I want to know why there is a maximum and then decrease.


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    $\begingroup$ The exact content of the answer will differ slightly depending on the source of the neutrons (simple instability to ejection, beta-+ decay, spallation, etc...) but they will all depend in part on the uncertainty of the momentum associated with the neutrons initial state. Perhaps a better question is "Why would you expect anything else?" $\endgroup$ – dmckee Feb 15 '18 at 0:12
  • $\begingroup$ This is very brief and cryptic. $\endgroup$ – Ben Crowell Feb 15 '18 at 2:18
  • $\begingroup$ What do you mean by "neutron spectra"? Do you mean the neutron absorption spectra of nuclei? Or do you mean the spectrum associated with decay of a neutron (i.e. the beta decay spectrum)? Or do you mean the spectrum of the neutrons emitted as fission products? $\endgroup$ – probably_someone Feb 15 '18 at 6:41

In nuclear and particle physics the only peaked energy spectra come from two body decays (spontaneous fission is a decay). One has a peak with a width due to measurement errors.

The neutrino was discovered because it was found that beta decay gave continuous spectra, no two body ones.


The particular shape of this decay is explained in the link, as it is easy to model since only electromagnetic and weak forces enter.

The functional form has a peak and diminution at the low and high energies.

In a fission fragmentation, there may be more than three fragments and the forces involved are the strong forces, the kinematics in the process will be more complicated than in the three body beta decay, but the balance of forces will give a similar form.

  • $\begingroup$ pl try a physical answer: I want to understand what is happening $\endgroup$ – ggs Feb 15 '18 at 5:56
  • $\begingroup$ In the beta decay, there is the electron, the rest of the nucleus, and they had to postulate a third particle because the energy did not give the unique kinematic peak at one energy,( with a gaussian shape due to error measurements) as expected from two body decays. When the kinetic energy gives a continuum , it means that more than two particles take energy away. One needs kinematics, and they are given in the formula in the link, which needs small corrections (from coulomb forces) to fit the data. $\endgroup$ – anna v Feb 15 '18 at 6:40
  • $\begingroup$ Similar formulae can be found for the fission fragments of your question. Physics needs mathematics, and kinematics is the most basic needed in studying physics. see iaea.org/inis/collection/NCLCollectionStore/_Public/20/082/… $\endgroup$ – anna v Feb 15 '18 at 6:40

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