Characterization of the muography background using the Muon Telescope (MuTe)
aa r X i v : . [ h e p - e x ] F e b Characterization of the muography backgroundusing the Muon Telescope (MuTe)
Jesús Peña-Rodríguez * a , Luis A. Núñez a , b , and Hernán Asorey c , d a Escuela de Física, Universidad Industrial de Santander, Bucaramanga-Colombia b Departamento de Física, Universidad de Los Andes, Mérida-Venezuela. c Departamento Física Médica, Centro Atómico Bariloche, Comisión Nacional de EnergíaAtómica, Bariloche-Argentina; d Instituto de Tecnologías en Detección y Astropartículas (ITEDA), Buenos Aires-Argentina.E-mail: [email protected] , [email protected] , [email protected] In this work, we estimate the background components in muography using the MuTe: a hybridmuon telescope composed of two subdetectors –a scintillator hodoscope and a Water CherenkovDetector (WCD). The hodoscope records the trajectories of particles crossing the telescope, whilethe WCD measures their energy loss. The MuTe hodoscope reconstructs 3841 different directionswith an angular resolution of 32 mrad for an inter-panel distance of 2.5 m. The spatial resolutioncan reach ∼ < * Speaker. © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ haracterization of the muography background using MuTe
Jesús Peña-Rodríguez
1. Introduction
Muography is a non-invasive technique for scanning anthropic and geologic structures. Itsapplications cover several fields: container inspection [1], archaeological building scanning [2, 3],nuclear plant examination [4], nuclear waste monitoring, underground cavities [5], overburden ofrailway tunnels [6], and volcanology [7].The measured flux gives information about the inner density distribution of the structure, butsuch a flux is affected by a particle background. This noise emerges as a result of multiple phe-nomena: upward-coming muons [8, 9], soft muons ( < e − , e + , γ ) of EAS [12] and, multiple particle events.Several methods have been implemented to reject the background in muography. Time-of-Flight systems reject upward-coming muons, absorbent layer installation stops low energy parti-cles, and extra sensitive layers decrease the probability of multiple particle events [12].In this paper, we estimate the contribution of the background components in muography usingMuTe. MuTe classifies the background by means of deposited energy and ToF measurements. TheWCD discriminates the electromagnetic and multiple particle events while the ToF differentiatesthe frontal and rear particle flux. The soft muons are rejected by a momentum threshold establishedusing the particle identity and velocity.
2. The Muon Telescope
Scintillator panelsFrontend electronics box PMT hausing Water CherenkovdetectorElevation angle supportTelescope base 2.5 m . m Figure 1: Mute mechanical structure. The hodoscope panels are mounted on a sliding rail allowinga separation distance configuration between 0.5 m to 2.5 m. The WCD stands near the center ofmass of the telescope to facility the elevation procedure. Eight supports affix the MuTe to theground.MuTe is composed of a hodoscope and a WCD as shown in Figure 1. The hodoscope is madeof two scintillator matrices of 30 ×
30 strips (120 cm × × . The dopants (1% PPO, 0 .
03% POPOP)set an absorption cut off ∼
40 nm and an emission maximum at 420 nm.1 haracterization of the muography background using MuTe
Jesús Peña-Rodríguez
Inside each scintillator strip a wavelength shifting (WLS) multi-cladding fiber (Saint-GobainBCF-92) transmits the light photons towards a silicon photomultiplier (SiPM, Hamamatsu S13360-1350CS). The SiPM has a photosensitive area of 1 . × . , 667 pixels, a fill factor of 74%, again from 10 to 10 and a photon-detection efficiency of 40% at 450nm.An ASIC MAROC3A amplifies and jointly discriminates the 60 signals from each panel. AnFPGA Cyclone 3 sets the ASIC slow control parameters. The data recording is managed by aRaspberry Pi 2 and stored in a central hard disk.The MuTe-WCD (120 cm side) has a Tyvek internal coating and an eight-inch photomultipliertube (PMT Hamamatsu R5912) as the sensitive element. The anode and last dynode PMT signalsare digitized by a 10-bit fast Analog-to-Digital converter with a sampling frequency of 40 MHz[13]. The ToF system ( ∼
97 ps resolution) is based on a Time-to-Digital converter implemented ona Xilinx FPGA Spartan 6 [14]. The MuTe data acquisition system is temporally synchronized byGPS. This architecture allows us to correlated the data recorded by the hodoscope and the WCDwhich operate individually. A GPRS/GSM ITEAD SIM900 module reports daily the telescopestate towards a remote server.
3. Background in muography
Particles impinging the muon telescope not only come from inside the scanning target. Severalunderlying phenomena cause an overwhelming particle background that affects the detector signal-to-noise ratio. Muography background is made of: low momentum muons scattered by the targetsurface, muons entering from the rear side of the detector, charged particles from EAS, and particlesarriving simultaneously as shown in Figure 2. m u o n s c a tt e r e d m u o n Detector Detector γp γ π + π - v(cid:3) e (cid:0) e (cid:1) e (cid:2) e (cid:4) π (cid:5)(cid:6)(cid:7)(cid:8) n e(cid:9)(cid:10)(cid:11)(cid:12)(cid:13) o n Detector m i n i s h o w e r γ p γ π + π - v (cid:14) e + e + e - e -0 π (cid:15) + (cid:16) - n m(cid:17)(cid:18)(cid:19) Detector
Figure 2: Background sources in muography: scattered muons, the electromagnetic component ofEAS, mini showers, and upward-coming particles.2 haracterization of the muography background using MuTe
Jesús Peña-Rodríguez
We performed a particle background characterization by means of Monte Carlo simulationsby using the CORSIKA code. The results showed that particles reaching the MuTe are mainlypositrons/electrons ( ∼
20 MeV average energy), and muons ( ∼ )deviate ∼
35 mrad [16].Scattered and quasi-horizontal ( θ ≥ ◦ ) muons also arrive from the rear side of the telescope.They have a mean momentum ∼
10 GeV/c greater than vertical muons ( ∼ < ◦ the inverse muon flux can exceed 50% [17].Multiple particles simultaneously crossing the telescope can create false-positive events. Thiscombinatorial background is classified as uncorrelated and correlated depending on the origin.Particles coming from independent sources (e.g., different EAS or soil radioactivity) generate theuncorrelated one. The relative arrival time between these particles is in the order of hundreds ofmicroseconds, allowing that ns-resolution detectors can reject them.Most of the correlated background is composed of muons that originated in the same EAS.For a < <
100 ns [18]. Elec-tron/positrons, generated few radiation lengths ( <
10 m) close to the telescope, arrive simultane-ously within a low relative angle, contributing in this way to the correlated background [19].
