.

Significance and Claims
The paper claims that single-crystal ZnO:Ga and near-single-crystal GaN:Si are viable, high-performance drop-in replacements for alpha detectors in API systems. The primary significance lies in the demonstration of sub-nanosecond decay times and timing resolutions in the range of 35–50 ps, which substantially outperform the widely used YAP:Ce.

The authors assert that these materials enable:

Higher Rate Capabilities: The fast decay constants reduce pulse pile-up, potentially enabling rate improvements of 10×.
Improved Image Resolution: The enhanced timing resolution allows for narrower coincidence windows, reducing random coincidences and improving the depth resolution in 3D elemental mapping.
Theoretical Potential: Cramèr-Rao lower bound (CRLB) analysis suggests that with photodetectors having a TTS FWHM < 100 ps, GaN:Si and ZnO:Ga could theoretically support DTR near or below 10 ps, approaching the aspirational targets for next-generation medical imaging and high-precision nuclear detection.

The study concludes that while practical limitations such as self-absorption and interface reflections currently reduce effective light yield, optical design optimizations (thinning, reflective layers, index matching) can measurably improve collection efficiency and position resolution, making these semiconductor scintillators promising candidates for future high-resolution, high-rate detection systems.

1. The Speed Test (Timing Resolution)

2. The Brightness Test (Light Yield)

3. The "Where" Test (Position Resolution)

4. The "Self-Absorption" Problem

The Bottom Line

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