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21 November 2009

Answer to Question #1259 Submitted to "Ask the Experts"

Category: Radiation Basics — Beta Particles (negative and positive) and Electrons

The following question was answered by an expert in the appropriate field:

When a positron is emitted in beta radiation, it seems that a proton is changed into a neutron. However, a neutron weighs slighty more than a proton. How is it possible then for a proton to change into a neutron and positron when a neutron and positron weigh more than the proton did? Where is the extra mass coming from?

The net effect of positron decay reduces the number of protons in a nucleus by one and increases the number of neutrons by one. Whereas the effect is "to change a proton into a neutron," the process does not occur by this mechanism. Rather, a positron and a neutrino are created in the nucleus at the moment of disintegration. This spontaneous nuclear process releases an amount of energy equal to the energy equivalent of the rest mass that disappears in the process. That positron decay is a nuclear process is consistent with the fact that the decay of free protons by positron emission is not observed in nature. On the other hand, the beta decay of free neutrons is a familiar fact.

Jim Turner, CHP

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