Neutron emission
Z → | 0 | 1 | 2 | 3 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
n ↓ | n | H | He | Li | 4 | 5 | |||||||||
0 | 1H | Be | B | 6 | 7 | ||||||||||
1 | 1n | 2H | 3He | 4Li | C | N | 8 | ||||||||
2 | 3H | 4He | 5Li | 6Be | 7B | 8C | 9N | O | 9 | ||||||
3 | 4H | 5He | 6Li | 7Be | 8B | 9C | 10N | 11O | F | 10 | |||||
4 | 5H | 6He | 7Li | 8Be | 9B | 10C | 11N
|
12O | 13F | Ne | 11 | 12 | |||
5 | 6H | 7He | 8Li | 9Be | 10B | 11C | 12N | 13O | 14F | 15Ne | Na | Mg | |||
6 | 7H | 8He | 9Li | 10Be | 11B | 12C | 13N | 14O | 15F | 16Ne | 17Na | 18Mg | 13 | 14 | |
7 | 9He | 10Li
|
11Be | 12B | 13C | 14N | 15O | 16F | 17Ne | 18Na | 19Mg | Al | Si | ||
8 | 10He | 11Li | 12Be | 13B | 14C | 15N | 16O | 17F | 18Ne | 19Na | 20Mg | 22Si | |||
9 | 12Li | 13Be | 14B | 15C | 16N | 17O | 18F
|
19Ne | 20Na | 21Mg | 22Al | 23Si | |||
10 | 13Li | 14Be | 15B | 16C | 17N | 18O | 19F | 20Ne | 21Na | 22Mg | 23Al
|
24Si | |||
11 | 15Be | 16B | 17C | 18N | 19O | 20F | 21Ne | 22Na
|
23Mg | 24Al
|
25Si | ||||
12 | 16Be | 17B | 18C | 19N | 20O | 21F | 22Ne | 23Na | 24Mg | 25Al | 26Si | ||||
13 | 18B | 19C | 20N | 21O | 22F | 23Ne | 24Na
|
25Mg | 26Al
|
27Si | |||||
14 | 19B | 20C | 21N | 22O | 23F | 24Ne | 25Na | 26Mg | 27Al | 28Si |
Neutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Two examples of isotopes which emit neutrons are beryllium-13 (mean life 2.7x10-21 sec) and helium-5 (7x10-22 sec).
Neutron emission usually happens from nuclei that are in an excited state, such as the excited O-17* produced from the beta decay of N-17. The neutron emission process itself is controlled by the nuclear force and therefore is extremely fast, sometimes referred to as "nearly instantaneous." The ejection of the neutron may be as a product of the movement of many nucleons, but it is ultimately mediated by the repulsive action of the nuclear force that exists at extremely short-range distances between nucleons. The life time of an ejected neutron inside the nucleus before it is emitted is usually comparable to the flight time of a typical neutron before it leaves the small nuclear "potential well," or about 10-23 seconds.[1] A synonym for such neutron emission is "prompt neutron" production, of the type that is best known to occur simultaneously with induced nuclear fission. Many heavy isotopes, most notably californium-252, also emit prompt neutrons among the products of a similar spontaneous radioactive decay process, spontaneous fission.
Most neutron emission outside prompt neutron production associated with fission (either induced or spontaneous), is from neutron-heavy isotopes produced as fission products. These neutrons are sometimes emitted with a delay, giving them the term delayed neutrons, but the actual delay in their production is a delay waiting for the beta decay of fission products to produce the excited-state nuclear precursors that immediately undergo prompt neutron emission. Thus, the delay in neutron emission is not from the neutron-production process, but rather its precursor beta decay which is controlled by the weak force, and thus requires a far longer time. The beta decay half lives for the precursors to delayed neutron-emitter radioisotopes, are typically fractions of a second to tens of seconds.
Nevertheless, the delayed neutrons emitted by neutron-rich fission products aid control of nuclear reactors by making reactivity change far more slowly than it would if it were controlled by prompt neutrons alone.
See also
References
- ^ "Neutron emission lifetime and why" (PDF). Retrieved 2012-09-17.
External links
The LIVEChart of Nuclides - IAEA with filter on delayed neutron emission decay
Nuclear Structure and Decay Data - IAEA with query on Neutron Separation Energy