Studies of the gas volume current density in the ATLAS Resistive Plate Chamber detector during the 2018 data-taking at the LHC

The ATLAS Resistive Plate Chamber (RPC) detector is a tracking detector, used to primarily trigger muons in the ATLAS barrel region (which corresponds to a pseudorapidity range of |η|<1.05) at the 40 MHz collision rate, and to provide η and ϕ coordinates. The RPC system consists of about 3700 gas volumes covering a sensitive surface of about 4000 m2. It is arranged in three concentric doublet layers at radius 7 m, 8 m and 10 m, and operating in an approximately 0.5–1 T toroidal magnetic field. RPCs provide up to 6 space points along the muon track with a space-time resolution of about 1 cm2 × 1 ns. This document studies systematically the gas volume current as a function of the electric field applied across the gas volume and of temperature both with and without the Large Hadron Collider (LHC) beam induced background and up to an instantaneous luminosity Linst = 2×1034 cm−2 s−1 (twice larger than the design LHC luminosity). These measurements have been used to study the RPC working conditions and to extrapolate the detector response to the High-Luminosity LHC regime with Linst = 7.5×1034 cm−2 s−1.

The ATLAS Resistive Plate Chamber (RPC) detector is a tracking detector, used to primarily trigger muons in the ATLAS barrel region (which corresponds to a pseudorapidity range of |η| < 1.05) at the 40 MHz collision rate, and to provide η and φ coordinates. The RPC system consists of about 3700 gas volumes covering a sensitive surface of about 4000 m 2 . It is arranged in three concentric doublet layers at radius 7 m, 8 m and 10 m, and operating in an approximately 0.5-1 T toroidal magnetic field. RPCs provide up to 6 space points along the muon track with a space-time resolution of about 1 cm 2 × 1 ns. This document studies systematically the gas volume current as a function of the electric field applied across the gas volume and of temperature both with and without the Large Hadron Collider (LHC) beam induced background and up to an instantaneous luminosity L inst = 2 × 10 34 cm −2 s −1 (twice larger than the design LHC luminosity). These measurements have been used to study the RPC working conditions and to extrapolate the detector response to the High-Luminosity LHC regime with L inst = 7.5 × 10 34 cm −2 s −1 .

K
: Resistive-plate chambers; Particle tracking detectors (Gaseous detectors); Trigger detectors emitter-coupled logic standard. The RPC is equipped with a Detector Control System [4] to monitor and safely operate around 380k readout channels. More details about the RPC can be found in Reference [5].
Efficient gas volume performance in various LHC running conditions is crucial for detector operation. The gas volume current is used in these studies to check the stability of the RPC as a function of the instantaneous luminosity (L inst ). Measurements of the gas volume current as a function of the electric field applied across the gas and of the temperature form the basis for the extrapolations of the RPC response to the High-Luminosity LHC (HL-LHC) regime. For studies with the LHC beam induced background, proton-proton data collected at √ s = 13 TeV during the second data-taking period of the LHC in 2018 are used.

