An improvised electroscope or film badge can demonstrate that ionising radiation is present or has been, and can reveal a gross change between two places or times, but it cannot measure a dose rate and cannot be calibrated without a known source. Because of this, safe areas cannot be distinguished from dangerous ones by improvised means, and distance, time and shielding have to be managed conservatively by assumption.
Part of Radiation Safety
Improvised detectors are home-made devices that respond to ionising radiation, such as the charged-leaf electroscope and film kept in light-tight wrapping. They are indicators, not meters.
The Electroscope Detects Ionised Air
The parent article, Nuclear Safety, describes the simplest improvised detector: a charged conductor with two hanging foil leaves in a jar. The leaves repel each other because they carry the same charge. Ionising radiation ionises the air inside the jar, and the ions carry the charge away, so the leaves fall together. Construction is in Electroscope Construction.
The parent calls it crude: it cannot tell radiation types apart or give precise dose measurements, and it is a go/no-go indicator. This section goes further: it is not even a reliable go/no-go for danger, for three reasons.
- Other things discharge it. Damp air, a dirty or damp stopper, dust, draughts and a charged object nearby all discharge it. The collapse of the leaves is therefore ambiguous.
- Its sensitivity is unknown. Nobody can say what dose rate it reacts to. A device that does not react may be sitting in a field that matters over hours or days.
- It cannot be calibrated. A calibrated version needs a source of known strength to set the scale against. The civil-defence literature has described foil-leaf meters made to be read in dose units, but what made them meters was calibration against a known reference, which a settlement will not have (recalled, not from the corpus).
A Film Badge Records What Already Happened
Photographic film kept in a light-tight wrapper darkens when ionising radiation reaches it. Worn for a period, then developed beside an unworn piece of film from the same batch, the badge can show that an exposure happened. But the film must be developed to be read, so the result arrives after the exposure, never as a warning. Film also fogs with age, heat and light leaks, which is why the unworn control matters. How dark it is says nothing reliable about dose without a calibrated reference.
What These Devices Are Good For
- Showing, qualitatively and by comparison under identical conditions, that one place discharges the device faster than another. Use the same instrument, the same charge, the same time and similar weather, and compare against a reading taken at a known quiet place on the same day, as the parent article advises.
- Teaching the idea of ionisation and keeping a record of relative changes.
- Nothing else.
The Consequence: Manage By Assumption
Without an instrument that reads in dose units, a settlement cannot tell safe from dangerous ground. The only consistent rule is conservative: assume risk where you cannot exclude it, minimise time and keep as far from suspected sources as the task allows. Do not turn a leaf-collapse time into a “safe/unsafe” boundary, and do not publish one.
Common Mistakes
- Declaring an area safe because the electroscope held its charge.
- Declaring it deadly because the leaves fell in seconds. This may be humidity; but retreat is the conservative response.
- Building a scale. Marking a scale on the jar invents a calibration.
- Wearing a film badge as a warning device.
Safety
Retreat on doubt, never advance on a quiet reading
Rapid collapse of the leaves in an area is a reason to withdraw and reassess. A slow collapse is not permission to stay. See Survey Procedures for a conservative approach to unknown ground.
What’s Next
- Learn next: Electroscope Construction — building and testing one
- Combine with: Survey Procedures — using indicators with discipline