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YOU CAN BUILD A MACHINE THAT MAKES RADIOACTIVITY VISIBLE

A desktop instrument that makes individual particles of radiation visible as white vapor trails, built from a CPU cooler, two Peltier modules and isopropyl alcohol.

by Curious Scientist

FULL CAD BOM FIRMWARE DOCS

ScienceWorkshop

Built with3D printing

difficulty
●●●○○
time
a weekend
license
license not specified
repo
repo 0 stars

WHAT YOU’LL NEED

Videos
1
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COMPAREE VERDICT

A cloud chamber is one of the most direct physics demonstrations you can build: you watch individual alpha and beta particles draw trails through supersaturated alcohol vapor, the same principle that won C.T.R. Wilson the 1927 Nobel Prize. Curious Scientist's build stacks two Peltier coolers (a TEC12715 under a TEC12703) on an ordinary air CPU cooler to chill the chamber floor to about -30°C, creating the temperature gradient needed for tracks to form. His blog post walks through the assembly with photos, exact voltages and currents, a parts list and downloadable 3D files for the small printed chamber. Frank Prins documented a larger version with water-cooled Peltier stacks, printable parts and a complete BOM that runs to about EUR 500, released as CC0. Both builds work and both show live footage of tracks. The single hardest part is not the assembly — it is getting the vapor density and Peltier voltages right, which takes trial runs. If you have never stacked Peltiers or tuned a supersaturated vapor, budget an extra evening. The reward is an instrument that makes the invisible visible, live, on your desk.

GOOD TO KNOW

  • —Curious Scientist's blog post shows the build process with photos, exact voltages and currents, a parts list, and downloadable 3D files for the printed chamber parts.
  • —Frank Prins's alternative build includes a detailed BOM (~EUR 500) and STL files for 3D-printed parts, licensed CC0 (public domain).
  • —Both builds document Peltier stacking, cooling loop assembly, and voltage supply setup. Neither provides circuit diagrams.
  • —The radioactive sources shown (americium smoke detector element, thorium mineral) are legal check sources in most jurisdictions, but sourcing rules vary by country.
  • —No firmware or code — this is a purely physical instrument.
  • —Frank Prins's parts list totals about 504 euros; Curious Scientist does not publish a total, but his build uses far fewer and cheaper parts.

Parts to buy

9 items

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

  • Two Peltier modulesFind
  • Air CPU cooler with fanFind
  • Thermal greaseFind
  • 2 mm clear plastic sheets for the wallsFind
  • 3D-printed clamp partsFind
  • Black nitrile or similar for the floorFind
  • Felt soaked in 99% isopropyl alcoholFind
  • Strong side light (a bike headlight works)Find
  • Legal radioactive check sourceFind

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Can I build this?

PrintCurious Scientist build: two small parts — the stack clamp and the top frame (files on his blog). Frank Prins build: chamber base and mounting parts (files on Thingiverse).
BuyTwo Peltier modules (TEC12715 for the bottom, TEC12703 for the top), an air CPU cooler with fan, 12 V 30 A power supply plus an adjustable DC-DC converter (SZBK07) for the top Peltier, thermal grease, 2 mm clear plastic sheets for the walls, the 3D-printed clamp parts, black nitrile or similar for the floor, felt soaked in 99% isopropyl alcohol, a strong side light (a bike headlight works), and a legal radioactive check source (americium-241 from a smoke detector or a thorium mineral)
ToolsSoldering iron, multimeter, hand tools for assembly, 3D printer if building the Prins version
SkillsIntermediate: you need to wire and power two stacked Peltiers correctly (reverse polarity will heat instead of cool, and the top one needs its own lower voltage), mount them flat with thermal grease, and seal a small chamber. No code.
TimeA weekend if you have all parts and some Peltier experience; a weekend-plus if this is your first time tuning vapor density or stacking coolers.
CostCurious Scientist's small build needs only a 12 V power supply, a CPU cooler, two Peltier modules, a DC-DC converter and small parts, so it can be done cheaply, especially with parts you already have. Prins's larger water-cooled version lists about 504 euros of new parts.
SafetyIsopropyl alcohol is flammable; the chamber is sealed during operation, but assembly and refilling must be done away from ignition sources. Radioactive check sources such as americium-241 from smoke detectors need real caution: the creator calls them hazardous, alpha emitters are most dangerous if ingested, so never open a source, handle it with tools or gloves, wash your hands and keep it away from children. Background radiation alone already gives you tracks. Verify the legality of any source in your jurisdiction. The bottom Peltier draws a lot of current; use an adequate power supply and check connections.

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

Videos

Curious Scientist's video walks through building the chamber and shows it running, with alpha tracks from an americium source.

