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A 200 DOLLAR OPEN-SOURCE GAMMA SPECTROMETER THAT IDENTIFIES WHICH ISOTOPE IS IN FRONT OF IT

A Geiger counter only beeps. This measures photon energy and therefore tells you which isotope is in front of it.

by OpenGammaProject

FULL CAD BOM FIRMWARE DOCS

ScienceOpen-hardware

Built withRaspberry Pi Pico / RP20403D printing

difficulty
●●●●○
time
a weekend-plus
license
GPL-3.0
repo
repo ACTIVE377 stars

WHAT YOU’LL NEED

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COMPAREE VERDICT

This is a gamma spectrometer — a device that measures photon energy and therefore identifies radioactive isotopes — built around a NaI(Tl) scintillator crystal, a silicon photomultiplier and a Raspberry Pi Pico 2, with a total parts cost the author puts at around two hundred dollars. The detector gives 4096 channels and up to about seven percent energy resolution at 662 keV, depending heavily on your SiPM and scintillator assembly; the example spectra show caesium-137, americium-241, sodium-22, lutetium-176 and a uranium glaze. The board files, firmware and a printable enclosure are all published, the hardware README covers potentiometer settings and scintillator assembly (with more steps on the Hackaday.io page), and REFERENCE.md explains calibration in the Gamma MCA software. If you have built and debugged mixed-signal boards before, that is enough. If you have not, this will be the project where you learn, and it will take longer than a weekend. The scintillator crystal and the photomultiplier are bought separately, and a damaged crystal or a SiPM assembly that is not light-tight will give you a spectrum full of noise. The SiPM bias is only around 30 volts — no high-voltage photomultiplier tube is involved. The single biggest trap is assuming this will work like a finished instrument: it is a reference design for someone who can read a schematic and chase down a signal.

GOOD TO KNOW

  • —GPL-3.0 licence: commercial use is allowed, but derivative works you distribute must remain GPL and include source.
  • —Board Gerbers, schematic, firmware, example spectra and a printable enclosure are all in the repo.
  • —Assembly notes are in the hardware README (potentiometers, scintillator coupling) with further instructions on the Hackaday.io project page.
  • —The NaI(Tl) scintillator crystal and the silicon photomultiplier must be sourced separately; the repo links some suppliers but availability shifts.
  • —Calibration is documented in REFERENCE.md: pick two or three known gamma peaks and assign their energies in the Gamma MCA calibration tab.
  • —Example spectra are provided for caesium-137, americium-241, sodium-22 and lutetium-176.

Parts to buy

8 items

From our check of the build. Exact quantities and part numbers are in the creator’s BOM.

  • PCB (Gerbers provided)from the repo files
  • NaI(Tl) scintillator crystalFind
  • Silicon photomultiplier (e.gFind
  • MICROFC-60035-SMT-TRFind
  • Raspberry Pi Pico 2Find
  • Optional OLED displayFind
  • Passives and ICsFind
  • Optical coupling compoundFind

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Can I build this?

Printan enclosure (STL files provided)
Buya PCB (Gerbers provided), a NaI(Tl) scintillator crystal, a silicon photomultiplier (e.g. MICROFC-60035-SMT-TR, optionally on the author's SiPM carrier board), a Raspberry Pi Pico 2, an optional OLED display, passives and ICs per the BOM, and optical coupling compound
Toolsa soldering iron, a multimeter, a way to order a PCB (JLCPCB, PCBWay or equivalent), and ideally an oscilloscope for debugging the photomultiplier output
Skillsintermediate to advanced electronics — you are building a mixed-signal board with a SiPM bias supply and debugging it from a schematic
Timea weekend to assemble and another few evenings to calibrate and match spectra, longer if the photomultiplier or crystal arrives faulty
CostMid-range budget, around 200 dollars in total parts according to the author, with the scintillator crystal and the silicon photomultiplier as the specialist items
SafetyThe board generates a SiPM bias of about 27.5 to 33.8 volts — low voltage, but avoid shorts while probing. The device only detects radiation and emits none. Do not handle radioactive sources unless you know what you are doing; the example spectra in the repo come from the author's own measurements.

Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.

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Gallery

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Start here

Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.

