What Is CR-39 (PADC) and How Is It Used in Radiation Detection?
CR-39 is a transparent thermoset plastic — chemically polyallyl diglycol carbonate (PADC) — used as a solid-state nuclear track detector. When ionizing particles such as alpha particles or neutrons (via recoil protons) pass through it, they leave latent damage trails that can be enlarged by chemical etching and then counted. It is chosen for radon measurement and neutron dosimetry because it is sensitive to these particles while producing a low background signal. The practical catch is that not all CR-39 is equal: dosimetry-grade material is manufactured and handled to strict tolerances, because detector quality directly affects how reliably tracks can be etched and counted.
The track-etch principle
CR-39 does not produce an electrical signal the way a Geiger counter does. Instead it stores radiation damage permanently in the plastic:
- Damage. An incoming charged particle breaks chemical bonds along its path, leaving a narrow latent track.
- Etching. The plastic is immersed in a chemical etchant (typically a hot alkaline solution). Etchant attacks the damaged material faster than the surrounding bulk plastic, so each latent track opens into a visible pit or cone.
- Counting. The etched tracks are counted — manually under a microscope or automatically with an image-analysis system — and the track density is converted into a radiation dose or concentration.
Because the tracks persist, the detector integrates exposure over time rather than measuring an instantaneous rate. That makes it well suited to long-term radon surveys and cumulative neutron dose monitoring.
Why CR-39 is used instead of other detectors
| Property | Why it matters |
|---|---|
| Sensitivity to alpha particles | Alpha particles from radon and its progeny are detected efficiently, which is the basis of radon measurement. |
| Sensitivity to neutrons via recoil protons | Fast neutrons collide with hydrogen in the plastic, producing recoil protons that leave etchable tracks — the basis of neutron dosimetry. |
| Low intrinsic background | Fewer spurious tracks means a lower detection limit and more reliable low-dose results. |
| Passive, integrating operation | No power or electronics needed at the measurement point; exposure accumulates over the deployment period. |
| Permanent record | The etched detector can be stored and re-examined if a result is questioned. |
The trade-off is that CR-39 gives you a cumulative track count, not real-time data, and the result depends on controlled etching and counting conditions.
Typical applications
- Radon measurement. CR-39 is used in passive radon detectors placed in buildings for weeks to months. Alpha particles from radon decay leave tracks that are etched and counted to estimate radon concentration.
- Neutron dosimetry. In neutron fields, recoil protons from neutron collisions create tracks, allowing cumulative neutron dose to be assessed.
- Nuclear technology and research. The same track-detecting property supports particle identification and other nuclear measurement work.
TASL (Track Analysis Systems Ltd), a Bristol-based specialist, manufactures dosimetry-grade TASTRAK PADC plastic — described on its site as CR-39 — and supplies it for radon measurements, neutron dosimetry, and nuclear technology, alongside the TASL Image system for analysing the etched tracks.
Practical considerations
- Dosimetry-grade quality. The material must be manufactured and processed to consistent standards; impurities or inconsistent thickness degrade track quality and reproducibility.
- Controlled etching. Etchant concentration, temperature, and time all affect track size and shape, so etching is a calibrated step, not a rough dip.
- Counting method. Tracks can be counted manually or by automated image analysis. Automated systems improve throughput and consistency for large batches.
- Accreditation context. TASL states it is ISO-accredited in laboratories across Europe, HSE-accredited in the UK, and NVLAP-accredited in the USA — relevant if you need results from an accredited measurement route.
If you are specifying CR-39 for a project, the key questions are the grade of the plastic, the etching and counting protocol, and whether the laboratory providing the result holds the accreditation your application requires.