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Hit-rate capability of a silicon strip detector module for decay positron detection in the J-PARC muon $g-2$ /EDM experiment

This paper presents the design specifications and experimental evaluation of a silicon strip detector module's hit-rate capability using a muon beam at J-PARC, demonstrating its ability to maintain high detection efficiency under the expected maximum rate of 1.4 MHz per sensor strip for the J-PARC muon $g-2$ /EDM experiment.

Original authors: Ryuto Azuma, Katsunori Awa, Shunsuke Doi, Yowichi Fujita, Seiso Fukumura, Yu Goto, Ryotaro Honda, Sohtaro Kanda, Tetsuichi Kishishita, Tatsuya Kume, Tsutomu Mibe, Yukiharu Murata, Shoichiro Nishimura

Published 2026-06-16

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Ryuto Azuma, Katsunori Awa, Shunsuke Doi, Yowichi Fujita, Seiso Fukumura, Yu Goto, Ryotaro Honda, Sohtaro Kanda, Tetsuichi Kishishita, Tatsuya Kume, Tsutomu Mibe, Yukiharu Murata, Shoichiro Nishimura, Shinji Ogawa, Yuta Okazaki, Naohito Saito, Maki Sakakibara, Osamu Sasaki, Taiki Sato, Yutaro Sato, Yoshiaki Seino, Hiroshi Sendai, Koichiro Shimomura, Shohei Shirabe, Masayoshi Shoji, Patrick Strasser, Taikan Suehara, Shiori Sugahara, Junichi Suzuki, Toshikazu Takatomi, Manobu M. Tanaka, Junji Tojo, Hiroyuki A. Torii, Takashi Yamanaka, Hiroshi Yamaoka, Takayuki Yamazaki, Tamaki Yoshioka

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

The Big Picture: Catching Tiny Particles

Imagine scientists are trying to solve a mystery about the universe: they want to measure exactly how a tiny particle called a muon spins and wobbles. To do this, they need to watch what happens when a muon dies (decays). When a muon dies, it shoots out a "child" particle called a positron.

The scientists at J-PARC (a giant particle lab in Japan) have built a special camera to catch these positrons. But there's a catch: the muons are dying so fast and so frequently that the camera has to be incredibly fast and tough. If the camera gets overwhelmed, it will miss the action, and the scientists won't get their answer.

This paper is about building and testing a specific part of that camera—a silicon strip detector—to make sure it can handle the chaos without dropping the ball.

The Problem: A Traffic Jam of Particles

Think of the positrons like cars trying to exit a highway.

Normal traffic: Usually, cars exit one by one. Easy to count.
The J-PARC traffic: In this experiment, the scientists are creating a situation where thousands of cars try to exit the highway at the exact same moment. Specifically, they expect up to 1.4 million cars (hits) per second on a single tiny lane (sensor strip).

If your camera is too slow, two cars will arrive at the same time, and the camera will only see one big blur or miss them both. This is called "pileup." The goal of this paper was to prove that their new camera lens can handle this massive traffic jam without losing too many cars.

The Solution: A High-Speed "Quarter Vane"

The scientists built a detector module they call a "quarter vane."

The Sensor: Imagine a piece of silicon the size of a large postcard, but sliced into 512 incredibly thin strips (like the slats of a Venetian blind). These strips are the "eyes" that see the positrons.
The Brain (Electronics): Attached to these strips is a super-fast electronic brain (called an ASIC). It's designed to react in less than 100 nanoseconds (a billionth of a second). Think of it like a photographer with a shutter speed so fast it can freeze a bullet in mid-air.
The Cooling: Because this brain works so hard, it gets hot. So, they attached "heat pipes" (like the cooling system in a high-end gaming computer) to keep it from melting.

The Test: A Real-World Stress Test

To see if this camera works, they didn't just run it in a quiet lab. They took it to the MuSEUM experiment at J-PARC.

The Setup: They placed their detector in a powerful magnetic field where a beam of muons was being fired.
The Challenge: They turned on the muon beam, creating a flood of positrons. They needed to see if the detector could count them all accurately, even when the "traffic" was heaviest right after the beam started.

The Results: Did It Pass?

The scientists looked at the data to see how many positrons the detector "missed" because of the traffic jam (pileup).

Noise Check: First, they made sure the detector wasn't just seeing "static" or random electrical noise. It was clean.
The Traffic Jam: They simulated the worst-case scenario. They found that when the hit rate was at the target level of 1.4 million hits per second, the detector missed about 10% of the hits due to the pileup.
The Verdict: Is missing 10% a big deal?
- Imagine you are trying to draw a picture of a car by counting its wheels. If you miss 10% of the wheels, you can still perfectly figure out what the car looks like and where it's going.
- Similarly, the scientists calculated that even with a 10% loss, they would still have enough positron tracks to perfectly reconstruct the path of the particles.

