Nuclear Inst.

and Methods in Physics Research, A 1048 (2023) 167995

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Nuclear Inst. and Methods in Physics Research, A

journal homepage: www.elsevier.com/locate/nima

A new cylindrical detector for borehole muon radiography

G. Saracino a,b ,∗, F. Ambrosino a,b , A. Anastasio b , L. Cimmino a,b , M. D’ Errico b , V. Masone b ,
M. Mirra b , L. Roscilli b
a Dipartimento di Fisica, Università degli Studi di Napoli ‘‘Federico II’, Italy
b Istituto Nazionale di Fisica Nucleare, sezione di Napoli, Italy

ARTICLE INFO ABSTRACT

Keywords: Muons of cosmic origin have a great capability to penetrate through matter. This property is exploited in muon
Muon radiography radiography, also known as muography, a technique which allows to highlight the presence of discontinuities
Muography of the mass density in the subsoil such as cavities, tunnels or rock masses. A detector of cylindrical geometry,
Borehole detector
optimized for borehole studies and with a diameter of 24 cm, was developed and tested . The scintillation
Geological prospecting
light is read out by 384 Silicon Photomultipliers, directly coupled to the bars. The front-end and acquisition
Geophysics
electronics, entirely housed inside the detector, are based on the EASIROC chip and are characterized by limited
energy consumption (about 30 W for the entire detector). The detector has been designed in such a way as
to simplify its construction as much as possible for its eventual mass production. In this article some details
concerning the construction and preliminary results of measurements conducted in the Mt Echia (Naples, Italy)
underground are presented.

1. Introduction of geometry simplifies assembly but does not optimize the geometrical
acceptance.
Muon radiography has been successfully applied in numerous fields A detector which, using arc-shaped plastic scintillators, optimizes
such as archaeology, the study of volcanoes, mining prospecting and angular acceptance and the sensitive surface has been realized and
the detection of underground cavities. For recent reviews see [1,2]. The patented. The detector was designed in order to simplify as much as
principle on which absorption muography is based is the measurement possible the assembling, in the optic to reduce cost for mass production:
of the flux of muons after they have crossed the volume to be inspected. photon sensors are directly coupled to the scintillators and a special
Indeed, the intensity of the flux depends on the average mass density container allows easy positioning of the scintillators.
encountered by the muon along its path. By measuring the direction
of the muons, a 2D map of the average density as a function of
2. Detector description
the direction is obtained, which allows to identify anomalies such as
cavities or mineral veins. Using measurements acquired from multiple
observation points it is possible to obtain 3D information [3]. In this section the detector is briefly described, for more details
Some applications require the detector to be placed inside a well. see [7,8]. The height and diameter of the detector directly affect
In these cases the optimization of the detector to the cylindrical shape its performance, as the number of muons detected per unit of time
of the hole is fundamental, since, with the same diameter of the well, is directly proportional to the surface area. On the other hand, the
the greater the sensitive surface and the angular acceptance, the shorter dimensions must be compatible with some practical aspects, such as the
the time required to acquire the necessary data. An interesting borehole diameter of the well, the maneuverability inside it, the transportability
detector technology based on scintillator fibers was proposed in [4,5]. and the cost. The dimensions of the detector, as regards the sensitive
This technology, however, requires a number of photon sensors and part, are approximately 20 cm in diameter and 1 m in length, while the
electronic channels that significantly increases with the size of the overall dimensions are 24 cm in height and 130 cm in height
diameter (in this specific case 384 electronic channels were used for a The detector has been designed for applications in urban areas and
14 cm diameter detector). Furthermore the use of fibers is not simple, for depths of the order of 50 m, with an angular resolution allowing the
requiring attention in their preparation and assembly. identification of cavities of the order of tens of meters in size at that
In [6] a borehole detector with scintillator bars and wavelength depths. Regarding the diameter, it was chosen in order to be inserted
shifting fiber assembled in a planar geometry is proposed. This kind inside a 25 cm well, which is a diameter that can be realized with

∗ Corresponding author at: Dipartimento di Fisica, Università degli Studi di Napoli ‘‘Federico II’, Italy.
E-mail address: giulio.saracino@na.infn.it (G. Saracino).

https://doi.org/10.1016/j.nima.2022.167995
Received 1 July 2022; Received in revised form 16 December 2022; Accepted 18 December 2022
Available online 26 December 2022
0168-9002/© 2023 Elsevier B.V. All rights reserved.
G. Saracino, F. Ambrosino, A. Anastasio et al. Nuclear Inst. and Methods in Physics Research, A 1048 (2023) 167995

Fig. 4. The trigger rate of the detectors as a function of the discriminator threshold,
expressed in arbitrary units of the DAC. The red and blue points correspond to a vertical
Fig. 1. Example of the plastic scintillators used in the detector. A: An arc-shaped and horizontal position of the detector, respectively. See the text for more details.
scintillator. B and C: A scintillator bar with rectangular cross section.

reason is that the detector is protected by a watertight steel cylinder.

