ARTICLE IN PRESS. Nuclear Instruments and Methods in Physics Research A

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1 Nuclear Instruments and Methods in Physics Research A 599 (29) 7 75 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research A journal homepage: High radiation tests of the MWPCs for the LHCb Muon System M. Anelli a, S. Baccaro b, P. Campana a, E. Dané c, C. Forti a, G. Lanfranchi a, G. Penso c,e, D. Pinci c, R. Rosellini a, M. Santoni a, A. Saputi a, A. Sarti a,d,, A. Sciubba a,d a INFN-Laboratori Nazionali di Frascati, Italy b ENEA-FIS/ION, C.R. Casaccia, S.Maria di Galeria (Roma), Italy c INFN, Sezione di Roma, Italy d Dip. Energetica, Sapienza Università di Roma, Roma, Italy e Dip. Fisica, Sapienza Università di Roma, Roma, Italy article info Article history: Received 29 September 28 Received in revised form November 28 Accepted 24 November 28 Available online 3 December 28 Keywords: Gas detectors MultiWire Proportional Chambers Tracking and position-sensitive detectors Elementary-particle and nuclear physics experimental methods and instrumentation abstract The MultiWire Proportional Chambers (MWPCs) for the LHCb Muon Detector were subjected to an ageing test, performed in the high-radiation environment of the Calliope Gamma Facility of the ENEA- Casaccia Research Centre. The test showed that the MWPCs can safely operate for many years in a high intensity particle flux. An additional test, performed with the CERN Gamma Irradiation Facility (GIF), confirmed these conclusions. & 28 Elsevier B.V. All rights reserved.. Introduction The aim of the LHCb experiment is to acquire a precise measurement of CP violation and of rare decays in the b b system. The LHCb Muon Detector [,2] consists of five stations (M M5) located along the beam axis and interspersed with iron filters (see Fig. ). Each station is divided into four regions (R ) of increasing distance from the beam (see Fig. 2). Station M is equipped with 2 Gas Electron Multiplier (GEM) detectors [3 5] and 264 two-gap MultiWire Proportional Chambers (MWPCs). The other stations comprise 4 four-gap MWPCs. The chambers are of different dimensions and read-out granularity. The geometry is projective and optimised in order to identify muons in the first level trigger (L) with a p T resolution of 2%. The L Muon trigger is designed to operate at an efficiency level of at least 95% within a 25 ns time window. This is achieved, in M2 M5, by using MWPCs with two OR-ed double-gaps, with all the corresponding pads of a double-gap ganged together (see Fig. 3). A time resolution lower than 4 ns (rms) is obtained [6,7] at kv and with a gas mixture of Ar=CO 2 =CF 4, 4/55/5% in volume. The MWPCs must have a stable performance during the lifetime of the experiment, when they will operate in conditions of high radiation. To check this requirement two different tests were performed: an ageing test, carried out at the Calliope Gamma Facility [8] of the ENEA-Casaccia Research Centre and an operational test, performed at the CERN Gamma Irradiation Facility (GIF) [9]. 2. The ageing test 2.. Experimental setup The experimental setup for the ageing test is shown in Fig. 4. Two different chambers were used: the chamber (T) to be tested and a reference chamber (REF). Chamber T was exposed to the g flux coming from an 8 TBq (at the time of the test) 6 Co radioactive source. The REF chamber was shielded from the radioactive source by a concrete wall, and hence exposed to a Corresponding author at: Dip. Energetica, Sapienza Università di Roma, Roma, Italy. Tel.: ; fax: address: asarti@lnf.infn.it (A. Sarti). The position of chamber T in relation to the radioactive source was chosen in order to optimise the g flux through the chamber and to perform the ageing test within a reasonable time interval /$ - see front matter & 28 Elsevier B.V. All rights reserved. doi:.6/j.nima

