Development of Glass Resistive Plate Chambers for INO
334 th International Conference on High Energy Physics, Philadelphia, 2008 Development of Glass Resistive Plate Chambers for INO
Satyanarayana Bheesette (for the INO collaboration)
Tata Institute of Fundamental Research, Mumbai, 400005, India
The India-based Neutrino Observatory (INO) collaboration is planning to build a massive 50kton magnetised Iron Calorimeter (ICAL) detector, to study atmospheric neutrinos and to make precision measurements of the parameters related to neutrino oscillations. Glass Resistive Plate Chambers (RPCs) of about 2m ×
2m in size are going to be used as active elements for the ICAL detector. We have fabricated a large number of glass RPC prototypes of 1m ×
1m in size and have studied their performance and long term stability. In the process, we have developed and produced a number of materials and components required for fabrication of RPCs. We have also designed and optimised a number of fabrication and quality control procedures for assembling the gas gaps. In this paper we will review our activities towards development of glass RPCs for the INO ICAL detector and will present results of the characterisation studies of the RPCs.
1. INTRODUCTION
ICAL is a 50kton magnetised iron tracking calorimeter, comprising of about 140 layers of low carbon 60 mm thick iron plates. Good tracking, energy and time resolutions as well as charge identification of the detecting particles are the essential capabilities of this detector. Sandwiched between these layers are glass RPCs, which are used as the active detector elements. About 27,000 RPCs of dimensions 2m ×
2m will be deployed in this detector. Lateral dimensions of this cubical geometry detector are 48m × ×
2. OVERVIEW OF OUR EARLIER WORK
We have started our detector R&D work by fabricating several dozen glass RPCs of dimensions 30cm × th International Conference on High Energy Physics, Philadelphia, 2008 characterised by their efficiency, leakage currents and noise rates, had not changed over a period of three and a half years, thus establishing their long-term stability. Finally, we had fabricated ten RPCs of dimensions 30cm × ×
1m (right)
3. DEVELOPMENT OF RPC MATERIALS AND ASSEMBLY PROCEDURES
RPC fabrication involves deploying a large number of materials as well as many assembly procedures. So, production of high performance and reliable chambers involves choosing the right type and quality of materials as well as optimisation of various assembly and quality control procedures involved in the fabrication. Materials such as glass used for electrodes, individual gases used for mixing and flowing the gas mixtures for the operation of the chambers, spacers, buttons, gas nozzles (see Figure 1) etc. which are needed for the assembling the chamber, resistive coat on the electrodes, epoxies used for gluing together different types of materials, pickup panels used for external signal pickup from the chambers, polyester films used for insulating the pickup panels from the resistive coated electrodes, to name some. We have studied a number of different type and quality of these materials and optimised most of these items. We have also designed a developed a large number of assembly and quality control procedures and invented a number of useful jigs that are extremely useful in production of good quality detectors. Coating of semi-resistivity paint on the electrodes, assembling and gluing of chambers, leak testing of the finished chambers are some of the important assembly procedures. We have worked closely with various R&D institutions as well as many industrial houses in developing these materials and designing and developing the assembly procedures. The RPCs are extremely disproportionate and heavy detector modules and hence pose serious problems in terms of mechanical rigidity and difficulties in the packing, transportation and installation of the modules in the detector. In addition, in spite of gluing the glass sheets together with a matrix of buttons throughout the area and using spacers on 4 th International Conference on High Energy Physics, Philadelphia, 2008 the four edges, the chamber tends to bulge outwards when the gas is flown through the chamber. These considerations call for a suitable and light weight casing for the chamber. Plastic honeycomb panels laminated on one side by aluminum sheet and the other side by copper sheet was developed by us with help of an industry. Pickup strips of 30mm width are realised by machining the copper sheet. The honeycomb panels were found to be an excellent solution to the mechanical support to the RPC, since it offered better rigidity to the detector with much lesser weight than that of aluminum. Resistive coating of the outer surfaces of the electrodes plays very crucial role in the operation of the RPC detector. We have collaborated with a local paint industry to develop a suitable paint as well as its application methods, which will be more adaptable for large scale production of the RPCs. We have also started designing paint automation techniques and developed prototype robotic machines for the purpose with help of local industry. We are also exploring the alternate and cost effective technique of coating the glass surface using screen printing method. This method can also be used to coat the paint on a PET film which can be stuck on the glass electrode surfaces [3]. CONSTRUCTION OF ICAL PROTOTYPE DETECTOR
