a I.N.F.N. Sezione di Roma Tre, Dipartimento di Fisica, Università di Roma Tre, Italy

1. vertical support Si module xy pair base beam direction rotator Chamber position (mm) T = -2 K drift time (tdc counts) T = -4 K Test beam: preliminary ○ <>  250 µm Ar-CF4 93-7%, HV=3080 V, p=3 bar ● <>  120 µm ■ <>  80 µm Systematic study of the calibration of the drift tubes for muon tracking in the ATLAS experiment at the LHC and possible use of fast gas mixtures for SLHC T.Baroncellia, P.Branchinia, S.di Luisea, E.Graziania, G.Morellob, F.Petruccia, D.Saccoc , M.Schioppab a I.N.F.N. Sezione di Roma Tre, Dipartimento di Fisica, Università di Roma Tre, Italy bDipartimento di Fisica Unical, Gruppo collegato I.N.F.N. Cosenza, Italy cISPSEL – DIPIA – UF X, Monte Porzio Catone and I.N.F.N. Sezione di Roma Tre, Italy The ATLAS Muon Spectrometer The MDT drift tube The autocalibration • Iterative procedure: • Straight line tangent to drift circles • Evaluate residuals • Compute mean value of residuals in time slices • Use it as correction to r-t relation Al tube R=15mm 400mm thick • Absolute pressure: 3 bar (reduce diffusion effects) • No hydrocarbons: avoid ageing • Ar-CO2 93-7% (high ionization density) • Gain 2x104 (HV=3080 V) • Discriminator threshold: 20 primary e- (~ 40 mV) • Max drift time: 700 ns • Single point resolution  80 µm The gas mixture Ar-CO2 93-7% makes the drift response function rt slow and highly non-linear, thus depending on environmental conditions such as temperature, pressure and local magnetic field. Anode wire 50 mm W The ATLAS Muon Spectrometer provides a stand-alone muon momentum measurement (Δpt/pt<10% up to pt=1 TeV/c) by means of the deflection of tracks in a large superconducting air-core toroid magnet. TDC ADC electron drift time signal amplitude The test beam experimental setup The chamber is enclosed in a box, to limit thermal exchanges. Two on-chamber probes monitor the temperature with a 0.1 K accuracy. Four Peltier cells are placed inside the box to keep the temperature at desired values. The goals of the test The chamber can slide vertically and rotate around an horizontal axis. The two axis system is controlled by step motors. The silicon tracker detectors and the scintillation counters are precisely aligned and fixed on the base of the mechanical support. The telescope consists of 5 xy plane of silicon microstrips with 50 µm pitch, originally developed for the CDF experiment at Fermilab. The trigger is based on 2 pairs of scintillation counters, 12 x 12 mm2 active area. The gas mixture is precisely set by mass flow meter controllers. Tracking performances :  10 µm point resolution • The main goal was the systematic study of the calibration of drift tubes with the aid of an external, precise, tracking detector. We brought (two data campaigns) a small MDT chamber at the 180 GeV/c muon beam at the Cern H8 North Area. We used a precise microstrip silicon telescope for external tracking and fast, tiny spot, scintillators as trigger devices. We intended: • measuring the tube drift properties with the standard gas mixture and compare with autocalibration • measuring the drift properties of faster gas mixtures for SLHC Test results Drift time spectrum vs T rt relation Temperature effects on rt relation The variations of the rt relations with the temperature are a very important subject for calibration. The expected pattern of such variations, as a function of the drift time, may be explained as follows: for large drift times the dominant effect is the decrease of the gas density with increasing temperatures and the consequent increase of the average drift velocity. For small drift times, in the region close to the anode wire, the behaviour is opposite, as the same effect of decreasing density with increasing temperature, produces an increase of the average distance between the primary ionization clusters, with a consequent increase of the drift times. The size of the effect and the shape of the functions agree well with the simulation and with the results of previous measurements. The maximum drift time length decreases with increasing temperature due to the dominant effect of the decrease of the gas density which, in turn, causes an increase of the average drift velocity. The test beam setupallows to directly reconstruct the space time relation rt of the drift tubes by associating the drift times, measured by the chamber tubes, with the externally estimated radii (distances to the wire or impact parameters), measured by the precise silicon telescope. Good overall (preliminary) agreement of the “signed” rt with the autocalibration and simulation results. In order to study detailed effects at  10 µm scale, a full comprehension of the test beam alignment issues is mandatory. Faster gas mixtures for SLHC: Ar-CF4 Garfield simulations The SLHC option implies a much higher detector occupancy for the ATLAS muon spectrometer Test beam results ■ Ar-CO2 93-7%, HV=3080 V, p=3 bar ○ Ar-CF4 93-7%, HV=2350 V, p=1 bar ● Ar-CF4 93-7%, HV=3080 V, p=3 bar • Need faster detector; • Redo FE electronics; • Redo L1/L2 trigger; • Keep ageing even more under control. resolution • CF4 based mixtures are known to be fast and linear; • linearity  less dependence on environmental parameters and space-charge effects; • Resolution has to stay good (requirements from physics); • Outside (S)LHC: one can think of a tight detector operated at atmospheric pressure, less HV, worse (but still good!) resolution; Many effects contribute to the resolution: space-time relation linearity, drift velocity, diffusion. While the average drift velocity of CF4 is more than 3 times larger, the resolution is only 50% worse if compared to CO2 based mixtures. The raw time spectrum (which already contains a lot of information about the rt relation) confirms the strong, expected, reduction of the maximum drift time. Much more work is currently in progress. One of the most important physical effects contributing to the resolution is the diffusion that electrons undergo while drifting towards the anode wire. The size of the effect decreases with the pressure. The Ar-CF4 mixture shows a smaller longitudinal diffusion coefficient than the standard ATLAS one. We investigated two possible choices (out of the many), both based on Ar-CF4 93-7%, operated to give ~ 200 ns max drift time: Ar-CF4 93-7%, HV=3080 V, p=3 bar Ar-CF4 93-7%, HV=2350 V, p=1 bar CF4 based mixtures result, as expected, faster and much more linear than CO2 ones Conclusions Now Neutron test at Tapiro • test beam results confirm (preliminarly) a good agreement between simulation (Garfield), autocalibration derived and externally, precisely measured Ar-CO2 drift properties; • the maximum drift time length decreases with the temperature as expected: tMAX/T = -2.3  0.2 ns/K; • differences between space-time relations rt at different temperatures behave as expected and agree both with the simulation and with previous test beam and cosmic ray results; • faster, Ar-CF4 based, gas mixtures look very promising for a possible use at SLHC; • for time spectrum lengths of  200 ns (which would allow the use of the MDT chamber information at the first level trigger) the single point resolution worsens only by 50%, with an increase of the average drift velocity of more than 300%; • test beam measurements confirm (preliminarly) the expectations of the simulation for the drift properties of CF4 mixtures; • the actual use of CF4 mixtures at SLHC would require tight limits on H20 content (dryer necessary). To do • all test beam results are preliminary; • detailed study of the drift properties of the Ar-CF4 mixture in the 10 µm range, by fully exploting the high resolution of the silicon tracker; • aging properties of CF4 mixtures, for both  and neutrons, must be demonstrated; • we plan to perform aging tests at Cern-GIF facility (for ’s) and at Enea-Casaccia (Tapiro reactor) for neutrons.