2 6 mrad 72 M. Anelli et al. / Nuclear Instruments and Methods in Physics Research A 599 (29) 7 75 M M2 M3 M4 M5 Detector GND nf CALORIMETERS Muon filter Muon filter 2 Muon filter 3 Muon filter mrad Gap D kω Polyurethane foam supplies Shielding/Safety GND HV power Fig. 3. Sketch of a four-gap chamber. nf k Ω Gold plated pad Gold plated wire R R2 R3 T 4.5 m y Pool water purification system REF 2 m Pool 6 Co source Shielded window z Fig.. Side view of the muon system. R3 R2 R VERTICAL STRIP LOGICAL PAD R R2 R3 BEAM PIPE HORIZONTAL STRIP Fig. 2. Front view of one quadrant of stations M2 and M3 showing the partitioning into sectors. In one sector of each region a horizontal and a vertical strip are shown. much lower g flux than chamber T. The 6 Co source can be shielded so that the gamma flux can be switched ON and OFF. Chamber T had an area of 2 cm 2 (2.4 l of gas) and a wire pitch of 2 mm. The REF chamber had an area of 5 cm 2 ( l of gas) with a wire pitch of.5 mm. The geometrical characteristics of the chambers are described in detail elsewhere [,2]. Both chambers Gas bottles Gas rack had four gaps 2 (A, B, C and D) with the anode wire (3 mm in diameter) planes placed in the centre of the 5 mm gas gaps. The materials used to build these prototypes were identical to those used for the final chambers of the LHCb Muon System. During the test the T and REF chambers were flushed in series in open-loop mode, with a gas mixture of Ar=CO 2 =CF 4 4/4/2% in volume. 3 The gas flow ð3 Volumes=hÞ was larger than that foreseen during the LHCb experiment, in order to take into account the accelerated ageing due to the higher particle flux. The ageing test was performed on gaps A, B and C of chamber T. To accelerate the ageing process the high voltage (HV) of these gaps was kept at 2.75 kv, a value 5 V higher than the normal working voltage []. With this HV the current in gaps A, B and C was about..5 ma with the source ON. In order to avoid a large voltage drop the current-limiting resistors placed on the HV lines were removed. Gap D of chamber T was used as a reference, in addition to the chamber REF. Starting from day 5 and for most of the duration of the test the HV of gap D was set to 2.5 kv to avoid ageing in this gap. HV PC Console Control room Fig. 4. Experimental setup at the Calliope Facility. The positions of the T and REF chambers are shown together with the other elements of the setup. 2 Only gaps A and B of the REF chamber were used for this test. 3 The final operating gas mixture (Ar=CO 2 =CF 4, 4/55/5% in volume) was chosen later.

3 M. Anelli et al. / Nuclear Instruments and Methods in Physics Research A 599 (29) Integrated charge (C/cm) <I A / I D > Fig. 5. Integrated charges in gaps A, B, C and D of chamber T during the 3 days of the test..8.6 B A C D <I A / I REF > <I C / I REF > <I B / I REF > Fig. 7. Ratios between the currents in gaps A, B and C of chamber T and the average current of gaps A and B of chamber REF. The average value of these ratios has been normalised to. <I C / I D > <I B / I D > Fig. 6. Ratios between the currents in gaps A, B, C and that in gap D of chamber T. The average value of these ratios has been normalised to. The HV of the REF chamber was set at 3: kv. This higher voltage was chosen to compensate [,2] the lower wire pitch of that chamber so that its gain is roughly equal to the gain of gaps A, B and C of chamber T. With the source ON the current per gap in the REF chamber was 2 ma. During the ageing test each gap in each chamber was powered by a different channel of a CAEN 4 SY2527 power supply having a current offset varying between and ma and a sensitivity of :2 ma in the measurement of the output current. LabVIEW-based software was used to manage the HV system and to acquire the currents of the different gaps Test results In order to assess the ageing properties of gaps A, B and C of chamber T, a large charge was accumulated over one month of exposure to the 6 Co source and the relative gain of each gap was constantly monitored during this period. In Fig. 5 we report the cumulative time distribution of the charge recorded in the four gaps of chamber T. At the end of the test the charge per unit 4 CAEN, 5549 Viareggio (LU), Italy.

