How Radtrak3® radon detectors work: CR-39 technology from the inside
A small device, a great science
Anyone who receives a Radtrak3® radon detector for the first time is usually surprised by its size: a black plastic capsule that fits in the palm of your hand, with no cables, no screen, no battery. It does not look like a measuring instrument. And yet, inside that unassuming housing lies one of the most reliable nuclear detection technologies in existence: CR-39 film.
What is inside a Radtrak3®
The Radtrak3® detector, manufactured by Radonova Laboratories, is a passive alpha track measurement device. Its interior is surprisingly simple and consists of two fundamental elements.
The first is the antistatic plastic housing, a small capsule made from a special material that allows radon gas to enter by diffusion while blocking dust and the radon decay products already present in the surrounding air. This housing acts as what is technically known as a diffusion chamber.
The second element, and the true heart of the detector, is the CR-39 film: a small, transparent sheet made from a polymer called PADC (polyallyl diglycol carbonate). This is the component that physically records the presence of radon in the air.
There is no electronics, no battery, no active components. The detector operates in a completely passive way, making it immune to electrical interference, power outages, or accidental tampering.
CR-39: a plastic with a history
The name CR-39 comes from Columbia Resin #39, the thirty-ninth resin formula developed in the laboratories of the Pittsburgh Plate Glass Company. This material was first used during the Second World War in the fuel tanks of bomber aircraft, valued for its transparency, strength, and light weight.
Over time, an extraordinary property of this polymer was discovered: when an alpha particle passes through its structure, it breaks the molecular bonds of the plastic and leaves a trail of microscopic damage — invisible to the naked eye but perfectly detectable in a laboratory. This characteristic made CR-39 the most widely used solid-state nuclear track detector (SSNTD) in the world.
Today, beyond radon detection, CR-39 continues to be used in personal radiation dosimetry, nuclear physics research, and even in the manufacture of high-quality ophthalmic lenses.
How it records radon: alpha tracks
The detection process is based on the physics of radioactive decay and unfolds in four stages.
It all begins when radon-222 (²²²Rn) gas present in the air slowly diffuses through the detector housing. The diffusion chamber filter allows the gas to pass through while blocking solid particles and the radon progeny already present in the environment.
Once inside the chamber, radon atoms spontaneously decay. In the decay chain of radon-222, high-energy alpha particles (helium nuclei) are emitted — both from the radon itself and from its short-lived progeny: polonium-218 (²¹⁸Po) and polonium-214 (²¹⁴Po).
Each alpha particle that strikes the CR-39 film breaks the chemical bonds of the polymer along its path, leaving what is known as a latent track: a trail of molecular damage invisible to the naked eye.
The higher the radon concentration in the environment and the longer the detector is exposed, the more tracks accumulate on the film. This proportional relationship between tracks and exposure is the basis for all subsequent calculations.
Laboratory readout: chemical etching
When the detector is sent to the laboratory after the exposure period (between 2 and 12 months for the Radtrak3®), a fascinating process begins.
Chemical etching
The CR-39 film is immersed in a concentrated sodium hydroxide (NaOH) solution, typically at a concentration of 6.25 N and a temperature of 80 °C for several hours. This alkaline solution attacks and dissolves the polymer uniformly across its entire surface. However, in the areas where alpha particles have left latent tracks, dissolution progresses much faster, because the molecular bonds were already broken.
The result is the formation of small conical pits visible under a microscope on the surface of the film. Each pit corresponds to the impact of a single alpha particle.
Automated counting
Once the film has been etched, it is analyzed using a high-resolution image scanner connected to a computer system. The software automatically identifies, counts, and classifies the tracks, discarding imperfections and artifacts. The number of tracks, combined with the volume of the diffusion chamber and the duration of the exposure, allows the mean radon concentration during the measurement period to be calculated with precision, expressed in Bq/m³.
We recommend this video by José Luis Gutiérrez Villanueva, a radon measurement specialist at Radonova, in which he explains how alpha track detectors work:
Why the Radtrak3® and not another method
Alpha track technology in CR-39 has specific advantages that make it particularly well suited for radon measurement in homes and workplaces.
Because it operates in a completely passive way, the Radtrak3® requires no electricity or battery. You place it and forget about it. Gamma radiation and ambient humidity do not affect it either, since CR-39 is selective to alpha particles and inert to other forms of radiation. This robustness makes it an exceptionally reliable detector in any environment.
Its capacity for long-term measurement is another key advantage. The Radtrak3® can record tracks over periods of 2 to 12 months, making it possible to obtain a true annual average of radon concentration — which is precisely what Spanish regulations require. The measurement range spans from 15 Bq/m³ to 25,000 Bq/m³ over 3-month periods, covering both low levels and high-concentration situations.
Radonova Laboratories is accredited under ISO/IEC 17025 and regularly takes part in international intercomparisons, ensuring the traceability and reliability of every result. And because the device contains no electronics, it is inexpensive to manufacture, enabling large-scale measurement campaigns at an accessible cost.
The importance of long-term measurement
Radon concentrations in an indoor space can vary considerably throughout the day, the week, and across seasons. Factors such as ventilation, outdoor temperature, atmospheric pressure, and building use all directly influence radon levels.
For this reason, both Spanish regulations (Real Decreto 1029/2022) and Instruction IS-47 from the CSN establish that the estimation of the annual average must be based on measurements lasting at least 3 months. A detector like the Radtrak3®, capable of measuring for a full year, provides the most accurate picture possible of actual exposure.
A passive alpha track detector like the Radtrak3® does not measure radon concentration in real time. It records the cumulative exposure over the entire measurement period, and the mean value is calculated in the laboratory. This is precisely its strength: it delivers a representative figure that does not depend on momentary spikes or transient conditions.
In summary
Inside a Radtrak3® there is, essentially, a small sheet of CR-39 plastic protected by a housing that acts as a diffusion chamber. The alpha particles emitted by radon and its decay products leave microscopic tracks in the plastic, which are then revealed through chemical etching and automatically counted in the laboratory. The result is a precise, reliable, and representative measurement of long-term mean radon concentration.
It is a technology with more than half a century of development, validated by decades of scientific research and millions of measurements worldwide. Simple in concept, rigorous in execution.
References:
Radonova - El detector de radón CR-39: características y curiosidades