New readout scheme for large area timing & position RPCs

Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima New readout scheme for large area timing & position RPCs João Saraiva ∗, Alberto Blanco LIP, Laboratory of Instrumentation and Experimental Particle Physics, Portugal ARTICLE INFO Keywords: Gaseous detectors Resistive Plate Chambers Readout codification Muon Scattering Tomography ABSTRACT A new readout technique was developed with the primary aim of keeping the number of channels in the front-end electronics as low as possible when scaling up the sensitive area of a Resistive Plate Chamber (RPC). The readout method here presented significantly reduces the dependence between the detector area and the number of electronic channels, without substantial reduction of its performance: a 30 ×30 cm2double stack multi-gap timing RPC was operated during weeks with cosmic rays, achieving a 2D spatial resolution well below 1 mm and time resolution lower than 100 ps, while its efficiency was kept above 98%. 1. Introduction Resistive Plate Chambers (RPCs) are deployed across diverse fields, for instance in High Energy Physics (HEP) for particle triggering and tracking over large areas, but also for Particle Identification (PID), if built to achieve high timing resolutions [1]. In fact, with the capability of measuring simultaneously and accurately position and time, RPCs could actually perform PID without the need of additional detectors. In the field of the Muon Scattering Tomography (MST), RPCs were first employed in 2012 to infer the presence of a small lead block within a scanned volume at the center of the detector [2]. More recently, the MST technique was used anew with RPCs, showing the presence of several high-Z materials after few minutes of acquisition [3]. RPCs are indeed well suited for muographic techniques since they can be built at relatively low cost, covering large areas with high efficiency, spatial and time resolutions. However, the front-end electronics (FEE), which constitute the major expense of the detector, can reach significant amounts when covering a surface well above 100 m2, the area that must be instrumented to scan, for example, a shipping container. A novel RPC readout codification was therefore designed and tested with the main purpose of addressing the aforementioned problem, partially decoupling the number of FEE channels from the sensitive area of the detector, while keeping high spatial and timing resolutions. 2. Novel readout scheme With this new method, the strips of the readout electrodes are grouped in parallel by the Signal Merging Printed Circuit Board (SMPCB) as shown in Fig. 1. In this way, each FEE channel reads out several strips in parallel, one per group, leading to a significant decrease of the number of preamplifiers, equivalent to the number of groups created by ∗Corresponding author. E-mail address: [email protected] (J. Saraiva). the SMPCB. However, the ambiguity that arises by grouping together strips of the thin-strip readout boards must be disentangled in order to determine in which group the signal was in fact induced. This is the role of the wide-strip readout electrode, which provides the 2D raw position of each event, allowing the impinged group to be identified in both directions. In this setup, 120 thin strips were subdivided into 5 groups, resulting in the use of only 24 preamplifiers to read out all the strips. The reduction factor of the number of electronic channels could be even higher, by simply adding more groups of strips in parallel. In this way, a larger detector area would have been covered while keeping the number of FEE channels unchanged, decoupling both quantities, as pretended with this technique. 3. Experimental setup Fig. 2 shows the setup, composed of a stack of two multi-gap timing RPCs and three readout electrodes connected to the respective FEE. With an active area of 30 ×30 cm2, the RPCs have 6 gas gaps each, 300 μm wide, and resistive electrodes made of float glass, 1 mm thick (𝜌≈4×1012 Ωcm at 25 ◦C). The detector was operated in open gas flow, with a gas mixture of 95.5% of R-134a and 4.5% of SF6. Two types of pick-up electrodes were used in the setup: •wide-strip readout PCB: constituted by 5 strips 5.9 cm wide and pitch of 6.1 cm; located between the RPCs, it is read out by fast https://doi.org/10.1016/j.nima.2024.169803 Received 21 June 2024; Received in revised form 3 August 2024; Accepted 24 August 2024 Nuclear Instruments and Methods in Physics Research A 1068 (2024) 169803 Available online 26 August 2024 0168-9002/© 2024 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies. J. Saraiva and A. Blanco Fig. 1. Process of grouping in parallel the thin strips of two readout electrodes transversely oriented. A SMPCB is used for each direction. The wide strips used for disentanglement between groups are represented in orange. Fig. 2. Left: Top view with the detector at the center and two SMPCBs grouping together the thin strips of the readout electrodes transversely oriented; Right: Detailed view of the stack, from the bottom to the top: (1) thin-strip PCB, (2) RPC_bottom, (3) wide-strip PCB, (4) RPC_top, (5) transversal thin-strip PCB. preamps (based on HADES FEE [4]) in both sides of the strips, providing simultaneously time and coarse 2D position1of the interactions occurring in the sensitive volume of the detector, and making it possible to identify unequivocally, for each event, the group of thin strips where the signal induction took place; •thin-strip readout PCB: composed of 120 strips 1.54 mm wide and 2.54 mm pitch; two of them were used, one located at the bottom of the stack with the strips oriented in the same direction as the wide ones, and a second board at the top of the stack, with the strips transversely oriented with respect to the previous ones; both pick-up electrodes are connected to a SMPCB, reducing the number of channels from 120 to 24 (see Section 2); each of these channels (2 ×24) is then read out by slow electronics2(customdesigned), integrating both components of the induced signals (electronic & ionic), allowing, in this way, for submillimetric precision in positional measurements in both dimensions: X & Y. The detector was operated during several weeks with cosmic rays, using a coincidence trigger generated externally by scintillators located 1having time measurements from both strip ends (front (f) and back (b)), one can compute two quantities: the longitudinal position along the strips (𝑌= (𝑇𝑓−𝑇𝑏)∕2) and the time of the event (𝑇= (𝑇𝑓+𝑇𝑏)∕2). The charge is also measured (via the Time over Threshold (ToT) method) and used to determine the transverse position relative to the strips (through charge interpolation). 