Ask A Scientist Physics Archive

         
name         Sungho J.
status       other
age          30s
 
Question -   Hi.
If an electron with enough energy collides with a proton, then what happens?
They just form a hydrogen atom?
Or they form a neutron?
If both are possible, then which factor controls the process?
I am asking because I heard that a neutron can decay to a proton and an
electron and the reverse is possible.
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To form a hydrogen atom, it is required that the electron and proton have 
almost no energy, almost no velocity relative to each other.
A hydrogen atom ionizes at less than 20 electron-volts of energy, so 
reverse ionization requires energy less than about 20 electron-volts (eV)
AND freedom and luck to radiate a photon of the right energy to render the 
electron "captured".
It happens all the time in every electrified gas lamp (fluorescents, 
neons, mercury-arc, etc), and in the surface layers of the sun.

When a neutron decays into a proton, electron, and neutrino, it also 
releases energy, 780,000 eV, as the sum of the kinetic energy of the 3 
particles.
It is unreasonably difficult to get 3 separate particles to collide 
simultaneously, so the exact reverse of this never happens to a 
significant extent.
It is particularly difficult to get the ghost-like neutrino to react on 
command with an electron and proton .
Neutrinos are the particles that sail all the way through the earth, 
almost never bumping anything.

However, it is not so difficult for a balanced neutrino / anti-neutrino 
pair to be accidentally made
out of pure excess energy, from the collision between an electron and a 
proton.
Then you have the situation of the electron and proton and neutrino in the 
same place, merging to form a neutron,
and an anti-neutrino flying away free carrying any excess energy (beyond 
the 780 keV that was needed to make up the neutron).
I think this is one of the processes which together make sun-sized masses 
of neutrons when a neutron star is formed in a supernova explosion.
Actually, in that situation the electron and proton are steadily squeezed 
together by pressure of others around them.
Increasing pressure and temperature can smoothly change the energy of 
repeated collisions until the best energy is found,
and the conversion becomes quite rapid and energy-efficient.

If the excess energy of collision is over 1,000 keV, other random 
particles might be made from the energy too.
It only takes 1,020 keV to make an electron-positron pair, for example.
I'm not sure what it takes to start emitting excess energy as gamma-ray 
photons.
That might happen too.

Jim Swenson
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