Development, Integration and Performance Verification of the Expanded GRAPES-3 Muon Telescope

The GRAPES-3 muon telescope is undergoing a major expansion that nearly doubles its active detection area from 560 m² to approximately 1130 m² and increases the number of proportional counters from 3712 to 7488. This work presents the large-scale development, integration, and detector-level performance verification of the expanded system, including refurbishment of legacy proportional counters, upgraded front-end electronics, individual-counter characterization, and progressive integration with the existing telescope.

The development has been guided by the continuous 24×7 operation of the existing GRAPES-3 muon station. During 2011–2022, data were routinely monitored through weekly remote performance meetings involving collaborators from Japan, Mumbai, and the experimental site, providing direct feedback on counter behaviour, electronics, operational stability, and failure modes. This experience was extensively incorporated into refurbishment procedures, verification methods, and acceptance criteria for the expanded system.

The refurbishment involved opening and systematically cleaning legacy proportional counters, with the methodology validated using XRD, FTIR, SEM-EDX, and pulse-height analysis. Approximately 3,300 counters were processed using a scalable vacuum-manifold system achieving a vacuum better than 3.6 × 10⁻⁷ mbar at a rate of up to 10 counters per day. Following evacuation, helium leak testing, thermal degassing, and P10 gas filling, more than 97% of the processed counters satisfied predefined acceptance criteria for leak integrity, gas retention, and pulse-height response.

The upgraded front-end electronics, incorporating a spectroscopic amplifier and Maestro-based MCA, reduced the electronic noise floor by up to 30%. Counter characterization showed stable pulse-height spectra, with six-month monitoring confirming gain stability and count-rate variations below 2%. The present overburden gives a vertical muon energy threshold of ~0.5 GeV; remaining concrete overburden will raise it to ~1 GeV, matching the existing muon station. Full-system performance verification and major physics results will follow completion of the overburden and final commissioning.