4. Background characterization
200 400 600 800 1000 E d [MeV] -1 c o un t s WCD dataγ, e ± ToF [ns] c oun t s CorrelatedUncorrelated
Figure 3: (Left) Deposited energy histogram for the events crossing the MuTe during one hour.The black-line hump ( <
180 MeV) corresponds to the electromagnetic component ( e ± , γ ). Thegreen-line hump (180 MeV < E d <
400 MeV) is the deposited energy of muons. Multiple particleevents deposit energy above 400 MeV. The blue-line hump represents two-muon events. (Right)Time-of-Flight of particles traversing the MuTe hodoscope. Single-particle events and correlatedbackground (blue) have a ToF <
30 ns, while the uncorrelated background (red) has a ToF >
300 ns.3 haracterization of the muography background using MuTe
Jesús Peña-Rodríguez
The MuTe recorded data to characterize the particle background at an elevation angle of 15 ◦ and an inter-panel distance of 2.5 m during two months. Figure 3-left displays the energy depositedin the WCD for the particles crossing the hodoscope during one hour. The first hump ( <
180 MeV)belongs to electromagnetic particles ( e ± , γ ) and represents the 36% of the events.The muonic component spans from 180 MeV to 400 MeV and contains 33% of the events.The mean of the muonic hump corresponds to the energy released by a vertical muon (VEM)traversing 120 cm of water – ∼
240 MeV. Multiple particle events lose energy above 400 MeV witha significant contribution at 480 MeV (2 VEM). These add up the 30.4% of the events.The ToF distribution of single-particle events and the correlated background has an uppercutoff at ∼
30 ns, and an average value of ∼
10 ns as shown Figure 3-right. The expected ToF ofsingle-particle events spans 2.53 ns to 20.9 ns. This estimation was carried out taking into accountthe scintillator transmission delay ( ∼ × ∼
50 ps). The uncorre-lated background starts at ∼
300 ns increasing its occurrence while the time difference grows. Theprobability that an uncorrelated event occurs below 200 ns is roughly 0.05%. (cid:20)(cid:21)(cid:22)(cid:23)(cid:24) (cid:25)(cid:26)(cid:27)(cid:28)(cid:29) (cid:30)(cid:31) !" -5.0 0.0 5.0 -5.00.05.0
NOPQRSTUVWXYZ[\]
Figure 4: Angular distribution of the frontal (left) and inverse (right) particle flux traversing theMuTe hodoscope for an elevation angle of 15 ◦ . The hodoscope axis is located at ( Θ x = Θ y = <
240 MeV.Figure 4 compares the frontal (left) and the inverse (right) flux traversing MuTe. The inverseflux represents the 22% of the particle events impinging the detector. Most of the particles withenergy <
240 MeV are absorbed by the WCD water volume. The inverse flux is higher for inclinedtracks than for quasi-perpendicular tracks because of the shadowing effect of the WCD.The incident particle momentum was estimated using the ToF measurements, the trajectoryreconstruction, and the particle mass [14]. We set a threshold to reject particles with momentumbelow 1 GeV/c as shown in Figure 5-left. The 54% of the recorded events have a momentum > <
17 mrad. We summarise the results of the muographybackground characterization in Figure 5-right. 4 haracterization of the muography background using MuTe
Jesús Peña-Rodríguez p [GeV/c] T o F [ n s ]
6% 33%30%46% 54%22% 78% data ^_‘abcdfg -dE/dx Momentum
BackwardForwardElectromagnetic < 180 MeVMuons 180 MeV
17 mrad. (Right) Muography background classi-fication. The frontal flux (78%) is composed of electromagnetic particles (36%), single-muons(33%), and multiple particle events (30%).
5. Conclusions
We designed and built a muon telescope capable to identify and reject the muography back-ground. The identification methodology of MuTe is based on particle identification techniques– Time-of-Flight and deposited energy measurements. We found the background caused by theelectromagnetic component of EAS represents ∼
36% of the recorded data, while the correlatedmultiple particle events were ∼ ◦ it was ∼
22% taking into account the WCD water volumeabsorbs particles with energy <
240 MeV. The multiple particle background was caused by twosources: correlated particles with a relative arrival time <
100 ns, and uncorrelated particles gen-erated by the cosmic ray background or soil radioactivity. As future work in muography, we aredeveloping machine learning techniques to separate automatically the signal from the background.
Acknowledgments
The authors acknowledge the financial support of Departamento Administrativo de Ciencia,Tecnología e Innovación of Colombia (ColCiencias) under contract FP44842-082-2015 and to thePrograma de Cooperación Nivel II (PCB-II) MINCYT-CONICET-COLCIENCIAS 2015, underproject CO/15/02. We are particularly thankful to the Latin American Giant Observatory Collabo-ration and to Pierre Auger Observatory for their permanent support and inspirations.
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