Nuclear Physics - Difference between electron capture and beta plus decay

[emilypearson]

So my question has a few parts to it.
First, if an atom is decaying and the proton (Z) number is decreasing in the decay process, am I correct in assuming that the nucleus is either decaying by electron capture of beta plus emission?
Secondly, I understand that beta plus emission can only occur is the mass of the original atom is at least 2 electron masses larger than the final atom. Therefore if the final mass is under 2 electron masses, the atom decays by electron capture. However, if the final mass is larger than 2 electron masses, how do you know if it is decaying by beta plus emission or electron capture?
Thanks in advance for any help.
Emily.

 

[Bob S]

Hi Emily-
Cu⁶⁴ is odd-odd, and decays to both Zn⁶⁴ (even-even) by beta-minus decay, and by positron AND electron capture to Ni⁶⁴ (even-even). See http://en.wikipedia.org/wiki/Copper-64. I think V⁵⁰ is similar.

 

[emilypearson]

Thanks, but is there any way of predicting which is more likely? For example Ce-137 (Z=58) decays by positron decay (according to a search engine). How would you know it is beta plus not electron capture? Or is there no way of working it out (via masses etc)?

 

[Bob S]

Cu⁶⁴ decay modes show that electron capture is about 45%, meaning that only about 55% of decays emit a typical charged beta ± decay lepton with a contiunuous energy spectrum. The best signature would be to look at the unique decay neutrino energy spectrum in electron capture (LOL).

 

[emilypearson]

So I guess for first year university physics I'm ok just to write 'beta plus and/or electron capture' and hope for the best in the exam next week! Thanks very much for your help.

 

[K^2]

Yes, computing actual branching fractions, or even estimating them, is way beyond what you might be expected to do or know how to do. You might need to know how to measure these experimentally, though.