# How are free electrons reincorporated into orbits?

In particular, I'm curious about what happens to beta particles once they lose most of their kinetic energy. As I understand, they'll eventually find their way into orbits of surrounding atoms, but how exactly does it happen and what are the prerequisites for it to happen? Should an electron's energy just fall below certain threshold? And what determines the value of that threshold?

Also, am I right to understand that in order to be caught, an electron's energy must be larger than the energy of the orbit it's about to occupy - otherwise it won't be caught. If so, what happens to electrons that were slowed down so much that they don't have enough energy to occupy any orbit in vicinity? Do they just stay in free state indefinitely?

P.S. Also, how is this process called properly? Is it... electron reincorporation? Or what?

• It's called recombination. – Ruslan Mar 11 at 20:45
• Ruslan, according to this Wikipedia article, it refers specifically to conversion from plasma back to gas. Are you sure it would be the same term when referring to recombination of beta particles with surrounding atoms and molecules? – Slavus Mar 11 at 20:56
• That's only one use case for this term. There's also carrier generation and recombination in semiconductors. Whenever I heard about electrons getting into a positive ion's electron shell, it was always called recombination. – Ruslan Mar 12 at 5:44

The cross section for the process is relatively small. It can be studied for UV-photons or for photons in the x-ray range (often for example the Al K$$\alpha$$ line) and then a different name is Bremsstrahlung Isochromat Spectroscopy, which is a technique to measure the density of unoccupied states of a material.