TECHNIQUES OF MUOGRAPHY

MUON GENERATION IN THE ATMOSPHERE

Protons and alpha particles accelerate at nearly the speed of light originating from very energetic cosmic events such as supernovae. These particles are called cosmic rays and they travel more than a million years before arriving at the Earth’s atmosphere, its outermost boundary. Within the atmosphere, cosmic rays interact with the atmospheric nuclei such as nitrogen or oxygen. This generates very unstable particles called pions and kaons which decay into muons and neutrinos around 1/100 millionth of a second later. Muons generated in the Earth’s atmosphere are called atmospheric muons (or cosmic ray muons), and precipitate downwards like rain towards the Earth’s surface almost constantly. For example, if we sleep about 8 hours, we can expect about one million muons to pass through our body every night.

MUON PROPAGATION THROUGH MATTER

As with electrons, muons have electric charge but no substructures. Both are charged leptons that are free from strong interaction but sensitive to electromagnetic force. Mass and lifetime differ between electrons and muons . The muon's mass is approximately 200 times larger than the electron, and this feature enables high energy muons to pass through volcanoes.  However, muons decay in 2.2 microseconds. This means muons can travel only 660 meters even at the speed of light.
Einstein's theory of special relativity predicts that the lifetime of an object moving near the speed of light will be delayed. Therefore, muons may survive long enough to make the journey from the atmosphere through a volcano and into a muon detector. In radiography, X-ray transmission through the human body shows the shadows of the internal organs of greater thickness or density, for example bones, and the presence of voids. However, it cannot be used as a probe for subterranean imaging. The unusual characteristics of muons make it possible for it to be used to image the interior structure of volcanoes and other gigantic objects.

MUON DETECTION

The primary task of the muography observation systems is to record the trajectories of incoming muons efficiently with a minimum power consumption and at the lowest costs. Three types of muon detectors have been used: (a) nuclear emulsion, (b) scintillation detectors, and (c) gaseous detectors.

When a charged particle passes through the nuclear emulsion incorporated into the first type of muon detector, part of the AgBr micro-crystals incorporated in the emulsion will record the muon trajectories and these trajectories can be read when the emulsion is later developed. The nuclear emulsion detectors have the major advantages of being able to function without any electric power, and thus they are used at locations where commercial electricity is not available. However, the nuclear emulsions integrate all muon events without recording timing information. Consequently, the most important criteria for selecting this detector scheme depend on (a) whether the desired muographic image should contain time information, and (b) the availability and affordability of the commercial electricity.

With scintillation detectors, the events are recorded as a flash of light generated by the incident charged particles and subsequently detected by photo-multiplier tubes. Particle trajectories can be tracked by connecting two or more vertex points determined by the geometrical address of the scintillator bars and their corresponding timings. Unlike nuclear emulsion detectors, scintillation detectors yield the timing of individual events.

Gaseous detectors are also utilized as real-time detectors based on a tracking technique similar to scintillation detectors. Although the detector requires a constant gas flow over the entire period of the measurement, the gaseous detector has a potential to produce high-definition muographic volcano images.