I used bananas as source of radiation to test detection of radiation by pair of UKRAA Cosmicwatch coincidence detectors – bananas placed on top of detectors.
Download my results by clicking below:
Comment from Richard on my banana experiment 29/1/2024:
I love bananas – fantastic food source – especially when over ripe (almost black) and very sweet – not everybody’s taste.
Bananas contain both potassium-40 (K-40) and carbon-14 (C-14), both of these isotopes are radioactive – emitting ionising radiations.
For C-14, the decay is via beta particle at Emax=156KeV.
For K-40, the decay is via beta particle at Emax=1.3MeV (about 90% of the time) and gamma ray at E=1.5MeV (about 10% of the time).
So how much C-14 and K-40 is in the “average” banana? Wiki cites C-14 as 5Bq (won’t consider this anymore as small activity and low energy) and K-40 as 15Bq – where a Bq (becquerel) is a unit of radioactivity – effectively, the number of decays per second.
So the K-40 in the “average” banana will produce, on average, 15 decays per second, of which about 90% will be Emax=1.3MeV beta particles.
The questions to then ask is; how far do these K-40 Emax=1.3MeV beta particles travel?
There are some “rules of thumb” that can be used to evaluate this question.
Rule of thumb : Range (in cm) = (Emax/2) / density.
Note: (Emax/2) is used to give an average energy for the beta particles – because they are not monoenergetic. The 1.3MeV is shared between the beta particle and the anti-neutrino that leave the nucleus at decay.
Air has a density of about 0.0012 g/cm^3. So the K-40 beta particle range in air = (1.3/2)/0.0012 = 540cm
What about range of beta particle through the scintillator – this has a density of 1.032 g/cm^3. So the K-40 beta particle range through BC-408 scintillator = (1.3/2)/1.032 = 0.63cm
This means that one K-40 beta particle cannot traverse through one detector scintillator (1.0cm thick) to the second detector scintillator and be recorded as a coincidence event.
As beta particles undergo inelastic collisions – i.e. change direction when they collide with atomic electrons, I suspect that the reason you have an increase in coincidence measurements is because you have effectively increased the background radiation level around your detectors with the addition of the bananas – by about 40Bq (3x15x0.9) – so increasing the chances of there being two independent events triggering the master and slave unit at the same time (or within the time period allowed for coincidence).
As K-40 beta particles are emitted isotropically from the banana, some of the K-40 beta particles are being ejected from the banana sideways, undergoing inelastic collision with an air molecule and being scattered towards the second detector at the same time as a K-40 beta particle is being ejected from the banana towards the first detector (or vis-a-versa) – hence being recorded as a coincidence event..