How Does Uranium-238 Capture Neutrons and What Happens After?

[wdlang]

can anyone give an example in which a nucleus absorbs a neutron and becomes a stable nucleus?

or the combination must break into parts?

 

[Vanadium 50]

H + n --> D + gamma

 

[wdlang]

H + n --> D + gamma

in this case, a gamma photon is emitted

is it possible that no particle is emitted at all?

just A+B ----> C ?

 

[Vanadium 50]

No, because energy-momentum would not be conserved.

 

[mfb]

You would need exactly the energy of the final particle, and you don't get this. For short-living particles, the energy can vary a bit, and it is possible.

 

[Astronuc]

can anyone give an example in which a nucleus absorbs a neutron and becomes a stable nucleus?

or the combination must break into parts?

There is usually a prompt gamma released upon a neutron combining with a nucleus. The product nucleus increases it's mass, but by less than the mass of the neutron. The gamma represents the binding energy needed to remove that neutron.

The product nucleus could be stable, i.e., not undergo β-decay or EC.

Some nuclei have very low energy capture gammas (but the product nucleus isn't necessarily stable).
http://www.nndc.bnl.gov/capgam/bye/page01.html

There is also the consideration of the kinetic energy and momentum of the original nucleus and neutron.

 

[snorkack]

Uranium 238 is supposed to have resonant capture of neutrons at 7 eV, which is why it is so important to moderate neutrons to thermal speed before uranium 238 captures them.

Does it mean that uranium 239 has an excited state exactly 7 eV above uranium 238?

If uranium 238 absorbs neutrons at, say, 9 or 10 eV, will the nuclei be emitting light rather than gamma rays?

 

[Astronuc]

Uranium 238 is supposed to have resonant capture of neutrons at 7 eV, which is why it is so important to moderate neutrons to thermal speed before uranium 238 captures them.

Neutrons are moderated to thermal energies in order to take advantage of the higher fission cross-section of U-235 (or Pu-239) for thermal neutrons. The resonance absorption of neutrons is just a complication in a moderated system. Fission neutrons are born in the MeV range, and must be slowed to < 0.1 eV to take advantage of the high cross-sections in the thermal range.

Does it mean that uranium 239 has an excited state exactly 7 eV above uranium 238?

If uranium 238 absorbs neutrons at, say, 9 or 10 eV, will the nuclei be emitting light rather than gamma rays?

The energy 7 eV or 9 eV has nothing to with the nuclear energy levels within the nucleus, only with the neutron energy. If U-238 absorbs 7 eV or 9 eV neutron, the emitted capture (or prompt) gamma would have the same energy. However, each nuclei has a unique spectrum of capture gammas, meaning that the nucleus has a number of internal excited states, which would be reflected in the subsequent decay of that nucleus.

http://www.nndc.bnl.gov/capgam/byn/page255.html
A list of levels, a level scheme and decay radiation information are available
http://www.nndc.bnl.gov/chart/reCenter.jsp?z=92&n=146

Visible light photon energies are on the order of 1.5-3.5 eV