Table of Contents • Introduction to CERN • Detectors • Optimization of the closed-loop gas system for the RPCs at LHC • Test setup • Optimized purifier configuration • Validation of the Purifiers • RPC performance at long term • Monitoring Impurities • Conclusions • CF4 recuperation unit for the CSC of CMS • Test Setup • Further Tests • Word of Thank

Introduction to CERN

Introduction to CERN • Standard Model • Why do particles have mass? • Higgs Boson • Supersymetry

Introduction to CERN • Gold plated Higgs Boson decay, no missing momentum H→ZZ*→4μ • Excellent detecting properties • Low background signal

Introduction to CERN CMS

Introduction to CERN • Muon Spectrometer • 250 Drift tubes • 540 Cathode Strip Chambers • 610 Resistive Plate Chambers

Detectors • Principles of particle detection • Interaction with matter • Energy transfer to an electron • Electron acceleration • Charge multiplication • Charge collection Some examples: • Gaseous detectors; Drift Tubes, Resistive Plate Chambers and Cathode Strip Chambers. • Cherenkov detector, Silicon detector.

Detectors • Resistive Plate Chambers(RPCs) • Planar gaseous detector • Resistive plates made out of Bakelite • 2 mm wide gap between the plates • Read-out strips glued on the outside of a gap • 2 gaps per set of read-out strips • Suitable spatial resolution (~ 1 cm) • Can be operated in a high background environment • High rate capability • Long Term performance (>10 LHC years)

Detectors • Gas flow through the gap • SF6 (0.3%) • C2H2F4 (94.7% • iC4H10 (5.0%) • Voltage applied across the plates • Incoming particle result in the production of an avalanche • The avalanche production and movement of charges induces a charge on the read-out strips

Detectors • Cathode Strip Chambers (CSCs) • Multiwire Proportional Chamber with stripped cathode read-out, planes as cathode and wires as anodes

Detectors • Gas consists of: • Ar (40%) • CF4 (10%) • Added to prevent the accumulation of polymers on the anode wires • CO2 (50%) • Avalanche productions occurs close to the wires • Avalanche creation and movement results in an induced charge on the cathode strips

Problems: closed-loop gas system for RPCs • LHC RPCs are operated in a closed-loop set-up • Large detector volume (~16 m3) • Relative expensive gas mixture • Tests have shown that when chambers irradiated at LHC levels, impurities are formed inside the gas, F- radicals and Hydro-Carbons • Potentially dangerous for the chambers • Purifiers to remove the impurities from the gas. • Purifiers have to be validated in a long-term test for optimal usage Supply Exhaust Resistive Plate chambers Purifiers

Problems: closed-loop gas system for RPCs Fresh gas (blue) and a sample of gas after the irradiated RPC chambers with the impurities (green) Impurities created inside the gas C2H6 C3H6 C2H2F2 CH4 C2H3F3 CH2F2 C2HF3

Test Setup: closed-loop gas system for RPCs • Twelve RPC gaps are exposed to radiation conditions close to the conditions at LHC • Gamma Irradiation Facility (GIF) • 650 GBq 137Cs source at the center of the facility • Radiation levels can be controlled via a set of filters • Chambers are placed in a tent in order to control the temperature (~20 °C) and humidity (35-40%) • Acceleration factor of ~30 to speed up charge collection • Allows to do ageing test in an accelerated way

Test Setup: closed-loop gas system for RPCs • Gas system similar to the one at LHC • Three double gaps RPC are operated in open mode and the other three are operated in a closed-loop • Open mode: no gas circulation, used as validation for the closed loop • Refresh rate of 1 volume change per/hour (~6 L)

Mixture from RPC Mixture to RPCs Ni Al2O3 MS 3 Å Cu-Zn R12 Cu R11 MS 5 Å P2 P1 Measured performance of purifiers with the RPC gas mixture • Current purifier configuration at LHC • Lifetime of the MS ~1.5 day • Lifetime of the metallic catalysts 15 days • Drawbacks • An increase in flow would result in a lower lifetime beyond the regeneration time • Still a large amount of impurities when the chambers are irradiated • Capacity to absorb water is low

Search of an optimized Purifier configuration Removal of certain types of impurities for some purifiers

Search of an optimized Purifier configuration • Objective for new purifier sets • Perfect H2O and O2 filtering • The removal of as much impurities as possible • Stable chamber currents over a long period of LHC operation • Optimized configuration and currently being tested: • 3 cartridges are available • MS 3 Å is replaced by MS 4 Å which removes more impurities. • Double the volume for Cu and Ni Al2O3. • H2O filtering is not improved

Search of an optimized Purifier configuration • Proposed new purifier configuration • Double capacity for H2O absorption • Optimized capacity of MS 4 Å, more absorption of F- and Hydro Carbons • Removal of the Ni Al2O3, it sometimes enhances to production of impurities • Has yet to be tested

