⚛️ nuclear experiments

Timing Challenges in Transmission Silicon Detectors for Light Ions

This paper investigates how particle energy and detector non-uniformity affect signal rise time in transmission silicon detectors, aiming to mitigate time-walk effects that degrade time-of-flight measurements in light-ion experiments at the NFS facility.

Original authors: Diego Tarrio (Uppsala University, Uppsala, Sweden), Friedrich Karl Moraht (Uppsala University, Uppsala, Sweden), Lucas de Arruda (Uppsala University, Uppsala, Sweden), Stephan Pomp (Uppsala University

Published 2026-10-08

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Diego Tarrio (Uppsala University, Uppsala, Sweden), Friedrich Karl Moraht (Uppsala University, Uppsala, Sweden), Lucas de Arruda (Uppsala University, Uppsala, Sweden), Stephan Pomp (Uppsala University, Uppsala, Sweden), Xavier Ledoux (Grand Accelerateur National d'Ions Lourds, Caen, France), Diego Ramos (Grand Accelerateur National d'Ions Lourds, Caen, France)

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

In the world of nuclear physics, scientists often need to know exactly how fast a tiny particle is moving to understand what happened when it collided with an atom. To measure this speed, they rely on a technique called time-of-flight, which is essentially a very precise stopwatch. The idea is simple: if you know the distance a particle travels and you know exactly when it started and when it arrived, you can calculate its velocity. In many experiments, this timing information is captured by thin slices of silicon, which act as electronic tripwires. When a particle passes through, it leaves a tiny electrical signal. The moment that signal crosses a certain threshold is recorded as the start or stop time. For this system to work perfectly, the electrical signal needs to rise at a consistent speed, regardless of the particle's energy or where it hit the sensor. If the signal rises slowly for one particle and quickly for another, the stopwatch gets confused, leading to errors in the calculated speed and, consequently, in the identification of the particle itself.

A team of researchers recently investigated why these silicon sensors were giving inconsistent timing readings during experiments at a facility in France called GANIL. They were using a setup known as Medley, which consists of several telescopes made of stacked silicon detectors designed to catch light ions like protons, deuterons, and tritons produced by neutron collisions. While analyzing their data, they noticed a strange pattern: the timing measurements were shifting in a way that depended on the energy of the incoming particles, particularly when the particles had just enough energy to punch completely through the silicon layer. This suggested that the shape of the electrical signal was changing, but the researchers wanted to know if this was the only cause of the problem or if the sensors themselves had hidden flaws.

To get to the bottom of this, the scientists combined their live experimental data with a series of controlled tests performed in a laboratory. In the live experiments, they observed that when protons hit the second silicon detector in their telescope, the timing signal behaved erratically right around the energy level where the protons could just barely pass all the way through the material. Below this threshold, the protons stopped inside the silicon; above it, they punched through. The researchers found that the time it took for the electrical signal to rise from a low level to a high level changed significantly depending on whether the particle stopped or passed through. This happens because the electrical signal is created by the movement of charged particles inside the silicon. When a particle stops, the charge is generated only along its path, and the time it takes for that charge to reach the sensor depends on how deep the particle went. However, when a particle punches through, the charge is generated all the way through the entire thickness of the detector, creating a different signal shape that confuses the timing electronics.

But the story did not end with particle energy. The team also took spare silicon detectors, identical to the ones used in the experiment, and scanned them with a radioactive source that fired alpha particles at them. These particles were stopped completely within the detector, so the "punch-through" effect could not be the cause of any timing issues. By moving the source across the surface of the silicon, they mapped out how the detector responded at different spots. They discovered that the timing was not uniform across the sensor. In a thicker detector, measuring 1000 micrometers, the time it took for the signal to rise varied dramatically depending on where the particle hit, ranging from about 180 nanoseconds to 1.4 microseconds. This variation was so large that it created a pattern on the sensor's surface that looked exactly like the pattern of errors in the detector's ability to measure energy. This suggested that the silicon material itself was not perfectly uniform; some areas might have slightly different electrical properties or tiny regions that were not fully active, causing the signal to drift slower or faster depending on the location.

The researchers concluded that the timing errors observed in their experiments were not caused by a single factor but by a combination of two distinct issues. First, the energy of the particle matters, especially near the point where it can penetrate the detector, because the depth of the interaction changes the shape of the signal. Second, the physical quality of the detector matters, as imperfections in the silicon can cause the signal to rise at different speeds even for identical particles hitting different spots. This finding is crucial because the standard method used to fix timing errors, which assumes that all signals look roughly the same, cannot fully correct for these large variations in signal shape. The team suggests that future experiments will need to account for both the energy of the particles and the specific location where they hit the detector to achieve accurate timing. While the current setup can be corrected with rough estimates, a more precise understanding of these signal shapes will be necessary for the next generation of nuclear physics experiments.

