On Radioactive Music: Ray Of Light MCA/S

On Radioactive Music: chapter 1
Gamma spectrometry and electronic music systems

Presented for the first time at the DATAMI Artist in Residence/Resonance JRC-Ispra, January 24th, 2019

From spectralism to spectrometry

Computers have been used in music since the early 1950s and long before those gigantic mainframes, we used calculations, permutations and mechanical devices to create sound and aid in the composition of music. Computers are employed not only in the production and post-production of sound, but also as vectors of development of new musical theories and ideas. Science is the most important actor here, and it should not surprise us, if we come to realize that electronic music was indeed born using scientific equipment and scientific ideas.

In the Quadrivium of the Middle Ages, mathematics was the study of numbers, geometry the study of numbers in space, music the study of numbers in time and astronomy the study of numbers in space and time. Centuries have passed and mathematics and science are more than ever connected to music. Every technology that we use in music has a profound relation with telecommunications and physics.

By the 1970s, a group of composers who worked with the Ensemble l’Itineraire gave birth to one of the most important post-serial musical aesthetics now recognized as the ”French School”:spectralism.

Spectral music is a set of musical ideals and compositional practices that seek to bring timbre to the forefront of composition. Its’ language often centers on orchestrations of harmonic series partials as its’ pitch material (rather than tonal or atonal pitch collections) that are in turn varied and transformed throughout the composition. By creating instrumental scoring combinations built on specified pitch frequencies, spectral textures “melt to- gether” sounding more like new instrumental timbre composites rather than a vertical sonority/harmony. Originators of this genre performed computer-based analyses, such as the Fast Fourier transform on sound sources to see a visual spectrogram of har- monic frequencies that define their timbres.

Why radioactivity?

While the spectralists wished ”a sound for sound,” most diligently investigating only the intimate reasons of the acoustic phenomena, we turn our attention to the physics of nuclear reactions at the core of matter. Looking at patterns of radiation, we realize that we constantly live in an ocean of en- ergy that surrounds and penetrate us and we want to extend our perceptions to the most intimate parts of the cosmos, so that with our experiments, we become listeners of the perpetual act of transformation of nature.

But what is an experiment? An experiment is an action of which the outcome is unknown. Following John Cage’s definition: experimental music is a music indeterminate in respect of its score, hence this form of music deals with indeterminacy which is employed with different techniques like: impro- visation, graphical scores, written instructions and computer algorithms.

As we know, computers are not able to output real random numbers. These are instead, pseudo-random sequences of numbers created with spe- cial equations. At some point, after a long time, these numbers repeat. Radioactivity on the other hand, is a real random process because we can’t predict the exact moment when an atom will decay. Also, radioactive sources produce stochastic patterns which describe the distinctive footprint activity of an isotope. These probabilistic patterns of energies are unique for every radioactive element.

We may say that these patterns have something in common to forms of timbre or tone colors.
However, the analogy with sound does not end here. If we talk of photons, light shares some common principles with sound:

  1. Both are forms of energy that travel in waves.
  2. Both have properties of wavelength, frequency and amplitude, thus sharing the concept of spectrum.
  3. Both shares spherical divergence/inverse square law, where the inten- sity is inversely proportional to the square of the distance from the source.
  4. Both can be subject to interference, diffraction, refraction.

Measuring instruments

The most famous ionizing radiation detector is the Geiger-Muller counter. Commonly known as the Geiger counter. This instrument uses one or more tubes filled with low pressure inert gas such as helium, argon or neon to which a high voltage is applied. When ionizing radiation hits the counter, an electrical discharge is generated and the user reads the count number of the events or the equivalent dose on a display or gauge.

Geiger counters have been often used in art and music as random event triggers taking advantage of the aleatory nature of radioactive decay. From a musical point of view, this equipment is useful as a random clock and nothing more, because the counter can’t discriminate the energies. Because our goal is to hear X and gamma rays, we have adopted a more sophisticated kind of measuring instrument: the gamma spectrometer.