  1. 1.Read the hardware documentation (Schematic, Gerbers, BOM, potentiometer settings and scintillator assembly notes.)
  2. 2.Order the PCB and source the scintillator and photomultiplier(The repo links some suppliers but availability changes. Verify the photomultiplier matches the board footprint.)
  3. 3.Flash the firmware to the Pico 2 (Precompiled UF2 provided. Drag and drop.)
  4. 4.Assemble the board and test the photomultiplier output(An oscilloscope helps. If you see no pulses, check the bias voltage and the scintillator coupling.)
  5. 5.Match your spectrum to the example files(Caesium-137, americium-241, sodium-22 and lutetium-176 spectra provided. Calibration is manual.)

Resources

Documentation, files and community threads for this build — we link straight to the original sources and never rehost the creator’s files.

KNOWN ISSUES

  • The scintillator crystal and the silicon photomultiplier are specialist parts — a counterfeit or damaged crystal will give you noise and no clear way to know why.
  • Assembly guidance is split between the hardware README (potentiometer settings, scintillator and SiPM assembly) and the Hackaday.io instructions page. It is written for people who can work from a schematic, BOM and photos — if that makes you nervous, pick a different project.
  • Calibration is manual: the FAQ explains how to calibrate in Gamma MCA using two or, better, three known gamma peaks, so you need a reference sample with known energies.
  • The board generates a SiPM bias of roughly 27.5 to 33.8 volts — low voltage, but enough to damage parts if you short it — so probe carefully and set the bias with the potentiometer as the hardware README describes.
  • If you have never debugged a mixed-signal board before, expect this to take longer than a weekend — possibly much longer if something is wrong with the photomultiplier or the crystal.
  • The GPL-3.0 licence means derivative works must also be GPL. If you plan to sell a modified version, read the licence first.

What is the difference between this and a Geiger counter?

A Geiger counter registers that radiation is present. A gamma spectrometer measures the energy of each photon, and because every isotope emits at its own signature energies, the shape of the spectrum identifies it. Caesium-137 peaks at 662 keV, americium-241 at 26 and 60, sodium-22 at 511 and 1275.

Is seven percent resolution good?

Good enough to separate the common isotopes. The author says this design typically gives 8 to 10 percent at 662 keV, up to about 7 percent with a good SiPM and crystal, and that around 6 percent is the limit for most NaI or CsI scintillators anyway.

Where do I get radioactive sources to test it?

Do not. The example spectra in the repo come from the author's own measurements. Some smoke detectors contain americium-241, some old camera lenses contain thorium, and potassium-40 is in bananas and salt substitutes, but if you do not already know how to handle radioactive materials safely, this is not the project to learn on.

Can I use this to measure dose or check if something is safe?

No. It measures energy spectra, not dose rate, and it is not calibrated for survey work. If you need to know whether something is safe, you need a professional survey meter.

What is the biggest single thing that will go wrong?

A faulty or counterfeit scintillator crystal or photomultiplier. If either is bad, you will see noise and nothing else, and distinguishing a bad part from a bad solder joint or a firmware bug is hard without an oscilloscope and experience.

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Discussion1

FROM THE COMPAREE TEAM

Four thousand channels of resolution, for two hundred dollars. What isotope would you point it at first?

CompareeTEAM2mo agoedited

Practical notes from our verification: the repository contains Gerbers, EasyEDA source files, firmware for the Raspberry Pi Pico 2, example spectra and printable enclosures, and the hardware folder documents assembly and potentiometer settings, with more build notes on the project's Hackaday.io page. Calibration is described in REFERENCE.md: you use the Gamma MCA software and two or three known peaks. The single biggest variable is the scintillator crystal and the silicon photomultiplier: both are bought separately from specialist distributors, and your energy resolution depends heavily on that assembly. If you have built mixed-signal boards before, this is a manageable project; if you have not, expect it to take longer than a weekend. The board generates the SiPM bias voltage (the supported range is roughly 27.5 to 33.8 volts), so do not probe it carelessly while powered. Correction (4 October 2026): we re-checked this page line by line against the project's own repository, documentation and videos, and fixed errors in earlier versions.

OpenGammaProject

An open hardware project to make gamma spectroscopy accessible outside professional laboratories. The design is built around widely available components and a Raspberry Pi Pico 2.

GitHub

Star the project on GitHub

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  • Information is provided without warranty and may become outdated as projects evolve. Prices are indicative bands only — always check the creator’s parts list for current costs.
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