Matthias Rosezky's Open Gamma Detector Turns a Raspberry Pi Pico Into a Gamma-Ray Spectrometer

Physicist Matthias Rosezky has put a Raspberry Pi Pico, and its RP2040 microcontroller, to work in a somewhat unusual project: gamma-ray spectroscopy.

"Not only does the RP2040 have lots of compute power, it hasn’t suffered the kind of shortages afflicting other chips," Rosezky writes of his decision to build the tool, designed to detect and identify sources of gamma-ray radiation, on Raspberry Pi's first in-house microcontroller. "So when I decided to build a cheap DIY scintillating gamma spectrometer, it was the natural choice."

At its simplest, a gamma-ray spectrometer can be used like a Geiger-Müller tube to detect radiation. With a highly-capable sensor at its heart, though, it's considerably more sensitive — enough so that it's possible to identify the composition of isotopes making up a gamma-ray source.

Ordinarily, a gamma-ray spectrometer would be a piece of specialized and expensive lab equipment. Rosezky, though, has been working on making the technology more accessible through the Open Gamma Project — resulting in an open-source relatively low-cost tool driven by the Raspberry Pi Pico.

A sodium iodide crystal scintillator, purchased for $40, is inserted into a silicon photomultiplier (SiPM) — designed to replace the traditional vacuum tube used for the task, being smaller, cheaper, and not requiring high-voltage electronics to operate — and attached to a carrier board for the Raspberry Pi Pico.

"The finished SiPM carrier board is there to allow for easier packaging with the scintillator as well as to be reusable for different detectors," Rosezky explains, "as that's by far the most expensive part and you'll want to use it as long as possible. The software aims to be as simple as possible to understand and maintain."

One wrinkle, however, comes in a known glitch in the design of the Raspberry Pi Pico's analog to digital converter — caused, it is theorized, by capacitors, which are a mere 0.8 per cent out of spec. "Four channels [are] much more sensitive (wider AC input range) than the rest," Rosezky explains of the problem. "For now the simplest solution was to discard all four of them, by printing a 0 when any of them comes up in the measurement (to not affect the CPS [count per second] readings)."

The board designs and source code for the Open Gamma Detector are published to the Open Gamma Project's GitHub repository under the reciprocal GNU General Public License 3. More details are available in Rosezky's write-up for IEEE Spectrum.