Through the sonification of cosmic rays, the work Hotspot (2019 [fig. 30, 33, 34]) aims to make exhibition visitors aware of the constant invisible background radiation surrounding us, of an unknown aspect of the universe that is not detectable by the unaided human sensorium. Interstellar space contains a permanent flux of high-energy elementary particles called cosmic rays, travelling at the speed of light and originating from distant supernovas. When colliding with the earth's atmosphere, cosmic rays create showers of secondary particles. Although they are partly absorbed by the atmosphere, these showers induce a large range of phenomena, the main one being a flux of muons. These heavy electrons, which are not present in usual matter because of their short lifespan, are produced at a high rate in cosmic showers. Because of their excellent penetration capacities, muons are able to reach the earth's surface. Human beings, however, are unaware of these cosmic particles crossing their body.
What follows is a short description of the technical setup used to create Hotspot. Two detector arrays were each composed of a photomultiplier tube combined with a slat of plastic scintillator – a luminescent material that, when struck by an incoming muon, absorbs its energy and re-emits this energy in the form of light. A particle passing through the scintillators triggered the emission of some of the photons it contains. These photons were then guided along the scintillators by an optic fibre cable until they reached the photomultiplier tube, which generated an electrical impulse [fig. 31, 32]. Parasitic noise from natural terrestrial radioactivity was eliminated because muons travel at a similar speed to that of light, and therefore cross almost simultaneously. When a muon hit both photosensitive detectors at the same time, a threshold that separates background noise from pulse was applied to clean up the received signal. Only when the input signal from both photodetectors surpassed the installed threshold was a block pulse generated as output. This negative pulse needed to be converted to a positive voltage of maximum 5V for the Arduino microcontroller to be able to read it. A fast data acquisition program then checked whether or not a signal hit the Arduino pin, saving the interval between two impacting muons. Each interval was assigned a specific tonal value, which the microcontroller sent as an analogue pulse to an audio driver that amplified the signal before transmitting it as a MIDI format to a loudspeaker. Every sent pulse had the same length of 300 milliseconds: the only variation was the interval between, and the frequency of, the notes.
The test network described here was installed for the period of the Harbinger exhibition in one of the greenhouses of Ghent University’s Botanical Garden. In real time, it translated the ambient cosmic ray activity into sound, broadcast through an outdoor garden loudspeaker [fig. 33, 34]. The title of the work, Hotspot, not only refers to a radiation hotspot in orbit, a place where the flux of cosmic rays is 10 to 100 times greater than the rest of the orbital path, but also to the hotspot's definition as 1. the place to be, a tourist attraction; 2. a place of significant activity, danger, or violence; 3. a public place where a wireless signal is made available. I decided to show the underlying, specialized technology necessary to create the soundscape, while the listeners can experience the physical phenomenon itself. This added to the surprising and at the same time humorous effect of the installation, as there is a large discrepancy between the high-tech part of the work and the deadpan sound it produces, resembling the popping of corn.
Several sound recordings were made in the course of the project [fig. 35] and later converted into sheet music scores [fig. 36]. I also ran a recording through a photosounder software program to create a visual output of the captured cosmic radiation, which was printed on a strip of photo paper 7.25 cm by 585 cm in size [fig. 37]. The different versions of Hotspot illustrate that each image-making framework prescribes how we (can) visualise the world, in the sense that it also constructs and interprets reality in a preconditioned way.