RPC Detector Research Articles

A multi Gbit/s backplane for the RPC DAQ and L1 Trigger of the ATLAS Muon Spectrometer

The Read Out Driver (ROD) of the RPC detectors in the ATLAS Muon Spectrometer is a VME64x board designed to perform sub-event building for one of the 32 sectors in which the apparatus is divided. Each ROD receives data fragments from two companion boards placed in the adjacent VME slots. These two boards also send trigger data to the nearest slot, respectively at their right and left hand side. Due to the large amount of data to be transferred with a known latency, an auxiliary bus named RODbus has been designed and deployed in order to link the five boards.In this paper we present the logical and physical architecture of the RODbus, which implements multiple point-to-point LVDS links for high-speed data transfer and parallel TTL busses for commands, flags and flow-control signals. A prototype of the bus embedded into a VME64x 21-slot backplane has been recently developed together with HARTING, the firm which designed the connectors specified by the VME64x standard. We also discuss the test results on signal integrity, crosstalk and impedance profile made with Time Domain Reflectometry.

The ATLAS RPC detector control system: Problems, solutions and new opportunities

The development of large detector systems for the LHC experimental apparatus induced to consider the problem of monitoring and controlling the different sub-detectors under a new perspective. On one side the necessity of a powerful tool which permits to a relatively experienced user to safely manage the detector for all the standard purposes and inform on the detector status and quality of the data pushed the LHC scientific community to adopt a common development platform and as much as possible uniform front ends. On the other side the monitoring of short- and long-term behavior of unprecedented complex detectors systems built by thousands of independent detectors offered a problem and an opportunity: the problem is in terms of careful and optimized design of the control system combining a rational use of software and database tools and all the experience accumulated previously in the long ageing studies which evidenced case by case the main source of problems and the ideal tools to control them; the opportunity consists in the availability of such a statistically large number of detectors which would expand the chances to understand deeply the systematic aspects of the detector physics hardly reachable in small detector sample tests even though on a long-time scale. Here we will present the architecture, the present status and the perspective applications and potentialities of the ATLAS RPC detector control system in view of the forthcoming startup of the ATLAS experiment.

RPC gas recovery by open loop method

RPC detectors require to be flushed with small but continuous flow of gas mixture. Dealing with large number of detectors, gas consumption to very large volumes. Gas flow is a running expense and constituent gases are too expensive to be treated as consumables. Exhaust gas mixture from detectors is a potential environmental hazard if discharged directly into the atmosphere. Storage of gases on a large scale also leads to inventory- and safety-related problems. A solution to these problems is the recovery and reuse of exhaust gas mixture from RPC detectors. Close loop method employs recirculation of exhausted gas mixture after purification, analysis and addition of top-up quantities. In open loop method, under consideration here, individual component gases are separated from gas mixture and reused as source. During open loop process, gases liquefiable at low pressures are separated from ones liquefiable at high pressure. The gas phase components within each group are successively separated by either fractional condensation or gravity separation. Gas mixture coming from RPC exhaust is first desiccated by passage through molecular sieve adsorbent type (3A+4A). Subsequent scrubbing over basic activated alumina removes toxic and acidic contaminants such as S 2F 10 produced during corona (arcing) discharge. In the first stage of separation isobutane and freon are concentrated by diffusion and liquefied by fractional condensation by cooling upto −30 °C. Liquefied gases are returned to source tanks. In the second stage of separation, argon and sulphur hexafluoride, the residual gases, are concentrated by settling due to density difference. SF 6 is stored for recovery by condensation at high pressure while argon is further purified by thermal cracking of crossover impurities at 1000 °C followed by wet scrubbing.

The ATLAS trigger - commissioning with cosmic rays

The ATLAS detector at CERN's LHC will be exposed to proton-proton collisions from beams crossing at 40 MHz. At the design luminosity there are roughly 23 collisions per bunch crossing. ATLAS has designed a three-level trigger system to select potentially interesting events. The first-level trigger, implemented in custom-built electronics, reduces the incoming rate to less than 100 kHz with a total latency of less than 2.5μs. The next two trigger levels run in software on commercial PC farms. They reduce the output rate to 100-200 Hz.In preparation for collision data-taking which is scheduled to commence in May 2008, several cosmic-ray commissioning runs have been performed. Among the first sub-detectors available for commissioning runs are parts of the barrel muon detector including the RPC detectors that are used in the first-level trigger. Data have been taken with a full slice of the muon trigger and readout chain, from the detectors in one sector of the RPC system, to the second-level trigger algorithms and the data-acquisition system. The system is being prepared to include the inner-tracking detector in the readout and second-level trigger.We will present the status and results of these cosmic-ray based commissioning activities. This work will prove to be invaluable not only during the commissioning phase but also for cosmic-ray data-taking during the normal running for detector performance studies.

