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Intrinsic linearity of bakelite Resistive Plate Chambers operated in streamer mode
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
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Intrinsic linearity of bakelite Resistive Plate Chambers operated in streamer mode B. Bartoli a,b , P. Bernardini c,d , X.J. Bi e , P. Branchini h , A. Budano h , P. Camarri j,k , Z. Cao e , R. Cardarelli k , S. Catalanotti a,b , S.Z. Chen e , T.L. Chen l , P. Creti d , S.W. Cui m , B.Z. Dai n , A. D’Amone c,d , Danzengluobu l , I. De Mitri u,v , B. D’Ettorre Piazzoli a , T. Di Girolamo a,b , G. Di Sciascio k , C.F. Feng o , Zhaoyang Feng e , Zhenyong Feng p , Q.B. Gou e , Y.Q. Guo e , H.H. He e , Haibing Hu l , Hongbo Hu e , M. Iacovacci a,b ,∗, R. Iuppa j,k , H.Y. Jia p , Labaciren l , H.J. Li l , G. Liguori f,g , C. Liu e , J. Liu n , M.Y. Liu l , H. Lu e , L.L. Ma e , X.H. Ma e , G. Mancarella c,d , S.M. Mari h,q , G. Marsella c,d , D. Martello c,d , S. Mastroianni b ,∗∗, P. Montini h,q , C.C. Ning l , M. Panareo c,d , L. Perrone c,d , P. Pistilli h,q , F. Ruggieri h , P. Salvini g , R. Santonico j,k , P.R. Shen e , X.D. Sheng e , F. Shi e , A. Surdo d , Y.H. Tan e , P. Vallania r,s , S. Vernetto r,s , C. Vigorito s,t , H. Wang e , C.Y. Wu e , H.R. Wu e , L. Xue o , Q.Y. Yang n , X.C. Yang n , Z.G. Yao e , A.F. Yuan l , M. Zha e , H.M. Zhang e , L. Zhang n , X.Y. Zhang o , Y. Zhang e , J. Zhao e , Zhaxiciren l , Zhaxisangzhu l , X.X. Zhou p , F.R. Zhu p , Q.Q. Zhu e , G. Zizzi i , The ARGO-YBJ Collaboration a
Dipartimento di Fisica dell’Università di Napoli ‘‘Federico II’’, Complesso Universitario di Monte Sant’Angelo, via Cinthia, 80126 Napoli, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Complesso Universitario di Monte Sant’Angelo, via Cinthia, 80126 Napoli, Italy c Dipartimento Matematica e Fisica ‘‘Ennio De Giorgi’’, Università del Salento, via per Arnesano, 73100 Lecce, Italy d Istituto Nazionale di Fisica Nucleare, Sezione di Lecce, via per Arnesano, 73100 Lecce, Italy e Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, 100049 Beijing, PR China f Dipartimento di Fisica dell’Università di Pavia, via Bassi 6, 27100 Pavia, Italy g Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, via Bassi 6, 27100 Pavia, Italy h Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy i Istituto Nazionale di Fisica Nucleare - CNAF, Viale Berti-Pichat 6/2, 40127 Bologna, Italy j Dipartimento di Fisica dell’Università di Roma ‘‘Tor Vergata’’, via della Ricerca Scientifica 1, 00133 Roma, Italy k Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy l Tibet University, 850000 Lhasa, Xizang, PR China m Hebei Normal University, Shijiazhuang 050016, Hebei, PR China n Yunnan University, 2 North Cuihu Rd., 650091 Kunming, Yunnan, PR China o Shandong University, 250100 Jinan, Shandong, PR China p Southwest Jiaotong University, 610031 Chengdu, Sichuan, PR China q Dipartimento di Fisica dell’Università ‘‘Roma Tre’’, via della Vasca Navale 84, 00146 Roma, Italy r Osservatorio Astrofisico di Torino dell’Istituto Nazionale di Astrofisica, via P. Giuria 1, 10125 Torino, Italy s Istituto Nazionale di Fisica Nucleare, Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy t Dipartimento di Fisica dell’Università di Torino, via P. Giuria 1, 10125 Torino, Italy u GSSI, Gran Sasso Science Institute, Via Iacobucci 2, L’Aquila, Italy v INFN, Laboratori Nazionali del Gran Sasso, Assergi, L’Aquila, Italy b
ARTICLE Keywords: RPC detector Streamer mode Intrinsic linearity Calorimetry
INFO
ABSTRACT Resistive Plate Chambers have largely been used in High Energy Physics and Cosmic Ray research. In view of using this detector for calorimetry applications it is important to know the maximum measurable particle density, or its intrinsic linearity limit, which is tightly related to the dimension of the discharge region. In this paper we report the results of measurements performed at the Beam Test Facility (INFN National Laboratory of Frascati, Italy) where the intrinsic linearity of bakelite RPCs operated in streamer mode has been tested at different impinging particle densities.
∗ Corresponding author at: Dipartimento di Fisica dell’Università di Napoli ‘‘Federico II’’, Complesso Universitario di Monte Sant’Angelo, via Cinthia, 80126 Napoli, Italy. ∗∗ Corresponding author. E-mail addresses: [email protected] (M. Iacovacci), [email protected] (S. Mastroianni).
https://doi.org/10.1016/j.nima.2019.162746 Received 25 August 2018; Received in revised form 4 September 2019; Accepted 9 September 2019 Available online 13 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
B. Bartoli, P. Bernardini, X.J. Bi et al.
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
1. Introduction
current electron–positron linear accelerator. The Beam Transfer Line, that is the BTF, is capable of producing electron or positron bunches within an energy range of 50 to 800 MeV. The emitted particle bunches have a Poisson distribution with a mean value dependent on the slit setting. The typical pulse duration, 10 ns, is comparable to the time tickness of the EAS front at its center, while the maximum repetition rate is limited to 50 Hz. The experimental setup used for the tests described in this paper [5] consisted of four small RPCs of dimensions 63 × 57 cm2 , closely stacked and placed near the exit of the beam line, and of a lead glass block, length 37 cm and base 11 × 11 cm2 , positioned just after the RPCs , with its major axis aligned along the beam direction. These RPCs have the same layout, see Fig. 1, of the large RPCs used in the ARGO-YBJ experiment [6], with eight pickup strips (63 × 6.8 cm2 ) for digital readout facing one side of the gas volume, and a single pick-up electrode (pad, 63 × 57 cm2 ) facing the opposite side of the same gas volume. A 3 mm foam layer was inserted between the RPC and the pad to reduce cross-talk effects (more details in [6]). The beam spot had the same dimensions as the vacuum pipe (5 × 3 cm2 ) and was entirely unfocused as a result of all transfer line quadrupoles being shut off. The slit apertures were operated in a manner so that the bunches, emitted at a rate of 1 Hz, could contain from single to hundreds of particles in the energy range of 450–500 MeV. The glass block itself (24 radiation lengths) was taken from the OPAL experiment [31] and consists of a Schott SF57 glass with the shape of truncated prism. A Hamamatsu R2238 photomultiplier tube was used to read the light signals produced in the glass by the impinging particles. The lead glass block was used as a calorimeter, whose signals allowed both the measurement of the beam intensity and a comparison with the RPC signals, in particular with the first RPC along the beam line. The analog signals coming from the RPCs were processed by a custom board [32,33], based on a Peak and Hold circuit, which continually samples the amplitude (voltage on 50 𝛺), maintains the highest reached value for 2 𝜇𝑠 and digitizes it. After being adapted, the calorimeter signals were collected by the same electronics used to handle the RPC analog signals. Instead, the standard electronics of ARGO-YBJ was used to readout the RPC digital signals. The data were recorded at each trigger occurrence by a custom system utilizing a VME crate in a way simplified with respect to that described in [34]. The trigger signal was provided by the accelerator complex at each electron–positron bunch delivery.
