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Units of Measure

Activity (Bq, Ci, Rd) & Specific Activity (Bq/g)

The activity is the amount of radiation a nuclide emits per unit of time. It is thus a physical property of that nuclide. It is measured in becquerel (Bq), which is just the number decays per second. Non-standard units are also the curie (Ci), which is equal to 37 GBq, or the rutherford (Rd), which is equal to 1 MBq. It is also useful to express the activity per unit of mass, thus in becquerel per gram (Bq/g). This allows to quickly get the activity of a sample, just multiplying its specific activity in Bq/g for the mass of the sample. The specific activity SA is deeply connected to the half-life T1/2 of the nuclide: \[ S_A = \frac{N_A\,\ln 2}{T_{1/2}\,m} \] where NA the Avogadro’s number, and m the molecular weight of the nuclide. Espressing the half-life in seconds and the molecular weight in grams per mole (g/mol), the specific activity in Bq/g can be obtaied by \[ S_A\,[Bq/g] = \frac{4.17\cdot 10^{23}\,[1/mol]}{T_{1/2}\,[s]\,\,m\,[g/mol]} \] This allows easy calculations of the activity of radioactive samples, for example:

NuclideHalf-life (years)Molecular
Potassium-401.252 10940 g/mol264.5 KBq/g0.117 mg31 Bq
Thorium-23214 109232 g/mol4.06 kBq/g1 g4 kBq
Neptunium-2372.144 106237 g/mol26 MBq/g8.7 ng0.23 Bq
Americium-241432.2241 g/mol126.9 GBq/g0.29 μg37 KBq

The masses in this table are picked arbitrarily, usually matching numbers shown below.

Count rate (CPM)

The count rate is the most immediate unit of measure of radioactivity. It is usually expressed in counts per minute (CPM) or counts per second (CPS). As the name says, it is the number of events your detector measure per unit of time. This is exactly what your detector measure. Unfortunately, it is difficult to compare count rate with other detectors or reference data. Since each detector is different from another the count rate can change significantly between two detectors.


STS-5 tube from Radioactive@Home BOINC project (from 2012, version 2.51).

STS-5 is very similar to the more common SBM-20; both russian detectors. Detector is cilindrical tube covered by a thin metal shield. Moreover the tube and circuits are inside a plastic case for protection. This implies α radiation is mostly shielded, as well as most of β (probably). This makes the setup essentially a γ detector only. Any other tube rated for 400V can be used with the ame circuitry.

Detector is powered by USB, have a nice LCD display, make noise buz at each detection, and –most importantly– send data to computer for logging. The interface to the computer is via a hid interface (see code on github).

Conversion factor from CPM to μSv is 171.2 (magic number, just use it…).

Last update: 29 June 2018