COSMICWATCH V3X — DESKTOP COSMIC-RAY MUON DETECTOR
A pocket-sized particle physics detector you can build yourself — it counts cosmic-ray muons hitting your desk in real time.
ScienceOpen-hardware
- difficulty
- ●●●●○
- time
- a weekend-plus
- license
- CC-BY-NC-4.0
- repo
- repo ACTIVE72 stars
●●●●○ · a weekend-plus · CC-BY-NC-4.0 · 72 stars · repo ACTIVE
WHAT YOU’LL NEED
- 3D printer + filament — printable parts — files are in the repo
- Electronic parts — full list with part numbers in the repo BOM
- Dev board / microcontroller — runs the project firmware
Partner
COMPAREE VERDICT
CosmicWatch is the real thing: an MIT-designed muon detector that counts particles born when cosmic rays smash into the atmosphere kilometres above you. A plastic scintillator flashes when a muon passes through, a silicon photomultiplier catches the light, and the screen logs every hit. Put two units side by side in coincidence mode and you can prove the particles are coming from above, not from the detector itself. The build is entirely open — Gerbers, firmware, BOM, assembly videos — and the paper says a beginner's first build takes about four hours of soldering. That is optimistic unless you have done SMD work before; the boards are dense and the SiPM is light-sensitive, so you cannot test it under a bench lamp while powered. The single hardest part is sourcing: the scintillator and silicon photomultiplier are not hobby-store parts, and the BOM does not link to current suppliers. Once built, the detector runs on half a watt over USB and writes timestamped counts to a microSD card. This is particle physics you can hold in your hand, but the licence is CC BY-NC — you can build it, you can teach with it, but you cannot sell it or use it in a product. If you want to measure muon flux as altitude changes or compare count rates across cities, this is the weekend project. If you have never soldered 0805 components, start with the author's videos and expect a second weekend.
IN THE REPO
NOT IN THE REPO
- —Full bill of materials, Gerber files, firmware, and assembly instructions are in the v3X repository.
- —The paper (arXiv:1801.03029) walks through the physics and build process for the v2; v3X is the current design.
- —The scintillator and SiPM are specialised parts — you will not find them at a local electronics shop.
- —Licensed CC BY-NC 4.0 — free to build for personal or educational use, but you cannot sell the device or use it commercially.
- —Spencer Axani (the creator) has a full video series on soldering the boards, but you need real SMD soldering experience.
- —No official project page for ordering a kit — you source everything yourself from the BOM.
Can I build this?
Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.
Partner · KickstarterHeyGears G1: 10M+ colors and transparent parts in one print, plus UV printing on flat objects. Figures, parts and labels — no painting.
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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.Read the arXiv paper (1801.03029) to understand what you are building and why coincidence mode matters. (The paper covers the v2 design, but the physics and measurement principles are identical.)
- 2.Clone the v3X repository and review the BOM — source the scintillator and SiPM first, because lead times can be long. (The BOM is in the repo; part numbers are listed but supplier links may be stale.)
- 3.Order PCBs from the Gerber files and watch Spencer's assembly videos before the boards arrive.(The author's YouTube channel has a step-by-step series; start there if you have not done SMD soldering.)
- 4.Assemble the boards in dim light and test the SiPM bias circuit before powering the sensor under a lamp.(The SiPM is the single most expensive component and the easiest to destroy — do not skip this step.)
KNOWN ISSUES
- The scintillator and SiPM are not standard hobby parts — expect to order from specialised suppliers, and expect a week or more of lead time. Do not start the build assuming you can find them locally.
- The SiPM is ruined if you power it under bright light. Test the bias circuit with the sensor covered or in a dark room, and do not bench-test the assembled detector under a work lamp.
- The licence is CC BY-NC — you can build it, teach with it, publish results, but you cannot sell the device or use it in a commercial product. If you are building for a startup or a client, this licence blocks you.
- The BOM in the repo lists part numbers but not always current suppliers — some parts have been discontinued since the v3X design was published. Budget time to cross-reference and find equivalents.
- Coincidence mode requires two detectors and careful alignment — if you are building one unit to see muons, that works, but proving the particles come from above (not from the detector's own noise) needs two boxes and a geometry that rules out random overlaps.
- The paper's four-hour build time assumes you have done SMD soldering before and have all parts on hand. A first-time builder should expect two evenings: one for assembly, one for debugging.
Do I need a radioactive source to test it?
No — cosmic-ray muons pass through the detector at sea level at about one per minute per square centimetre. If the detector is working, you will see counts within seconds of powering it on. A check source can help verify the scintillator response, but it is not required.
Can I use a different scintillator or SiPM?
Yes, within limits — the scintillator needs to emit in the SiPM's sensitive range (typically blue-green for plastic scintillators), and the SiPM's active area and gain need to match the readout circuit. The BOM lists known-good parts; substitutions are possible but require checking datasheets.
What does coincidence mode actually prove?
Two detectors stacked or placed side by side will both fire when the same muon passes through both. Random background counts (electronic noise, low-energy photons) will not trigger both at the same nanosecond. Coincidence logic filters those out, leaving only real particle tracks.
The licence says non-commercial — does that include educational use?
Educational and research use is explicitly allowed under CC BY-NC. You can build it for a university lab, a school demo, or a science fair. You cannot sell the detector, use it in a commercial service, or incorporate it into a product you charge for.
Community builds
No community builds yet — be the first, we feature the best ones.
Discussion1
FROM THE COMPAREE TEAM
The paper says a first build takes about four hours, and the detector counts roughly one muon per minute per square centimetre at sea level — have you built one, and did the count rate match what the altitude predicts?
Spencer Axani
Spencer Axani designed CosmicWatch as a PhD student at MIT, working to make particle physics accessible outside the large-detector collaborations that dominate the field. The project grew out of a desire to put real cosmic-ray measurements in the hands of students and citizen scientists — muons have been a teaching staple for decades, but the detectors were expensive and institutional. Axani published the design, the build process, and the educational context in Physical Review Applied, and the project has since been built by high schools, universities, and makers worldwide.
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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.
- Building and operating any project is at your own responsibility. Protective equipment, safe workshop practice and compliance with local regulations are the builder’s responsibility.
CompareeTEAM1mo agoedited
Practical notes from our verification: the v3X repository is the current design (v2 is deprecated), the Gerbers and BOM are complete, and the firmware is in the repo ready to flash to the Teensy. The single hardest part is sourcing the scintillator and SiPM — the BOM lists part numbers but not always live supplier links, and some parts have longer lead times than the rest of the build. Spencer Axani has a YouTube playlist with step-by-step assembly videos, which is the best place to start if you have not done SMD work before. The project has been built by schools and labs worldwide, so there is a real community around it, but there is no official kit or project page — you are sourcing everything yourself. The licence is CC BY-NC, which is fine for personal or educational builds but blocks commercial use outright.