Neutron emission

Neutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Neutron emission is one of the ways an atom reaches its stability. An atom is unstable, therefore radioactive, when the forces in the nucleus are unbalanced. The instability of the nucleus results from the nuclei having extra neutrons or extra protons. Two examples of isotopes that emit neutrons are beryllium-13 (mean life 2.7×10−21 s) and helium-5 (7×10−22 s). Commonly, it is abbreviated with a lower case n.

As only a neutron is lost in this process, the atom does not gain or lose any protons, and so it does not become an atom of a different element. Instead, the atom will become a new isotope of the original element, such as beryllium-13 becoming beryllium-12 after emitting one of its neutrons.[1]

Neutron emitters to the left of lower dashed line (see also: Table of nuclides)
Z → 0 1 2
n ↓ n  H  He 3 4
0 1H 2He Li Be 5 6
1 1n 2H 3He 4Li 5Be B C 7
2 2n 3H 4He 5Li 6Be 7B 8C N 8
3 4H 5He 6Li 7Be 8B 9C 10N O 9
4 4n 5H 6He 7Li 8Be 9B 10C 11N 12O F 10
5 6H 7He 8Li 9Be 10B 11C 12N 13O 14F Ne 11
6 7H 8He 9Li 10Be 11B 12C 13N 14O 15F 16Ne Na 12
7 9He 10Li 11Be 12B 13C 14N 15O 16F 17Ne 18Na Mg 13
8 10He 11Li 12Be 13B 14C 15N 16O 17F 18Ne 19Na 20Mg Al 14
9 12Li 13Be 14B 15C 16N 17O 18F 19Ne 20Na 21Mg 22Al Si
10 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 19C 20N 21O 22F 23Ne 24Na 25Mg
26Al
 27Si
14 20C 21N 22O 23F 24Ne 25Na 26Mg 27Al 28Si

Neutron emission in fission

Neutron emission usually happens from nuclei that are in an excited state, such as the excited 17O* produced from the beta decay of 17N. The neutron emission process itself is controlled by the nuclear force and therefore is extremely fast, sometimes referred to as "nearly instantaneous". This process allows unstable atoms to become more stable. 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.[2]

Induced fission

A synonym for such neutron emission is "prompt neutron" production, of the type that is best known to occur simultaneously with induced nuclear fission. Induced fission happens only when a nucleus is bombarded with neutrons, gamma rays, or other carriers of energy. Many heavy isotopes, most notably californium-252, also emit prompt neutrons among the products of a similar spontaneous radioactive decay process, spontaneous fission.

Spontaneous fission

Spontaneous fission happens when an atom's nucleus splits into two smaller nuclei and generally one or more neutrons.

Delayed neutrons in reactor control

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. About 0.65% of neutrons are released in a nuclear chain reaction in a delayed way due to the mechanism of neutron emission, and it is this fraction of neutrons that allows a nuclear reactor to be controlled on human reaction time-scales, without proceeding to a prompt critical state, and runaway melt down.

See also

References

  1. ↑ "Neutron Emission" (webpage). Retrieved 2014-10-30.
  2. ↑ "Neutron emission lifetime and why" (PDF). Retrieved 2012-09-17.
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