[Paper Review] A Proportional Wire Chamber Array: GRAND's Status

J. Poirier, C. D’Andrea+3 more|arXiv (Cornell University)|Jun 18, 2003

Vibration and Dynamic Analysis3 citations

TL;DR

This paper presents GRAND, a 100m × 100m array of position-sensitive proportional wire chambers at the University of Notre Dame, designed to detect extensive air showers and muon tracks via two simultaneous triggers. It achieves 96% operational efficiency, measures muon angular resolution at 0.26°, and uses single muons as a signature for high-energy gamma rays, demonstrating a 6σ correlation with a solar flare and confirming near-isotropy in muon arrival directions with only 0.23% variation in right ascension.

ABSTRACT

Project GRAND is a 100m x 100m air shower array of position sensitive proportional wire chambers (PWCs) located at 41.7 degrees North and 86.2 degrees West at an elevation of 220m above sea level. Its convenient location adjacent to the campus of the University of Notre Dame makes it a good training ground for students. There are 64 stations each with eight 1.29 m^2 PWCs. The geometry of the stations allows for the angles of charged secondaries to be determined to within 0.26 degrees in each of two orthogonal planes; muons are differentiated from electrons and hadrons by means of a steel plate. Two triggers are run simultaneously: a multiple hit coincidence trigger, rich in extensive air showers, and a single track trigger, rich in secondary muon tracks. The former trigger is sensitive to primary energies greater than ~100 TeV, the latter to energies greater than ~10 GeV.

Motivation & Objective

• To develop a high-efficiency, student-friendly cosmic ray detection array for training and research.
• To measure the composition of cosmic rays near the knee of the spectrum (around 100 TeV) using muon tracking and shower reconstruction.
• To detect high-energy gamma rays via their secondary muon signatures using single-muon triggers.
• To study anisotropies in muon arrival directions, including solar and sidereal effects.
• To validate the use of single muons as a signature for gamma-ray primaries through correlation with transient events like solar flares.

Proposed method

• The array uses 64 stations, each with eight 1.29 m² proportional wire chambers (PWCs) arranged in orthogonal x and y planes, with a 50 mm steel plate above the bottom x-plane to distinguish muons from electrons/hadrons.
• Two simultaneous triggers are employed: a shower trigger requiring three stations in time coincidence and four tagged as coincident, and a single track trigger using an 800 Hz oscillator to read out all 40,960 wires in 7 μs.
• A shift register memory stores wire status until readout, and absolute time is recorded with 1 ms precision using WWVB-synchronized clocks.
• The shower trigger records compressed data to 8 mm Exabyte tapes, while the single track trigger selects stations with one or two hits per plane, storing 900 candidates before writing to tape.
• Monte Carlo simulations using CORSIKA and FLUKA model GRAND’s response to primary cosmic rays, including gamma rays and protons, with predicted muon yields of 0.23 per TeV gamma ray.
• Data are analyzed for isotropy by binning muons into 1° × 1° right ascension/declination bins and applying a smoothness test (σ/average < 0.03) to exclude spurious data.

Experimental results

Research questions

• RQ1 What is the angular resolution and efficiency of GRAND in detecting muons from extensive air showers?
• RQ2 Can single muons serve as a reliable signature for high-energy gamma-ray primaries?
• RQ3 What level of anisotropy exists in the arrival directions of muons, and what causes variations in right ascension and solar time?
• RQ4 How well does GRAND correlate transient muon bursts with known astrophysical events like solar flares?
• RQ5 What is the sensitivity of GRAND to primary cosmic ray composition near the knee of the spectrum?

Key findings

• GRAND achieved 96% operational efficiency and recorded 101 billion muons over 1,295 valid data files after applying a 3% smoothness cut to exclude data with high statistical variation.
• The array measures muon angular resolution at 0.26°, though atmospheric and geomagnetic effects degrade the effective resolution to ±3°–5° for primary direction reconstruction.
• The distribution of muons in right ascension shows only 0.23% variation from isotropy, indicating extreme isotropy in the cosmic ray arrival direction.
• The largest variation occurs in the hour-of-solar-day distribution (0.55%), likely due to solar magnetic field effects, which is more than twice the sidereal variation.
• A 6σ correlation was observed between a 0.6-hour muon burst and a solar flare on April 15, 2001, confirming GRAND’s capability to detect transient high-energy events.
• Single muons were found to be a detectable signature for gamma-ray primaries, with FLUKA simulations predicting 0.23 muons per TeV gamma ray at normal incidence.