The Positron

The Field Guide to Particle Physics : Season 3
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The Positron

The positron is the antiparticle partner to the electron.

Ostensibly, positrons have the same mass as the electron, around 511 keV.  They also have the same electric charge - at least up to a minus sign. The positron is of course positively charged.

Positrons also carry equal and opposite magnetic dipole moments to the electron: that little magnetic field carried often carried by elementary particles.

Like the electron, positrons are stable. They do not decay. But of course, we don’t see may of them around. When electrons and positrons collide, they annihilate each other! That is, they convert into a pair of photons, each with 511 keV of energy.

Because it is *extremely* rare for photons to interact with each other, this reaction almost never goes in reverse, which explains why positrons don’t accumulate here on Earth.

As you might be aware, the matter to antimatter ratio of our universe is way out of whack - which is great for us! - but makes it a little hard to study antimatter particles like the positron.

Sources of Positrons

Some positrons are produced by the decay of cosmogenic muons - or antimuons, more precisely - that are formed when the pi-plus - the positively charged pion  decays. Those pions are in turn produced in collisions with cosmic rays in the upper atmosphere.

Sometimes positrons are produced in nuclear decays, like an antimatter version of beta decay. Fluorine-18 - which has 9 protons and 9 neutrons - is one such unstable nucleus. Oxygen-15 - which has 8 protons and 7 neutrons is another. A more exotic case is Rubidium-82,  which forms when a strontium-82 nucleus absorbs an electron, converting one of its 38-protons into a neutron. Rubidium-82 then decays by positron emission, converting another proton to a neutron, resulting in the noble gas Krypton-82.

Because the mass of the neutron is higher than that of the proton, positron emission is a form of radioactive beta decay that requires *extra* input energy, which is typically supplied by the remainder of the nucleus. It’s a curious concept that we’ll come back to in a future episode.

In medicine

Because the photons emitted by the annihilation of a positron-electron pair have a very specific energy, scientific instruments can be calibrated to detect them. Positron Emission Topography is an imaging technique that specifically looks for these pairs of 511 keV photons - these gamma rays if you like. By injecting a radioactive substance that decays by positron emission, PET devices back calculate the gamma ray trajectories to build a three-dimensional model of whatever that tracer was injected into. Typically the human body!

Fluorine-18, oxygen-15 and rubidium-82 are manufactured by particle accelerator for direct use in medical PET imaging. Sometimes those accelerators are RIGHT INSIDE THE MEDICAL FACILITY.

That’s right. Particle physics isn’t just for lab rats or abstruse aloof theorists. It’s crucial for medicine too! You can be a medical doctor AND study particle physics.

Positronium

Finally, electrons and positrons can form a bound state - an atom if you like - called positronium. Positronium doesn’t last very long - typically it decays by annihilation into an assorted number of gamma rays in a time that’s measured in nanoseconds .

The precise dynamics of positronium decay is a well studied science used in precision tests of quantum electrodynamics. We’ll learn more about positronium later this season!