How does positron decay work?

1 Answer
Mar 5, 2017

Positron decay is when a proton decays into a neutron, a positron and an electron neutrino.


#p -> n + beta^+ + v_e#

In nuclear reactions, some fundamental quantities are conserved. One of these is charge.

Before the reaction, the net charge is the charge of the proton which is #+1#. After the reaction, the neutron and neutrino have no charge, and the positron has a charge of #+1#, so the charge is conserved.

Another quantity that is conserved is baryon number. Baryons (protons, neutrons) have a baryon number of #+1#, because they are, unsurprisingly, baryons - particles consisting of three quarks.

Before the reaction, the baryon number of the proton is #+1#. Afterwards, the baryon number of the neutron is #+1#, and the electron and electron neutrino are leptons so have no baryon number. Therefore, the baryon number is conserved.

The final quantity conserved is lepton number. The proton isn't a lepton, so has no lepton number. Neither does the neutron. The positron is an antimatter electron so has a #-1# lepton number, and the electron neutrino has a #+1# lepton number, which cancel to #0#. Therefore, the lepton number is also conserved.

For these reasons a proton can decay into a neutron and an electron neutrino.

The overall mass of the nucleus doesn't change, though the atomic number decreases by one, because a proton is lost to become a chargeless neutron.