The heat exchange with the outside is therefore limited and excessive
energy consumption would lead to unacceptable overheating inside the
detector.
The FEE and DAQ was developed on the experience maturated in
previous muon radiography projects [9,10]. The FEE is based on the
EASIROC ASIC specific for SiPM readout and characterized by low
power consumption and compactness. The detector has, in total, 384
Fig. 2. Example of the determination of the cylindrical coordinates of the impact points Silicon photomultipliers. Each EASIROC can read 32 SiPMs and a total
of a muon across the detector.
of 12 boards are used for the detector. Each board produces a fast
logic signal (OR32) if at least one of the 32 SiPM signals exceeds a
programmable voltage threshold produced by a Digital to Analogical
Converter (DAC). The FEE boards are controlled by a MASTER board
based on a FPGA for the data transfer with the FEE and a RaspberryPi-2
computer for the control of the DAQ and internet communication. The
MASTER receives all the 12 OR32 logical signals from the FEE boards
and produces a trigger logic signal according to a programmable trigger
logic in order to start data acquisition from all FEE boards. The total
power consumption, including the RaspberryPi and the Arduino-UNO
controller for the temperature and humidity measurements inside the
detector, is about 30 W. At start-up, the detector temperature increases,
with respect to the environmental one, due to the power dissipation
of the electronic hosted inside the detector, reaching a stable value
Fig. 3. Left: the detector without the steel cylindrical shell. Right: the detector that depends mainly on the environmental temperature. As an example,
assembled into the shell. for an environmental temperature of 20 ℃. The temperature measured
inside the detector cylinder was around 35 ℃. The SiPM bias voltage
was set in order to maintain the same photo-detection efficiency at 20
standard tools, as suggested by our industrial partner TECNO-IN. With ℃. No degradation in the detector performance was observed.
respect to the length, it was mainly constrained by the efficiency of the
scintillator bars and to the operability of the detector. 3. Measurements results
The detector is formed by arc-shaped plastic scintillators arranged
one above the other horizontally and two lines of plastic scintillator
During the detector commissioning the muon rates in open sky
bars, with a rectangular section and arranged vertically to form two
mode was measured. The detector was positioned at the ground surface
half cylinders (see Figs. 1 and 2). Considering the detector positioned
with no object between the detector and the sky, with the exception
with its axis vertically, the impact point of the muon is fully determined
of the roof of the container where the detector was positioned. The
by the cylindrical coordinates (𝜙, 𝑧) where the 𝜙 angle is measured by
measurements confirmed the good performances of the detector, as
the bars and 𝑧 by arcs. The average angular resolutions of the muon
shown below. The trigger rate of the detector as a function of the
direction are about 0.7◦ and 3◦ in azimuth and zenith respectively.
All the scintillators are inserted in specific containers, produced discriminator thresholds (see Section 2) is shown in Fig. 4. A common
in acrylonitrile butadiene styrene (ABS) by a 3D printer, that define value for the 12 thresholds of the discriminators is changed. The trigger
the correct position of the elements and host the front end electronic logic selects events where a muon crosses at least one of the two
inside The containers with all the electronic boards are protected by a semicylinder. In the plot it is possible to observe a region of low
watertight steel cylinder. The final assembling of the detector is shown threshold values where accidental coincidences, due to SiPMs dark
in Fig. 3. counts, are dominant. For higher threshold values (corresponding at
Silicon photomultipliers (SiPM) (Hamamatsu S13360-3050PE) with about more than 5 photoelectrons), the detector shows a large plateau
a sensitive area of 3 × 3 mm2 are directly coupled to the scintillators. where the rate of detected muons is stable. Figs. 5 and 6 show the
Front End Electronic (FEE) and Data Acquisition (DAQ) require good agreement between the measured and the expected muon rates in
particular characteristics for this type of application. Compactness is open-sky. The expected flux was computed with the Gaisserś modified
a must, in order to house the electronic boards in the small space model [11] and a simplified detector model that takes in account only
inside the cylinder. Low power consumption is also required for at least the geometrical efficiency of the detector, including dead spaces.
two reasons. First of all, the detector can operate in places where the Successively a measurement campaign in the underground of Mt
electricity grid is not available and therefore it is necessary to resort Echia was performed. Mt Echia is an ancient and complex system of
to alternative sources, such as photovoltaic or wind power. The second galleries and cavities, where known cavities were previously measured

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G. Saracino, F. Ambrosino, A. Anastasio et al. Nuclear Inst. and Methods in Physics Research, A 1048 (2023) 167995

Fig. 8. Total energy distribution, expressed in number of photoelectrons, of the

reconstructed track. This is obtained as the sum of all clusters energies involved in
the track. Blu line:open sky sample. Red: underground sample. See the text for more
Fig. 5. Comparison between expected (top figure) and measured (bottom) muon rates details.
as a function of the elevation and azimuth angles.

