Why not use proton bombardment to trigger nuclear fission?

[Bararontok]

Fission type nuclear bombs and nuclear reactors use neutron bombardment to trigger the fission reaction but why not protons? Protons can repel the like electromagnetic fields of the other protons in the atomic nucleus so why not bombard the nucleus with protons instead?

 

[M Quack]

That works perfectly well, only you need much more energy to accelerate the protons than you get out of the fission. It is therefore used only for research neutron sources.

Look at the "production of neutrons" section in this Wiki article
http://en.wikipedia.org/wiki/Spallation

 

[Bararontok]

Why are the neutrons easier to release from the atoms in the spallation process?

Because in another thread,

https://www.physicsforums.com/showthread.php?t=594030,

also in this forum, atoms with more protons are more unstable because the protons repel each other due to their like charges. So should this not make it easier for protons to be released from the atom?

In the design of fission nuclear bombs it is said that it is the neutrons added to the radioactive isotopes that makes them unstable enough to fission when the atoms are collided together using the gun-type and implosion assembly methods. But why is this so when atoms with more protons are the ones that are supposed to be unstable? Should it not be the atoms with more protons that are more likely to fission during nuclear spallation?

Could it be that the protons are more difficult to use for bombardment because they may repel each other but the electrons in the atoms are keeping them bound to the atom because of the attraction of opposite charges?

But if this is the case, why should the neutrons be easier to release if there are no repulsion forces making it difficult for them to bind?

 

[M Quack]

If you want protons, you just strip the electron off a Hydrogen atom, at the cost of 13 eV or so. No need for nuclear physics.

If you want neutrons, (and a lot of experiments do) you have to use nuclear physics.

Lighter nuclei have a lower ratio of neutrons/protons, therefore fission tends to produce extra, free neutrons.

The question is how to induce fission. Either by neutrons, which is easy because the neutron has no charge (but you need a source of neutrons to do this), or by spallation with protons.

Protons are more difficult to use because of their charge. It takes a lot of energy to get the proton close enough to the (positively charged) target nucleus before any nuclear reaction can happen.

 

[Bararontok]

That is correct. The neutron from a source can be collided into an atom more easily because it will not be repelled like the proton since it has no charge. If a charged particle is used, either the positive charge of the protons or the negative charge of the electrons will repel the charged particle and prevent spallation from happening in the first place.

 

[Karimspencer]

It needs a lot of energy for protons

 

[Bob S]

It needs a lot of energy for protons

Yes. The best neutron yield per unit energy is for ~ 2 GeV protons on a tungsten (or lead) target, and I think the optimum yield is ~ 20 spallation neutrons per proton per GeV (= 50 MeV per neutron). See plot for lead on ~page 7 in http://www.physics.indiana.edu/~p537/notes/20060927_Pynn.pdf