Beta or positron induced reactions?

[arivero]

Looking at the table of isotopic masses table it seems that certain atoms, as 64Zn or 58Ni, could be able to capture an electron and then release a positron with an energy higher, in average, that the initial electron.

On the other side, some nucleus are able to capture a positron and release an electron, for instance 124Sn or 116Cd. Of course, these are just the atoms which can undergo double-beta, but here we have an energy input from the initial particle so the decay rate will be higher.

We could do a pile of both kinds of material and then try a chain reaction.

The energy gain in this kind of reactions is about 1MeV / atom, thus two orders of magnitude lower than fission, and one order lower than fusion, but still orders higher than chemical sources of energy. True, there are dissipative mechanisms against this, the most relevant being neutrino radiation, for which I do not know on any efective trick to recapture :-(

In any case, I wonder if someone has anytime read about this mechanism, even if only to dismiss it. Any pointer there? It seems worth to look a little about how to overcome the dissipation problems.

 

[mathman]

I am not familiar with the details of the process you are describing. However, whenever positrons are involved, they would be much more likely to collide with electrons resulting in mutual anihilation than in any nuclear reaction. As a result I can't see any chain reaction occurring.

 

[arivero]

well. that gamma could be absorbed again, causing an atom to decay, or be used to get energy. Or perhaps some "electron free" paths could be created by solid state effects. Also, a fast electron could be induced to emit a pair, so it could compensate the loss. This is a thing you can not do with neutrons, to create a pair at these energies.

The real problems are neutrinos. They are not going to escape because its low low cross section, carriying energy away. I can not see if there is any solution to it.

 

[mathman]

Gamma rays from electron-positron reaction are 511 kev. They can't cause any nuclear reaction. All they can do is excite electrons.

 

[arivero]

Ah,no, excitation energies for atomic electron are of the electronvolt order, even if a deep electron is involved, it should be only some tens of electronvolts. 0.511 MeV is already the order of excitation of protons and neutrons in the nucleus. The usual thing for this ray should be to excite some levels *in the nucleus* then getting a secondary cascade of Xrays.

 

[mathman]

The two principal reactions of gamma rays with atoms at these energies are Compton scattering (gamma ray loses energy by transfering some of it to an electron) and photoelectric effect (gamma ray is completely absorbed by electron being knocked out of the atom). Nuclear reactions would be extremely rare (if at all).

 

[arivero]

The most I look, the most I tend to agree. The barrier to do a double beta (alt a double capture) is usually about hundreds of KeV only, but it is difficult to jump it. As you say, compton scattering is the most likely effect, jointly with complete electron ejection (energy is too hard for just an atomic excitation, that is eV only) and perhaps some nuclear excitation when/if frequencies match.

I wonder if we could expect surprises from the electroweak force. The carriers W,Z have a (virtual) mass around 90 atomic mass units. Thus momenta transfer mechanisms could be different depending of the mass of the nucleus and its internal state. Current research have not found discrepances, but it is done mainly in the low atomic mass range.