Muography and the Paarl African Underground
Laboratory

Lumkile Msebi, Stephan Jonker, Lerothodi Leonard Leeuw, Robert Lindsay, Fairouz Malek, Jacques Marteau, Richard Newman, Shaun Wyngaardt, J.J van Zyl

► To cite this version:

Lumkile Msebi, Stephan Jonker, Lerothodi Leonard Leeuw, Robert Lindsay, Fairouz Malek, et al. Muography and the Paarl African Underground Laboratory. 68th Annual Conference of the South African Institute of Physics (SAIP2024), Rhodes University, Jul 2025, Grahamstown, South Africa. pp.304-309. [hal-05362875](https://hal.science/hal-05362875v1)

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Submitted on 25 Nov 2025

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# Muography and the Paarl African Underground Laboratory

L. Msebi¹, S. Jonker², L. Leeuw¹, R. Lindsay¹, F. Malek²,³, J. Marteau⁴, R. Newman², S. Wyngaardt², J.J. van Zyl²

¹ Department of Physics and Astronomy, University of the Western Cape, P/B X17, Bellville, ZA7535 South Africa.

²Physics Department, Stellenbosch University, P/B X1, Matieland, 7602, South Africa.

³LPSC Grenoble, CNRS and Grenoble Alpes University, France.

⁴IP2I, CNRS-IN2P3, Université Claude Bernard Lyon, France

E-mail: lmsebi@uwc.ac.za

**Abstract.** Shielding from cosmic background is an essential element in rare event searches. One of the ways to do this is to conduct experiments in underground laboratories which provide shielding by hundreds of meters of rock. To this end, there has been a commitment to develop such laboratory in South Africa called the Paarl African Underground Laboratory (PAUL). Muons, which are secondary particles stemming from cosmic rays, are able to penetrate several kilometres of rock due to their highly energetic nature. Muon imaging or muography is one of the powerful and complementary methods that can help unravel the internal secrets of natural edifices. As part of a broader plan to measure the muon flux at the overburden in the Huguenot Tunnel, open sky measurements were undertaken using a muon telescope. These measurements were done at Stellenbosch University and at the University of the Western Cape and they form part of the calibration process. The muon telescope is currently stationed inside the tunnel with an endeavour to find the muon flux rate therein and help map the topography of the overburden. This information will be used to decide the location of PAUL inside the tunnel.

## 1. Introduction

Atmospheric muons belong to a family of charged leptonic particles that are ubiquitous and are known to have a wide range of energies. The highly energetic component of the muon distribution is able to penetrate kilometers of rock deep underground. As they traverse through the earth's atmosphere, muons interact with electrons and other nuclei, thereby losing a fraction of their energy. Once they are inside matter, their energy loss is determined by, amongst others, the density and composition of the matter. The conventional approach to quantify the energy loss of charged particle through matter is to make use of the stopping power, whose generic form is given by [1]:

$$ \langle \frac{dE}{dx} \rangle = a(E) + b(E)E. \qquad (1) $$

If a particle propagates through a non-uniform medium, it is convenient to express Equation 1 in terms of opacity, which is the density per unit length [2]:

$$ \rho(L) \equiv \int_L \rho(\xi) d(\xi), \qquad (2) $$
where $\rho$ is expressed in g.cm$^{-2}$, $\rho$ is the density and $\xi$ the co-ordinate measured along the trajectory L of the muon crossing the volume of rock. Recently, there has been a huge interest in rare event searches such as neutrinoless double-beta decay and dark matter. This has resulted in building of underground laboratories to ensure maximum background shielding. Muons, due to their highly penetrative nature, form part of this background, however, they are useful source of calibration and target imaging.

Plans are underway to build the first underground laboratory in Africa. The proposed site for this laboratory is the Huguenot Tunnel that is located along the N1 national road that passes through the Du Toitskloof mountain range that separates Paarl from Worcester, near Cape Town, South Africa. The mountain is endowed with erosion resistant quartzite sandstone of the Table Mountain Group that form the skyline as well as the underlying granite base [3] (see Figure 1). With a height of around 800m, this interesting geology makes it appealing for an underground laboratory for rare event searches since it provides substantial shielding from unwanted background.

Furthermore, the density variation of the rock above the tunnel makes it ideal for muography, which is also known as muon radiography. Muons can cross hundreds of meters (up to kilometres) of rock and their attenuation is a consequence of the energy loss throughout their trajectory. We can take advantage of this to characterize the geological overburden above the tunnel. Such a technique has proven to be useful tool for investigation across a wide spectrum from natural edifices such as volcanoes [4, 5] to archaeology [6], nuclear reactors [7] and pyramids [8]. Muography is a non-invasive technique that can be used to characterize overburden density anomalies. The present work outlines the ongoing efforts to explore the applications of muon radiography in the broader context of the Paarl African Underground Laboratory initiative.

<figure><img src="image_1.png" /><figcaption>**Figure 1.** Geological mapping of the Huguenot Tunnel showing the initial position when the muon detector was placed at VCC2 [3].</figcaption></figure>
<figure><img src="image_2.png" /><figcaption>**Figure 2.** (a) A schematic drawing depicting the dimensions of the muon detector. (b) The muon detector next to the Merensky Building at Stellenbosch University.</figcaption></figure>

**2. Experimental Set Up and Results**

There are several detection methods for muons including scintillators, Cherenkov detectors and gaseous systems. For this study, a muon telescope was used see Figure 2, which consists of three plastic scintillator detection matrices. Each detection matrix consists of two series of strips, aligned in the *x* and *y* directions and forming an array of pixels.