4 74 M. Anelli et al. / Nuclear Instruments and Methods in Physics Research A 599 (29) 7 75 Table Current density (na/cm of wire) in the MWPCs expected at the LHC with a luminosity of 2 32 cm 2 s and no safety factor. Average current density (na/cm of wire) in the most irradiated chamber (in the most irradiated cm 2 ) R R2 R3 M 3: :8 :37 :9 :637 :7 ð6:48 :36Þ ð2:54 :5Þ ð:238 :89Þ M2 :8 :82 :392 :37 :85 :5 :282 :76 ð2:44 :4Þ ð:943 :62Þ ð:336 :3Þ ð:39 :8Þ M3 :277 :24 :78 : :24 :67 :87 :27 ð:7 :73Þ ð:295 :29Þ ð:48 :8Þ ð:98 :7Þ M4 :22 :23 :6 : :8 :56 :56 :9 ð:886 :6Þ ð:33 :29Þ ð:48 :8Þ ð:9 :7Þ M5 :3 :4 :355 :6 :69 :58 :42 :7 ð:599 :44Þ ð:254 :28Þ ð:56 :8Þ ð:98 :7Þ The current densities are averaged on the most heavily irradiated chambers (the most heavily irradiated cm 2 ) of the different regions (R ) and stations (M M5) of the muon system. The data refer to a gas mixture of CO 2 =Ar=CF 4, 55/4/5% in volume and to a gain of 7: 4 corresponding [] to an HV of 2.6 kv. In region R of station M the muon detector is composed of GEM chambers. wire length accumulated in gaps A and B was about.5 C/cm. The small difference between the charges integrated in gaps A and B is due to slight geometrical differences in the gap (e.g. nonperfect panel planarity) that affect the gain. During the 22nd day the HV of gap C was turned off when one of the wires in it broke. This was most probably caused by the absence of the currentlimiting resistors. In gap D, which was used as a reference gap, the charge integrated per unit wire length was about one sixth of that in gaps A and B. To measure the gain stability of gaps A, B and C of chamber T independently of the variation in gas conditions (pressure, temperature and composition of the gas mixture), the currents in these three gaps were compared every 2 4 days with that in gap D. During these measurements the HV of gap D was raised to 2.75 kv (i.e. the same as gaps A, B and C) for the time necessary to stabilise and measure its current. In Fig. 6 we report the ratio of the current in gaps A, B and C to that in gap D during the ageing test. Within 5% the currents (and therefore the gain) in gaps A, B and C behave in the same way as that in gap D, independently of the charge accumulated during the test. During the same exposure to the 6 Co source another test was performed to compare the currents of gaps A, B and C of chamber T with the current drawn by gaps A and B of chamber REF. In Fig. 7 we show the ratio between the currents in gaps A, B and C of chamber T and the average current of gaps A and B of chamber REF. During the test period the measured variation of the ratios was less than %. These results allow us to conclude that no evident gain variation is observed within the accumulated charge. To evaluate the significance of this result, the charge integrated during the test must be compared with the values expected at the LHC, in the different regions of the muon detector. These values were calculated taking into account the local particle rate, the ionisation produced by a charged particle crossing the gas gap and the gain of the chambers. The local particle rate was estimated [3] from a Monte Carlo sample of minimum bias events, assuming a luminosity of 2 32 cm 2 s and no safety factor. 5 The ionisation of a minimum ionising particle traversing the 5 mm gas gap, calculated [] with HEED [4,5], is, for the final gas mixture, 36 2 electrons ions pairs. The gain of an MWPC, with the final gas mixture, was measured precisely [] as a function of the HV. At the working voltage of the chamber ( kv) the gain varies from 7: 4 to 9:7 4. The results of these calculations, summarised in Table, show that the largest charge accumulated in gap B of chamber T (:44 C=cm, Fig. 5) corresponds to about 7 years 6 of run in the most heavily irradiated cm 2 of the most heavily irradiated chamber of region R2 in station M. If the current density averaged in the most heavily irradiated chamber (rather than the most heavily irradiated cm 2 ) of MR2 is considered (see Table ) the charge accumulated during the test in gap B of chamber T is equivalent to 5 years of run. No significant gain variation is therefore expected for the MWPCs of the muon detector during their expected lifetime at the LHC. Ten days after completion of the test, the dark currents were measured with a CAEN N47A HV power supply with a sensitivity of na, and were all found to be lower than na. After the ageing test, chamber T was opened and visually inspected. In Fig. 8 a picture of a cathode plane shows the etching of the epoxy on the F board due to the ageing process. This etching was present in all the gaps but was not correlated with the direction of the gas flow. No traces of discharges were observed on the cathode plane and the anode wires appeared clean. 3. The operational test The aim of the operational test was to check the behaviour of the chamber when the intense particle flux crossing it is first removed and then restored, so as to verify the possible on set of long-term self-sustaining currents, even in the absence of radiation. The test was performed at the CERN GIF [9], in experimental conditions (chamber HV and irradiation) similar [6] to those expected at the LHCb collider. Six different chambers (four MR3 chambers, one M3R3 and one M5), were placed in the gamma flux coming from the 63 GBq (at the time of the test) 37 Cs radioactive source of the GIF. Special absorbers, placed in front of the source, allowed the gamma flux to be switched ON and OFF. The chambers were tested with the final gas mixture (Ar=CO 2 =CF 4, 4/55/5% in volume) and at different HV. 5 The multiplicative safety factor is 2 in M and 5 in M2 M5. 6 One year of run is conventionally equivalent to 7 s.

