Prompt and Delayed Neutrons
But not all neutrons are released at the same time following fission. Even the nature of creation of these neutrons is different. From this point of view we usually divide the fission neutrons into two following groups:
- Prompt Neutrons. Prompt neutrons are emitted directly from fission and they are emitted within very short time of about 10-14 second.
- Delayed Neutrons. Delayed neutrons are emitted by neutron rich fission fragments that are called the delayed neutron precursors. These precursors usually undergo beta decay but a small fraction of them are excited enough to undergo neutron emission. The fact the neutron is produced via this type of decay and this happens orders of magnitude later compared to the emission of the prompt neutrons, plays an extremely important role in the control of the reactor.
Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library
Precursors of Delayed Neutrons
In addition current nuclear physics facilities can produce more neutron-rich isotopes that can emit multiple neutrons. Currently about 18 2n-emitters are experimentally confirmed [IAEA – INDC(NDS)-0599], but only two of them are also fission products.
As an example, the isotope 87Br is the major component of the first group of precursor nuclei. This isotope has half-life of 55.6 seconds. It undergoes negative beta decay through its two main branches with emission of 2.6 MeV and 8 MeV beta particles. This decay leads to the formation of 87Kr* and 87Kr (ground state) and the 87Kr nucleus subsequently decays via two successive beta decays into the stable isotope 87Sr. But there is also one possible way for the 87Br nucleus to beta decay. The 87Br nucleus can beta decay into an excited state of the 87Kr nucleus at an energy of 5.5 MeV, which is larger than the binding energy of a neutron in the 87Kr nucleus. In this case, the 87Kr nucleus can undergo (with probability of 2.5%) a neutron emission leading to the formation of stable 87Kr isotope.
According to the JEFF 3.1 database, about 240 n-emitters are known between 8He and 210Tl, about 75 of them are in the non-fission region. Furthermore 18 2n-emitter, and only four 3n-emitters ( 11Li, 14Be, 17B, 31Na) are experimentally confirmed. These numbers are not certainly final. Since new IAEA Co-ordinated Research Project (CRP) on Beta-delayed neutron emission evaluation has been started in 2013, it is expected these numbers will change significantly.
See also: IAEA, Beta-delayed neutron emission evaluation, INDC(NDS)-0599
As can be seen it was identified many precursor nuclei. Not all of them are fission products (about 75 of them in the non-fission region A<70), but there are also many precursor nuclei, that are in the fission region between A=70-150. Their half-lives range between tenths of second (0.12 s) and tens of seconds (55.6 s), therefore their delayed neutrons appear with considerably differing delay times.
Six Groups of Delayed Neutrons – Parameters
Therefore delayed neutrons are traditionally represented by six delayed neutron groups, whose yields and decay constants (λ) are obtained from nonlinear least-squares fits to experimental measurements. This model has following disadvantages:
- All constants for each group of precursors are empirical fits to the data.
- They cannot be matched with decay constants of specific precursors.
- These constants are different for each fissionable nuclide.
- These constants change also with the neutron energy spectrum.
Eight Groups of Delayed Neutrons – Parameters
The eight-group representation uses a set of eight-group half-lives for all fissioning systems, with the half-lives adopted for the three longest-lived groups corresponding to the three dominant long-lived precursors: 87Br, 137I and 88Br (these precursors was separated into single groups).
See also: Delayed Neutron Data for the Major Actinidies, NEA/WPEC–6. OECD 2002.
Source: DELAYED NEUTRON DATA FOR THE MAJOR ACTINIDES, NEA/WPEC–6. Subgroup 6, NEA.