Resistive Plate Chambers (RPCs) were initially conceived as particle counters able to provide a good time resolution [1,2]. Thanks to their robustness, simplicity and low cost they have been used in experiments with large instrumented surfaces, typically of the order of (5–10)×103 m2 , both in the context of accelerator [3,4] and cosmic ray physics [5– 7]. Being used in environments with a low density of particles, mainly for triggering, tracking or vetoing, the spatial resolution requested to them has been mostly systematically delegated to the size of the pickup electrodes, typically strips with dimensions of a few centimeters (width) by some decimeters (length). Motivated by the need to use RPCs for the construction of apparatuses or devices with high spatial resolution, specific studies were initiated some years ago [8–15] and the dimensions of the RPC discharge were measured in different ways operating the detector either in streamer or avalanche mode. The measurements agree in reporting the transversal size of the discharge within a couple of millimeters (1–2 mm) in case of operation in avalanche mode and many millimeters (4–5 mm) in case of streamer mode. In the framework of the astroparticle applications of RPCs, the ARGO-YBJ experiment [5,6] is certainly the most important in terms of dimensions and duration. The ARGO-YBJ detector, made of one layer (6700 m2 ) of bakelite RPCs, was operated in streamer mode at the Yangbajing Cosmic Ray Laboratory (Tibet, P.R. China, 4300 m.a.s.l. , 606 g/cm2 ), taking data with very high stability from November 2007 to February 2013. The choice of the streamer operation [16] was dictated by the needs of stability and ease of operation. The dense sampling area and the high altitude allowed for a study of cosmic radiation from a few hundred GeV to the PeV range [17–24]. The detection of particles of Extensive Air Showers (EAS) was handled by both a digital readout, consisting of a pick-up strip pattern, and an analog readout, made by means of large pick-up electrodes, to measure the total charge induced by the passing particles. The factors limiting the maximum measurable EAS energy are respectively, for the two readout modes, the strip density (23 strips/m2 ) for the former, where the EAS energy is limited to a few hundred TeV [25], and the maximum number of streamers per unit surface (streamer/m2 ) for the latter [5]. In the analog readout mode the upper measurable energy depends on the intrinsic linearity of the RPC response, which is also related to the size of the streamer. Indirect measurements of the streamer size in bakelite RPCs, as the ones used in ARGO-YBJ, have been obtained by means of a test carried out at the Beam Test Facility (BTF) of the Frascati INFN Laboratory. The primary goal of the test was to determine the saturation effects of the RPC response when operated in streamer mode. Results concerning the linearity of RPCs up to particle densities less than 2 × 104 /m2 have been already reported [5]. These results have been extensively exploited to measure with great detail the lateral distribution of charged particles around the core of high energy EAS [26] and to measure the energy spectrum of the cosmic ray light component (p + He) up to a few PeV [23]. The prospect of new initiatives [27–29] aimed to define EAS experiments in the Southern Hemisphere at high altitudes, as in the Andes, 5000 m.a.s.l. or more, raises the question of the maximum particle density that can be measured by bakelite RPCs operated in streamer mode before attaining saturation effects. Indeed, large continuous detectors, primarily planned for gamma astronomy, could sample very dense showers induced by primary cosmic rays with energies of 10 PeV or more. After analyzing data from other tests carried out at the BTF, we can here report on the results concerning RPCs exposed to beams with particle density up to 8 × 104 /m2 .
3. RPC calibration Prior to the beam exposure, the RPCs were calibrated with m.i.p., i.e. the passing through cosmic muons. This was done in the same experimental hall where the detectors were subsequently exposed to the beam. Operating conditions were carefully kept constant during both the calibration phase and the subsequent exposure to the particle beam, that means we paid attention to keep the ratio Temperature/Pressure (T/P) stable within 0.5%, which translates in 𝛥P < 5 mbar and 𝛥T < 1.5 C◦ [35,36]. The four RPCs were aligned one on top of another, so to make a small vertical muon telescope. The top RPC and the bottom one were respectively the first and the last (along the beam line) in the subsequent exposure to the particle beam. A 3 cm thick lead slab was put above the lowest RPC in order to filter low energy particles. Finally, four additional small RPCs, surrounding the telescope, were used to anti-coincide EAS events as shown in Fig. 2. The muon trigger was set as the 3-fold coincidence of the four stacked RPCs in anticoincidence with any of the four additional small chambers surrounding the telescope. This condition limits the surviving EAS contamination. To improve the selection of good events and provide adequate sample purity, the lowest RPC is always required to be in the trigger coincidence and no more than two contiguous fired strips are allowed in each RPC. The telescope geometry and the trigger conditions guarantee the selection of muons with zenith angle not greater than 60◦ . The RPCs were filled and fluxed with a gas mixture 75% Tetraflouroethane, 15% Argon and
2. Experimental setup The test-beam described here was carried out at the Beam Test Facility (BTF) [30], located at the DAFNE-factory complex within the INFN National Laboratory at Frascati. The DAFNE facility consists of two storage rings, a 510 MeV electron–positron accumulator and a high 2
B. Bartoli, P. Bernardini, X.J. Bi et al.
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
Fig. 1. Layout of the bakelite RPCs under test, with High Voltage, Low Voltage and grounding connections (see [6] for details).
Fig. 2. Sketch of the RPC vertical telescope along with anticoincidence chambers and absorber.
