[Paper Review] Operating Water Cherenkov Detectors in high altitude sites for the Large Aperture GRB Observatory

D. Allard, C. Álvarez+39 more|ArXiv.org|Jun 4, 2009

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper presents the operation and stability of Water Cherenkov Detectors (WCDs) at high-altitude sites (4550–5280 m a.s.l.) within the Large Aperture GRB Observatory (LAGO), demonstrating their effectiveness in detecting high-energy gamma-ray bursts (GRBs) via atmospheric cascades. The study confirms stable long-term operation under high-background conditions, identifies light leaks and atmospheric pressure effects as key challenges, and introduces a low-power, GPS-synchronized FPGA-based DAQ system for improved data acquisition and calibration.

ABSTRACT

Water Cherenkov Detectors (WCD) are efficient detectors for detecting GRBs in the 10 GeV - 1 TeV energy range using the single particle technique, given their sensitivity to low energy secondary photons produced by high energy photons when cascading in the atmosphere. The Large Aperture GRB Observatory (LAGO) operates arrays of WCD in high altitude sites (above 4500 m a.s.l.) in Bolivia, Mexico and Venezuela, with planned extension to Peru. Details on the operation and stability of these WCD in remote sites with high background rates of particles will be detailed, and compared to simulations. Specific issues due to operation at high altitude, atmospheric effects and solar activity, as well as possible hardware enhancements will also be presented.

Motivation & Objective

• To enable sensitive, ground-based detection of high-energy gamma-ray bursts (GRBs) in the 10 GeV–1 TeV range using water Cherenkov detectors (WCDs) at high-altitude sites.
• To address operational challenges in remote, high-altitude locations with high particle background rates and extreme environmental conditions.
• To improve detector stability and data acquisition by implementing a low-power, GPS-synchronized FPGA-based DAQ system with real-time calibration.
• To investigate the impact of atmospheric pressure, temperature, and solar activity on detector performance and signal stability.
• To validate the performance of WCDs through comparison of real data with Geant4 simulations and to identify hardware enhancements for future deployment.

Proposed method

• Deployment of WCD arrays at three high-altitude sites: Sierra Negra (4550 m), Chacaltaya (5280 m), and Mérida (4780 m), all above 4500 m a.s.l.
• Utilization of the single-particle technique to detect low-energy secondary photons produced by high-energy gamma-ray cascades in the atmosphere.
• Implementation of a new FPGA-based data acquisition (DAQ) system with 200 MSPS sampling, GPS time tagging (50 ns accuracy), and integrated pressure and temperature sensors.
• Calibration of WCDs using real data histograms to identify threshold positions and VEM (photoelectron) response, validated against Geant4 simulations.
• Monitoring of scaler rates at multiple thresholds to assess detector stability and detect anomalies such as light leaks or electronic drifts.
• Correlation of scaler rate variations with atmospheric pressure and time-of-day to investigate environmental effects on signal response.

Experimental results

Research questions

• RQ1 How stable are Water Cherenkov Detectors (WCDs) in high-altitude, remote locations with high cosmic ray background rates?
• RQ2 What are the dominant environmental and instrumental effects—such as light leaks or temperature variations—on WCD performance at high altitudes?
• RQ3 To what extent does atmospheric pressure modulate the flux of secondary particles detected at ground level, and how is this reflected in scaler rate data?
• RQ4 Can a low-power, FPGA-based DAQ system achieve reliable, high-precision data acquisition and calibration for WCDs in remote, high-altitude observatories?
• RQ5 How do real detector data compare with Geant4 simulations in terms of charge distribution and threshold response?

Key findings

• The WCDs at Sierra Negra and Chacaltaya demonstrated stable operation over extended periods, with consistent scaler rate averages and low standard deviations, indicating reliable performance.
• WCD 3 at Chacaltaya exhibited a noisy behavior during daytime with increased standard deviation and spikes in the lower threshold rate, which was attributed to a confirmed light leak.
• A clear anti-correlation was observed between the second-threshold scaler rate and atmospheric pressure at Chacaltaya, with stronger daytime peaks suggesting additional effects from temperature or light leaks.
• The new FPGA-based DAQ system achieved 200 MSPS sampling with GPS time synchronization (50 ns uncertainty), enabling precise event tagging and improved data format compatibility.
• Calibration histograms from the new DAQ system at Sierra Negra showed a well-defined VEM peak, confirming accurate threshold calibration and signal response.
• The prototype HAWC experiment (Proto-HAWC) at Sierra Negra is located in proximity to LAGO, suggesting potential for future complementarity in high-energy transient detection.