Performance of the CMS RPC upgrade using 2D fast timing readout system

Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx

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Performance of the CMS RPC upgrade using 2D fast timing readout system K. Shchablo ∗, I. Laktineh, M. Gouzevitch, C. Combaret, L. Mirabito, on behalf of the CMS Muon group Institute of Nuclear Physics of Lyon, Batiment Paul Dirac 4, Rue Enrico Fermi 69622 Villeurbanne Cedex, France

ARTICLE Keywords: RPC iRPC PETIROC CMS upgrade

INFO

ABSTRACT We present a new generation of Resistive Plate Chambers (RPC) that are able to withstand high particle fluxes (up to 2 kHz cm−2 ). These chambers will be instrumented with a precise timing readout electronics and they are proposed to equip two of the four high eta stations of the CMS muon system. Two-gap RPC detectors, with each gap made of two 1.4 mm High Pressure Laminate (HPL) electrodes and separated by a gas gap of the same thickness, are proposed. The new scheme reduces the amount of the avalanche charge produced by the passage of a charged particle through the detector. This improves the RPC rate capability by reducing the needed time to absorb this charge. To keep the RPC efficiency high a sensitive, low-noise and high time resolution Front-End Electronics Board (FEB) is needed to cope with the low charge signal. An ASIC called PETIROC that has all these characteristics is proposed to read out the new chambers. A thin (0.6 mm) Printed Circuit Board (PCB), 165 cm long, equipped with pickup strips of average pitch of 0.75 cm is inserted between the two RPC gaps. The strips are read out from both ends and the arrival time difference of the two signals is used to determine the particle position along the strip (𝜂 position). The absolute time measurement will also be used to reduce the data ambiguity due to the expected high pileup at the High-Luminosity phase of the Large Hadron Collider (HL-LHC).

1. Introduction During the HL-LHC phase the instantaneous luminosity in the CMS experiment is expected to increase up to a maximum of 5 − 7 × 1034 cm−2 s−1 . To cope with the expected high particle rate in the very forward region of CMS the new improved Resistive Plate Chamber (iRPC) will be installed during the shut-down periods (2022–2023) and (2024–2026) of the LHC [1]. The installation of iRPC in the last two stations RE3/1 and RE4/1 (Fig. 1) covering 1.8 < |𝜂| < 2.4 with a good intrinsic time resolution would improve the rejection of background hits and low transverse momentum tracks. It would also help to improve the Endcap trigger efficiency for the identification of multiple tracks and would allow the reconstruction of slowly moving Heavy Stable Charged Particles (HSCP). At these two stations the expected rate (including a safety factor of three) during the HL-LHC program will not exceed 2 kHz cm−2 . The new stations are required to detect muons with an efficiency above 95% and to sustain a total integrated charge of 1 C cm−2 corresponding to the full period of operations during the HL-LHC program with the given rate in this region.

Fig. 1. Layout of a quadrant of the CMS muon detector.

2. iRPC prototype 2.1. Gas detectors Thick HPL (2 mm) with an electric resistivity range of (1 ∼ 6) × 1010 Ω cm has been used for the present CMS RPC system. However,

∗ Corresponding author. E-mail address: [email protected] (K. Shchablo).

https://doi.org/10.1016/j.nima.2019.04.093 Received 30 March 2019; Received in revised form 23 April 2019; Accepted 24 April 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: K. Shchablo, I. Laktineh, M. Gouzevitch et al., Performance of the CMS RPC upgrade using 2D fast timing readout system, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.093.

K. Shchablo, I. Laktineh, M. Gouzevitch et al.

Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx

using thinner HPL (1.4 mm) with a lower resistivity range of (1 ∼ 3)×1010 Ω cm for the present double-gap iRPCs (Fig. 2) will improve the rate capability by about a factor of 2 when considering the reduction of the resistance of the electrode material. Indeed the charge produced by the passage of the charged particle through a thinner gas gap is reduced and it is absorbed faster through a thinner electrode. This increases the rate capability of the detector but reduces the charge left by a muon. Therefore a more sensitive electronics is required to keep the efficiency as high as before [2]. Fig. 4. Photograph (left) and dimensions (right) of a prototype pickup-strip PCB.

