No human sense detects ionising radiation, and the improvised methods available without a calibrated instrument, an electroscope, a film badge and observation of plants and animals, can show at most that radiation is or was present, not how strong it is. Without a calibrated instrument, safe and dangerous areas cannot be told apart, so distance, time and shielding must be managed by conservative assumption rather than by measurement.
Part of Radiation Safety
Detection means finding out whether and how strongly radiation is present. Measuring a dose rate takes a calibrated instrument; everything cruder is an indicator.
What Each Method Can And Cannot Tell You
| Method | Can show | Cannot show |
|---|---|---|
| Calibrated survey instrument (salvaged) | Dose rate or count rate, if working and checked | Anything if flat battery, wrong range or unchecked |
| Electroscope | That ionising radiation is present, or a gross change | A dose rate; it cannot be calibrated without a known source |
| Film badge | That film was exposed, after it is developed | A warning; it records only what has already happened |
| Plants and animals | That something harmed them, weeks later | Radiation as the cause, or any current level |
| Knowledge of the site | What might be there | What is there now |
A Salvaged Instrument Is Valuable And Still Needs Checking
Hospitals, laboratories, industrial sites and civil-defence stores held survey meters. A salvaged meter is the only way to get a real dose-rate reading, but treat it with suspicion: check the battery, check that it responds to anything, and check which unit and range the dial shows (see Units of Measurement). A Geiger-type counter usually reports counts, not dose. Some Geiger tubes in a very intense field can read low or zero (recalled, not from the corpus), so a reading that falls to nothing near a suspected strong source must not be trusted as clean. Without a known source to test against, the instrument’s calibration is also unknown.
Improvised Methods Are Indicators, Not Meters
An electroscope charged by hand loses its charge faster when ionising radiation ionises the air inside; see Improvised Detectors and Electroscope Construction. Humidity does exactly the same, so the leaf collapsing does not prove radiation. Photographic film kept in a light-tight wrapper darkens with exposure, which is how radioactivity was first noticed in the 1890s (general history, recalled), but it must be developed afterwards, which means it can never warn you in time.
The Decision You Face Without An Instrument
If you cannot measure, you must decide by assumption. The honest rules are the conservative ones:
- Assume ground and air in any affected area are hazardous until you have good reason, such as distance from the event and time elapsed, to think otherwise. The 7-10 rule says that rate falls by a factor of 10 for each sevenfold increase in time, so waiting helps; see Fallout Patterns.
- Keep as far away from any suspected source as the task allows.
- Keep every visit short, and record who went where and for how long.
- Take your decisions from timing and site knowledge, not from an improvised reading.
That is an argument for conservatism, not for improvising a number.
Common Mistakes
- Reading a slow leaf collapse as “safe”. An electroscope of unknown sensitivity does not detect dangerous gamma fields reliably.
- Reading a fast collapse as a measurement. It could be damp air.
- Trusting a dead or unchecked meter.
- Treating “no reading” as “no contamination”. Some contamination is not detected by a particular instrument at all.
Safety
No result from an improvised device clears an area
A device that gives no alarm cannot clear a place for people to live or work. Use distance and time since the event as your primary controls, and enter unknown areas only for a specific purpose, briefly.
What’s Next
- Learn next: Improvised Detectors — what the home-made devices can do
- Combine with: Survey Procedures — a systematic approach to an unknown area