Cosmich Watch is simple, physics-motivated machine- and electronics-shop project for university students and schools. Our detector is a self-contained apparatus that employs plastic scintillator as a detection medium and a silicon photomultiplier for light collection. These detectors can be battery powered and used in conjunction with the provided software to make interesting physics measurements. The total cost of each counter is approximately $100.

Cosmic muons

The desktop muon counter is triggering on muons that are produced when high energy astrophysical particles, called cosmic rays, collide with the Earth’s atmosphere producing particles that decay to muons. In his 1950 Nobel Lecture, C.F. Powell described cosmic rays as a “thin rain of charged particles”. Most cosmic rays are produced in our galaxy and are nuclei expelled in supernova explosions. About 90% of cosmic rays are protons, 9% are helium nuclei, and the remaining 1% are heavier nuclei. When the cosmic rays hit the nuclei of the atmosphere, a shower of particles are produced including pions and kaons. These are the progenitors of the muons.

The muons that are ultimately produced in the shower are fundamental particles that carry electric charge of +1 or −1 and have mass that is about 200 times that of the electron. To briefly learn more about muons and their place within the Standard Model of particle physics, we recommend to visit the Particle Adventure website. Muons are unstable and will decay to an electron, a neutrino and an anti-neutrino. At rest, the lifetime of the muon is approximately 2.2 microseconds. Given that the muons are produced in the shower at more than 10 km above the Earth’s surface, Galilean relativity calculations will show a very small probability of survival to reach the desktop muon counter. However, because the muons are produced at high energies, relativistic time dilation extends their lifetime. As a result, muons can survive to be detected on Earth.

The muon flux at sea level is about one per square centimeter per minute for a horizontal detector. This constant bombardment by muons has pros and cons for a particle physicist. On the plus side, cosmic ray muons are commonly used in surface-based particle physics experiments in order to commission and calibrate detectors before they are exposed to beam produced by accelerators. Often the muons that survive to sea level are accompanied with other particle debris, such a photons and protons. A relatively small amount of shielding material is often used to remove this accompanying debris, leaving only the muon for use in calibration. On the other hand, many particle physics experiments are looking for rare events, and the rare signal can be swamped by the muon signal. These experiments must be located in deep underground laboratories.

The desktop muon detector was initially built as a Muon Tagging Optical Modules (MTOMs) for PINGU, the proposed low energy upgrade for IceCube experiment, in order to tag the position of muons with high precision (within a few cm). This is inspired by a recent study using the IceCube dark matter detector, DMIce, which consists of two 5 inch NaI crystals. The primary purpose of DMIce is the direct detection of dark matter; however, the crystals detect 12 muons per day that also trigger IceCube. A study of the impact in energy, direction, and position uncertainty of muons using the DMIce detector in conjunction with IceCube. It also showed a reduction in misreconstructed events, when tagging the muon using the NaI crystals, as well as an improvement in the track reconstructed position. Given that the mTOMs size is 10 times smaller than the NaI crystal, the track can be better localized.

The mTOMs in PINGU will be constructed of solid, 5x5x1 cm scintillator block placed within a lighttight reflective aluminium casing. This prototype has been built in the context of a stand alone desktop muon counter. The optical readout will be through a silicon photomultiplier (SiPM). These modules have a thin profile, so that they require very little space within the module making it possible to have four modules per PINGU DOM. The detector will be hardwired into the standard PINGU DAQ.

The desktop muon detector is an extension of the mTOMs. The scintillator and SiPMs are those found in the mTOMs, but the read-out electronics and software is unique. In essense, it is a self-contained, USB powered muon detector simple enough that students familiar with simple electronics can construct them.