10% Isobutane, as in the ARGO-YBJ experiment. First, some efficiency checks were performed on the RPCs under test. The efficiencies of the three uppermost RPCs were measured by excluding in turn one of them from the trigger and the values were found compatible among each other after correction for acceptance. As an example, the efficiency of the top RPC is shown in Fig. 3. Starting from 9.2 kV the efficiency is larger than 95% and keeps stable within 5%; the voltage starting from which the efficiency keeps stable within 5% was assumed as our definition of operating voltage, therefore we will refer to 9.2 kV as the operating voltage from here on. The distribution of the pad signal amplitude of the top RPC, at 9.2 kV, is shown in Fig. 4. By applying a gaussian fit to the first peak, an average amplitude of 3.7 ± 0.2 mV has been obtained while the fit to the second peak results in a mean value of 7.2 ± 0.5 mV, the two peaks corresponding respectively to single and double streamer events. Then, the dependence of the pad signals on the applied voltage has been studied. The results are given in Fig. 5, where the signal amplitudes (voltage on 50 𝛺) of the first and second peak are shown, and in Fig. 6, where the percentage occurrences of the first and second peak are reported. For an applied voltage larger than 9.0 kV double streamers occur in about 15% of the events and this percentage is approximately stable. If we consider a 9.2 kV operating voltage (efficiency 96%) we find that about 72% of events are with 1 streamer, 16% of events with 2 streamers and, even if not highlighted by any fitting line, about 4% are 3 streamer events, namely the little blob between 8 and 11 mV. The remaining 3%–4% are residual shower events and appear in the distribution at amplitudes greater than 11 mV, very widely spread . They all sum up to 96%, a remaining 2% is due to the geometry (spacers)
Fig. 3. Top (first along beam line, see text for details) RPC efficiency versus applied voltage.
and only what remains (about 2%) is detector intrinsic inefficiency. We define a calibration coefficient (mV/m.i.p.) as the weighted average of the signal peak amplitude on events with 1, 2 and 3 streamers at 9.2 kV, which results in 4.2 mV/m.i.p. with a statistical uncertainty of 5%. This amplitude, according to the measurements made at the ARGO-YBJ site and reported in [5], does not change with incidence angle, at least up to a zenith angle of 60◦ . Being the applied voltage stable within ±5 V and 3
B. Bartoli, P. Bernardini, X.J. Bi et al.
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
Fig. 4. Pad signal amplitude for minimum ionizing muons at 9.2 kV. The bin size is ∼0.2 mV. Two peaks are clearly visible, their position identified by a gaussian fit (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Fraction of occurrences of events in the first and second peak versus HV.
to the particle beam of the BTF and a data taking, driven by the bunch-crossing signal of the BTF complex, was started, operating the RPCs at 9.2 kV with the mentioned gas mixture. In spite of the beam being totally unfocused, the beam geometry [30] allowed for a nearly complete containment by the lead glass calorimeter (Schott SF57 glass has a Moliere radius of 2.61 cm) with only marginal loss. The PMT of the calorimeter was operated at 1.2 kV, as suggested by the BTF experts, therefore providing a single particle signal of about 70 mV at the mean energy of the beam, namely 480 MeV. The calorimeter signal was attenuated by roughly a factor 30 and adapted to fulfill the requirements of the readout electronics. Data were taken with different settings of the slits which defined the number of delivered particles at the exit windows. Only electron runs have been considered for the following analysis. The signal amplitude of the foremost RPC versus the calorimeter signal is shown in the scatter plot of Fig. 7. Data refer to four runs (represented by different colors) where, according to the estimation done by the beam monitoring system of the BTF, the number of electrons per bunch was changing from 7 to 20 (gray points), from 15 to 40 (blue points), from 45 to 70 (red points) and from 60 to 120 (green points). Also, to better highlight the relation between the two signals, the profile points (the mean value of Y for each bin in X [37]) are reported by the full black triangles; in the specific case the RPC mean value is computed in intervals of 0.4/0.6/0.8/2.1 mV/bin of the calorimeter signal amplitude for the gray/blue/red/green points. On the right scale of Fig. 7 the corresponding particle density, estimated according to a mean amplitude of 4.2 mV/particle for the RPC signal, is reported. Although we measured a maximum density of 120 particles on 15 cm2 , we assume the scalability of the results to a wider surface, that is we consider still valid a single cell model for the RPC response [38] as data show no deviation from linearity. In order to check the response linearity, a linear fit (red line in Fig. 7) to the RPC mean amplitudes (full black triangles) has been performed. The distribution of the normalized residuals, that is the difference between the fit values and the measured values normalized to the fit values, is shown in Fig. 8. It follows fairly well a gaussian distribution for normalized residual values smaller than 0.05 (about 2.2 𝜎) from the center, which corresponds to a statistics of 330 points over 370. About 40 points are left out, mainly on the left tail of the distribution, which contains the points where the RPC response is lower than the one expected according to calorimeter signal. This could be an indication of non linearity, but we have verified that this happens for points in the central part of the scatter plot in Fig. 7 and therefore it is more likely to be attributed to the effect of the beam structure or of its purity.
Fig. 5. First and second peak of the pad signal amplitude versus applied voltage. The peak values have been identified by gaussian fits as shown in Fig. 4. Data refers to the top RPC. Error bars are the standard deviations coming from the gaussian fit.
the percentage variation of the mean peak amplitude (for both 1 and 2 streamer events) contained within ±7% for a ±100 V variation of the operating voltage, and given the stability of the percentages of 1 and 2 streamer events in 9.2 ± 0.1 kV, we estimate the systematic uncertainty on the calibration coefficient to be less than 1%. Also, considering that our measurements were done in stable conditions of temperature and pressure, and that variations of these parameters translate into an equivalent variation of the applied voltage, we can assume that a maximum of 1% systematics can be due to environmental parameter variations. Therefore the overall systematic error can be quoted to be less than 2%. 4. Exposure to the beam and data analysis After calibration, the four RPCs were rotated and put vertically with their surface orthogonal to the beam line. The lead glass calorimeter described before was mounted behind the fourth RPC and connected to the DAQ system. Then the whole experimental setup was exposed 4
B. Bartoli, P. Bernardini, X.J. Bi et al.
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
Fig. 7. RPC signal versus the calorimeter signal. Data refer to four runs, represented by the different colors, with number of electrons ranging in different intervals. A straight line fit (in red) was performed to the profile points (full black triangles). On the right scale is reported the particle density estimated using the RPC signal amplitude and the calibration constant of 4.2 mV/particle. See text for more details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Distribution of the normalized residuals after a straight line fit has been performed to the profile points in Fig. 7 for calorimeter signals > 170 mV.
