miniMuTe: A muon telescope prototype for studying volcanic structures with cosmic ray flux.

MiniMuTe: un prototipo de telescopio de muones para el estudio de estructuras volcánicas con flujos de rayos cósmicos

Muography or muon tomography is a technique which uses atmospheric muons to study geological and/or anthropic structures. Applications of muography range from archeology to civil engineering, however, geology is presently the field with more ongoing experiments because no-invasiveness, high spatial resolution, and penetration introduce it as an excellent method to study volcanoes. The most common tools for studying volcanoes as gravimetry and electrical tomography lack the spatial resolution [1], penetration and offers a complex inversion problem due to data interpretation. Another issue is their invasive nature, which is a hazardous task considering that the information must be recorded over volcano domes [2].

Several experiments based on muography have been developed, L. Alvarez and collaborators made the earliest on the Chephren pyramid looking for hidden cavities. They did not find any cavity but advertise the technique [3]. More recently, Tanaka et al. [4], [5] and Lesparre et al. [6], [7], [8] mark the landmark in the muography field with several studies about the feasibility of muon imaging. Today the most representative muon telescopes to study volcano structure (that we shall describe in the present work) are Muon Radiography (MuRay), ToMuVol and Diaphane.

The MuRay detector [9] was designed to perform muography on the Mount Vesuvius. It has 3 XY planes of 128 triangular plastic scintillators, 64 vertical and 64 horizontal. The planes have a separation distance of 1 m. The newest feature in MuRay is the capability to measure the Time-of-Flight to filter "albedo muons" (muons incoming from the detector back) and other false positive events. Each scintillator bar couples to a Silicon Photomultiplier for converting the scintillation light-beam into an electrical signal. Muon flux information coming from panels are processed by the EASIROC ASIC and an FPGA Xilinx Spartan III [10].

ToMuVol -located in the south of France in the Puy de Dôme- has four layers about 1 m of single Resistive Plate Chambers (RPCs). The readout system is a 64 channels Hardroc2 ASIC which efficiently discriminates events from noise. Moreover, the detector needs a Programming Logic Controller (PLC) to monitor gas, high voltages, and environmental conditions. The RPCs need a high voltage (7.5 kV) to operate, this fact is a drawback concerning the power consumption of the telescope. Another issue is about the mechanical robustness of the detector and its limits to operate in harsh environments. It achieves an angular resolution of 1 deg in 1000 days [11].

The DIAPHANE is an ongoing experiment at the Soufrière of Guadeloupe, an active volcano. These detectors have two matrices of 16x16 pixels of scintillators where the light signals from each scintillator are collected through of a wavelength shifting (WLS) optical fiber and converted to electrical signals using a 64 channel Multi-Anode Photomultiplier (MAPM). Each MAPM is readout by a multichannel frontend electronics for discrimination and trigger generation. DIAPHANE has an angular resolution of 0.1 rad and low power consumption about 36 W [8]. The principal disadvantage of DIAPHANE is that a fraction of fibers length is out of the scintillation bars and this exposes them to external light noise sources, and a decrease in the signal-to-noise ratio (SNR) could occur.

In this paper, we shall briefly discuss the concept of a hybrid muon telescope which solves some of the main drawbacks of the detectors mentioned above. MuTe (for Colombian Muon Telescope) combine two detection techniques: a hodoscope formed by two planes of plastic scintillator bars, and a WCD which allows us to isolate the muon from the electromagnetic component and to estimate the energy of this component. MuTe project aims to study Colombian active volcanoes [12, 13] and in this note, we report the first test of the hodoscope electronic system and measurements of the background flux at Cerro Machin Volcano made through the miniMuTe prototype.

The first test made at Cerro Machin was to measure the temporal evolution of the background flux. The panels were placed one next to the other attaining a sensitive area of 288 cm² and registered an average flux of 0.6 hits/min-cm at an altitude of 2.650 m.a.s.l, during 7.5 hours with an aperture angle of 68°. Fig. 8. shows the number of hits per minute.

The second test, --displayed in Fig. 6. and performed having the two panels one over the other-- records 25 trajectories in a 2-dimensional histogram which has a maximum in and due to flux variation from 0° to 90° zenith modulates by a square cosine factor.

Fig. 8.
Background flux at Cerro Machin Volcano.

Flux raw rate (blue line). Average flux rate with a temporal sliding window of 60 minutes (green line).

However, the 2D-histogram obtained shows a maximum value near the right-bottom corner, see Fig. 9. This phenomenon occurs due to the position of SiPMs; light produced by muons crossing in the opposite direction to the SiPMs location have less probability of reaching the photo-sensors; therefore, a bias appears in the acceptance of the hodoscope. This bias in the acceptance maximum is corrected by applying a variable gain mask on the MAROC3 amplification stage, that is, to find the maximum gain per each channel.

Fig. 9.
Background flux histogram depending on the muon incident trajectory.

The color bar shows the flux in hits per day.

Finally, the last test was pointing the miniMuTe to a section of the volcano dome, for recording data from a fraction of clear sky and, at the same time, from a fraction of the dome during 24 hours.

Fig. 10. illustrates the resulting 2D-histogram. In this case, the histogram per day has decreased in comparison with the background measurement due to the flux is small for angles near to 90° zenith. However, a clear contrast between clear sky and dome section appears.

The angle of the incident muons crossing the volcano dome is an important issue to determine the time of exposure of the instrument to collect enough information to do a muography of the Cerro Machin volcano.

The background flux histogram provides information on the acceptance of the detector and how to carry out the calibration process to get a fair distribution of hits rate.

Fig. 10.
2D flux histogram depending on the muon trajectories as result of pointing the miniMuTe to a fraction of the Cerro Machin volcano dome.

With this fist evaluation of the hodoscope electronic system, we have checked the stability of its performance, particularly of the data acquisition system through the SiPM and the reliability of the operation in recording the data. We have also tested the field response SiPM with the variation of the temperature, which was of 10°C --between 5 °C and 15 °C. This behavior affects the breakdown voltage point for the SiPMs and, in this way, the acceptance of the detector; To correct this effect we implement a high voltage corrector depending on temperature on the Raspberry Pi SCB.

The authors acknowledge the financial support of Departamento Administrativo de Ciencia, Tecnología e Innovación of Colombia (ColCiencias) under contract FP44842-051-2015 and to the Programa de Cooperación Nivel II (PCB-II) MINCYT-CONICET-COLCIENCIAS 2015, under project CO/15/02. We are also very thankful to LAGO and to Pierre Auger Collaboration for their continuous support. Finally, we would like to thank Vicerrectoría Investigación y Extensión Universidad Industrial de Santander for its permanent sponsorship. DSP wants to thank the Escuela de Física, the Grupo de Investigación en Relatividad y Gravitación, Grupo Halley de Astronomía y Ciencias Aeroespaciales and Vicerrectoría Investigación y Extensión of Universidad Industrial de Santander for the hospitality during my post-doctoral fellowship.