Hardware Hacking

Muons created by the interaction of cosmic rays and our atmosphere lose their energy gradually by ionisation of the material through which they pass. As they start with high energies they have the capacity to ionise many atoms before their energy is exhausted. Also, as they travel at nearly the speed of light, they tend not to ionise very efficiently and hence can travel through substantial lengths of matter, some metres of lead, before being stopped.

The process is a little different for the electrons. Theory shows that ionisation of the medium is not the predominant energy loss mechanism for relativistic electrons, high energy electrons lose energy more efficiently through the emission of electromagnetic radiation as they are decelerated in the presence of matter. If ionisation had been the main mechanism for slowing the electrons down then they might have had penetrative properties comparable to those of the muons, but the radioactive energy loss mechanism ensures that about 15cm of lead is sufficient to stop them.

The different energy loss mechanisms allow the sea level particle flux to be split into two main components. The penetrating muon part of the ground level flux is referred to as the ‘hard’ component and the easily absorbed electron part is called the ‘soft’ component.

The decay of a neutral pion into a pair of high energy gamma rays is the starting point of an avalanche of electrons, positrons and further gamma rays. This avalanche is known as an electromagnetic cascade. The process starts when one of the gamma rays passes close to the nucleus of an atom. Even though the gamma ray carries no electric charge its electromagnetic nature allows it to interact with the strong electric field of the nucleus to cause the materialization of an electron-positron pair. The energy required for pair creation is about 1 MeV, the gamma rays can have a thousand times that energy and hence the electron-positron pair produced can move on sharing nearly all the energy of the initiating gamma ray. If these fast moving electrons and positrons go on to pass close to other nuclei then they will suffer accelerations due to the positive charge of the protons. An accelerated charged particle will emit electromagnetic radiation. The intense accelerations can produce further gamma rays capable of producing more electron-positron pairs. The cycle of pair-production and gamma ray generation continues with the original gamma ray energy eventually manifesting itself as many particles.