Acknowledgments We thank the DAFNE-BTF group, especially G. Mazzitelli and P. Valente for their invaluable aid during and after the test beam. References [1] R. Santonico, R. Cardarelli, Development of resistive plate counters, Nucl. Instrum. Methods Phys. Res. A 187 (1981) 377, http://dx.doi.org/10.1016/0029554X(81)90363-3. [2] R. Cardarelli, R. Santonico, Progress in resistive plate counters, Nucl. Instrum. Methods Phys. Res. A 263 (1988) 20, http://dx.doi.org/10.1016/0168-9002(88) 91011-X. [3] ATLAS Collaboration, Technical Design Report, CERN/LHCC/(1998) 1998-14. [4] CMS Collaboration, Technical Design Report, CERN/LHCC/(2000) 2000-38. [5] B. Bartoli, et al., The analog Resistive Plate Chamber detector of the ARGOYBJ experiment, Astropart. Phys. 67 (2015) 47, http://dx.doi.org/10.1016/j. astropartphys.2015.01.007. [6] G. Aielli, et al., Layout and performance of the RPCs used in the ARGOYBJ experiment, Nucl. Instrum. Methods Phys. Res. A 562 (2006) 92, http: //dx.doi.org/10.1016/j.nima.2006.02.136. [7] A. Bertolin, et al., The RPC systems of the OPERA experiment, Nucl. Instrum. Methods Phys. Res. A 602 (2009) 631, http://dx.doi.org/10.1016/j.nima.2008. 12.071. [8] R. Arnaldi, A. Baldit, V. Barret, N. Bastid, G. Blanchard, E. Chiavassa, Spatial resolution of RPC in streamer mode, Nucl. Instrum. Methods Phys. Res. A 490 (2002) 51, http://dx.doi.org/10.1016/S0168-9002(02)00917-8. [9] A. Blanco, N. Carolino, C.M.B.A. Correia, R. Ferreira Marques, P. Fonte, D. Gonzalez-Diaz, et al., RPC-PET Prototype with high spatial resolution, Nucl. Instrum. Methods Phys. Res. A 533 (2004) 139, http://dx.doi.org/10.1016/j. nima.2004.07.016. [10] R. Cardarelli, G. Aielli, P. Camarri, A. Di Ciaccio, B. Liberti, R. Santonico, Improving track resolution in the RPC chamber, Nuclear Phys. B 158 (2006) 25, http://dx.doi.org/10.1016/j.nuclphysbps.2006.07.023. [11] Q. Li, Y. Ye, C. Wen, Study of spatial resolution properties of a glass RPC, Nucl. Instrum. Methods Phys. Res. A 663 (2012) 22, http://dx.doi.org/10.1016/j.nima. 2011.10.006. [12] Y. Jin, C. Jianping, Y. Qian, L. Yuanjin, Jin, W. Vi, Study on the position resolution of resistive plate chamber, Nucl. Instrum. Methods Phys. Res. A 591 (2008) 411, http://dx.doi.org/10.1016/j.nima.2008.02.102. [13] I. Kitayama, H. Sakai, Y. Teramoto, et al., Optical observation of discharge in resistive plate chamber, Nucl. Instrum. Methods Phys. Res. A 424 (1999) 474, http://dx.doi.org/10.1016/S0168-9002(98)01379-5. [14] S. Narita, M. Shoji, Y. Hoshi, D. Miura, Y. Kikuchi, K. Neichi, A. Yamaguchi, Measurements of induce charge profile in RPC with submilli-strips, IEEE Trans. Nucl. Sci. 57 (2010) 2210–2214, http://dx.doi.org/10.1109/TNS.2010.2052111. [15] S. Narita, Y. Hoshi, K. Neichi, A. Yamaguchi, Induced Charge Profile in Glass RPC Operated in Avalanche Mode, 2012 IEEE (NSS/MIC) N(1146) 14-128. doi:10.1109/TNS.2013.2282638. [16] C. Bacci, et al., High altitude test of RPCs for the Argo YBJ experiment, Nucl. Instrum. Methods Phys. Res. A 443 (2000) 342, http://dx.doi.org/10.1016/ S0168-9002(99)01079-7. [17] B. Bartoli, et al., TeV Gamma-ray survey of the northern sky using the ARGO-YBJ detector, Agron. J. 779 (2013) 27, http://dx.doi.org/10.1088/0004-637X/779/ 1/27.
Fig. 8. Distribution of residuals with respect to a line fit performed to the profile points in Fig. 7. The residuals are normalized to the fit value.
On the other side, for positive values, we have a good agreement and the tail is roughly gaussian (as expected, about 3% of the values are beyond the 2.2 𝜎 where the fit region ends ). To definitely confirm no deviation from linearity we performed a linear fit only to the right part of the plot, namely for calorimeter signals greater than 170 mV; the distribution of normalized residuals (reported in Fig. 9) is well fitted by a gaussian (black line) and the 𝜒 2 of the linear fit results to be 74.6, with 70 d.o.f., which also is a good value as compared to 90.53, the 2 for 95% c.l.. The parameter values of the linear fit are consistent, 𝜒𝑙𝑖𝑚 within the experimental errors, with those obtained by fitting all data over the entire range. 5. Conclusions A test of the intrinsic linearity of the bakelite Resistive Plate Chamber response has been conducted at the Beam Test Facility of the INFN National Laboratory in Frascati, Italy. The detector was operated in streamer mode. No evidence for deviation from a linear behavior was observed up to 120 particles/15 cm2 , implying a linear response up to a particle density of 8×104 ∕m2 . This limit corresponds, at an altitude of 4300 m a.s.l., to the particle density at the core of EASs induced either by protons with energy E𝑝 of 7–10 PeV or by Fe nuclei with energy E𝐹 𝑒 of 15–20 PeV [5]. 5
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[18] B. Bartoli, et al., Identification of the TeV gamma-ray source ARGO J2013+4157 with the Cygnus Cocoon, Agron. J. 790 (2014) 152, http://dx.doi.org/10.1088/ 0004-637X/790/2/152. [19] B. Bartoli, et al., Study of the diffuse gamma-ray emission from the galactic plane with ARGO-YBJ, Agron. J. 806 (2015) 20, http://dx.doi.org/10.1088/0004637X/806/1/20. [20] B. Bartoli, et al., 4.5 years of multi-wavelenght observations of mrk 421 during the ARGO-YBJ and fermi common operation time, Astrophys. J. Suppl. 222 (2016) http://dx.doi.org/10.3847/0067-0049/222/1/6. [21] B. Bartoli, et al., Search for gamma-ray bursts with the ARGO-YBJ detector in shower mode, Astrophys. J. 842 (2017) 31, http://dx.doi.org/10.3847/15384357/aa74bc. [22] B. Bartoli, et al., Cosmic ray proton plus helium energy spectrum measured by the ARGO-YBJ experiment in the energy range 3-300 TeV, Phys. Rev. D 91 (2015) 112017, http://dx.doi.org/10.1103/PhysRevD.91.112017. [23] B. Bartoli, et al., Knee of the cosmic hydrogen and helium spectrum below 1 PeV measured by ARGO-YBJ and a Cherenkov telescope of LHAASO, Phys. Rev. D 92 (2015) 092005, http://dx.doi.org/10.1103/PhysRevD.92.092005. [24] B. Bartoli, et al., Galactic cosmic ray anisotropy in the northern hemisphere from the ARGO-YBJ experiment during 2008-2012, Agron. J. 861 (2018) (2018) 93, http://dx.doi.org/10.3847/1538-4357/aac6. [25] L. Saggese, et al., Feasibility of measurements of cosmic ray composition by means of RPC digital read out in ARGO-YBJ, Nucl. Instrum. Methods A 533 (2004) 55–59, http://dx.doi.org/10.1016/j.nima.2004.06.129. [26] B. Bartoli, et al., EAS Age determination from the study of the lateral distribution of charged particles near the shower axis with the ARGO-YBJ experiment, Astrop. Phys. 93 (2017) 46, http://dx.doi.org/10.1016/j.astropartphys.2017.06.003. [27] M. Mostafa for the HAWC Collaboration, On the scientific motivation for a wide field-of-view TeV gamma ray observatory in the Southern Hemisphere, ICRC 2017 PoS 851.
