Gamma Spectrometry Forum

Hello all,

I am not a physicist and my question is probably pretty dumb, but I am really puzzled...

When an atom undergoes a beta+ decay, a positron and a neutrino are expelled. The positron then annihilates with an electron, releasing two gamma photons with the characteristic 511 kev energy.

A single gram of potassium with the natural potassium isotope composition has 31Bq of gamma activity from electron capture and about 9 times higher beta activity. Wouldn't that mean that for each 1461 kev gamma event registered by the detector, there should be at least 9 times more 511 kev annihilation events registered? But that definitely doesn't correspond to the K-40 spectrum I get (and also with virtually every K-40 spectra I've seen online). The 511kev photopeak is usually negligible as compared to the 1461 one.

What's the reason for this? Or perhaps I am getting something very wrong...

Hello,

K-40 has three independent kinds of decay:
beta- (for K 28.2 decays per gram and s, for KCl 14.8 decays per gram and s)
electron capture (gamma) (for K 3.33 decays per gram and s, for KCl 1.75 decays per gram and s)
beta+ decay is very rare (10 decays from a million) and can not be seen with simple equipment

I don`t know any usual material showing a visible beta+ decay.

Greetings from Germany

Hans-Jörg

gwgw wrote: ↑

10 Aug 2019, 05:17

CUT
The 511kev photopeak is usually negligible as compared to the 1461 one.

What's the reason for this? Or perhaps I am getting something very wrong...

Hi, the branching ratio for the Beta+ decay is only 0.001% .


https://www.nucleonica.com/Application/ ... 40_TXT.htm

Thanks. Apparently I've got that very wrong. 0.01% branching ratio makes a lot of sense, it explains it all.

Hans-Jörg wrote: ↑

10 Aug 2019, 21:40

I don`t know any usual material showing a visible beta+ decay.

Yeah, I don't think there's anything common in nature. I think I recall reading the Joliot-Curies' Nobel Prize was awarded for discovering "artificial radioactivity" when they created a positron emitting nuclide - I think they were using the Al(alpha,n) reaction to do that.

The most "usual" source for beta+ I've seen would be manufactured Na-22 sources - quite a common one for demonstrating annihilation coincidence measurements.