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YOU CAN BUILD A PARTICLE DETECTOR THAT COUNTS PIECES OF SPACE PASSING THROUGH YOUR DESK

A particle detector you can solder together yourself that counts muons from space as they pass through your desk.

by Spencer Axani

FULL CAD BOM FIRMWARE DOCS

ScienceOpen-hardware

Built withRaspberry Pi Pico / RP2040

difficulty
●●●○○
time
a weekend-plus
license
CC-BY-NC-4.0
repo
repo ACTIVE84 stars

WHAT YOU’LL NEED

Videos
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COMPAREE VERDICT

This is a real particle physics instrument scaled down to a desktop build, designed by physicist Spencer Axani so students can assemble their own detectors. A block of plastic scintillator emits a faint light pulse when a muon passes through, a silicon photomultiplier catches it, and a Raspberry Pi Pico-based board counts the events, logs them to microSD and shows the rate on an OLED. The author's purchasing list puts the parts at roughly 200 dollars for a single unit (less when bought in bulk for a class), and most of the build time goes into careful SMD soldering and making the scintillator light-tight. The single biggest trap is the SiPM: it is the priciest part, and if you order a different model the detector will not work as designed. The second is the licence: CC BY-NC 4.0, so personal and educational builds are fine but selling kits needs the author's permission. If you want coincidence detection, you need two complete units linked by an Ethernet cable. What you get is an instrument that genuinely detects cosmic ray muons, and the author has flown them to watch the count climb with altitude.

GOOD TO KNOW

  • —PCB Gerbers, circuit diagram, Raspberry Pi Pico firmware (.uf2), a Python GUI, an illustrated instruction manual and a full purchasing list are in the v3X repository.
  • —Licence is CC BY-NC 4.0 (README and LICENSE file): build and modify for personal or educational use; commercial use and redistribution need the author's permission.
  • —The v3X purchasing list prices parts in bulk for classes; the author notes a single unit costs roughly 200 dollars.
  • —The silicon photomultiplier (SiPM) is the most expensive single part (on Digi-Key); the plastic scintillator comes from specialist sellers (the list links eBay), so check stock and lead time.
  • —Two-detector coincidence mode needs two complete units connected with an Ethernet cable; the same firmware handles it when both are reset within a second.
  • —Active repository with recent commits; Spencer Axani responds to issues.

Parts to buy

10 items

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

  • Custom PCB (Gerbers provided)from the repo files
  • SensL MicroFC-60035-SMT SiPMFind
  • 50x50x10 mm plastic scintillatorFind
  • Raspberry Pi Pico (RP2040)Find
  • 128x64 OLEDFind
  • BMP280 sensorFind
  • AccelerometerFind
  • MicroSD socketFind
  • Passives per the purchasing listFind
  • Optional aluminium split-body case with acrylic end platesFind

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Can I build this?

Printnothing required; the optional case is an aluminium split-body enclosure with laser-cut acrylic end plates (files for the plates are in the Enclosure folder)
Buycustom PCB (Gerbers provided), SensL MicroFC-60035-SMT SiPM, 50x50x10 mm plastic scintillator, Raspberry Pi Pico (RP2040), 128x64 OLED, BMP280 sensor, accelerometer, microSD socket, passives per the purchasing list; optional aluminium split-body case with acrylic end plates
Toolssoldering iron (fine SMD work), multimeter, PCB fabrication (upload Gerbers to JLCPCB/similar), optional laser-cutting service for the case end plates
Skillsintermediate soldering (mostly 0805 SMD parts; the SiPM is small and static-sensitive), drag-and-drop firmware upload to a Raspberry Pi Pico, careful assembly following illustrated steps; no prior particle physics knowledge required
Timeexpect several sessions for a first build: hand-soldering the mostly 0805 SMD parts, wrapping the scintillator light-tight and testing; PCB fabrication adds a week or more, scintillator shipping can add another
Cost$$ — roughly 200 dollars for a single unit per the author's purchasing list (cheaper per unit when bought in bulk for a class); the SiPM and scintillator dominate; a second detector for coincidence mode doubles the cost
SafetyThe SiPM runs at ~30V DC bias, which is low current but the voltage requires care during assembly and testing. The scintillator is non-toxic plastic. Static discharge can destroy the SiPM during soldering. No mains voltage, no radiation hazard, no lasers — the detector measures natural background muons, not a radiation source.

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

Videos

CosmicWatch -- Part 1: Introduction9:28

Spencer Axani's 2018 introduction to the earlier CosmicWatch detector; the v3X hardware differs (Raspberry Pi Pico board, new sensors), but the physics and the build idea carry over.

