Cosmos is permeated by streams of very energetic particles (gamma-rays, protons, alpha particles, and heavier nuclei) which are produced in our Sun, in supernovae explosions, in gamma-ray bursts, and in the vicinity of black holes. The collisions of these primary cosmic rays with the gas atoms in the upper layers of Earth atmosphere produce cascades of secondary particles which propagate through the atmosphere towards the Earth surface. At sea level the secondary cosmic rays consist of mu-mesons arriving at the rate of about one mu-meson per square cm, per minute. There are two kinds of phenomena that are worth studying with these secondary cosmic rays: the cosmic ray showers and the decay of the mu-mesons.
Cosmic Ray Showers
Since the secondary cosmic rays are produced in collisions of very energetic particles with the atmospheric atomic nuclei, it makes sense that the produced particles are also very energetic. These particles hit other nuclei along their way down to the Earth, which makes even more secondary particles. In the end, lots and lots of secondaries can be produced from the primary cosmic ray. Such cascades of secondaries are named cosmic ray showers. A cosmic ray shower can cover an extended area of hundreds of meters at the Earth surface. One of the scientific experiments which is recording such showers is named the High Altitude Water Cherenkov Gamma-Ray Observatory (HAWC). It operates near the Sierra Negra mountain in Mexico. On the official HAWC website you can see the extensive array of water tanks serving as detectors. We cannot (yet) build this kind of the multidetector observatory in our backyard. We will come back to this endeavor somewhat later, after we complete a series of less demanding table top experiments on the cosmic ray mu-meson capture and decay, described in the next section.
Cosmic Ray Capture and Decay
A grapefruit-size detector is sufficient to record the cosmic ray capture and decay in a table top experiment. Most mu-mesons will penetrate and then leave the detector. Such "punch-through" mesons will produce single pulses during their passage through the detector medium. Once in a while a mu-meson will come to rest within the detector volume. The stopped meson will decay into an electron and a neutrino after spending a few microseconds within the detector. Such events produce two pulses in sequence. The first pulse is induced when the mu-meson hits the detector. The subsequent pulse is produced when the mu-meson decays. In our experiments we will collect both kinds of events, the single-pulse events due to the punch-through and the double-pulse events due to the decay of the stopped mu-meson.