radiation emission by matter

[bwanaaa]

as described by einstein in his paper published a few days ago in 1917, photon emission can be spontaneous or stimulated. In stimulated emission, atoms can be in an 'excited' state and the passage of a photon through such a population can can cause the ejaculation of energy in the form of an additional photon with the same energy and phase as the incident photon. This chain reaction leads to coherent radiation (masers, lasers, etc).

But what happens if instead of whole atoms in the lasing medium, one just puts nuclei. Can a nucleus be in an excited state? (like bombarding it with neutrons first) If not, can one isolate nuclei from unstable isotopes and use those? Will it emit photons in the same way or particles like neutrons?

A corollary question is what happens when electrons instead of photons are used to bombard an excited population of whole atoms? Do we get a cascade of electrons? An electron laser?

We know what happens when we throw neutrons at excited (unstable) atoms like Pu239.....

 

[BarkingGhostar]

as described by einstein in his paper published a few days ago in 1917, photon emission can be spontaneous or stimulated. In stimulated emission, atoms can be in an 'excited' state and the passage of a photon through such a population can can cause the ejaculation of energy in the form of an additional photon with the same energy and phase as the incident photon. This chain reaction leads to coherent radiation (masers, lasers, etc).

But what happens if instead of whole atoms in the lasing medium, one just puts nuclei. Can a nucleus be in an excited state? (like bombarding it with neutrons first) If not, can one isolate nuclei from unstable isotopes and use those? Will it emit photons in the same way or particles like neutrons?

A corollary question is what happens when electrons instead of photons are used to bombard an excited population of whole atoms? Do we get a cascade of electrons? An electron laser?

We know what happens when we throw neutrons at excited (unstable) atoms like Pu239.....

 

[Mark R]

Nuclei can be excited, even without the emission of nucleons or beta particles. There are a number of nuclear isomers which are purely excited nuclei which spontaneously emit photons to reach their ground state; these are potentially advantageous because they are pure gamma emitters. As a result, they find use in medical imaging, because there are no beta or alpha decays which would contribute to absorbed dose without contributing to the image. The classic example if Tc-99m which emits a 150 keV gamma ray to decay to Tc-99 which is (near enough) stable.

Fluorescent gamma emission from excited nuclear isomers is known, but largely impractical due to the inherent radioactivity of such isomers. However, Hf-178m2 has been proposed for use in such a quantum nucleonic reactor, or gamma ray amplification by stimulated emission of radiation (GRASER) device due to its practically long 31 year half life and capacity for stimulated emission of a 2.45 MeV photon. However, there has been no attempt to build a practical device, despite fleeting interest in aircraft propulsion and similar schemes.

In the case of a GRASER, the electrons are largely irrelevant, as it is only the nuclei which participate in stimulated emission.

Electrons are fairly similar to photons in terms of effects on atoms, fast moving electrons will knock electrons out of the atom; however, because of the mass and charge of electrons, they cause a higher number if ionisations per mm of travel, and so consequently have a shorter range before their energy is depleted. Of course, if a photon hits an electron and ejects it from an atom via photoelectric or Compton interactions, then the ejected electron is indistinguishable from an externally bombarding electron. The result of the multiple ionisations is a shower of lower-energy photons as the electrons fall back into orbit, and those photons may induce additional electron transitions, until the average energy left is less than the work function of the material and everything is dissipated as thermal energy.