ACTA PHYSICA DEBRECENIENSIS XLI, 11 (2007)

Transcription

1 ACTA PHYSICA DEBRECENIENSIS XLI, 11 (2007) RESEARCH, DEVELOPMENT AND CONSTRUCTION OF THE BARREL MUON POSITION MONITORING SYSTEM AND PARTICIPATION IN THE PROJECT ON THE VELOCITY DRIFT CHAMBER FOR THE CMS EXPERIMENT AT CERN On behalf of the CMS-Debrecen Muon group* Z. Szillasi, Gy. Zilizi Institute of Experimental Physics, University of Debrecen, Debrecen, Hungary Abstract We describe the processes of design, production, calibration, installation and operation of the Muon Barrel Alignment System on the Compact Muon Solenoid detector at the Large Hadron Collider. We present the results of the CMS Magnet Test and Cosmics Challenge. We also present our contribution to the construction of the drift velocity monitors in the CMS muon detector chambers. I. Introduction The Compact Muon Solenoid (CMS) experiment (Fig. 1) is one of two large general-purpose particle physics detectors being (as of 2008) built on the proton-proton collider Large Hadron Collider (LHC) at CERN in Switzerland. Approximately 2300 people from 159 scientific institutes form the collaboration building the experiment located in an underground cavern at Cessy in France, just across the border from Geneva. The completed detector is cylindrical, 21 metres long and 16 metres diameter and weighs approximately tonnes. 11

2 12 Figure 1: The CMS detector.

3 The main goals of the experiment are to explore physics at the TeV scale, to discover the Higgs boson, to look for evidence of physics beyond the standard model, such as supersymmetry or extra dimensions, and, to be able to study aspects of heavy ion collisions. The main highlight features of the detector are its relatively small size, the powerful solenoid magnet and its optimization for tracking muons. To identify muons and measure their momenta, CMS uses three types of detector: drift tubes (DT), cathode strip chambers (CSC) and resistive plate chambers (RPC). DTs are used for precise trajectory measurements in the central barrel region, while the CSCs are used in the end caps. RPCs provide fast signal for trigger purposes when a muon passes through the muon detector, and are installed in both the barrel and the end caps. The optimal reach in momentum resolution for high-energy muons sets a severe position accuracy of the muon chambers. Thus, an adequate momentum-measurement (5-20% accuracy depending on the muon energy) demands a position accuracy comparable to the chamber resolution i.e µm depending on the radial distance of the chamber from the interaction point. This value should be compared to the scale of the muon detector (Fig. 2). Stability of the muon chambers at the 100 µm level cannot be guaranteed during detector operation. The expected movements and deflections of the muon spectrometer (e.g. due to magnet and temperature effects) will surely exceed the requirements. To cope with these movements, the CMS detector is instrumented with an optical position monitoring (also called alignment) system, which allows to determine the absolute positions of the chambers in the coordinate system of the full experiment and to perform continuous measurement of the chamber positions during its operation. The alignment information will be used for off-line correction in track reconstruction. II. Debrecen participation The Institute of Experimental Physics of the University of Debrecen (CMS-member since 1994) and the Institute of Nuclear Physics (ATOMKI, Debrecen, CMS member since 1997) in collaboration with CERN, University of Cantabria (Santander, Spain), Fermilab (USA) and other institutes 13

