Positron emission and proton decay are two distinct processes that occur in different contexts.
Positron emission is a type of radioactive decay where an unstable atomic nucleus emits a positron (a positively charged electron). This process typically occurs in certain types of radionuclides, such as isotopes of elements like potassium, chromium, and fluorine. During positron emission, a proton in the nucleus is converted into a neutron, and a positron is emitted to conserve both charge and baryon number. This process is commonly observed in positron emission tomography (PET) scans, which use positron-emitting isotopes for medical imaging.
On the other hand, proton decay is a hypothetical process that has been postulated in some unified theories of particle physics. According to these theories, protons, which are composed of quarks, can decay into lighter particles. However, proton decay has not been observed experimentally, and its occurrence, if it does happen, is expected to be extremely rare with a long lifetime.
The reason positron emission is not considered proton decay is because they operate on different scales and involve different fundamental interactions. Positron emission occurs within atomic nuclei and is governed by the weak nuclear force. It is a specific type of decay process associated with certain radionuclides. Proton decay, on the other hand, would occur at the level of elementary particles and is theorized to be mediated by processes beyond the Standard Model of particle physics, such as in certain grand unified theories.
In summary, while both positron emission and proton decay involve the conversion of a proton into another particle, they occur in different physical contexts and are governed by different mechanisms. Positron emission is an observed phenomenon in certain radioactive decays, while proton decay, if it exists, would be a rare process predicted by some theories of particle physics but has not yet been detected experimentally.