The structure of the prototype detector is built in the form of a 13 layer sandwich of 50mm thick low carbon iron plates and 12 glass RPCs of 1m ×
1m in area. The overall design of prototype magnet was kept as close to the conceived design of the full scale INO magnet. As the prototype magnet detects muons, it serves as the medium in which secondary charged particles can be separated on the basis of their magnetic rigidity. The detector is magnetised to 1.5Tesla, which enables momentum measurement of 1-10Gev muons produced by ν µ interactions within detector. Four sets of copper coils of 5 turns each, which were made from electrolytic copper conductor tubing having a central bore for flowing low conductivity water, are used for this purpose (see Figure 2). Figure 2: Prototype detector magnet in the assembled state (left) and its B-H curve (right) A sophisticated gas mixing and distribution system, which works on four different input gases has been designed and fabricated. It features molecular sieve based filter columns on the input gas lines, Nippon Tylan made model FC-760 Mass Flow Controllers to precisely mix the gases to required proportion, Parker made fine filters and on-line moister readout on the mixed gas manifold. Mixed gas flow into 16 pneumatically controlled output channels is controlled by 0.3mm diameter stainless steel capillaries. Each of the output gas channels is equipped with a pair of bubblers, one on the detector input side for protecting the RPC in case of a gas channel block and the other on the output to isolate the 4 th International Conference on High Energy Physics, Philadelphia, 2008 chamber from the atmosphere. The gas system also features a facility towhich is useful while working with the controlled by a PC using a dedicated hardware their entire dynamic range by water displacement and other methodsusing a gas mixture of R134:Isobutane:SF ELECTRONICS AND DATA
Role of the data acquisition system then is to generate the triggerand to record strip hit patterns as well as timing of individustability of the detector as well as laboratory ambient parameters such as temperature, relative humidity and barometric pressure, are the other important tasks of the Figure 3: Prototype detector stack (left)The signal readout chain essentially consists of a frontthreshold discriminator followed by the handles important tasks of latching the strip hit pattern on back-end. It is also here that the pre-trigger signals are generated, which are used by the backusing combinatorial circuits and produces master trigger. signal multiplexing required both during the event data acquisition as well as during strip signal rate monitoring. The timing data is acquired using the commercial TDC moduemploying many custom built modules, such as control and readout modules. end are sent to the back-end through appropriate router modules. background job using the scaler module in the backtemperature, relative humidity and barometric pressure along with high voltage and current is also routinely done and made detector stack of 12 RPCs, schematic of RESULTS
The prototype detector stack is in un-the on-line system described above. The recorded data is stored in a customised format International Conference on High Energy Physics, Philadelphia, 2008 chamber from the atmosphere. The gas system also features a facility to add controlled moister into the the bakelite RPCs. The entire operation of control and monitoring of gas system is hardware interface. All the input gas channels have beetheir entire dynamic range by water displacement and other methods [4]. The RPCs are operated in the avalanche modeIsobutane:SF in the proportion 95.5:5.0:0.5. ACQUISITION SYSTEMS
