DT HIT EFFICIENCY MEASUREMENTS WITH REDUCED HIGH VOLTAGE

See also: CMS DP-2019/041

For details on the method used to compute the hit efficiency, see: CMS DP-2018/016 or: this twiki page

DT hit efficiency dependence on anode High Voltage (measurements carried out with cosmic rays)

A hit signal is generated in a Drift Tube by the electrons produced in the gas by an ionising crossing track: these electrons will drift within the tube and reach the anode wire, where the signal is generated by charge amplification. The signal is then discriminated and readout.

It is therefore expected that the efficiency to produce a signal depends on the applied anode High Voltage (HV) and Front End (FE) threshold.

The hit efficiency is measured as a function of the HV in order to determine the optimal working point, i.e. a value which is at the same time well within the plateau (in order to guarantee performance stability) and as low as possible (in order to prevent detector early ageing).

The HV scan was repeated six times in 2017 and 2018, using cosmic rays: soon after the winter shutdown and at the end of the data taking periods with LHC colliding beams. The integrated luminosity accumulated in the corresponding time intervals was ~50 fb-1 in 2017 and ~68 fb-1 in 2018. Goals were: 1) to check any hint of early ageing 2) to check the effect on efficiency of lower FE thresholds applied at the beginning of 2018.

Results are shown for stations MB1 and MB4, containing the chambers most exposed to radiation.

The plots show superimposed results of the different HV scans, for the external wheels and for Wheel 0 separately; they are averaged on the sectors, i.e. on Φ.

In 2017 the HV of the MB1 chambers of external wheels and that of the MB4 chambers of top sectors were lowered from 3600 to 3550 V, in order to protect these chambers. This value is still well within the efficiency plateau, as it can be checked from the plots. In 2018 a further reduction of 50 V was applied to the external MB1s: this would have caused a small expected efficiency drop but this drop was prevented, as shown in the plots, by lowering the FE threshold from 30 to 20 mV.

No clear hint of ageing is shown by these results. The variations observed between different HV scans with the same FE threshold are probably caused by fluctuations related to atmospheric conditions, in fact in one case a higher hit efficiency was observed after a few months.

AllScans2017-2018_30_20mV.png

pdf version of this plot

DT Hit Efficiency as a function of anode HV at P5. Cosmic data 2017-2018. Results of the campaigns of cosmic data taking happening each year after the winter shutdown are superimposed to HV scans done at the end of the yearly LHC data-taking: the integrated luminosities were ~50 fb-1 in 2017 and ~68 fb-1 in 2018. No clear sign of ageing is visible. The lower FE threshold value applied in 2018 prevented the expected efficiency loss caused in the MB1 chambers of external wheels by the HV set at 3500 V, i.e. slightly below the efficiency plateau. The small decrease of efficiency observed at the end of 2018, as the increase observed instead in early 2018 w.r.t. end 2017, was likely due to different atmospheric conditions.

DT hit efficiency dependence on track inclination, observed with lower than nominal High Voltage (measurements carried out with a special pp collision run)

A hit signal is generated in a Drift Tube by the electrons produced in the gas by an ionising crossing track: this electrons will drift within the tube and reach the anode wire, where the signal is generated by charge amplification. The signal is then discriminated and readout.

It is therefore expected that the efficiency to produce a signal depends on the amount of the primary ionized charge.

This amount is not constant but depends in turn on the path that the crossing track travels within the drift tube.

In particular, when considering wires belonging to “phi super-layers”, which are laid along the beam direction and measure the track position on the transverse plane, we expect a larger hit efficiency in the high 𝛈 regions than at 𝛈≃0 for track coming from the Interaction Point (see sketch below).

Sketch.png

In order to increase sensitivity and make this effect visible, we set the anode voltage at 3450 V: a lower value than nominal, below the efficiency plateau.

During proton-proton data-taking, for one single LHC fill, the anode voltage of one layer of wires in each chamber was lowered from 3600 to 3450 V and the hit efficiency wasmeasured for each DT “ring” i.e. for each wheel and station.

The plots show the measured efficiency, station by station, as a function of the path within a single tube, of the track inclination and of the DT wheel. The corresponding trend obtained with HV = 3600 V (nominal value, within the efficiency plateau) is also shown for comparison.

Effi3450and3600.png

pdf version of this plot

Efficiency to detect a hit in the Drift Tubes with HV=3450V, i.e. ≈ 100V below the efficiency plateau, and with the nominal value of 3600V. The hit efficiency is shown station by station as a function of the wheel: with 3450V it is higher in the external wheels, due to the longer path travelled by crossing tracks within individual Drift Tubes. (Notice that some chambers of the MB4 station in Wheel 0 had their FE threshold reduced from 30 to 20 mV, which caused somewhat higher average efficiency in this region). No dependence is visible on the polar angle when the anode voltage is set at the nominal value within the efficiency plateau.

Effi3450_vsTheta.png

pdf version of this plot

Efficiency to detect a hit in the Drift Tubes when the anode voltage is set ≈ 100V below the efficiency plateau. The hit efficiency is shown station by station and wheel by wheel as a function of the average polar angle, measured in the Θ super-layer of the same chamber. The efficiency is lower for tracks that are normal to the chambers, due to the shorter path travelled within the Drift Tubes. The effect is consistent for stations 1, 2 and 3. (Notice that no such measurement is possible for the MB4 chambers as they have no Θ super-layer and don’t provide any z coordinate of the crossing tracks)

Effi3450_vsPath.png

pdf version of this plot

Efficiency to detect a hit in the Drift Tubes when the anode voltage is set ≈ 100V below the efficiency plateau. The hit efficiency is shown station by station and wheel by wheel as a function of the average path travelled by the track within one tube, on the zR plane (this path is computed from the polar angle, measured in the Θ super-layer of the same chamber). For tracks that are normal to the chambers this path is minimum and matches the tube’s depth of 1.3 cm. For inclined tracks the path increases and so does the efficiency. (Notice that no such measurement is possible for the MB4 chambers as they have no Θ super-layer and don’t provide any z coordinate of the crossing tracks)

-- FrancescaCavallo - 2019-11-19

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Topic revision: r5 - 2020-01-31 - FrancescaCavallo
 
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