Study on the position resolution of resistive plate chamber

A prototype RPC detector with a 2-mm readout strip pitch has been built for research into its position resolution. After determining the optimal threshold and working voltage, the efficiency and position resolution of this prototype RPC detector have been studied carefully. Using cosmic-ray muons with different directions and incident positions, we have obtained less than 0.5-mm average statistical errors of position resolution for two different RPC readout structures: 1- and 1.7-mm readout strip widths. The results show that the RPC can also give high position resolution with very little variation from its traditional structure. This will lead to possible applications of the RPC in medical imaging and other domains.

Comments on TPC and RPC calibrations reported by the HARP collaboration

The HARP Collaboration recently published calibrations of their TPC and RPC detectors, and differential cross-sections of large-angle pion production in proton-nucleus collisions. We argue that these calibrations are biased and cross-sections based on them should not be trusted.

Simulation of the neutron response of a glass-resistive plate chamber in the energy range 10−10 MeV

The neutron sensitivity of a glass-resistive plate chamber (RPC) has been simulated using GEANT4 MC codes for the neutrons in the energy range 10−10MeV–10 GeV. The glass-RPC has many advantageous features such as high rate capability, it is easy to handle and can be built on an industrial scale. For an evaluation of their efficiency, a simulation test of the glass-RPC detectors has been performed in order to investigate their performance for neutrons detection. When compared to previous works on bakelite-RPC the results are comparatively higher. As the density of the glass electrode is higher than the bakelite electrode, relatively higher sensitivity results at higher neutron energies were obtained using these types of RPCs. For an isotropic neutron source using a GEANT4 Standard MC package, sensitivity, sn<3.894×10−2 at <10 GeV by double-gap glass-RPC has been observed. For the same neutron source with a GEANT4 Low MC package, double-gap glass-RPC sensitivity, sn<4.042×10−2 at <10 GeV has been measured. Similar but lower simulation results of the RPC detector have been observed for parallel neutron source configurations. The obtained simulated results were applied to the large hadron collider (LHC)/(CMS) compact muon solenoid radiation environment both in the endcap and barrel regions. The average sensitivities, expected hit rates in those regions were estimated. Additionally, a comparison between the present glass-RPCs with the previous bakelite-RPCs neutrons simulation results has been performed.

Improving track resolution in the RPC chamber

A new very promising read out, in addition to the well known charge centroid method, is proposed for improving the spatial resolution in the RPC in the sub-millimeter range. The method is based on the read out of the signal propagating in the graphite electrode which was simulated using a distributed resistance-capacitance model in SPICE. The results show that a good space-time correlation is only possible by suitable signal processing. Three RPC detectors with a new layout and dedicated electronics were tested. The measured space resolution is in the order of a few 100 μm.

Active fibre optic splitter for the CMS RPC detector

An electronics module has been designed and tested for the CMS RPC detector readout. The module consists of twelve sub-blocks, each of which receives an optical signal at 1.6 GHz, converts it into electronic form for the splitting process and sends it forth to two or four destinations in optical form. It is a critical part in the trigger system of the experiment. Details of the design are presented, as well as test results confirming that the splitter fulfils all system requirements.

Attempt to correlate the ionic model with observations in BaBar RPC chambers and R&amp;D tests

The paper presents a model of ionic conduction in the Bakelite-based RPC detectors. This model explains why these detectors need to add water while they operate. To produce the Bakelite polymer, one needs a chemical reaction of the formaldehyde and the phenol molecules. Water is an important byproduct of this chemical reaction. The electrode current, according to the presented model, is formed mainly by the ionic sequence involving the phenol impurities left in the Bakelite and water, both left from the initial production. As long as these two ingredients are present in the original amount, the Bakelite resistance is constant. If either water or phenol impurity are depleted from various reasons, for example due to a large current or presence of a dry gas, the Bakelite resistance increases. According to the presented model, water should be added right from the beginning and not half way through the experiment. Similar ionic process is present in the Linseed oil polymer, where the current is carried by the fatty acid molecular impurities and water. The Bakelite RPC operation requires the entire ionic sequence to operate smoothly to keep the electrode resistance constant.

Production and test of one-third of barrel Resistive Plate Chambers of the CMS experiment at LHC

Resistive Plate Chambers have been chosen as dedicated trigger muon detectors for the Compact Muon Solenoid experiment at the Large Hadron Collider at CERN. The barrel RPC detector consists of 480 chambers of different forms and sizes, equipped with 75,000 strips and covering an area of about 2400 m 2 . About one-third of RPC barrel chambers have been produced up to the end of 2003 and these 150 chambers, produced and assembled in Italy, have been extensively tested at the two Italian test stands of Bari and Pavia by the RPC barrel collaboration. Preliminary results of the production and test of the chambers will be described here.

Performance of multigap RPC detectors in the HARP experiment

A time-of-flight system based on 46 multigap glass resistive plate chambers has operated for 2 years in the HARP experiment at the CERN PS. The system covers an area of 10m2 and comprises 368 readout channels. Systematic checks of the RPC performance are presented as well as preliminary results on its particle identification capabilities.

