Delayed Neutron
Delayed neutrons are neutrons that are not released as prompt neutrons (i.e. within about 10 −14 s) when an atomic nucleus splits , but much later (after milliseconds to minutes). They only make up about 1% or less of the total neutrons released, but are crucial for the controllability of nuclear reactors .
Explanation
The fission fragments of a nuclear fission are very unstable because of their large excess of neutrons and are gradually transformed into more stable nuclides through chains of successive beta-minus decays . In some cases, such a decay leads to an excitation state in the daughter nucleus that is above the binding energy of a neutron and therefore immediately decays further through neutron emission; these neutrons therefore appear with the half-life of the beta decay in question and much later than the original fission.
Mathematical description
For practical calculations, the temporal distribution of the delayed neutrons can be approximated by six time groups, each with a different half-life . The half-lives and the yield in each group depend somewhat on the split nuclide and on the energy of the neutron that triggers the split. The following applies to the fission of U-235 by a thermal neutron :
| group | Half-life (s) | Share (percent of all emitted neutrons) |
|---|---|---|
| 1 | 55.90 | 0.0221 |
| 2 | 22.73 | 0.1467 |
| 3 | 6.25 | 0.1313 |
| 4th | 2.30 | 0.2647 |
| 5 | 0.608 | 0.0771 |
| 6th | 0.230 | 0.0281 |
The sum of the proportions for this most important fission case in practice is 0.67% of all neutrons.
Energy spectrum
The delayed neutrons are emitted individually in transitions between discrete nuclear states. Their energy spectrum (all time groups taken together) can nevertheless be represented roughly like that of prompt neutrons by a Maxwell distribution . However, it is considerably softer. The mean energy is only about 0.45 MeV .
Importance for nuclear reactors
It is only thanks to the delayed neutrons that a nuclear reactor can be kept safely in the delayed critical operating state by technical means . The time scale of the prompt neutrons emission, around 10 −14 s, is far too short for technical control interventions.
While the delayed neutrons make up 0.67% for uranium-235, their percentage for plutonium-239 is only 0.22%. Therefore, in breeder reactors and also with the admixture of MOX fuel elements in light water reactors, the distance between the states is delayed critical and promptly critical smaller and requires finer control.
Individual evidence
- ^ EB Paul: Nuclear and Particle Physics . North-Holland, 1969, p. 249
- ↑ The technical term in physics and nuclear engineering is 'split', not 'split'
- ↑ A. Ziegler, H.-J. Allelein (Hrsg.): Reaktortechnik : Physical-technical basics . 2nd edition, Springer-Vieweg 2013, ISBN 978-3-642-33845-8 , page 48 ff.
- ↑ B. Akdeniz: PhD Thesis, Pennsylvania State University (2007), page 54, PDF download 1.5MB