H4 beamline and a 13.9 TBq 137 Cs source at the Gamma Irradiation Facility [5] of CERN. The rate of gammas converted in the detector ranged from 0 to 3 kHz cm−2 as shown in Fig. 5.

Fig. 2. Scheme of the 2-gap HPL-based iRPC proposed to equip two of the high 𝜂 CMS muon stations.

2.2. Readout electronics and strip panel A new readout system that uses the OMEGA PETIROC2A ASIC [3] allows the readout of the signal from the two ends of the strips. This ASIC has 32 readout channels. Each channel includes a fast preamplifier with an overall bandwidth of 1 GHz and a gain of 25. It has a dynamic range from 60 f C to 160 pC. These characteristics of the PETIROC2A qualify it for the iRPC readout. A common 10-bit Digital Analogue Converter (DAC) system is used by the ASIC to ensure a global triggering level adjustment in the dynamic range of the ASIC. Furthermore, an individual 6-bit DAC is used in each channel to achieve a similar response of all the ASIC’s channels (Fig. 3). The ASIC is controlled with a Cyclone II Altera Field Programmable Gate Array (FPGA) that includes a Time-to-Digital Converter (TDC) with an embedded temperature correction system [4].

Fig. 5. Gamma background rate as a function of reciprocal of gamma-attenuation factor.

A muon trigger system (Fig. 6) was built from four scintillators: two externals (outside GIF++ bunker area) and two internals. Using reference signals of the scintillators-based coincidence unit the arrival time of the detector associated signals of each of the two strip ends were recorded (Fig. 7). The time distribution for each of the two ends can be described by a Gaussian function plus a constant function.

Fig. 6. Setup in Gamma irradiation facility (GIF++).

The true muon efficiency in the presence of the gamma background is calculated as follows, Fig. 3. The Front-End Electronics Board (FEB) that hosts one PETIROC ASIC and the FPGA that includes the TDC(top right) and the schematics of the PETIROC ASIC (left).

𝜀=

The detector signals read from the pickup strips inserted in the middle of 600 μm FR4 plate (Fig. 4) are transferred to the FrontEnd Board (FEB) via shielded return traces embedded in the middle layer of the pickup-strip PCB. The FEB input impedance for the signals is precisely adjusted to avoid reflections. This design presents the advantage of having a good protection against noise and an easier impedance adaptation solution to avoid reflections. The noise rate of the present prototype iRPC measured at a threshold of 81 f C does not exceed 5 Hz cm−2 . The difference in arrival times of the signals from two ends of each strip allows us to determine the hit position of the particle on the 𝜂 direction. Also, comparing signals from the two ends of the strips gives an additional possibility to filter out the electronics noise.

𝑁 𝑁𝑡𝑟𝑖𝑔

1−

−

𝑁𝑏𝑘𝑔 𝑁𝑡𝑟𝑖𝑔

𝑁𝑏𝑘𝑔

(1)

𝑁𝑡𝑟𝑖𝑔

Here, 𝜀, 𝑁, 𝑁𝑡𝑟𝑖𝑔 , 𝑁𝑏𝑘𝑔 are the measured muon efficiency, the number of the measured events, the number of the muon triggers and the number of the gamma background hits expected in the time interval, respectively.

3. Performance of iRPC 3.1. Rate capability Fig. 7. Time profiles of strips signals associated to the beam. Left: time signal from left end. Right: time signal from right end.

The prototype detector was tested with 100 GeV (mean energy) muons and background gamma rays respectively provided by the SPS 2

Please cite this article as: K. Shchablo, I. Laktineh, M. Gouzevitch et al., Performance of the CMS RPC upgrade using 2D fast timing readout system, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.093.

K. Shchablo, I. Laktineh, M. Gouzevitch et al.

Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx

To estimate the efficiency independently of pressure and temperature variations during the beam exposure, effective values of the high voltage were calculated according to the following formula: 𝐻𝑉𝑎𝑝𝑝

𝑃 𝑇0 = 𝐻𝑉𝑒𝑓 𝑓 ((1 − 𝛼) + 𝛼 ) 𝑃0 𝑇

3.3. Absolute time resolution To estimate the absolute time resolution of our detector we measured the arrival time difference of the signals of two chambers crossed by the same particle. The absolute time resolution is deduced by √ dividing the estimated resolution of this time difference by 2 with the assumption that both electronics are identical and uncorrelated.

