Radioactivity is all around us but how do we actually measure it? You probably heard people talk about Siverts, Curies, Becquerels, Count per Minute and many other types units, but which ones should we actually use?
Most units can be put into two categories, activity units and dose units.
Activity units
To measure the activity of radioactive objects we use unit called Becquerels, where 1 Bq is equal to 1 decay per second. If you are located in US, you are probably more used to using Curies where 1 Curie is equal to the activity of 1g of pure Radium 226, which is 37 billion (3.7 x 1010) Bequerels.
Many radiation meters use counts per minute (CPM) or counts per second (CPS) instead of Bequerels. This is because these units show the exact amount of radiation decay events detected by the geiger tube or scintillator used by the meter. Depending on the type of detector, its sensitivity will vary, meaning that some can show 200CPM while others can show 1000CPM for the same source.
When it comes to geiger muller tube detectors, one of the most commonly used by the scientific community is the LND7311, which is often found in the pancake style probes such as Ludlum 44-9.
The activity units are the best way to measure how active an object or an area is.
Dose units
The most common dose units are Sivert and REM (Roentgen Equivalent Man) and 1 Sivert is equal to 100 REM. Both Sievert and REM are used to measure the biological effect of ionising radiation on human tissue. Sievert is a SI unit, while the REM is a part of the older Centimeter-Gram-Second system or CGS for short. Similarly to the case with Becquerels and Curies, Siverts are used in most countries around the world, while the US still sticks with REMs.
Some geiger counters, such as Terra-P, show the readings only in dose units as they have been designed to be used in nuclear contaminated environment and quickly inform the user about the dose they are being exposed to.
These readings can be widely inaccurate when measuring radiation coming from other isotopes to which the meter has not been calibrated too, due to the differences in gamma ray energies that different isotopes produce. Here is an example, most geiger counters are usually calibrated for Cs137 which has gamma energy of 662keV. Americium 241 on the other hand has an energy of 59.5keV, while Cobalt 60 has energies of 1173 and 1332kev. This means that the dose emitted from these isotope wont be presented accurately when measured with a geiger counter that has been calibrated for Cs137. Some GM tubes are gamma compensated and will show more accurate results even when used to measure isotopes outside of their original calibration source, such is the case with the CDV-700 geiger counter.
Distance and inverse square law
When measuring samples, it is important to keep a distance between a source and the detector. This ensures accuracy and consistency, as it adheres to calibration standards and minimises errors from scattering and absorption. Furthermore, it optimises the detector’s sensitivity and efficiency while reducing the risk of contamination or accidental exposure, ensuring reliable and safe measurements.
Conclusion
Personally I try to always give the measurements in counts per minute which I get on my Ludlum Model 3 meter with a 44-9 Pancake Probe at 1cm distance from the source. This way I can ensure that the results are accurate, consistent and comparable between different test. Sometimes I also include a dose rate in uSv/h which I measure using my RAYSID gamma spectrometer which factors in the different gamma energies of the isotopes being detected.
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and remember, stay active!