2with a charge collection and integration process extending over a few microseconds, the electronics perform well due to the intrinsically low rate of the muons at sea level (∼1μ/cm2/min). Fig. 3. Left: Time resolution of the RPCs, decreasing from 85 to 74 ps (𝜎) after removal of the scintillator contribution; Right: On the right side, the image shows a 2D position map of the two groups of scintillators (projected shadows). A close-up view is shown at the bottom side, where the location of the nylon monofilament spacers of the RPCs is identified by red arrows. On the left side, the 2D event maps of the SiPMs and PMTs are projected onto the Y axis, providing clearer evidence of the presence of the 300-μm gas gap spacers. directly above and below the RPCs. Four plastic scintillators were used, two parallelepipeds of 8 ×4×1 cm3and two 8 ×2×3 cm3, coupled to silicon photomultipliers (SiPM) and photomultiplier tubes (PMT), respectively. The reduced electric field was set to approximately 379 Td (around 2.75 kV/gap, 92 kV/cm) and the respective efficiency was slightly above 98%. 4. Results The time precision of the detector was determined by measuring the time difference between the RPCs and one of the scintillators. A time resolution of 74 ps (𝜎) was achieved (see Fig. 3) after applying the time walk correction and removing the scintillator contribution. A 2D position map of the scintillator events is also provided in Fig. 3. In the reconstructed image of the scintillators, it is possible to identify the presence of the 300 μm-diameter spacer lines used between the glass electrodes of the RPCs, providing a clear indication of the submillimetric spatial resolution of the detector. 5. Application - Muon scattering tomography With the proposed codification, it is possible to decouple two quantities usually directly correlated: the detector surface and the number of FEE channels needed to instrument the respective sensitive area. The decoupling is partial since, as seen previously, the channels connected to the wide strips have to increase proportionally with the number of groups of thin strips added in parallel. However, due to the large dimensions of the strips used for disentangling the groups, the increase of electronic channels achieved with this technique is by far lower than if each thin strip would have been connected to its own channel. Considering the above, and knowing that the FEE is the driving cost of gaseous detectors such as RPCs, the new readout scheme outlined in this document can be used in any application requiring large sensitive areas. Typical applications are linked to the HEP field, however another strong candidate is the MST, since: •the area required for the MST of shipping containers is around 130 m2, on par with the 141 m2covered by RPCs in the ALICE experiment at CERN [5]; •the spatial resolution requirements for a muon tracking detector is dictated by the precision needed to measure the small scattering angles between the incoming and exiting muon trajectories; submillimetric spatial resolution allows both: (1) keeping the distance between detector planes, below and above the fiducial region, at few tens of centimeters (maximizing the detector acceptance and its compactness), (2) having the ability to detect small shifts of the muon direction, in the order of one degree (around 17 mrad). Nuclear Inst. and Methods in Physics Research, A 1068 (2024) 169803 2 J. Saraiva and A. Blanco Fig. 4. Two step simulation using the FLUKA cosmic ray source and Earth atmospheric model [6]; geometry: MST tracking detector of four 2 m2RPCs, with a distance of 45 cm between planes and a tungsten block of 10 ×10 ×10 cm3at the center; applied restrictions: only scatters above 1.5◦(Left), removing also muons below 500 MeV (Right). •a time resolution below 100 ps allows, via the Time Of Flight (TOF) technique, to reject low energy muons (a few hundred of MeV) since they are highly scattered within the material budget of the detector, resulting in false events all around the fiducial region. The impact of removing low energy muons, obtained via Monte Carlo calculation, is illustrated in Fig. 4. 6. Conclusion A new readout scheme was developed to reduce the number of FEE channels (driving cost of the detector), and tested with a multi-gap timing RPC of 30 ×30 cm2. With the presented codification, 24 + 24 charge sensitive preamps were used to read out 120 + 120 thin strips transversely oriented, providing submillimetric 2D positioning. An additional pick-up electrode was used with 5 + 5 current sensitive fast preamps to read out 5 large strips and resolve the ambiguity introduced by this technique. The experimental setup made it possible to achieve a 2D high spatial resolution (<1 mm), along with a time precision of 74 ps (𝜎) enabling PID, and an efficiency above 98%. The next step includes using the same readout method with a large scale RPC (120 ×90 cm2), without increasing the number of FEE channels connected to the thin strips, i.e. keeping the number of electronic channels independent of the sensitive area of the detector, as intended. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Foundation for Science and Technology (Portugal) (CERN/FIS-INS/0006/2021) and the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement AIDAinnova n.◦101004761. References [1] A. Akindinov, et al., NIM A 602 (2009) 709–712. [2] P. Baesso, et al., JINST 7 (2012) P11018. [3] J. Saraiva, et al., NIMA 1050 (2023) 168183. [4] D. Belver, et al., IEEE Trans. Nucl. Sci. 57 (2010) 2848–2856. [5] The ALICE Collaboration, et al., JINST 3 (2008) S08002. [6] G. Battistoni, et al., Ann. Nucl. Energy 82 (2015) 10–18. Nuclear Inst. and Methods in Physics Research, A 1068 (2024) 169803 3