> Cosmic rays with energies of more than 100 EeV are rarely spotted — fewer than one of these particles arrives on each square kilometre of Earth each century.
How do they explain detecting such a particle at all? I would assume that the surface of the Earth has much less than 1 square kilometer worth of detectors, so on average they shouldn't have detected any 100 EeV particles since the invention of cosmic-ray detectors.
It uses photomultiplier tubes to record interactions of the cosmic rays with a good-sized chunk of atmosphere, plus detectors for the shower of particles when a cosmic ray interacts with the air. https://en.wikipedia.org/wiki/Telescope_Array_Project says it uses "a 762 km2 grid array with 1.2 km between each unit".
> The Telescope Array project ... is designed to observe air showers induced by ultra-high-energy cosmic ray using a combination of ground array and air-fluorescence techniques. ... When a cosmic ray passes through the Earth's atmosphere and triggers an air shower, the fluorescence telescopes measure the scintillation light generated as the shower passes through the gas of the atmosphere, while the array of scintillator surface detectors samples the footprint of the shower when it reaches the Earth's surface.
A very different and really neat concept that hasn't become real yet is JEM EUSO, a telescope that would be mounted on a space station, pointed at Earth, would detect air showers via fluorescence like Auger's fluorescence telescopes do on the ground. This could theoretically cover a much larger area than traditional CR observatories. https://en.wikipedia.org/wiki/JEM-EUSO
Fluorescence detectors are volumetric though only operate during moondown at night, and in the case of both Auger and TA, are paired surface detectors. Auger uses more water tanks with a PMT lined with Tyvek and detect electronic and muonic secondary particle components for the EAS. TA has scintillation detectors with PMTs, which are polyvinyl toluene sheets embedded in a steel casing. In both cases, they have individual triggers, usually around 1 MIP (minimum ionizing particle), which will trigger communication to a tower, which would then poll nearby detectors for events greater than 1/3 MIP.
Or, at least this is how it was setup 15 years ago. Both experiments have added new fluorescence and surface detectors since then.
A century is about 3 billion seconds, and Earth's surface area is about half-a-billion square kilometres, so one of these hits Earth about every 6+ seconds.
How do they explain detecting such a particle at all? I would assume that the surface of the Earth has much less than 1 square kilometer worth of detectors, so on average they shouldn't have detected any 100 EeV particles since the invention of cosmic-ray detectors.