[Paper Review] Simulations of the COMET veto counter

Oleg Markin, Kirill Tarasov|arXiv (Cornell University)|Feb 22, 2014

Particle Detector Development and Performance1 references3 citations

TL;DR

This paper presents a detailed Monte Carlo simulation of a scintillator strip veto counter for the COMET experiment, modeling muon detection efficiency across four stacked layers with optimized strip geometry to minimize inefficiency. The simulation achieves a cosmic muon veto inefficiency below 0.0001 at an 11-photoelectron threshold, meeting COMET's stringent requirements while balancing noise-induced dead time and neutron background effects.

ABSTRACT

A computer model of a scintillator strip veto counter was built in order to verify the efficiency of the cosmic muon veto for the COMET experiment. To tune the model, experimentally measured data were utilized. Three different geometrical configuration of the counter were considered. For one of the configurations the simulation gave the inefficiency of the cosmic muon registration being below 0.0001, which meets requirements of the experiment.

Motivation & Objective

• To validate the cosmic muon veto efficiency of the COMET experiment's scintillator strip counter using detailed simulation.
• To optimize the geometric layout of scintillator strips across four stacked layers to minimize inefficiency.
• To evaluate the impact of noise and neutron-induced signals on data-taking efficiency.
• To determine the optimal signal threshold that balances muon detection efficiency and dead time from false coincidences.
• To guide future design improvements through simulation-based optimization of strip alignment and inactive zone management.

Proposed method

• A simplified 3D model of a single horizontal layer and a full four-layer stack of scintillator strips was developed, incorporating measured signal spectra from real detector components.
• Muon trajectories were generated using a realistic cosmic muon angular distribution (cos²θ dcosθ dϕ), with random entrance points and directions.
• Signal amplitudes were computed by interpolating experimentally measured pulse height spectra along 2218 mm strips, then scaled by muon range in the strip to account for energy deposition.
• Inefficiency was calculated as the fraction of muons with signals below threshold in at least three of four layers, with threshold set at 11 photoelectrons to model noise from neutron irradiation.
• The model included realistic inactive zones (0.3 mm inter-strip, 5 mm inter-module) and explored three strip offset configurations to reduce signal loss.
• Signal attenuation along strips was modeled using measured spectra, with adjustments for range-dependent signal broadening and scaling of fit parameters.

Experimental results

Research questions

• RQ1 What is the minimum achievable inefficiency of the COMET veto counter for cosmic muon rejection using four-layer coincidence?
• RQ2 How does strip geometry, including inter-strip and inter-module inactive zones, affect muon detection efficiency?
• RQ3 What signal threshold minimizes data loss from false coincidences due to noise and neutron-induced signals while maintaining high muon detection efficiency?
• RQ4 How does neutron irradiation affect the frequency of false veto signals and the resulting dead time in data acquisition?
• RQ5 Can optimized strip layer offsets reduce inefficiency below the COMET requirement of 0.0001?

Key findings

• The simulation demonstrated a cosmic muon veto inefficiency below 0.0001 for one of the three tested strip configurations, satisfying the COMET experiment's requirement.
• Operating at an 11-photoelectron threshold keeps the fraction of lost data time to a few percent, even under neutron irradiation up to 2×10¹¹ p/cm².
• The frequency of false coincident noise signals in two layers is estimated at approximately 10,400τn² for s=2, with k=15 adjacent strips to consider for coincidence.
• The model shows that higher thresholds reduce noise-induced dead time and neutron registration efficiency, improving data-taking efficiency.
• The simulation confirms that the four-layer coincidence scheme with properly offset strip layers can achieve the required low inefficiency despite signal attenuation and inactive zones.
• The study identifies that inter-layer offset alignment is critical to mitigate signal loss in wide inactive zones (5 mm inter-module gaps).