Question 26.6: Positron Decay and Electron Capture Discuss the quark transf...
Positron Decay and Electron Capture
Discuss the quark transformation that occurs during positron decay and electron capture.
Recall from Chapter 25 that positron decay can be viewed as a proton changing into a neutron, accompanied by emission of a positron. Lepton conservation (Section 26.3) dictates that a neutrino is also emitted, so the process looks like this:
p^{+} \rightarrow n+\beta^{+}+\nu_{e}.
The positron and neutrino don’t involve quarks. The transformation of a proton into a neutron involves a change of quarks: u u d \rightarrow u d d It appears that an up quark has transformed into a down quark. Similarly, electron capture can be viewed as
p^{+}+e^{-} \rightarrow n+\nu_{e}.
where again the neutrino is required by lepton conservation. The quark transformation is u u d \rightarrow u d d, which again indicates an up quark changing to a down quark.
REFLECT It shouldn’t be surprising that positron decay and electron capture involve the same underlying process, because on a larger scale, both involve the transformation of a proton to a neutron.
In Chapter 25 you learned that both these processes are likely for nuclei that are relatively proton-rich and need to change a proton to a neutron to approach stability.