4 Figure 2: High energy muon passing through the barrel region and bending in 4T magnetic field. 14

5 is playing a leading role in the construction of the position monitoring system for CMS. Its task has been to propose, study, develop, design, build and install the monitoring system for the barrel part of the muon spectrometer. Beside the Muon Barrel Alignment Project in 2005 our group has joined the project on developing a drift velocity monitoring system for the CMS DT chambers lead by the RWTH Institute in Aachen, Germany. III. The Muon Barrel Alignment Project During the past more than 10 years the project which was supported by OTKA-grants T026178, T and T has arrived from the start to close-to-finish state of construction. This period can be divided into three -though largely overlaping- phases. The first years ( ) were devoted to determination of the proposed system. The barrel muon chambers are objects of 2,5x3x0,3 m 3 typical size embedded in the return yoke of the detector magnet. The size and configuration of the chambers as well as the additional circumstances and requirements like radiation background, long-term autonomous operation and acceptable price have made it necessary to develop new methods to build the position monitor system. The measurements are made by means of about light sources mounted on the chambers and observed by an opto-mechanical network composed of composite rigid structures and 600 video-cameras (Fig. 3). The operation and synchronisation of the elements of the system, also the data-taking and preliminary evaluation is executed by 36 PC-modules mounted on the detector and connected to each other and to the main control computer. During the period the main activity was concentrated around the research and development of the elements of the system. The optimal solutions were worked out and the prototypes have been built and tested. In particular, all the materials and electronics parts were checked for radiation hardness and functionality in particle beam at the ATOMKI cyclotron and 15

6 Rigid structures (MABs, z bars) Videocamera boxes (on the MABs) LED holders (called forks) on the chambers Diagonal and z LED holders (on the MABs and z bars) Board computers (one for each MAB) Figure 3: Shematic view of the Barrel Muon Position Monitoring System. in collaboration at the Uppsala University (Uppsala, Sweden). As the necessary accuracy during the mass-production of the elements cannot be guaranteed it is very important to calibrate them after manufacturing and the principle of calibration and its technique had to be worked out. In the period of the attention turned gradually to the engineering aspects of the project. By the end of 2005 the design, production, assembly and calibration of most of the elements of the system was finished. IV. Activity in 2006 This year was a major milestone in the construction of the CMS detector. Before lowering it in the underground cavern the detector was assembled and tested on the surface. The most critical part of the program was to test the 4T solenoid magnet which was switched on for the first time. Also, many subparts were checked and a considerable part of the calorimeters and the muon system were running with cosmic particles. This program called Magnet Test and Cosmic Challenge (MTCC) gave a possibility to assemble and test about 25% of the Barrel Muon Position Monitoring System in real conditions. In harmony with the general CMS planning the bottom part (sectors 10 and 11) of the system was built (Fig. 4). 16

7 Figure 4: MABs installed on the central wheel of the barrel part of CMS for MTCC. (The picture was taken during the preparation before closure of the barrel.) It consisted of 10 MABs and 2 z-bars including 152 videocamera boxes and 1764 LED light sources of which 1680 were mounted on 42 barrel muon chambers. 17

8 Figure 5: Logical scheme of the control and readout during the MTCC. The logical scheme of the control and read-out of the system is shown in Fig. 5. The MABs are controlled and the read-out and pre-processing of the video-images is performed by PC-104 type PCs (1/MAB) called Board PCs (BPC). The BPCs are connected to the Main workstation through local ethernet network. The control of the LED light sources mounted on the muon chambers was performed in two different ways. As part of the MTCC the barrel muon chambers in sectors 10 and 11 mounted on one outer wheel (wheel YB+2) and in sector 10 on the neighbouring wheel (YB+1) - 14 chambers altogether- were operated and took data generated by cosmic rays. On these chambers the LEDs were operated according to the final CMS-scheme using the control and readout (so-called MiniCrate) of the chambers. As for the other 28 chambers the standard way of control could not be used as the MiniCrates of those chambers were not operational. 18