(2)

Here, 𝐻𝑉𝑎𝑝𝑝 , 𝐻𝑉𝑒𝑓 𝑓 , 𝑃 , 𝑃0 , 𝑇 , 𝑇0 , 𝛼 are the high voltage applied, high voltage effective, local pressure, reference pressure equal to 990 mbar, local temperature, reference temperature equal to 293.15 K, correction coefficient for RPC equal to 0.8. Fig. 8 shows the efficiency at different particle rates as a function of the effective high voltage applied on each gap. These results show that efficiencies around 95% are reached for a particle rate of 2 kHz cm−2 . The rate is estimated by counting the number of clusters of fired strips in a given time interval normalized to the instrumented surface and to the efficiency.

𝐻𝑅 𝐻𝑅 𝐿𝑅 𝐿𝑅 𝛥𝑡 = ((𝑇𝑝𝑟𝑜𝑡.1 − 𝑇𝑝𝑟𝑜𝑡.2 ) + (𝑇𝑝𝑟𝑜𝑡.1 − 𝑇𝑝𝑟𝑜𝑡.2 ))∕2

(3)

⟨𝜎 ⟩ ⟨𝜎𝑡 ⟩ = √𝛥𝑡 2

(4)

The result is drawn in Fig. 10. One can thus estimate that the absolute time resolution of each of these large two detectors is 368 ps. To determine this absolute time resolution (4) of iRPC detectors we use the fact that the muon beam is more or less perpendicular to the chambers planes.

Fig. 8. Efficiency curves as function of the effective high voltage for the prototype shown for different background rates (Left). Efficiency at plateau for prototype at different background rates (Right).

3.2. Linearity and along-strip resolution The measurements of the time characteristics of the chamber were carried out using muon beams provided by the SPS H2 beamline. The chamber was mounted on a 100 micron-precision movable table that allows changing the positions of the detector with respect to the beam. The scintillators scheme is similar to that used in GIF++. Fig. 9

Fig. 10. Time difference Eq. (3) of two detectors. The RMS value divided by square root of 2 provides the 2-gap HPL iRPC absolute time resolution.

4. Conclusion A new RPC technology to equip two stations of the high 𝜂 region of the CMS muon spectrometer is proposed and validated on real size prototypes. The resulting detector fulfills all the requirements: space resolution better than 1.7 cm, absolute time resolution of ≈ 370 ps and an efficiency of 95% for the maximum expected background rate of 2 kHz cm−2 . The detector is found to be uniform over 1 m length. References

Fig. 9. Time resolution of the iRPC prototype at the center of the detector (left). Correlation between the average 𝛥T measurement and the position of the beam with respect to a fixed reference.

[1] CMS Collaboration, The Phase-2 Upgrade of the CMS Muon Detectors, Tech. Rep. CERN-LHCC-2017-012. CMS-TDR-016, CERN, 2017. [2] K.S. Lee, et al., Study of thin double-gap RPCs for the CMS muon system, J. Korean Phys. Soc. 73 (2018) 1080. [3] J. Fleury, et al., Petiroc, a new front-end asic for time of flight application, in: 2013 IEEE Nuclear Science Symposium and Medical Imaging Conference, Conference Record, 2013, p. 15. [4] Pan Weibin, Guanghua Gong, Jianmin Li, A 20-ps time-to-digital converter (TDC) implemented in field-programmable gate array (FPGA) with automatic temperature correction, IEEE Trans. Nucl. Sci. 61 (2014) 1468. [5] M.R. Jakel, et al., CERN-Gif++: a new irradiation facility to test large-area particle detectors for the high-luminosity LHC program, PoS TIPP2014 (2014).

(left) shows the observed time resolution (177 ps) at the center of the detector. This excellent resolution allows the determination of alongstrip position with a resolution of ≈ 1.7 cm. Fig. 9 (right) shows the average position of the beam estimated from the arrival time difference (𝛥T) as a function of the table position. A good linearity is observed. Maximum deviation is found to be less than 2 cm with respect to the exact values.

3

Please cite this article as: K. Shchablo, I. Laktineh, M. Gouzevitch et al., Performance of the CMS RPC upgrade using 2D fast timing readout system, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.093.