During the measurements an anomalous trigger rate, with respect

to the expected muon flux was observed (∼ 100 Hz instead of ∼
1 Hz with a trigger logic that requires at least two energy clusters
produced simultaneously). This phenomenon has been attributed to
environmental radioactivity. The hill of Mt Echia is of yellow tuff of
volcanic origin. As reported for example in [13], this rock contains
many radioactive isotopes as 232 Th 226 Ra and 40 K. Since the detector is
protected by a 3 mm thick steel tube, watertight, the observed signals
are not attributed to 𝛼 or 𝛽 emissions but to 𝛾. For instance 232 Th
daughters 212 Bi, 208 Tl and 228 Ac produce 𝛾 of 727, 860 and 911 keV
respectively, and can trigger the detector. Most of these events are not
Fig. 6. Comparison between expected (top figure) and measured (bottom) muon rates able to produce a track (that requires at least four energy clusters)
as a function of the azimuth angles at the zenith angle of about 46.5◦ . and are rejected during the analysis. Anyway a small amount of tracks
presents a total energy deposition that is not compatible with the one
observed with opensky sample (see Fig. 8). These tracks have been
attributed to the radioactivity background and have been discarded
applying an energy cut. The small difference between the distributions
of the energies deposited by muons is probably due to the different
angular distribution of the muons in the underground and in free-sky,
since the cluster energy depends on the angle of impact of the muon
with the scintillators.

Fig. 7. Example of a muon radiography obtained in the Mt Echia underground. The 4. Conclusions
Relative Transmission is plotted as a function of the elevation and azimuthal angles.
Regions labeled with the letters A, B and C correspond to angular regions where known A new type of borehole detector, based on plastic scintillator el-
cavities are present and are characterized by values of R greater than 1.
ements and with a cylindrical geometry, was realized and tested for
muography applications. This specific geometry optimizes the sensitive
area and the angular acceptance of the detector. The detector has
wit a planar detector of 1 m2 of sensitive area [3,12]. In order to iden- an average angular resolutions of about 0.7◦ and 3◦ in azimuth and
tify the presence of cavities, the relative transmission 𝑅(𝛼, 𝜙) is used, zenith respectively and a geometrical acceptance of 360◦ in azimuth and
defined as the ratio, for each direction (𝛼, 𝜙), between the measured between 15◦ and 90◦ in zenith, in vertical position. Tests conducted in
transmission and the transmission obtained from simulations where no open sky have shown a good agreement with the expected muon rates
cavities are included. The measured transmission is obtained by the and stable detector’s performances. Excellent results were obtained
ratio between the muons rates measured in the underground and the during a measurement campaign in the underground of a tuff hill in
Naples (IT), where known cavities have been detected in few days of
one observed in open sky.
data acquisition. A small amount of events due to natural radioactivity
As an example Fig. 7 shows the relative transmission obtained background was observed and rejected in data analysis.
placing the detector in a position where known voids, with a volume
of the order of one hundred of m3 and distances from the detector in Declaration of competing interest
the range between 5 to 30 m, are in the angular acceptance of the
detector. Values of 𝑅(𝛼, 𝜙) ∼ 1 correspond to directions where the The authors declare that they have no known competing finan-
measurement values and model ones are in agreement, i.e. no cavities cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
are crossed by muons. Instead values of 𝑅(𝛼, 𝜙) > 1 correspond to an
excess of muons with respect to the model, i.e. a cavity is present in
Acknowledgments
that direction. All the known cavities are well identified and the results
are in agreement with the results obtained with the planar detector, The realization of the detector prototype was supported by the
confirming the capability of the cylindrical detector to detect voids (for STRESS S.C.a R.L. company, in the frame of the project METROPOLIS
more details see [7,8]). - PON 03PE 00093 4 and TECNO-IN S.P.A. The authors gratefully

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G. Saracino, F. Ambrosino, A. Anastasio et al. Nuclear Inst. and Methods in Physics Research, A 1048 (2023) 167995

acknowledge the Associazione Culturale Borbonica Sotterranea for the [6] A. Bonneville and R et al, J. Kouzes, C. Yamaoka, E. Rowe, J. Guardincerri,
measurements in Mt Echia. C. Durham, D. Morris, K. Poulson, D. Plaud-Ramos, J. Morley, J. Bacon, J.
Bynes, C. Cercillieux, K. Ketter, I. Le, G. Mostafanezhad, J. Varner, A. Flygare,
A. Lintereur, A novel Muon detector for borehole density tomography, Nucl.
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