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A positron is created in a pair production event or in beta+ nuclear decay (which is called positron emission). It (the positron) appears "out of nowhere" with an associated electron under certain conditions in pair production. And in the nuclear decay schemes of some radionuclides, it is generated spontaneously within the (unstable) nucleus and exits that nucleus in the decay event. Curious? Let's look further.

In positron emission (beta+ decay), a proton in an atomic nucleus experiences a change mediated by the weak interaction (the weak force), and one of its up quarks is transformed into a down quark. The change results in the "conversion" of a proton into a neutron. This causes atomic number to go down by one because there is one fewer proton in the atomic nucleus than just before the event. Here's an example:

In the beta plus decay of carbon-11, a new element, boron-11, is created. A positron, a neutrino, and a gamma ray will be ejected from the nucleus. Here's the equation for it:

6C => 5B + e+ + ve + 0.96 MeV

An atom of carbon becomes an atom of boron. The e+ is the positron and the ve is the neutrino. The gamma ray has an energy of 0.96 MeV (million electron-volts). There aren't many nuclei that do this. It is only seen in carbon-11, potassium-40, nitrogen-13, oxygen-15, fluorine-18, and iodine-121. That's it. Beta+ decay isn't all that tough to understand. What about pair production?

Pair production is the "making" of a positron and an electron out of a high energy gamma ray. Both pair production and beta plus nuclear decay occur naturally, so the positron can be said to occur in nature. Remember that the positron is an antiparticle - it's antimatter - and it will, after appearing, slow down via scattering and will eventually combine with an electron in mutual annihilation. The positron has a short mean lifetime and a short mean path of travel. They usually don't last long after they're created. But lets look at the creation of the particle pair.

The energy of the photon that creates the electron pair must have must meet a minimum threshold. And the threshold energy necessary for this even to be possible is 1.022 MeV. That's a lot of energy, and all that energy will be converted into mass - the rest mass of the electron and the rest mass of the positron. Higher energy gamma rays might still initiate pair production, but the extra energy would be accounted for in the kinetic energies of the pair of particles produced.

A gamma ray of sufficient energy passes near an atomic nucleus and the pair is produced. Note that pair production is not the spontaneous "option" that high energy gamma rays have. The photons must pass close by an atomic nucleus for there to be a probability that pair production will occur. This is because momentum must be conserved, and the "assisting" nucleus will handle this chore.

We should also note that researchers using high powered lasers on gold target material are able to produce considerable quantities of positrons for research, and this work is continuing. Links are provided to associated Wikipedia articles and related questions.

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โˆ™ 2010-07-07 00:38:51
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Q: How is a positron produced?
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What happens to an atomic nucleus when a positron is produced?

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