How to produce beam of neutrons?

calvinjhfeng

I am just curious, for nuclear power plant or nuclear weapon/detonation, how to create a beam of neutron that's energetic enough to go inside the U-235 atom and trigger the chain reactions?

What was the most conventional or the oldest way to create this beam of neutrons in lab back in the 1900s? What kind of decay is this?

Side question: How was neutron even detected? It doesn't interact with most things.

Thanks.

QuantumPion

Neutrons are uncharged so the only way to make a beam of them is to have a beam port in front of an enclosed neutron source. They can be collimated to some degree using neutron reflectors though.

You can make neutrons with an alpha radiation source such as radium mixed with Beryllium to produce the (a,n) reaction. The neutron was not discovered until 1932.

Note that U-235 does not require high energy neutrons to fission, hence why it is so important for reactors and bombs.

Neutrons may be uncharged but they still interact with nuclei by elastic scattering, as well as inelastic interactions such as fission.

mfb

"Chain reaction" is the key here. You do not need a beam of neutrons, some single neutrons are sufficient to begin the chain reaction. Both uranium and plutonium have isotopes that can decay via spontaneous fission, releasing neutrons. Nuclear weapons with plutonium have the issue that there are too many neutrons - without fancy setups, the chain reactions begins too early, and the bomb explodes before most of the material was fissioned.

Oh, and you do not need high energetic neutrons to trigger fission, the neutrons are not repelled by the nucleus - it is the other way round, slower neutrons have a higher probability to induce fission.

DLHill

From what I have read, you can use a piece of americium 241 in some sort of chamber (probably a vacuum tube to avoid collisions with air molecules) and have them exit through a small hole (covered with a metal foil to strip the proton from the alpha particle) at one end and through a tube where your uranium. I cannot be assured that this will have enough energy to do much, but it's worth a shot (no pun intended). Americium is also fairly easy to obtain.

ZapperZ

Closed, pending moderation.

Zz.