Characterization of the existing and exploration of the new materials for CMS muons detector upgrade at LHC

Materials play an important role in the elementary particle detectors technology. The detector is one of the basic instrument in experimental particle physics research. The detector material choice is based on detection requirements, i.e., precision, efficiency, dimensions and cost. Many types of detectors are being used to identify and study elementary particles in diverse fields, such as medical, homeland security, cosmology, nuclear, subnuclear, astro and particle physics. The studies presented in this thesis concern materials that are used (or planned to be used) in two types of particle detection technologies such as Resistive Plate Counters (RPC) and the Gas Electron Multiplier (GEM) detectors for muon particles detection at the Compact Muon Solenoid (CMS) experiment. CMS records and identifies collisions between protons accelerated at the highest energies ever reached by the Large Hadron Collider (LHC), located at the European Centre for Nuclear Research (CERN) in Geneva, Switzerland. CMS is one of the four main particle detectors at the LHC. Goal of CMS and the LHC is to produce, detect and study the Higgs boson, and the elusive new particles that could explain the dark matter enigma in the Universe. The Higgs boson was observed in 2012, and since then new statistically significant data was accumulated. The new particles are being searched for in the data taking period just started (March 2017) that will continue for more than 10 years at ever increasing intensity and energy. CMS is composed of many sub-detectors systems such as silicon tracker, electromagnetic & hadronic calorimeter, muon systems, all immersed in the most powerful magnetic field ever built for momentum measurement. The muon system is of paramount importance, because of the prevalence of the new particles to decay in final states with a large content of muon particles. The CMS muon system uses three particle detection technologies, such as Drift Tubes (DTs) detectors, Cathode Strips Chambers (CSCs) and Resistive Plate Chambers (RPCs). The DTs and CSCs provide a precise measurement points for muon triggering and identification; the DTs and CSCs are installed in barrel and endcap region respectively and give coverage up to 0 < ||< 2.4, with being the pseudo-rapidity. The RPCs detectors provide an extra muon trigger, these are installed in both barrel and endcap regions. The existing RPC system has a coverage up to || < 1.6. Beyond this region the eight endcap stations are empty. To cover the high region, novel detectors are required in order to cope with high radiation level resulting by high density of particles. The Gas Electron Multiplier (GEM) technology was proposed and approved. This thesis reports on material studies carried on to characterize both the RPC and the GEM detectors. In the first phase of upcoming data taking periods (so called long shutdown 2 or LS2, scheduled for 2019), a GEM chamber station called GE1/1 will be installed. In the second phase called High-Luminosity LHC and scheduled in LS3 (2024), additional subsystems with GEM and RPC technology have been proposed as candidates for additional stations. The study of the main material composing a GE1/1 detector (the GEM foil) is the first part of the thesis. The GEM foil is the basic component of the detector, and it plays a vital role for detection of the particles. The foil is composed of 50 μm thick polyimide film, coated with 5 μm copper on both sides, it is perforated with the micro size holes such as outer diameter of the hole is 70 μm, inner diameter 50 μm and pitch (distance between two consecutive holes) is 140 μm. In one detector a stack of three GEM foils is used, the area of the foil is about 0.345 m2 (short GE1/1 detector), 0.409 m2 (long GE1/1 detector). By design the spacing among the foils in the stack will be 1-3 mm. To assemble the detector the foils stack is stretched with a specially designed pull-out arranged around the detector, which uses 58 lateral screws (for short GE1/1 detector). During stretching, if the applied force is beyond the elastic limit or force applied non-uniformly across the screws, then the micro holes could have deformed. The electric field lines produced during detector operation, will be also deformed/dispersed accordingly, and this will directly affect the detector performance. The detectors have to perform for about 20 years at CMS in high radiations environment, therefore it is important to study the radiation impacts on the tensile properties of the foil. The long-term stability test is also of paramount importance. The GE1/1 foil is an asymmetric mechanically (due to the formation of the HV sectors with non-uniform spacing), so the asymmetry studies are important to know that in which orientation the foil can deform differently by applying uniform stress. The studies on GEM materials are performed by using both conventional and non-conventional techniques. In conventional techniques, we used stretching machine for tensile characterization and same setup was used for the creep test by adjusting the stress accordingly. For GEM foil holes deformation study a high resolution microscope was used along with the tensile test setup as mentioned above. In the non-conventional technique we used Moir´e interferometry to measure the flatness of the top foil of the stack, this optical method was used to characterize GE1/1 foils in situ, i.e., inside the GE1/1 assembled full size chambers under varying stress along the lateral screws. This test is helpful to optimize and to attain maximum possible flatness of the foil under nominal stress. Furthermore, Moir´e interferometry does not require physical contact with foil during the measurement, due to this reason this method is safer with respect to damaging the foil. This method allows to verify that about 80 μm precision can be reached on the transverse plane, well suited to