How does the cosmic ray detector at the Museum work?
In the previous question we explained how an individual detector, a Geiger tube, works. The passage of a particle through a tube gives rise to a pulse of electric current which can be recorded and displayed by light, sound, etc. But why sixteen tubes? Why form two parallel and overlapping rows of eight each?
Fig B2-1. Detector Image of the CRIS detector (acronym for Cosmic Ray Interactive Station). Device designed and built for the exhibition "In/visibility. Arturo Duperier and cosmic rays".
We are interested in cosmic rays, but if we had only one tube, most of the signals would be accidental, due, for example, to noise from electronics or environmental radioactivity of terrestrial origin, which we are not interested in.
To minimise these spurious signals, the tubes are arranged in coincidence, i.e. only if a pair of tubes registers a signal within a very short time interval (1), that signal is recorded with its time stamp and we attribute it to a particle that has passed through the detector - although it is never possible to say with absolute certainty that a given event is due to a secondary cosmic ray particle and not to anything else.
But in addition, asking a pair of tubes to "be triggered in coincidence" allows us to define to some extent the direction from which the particle has arrived: along the straight line joining the tubes. As we will see later, this is very important for the use we want to make of the data.
Fig.B2-2 Tube detector scheme
The pair of tubes that is activated "simultaneously" (in a very short interval predefined according to the characteristics of the detector) defines the incoming direction of the particle responsible for the activation.
Finally, we have used tube trays to increase the effective area of the detector and to achieve a detection rate high enough for an exhibition at a Museum. According to the specification sheet of our Geiger tubes, each has an active area - the area of the "target" they present to cosmic rays - of about 9 cm2. The average vertical cosmic ray flux at the Earth's surface is of the order of 0.01 /(s-cm2). This means that, under ideal efficiency conditions, a single tube would record a coincidence about every ten seconds [9 cm2 x 0.01 / (s·cm2) ~ 0.1 /s]. This means that in the museum it would not be unusual to have to wait more than 10 s to see a match, which is far too long. Putting eight pairs - which we can consider independent - the maximum expected coincidence rate would be:
8 × 9 cm2 x 0,01 / (s·cm2) ~ 0,7 /s
which in practice gives us in the order of one coincidence every 2 seconds, a much more appropriate value for an exposure and which has been confirmed by the experience of previous tests.
As for the electronics of the detector, suffice it to say that it contains the necessary elements to generate the high voltage that feeds the tubes, convert their analogue electrical pulses into digital signals and combine these in a logic circuit to define their coincidences and end up registering them in a computer (Raspberry Pi 3B) with a time stamp and the indication of the pair of tubes involved. Additionally, there are two outputs to convert the coincidences into light and audio signals that are used in their sensory representation for the public of the exhibition.
(1) Note that the particles we detect, mostly muons, have typical energies of the order of a few GeV, which means that they travel at almost the speed of light, about 300 000 km/s. The time they take to cross the distance between two adjacent tubes (1.6 cm) is negligible compared to the response times of Geiger tubes and electronics.