Positron

Positrons are in the current AP Physics 2 framework, at 15.8.A.1.iv, and the statement is short enough to hold whole: positrons, or antielectrons, are subatomic particles that have an electric charge opposite that of an electron, have the same mass as an electron, and are symbolised by $e^{+}$ or $\beta^{+}$.

Three consequences follow from that sentence and no fourth is required.

• Charge $+e = +1.60 \times 10^{-19}$ C, so in a decay equation the positron sits in the charge column as $+1$.
• Mass equal to the electron mass, $9.11 \times 10^{-31}$ kg on the sheet, which is small enough that its mass number in nuclear notation is 0.
• Two symbols for one particle. A question may write either, and $\beta^{+}$ never means anything else.

Where it comes from is 15.8.A.2.iv: beta-plus decay occurs when a proton changes to a neutron by emitting a positron and a neutrino. Note the pairing. A positron travels with a neutrino, and an electron from beta-minus decay travels with an antineutrino. Getting that pair the wrong way round is the standard slip on beta decay equations.

What the CED leaves out is as useful as what it puts in. Annihilation with an electron, the pair of photons that results, and the use of positrons in medical imaging are all absent from the required course content, and so is any account of the weak force that produces the particle. The Topic 15.8 boundary statement excludes the last of those in so many words.