Xiong Zuo IHEP, CAS, for the LHAASO Collaboration 2017-07-17 ICRC2017 Calibration and monitoring of LHAASO-KM2A muon detectors with muon decay events Xiong Zuo IHEP, CAS, for the LHAASO Collaboration 2017-07-17

Outline Introduction MC simulation Experiment setup Data selection 1 Introduction 2 MC simulation 3 Experiment setup 4 Data selection 5 Conclusion

P art 1 Introduction

1.1 LHAASO MD array MD 4400 m a.s.l 1171 MDs, ~40000 m2 Effect drawing of LHAASO

1.2 MD unit MD Two prototype muon detectors in Tibet Schematic of LHAASO muon detector 5

1.3 Physical requirements of MD unit Detection efficiency > 95% Signal charge resolution for single muon < 25% Time Resolution < 10 ns Purity of number of muons detected Dynamic range 1-104 muons Signal attenuation in 10 years < 20% Signal charge distribution of vertical muons Performances meet the primary design well; Design and performances of prototype muon detectors of LHAASO-KM2A X. Zuo, et al., Nuclear Instruments and Methods in Physics 284 Research Section A 789 (2015) 143-149. Normalized NPE of single muon signals varying with time

1.4 Calibration requirements Parameters Requirements Statistical error (experiment data) System error Signal charge of single muon < 1% < 0.3%@50000 events < 0.5% Time offset < 3ns < 0.1ns@50000 events < 2ns A/D ratio ~ 0.4%@50000 events < 0.2% Water absorption length < 15%@100m < 1%@100m@4000 waveforms ~ 11.5%@100m Water level < 2cm ? Calibration accuracy meets the requirement well for each parameter except the water level.

1.5 Calibration of the water level So large area of MD array in tough high-altitude environment; Deeply overburdened by soil; Liquid-level sensor increase the burden on installation, maintenance, cost, DAQ and electronics; On-line muon decay method possess simplicity and convenience.

1.6 Muon decay method Typical life time: 2.2 μs; Michel electron: Eave=37 MeV and Emax =53 MeV; Charge of vertical equivalent muon (VEM)∝ water level; Michel electron deposits all energy in MD; QVEM/Qe is only related to the water level.

P art 2 MC simulation

2 MC simulation R= 97.5%, AL= 150 m; θ~cos6θ, ϕ~0-360˚; E~ energy spectrum of secondary muons; D~109-120 cm; QVEM/Qe~=6.8 (D=120 cm); Probability: 6.5%. Charge distribution of Michel electron (blue line) and VEM (red line) in simulation. QVEM/Qe versus the water level

P art 3 Experiment setup

3 Experiment setup 500 MHz 12-bit FADC; pedestal + time + charge+…. => hit package ; 5 μs waveform per second. Schematic view of MD FEE and DAQ system

P art 4 Data selection

4.1 Selection using waveforms Stopping muon (first) and Michel electron (second); Integral windows: 400-1200 ns and 1400-4500 ns; 3.87 million waveforms in 2 months with D=120 cm; Backgrounds dominant; Other features needed. 5 μs T1/Q1 T2/Q2 Schematic view of candidate decay events Single muon Schematic view of MD FEE and DAQ system

4.2 New statistical variable Statistical variable Tc: mean hitting time of PMT by cluster photons; Tsum - sum of the time intervals; Ncluster - number of photoelectrons (PEs) clusters. Characteristic peak around 50 ns for the data selection. Stopping muon（Q1） Michel electron（Q2） A waveform of muon decay event in simulation Tc distribution of the single pass-through muon (red one) and the Michel electron (blue one) in simulation

4.3 Michel electron events Cut-offs: Q1 < 50 PEs, 20 ns < Tc < 120 ns; Michel electron signal: ~ 10.5 PEs; QVEM/Qe~=6.8 (D=120 cm); Q2 < 25 PEs => Time index ~= 2.2 μs; Event rates: 0.208% (experiment) and 0.199% (simulation). Charge of the second events after several cut-offs Time difference between the first and second event

4.4 Water level experiment D: 120 cm to 112 cm; Qe decreases as the water quality deteriorates. Simulation agree with experiment data; ~ 1.3 cm accuracy for calibration of water level. Charge of VEM signal verses water level Charge of the Michel electron signal verses water level

5 Conclusion The large area of LHAASO MD array would contributes to Υ/p discrimination, primary hadron compositions discrimination and HIM test; All the calibration parameters meet the design requirement; The on-line muon decay method based on waveforms, studied by MC simulation, is presented to calibrate and monitor the water level. The statistical variable Tc has been constructed and suppressed successfully the backgrounds; Qe ~= 0.147 QVEM, QVEM/Qe has a linear correlation with the water level, experiment data validates the simulation.

Thanks!!!