Scintillators are employed in gamma spectrometry as detectors. These de- vices use a special crystal that produce fluorescence when struck by a high energy photon. A photomultiplier circuit detects light and transduce it then, into a pulse of electricity, which can be analyzed and counted. What makes scintillators interesting is that the output voltage is directly proportional (ideally linear) to the energy of the photon. So if we count the amplitudes, we are counting the photons’ energies, and by graphing those numbers in a histogram, we get a clear picture of the distribution of the energies and therefore we can recognize the element which has emitted them. This technique is known as pulse height analysis or PHA.

On the internet we can find a vast community of amateur physics who like to measure and study radioactivity. Thanks to this large catchment area, gamma spectrometry has become affordable and easy. To keep costs low, as a matter of fact, the FFT analysis is done in a personal computer. The spectrometer is connected to the computer with a common mini-jack audio cable then plugged into the computer’s audio interface. The results of the analysis are saved as graphic and text files.

Ray of Light MCA/S

In order to translate the spectra of the decay into audible frequencies, we have created a special software called Ray of Light MCA/S (Multi Channel Analyzer/Synthesizer), which is connected to a gamma spectrometer with a 1.5” NaI(Tl) scintillator. The software was written with MaxMSP (Max8), a high level programming language originally developed at IRCAM for realtime audio synthesis and control. The sound I/O is handled by an RME Fireface UC professional audio interface at a sampling frequency of 192 KHz. The computer runs a Mac OSX High Sierra operating system.

The software is composed by three modules:
1. A pulse recognition module
2. An FFT analysis module
3. A post-processing module

In the pulse recognition module we separate the scintillator’s pulses from the background electrical noise. It’s a simple threshold detector for now. The signal is fully rectified before going into the analysis module.

The analysis module uses a 1024 bins FFT which measures the amplitudes of every incoming pulse and accumulate them in 512 separate registers’ chan- nels. The output is a list of audio frequencies and relative amplitudes for the synthesis section, and one of energies and occurrences, for the histogram display. In Ray of Light the data is not recorded on a file but instead it is directly forwarded to a bank of 512 sinusoidal oscillators that provide the synthesis engine for realtime sound.

The post-processing module has three sub modules: 1. The synthesis module
2. The mixer and effect module
3. The loopers module

The synthesis module uses one oscillator bank made by 512 cosine func- tion generators connected to their respective amplifiers. The energy range (which corresponds to the pulse dynamic range in the spectrometer) is scaled in the audio frequency at the will of the user and applied to the oscillators. We can chose a direct KeV to Hz correlation (i.e., 662KeV will be trans- lated to 662 Hz) or stretch/compress the range freely. The most important factor is not the output bandwidth, but rather the proportions between the energies. The generators’ magnitude can be smoothed in time with an enve- lope function. With these shaping functions it is possible to craft drone like sounds, short percussive tones, and fast evolving timbres.

The oscillators’s signal is sent to a small mixer which has a number of channels used to control the amplitude of the synthesis engine, a stereo re- verberation, a stereo delay and three looping samplers. The master output has one stereo post-fader full parametric equalizer, for further tone shaping.

The loopers module has three mono looping table players with selectable speed and pitch control (now with time/pitch stretching). The samplers record any sound directed to the master output and play the table in loop.
The in/out points are user’s selectable. These sample players are used to thicken the soundscape with fragments of processed sound and during a performance can be used to sustain the soundscape while preparing the spectrometer for a new realtime measurement.

Results and developements

So far we have analyzed and synthesized a number of sources that we present here to the reader as a starting point in programming sound with gamma spectrometry. We have collected a catalogue of sounds using a re-synthesis approach, but the technique can be adapted to work by subtraction (with filters) o creating more sophisticated layers with the use of granular synthesis. In that case we would use 512 sample players pitched to 512 different values.

The possibility of reading energies levels opens the way to the sonification of specific experiments, such as the production of couples positron-electron, with their corresponding annihilating photo peak, by using, for example, a source of Co-60. Shielding must be implemented to test the possibility of exploration of naturally radioactive food and to maximize the scintillator’s resolution. Other software is currently under development to extend the research to cosmic radiation, with muon detectors.

All the operations with radionuclides follow the ALARA guidelines for the safe use of radioactive sources.

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