Conclusion

The paper concludes that the new silicon strip detector is ready for the job. It is fast enough and tough enough to survive the intense traffic of the J-PARC muon experiment. It successfully proved that it can keep track of the particles even when they are arriving at a rate of 1.4 million per second, ensuring the scientists can finally get their precise measurements of the muon's behavior.

Technical Summary: Hit-rate Capability of a Silicon Strip Detector Module for the J-PARC Muon $g-2$ /EDM Experiment

Problem Statement
The J-PARC muon $g-2$ /EDM experiment aims to measure the muon anomalous magnetic moment and electric dipole moment using a unique technique involving a thermal muon beam accelerated to 300 MeV/ $c$ . This approach utilizes a compact 66 cm diameter storage ring with a 3 T magnetic field, eliminating the need for electric fields that introduce systematic biases. A critical component of this setup is a full tracking detector designed to reconstruct positrons from muon decay within a volume of 290 mm radius and 400 mm height.

The primary challenge addressed in this work is the extreme instantaneous hit rate expected during data acquisition. Near the muon beam orbit, the decay positron flux creates a spatial density of 15 MHz/cm², resulting in a maximum hit rate of 1.4 MHz per sensor strip. Under these conditions, signal pileup in the analog front-end electronics can lead to the loss of hit signals, potentially causing failures in track reconstruction. The experiment requires a detector capable of maintaining high detection efficiency despite these high-rate conditions.

Methodology
To address these requirements, the authors developed a "quarter vane" detector module, which serves as the smallest unit of the radial tracking system. The module consists of:

Sensors: Four single-sided p-on-n silicon strip sensors (Hamamatsu S13804) with a 190 $\mu$ m strip pitch and 48.365 mm strip length, arranged on a glass-epoxy frame.
Front-end Electronics: Custom ASICs (SliT128D) featuring 128 channels each, equipped with CR-RC shaping amplifiers (pulse width < 100 ns) and differentiators for time-walk correction. The system operates at a 200 MHz sampling rate.
Readout: An FPGA-based board (FRBS) utilizing SiTCP protocol for data transmission.

The performance of this module was evaluated using a muon beam test at the J-PARC MLF H-line within the MuSEUM experimental setup. This setup provided a high-intensity pulsed muon beam (approx. $1 \times 10^8$ $\mu^+$ /s) with a double-bunch structure, simulating the high-rate environment relevant to the $g-2$ /EDM experiment. The evaluation focused on:

Noise Characterization: Measuring equivalent noise charge (ENC) and optimizing Time-over-Threshold (ToT) cuts to reject non-beam noise.
Hit-Rate Capability: Analyzing the time spectrum of signal hits to quantify efficiency loss due to pileup. The analysis window was set to 20 $\mu$ s starting 400 ns after the second muon bunch arrival. The observed time spectrum was fitted with a model incorporating an efficiency loss term proportional to the hit rate.

Key Contributions and Results
The paper presents the design, construction, and performance validation of the silicon strip detector module. Key findings include:

Noise Performance: The equivalent noise charge was measured at an average of 0.200 fC (approx. 1/18 of a MIP charge), confirming that electronic noise is negligible compared to the signal. A ToT cut of > 20 ns was established, yielding a signal-to-noise ratio greater than $9 \times 10^4$ while maintaining > 90% signal efficiency.
Pileup Characterization: The study demonstrated that signal hit loss due to pileup is approximately proportional to the hit rate.
Efficiency at Target Rate: At the experiment's expected maximum hit rate of 1.4 MHz per strip, the measured maximum efficiency loss is 10%.
Impact on Reconstruction: Given that the expected number of hits from signal momentum tracks (200–275 MeV/ $c$ ) is approximately 50, the authors estimate that a 10% hit loss has a negligible impact on the overall track reconstruction efficiency.

Significance
The paper concludes that the developed silicon strip detector module satisfies the stringent performance requirements of the J-PARC muon $g-2$ /EDM experiment. By successfully demonstrating a 10% efficiency loss at the target hit rate of 1.4 MHz, the module proves capable of withstanding the high pileup conditions inherent to the experiment's design. This validation is essential for ensuring the reliable reconstruction of full positron tracks, which is fundamental to the precision of the $g-2$ and EDM measurements. The work confirms that the combination of fast-shaping front-end electronics and the specific sensor geometry can operate effectively in the high-flux environment of the J-PARC storage ring.