9 For these chambers a special unit, a PIConNET based I2C control unit has been developed. During the MTCC the switch unit was at about 25m from the CMS barrel. At this distance the field even at full magnet current is below 50 Gauss. In the final configuration, however, the switch units must be close to the barrel where the nominal field is above 600 Gauss. A separate test channel has been installed during the MTCC to check the operation of commercial ethernet switch units in magnetic field. This test was successful, the candidate unit worked correctly. The MTCC was split into two periods called phase 1 in July-August 2006 and phase 2 in October During these periods about 500 full measurement cycles were taken and recorded by the barrel muon position monitoring system. As the detailed evaluation of the data is still in progress, here we can present some results of common interest that could be achieved already at the present level of analysis. Fig. 6 shows the response of the CMS barrel due to thermal changes. The relative movement of the video-sensor mounted on the MAB and the LED mounted on the chamber is about 15 micrometers which is well below any value relevant for CMS physics but shows the sensitivity and the resolution of the position monitoring system. The temperature value is measured on the MAB close to the observed object but in the air. Fig. 7 shows the period phase 1 when the CMS magnet was switched on for the first time. The magnet was powered during short periods and each time with bigger current until it reached near A corresponding to 3.8T field. The bottom part of the picture shows the magnet operation. The upper part shows the one of the outer LEDs of the z-bar measured by the corresponding video-sensor on the external MAB fixed to the outer side of the YB+2 wheel. The relative movement shows the movement of the wheel in the direction towards the detector centre, in other words the shrinking of the barrel. This shrinking does not recover when the magnet is switched off. The measurements were made only during the periods when the magnet was off. This result can be compared with the result of the measurement during phase 2 that is shown in Fig. 8. During this period the magnet was operated for longer periods and the data was taken continuously. As it can be seen 19

10 20 Figure 6: Thermal movements detected during the MTCC.

11 a further shrinking of the barrel can be observed, however, this additional shortening recovers when the magnet is off. The values measured for nonelastic and elastic shrinking are 2.7 mm and 2.9 mm respectively. Both effects and the values have been predicted during the magnet design by finite-element analysis. These results confirm both those calculations and the usefulness of the position monitoring system. VI. The CMS Velocity Drift Chamber In 2005 we joined the effort on the development of the drift velocity monitor system for the barrel muon chambers. As our group already had participated in the development of a similar device in the 90 s for the muon chambers of the L3 detector, and as certain respect has been obtained inside the CMS by the work on the position monitoring system of the muon chambers, the collaboration reacted positively to the willingness of the Debrecen group to participate in this project. By the beginning of 2006, the first prototype has been developed in collaboration with the RWTH Institute in Aachen, Germany. The first tests, performed in Aachen, have fulfilled our best expectations and have shown that the proposed solution meets all the requirements. Presently we work on the construction and operation of the equipment to be installed on CMS. According to the plans, 7 small-size drift chambers will be constructed together with the trigger, gas and high voltage part (this is the task of the Aachen partner) and the control and data acquisition system including the control and evaluation software (the task of Debrecen). Our work in more detail: Installation of a complete system in Debrecen to be used for the development of the control and data acquisition hardware and software. After completion, this system will serve educational purposes as a special lab exercise for our physics students. Development of the control and data acquisition system of the VDC. Later it will become another task to integrate it into the CMS Detector Control System (DCS). Participation in the installation, running and maintenance of the system at the CMS detector. 21

12 22 Figure 7: Non-elastic schrinking of the barrel.

13 Figure 8: Elastic schrinking of the barrel. 23

14 VII. Design of the CMS Velocity Drift Chamber Drift velocity is a key parameter for accurate measurement of muon particle tracks in the CMS muon chambers. This value, however, can change due to changes of different factors, e.g. the parameters of the electric or magnetic field or due to changes of gas parameters. A small test chamber, the CMS Velocity Drift Chamber (VDC) is used for monitoring and recording drift velocity values in the gas mixture of the CMS muon chambers. The basic principle of drift velocity measurement in the CMS VDC is the measurement of time differences. The time difference between the arrival at a signal wire of drifting electrons created by two β-particle beams at certain positions is measured. The drift times are measured relative to a trigger signal caused by the beam particles after traversing the chamber. Knowing the distance between the beams the drift velocity can be calculated. For the installation at CMS, six series production chambers have been produced and a seventh series production chamber is foreseen for reference measurements. At the L3 experiment at LEP, similar test chambers were used for monitoring the drift velocity; one for the muon chambers and one for the Time Expansion Chamber (TEC), which was part of the inner tracker of the L3 detector. The CMS VDC is based on the L3 TEC VDC. Improvements were needed because for CMS much higher E-fields are expected and the electric strength of the L3 TEC VDC was not adequate. So changes in the cathode region had to be done, and also the homogenity of the electric field had to be improved. VIII. Requirements The VDC system for the barrel muon system of the CMS detector should fulfill the following requirements: Very homogeneous E-field Same region for the maximum reachable E-field as the maximum E- field in the CMS muon chambers. 24