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Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Intrinsic linearity of bakelite Resistive Plate Chambers operated in streamer mode B. Bartoli a,b , P. Bernardini c,d , X.J. Bi e , P. Branchini h , A. Budano h , P. Camarri j,k , Z. Cao e , R. Cardarelli k , S. Catalanotti a,b , S.Z. Chen e , T.L. Chen l , P. Creti d , S.W. Cui m , B.Z. Dai n , A. D’Amone c,d , Danzengluobu l , I. De Mitri u,v , B. D’Ettorre Piazzoli a , T. Di Girolamo a,b , G. Di Sciascio k , C.F. Feng o , Zhaoyang Feng e , Zhenyong Feng p , Q.B. Gou e , Y.Q. Guo e , H.H. He e , Haibing Hu l , Hongbo Hu e , M. Iacovacci a,b ,∗, R. Iuppa j,k , H.Y. Jia p , Labaciren l , H.J. Li l , G. Liguori f,g , C. Liu e , J. Liu n , M.Y. Liu l , H. Lu e , L.L. Ma e , X.H. Ma e , G. Mancarella c,d , S.M. Mari h,q , G. Marsella c,d , D. Martello c,d , S. Mastroianni b ,∗∗, P. Montini h,q , C.C. Ning l , M. Panareo c,d , L. Perrone c,d , P. Pistilli h,q , F. Ruggieri h , P. Salvini g , R. Santonico j,k , P.R. Shen e , X.D. Sheng e , F. Shi e , A. Surdo d , Y.H. Tan e , P. Vallania r,s , S. Vernetto r,s , C. Vigorito s,t , H. Wang e , C.Y. Wu e , H.R. Wu e , L. Xue o , Q.Y. Yang n , X.C. Yang n , Z.G. Yao e , A.F. Yuan l , M. Zha e , H.M. Zhang e , L. Zhang n , X.Y. Zhang o , Y. Zhang e , J. Zhao e , Zhaxiciren l , Zhaxisangzhu l , X.X. Zhou p , F.R. Zhu p , Q.Q. Zhu e , G. Zizzi i , The ARGO-YBJ Collaboration a
Dipartimento di Fisica dell’Università di Napoli ‘‘Federico II’’, Complesso Universitario di Monte Sant’Angelo, via Cinthia, 80126 Napoli, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Complesso Universitario di Monte Sant’Angelo, via Cinthia, 80126 Napoli, Italy c Dipartimento Matematica e Fisica ‘‘Ennio De Giorgi’’, Università del Salento, via per Arnesano, 73100 Lecce, Italy d Istituto Nazionale di Fisica Nucleare, Sezione di Lecce, via per Arnesano, 73100 Lecce, Italy e Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, 100049 Beijing, PR China f Dipartimento di Fisica dell’Università di Pavia, via Bassi 6, 27100 Pavia, Italy g Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, via Bassi 6, 27100 Pavia, Italy h Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy i Istituto Nazionale di Fisica Nucleare - CNAF, Viale Berti-Pichat 6/2, 40127 Bologna, Italy j Dipartimento di Fisica dell’Università di Roma ‘‘Tor Vergata’’, via della Ricerca Scientifica 1, 00133 Roma, Italy k Istituto Nazionale di Fisica Nucleare, Sezione di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy l Tibet University, 850000 Lhasa, Xizang, PR China m Hebei Normal University, Shijiazhuang 050016, Hebei, PR China n Yunnan University, 2 North Cuihu Rd., 650091 Kunming, Yunnan, PR China o Shandong University, 250100 Jinan, Shandong, PR China p Southwest Jiaotong University, 610031 Chengdu, Sichuan, PR China q Dipartimento di Fisica dell’Università ‘‘Roma Tre’’, via della Vasca Navale 84, 00146 Roma, Italy r Osservatorio Astrofisico di Torino dell’Istituto Nazionale di Astrofisica, via P. Giuria 1, 10125 Torino, Italy s Istituto Nazionale di Fisica Nucleare, Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy t Dipartimento di Fisica dell’Università di Torino, via P. Giuria 1, 10125 Torino, Italy u GSSI, Gran Sasso Science Institute, Via Iacobucci 2, L’Aquila, Italy v INFN, Laboratori Nazionali del Gran Sasso, Assergi, L’Aquila, Italy b
ARTICLE Keywords: RPC detector Streamer mode Intrinsic linearity Calorimetry
INFO
ABSTRACT Resistive Plate Chambers have largely been used in High Energy Physics and Cosmic Ray research. In view of using this detector for calorimetry applications it is important to know the maximum measurable particle density, or its intrinsic linearity limit, which is tightly related to the dimension of the discharge region. In this paper we report the results of measurements performed at the Beam Test Facility (INFN National Laboratory of Frascati, Italy) where the intrinsic linearity of bakelite RPCs operated in streamer mode has been tested at different impinging particle densities.