Results with LHC beam induced background
The current is obtained individually for each gas volume, by measuring the voltage across a 100 kΩ resistor in serial connection with the gas volume and ground. For uniformity all currents are normalized to the gas volume area. Currents in gas volumes measured at the working RPC voltage but without proton-proton collisions are subtracted to show the net effect of the luminosity. The RPCs disconnected from the high voltage (HV) or those exhibiting low efficiencies are excluded from the analysis. These amount to about 10% of the chambers. The expected linear increase of the gas volume current densities is observed during stable RPC operation with L inst up to 2.1 × 10 34 cm −2 s −1 . Figure 2 (left) shows the gas volume current densities as a function of L inst for individual RPCs. Their increase with η is due to thinner shielding materials in the forward region of the RPC. Additionally, charged particles at a larger η have a longer track length in the gas volume, producing more gas ionization. Figure 2 (right) shows averaged current densities over gas volumes for various RPC chambers in the η station 1 as a function of L inst , shown separately for the middle (BMx) and outer (BOx) layers, and large (L) and small (S) chamber types. This plot shows that the gas volume current density decreases with increasing distance from the proton-proton collision point. This effect is explained by the particle flux reduction at larger radii. Discrepancies in current densities between small and large chambers in the same layer are explained by their location with respect to the barrel coils, which systematically shadow the small sectors. Additionally small chambers are located at slightly larger radii from the interaction point than large chambers.
Results shown in figure 2 confirm the expected performance in the highest luminosity conditions during the second period of LHC operation and allow to extrapolate the gas volume current density to the HL-LHC operation conditions, where L inst is expected to reach ∼ 7.5 × 10 34 cm −2 s −1 .
RPC current densities have been measured at different operating voltages. The range, from 9.0 kV up to 9.6 kV, is chosen to address future possible RPC working conditions. All the current measurements are scaled to the same L inst = 1.8 × 10 34 cm −2 s −1 , with the largest correction of 3%. Voltages are also corrected for temperature and pressure effects. Each current density measurement is performed twice to estimate an uncertainty, arising from the variations of instantaneous luminosity within the short time interval, when it assumed to be constant in relation to the current density. This uncertainty is defined as half of the difference between the two current density measurements separated by approximately 60 s and scaled to the same L inst . Figure 3 (left) shows the gas volume current densities as a function of the voltage for 3 individual BML RPC gas volumes in different η stations and the same φ sector. It is observed that the gas volume current during the beam collisions increases with the high voltage approximately linearly and uniformly across the whole RPC detector in the voltage range from 9.0 to 9.6 kV. Therefore an extrapolation of the gas volume current can be performed individually per gas volume or with one global scale factor (SF). A comparison between these two procedures is shown in figure 3 (right). A global SF of 2.1 is defined as the mean value of the ratio, per gas volume, of the current densities at 9.6 kV and 9.2 kV. As it can be seen from the plot individual scaling provides a more accurate extrapolation for current densities in this voltage range. Figure 4 (left) shows distributions of the gas volume current densities for the whole RPC system during data-taking in 2018. Very few gas volumes have a current density higher than 12 µA/m 2 at 9.6 kV and L inst = 1.8 × 10 34 cm −2 s −1 . To build the extrapolations to the HL-LHC regime, gas volume current densities measured at 9.2 kV and 9.6 kV are scaled by a factor 4.2, coming from the ratio between L inst = 7.5 × 10 34 cm −2 s −1 and 1.8 × 10 34 cm −2 s −1 . Figure 4 (right) shows the resulting distributions of extrapolated current densities for the whole RPC system. Some gas volume current densities exceed 30 µA/m 2 , as reported in the Technical Design Report for the Phase-II Upgrade of the ATLAS Muon Spectrometer [7] as a safe limit for RPC. The overcurrent in the affected gas volumes can be addressed by lowering the voltage from 9.6 kV to 9.2 kV. This change will lead to a loss in efficiency that can partly be recovered by adjusting the thresholds of the discriminator [9]. Figure 5 shows the expected gas volume current densities for the HL-LHC program as a function of η and φ for gas volumes belonging to the middle layer (averaged over two sub-layers) at V = 9.2 kV (left) and 9.6 kV (right). Expectations are based on individual linear fit approximations. These two-dimensional η-φ maps show in details hot gas volumes at 9.6 kV. Figure 5 also shows that the expected current density slightly decreases for the gas volumes located at the bottom of the ATLAS detector, which is compatible with the temperature gradient in     Figure 6 shows current densities for the RPC BML gas volumes averaged over all φ sectors in η stations 1, 4, 6 as a function of the temperature.

Results without LHC beam induced background
Gas volume current density studies without proton-proton collisions provide a unique and efficient instrument to monitor the state of the gas volumes. It is found that through the entire detector the mean of the gas volume current density distribution without beam induced background increases by 15% from V = 9.2 kV to V = 9.6 kV, where a 5% increase comes from the ohmic contribution, while the other 10% are from the avalanche growth. These results are shown in figure 7 (left) and confirm the overall good quality of gas volumes in the RPC detector. It is found that the dependency of the gas volume current on the voltage at these conditions can be linearly described up to V = 6 kV, then the current density starts to increase exponentially with the voltage. Figure 7 (right) shows the gas volume current density as a function of the voltage for 2 individual RPC middle large gas volumes without beam induced background. The discrepancies between these two curves indicate a difference in operation mode of these gas volumes. In general, discrepancies in linear slopes (V < 6 kV) are connected to a different level of cleanliness or deposition on the frame and spacer surfaces within the gas gap volume.

Conclusion
RPC gas volume current densities are studied as a function of the instantaneous luminosity, voltage and temperature with and without LHC beam induced background. Studies at various luminosities and fixed voltage confirm the expected linear increase of the gas volume current density up to the highest instantaneous luminosity during the second data-taking period in 2018 at the LHC for various RPCs. Results show that the dependency of the gas volume current density on the voltage with LHC beam induced background can be linearly described in the voltage operation condition range 9-9.6 kV. Studies of gas volume current density extrapolations to HL-LHC conditions show that lowering the voltage to 9.2 kV will help address the overcurrent in affected gas volumes at HL-LHC, however with some loss of efficiency. A consistent increase of the gas volume current density with temperature across the whole RPC is observed (≈ 6 µA/m 2 per 10 • C). Studies of gas volume currents as a function of the voltage without LHC beam induced background are an efficient instrument to monitor the state of gas volumes.