15 Figure 9: Schematics of the VDC. 25

16 The drift time measurement should be as short as possible The chamber volume should be small so that a fast gas exchange is possible to test the gas from different muon chambers in short time intervals. The chamber should be usable for the whole period of the CMS experiment. The running system should produce no high service costs. The complete system should be controllable, all data should be reachable from outside. IX. Principle of Operation To generate drift electrons, two β-sources ( 90 Sr, half-life period: 28,5 years) with an activity of 5 MBq for each source are placed outside the chamber. The chamber has outer dimensions of around 200mm 120mm 100mm and an inner volume of around 1 litre. In front of the sources there is a round collimator with a diameter of 1 mm and a length of 17 mm leading into the chamber. At the opposite side of the VDC, at a distance of 109 mm from the source, there is a rectangular second collimator of 2 mm 10 mm. Because of the first collimator the beams are focused to a circle with a diameter of 6,42 mm on the second collimator. For two 5 MBq sources, a β-particle rate of 430 Hz is expected behind the second collimator. The inlet and outlet of the β-beams are covered by very thin film of mylar. The outgoing electrons pass through the scintillator and create light in it, which light is detected by two light detectors at the ends of the scintillator. The logical AND of both light detectors is the trigger signal, which defines the time t = 0. In the gas-filled volume of the VDC between the two β- beams there is a very homogeneous electrical field around the middle of the chamber. Along the track of the β-particles the gas is ionized. Because of the electrical field the created free electrons drift toward the anode. The ions drift towards the cathode where they are neutralized. In the homogeneous E-field the electrons have on average a constant macroscopic drift velocity. The anode is separated from the drift room by a wall by a 2,5 mm wide slit, so that only drifting electrons from the regions in the middle 26

17 Figure 10: Drift velocity (ν d ) measurement. of the drift room can reach the anode. Near the anode ( 1 mm distance) a gas avalanche occurs and an analog electrical signal can be read out at the anode wire. Because of the slit, only drifting electrons created by the β-beams from two small regions along the central path of the chamber can reach the anode wire. Two different travel times corresponding to the two β-beams arriving at the anode are measured. The measured drift times are filled into a histogram, the drift-time spectrum. From Gaussian fits to the peaks, the times of the arriving drift electrons can be obtained (Fig. 10). X. Impact on the education Is is important to note that all the tasks mentioned above yield considerable impact on the university education program in the form of diploma 27

18 and doctoral projects. Already up to now several students have been able to take part in the CERN Summer Student program spending several months at CERN and working on these programs. The drift velocity monitor to be installed at Debrecen is in fact a complete particle detector unit from the drift chamber through the trigger system to the data acquisition and evaluation, therefore it is a very suitable tool for being the core equipment of a laboratory of educational purpose. Acknowledgements Our work has been supported by the Hungarian Scientific Research Fund (OTKA) through grants T , T and T We wish to express our gratitude to Mr Z. Biro, K. Kiss, V. Sass and B. Szabo for their professional contribution during the construction phase of the project. * The CMS-Debrecen Muon group N. Beni, A. Kapusi, P. Raics, Zs. Szabo, Z. Szillasi, Gy. Zilizi Institute of Experimental Physics, University of Debrecen, Debrecen, Hungary A. Fenyvesi, J. Imrek, J. Molnar, D. Novak, G. Szekely ATOMKI Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen, Hungary Gy. Bencze RMKI Institute of Particle and Nuclear Physics, Budapest, Hungary Visiting scholar at the Institute of Experimental Physics, University of Debrecen References [1] CMS The Muon Project, Technical Design Report CERN/LHCC [2] CMS Detector Performance and Software Physics, Technical Design Report, Vol. 1. CERN/LHCC