5 M. Anelli et al. / Nuclear Instruments and Methods in Physics Research A 599 (29) The test was initiated at an HV of 2.2 kv with the source OFF. The HV was then raised to 2.3 kv for about 5 min. During this period the source was switched OFF for several minutes and then turned ON again. This enabled us to check that the current in the chamber returned to zero when the source was OFF. This procedure was repeated at HV ¼ 2:4, 2.5, 2.6 and 2.65 kv. Between each step the HV was brought down to 2.3 kv and then increased in steps of V to the next HV value in order to train the chamber gradually in the high-rate environment. The full procedure is reported in Fig. 9, together with the current measured in one of the chambers tested (belonging to M5). Similar results were obtained for the other five chambers. For all the chambers tested the current dropped to zero when the source was turned OFF and returned to its original value when the source was turned ON again. Fig. 8. Etching of epoxy layer on F glass epoxy: the arrows indicate the border of the etched area. Below the line the glue that seals the gap protects the F and no etching is observed. HV (kv) Conclusion An ageing test of an MWPC of the LHCb Muon System was performed in the high-radiation environment of the Calliope Facility. The results show that the gain of the chamber does not vary after accumulating a charge of 4 C/cm of wire, i.e. a similar amount to that expected over 5 years (without safety factors) of operation at the LHC. This result is in agreement with previous data obtained [7] with an Ar=CO 2 =CF 4 gas mixture. An operational test performed at the GIF showed that the good behaviour of the chambers can also be satisfactorily reproduced in a high-radiation environment. It also showed that no long-term self-sustaining currents are generated after the chambers are exposed to the high-radiation environment. Acknowledgements We would like to thank the staff of the Calliope Facility and, in particular, A. Pasquali and F. Zarbo for their generous help in the preparation and performance of the test. Current (μa) Time (minute) Fig. 9. Lower chart: measured current as a function of time during the operational test at the GIF. The upper chart shows the setting of the chamber HV during the test. The source was turned OFF during the time intervals corresponding to the shaded zone, and was ON at all other times. They were powered with a CAEN N47 HV module and currents in the na ma range were read and acquired with a custom nanoammeter. References [] LHCb Collaboration, LHCb Muon System TDR, CERN/LHCC 2-, 2; Addendum to the Muon System TDR, CERN/LHCC 23-2, 23; Second Addendum to the Muon System TDR, CERN/LHCC 25-2, 25. [2] LHCb Collaboration, J. Instrum 3 (28) S85. [3] F. Sauli, Nucl. Instr. and Meth. A 386 (997) 53. [4] G. Bencivenni, et al., Nucl. Instr. and Meth. A 53 (23) 264. [5] M. Alfonsi, et al., IEEE Trans. Nucl. Sci. NS-5 (24) 235. [6] M. Anelli, et al., IEEE Trans. Nucl. Sci. NS-53 (26) 33; D.E. Hutchcroft, et al., Results obtained with the first four gap MWPC prototype chamber, LHCb Note, CERN-LHCb-2-24, 2.; M. Anelli et al., Test of MWPC prototypes for region 3 of station 3 of the LHCb muon system, LHCb Note, CERN-LHCb-24-74, 24. [7] R. Antunes Nobrega, et al., in: 27 IEEE Nuclear Science Symposium Conference Record, vol., 27, p. 564, DOI:.9/NSSMIC [8] A. Tata, et al., Radiation technology facilities operating at the Italian ENEA- Casaccia research center, ISSN/2-557, available on CERN Document Server: LAL-RT-INN-98-, 998. [9] S. Agosteo, et al., Nucl. Instr. and Meth. A 452 (2) 94. [] E. Dané, et al., Nucl. Instr. and Meth. A 572 (27) 682. [] W. Riegler, Detector physics and performance simulations of the MWPCs for the LHCb muon system, LHCb Note, CERN-LHCb-2-6, 2. [2] W. Riegler, Crosstalk, Cathode structure and electrical parameters of the MWPCs for the LHCb muon system, LHCb Note, CERN-LHCb-2-6, 2. [3] G. Martellotti, et al., LHCb Note, CERN-LHCb-25-75, 25. [4] I.B. Smirnov, Nucl. Instr. and Meth. A 554 (25) 474. [5] I.B. Smirnov, CERN Comput. Newslett. 226 (996) 3 Available: hhttp:// consult.cern.ch/writeups/heed/i. [6] M. Anelli, et al., Nucl. Instr. and Meth. A 593 (28) 39.6/j.nima [7] For a review, see A.S. Schwartz, Nucl. Instr. and Meth. A 55 (23) 239.