Question:
Hello.
I have a question and I was hoping you could help me out. Why does beta+ emission (positron)
occur only with artificially produced rather than naturally occurring radioactive isotopes?
Replies:
I am not a nuclear physicist, so I would not presume to declare that positron emission NEVER
occurs in the decay scheme of naturally occurring radioactive isotopes. However, that does not
appear to be common. Positron emission occurs when a "light" element has "too many" protons
compared to the stable number of protons and neutrons. Or equivalently, it has "too few"
neutrons. Because a positron is anti-matter it will annihilate if it comes in the vicinity of
regular matter -- so it could not remain in the nucleus for very long.
If the element is irradiated, it can emit a positron which in the presence of regular matter
will undergo e(-) + e(+) -------> 2 photons of energy E~ 2Me*c^2 where Me and c are the mass of
the electron = mass of the positron and c = speed of light.
These photons are easily detected because the move exactly 180 degrees in the opposite directions.
This is the basis of a fairly new medical diagnostic called Positron Emession Tomography (PET)
which you can find on a word search.
It is a very useful tool.
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