the operation specifications of the GE1/1 foils. In the second non-conventional method, we used Fiber Bragg´s Grating (FBG) sensors to measure the strain at various points on the three GEM foils stack simultaneously during tightening the GEM foils stack. The demonstration of installing and measuring of the strain variation from the FBGs sensors on the GE1/1 three foils simultaneously also leads to the idea to install the FBG sensors on some of the final version of the GE1/1 chambers and to monitor the affects of high magnetic field at the GEM foil during operation at the CMS. For tensile characterization we not only used the GEM foil but also the kapton (non-copper coated and non-perforated) which is the base material of the GEM foil. The purpose of GEM material characterization is to see the tensile trends by applying continuously increasing stress and the estimation of the elastic limit, and the aging affects due to radiation such as gamma and neutron. The different environmental conditions could affect the tensile trends and elastic limit therefore the samples are conditioned not only in radiation but also in different temperature and humidity environment. In addition to all these conditioning, one set of the vii sample is treated in the harsh environment such as kept in the oven at 360o C. These various types of conditioning help to understand deeply the mechanical properties of the GEM material. The tensile tests provided the Young’s modulus of GEM and kapton foils. It was observed the degradation due to neutron irradiation in the elastic region for both kapton and GEM foil. Neither moisture nor dryness affect significantly the tensile properties. However, the heat treatment drastically degraded the material in the elastic regime and beyond. The long term stability test of the GE1/1 foil under constant stress (creep test) was performed. For this test, samples are taken from unused GE1/1 foil, two tests were performed separately in both longitudinal and transverse directions. The reason to perform the two tests is the asymmetry in the GE1/1 foil. The asymmetry arises in the foil due to HV sectors formation, and each sector is separated by about 1-2 mm wide lines without copper coating and the holes. Creep test provide the characterization of GE1/1 foils for long term stability under constant load. The second part of this thesis is focussed on developing and characterizing the RPC detector new material. The RPCs are already in operation in many experiments particularly at the LHC experiments such as CMS and ATLAS. In future the RPC are being considered in the upgrade project of the muon system particularly for the CMS. The CMS upgrade plan is linked with the LHC upgrade in which the luminosity will be enhanced, this enhanced luminosity era of the LHC is called the HL-LHC. During HL-LHC high rate of particles is expected and therefore faster detectors are required, which should have better capability of sustaining in very high particle rates without suffering from electrical discharges and aging. To make the RPCs faster a lot of R & D programs are on going. Along with other things such as fast front end electronics and detector design (reducing the gap size), the most important thing is the electrodes material. A good quality internal surface of the gaps (electrodes) and moderate resistivity could improve the RPCs performance. The efforts to modify the existing material and to develop a new material will be not only useful to meet the HL-LHC challenges particularly, but also good for the RPC detector technology in general. Therefore the focus of my work in last part of thesis is to modify the existing material and to develop the new material and its characterization. The existing and the standard RPC electrodes are made with two Bakelite electrodes which have resistivity of 1010 − 1012W.cm, the electrodes are coated internally with linseed oil and externally with graphite. The linseed oil is used to make the surface smooth and graphite coating is necessary to enhance external surface conductivity which help to distribute high potential (10 kV) uniformly across the electrodes to generate a uniform electric field along the entire gas gap. The particle detection rate capability depends on the electrodes material surface quality and its resistivity. For the RPCs material study a two-fold approach was adopted such as: modification of the existing material, and development of a new material for the electrodes. In the first approach, we replaced the linseed oil with polyimide and, to control resistivity, we added carbon black and graphene in the polyimide. To coat the electrodes (bakelite sheet) with polyimide and polyimide mixed additives different techniques are adopted such as spray, spin coating etc.., after the coating treated viii the samples surface in control temperature and humidity environment to make it dry and sticky with the base material. The inspection of surface morphology and roughness SEM and AFM respectively were used. The chemical structure of the surface was studied via FTIR analysis. For resistivity measurement arranged a setup in which the surface and bulk resistivity was measured separately. For moisture absorption rate measurements a dedicated setup was built. In the second approach of the strategy we developed a new material for the RPCs electrodes. It consists of polyimide and additives. To make the self sustained samples we used the thick the solutions of polyimide and uniformly mixed the additives (carbon black and graphene pellets). Samples were dried adopting two different procedures: room temperature and oven desiccation. After that FTIR, AFM, SEM, resistivity and moisture absorption tests were performed. The detail procedure and results are reported in the chapter 7, the results are very encouraging. Future plan includes to keep continue this research by building a prototype by using the new material and to study and validate the material by looking the performance of the detector.