∗ Corresponding author at: Dipartimento di Fisica dell’Università di Napoli ‘‘Federico II’’, Complesso Universitario di Monte Sant’Angelo, via Cinthia, 80126 Napoli, Italy. ∗∗ Corresponding author. E-mail addresses: [email protected] (M. Iacovacci), [email protected] (S. Mastroianni).
https://doi.org/10.1016/j.nima.2019.162746 Received 25 August 2018; Received in revised form 4 September 2019; Accepted 9 September 2019 Available online 13 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
B. Bartoli, P. Bernardini, X.J. Bi et al.
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
1. Introduction
current electron–positron linear accelerator. The Beam Transfer Line, that is the BTF, is capable of producing electron or positron bunches within an energy range of 50 to 800 MeV. The emitted particle bunches have a Poisson distribution with a mean value dependent on the slit setting. The typical pulse duration, 10 ns, is comparable to the time tickness of the EAS front at its center, while the maximum repetition rate is limited to 50 Hz. The experimental setup used for the tests described in this paper [5] consisted of four small RPCs of dimensions 63 × 57 cm2 , closely stacked and placed near the exit of the beam line, and of a lead glass block, length 37 cm and base 11 × 11 cm2 , positioned just after the RPCs , with its major axis aligned along the beam direction. These RPCs have the same layout, see Fig. 1, of the large RPCs used in the ARGO-YBJ experiment [6], with eight pickup strips (63 × 6.8 cm2 ) for digital readout facing one side of the gas volume, and a single pick-up electrode (pad, 63 × 57 cm2 ) facing the opposite side of the same gas volume. A 3 mm foam layer was inserted between the RPC and the pad to reduce cross-talk effects (more details in [6]). The beam spot had the same dimensions as the vacuum pipe (5 × 3 cm2 ) and was entirely unfocused as a result of all transfer line quadrupoles being shut off. The slit apertures were operated in a manner so that the bunches, emitted at a rate of 1 Hz, could contain from single to hundreds of particles in the energy range of 450–500 MeV. The glass block itself (24 radiation lengths) was taken from the OPAL experiment [31] and consists of a Schott SF57 glass with the shape of truncated prism. A Hamamatsu R2238 photomultiplier tube was used to read the light signals produced in the glass by the impinging particles. The lead glass block was used as a calorimeter, whose signals allowed both the measurement of the beam intensity and a comparison with the RPC signals, in particular with the first RPC along the beam line. The analog signals coming from the RPCs were processed by a custom board [32,33], based on a Peak and Hold circuit, which continually samples the amplitude (voltage on 50 𝛺), maintains the highest reached value for 2 𝜇𝑠 and digitizes it. After being adapted, the calorimeter signals were collected by the same electronics used to handle the RPC analog signals. Instead, the standard electronics of ARGO-YBJ was used to readout the RPC digital signals. The data were recorded at each trigger occurrence by a custom system utilizing a VME crate in a way simplified with respect to that described in [34]. The trigger signal was provided by the accelerator complex at each electron–positron bunch delivery.
Resistive Plate Chambers (RPCs) were initially conceived as particle counters able to provide a good time resolution [1,2]. Thanks to their robustness, simplicity and low cost they have been used in experiments with large instrumented surfaces, typically of the order of (5–10)×103 m2 , both in the context of accelerator [3,4] and cosmic ray physics [5– 7]. Being used in environments with a low density of particles, mainly for triggering, tracking or vetoing, the spatial resolution requested to them has been mostly systematically delegated to the size of the pickup electrodes, typically strips with dimensions of a few centimeters (width) by some decimeters (length). Motivated by the need to use RPCs for the construction of apparatuses or devices with high spatial resolution, specific studies were initiated some years ago [8–15] and the dimensions of the RPC discharge were measured in different ways operating the detector either in streamer or avalanche mode. The measurements agree in reporting the transversal size of the discharge within a couple of millimeters (1–2 mm) in case of operation in avalanche mode and many millimeters (4–5 mm) in case of streamer mode. In the framework of the astroparticle applications of RPCs, the ARGO-YBJ experiment [5,6] is certainly the most important in terms of dimensions and duration. The ARGO-YBJ detector, made of one layer (6700 m2 ) of bakelite RPCs, was operated in streamer mode at the Yangbajing Cosmic Ray Laboratory (Tibet, P.R. China, 4300 m.a.s.l. , 606 g/cm2 ), taking data with very high stability from November 2007 to February 2013. The choice of the streamer operation [16] was dictated by the needs of stability and ease of operation. The dense sampling area and the high altitude allowed for a study of cosmic radiation from a few hundred GeV to the PeV range [17–24]. The detection of particles of Extensive Air Showers (EAS) was handled by both a digital readout, consisting of a pick-up strip pattern, and an analog readout, made by means of large pick-up electrodes, to measure the total charge induced by the passing particles. The factors limiting the maximum measurable EAS energy are respectively, for the two readout modes, the strip density (23 strips/m2 ) for the former, where the EAS energy is limited to a few hundred TeV [25], and the maximum number of streamers per unit surface (streamer/m2 ) for the latter [5]. In the analog readout mode the upper measurable energy depends on the intrinsic linearity of the RPC response, which is also related to the size of the streamer. Indirect measurements of the streamer size in bakelite RPCs, as the ones used in ARGO-YBJ, have been obtained by means of a test carried out at the Beam Test Facility (BTF) of the Frascati INFN Laboratory. The primary goal of the test was to determine the saturation effects of the RPC response when operated in streamer mode. Results concerning the linearity of RPCs up to particle densities less than 2 × 104 /m2 have been already reported [5]. These results have been extensively exploited to measure with great detail the lateral distribution of charged particles around the core of high energy EAS [26] and to measure the energy spectrum of the cosmic ray light component (p + He) up to a few PeV [23]. The prospect of new initiatives [27–29] aimed to define EAS experiments in the Southern Hemisphere at high altitudes, as in the Andes, 5000 m.a.s.l. or more, raises the question of the maximum particle density that can be measured by bakelite RPCs operated in streamer mode before attaining saturation effects. Indeed, large continuous detectors, primarily planned for gamma astronomy, could sample very dense showers induced by primary cosmic rays with energies of 10 PeV or more. After analyzing data from other tests carried out at the BTF, we can here report on the results concerning RPCs exposed to beams with particle density up to 8 × 104 /m2 .
3. RPC calibration Prior to the beam exposure, the RPCs were calibrated with m.i.p., i.e. the passing through cosmic muons. This was done in the same experimental hall where the detectors were subsequently exposed to the beam. Operating conditions were carefully kept constant during both the calibration phase and the subsequent exposure to the particle beam, that means we paid attention to keep the ratio Temperature/Pressure (T/P) stable within 0.5%, which translates in 𝛥P < 5 mbar and 𝛥T < 1.5 C◦ [35,36]. The four RPCs were aligned one on top of another, so to make a small vertical muon telescope. The top RPC and the bottom one were respectively the first and the last (along the beam line) in the subsequent exposure to the particle beam. A 3 cm thick lead slab was put above the lowest RPC in order to filter low energy particles. Finally, four additional small RPCs, surrounding the telescope, were used to anti-coincide EAS events as shown in Fig. 2. The muon trigger was set as the 3-fold coincidence of the four stacked RPCs in anticoincidence with any of the four additional small chambers surrounding the telescope. This condition limits the surviving EAS contamination. To improve the selection of good events and provide adequate sample purity, the lowest RPC is always required to be in the trigger coincidence and no more than two contiguous fired strips are allowed in each RPC. The telescope geometry and the trigger conditions guarantee the selection of muons with zenith angle not greater than 60◦ . The RPCs were filled and fluxed with a gas mixture 75% Tetraflouroethane, 15% Argon and
2. Experimental setup The test-beam described here was carried out at the Beam Test Facility (BTF) [30], located at the DAFNE-factory complex within the INFN National Laboratory at Frascati. The DAFNE facility consists of two storage rings, a 510 MeV electron–positron accumulator and a high 2
B. Bartoli, P. Bernardini, X.J. Bi et al.
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
Fig. 1. Layout of the bakelite RPCs under test, with High Voltage, Low Voltage and grounding connections (see [6] for details).
