There are many useful things you can do with a Raspberry Pi Pico (opens in new tab), as our listing of the best Raspberry Pi Projects (opens in new tab) underlines. However, here’s one we admit we’d never thought of: detecting radiation. Physicist Matthias Rosezky, AKA Nuclear Phoenix (opens in new tab), whose work has also been covered by Hackaday (opens in new tab), has written up a detailed account of building a DIY gamma-ray spectrometer in IEEE Spectrum (opens in new tab).
The device acts a little like a Geiger counter but is more sensitive and can identify the exact combination of isotopes that makes the detector click. Rosezky described the Pico as the ‘natural choice’ for a microcontroller when creating this project. He purchased a small sodium iodine crystal from eBay for $40 and combined it with a silicon photomultiplier. All this was connected to a carrier board, into which the Pi Pico was inserted. A gamma ray produces an electron with proportional energy in the crystal, which excites the atoms as it moves through the structure. This causes photons - light - to be emitted, and by counting the photons, you can know the energy of the gamma ray.
Known as the Open Gamma Detector (opens in new tab), the aim behind the project is to keep the price down, as the devices can cost over $1,000 if purchased from a lab supplier. Measuring just 6cm x 6cm (2.3in square), the custom detector PCB makes up most of the device’s area, as the Pico itself slots into it.
The photons that come out of the crystal are amplified and measured by a photomultiplier, which releases a voltage. This is then increased to a detectable level using a non-inverting operational amplifier, which is picked up by a peak detector and pulse discriminator connected to the microcontroller. Any detections are sent to a recording voice over USB and can also make an audible CLICK sound as a warning. No external sound hardware is required.
Programming comes via the Arduino IDE. Rosezky wrote his own library to calibrate the detector, and all code, as well as specifications for the carrier board, are available on GitHub (opens in new tab), including a sample web app that plots spectra.
One notable issue with the Pico is a differential non-linearity error, speculated to be something to do with the board’s capacitors, that leads to four (out of 4,096) input channels being much more sensitive than the rest. Rosezky utilizes the simple solution of discarding the signals from these channels so as not to have spikes that skew the readings and hopes it will get fixed in hardware soon.