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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 both build guides (Curious Scientist's post is the small, cheap version on an air CPU cooler. Frank Prins's post (prinsfrank.nl) is a larger water-cooled build with a full parts list totalling about 504 euros and printable parts. Pick one based on your budget.)
  2. 2.Source the Peltier modules and cooler first(Curious Scientist uses a TEC12715 at the bottom and a TEC12703 on top, cooled by a regular air CPU cooler; Frank Prins uses 12715/12712 stacks on AIO water coolers. Match the modules to your power supply.)
  3. 3.Verify local legality of radioactive check sources before buying(Americium-241 from ionization smoke detectors and thorium minerals (e.g. from geology suppliers) are legal in many places, but rules vary. Research your jurisdiction before ordering.)
  4. 4.Test the Peltier stack on the bench before final assembly(Wire the modules in series or parallel (depending on your power supply voltage), apply thermal paste, mount to a heatsink, and verify you get sub-zero temperatures with a thermometer. Reverse polarity will heat instead of cool.)

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

  • Peltier polarity: wiring them backwards turns them into heaters. Test on the bench with a thermometer before sealing the chamber.
  • Vapor tuning takes multiple tries. Too little alcohol and no tracks form; too much and the chamber fogs opaque. Start with the volumes in the blog posts and adjust from there.
  • If you build Frank Prins's water-cooled version, test the AIO pump and radiator outside the final enclosure before committing; Curious Scientist's version uses a plain air CPU cooler and avoids this.
  • Radioactive source strength varies. A weak source (old americium button, low-activity mineral) will show fewer tracks. If you see nothing after an hour, the source may be too weak or the vapor density wrong.
  • The chamber floor (a black surface on the cold Peltier — a piece of black nitrile glove in Curious Scientist's build) must stay very cold. If the Peltier stack cannot pull the floor below -25°C, tracks will be faint or absent. Verify temperatures before blaming the source.

Is this safe to run indoors?

Yes, with normal precautions. The radioactive sources shown emit alpha and beta particles that travel only millimeters in air and are stopped by the chamber walls. The Peltier stack draws around 9 A from a 12 V supply, so use adequate wiring. The main hazard is alcohol vapor during assembly and refilling — do that in a ventilated space away from ignition sources. During operation the chamber is sealed.

Can I use a different cooling method instead of Peltiers?

Dry ice works and is simpler (no power supply, no stacking), but it is consumable and you need a source. Peltiers let you run the chamber continuously without resupply. Liquid nitrogen is overkill and adds handling complexity.

What am I actually seeing in the vapor?

Alpha particles (helium nuclei) leave thick, short tracks. Beta particles (electrons) leave thin, erratic tracks. Cosmic ray muons occasionally leave long, straight trails. You cannot see gamma rays directly because they do not ionize the vapor strongly enough.

How long does the alcohol last?

The felt strip at the top of the chamber holds a few milliliters and that evaporates over hours. You will refill every few sessions. Keep a squeeze bottle of 99% isopropyl nearby.

Can I use this to measure radiation levels?

No. A cloud chamber is qualitative — it shows that radiation is present and lets you see particle types, but it does not count rate or dose. For quantitative measurement you need a Geiger counter or scintillation detector.

Community builds

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Discussion1

FROM THE COMPAREE TEAM

Curious Scientist shows alpha tracks from an americium source taken out of a smoke alarm, but he also notes that background radiation alone gives you enough particles to see. What would you point yours at first — a vintage watch dial, a piece of uranium glass, or just cosmic rays?

CompareeTEAM1mo agoedited

Practical notes from our verification: neither build lives in a code repository — Curious Scientist documents his on a blog with a 27-minute build video and a downloadable zip of the two 3D-printed chamber parts, and Frank Prins wrote up his 2022 version on his own blog under CC0, with a parts list and the 3D models on his Thingiverse profile. The two take different routes to the cold: Curious Scientist stacks two different Peltiers (a TEC12715 under a TEC12703) on an air-cooled CPU cooler, while Prins uses AIO water coolers. Neither uses a high-voltage grid. One trap Prins describes is worth reading before you buy parts: he could not get past about -20 to -25 °C until he stopped using four identical elements. Getting the alcohol-soaked felt or paper and the cooling right takes trial runs, so budget an extra evening for iteration, and respect the Peltier current (around 9 A on the bottom element) and the flammable alcohol. 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.

Curious Scientist

Curious Scientist documents electronics and physics builds on his blog and YouTube channel, with a focus on instruments that make invisible phenomena visible. His Peltier cloud chamber was built as a demonstration of accessible physics hardware. Frank Prins independently published a larger water-cooled version with printable parts and a detailed BOM, released as CC0.

Web

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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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