Experimental methods: closed-loop gas system for RPCs • Validation of the configuration • 6 sampling points • 1: After the Mixer called Freshmix • 2: After the chambers in open mode called Omreturn • 3: After the chambers in Closed-loop • 4/5/6: Just after respectively purifier 1/2/3 • Purifier 1 contains 10% MS 5 Å and 90% MS 4 Å • Purifier 2 contains Cu (R11) • Purifier 3 contains Ni and Al2O3 • Sampling done with a μGC • Currents, temperature and humidity are constantly monitored 1 4 2 6 5 3

Experimental methods: closed-loop gas system for RPCs • Analysis methods • Daily checklist • Resistivity measurements of the bulk resistivity of the chambers • HV scans at different radiation levels • F- ion concentration measurements because • Gas analysis with a microGC/MS

Experimental methods: closed-loop gas system for RPCs Sample Injection with a carrier gas Argon • microGas Chromatograph/Mass Spectrometer • 3 columns used to improve the separation • OV1, used to separate Hydro-Carbons • PPU, used to separate Hydro-Carbons but in a smaller range • MS, used to separate Noble gasses and molecules like O2 • Thermal Conductivity Sensor (TCD) measures the thermal conductivity of a gas versus a reference flow (He). • Possibility to place a Mass Spectrometer in addition to the TCD to indentify peaks, this function is available for 2 columns Gas passes through the columns and gets separated Gas passes the TCD. Signal is recorded Gas is exhausted

Validation of the purifiers Ageing is related to the total charge collected by the detector Chambers run in open mode as an validation point Chambers run in closed-loop with the current LHC purifier configuration Chambers run in closed-loop with the optimized purifier configuration 3 1 2

RPC performance at long-term

Monitoring Impurities • Concentration of iC4H10 is constant • The main components decrease in concentration with respect to the Freshmix • The lowest concentration can be found in the Clreturn sample. • Incoming radiation breaks the main components

Monitoring Impurities Human interventions N2 level is defined by the refresh rate of the system. After an human intervention it takes about 1-2 weeks until the N2 returns to normal levels

Monitoring Impurities • Oxygen levels are in general lower due filtering with the purifiers • In addition the levels of oxygen are also decreasing faster after an human intervention, around 1 day to go to normal values • Human interventions are still visible

Monitoring Impurities • Water is monitored with the mass spectrometer in the Clafterp1 channel. • When the purifier is saturated traces of water are found MS saturation Counts Date

Monitoring Impurities • A typical example of an impurity (CH2F2) • Saturation of the MS is clearly visible • The slope is characterized by the amount of CH2F2 created inside the gaps and removed in the purifiers MS saturation

Conclusion • A systematic study of the RPC gas mixture has been performed. Several impurities have been identified. Their concentration monitored along time and correlated with the saturation of the purifiers. • Detector performances seem not to be affected by this level of concentration. After correction for environmental conditions, the RPC currents are very stable over all the test period. • An optimized configuration of purifiers is under test since 6 months: it allows improving the purifiers run cycle and the filtering performances on the LHC RPC detectors. It will be soon implemented in the LHC experiments.

Problems: CF4-recuperation unit for the CSC of CMS • The CSCs of CMS are operated in a mixture of CF4/CO2/Ar (10/50/40) • The cost of this mixture is almost totally defined by the cost of the CF4 • Large detector volume • Closed-loop operation is mandatory • N2 is diffusing through some parts of the loop and affecting the detectors performance • A recuperation system for CF4 has been proposed in order limit the operating costs and increase the flow through the system • With the CF4 being recuperated the injection of “fresh” CO2 and Ar can be increased • An increase in flow results in a better operation of the detector

Test Setup: CF4-recuperation unit for the CSC of CMS CSCgas system at CMS, the input to the CF4 recuperation plant is ~ 570 l/h

Test Setup: CF4-recuperation unit for the CSC of CMS Input from the system Membrane to separate CF4 from the other gasses The efficiency of the membrane is not 100%. The gas has to be further purified with a MS 4 Å To trap the CF4 a MS 13X is used. This MS also captures CO2

Ar CO2 CF4 Membrane • Membrane consists of many thin tubes spanned across the total length of the membrane • The walls of the thin tubes are permeable for some gasses • Permeable gasses are pumped out at the wall of the membrane while non permeable gases will be exiting at the end of the thin tubes

Test Setup: CF4-recuperation unit for the CSC of CMS • Recovery of the CF4 • When MS 13X is saturated a valve will be closed • The MS 13 X will be pumped down. During the process CF4 will be released by the MS and can be stored Valve

Test Setup: CF4-recuperation unit for the CSC of CMS • Valves are operated with Labview • Expected lifetime 1.5 h for the MS 13X • 42 l cardtridge

Further tests • Separation tests of a new cartridge • Efficiency will be tested at different pressure levels. Has been tested at 50 mbar. Will be tested up to 200 mbar. • 4 Cartridges will be needed for the final set-up • N2 absorption of the MS 13X. • Depends the absorbed amount of N2 on the percentage of N2 in the gas? Yes, cryogenic installation • Or is it a steady amount?

further tests • Recovery phase • 3 sub steps • N2 increase after certain amount of time

Word of thank A special word of thank to my supervisors at CERN: Dr. Mar Capeans Garrido Dr. Roberto Guida and to my supervisor at Fontys: Ing. Pé Philipsen

Table of Contents