GM tube and dual probe scintillator

[jacob1]

Recently I measured a range of sources with two detectors comparing their efficiency. One of the sources used (C-14) was a pure beta emitter and it made me wonder. If a radioactive source decays by beta then gamma, will one decay register as two counts as two radioactive particles are produced. The results showed a relatively low efficiency for the C-14 on both detectors but could this just be down to the low energy beta being harder to detect.

 

[gleem]

If a radioactive source decays by beta then gamma, will one decay register as two counts as two radioactive particles are produced

In general no. There are at least two reasons for this, one particular to GM counters is that when an event is registered the detector is "dead" for a certain amount of time. It is unable to register another event until the present ionization is cleared from the chamber. The length of time to clear the ionization can be as much as 200 microseconds. If particles come in faster than one every 2x10^-4 second the subsequent arriving particles will not be registered. Multiple radiations emitted from a decay are usually very fast.

The other reason and this is more applicable to scintillation detectors or solid-state detectors since the dead times are very short is that the radiation may not be emitted in the solid angle that the detector subtends although it is more likely you will register multiple radiations per decay.

The results showed a relatively low efficiency for the C-14 on both detectors but could this just be down to the low energy beta being harder to detect.

Yes. Typically the window of the detector for betas of energy less than 250 Kev should have an areal density of about 5 mg/cm² or less. Very low-energy Betas are most easily detected with liquid scintillation detectors or gas flow counters which are windowless.

 

[Vanadium 50]

One of the sources used (C-14) was a pure beta emitter

I'm really surprised. C-14 is not a commonly used source, as it is more trouble than it's worth. The beta is low energy, the activity is low (and thus the cost is high), you need to protect against biological uptake, etc. I don't even see it in the usual catalogs for academic sources.

Nothing is a pure beta emitter. The particles exit the source, ionize the material surrounding it (or the detector, like the tube) and as the electrons repopulate those atoms you get x-rays. But as @gleem points out, multiple pluses that happen in the same time window register as a single pulse.