CR-39, Explained
CR-39 is a clear plastic that can do something most plastics can't: record the exact path of a single radiation particle passing through it, permanently, with no power source, and reveal it later as a visible mark under a microscope. It's the same base material used in eyeglass lenses — but manufactured and used a very different way when the goal is detecting radiation instead of correcting vision.
This page explains what the material is, where the name comes from, and how it works. If you already know that and want specifications, pricing, or to order detectors, see the CR-39 product overview or ordering page.
The Material
"CR-39" is short for Columbia Resin #39 — the 39th formulation in a research series run by the Columbia Southern Chemical Company (later part of PPG Industries) in the early 1940s. Chemically, it's poly(allyl diglycol carbonate), a thermoset polymer originally developed for its optical clarity, low weight, and resistance to scratching and impact. Those same properties made it a standard eyeglass lens material for decades, and lens-grade CR-39 is still common today.
Its second career started when researchers found that ionizing radiation passing through the plastic leaves permanent, chemically-reactive damage along its path — and that this damage can be revealed and made visible. That discovery turned an optical plastic into what's called a solid-state nuclear track detector (SSNTD): a passive sensor that needs no electricity, no batteries, and no maintenance while it's deployed, because it isn't "reading" anything in real time. It's simply recording.
The Mechanism
Three steps turn an invisible event — a single particle crossing a sheet of plastic — into a mark you can see and count.
A charged particle — an alpha particle, a heavy ion, a fission fragment — plows through the polymer and breaks chemical bonds along a narrow trail. This is called a "latent track": it's already there, but invisible to the eye and to a microscope at this point.
The damaged material along the track is chemically more reactive than the undamaged bulk plastic around it. Soaking the chip in a heated etching solution (commonly sodium hydroxide) dissolves the damaged trail faster than the surrounding plastic, carving it out into a visible pit.
What was an invisible damage trail is now a pit or track large enough to see and measure under an ordinary microscope. Counting tracks per unit area — or classifying their size and shape — tells you how much radiation the chip was exposed to, and what kind.
Selectivity
Not every kind of radiation leaves a track CR-39 can resolve — and that selectivity is usually an advantage, not a gap.
| Radiation type | Detected by plain CR-39? | Why |
|---|---|---|
| Alpha particles | Yes | Heavy, densely-ionizing — leaves a strong, easily-etched track |
| Heavy ions / fission fragments | Yes | Even more densely-ionizing than alpha particles; strong track response |
| Neutrons | Yes, with a boron coating | Boron-coated CR-39 converts neutrons to detectable alpha particles via the 10B(n,α)7Li reaction — see BSI's passive neutron detection page |
| Beta particles | Generally no | Too lightly-ionizing to leave a resolvable track in standard use |
| Gamma rays / X-rays | Not directly | Lightly-ionizing; CR-39 is essentially transparent to them for track-counting purposes |
In practice, this means CR-39 gives you a clean measurement of alpha, heavy-ion, and (with a boron coating) neutron exposure, largely undisturbed by the beta and gamma background that's often present alongside them.
Real-World Use
A small CR-39 chip left in a home or building for weeks to months integrates alpha exposure from radon decay products — see BSI's radon detection page.
Worn as a badge, CR-39 gives a passive, cumulative record of alpha or neutron exposure over a work period, with no batteries to fail.
Students see individual particle tracks instead of a trusting a black-box counter number — see BSI's teaching lab page.
From fission-fragment studies to SEM-based automated track analysis, CR-39 remains an active research tool — see BSI's published work.
Frequently asked questions
Yes, chemically. CR-39 is poly(allyl diglycol carbonate), and it has been one of the most common plastic eyeglass lens materials since the 1940s because it is lightweight, optically clear, and scratch-resistant compared to earlier plastics. The lens-grade material and the radiation-detector-grade material are the same base polymer; detector-grade CR-39 is manufactured and cast with the uniformity and low background track density that a track-counting measurement needs.
No. CR-39 is a passive detector — it records damage from particles as they pass through it, with no power source, electronics, or maintenance required while it's deployed. The tradeoff is that you don't get a live reading: the result only becomes visible after the chip is chemically etched and examined, after exposure is complete.
A Geiger-Müller counter is an active, powered instrument that gives you a real-time count rate the moment radiation is present. CR-39 is a passive integrating detector: it accumulates a permanent record of particle damage over the entire exposure period, but you only see the result afterward, once the chip is etched and the tracks are counted under a microscope. Geiger counters are good for immediate rate monitoring; CR-39 is good for unattended, long-duration, or distributed measurements where you care about total exposure rather than a live reading.
Not directly, and that's normally a feature rather than a limitation. CR-39 responds to densely-ionizing particles — alpha particles, heavy ions, protons, and fission fragments — because those particles leave a concentrated damage trail dense enough to etch into a visible track. Gamma rays, X-rays, and beta particles are lightly ionizing by comparison and generally don't produce a track CR-39 can resolve, so a CR-39 measurement is naturally selective for the particle types it's used to detect.
No. CR-39 is an ordinary, stable plastic and is not radioactive. It is a passive sensor for radiation coming from an external source (a sample, a room, a soil gas, an exposed material); the plastic itself poses no radiation hazard before, during, or after use.
Next step
BSI manufactures radiation-grade CR-39 in the US — standard and boron-coated chips, custom sizes, and class sets for teaching labs, from $2/cm².