Role of the data acquisition system then is to generate the trigger, based on the hit pattern of the RPC pickup strips and to record strip hit patterns as well as timing of individual RPCs with reference to the trigger. Monitoring the stability of the detector as well as laboratory ambient parameters such as temperature, relative humidity and barometric pressure, are the other important tasks of the on-line data acquisition system. (left), schematic of its DAQ system (middle) and a muon track in the stack The signal readout chain essentially consists of a front-end fast high gain HMC based preampfollowed by the digital front-end. The digital front-end built around a couple of CPLD chips f latching the strip hit pattern on master trigger as well as serially transferring the data to trigger signals are generated, which are used by the backuces master trigger. Finally, the digital front-end system also handles the entire signal multiplexing required both during the event data acquisition as well as during strip signal rate monitoring. The acquired using the commercial TDC modules. The data acquisition is done using a CAMAC backend, y custom built modules, such as control and readout modules. The multiplexed signals from the frontend through appropriate router modules. Monitoring of strip signalusing the scaler module in the back-end. Monitoring of ambient laboratory parameters such as temperature, relative humidity and barometric pressure along with important operating parameters of the RPCs also routinely done and made available on-line on web. Shown in Figure 3 schematic of its data acquisition system and typical muon track in the stack.-interrupted operation now for about a year and the datline system described above. The recorded data is stored in a customised format by the on controlled moister into the gas mixture, bakelite RPCs. The entire operation of control and monitoring of gas system is All the input gas channels have been precisely calibrated over perated in the avalanche mode based on the hit pattern of the RPC pickup strips al RPCs with reference to the trigger. Monitoring the stability of the detector as well as laboratory ambient parameters such as temperature, relative humidity and barometric muon track in the stack (right) preamplifier and low level end built around a couple of CPLD chips trigger as well as serially transferring the data to trigger signals are generated, which are used by the back-end trigger module built end system also handles the entire signal multiplexing required both during the event data acquisition as well as during strip signal rate monitoring. The The data acquisition is done using a CAMAC backend, The multiplexed signals from the front-ignal rates is done as a cyclic laboratory parameters such as important operating parameters of the RPCs such as Shown in Figure 3 are the prototype and typical muon track in the stack. operation now for about a year and the data is being acquired using by the on-line software and is 4 th International Conference on High Energy Physics, Philadelphia, 2008 analysed off-line in detail using sophisticated ROOT based analysis software. Some of the aspects that are analysed on day to day basis are the RPC efficiencies for cosmic ray muons, absolute and relative timing resolutions as well as the stability of RPCs based on the monitoring data of the individual strip rates. Timing resolution plots for four individual RPCs is shown in Figure 4. These RPCs show timing resolutions of about 2nSec with reference to scintillator paddle based trigger signal. Shown in Figure 4 are also the strip rate monitor profiles of an RPC. As can be seen from the plots, the noise rates are very stable over the period of monitoring inferring the stability of operation of the RPCs under test. Figure 4: Time resolution (left) and noise rate monitor profile (right) plots of RPCs CONCLUSIONS AND OUTLOOK
Large area RPCs of dimensions 1m ×
1m required for the ICAL prototype detector were successfully developed, built and characterised. They were operated stably in avalanche mode for long period of time without any signs of aging and other problems. Electronics, trigger, data acquisition and monitoring system hardware required to operate the detector has been designed and developed indigenously and commissioned. Necessary data analysis tools have also been developed. The prototype detector magnet has been designed, fabricated, installed and was found to produce the designed field. Final integration of the magnet, the active detector elements and the electronics systems is in progress and we expect to start acquiring the data from the prototype detector soon.
Acknowledgments
The author would like to thank all his INO collaboration colleagues for their valuable contributions towards the work reported in this paper.
References [1] INO Collaboration, “INO Project Report”, INO/2006/01, May 2006. [2] N.K.Mondal, “India-Based Neutrino Observatory (INO) Plans & Status”, These proceedings. [3] S.D.Kalmani et al , “Development of conductive coated polyester film as RPC electrodes using screen printing”, RPC2007, Mumbai, February 2008. [4] P.Verma et alet al