Muography applications developed by IFIN-HH

Abstract

Cosmic-ray muons have been studied at IFIN-HH for more than 20 years. Starting as fundamental physics research, the muon flux measurements bring new directions of study regarding muography. Two new directions have been recently developed: underground muon scanning of old mining sites in order to detect the possible presence of unknown cavities and underwater scanning of ships in commercial harbours in order to prevent the illegal traffic of radioactive materials. The main goal of the first direction of study is to improve the security of underground civilian and industrial infrastructures, by starting the development of a new, innovative detection system that can be used to identify potentially dangerous conditions using a non-invasive, totally safe method. The method proposed uses information provided by a device placed underground that measures directional cosmic muon flux and identifies anomalies produced by irregularities in the geological layers above. For the second direction of study, the method proposed is based on the detection and analysis of the cosmic muon flux. The high-density materials (uranium, lead—used for radiation shielding, etc.) cause a decrease in the directional muon flux. The detection system will be submerged underneath the ship that will be scanned, being able to locate illegal radioactive materials without exposing any personnel to radiation or contamination. Correlated with simulations based on the known configuration of the ship scanned, the data provided by the detection system will provide the location and dimensions of the undeclared material transported.

This article is part of the Theo Murphy meeting issue ‘Cosmic-ray muography’.

1. Introduction

Cosmic-ray muons are produced when primary cosmic-ray particles interact with air molecules from atmosphere. As components of extensive air showers (EASs), they are cascading through the Earth's magnetic field following the direction of the primary particle. With a small interaction cross section compared to other EAS components, like electrons, gamma or hadrons, the secondary muons are ideal for tomography applications.

The method is based on the energy loss of high-energy cosmic muons in matter. Since the muon flux is steeply decreasing as a function of the energy, material traversed before interacting with the detector modifies the threshold energy for the detectable particles and correspondingly the flux.

Using different types of detectors or a combination of several detection techniques, this innovative method was successfully implemented in several research fields with great success: it has been used for scanning the pyramids in the Giza complex [1,2] and scanning the Pyramid of the Sun at Teotihuacan [3], in volcanology investigations [4], as well as for geophysical applications [5] and in the Fukushima power plant nuclear debris status investigations [6].

During the last years, muon tomography has been imposed as a new research direction at IFIN-HH, based on more than 30 years of experience in cosmic rays and muon detection [7,8]. Two directions were developed in parallel. The first one emerged from the necessity to solve a security vulnerability. More specifically, to reduce the risks implied by radioactive materials trafficking in harbours. Large ships have the capability to easily conceal nuclear materials and the ability to transport the huge weight associated with shielding materials used. In order to increase the security measures in international harbours, we propose the use of the muon tomography method. We aim to detect and identify nuclear materials trafficked or their shielding material based on their density and their atomic number.

The second one is proposing to improve the security of underground civilian and industrial infrastructures, by using information provided by a detector placed underground that measures directional cosmic muon flux and identifies anomalies produced by irregularities in the geological layers above. Correlated with accurate simulations based on the known configuration of the area scanned, the measured data will lead to identification of the location and the dimensions of undiscovered cavities.

2. Experimental set-ups

In the last decade, muon flux characterizations of different locations were performed using the mobile detector developed in IFIN-HH [9]. The detector consists of two 1 m² active layers, each composed of four 1 × 25 × 100 cm³ plastic scintillator sheets with 12 parallel grooves on the top, the scintillation signals being read out by photomultiplier tubes through 12 wavelength shifting optical fibres [10].

The altitude profile of the muon flux has been measured in locations with different elevations. Also, water equivalent depth estimations were made based on measurements in underground sites.

During measurement with the detector placed close to a wall in the Unirea salt mine at Slanic Prahova, directional muon flux variation was observed, the flux from the cavern being substantially bigger than the flux coming from the wall [11].

Another effect was observed when the Dej and Praid mines were characterized. Muon flux measurements were performed in three different locations situated at different depths for each mine.

As you can observe in figure 1, the muon flux depth profile measured at the Praid mine shows strange behaviour compared to the normal profile measured at Dej. The third point, measured at 100 m depth, shows a value compatible with a much lower depth, the muon flux value being comparable with the value obtained in a location from the same mine at 64 m below the surface. This indicates the presence of an unknown cavern of reasonable dimensions.

Figure 1.

The muon flux measured at three different locations for the Dej mine (a) and the Praid mine (b). (Online version in colour.)

In order to further investigate those findings, a new detector was required that could measure directional muon flux with very good angular resolution and characterize objects, like hidden caverns, with good precision.

