# Capturing Energy from a Positron-Electron Collision

According to the book Physics of the Impossible, the catastrophic collision between a electron and a positron yields an output of 1.02 MeV. Assuming you have an isotope like $^{22}\text{Na}$, which is known to emit photons during positron emission decay, will they annihilate upon any contact with electrons, or do they have to be accelerated at one another. Secondly, Is it possible to efficiently capture this energy yielded in the reaction. Also, in what form does this 1.02 MeV get emitted in? This question is excluding the fact that the generation of positions in very large amounts is not feasible at the moment.

Every $\beta^+$ decay generates positrons. Since they are not rare the reaction annihilation of a positron electron pair is far from catastrophic. They collide mainly because they attract each other due to their opposed charges.

The energy freed from the cpmplete mass loss is emitted antiparallely in two photons of 511 keV each(in the center of mass system of the electron positron pair), to preserve energy and momentum as well. The energy can be captured as as any high energy radiation, not too efficiently (with the current technology) apart from heating.

• Thank you! By catastrophic I was implying that the reaction was catastrophic to the particles, as they are destroyed. At Cambridge they developed 1% efficiency semi conductors that convert gamma ray photons to electricity. Jun 4, 2014 at 21:13

As @Lord_Gestalter said, the mass energy becomes two photons in opposite directions. At .5 MeV they fall in the traditional X-ray range but are nearly always called gamma rays.

They are not hard to capture. Nuclear medicine relies on the ability to detect these "coincidence" events. A patient is given an IV with a small amount of sodium 22 for example. When circular detector arrays (or detectors that rotate around the patient) pick up nearly simultaneous signals of the right energy, a line is drawn between the detectors and the methods of image reconstruction from projections is used to view the area of interest. With sufficient time discrimination you get an advantage over x-ray tomography and can tell where along the line the annihilation took place.

Patients often lie still for an hour to accumulate enough data. By timing with the pulse, a beating heart can be imaged in (averaged) motion.

Capturing the photons to do work, like an anti-mater/mater rocket is a different problem. We know the energy is there, but how do you direct it?

These positrons from beta decay get flung out into a sea of electrons - same mass, opposite charge, the rest is inevitable. As with the famous clam diggers, love was born as their shovels met in the mud.

• I interpreted "capture" as "using the energy". +1 for mentioning PET Jun 5, 2014 at 5:44