The top and bottom panels have a detection area of 80 cm by 80 cm and are composed of an array of 32 x 32 plastic-scintillator bars of 2.5 cm width, while the middle panel consists 16 x 16 plastic-scintillator bars of 5 cm width. The middle panel is used as a veto against random coincidences. The panels are separated by a vertical distance of 60 cm. The plastic scintillator bars are embedded with wavelength shifting fiber (WLS) to collect light towards a multi-anode photomultiplier (MAPMT), with 64 channels. A clear optical fibre connection is required for the MAPMT to guide light from the scintillator bars to the photosensor's entrance window. More details on the design and operation of the muon telescope can be obtained from [9].

**2.1. Muon Measurements**

The first muon measurements were done at Stellenbosch University outside the Merensky Physics Building. These measurements were done in open sky to ensure stability in detector performance and to check for any inefficiencies. Analog to Digital Converter (ADC) histograms were investigated to ensure consistency in the response of each channel and to obtain information about any possible variation of the signal strength. The spatial distribution measurements are shown in Figure 3, with a uniform spread in both *x* and *y* directions. Most of the muon events are concentrated at the centre with a gradual decline in all directions forming a roughly elliptical shape. An irregular pattern is seen in the Stellenbosch data, see Figure 3a, where a slight deviation from the elliptical shape is evident in the top right corner. This deviation
<figure><img src="image_3.png" /><figcaption>**Figure 3.** Preliminary results showing the spatial distribution of the number of muon events with their corresponding *x*, *y* directions at (a) Stellenbosch University outside the Merensky Building (b) University of the Western Cape open field.</figcaption></figure>

is due to the detector's proximity to the Merensky Building. Measurements that were done at the University of the Western Cape in an open field are shown in Figure 3b. In contrast to the measurements performed at Stellenbosch University, the spatial distribution follows an elliptical shape since there was no nearby obstruction. The preliminary analysis indicates that the number of events recorded for the open sky measurements was about 4000 events/hour. Subsequent measurements were done inside the Huguenot Tunnel. At first the detector was placed in (Vehicle Cross Connection) VCC2, see Figure 1, which cuts through the North bore and South bore of the tunnel. The number of events recorded in VCC2, which is 2 km from the Paarl Entrance, was 5 events per day, a factor of around 20000 suppression. The detector in now placed 300m away from VCC2 towards Paarl.

Event by event reconstruction of the tracks was done using a linear regression fit. The reconstruction algorithm pre-selects coincident hits in a 200 ns time window and an event is constituted by three hits (i.e when a muon passes through the three matrices) occurring within this time window. A muon track can be reconstructed in Cartesian coordinates (x, y, z) in the detector system, where x and y represent the coordinates in the detection plane while z axis is the detection plane number. If there are three pixels fired coincidentally that do not align, they are filtered as random coincidences. Should there be any difference by one or more pixels of the reconstructed tracks, either horizontally or vertically, the event is discarded [10]. The xz and the yz plane projections were taken into account for each track. Azimuth and zenith angular distributions of the muon tracks for one of the runs from the Stellenbosch data are shown in Figure 4.

It is important to consider the behaviour and the performance of a detector, since these are crucial for muon track and muon flux measurements. Key to this, is the solid angle, $\Omega$, covered by the telescope and the angular resolution, $\delta\Omega$. These factors depend on the number of pixels, $N_x$, $N_y$, the size of the pixels $d$, and the distance between the matrices $D$. The angular resolution, $\delta\Omega$, covers all muon trajectories that are able to hit a given number of pixels of the detection matrices.

In order to assign opacity, the acceptance of the muon detector is crucial. It is also useful
<figure><img src="image_4.png" /><figcaption>**Figure 4.** Preliminary results showing the frequency of the number of tracks that have angles within a specified data range is plotted against zenith and azimuth angular distributions.</figcaption></figure>

when considering the inverse problem which is deducing density of a target from the muon flux.
The acceptance can be obtained by multiplying the detection area by the angular resolution.
Figure 5 shows the angular resolution and acceptance of our muon detector.

<figure><img src="image_5.png" /><figcaption>**Figure 5.** (a) The Angular Resolution of a muon telescope consisting of three detector matrices, the top and bottom matrices are 32 x 32 by size with *d* = 2.5*cm* while the middle matrix is 16 x 16 *d* = 5*cm* (b) The geometric acceptance of the telescope.</figcaption></figure>
### 3. Future Outlook

The muon detector will be moved from the current position to 300m away from VCC2 towards Worcester. It may then be to another location that will be best suited for muography. With these different positions in the tunnel we will also be able to comprehend where the ideal location to build the laboratory is. Work on simulations that is meant to complement experimental results is ongoing using tools like COsmic Ray Simulation for KAscade (CORSIKA) [11]. A combined effort with other techniques used in geosciences to further characterize the geology and physics related to the Huguenot Tunnel is being instigated. The topology of the Du Toitskloof mountain range [3], is known to have a fault zone which can cause large discontinuities that affect rock properties. Such fault zones have far reaching and profound geological consequences [12]. The current work is but a start of a much bigger and concerted effort to unravel critical information that encompasses wide range of disciplines in the field of science. It shows how the Paarl African Underground Laboratory has the potential to build skills and expertise that are relevant to the present and future needs of the African continent.

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