Fig. 2. Sketch of the RPC vertical telescope along with anticoincidence chambers and absorber.
10% Isobutane, as in the ARGO-YBJ experiment. First, some efficiency checks were performed on the RPCs under test. The efficiencies of the three uppermost RPCs were measured by excluding in turn one of them from the trigger and the values were found compatible among each other after correction for acceptance. As an example, the efficiency of the top RPC is shown in Fig. 3. Starting from 9.2 kV the efficiency is larger than 95% and keeps stable within 5%; the voltage starting from which the efficiency keeps stable within 5% was assumed as our definition of operating voltage, therefore we will refer to 9.2 kV as the operating voltage from here on. The distribution of the pad signal amplitude of the top RPC, at 9.2 kV, is shown in Fig. 4. By applying a gaussian fit to the first peak, an average amplitude of 3.7 ± 0.2 mV has been obtained while the fit to the second peak results in a mean value of 7.2 ± 0.5 mV, the two peaks corresponding respectively to single and double streamer events. Then, the dependence of the pad signals on the applied voltage has been studied. The results are given in Fig. 5, where the signal amplitudes (voltage on 50 𝛺) of the first and second peak are shown, and in Fig. 6, where the percentage occurrences of the first and second peak are reported. For an applied voltage larger than 9.0 kV double streamers occur in about 15% of the events and this percentage is approximately stable. If we consider a 9.2 kV operating voltage (efficiency 96%) we find that about 72% of events are with 1 streamer, 16% of events with 2 streamers and, even if not highlighted by any fitting line, about 4% are 3 streamer events, namely the little blob between 8 and 11 mV. The remaining 3%–4% are residual shower events and appear in the distribution at amplitudes greater than 11 mV, very widely spread . They all sum up to 96%, a remaining 2% is due to the geometry (spacers)
Fig. 3. Top (first along beam line, see text for details) RPC efficiency versus applied voltage.
and only what remains (about 2%) is detector intrinsic inefficiency. We define a calibration coefficient (mV/m.i.p.) as the weighted average of the signal peak amplitude on events with 1, 2 and 3 streamers at 9.2 kV, which results in 4.2 mV/m.i.p. with a statistical uncertainty of 5%. This amplitude, according to the measurements made at the ARGO-YBJ site and reported in [5], does not change with incidence angle, at least up to a zenith angle of 60◦ . Being the applied voltage stable within ±5 V and 3
B. Bartoli, P. Bernardini, X.J. Bi et al.
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
Fig. 4. Pad signal amplitude for minimum ionizing muons at 9.2 kV. The bin size is ∼0.2 mV. Two peaks are clearly visible, their position identified by a gaussian fit (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Fraction of occurrences of events in the first and second peak versus HV.
to the particle beam of the BTF and a data taking, driven by the bunch-crossing signal of the BTF complex, was started, operating the RPCs at 9.2 kV with the mentioned gas mixture. In spite of the beam being totally unfocused, the beam geometry [30] allowed for a nearly complete containment by the lead glass calorimeter (Schott SF57 glass has a Moliere radius of 2.61 cm) with only marginal loss. The PMT of the calorimeter was operated at 1.2 kV, as suggested by the BTF experts, therefore providing a single particle signal of about 70 mV at the mean energy of the beam, namely 480 MeV. The calorimeter signal was attenuated by roughly a factor 30 and adapted to fulfill the requirements of the readout electronics. Data were taken with different settings of the slits which defined the number of delivered particles at the exit windows. Only electron runs have been considered for the following analysis. The signal amplitude of the foremost RPC versus the calorimeter signal is shown in the scatter plot of Fig. 7. Data refer to four runs (represented by different colors) where, according to the estimation done by the beam monitoring system of the BTF, the number of electrons per bunch was changing from 7 to 20 (gray points), from 15 to 40 (blue points), from 45 to 70 (red points) and from 60 to 120 (green points). Also, to better highlight the relation between the two signals, the profile points (the mean value of Y for each bin in X [37]) are reported by the full black triangles; in the specific case the RPC mean value is computed in intervals of 0.4/0.6/0.8/2.1 mV/bin of the calorimeter signal amplitude for the gray/blue/red/green points. On the right scale of Fig. 7 the corresponding particle density, estimated according to a mean amplitude of 4.2 mV/particle for the RPC signal, is reported. Although we measured a maximum density of 120 particles on 15 cm2 , we assume the scalability of the results to a wider surface, that is we consider still valid a single cell model for the RPC response [38] as data show no deviation from linearity. In order to check the response linearity, a linear fit (red line in Fig. 7) to the RPC mean amplitudes (full black triangles) has been performed. The distribution of the normalized residuals, that is the difference between the fit values and the measured values normalized to the fit values, is shown in Fig. 8. It follows fairly well a gaussian distribution for normalized residual values smaller than 0.05 (about 2.2 𝜎) from the center, which corresponds to a statistics of 330 points over 370. About 40 points are left out, mainly on the left tail of the distribution, which contains the points where the RPC response is lower than the one expected according to calorimeter signal. This could be an indication of non linearity, but we have verified that this happens for points in the central part of the scatter plot in Fig. 7 and therefore it is more likely to be attributed to the effect of the beam structure or of its purity.