Performance of shielded timing RPCs in a 12C fragmentation experiment

We show the feasibility of large area timing RPC detectors for experiments dealing with fragmentation-like primary interactions and/or high detector multiplicities, without significative degradation of the detector capabilities. The tested prototype is constituted by three independent cells placed into grounded aluminium boxes, which reduce the cross-talk to the level of 0.4%. Each 2×2×60cm3 box consists of a 4-gap glass–aluminium RPC.At 200Hz/cm2 the counters have shown a time resolution of 60ps<σt<80ps with 3σ-tails below 8%. High mechanical robustness and homogeneity of the physical properties along the longitudinal direction of the detector were also observed.

ATLAS Level-1 Muon Barrel Trigger robustness study at X5 test facility

The present paper describes the Level-1 Barrel Muon Trigger performance as expected with the current configuration of the RPC detectors, as designed for the Barrel Muon Spectrometer of ATLAS. Results of a beam test performed at the X5-GIF facility at CERN are presented in order to show the trigger efficiency with different conditions of RPC detection efficiency and several background rates. Small RPC chambers with part of the final trigger electronics are used, while the trigger coincidence logic is applied off-line using a detailed simulation model.

Results on long-term performances and laboratory tests of the L3 RPC system at LEP

The RPC detectors in the L3 experiment at LEP work as a trigger system for the Forward–Backward Muon Spectrometer. It consists of 192 bi-gap RPCs working in streamer mode. We monitored the behaviour of the system over seven years of data taking at LEP. To investigate the ageing of the RPCs after this long-term operation, we report the main results obtained from 1994 to 2000, together with the results of tests performed on some RPC chambers in our test site in Napoli with cosmic rays after the dismantling of L3.

Performances of RPCs in the BaBar experiment

The BaBar experiment uses a big system based on RPC detectors to discriminate muons from pions and to identify neutral hadrons. About 2000 m 2 of RPC chambers have been working at SLAC since the end of 1998. We report on the performances of the RPC chambers focusing on new problems discovered in the RPC behaviour. These problems started very soon after the installation of the chambers on the detector when the high-ambient temperature triggered an increase of dark currents inside the chambers and a reduction of the efficiency. Careful analysis of the BaBar data and dedicated R&D efforts in the laboratory have helped to identify the main source of the trouble in the linseed oil varnish on the bakelite electrodes.

The coincidence matrix ASIC of the level-1 muon barrel trigger of the ATLAS experiment

The ATLAS barrel level-1 muon trigger processes hit information from the resistive plate chamber detector, identifying candidate muon tracks and assigning them to a programmable p/sub T/ range and to a unique bunch crossing number. The trigger system uses up to seven detector layers and seeks hit patterns compatible with muon tracks in the bending and nonbending projection. The basic principle of the algorithm is to demand a coincidence of hits in the different chamber layers within a path. The width of the road is related to the p/sub T/ threshold to be applied. The system is split into an on-detector and an off-detector part. The on-detector electronics reduces the information from about 350 k channels to about 400 32-bit data words sent via optical fiber to the so-called sector logic (SL). The off-detector SL electronics collects muon candidates and associates them to detector regions-of-interest of /spl Delta//spl eta//spl times//spl Delta//spl Phi/ of 0.1/spl times/0.1. The core of the on-detector electronics is the coincidence matrix ASIC (CMA), which fulfils both the trigger algorithm and the readout of the RPC detector. Each CMA is able to process and readout 192 RPC strips from as many as four detector layers. In order to keep the full level-1 system latency below 2 /spl mu/s, the CMA has to find candidate muon tracks with a latency of a few 25 ns bunch crossing periods. The readout part of the CMA is able to time tag incoming RPC hits with a time interpolator running at the trigger pipeline frequency of 320 MHz and to send the data to the readout system via a serial link. The design of the trigger system and the performances of the ASIC, based on a CMOS 0.18 /spl mu/m technology, are presented.

First results on RB2 muon barrel RPC detector for CMS

The first CMS MB2 station, with one RPC and one DT module, has been tested with a muon beam under a high intensity photon flux at the CERN Gamma Irradiation Facility during the Autumn 2001 test. Results on efficiency, rate capability, cluster size and spatial resolution, for the RPC detector, are reported here. Studies with a small percentage of SF6 in the gas mixture, in order to decrease the noise rate, have also been carried out.

The RPC system for the CMS experiment at the LHC

The CMS detector at the LHC has a redundant muon system. Two independent muon systems are used in the L1 trigger. One of them is based on wire chambers, the other on RPC detectors. Properly combining the answers of the two systems results in a highly efficient L1 trigger with high flexibility from the point of view of rate control. Simulation results show, however, that the RPC system suffers from false triggers caused by coincidence of spurious hits. System improvements, which could avoid oiling the chambers, are possible. RPCs have also proved to be very useful for muon track reconstruction.

RPC performance at KLM/BELLE

The RPC detectors in the KL-muon (KLM) subsystem of the Belle detector and their performance are described in this paper. The Belle RPCs are built using glass electrodes. This system has operated stably with very high detection efficiency and low dark current since June 1, 1999.