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Gallery

github.com

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 Instruction Manual.pdf' in the repository root(Illustrated step-by-step with photos; follow the PCB population order and the SiPM handling steps carefully)
  2. 2.Order the PCB using the Gerber files in the PCB folder (Upload the Gerbers to JLCPCB or similar; minimum order is usually 5 boards)
  3. 3.Source the SiPM and scintillator from the BOM links(These are the two hardest parts to find; the BOM has supplier links but check stock and lead time before ordering the rest)
  4. 4.Flash the firmware from the Firmware folder (Copy the .uf2 file onto the Raspberry Pi Pico in boot mode; the Python GUI in the GUI folder runs on your computer to log and plot the muon rate.)

KNOWN ISSUES

  • The SensL MicroFC-60035-SMT SiPM is a specific model; ordering a different SiPM or a used/counterfeit part will result in a non-working detector or wrong count rates.
  • The scintillator must be real plastic scintillator, 50x50x10 mm as in the purchasing list; ordinary acrylic will not scintillate and the detector will see nothing.
  • The design is CC BY-NC 4.0: you cannot sell built detectors or kits, or use it commercially, without explicit permission from the author.
  • The SiPM is static-sensitive; ground yourself and use ESD precautions during soldering or a single touch can kill it before you power up.
  • The SiPM bias (about 30 V from the on-board boost converter) is fixed by precision resistors, so use exactly the resistor values in the purchasing list (154k and 6.65k, 0.1 %); a substitute changes the bias and the detector response.
  • For coincidence detection you need two complete detectors linked by a straight-through Ethernet cable; reset both within a second of each other and they boot into coincidence mode. The parts cost doubles.

Is this safe? Am I detecting dangerous radiation?

You are detecting cosmic ray muons, which are a natural background particle that has always been passing through you. The detector does not emit radiation and the muon rate it measures is normal environmental levels, not a hazard. It is not a dosimeter and does not replace radiation safety equipment.

Can I use a different SiPM or scintillator to save money?

Not reliably. The fixed bias of about 30 V and the analog front end are designed around the onsemi (formerly SensL) MICROFC-60035-SMT SiPM; another SiPM may have a different breakdown voltage and gain. The scintillator must be genuine plastic scintillator of the listed 50x50x10 mm size. Swapping either means re-engineering the detector.

What does coincidence mode actually give me?

Two detectors linked in coincidence mode flag only the events that both of them see within a three-microsecond time window. This rejects most radioactive background and electronic noise, so the coincident events are overwhelmingly cosmic-ray muons. It is the same idea used in large physics experiments. You need two full builds to do it.

What licence is it under?

Creative Commons Attribution-NonCommercial 4.0. Building and modifying for personal or educational use is fine; selling kits, commercial use or redistribution needs explicit permission from Spencer Axani.

Will I actually see muons, or is this theoretical?

You will see them. At sea level roughly one muon crosses each square centimetre per minute. A single v3X triggers about three times per second (muons plus some background), and with two detectors in coincidence mode the manual's example shows about 0.37 coincident events per second, which are overwhelmingly muons. The repo also ships data logged on an airplane flight, where the rate climbs with altitude.

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Discussion1

FROM THE COMPAREE TEAM

The repo even ships example data logged on a flight, where you can watch the muon rate change with altitude. If you built one, what would you measure first?

CompareeTEAM1mo agoedited

Practical notes from our verification: the v3X repository is active and comes from Spencer Axani's group at the University of Delaware. It ships the Instruction Manual, a Troubleshooting document and a Physics document as PDFs, Gerber files for the PCB, a purchasing list, ready-to-flash firmware for the Raspberry Pi Pico and a Python GUI for reading data. The licence is CC BY-NC 4.0, so personal and educational builds are fine but commercial use needs the author's permission. Older write-ups quote figures from the original 2016 CosmicWatch paper; for v3X the purchasing list puts a single detector at roughly two hundred dollars. The biggest practical hurdle is sourcing the silicon photomultiplier (onsemi MICROFC-60035) and the plastic scintillator, because these are not everyday electronics parts and stock varies, so order exactly what the list names. 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.

Spencer Axani

CosmicWatch started at MIT, where Spencer Axani, Katarzyna Frankiewicz and Janet Conrad designed a muon detector students could build themselves. Axani, now a professor at the University of Delaware, published this v3X redesign in 2025 with a Raspberry Pi Pico, microSD logging and environmental sensors. Detectors from the project have been used in teaching and taken on planes and high-altitude balloons, and v3X is meant as the backbone of a future citizen-science project.

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.
  • 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.
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