Fig. 5. First and second peak of the pad signal amplitude versus applied voltage. The peak values have been identified by gaussian fits as shown in Fig. 4. Data refers to the top RPC. Error bars are the standard deviations coming from the gaussian fit.
the percentage variation of the mean peak amplitude (for both 1 and 2 streamer events) contained within ±7% for a ±100 V variation of the operating voltage, and given the stability of the percentages of 1 and 2 streamer events in 9.2 ± 0.1 kV, we estimate the systematic uncertainty on the calibration coefficient to be less than 1%. Also, considering that our measurements were done in stable conditions of temperature and pressure, and that variations of these parameters translate into an equivalent variation of the applied voltage, we can assume that a maximum of 1% systematics can be due to environmental parameter variations. Therefore the overall systematic error can be quoted to be less than 2%. 4. Exposure to the beam and data analysis After calibration, the four RPCs were rotated and put vertically with their surface orthogonal to the beam line. The lead glass calorimeter described before was mounted behind the fourth RPC and connected to the DAQ system. Then the whole experimental setup was exposed 4
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Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162746
Fig. 7. RPC signal versus the calorimeter signal. Data refer to four runs, represented by the different colors, with number of electrons ranging in different intervals. A straight line fit (in red) was performed to the profile points (full black triangles). On the right scale is reported the particle density estimated using the RPC signal amplitude and the calibration constant of 4.2 mV/particle. See text for more details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Distribution of the normalized residuals after a straight line fit has been performed to the profile points in Fig. 7 for calorimeter signals > 170 mV.
Acknowledgments We thank the DAFNE-BTF group, especially G. Mazzitelli and P. Valente for their invaluable aid during and after the test beam. References [1] R. Santonico, R. Cardarelli, Development of resistive plate counters, Nucl. Instrum. Methods Phys. Res. A 187 (1981) 377, http://dx.doi.org/10.1016/0029554X(81)90363-3. [2] R. Cardarelli, R. Santonico, Progress in resistive plate counters, Nucl. Instrum. Methods Phys. Res. A 263 (1988) 20, http://dx.doi.org/10.1016/0168-9002(88) 91011-X. [3] ATLAS Collaboration, Technical Design Report, CERN/LHCC/(1998) 1998-14. [4] CMS Collaboration, Technical Design Report, CERN/LHCC/(2000) 2000-38. [5] B. Bartoli, et al., The analog Resistive Plate Chamber detector of the ARGOYBJ experiment, Astropart. Phys. 67 (2015) 47, http://dx.doi.org/10.1016/j. astropartphys.2015.01.007. [6] G. Aielli, et al., Layout and performance of the RPCs used in the ARGOYBJ experiment, Nucl. Instrum. Methods Phys. Res. A 562 (2006) 92, http: //dx.doi.org/10.1016/j.nima.2006.02.136. [7] A. Bertolin, et al., The RPC systems of the OPERA experiment, Nucl. Instrum. Methods Phys. Res. A 602 (2009) 631, http://dx.doi.org/10.1016/j.nima.2008. 12.071. [8] R. Arnaldi, A. Baldit, V. Barret, N. Bastid, G. Blanchard, E. Chiavassa, Spatial resolution of RPC in streamer mode, Nucl. Instrum. Methods Phys. Res. A 490 (2002) 51, http://dx.doi.org/10.1016/S0168-9002(02)00917-8. [9] A. Blanco, N. Carolino, C.M.B.A. Correia, R. Ferreira Marques, P. Fonte, D. Gonzalez-Diaz, et al., RPC-PET Prototype with high spatial resolution, Nucl. Instrum. Methods Phys. Res. A 533 (2004) 139, http://dx.doi.org/10.1016/j. nima.2004.07.016. [10] R. Cardarelli, G. Aielli, P. Camarri, A. Di Ciaccio, B. Liberti, R. Santonico, Improving track resolution in the RPC chamber, Nuclear Phys. B 158 (2006) 25, http://dx.doi.org/10.1016/j.nuclphysbps.2006.07.023. [11] Q. Li, Y. Ye, C. Wen, Study of spatial resolution properties of a glass RPC, Nucl. Instrum. Methods Phys. Res. A 663 (2012) 22, http://dx.doi.org/10.1016/j.nima. 2011.10.006. [12] Y. Jin, C. Jianping, Y. Qian, L. Yuanjin, Jin, W. Vi, Study on the position resolution of resistive plate chamber, Nucl. Instrum. Methods Phys. Res. A 591 (2008) 411, http://dx.doi.org/10.1016/j.nima.2008.02.102. [13] I. Kitayama, H. Sakai, Y. Teramoto, et al., Optical observation of discharge in resistive plate chamber, Nucl. Instrum. Methods Phys. Res. A 424 (1999) 474, http://dx.doi.org/10.1016/S0168-9002(98)01379-5. [14] S. Narita, M. Shoji, Y. Hoshi, D. Miura, Y. Kikuchi, K. Neichi, A. Yamaguchi, Measurements of induce charge profile in RPC with submilli-strips, IEEE Trans. Nucl. Sci. 57 (2010) 2210–2214, http://dx.doi.org/10.1109/TNS.2010.2052111. [15] S. Narita, Y. Hoshi, K. Neichi, A. Yamaguchi, Induced Charge Profile in Glass RPC Operated in Avalanche Mode, 2012 IEEE (NSS/MIC) N(1146) 14-128. doi:10.1109/TNS.2013.2282638. [16] C. Bacci, et al., High altitude test of RPCs for the Argo YBJ experiment, Nucl. Instrum. Methods Phys. Res. A 443 (2000) 342, http://dx.doi.org/10.1016/ S0168-9002(99)01079-7. [17] B. Bartoli, et al., TeV Gamma-ray survey of the northern sky using the ARGO-YBJ detector, Agron. J. 779 (2013) 27, http://dx.doi.org/10.1088/0004-637X/779/ 1/27.
Fig. 8. Distribution of residuals with respect to a line fit performed to the profile points in Fig. 7. The residuals are normalized to the fit value.
On the other side, for positive values, we have a good agreement and the tail is roughly gaussian (as expected, about 3% of the values are beyond the 2.2 𝜎 where the fit region ends ). To definitely confirm no deviation from linearity we performed a linear fit only to the right part of the plot, namely for calorimeter signals greater than 170 mV; the distribution of normalized residuals (reported in Fig. 9) is well fitted by a gaussian (black line) and the 𝜒 2 of the linear fit results to be 74.6, with 70 d.o.f., which also is a good value as compared to 90.53, the 2 for 95% c.l.. The parameter values of the linear fit are consistent, 𝜒𝑙𝑖𝑚 within the experimental errors, with those obtained by fitting all data over the entire range. 5. Conclusions A test of the intrinsic linearity of the bakelite Resistive Plate Chamber response has been conducted at the Beam Test Facility of the INFN National Laboratory in Frascati, Italy. The detector was operated in streamer mode. No evidence for deviation from a linear behavior was observed up to 120 particles/15 cm2 , implying a linear response up to a particle density of 8×104 ∕m2 . This limit corresponds, at an altitude of 4300 m a.s.l., to the particle density at the core of EASs induced either by protons with energy E𝑝 of 7–10 PeV or by Fe nuclei with energy E𝐹 𝑒 of 15–20 PeV [5]. 5
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