Technical Summary: Timing Challenges in Transmission Silicon Detectors for Light Ions

Problem Statement
Transmission silicon detectors are standard components in $\Delta E$ – $E$ telescope configurations used for particle identification and time-of-flight (TOF) measurements in nuclear physics. However, recent experiments using the Medley setup at the Neutrons For Science (NFS) facility (GANIL) revealed systematic deviations in timing behavior. Specifically, variations in signal rise time introduce time-walk effects that degrade TOF measurements. While empirical corrections have partially addressed these issues, the underlying physical origins—specifically the interplay between particle energy (particularly near punch-through thresholds) and detector non-uniformity—require further investigation to mitigate their impact on future experiments.

Methodology
The study employs a dual approach combining in-beam observations with off-beam detector characterization:

In-beam Measurements (Medley at NFS):
- Utilized the Medley setup, consisting of eight telescopes with $\Delta E$ – $\Delta E$ – $E$ configurations (two silicon transmission detectors followed by a CsI(Tl) crystal).
- Analyzed neutron–proton elastic scattering data using a CH $_2$ target. The scattered proton energy ( $E_p$ ) was correlated with the incident neutron energy ( $E_n$ ) via TOF.
- Timing signals were extracted from the second silicon detector in each telescope using a digital constant-fraction discriminator (CFD) algorithm within the Numexo2 acquisition system.
- The analysis focused on deviations from the expected linear relationship between $E_p$ and $E_n$ , specifically around the punch-through thresholds for protons, deuterons, and tritons.
Off-beam Characterization:
- Conducted dedicated mapping measurements on spare silicon detectors (identical to those in Medley) using a mixed $\alpha$ -source ( $^{239}$ Pu, $^{241}$ Am, $^{244}$ Cm).
- A motorized system scanned the detectors with a 2-mm collimator to map response across the surface in a vacuum environment.
- Full waveforms were recorded using a 1-GHz digitizer to extract pulse height, energy resolution, and rise time (defined as the 10% to 90% amplitude interval).
- This setup isolated position-dependent effects from energy-dependent punch-through effects, as the $\alpha$ particles fully stopped within the detectors.

Key Results

Energy-Dependent Timing Effects: In-beam data revealed significant deviations from the expected linear $E_p$ – $E_n$ correlation. The strongest deviations occurred for protons at approximately 12.3 MeV, corresponding to the punch-through threshold of the second silicon detector. Similar non-linearities were observed for deuterons and tritons at their respective thresholds. This indicates that signal rise time is heavily dependent on the depth of charge deposition; particles stopping inside the detector generate signals with different rise times compared to those punching through.
Spatial Non-Uniformity: Off-beam mapping demonstrated substantial position-dependent variations in rise time, even for particles that fully stopped within the detector.
- In a 1000 $\mu$ m thick detector, rise times varied from ~180 ns to 1.4 $\mu$ s across the surface, showing a strong spatial gradient.
- This spatial pattern in rise time correlated strongly with variations in energy resolution, suggesting a common underlying cause such as local electric field non-uniformities, resistivity variations, or partially depleted regions.
- In contrast, a 400 $\mu$ m thick detector exhibited much more uniform rise times (216–245 ns) and energy resolution, suggesting that thickness exacerbates the impact of inhomogeneities.

Key Contributions
The paper establishes that timing distortions in transmission silicon detectors arise from two distinct but compounding sources:

Particle Range Effects: The transition from stopping to punching through the detector alters the charge collection path, fundamentally changing the pulse shape and rise time.
Detector Non-Uniformities: Intrinsic spatial variations in detector properties (e.g., electric field distribution) cause significant rise-time fluctuations independent of particle energy.

The study clarifies that while digital CFD techniques are effective for correcting amplitude-based time walk, they rely on the assumption of similar signal shapes. The observed large variations in pulse shape and rise time limit the effectiveness of standard CFD corrections.

Significance and Claims
The authors conclude that accurate TOF measurements in future experiments cannot rely solely on empirical corrections or standard CFD algorithms if the detector exhibits significant non-uniformity or operates near punch-through thresholds. The work argues that:

Both particle range and detector non-uniformity must be explicitly accounted for in detector operation and analysis.
Future experiments would benefit from alternative approaches based on pulse-shape analysis to explicitly handle rise-time variations, rather than relying on methods that assume uniform signal shapes.
The findings suggest that thicker silicon detectors are more susceptible to these timing distortions due to inhomogeneities, though a broader study is required to confirm this trend definitively.

The paper does not propose specific new hardware designs but emphasizes the necessity of characterizing detector uniformity and adapting analysis strategies to accommodate the observed physical realities of signal formation in transmission silicon detectors.

More like this