A detector was built, consisting of two 1 m² XY planes, with 60 cm between top and bottom planes.

One plane is composed of two layers formed of 40 plastic scintillator bars each, tightly packed on the X- and Y-directions.

The bars are 2.5 × 1 × 100 cm³ of plastic scintillator material, with a central groove located on top along the length, where a 1.5 mm wavelength optical fibre is placed to read the formed signal and transfer it to a SiPM sensor.

LabVIEW-based data acquisition software is installed on a PC, communicating with an FPGA chip that sets the detection threshold and regulates the coincidences between layers through a mini-USB port.

The data are stored in text format, with time information of millisecond precision and the triggered channels for every detected event.

A metal enclosure (figure 2) was designed and built for underwater testing of the detector, communication with the enclosed PC and the power supply being achieved through a sealed cable.

Figure 2.

Transversal section through the bathyscaphe and the detector (a); reconstructed detector geometry using Geant4 (b).

With an angular resolution of 41 mrad (2.38°), 2.5 m position resolution, 1 × 1 m active area and 60 cm height, the detector qualifies for tomography applications.

3. Monte Carlo simulations

Simulations of the detector response to the interaction with the cosmic muons flux in underwater environment using the Geant 4 platform have been performed [12].

The muon flux at sea-level was obtained based on detailed Monte Carlo simulations using the CORSIKA code, a tool designed for four-dimensional simulation of EASs through atmosphere [13].

To limit the computing time, CORSIKA results were parametrized as follows: the muon flux was divided into different energy intervals (0.2–0.65 GeV/c; 0.65–2.1 GeV/c; 2.1–6.6 GeV/c; 6.6–20.8 GeV/c; 20.8–65.8 GeV/c; 65.8–208.1 GeV/c; 208.1–658.1 GeV/c) for angular intervals of 10° for zenith angle between 0° and 60°.

For the analysis, only the muons that interacted with both layers of the detector were used.

The next step was to simulate two objects, 10 × 10 × 25 cm³ and 40 × 40 × 25 cm³ lead rectangular parallelepipeds positioned at 1 m, 5 m and 10 m above the detector in the underwater environment in order to investigate how the reconstruction of the energy deposited in the detector by interacting muons is affected by the distance to which the screening object is placed and the size of this object.

For a 10 × 10 × 25 cm³ lead object (figure 3), at 1 m and 5 m there is a decrease in the recorded muon flux, but when the object was placed at 10 m away from the detector, the reduction of the muon flux recorded by the detector is annihilated by the large distance, the object being too small to be detected.

Figure 3.

The distribution of simulated muon hits with the detector placed underwater at 1 m (a), 5 m (b) and 10 m (c) below the scanned object, a 10 × 10 × 25 cm³ lead parallelepiped.

For a 40 × 40 × 25 cm³ lead object, one can see in figure 4 a correlation in the decrease of the recorded muon flux with the distance where the detector is placed.

Figure 4.

The distribution of simulated muon hits with the detector placed underwater at 1 m (a), 5 m (b) and 10 m (c) below the scanned object, a 40 × 40 × 25 cm³ lead parallelepiped.

Thereupon, the reconstructed image depends on the scanned object size and the distance respective to the detector.

4. Conclusion

Muography represents a modern technique used for scanning big objects for different applications. Directional muon flux variation was observed, indicating the presence of an unknown cavern of reasonable dimensions. Investigations are in progress using the developed detector at the Praid mine (Romania) in order to confirm the previous findings.

Complex Monte Carlo simulations have been performed for the detector response to the interaction with the cosmic muon flux in an underwater environment, validating the hypothesis on the basis of which the detector system was built and also the method. Measurements with the detector system in similar conditions are currently under analyses, raw data being presently under investigation.

Data accessibility

This article has no additional data.

Authors' contributions

B.M. and D.S. are the managers for the two research projects mentioned and they contributed to the design concept of the detectors. B.C. performed the mechanical design for the detectors. A.G.-L., T.M., A.M. and A.S. worked on data analyses and interpretation. M.N.-O. and A.B. carried out the electronics design and data acquisition process. R.M. and B.A. created the data analyse software. All authors read and approved the manuscript.

Competing interests

We declare we have no competing interests.

Funding

This work was supported by the Romanian National Authority for Scientific Research and Innovation, CNCS/CCCDI - UEFISCDI, projects number PN-III-P2-2.1-PED-2016-1922/2017, PN-III-P1-1.2-PCCDI-2017-0839/2017 and PN-III-P2-2.1-PED-2016-1659 within PNCDI III.