Improving time and position resolutions of RPC detectors using time over threshold information

Abstract

INO-ICAL is a proposed underground experiment to study the oscillation parameters of neutrinos by detecting those produced in the atmospheric air showers. The Iron CALorimeter (ICAL) is to have 151 layers of iron plates stacked vertically corresponding to a height of ∼14.5 m, with active detector elements inserted between the iron layers. The iron layers will be magnetized with a maximum magnetic field of 1.5 T to enable the measurement of the charge and momentum of the μ - (or μ +) produced by ν μ (or ν̅ μ ) interactions inside the detector throughout the operation. Resistive Plate Chambers (RPCs) have been chosen as the active detector elements as they are amenable to large area coverage, as well as due to their high particle detection efficiency, long-term performance and low cost of production. The major factors that weigh in on the physics potential of the ICAL detector are the efficiency, position resolution and time resolution of the large area RPCs. A prototype detector called mini-ICAL (with 11 iron layers) was commissioned in order to understand the engineering challenges in building the large-scale electromagnet and its ancillary systems, and also to study the performance of the RPC detectors and readout electronics developed by the INO collaboration. As part of the performance study of the RPC detectors, an attempt is made to improve their position and time resolutions. The designed position resolution for the ICAL detector is of the order of 1 cm and the required time resolution is ∼1 ns. Even a small improvement in the position and time resolution will help to improve the measurements of momentum and directionality of the neutrinos in the ICAL detector. In ICAL detector simulation, where muons traverse in all directions with on average 20 RPC layers, a 30 % improvement in position resolution results in ∼10 % improvement in the momentum resolution at 3 GeV. Due to large iron materials in between two RPCs, the resolution is dominated by the effect of multiple scattering at this momentum. Also for a muon with 10 layer hit, the charge ambiguity reduces from 0.04 % to 0.001 % when the time resolution improves from 1 ns to 0.7 ns. The Time-over-Threshold (ToT) of the RPC pulses is recorded by the readout electronics. ToT is a measure of the pulse width and consequently the pulse amplitude. This information is used to improve the time and position resolution of RPCs and consequently INO's physics potential.