Study of the effects of radiation on the CMS Drift Tubes Muon Detector for the HL-LHC

Abstract

The CMS drift tubes (DT) muon detector, built for withstanding the LHC expected integrated and instantaneous luminosities, will be used also in the High Luminosity LHC (HL-LHC) at a 5 times larger instantaneous luminosity and, consequently, much higher levels of radiation, reaching about 10 times the LHC integrated luminosity. Initial irradiation tests of a spare DT chamber at the CERN gamma irradiation facility (GIF++), at large (∼ O(100)) acceleration factor, showed ageing effects resulting in a degradation of the DT cell performance. However, full CMS simulations have shown almost no impact in the muon reconstruction efficiency over the full barrel acceptance and for the full integrated luminosity. A second spare DT chamber was moved inside the GIF++ bunker in October 2017. The chamber was being irradiated at lower acceleration factors, and only 2 out of the 12 layers of the chamber were switched at working voltage when the radioactive source was active, being the other layers in standby. In this way the other non-aged layers are used as reference and as a precise and unbiased telescope of muon tracks for the efficiency computation of the aged layers of the chamber, when set at working voltage for measurements. An integrated dose equivalent to two times the expected integrated luminosity of the HL-LHC run has been absorbed by this second spare DT chamber and the final impact on the muon reconstruction efficiency is under study. Direct inspection of some extracted aged anode wires presented a melted resistive deposition of materials. Investigation on the outgassing of cell materials and of the gas components used at the GIF++ are underway. Strategies to mitigate the ageing effects are also being developed. From the long irradiation measurements of the second spare DT chamber, the effects of radiation in the performance of the DTs expected during the HL-LHC run will be presented.

Background in the CMS muon detectors: simulation and measurements with pp collision data

“…Chambers of all the muon subdetectors are being irradiated at the GIF++. We refer to [11] for a detailed discussion of the longevity studies performed by the DTs. The CSCs have collected a The simulation is able to reproduce the shape of the data, with a quantitative agreement at the level of 10% to 30% in the barrel, and within a factor 2 in the endcap.…”

The Analytical Method algorithm for trigger primitives generation at the LHC Drift Tubes detector

“…The accumulated charge from this radiation can result in significant ageing effects that degrade the DT cell performance and lower the DT chamber efficiency. Measurements of the DT detector performance under high radiation have been conducted in the new CERN Gamma Irradiation Facility (GIF++) [6] and [7]. According to these results, ageing scenarios can be simulated by randomly removing DT hits, according to probabilities obtained by the expected inefficiencies in each detector region.…”

Study of the effects of radiation at the CERN Gamma Irradiation Facility on the CMS Drift Tubes muon detector for the HL-LHC

“…These results are an updated version with respect to the results presented at ref. [9] for taking into account the increase of the flux at HL-LHC suggested by the last CMS simulations. The simulation shows indeed an impact in the flux expected in the DT chambers with an 30% increase in the MB1 most external wheels (MB1 YB±2), chambers used as reference for these longevity results.…”

“…These efficiencies have been estimated considering a safety factor of 2 for the expected HL-LHC background rate (10 × 10 34 cm −2 s −1 ) and a safety factor 2 for the expected integrated luminosity (2 × 3000 fb −1 ), to cope with the various uncertainties in the final HL-LHC conditions and the data taking. In addition the hit efficiency presented here includes also the extrapolation of the muon beam results at GIF++ taking into account the effect of the average track angle in each chamber at CMS (angle corrections not included in [9]).…”