Neutron emission

Neutron emission is a mode of radioactive decay in which one or more neutrons are ejected from a nucleus. It occurs in the most neutron-rich/proton-deficient nuclides, and also from excited states of other nuclides as in photoneutron emission and beta-delayed neutron emission. As only a neutron is lost by this process the number of protons remains unchanged, and an atom does not become an atom of a different element, but a different isotope of the same element.

Neutrons are also produced in the spontaneous and induced fission of certain heavy nuclides.

Spontaneous neutron emission

As a consequence of the Pauli exclusion principle, nuclei with an excess of protons or neutrons have a higher average energy per nucleon. Nuclei with a sufficient excess of neutrons have a greater energy than the combination of a free neutron and a nucleus with one less neutron, and therefore can decay by neutron emission. Nuclei which can decay by this process are described as lying beyond the neutron drip line.

Two examples of isotopes that emit neutrons are beryllium-13 (decaying to beryllium-12 with a mean life 2.7×10−21 s) and helium-5 (helium-4, 7×10−22 s).[1]

In tables of nuclear decay modes, neutron emission is commonly denoted by the abbreviation n.

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

Double neutron emission

Some neutron-rich isotopes decay by the emission of two or more neutrons. For example hydrogen-5 and helium-10 decay by the emission of two neutrons, hydrogen-6 by the emission of 3 or 4 neutrons, and hydrogen-7 by emission of 4 neutrons.
Photoneutron emission
Main article: Photodisintegration

Some nuclides can be induced to eject a neutron by gamma radiation. One such nuclide is 9Be; its photodisintegration is significant in nuclear astrophysics, pertaining to the abundance of beryllium and the consequences of the instability of 8Be. This also makes this isotope useful as a neutron source in nuclear reactors.[2] Another nuclide, 181Ta, is also known to be readily capable of photodisintegration; this process is thought to be responsible for the creation of 180mTa, the only primordial nuclear isomer and the rarest primordial nuclide.[3]
Beta-delayed neutron emission

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.
Delayed neutrons in reactor control
Main article: Nuclear reactor physics

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.
Neutron emission in fission
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 a nucleus splits into two (occasionally three) smaller nuclei and generally one or more neutrons.
See also

Neutron radiation
Neutron source
Proton emission

References

"Neutron Emission" (webpage). Retrieved 2014-10-30.
Odsuren, M.; Katō, K.; Kikuchi, Y.; Aikawa, M.; Myo, T. (2014). "A resonance problem on the low-lying resonant state in the 9Be system" (PDF). Journal of Physics: Conference Series. 569: 012072. doi:10.1088/1742-6596/569/1/012072.

Utsonomiya, H.; Akimune, H.; Goko, S.; Yamagata, T.; Ohta, M.; Ohgaki, H.; Toyokawa, H.; Sumiyoshi, K.; Lui, Y.-W. (2002). "Photoneutron Cross Sections for Nuclear Astrophysics". Journal of Nuclear Science and Technology. Supplement 2: 542–545. doi:10.1080/00223131.2002.10875158.

External links

"Why Are Some Atoms Radioactive?" EPA. Environmental Protection Agency, n.d. Web. 31 Oct. 2014
The LIVEChart of Nuclides - IAEA with filter on delayed neutron emission decay
Nuclear Structure and Decay Data - IAEA with query on Neutron Separation Energy

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Alpha decay Beta decay Gamma radiation Cluster decay Double beta decay Double electron capture Internal conversion Isomeric transition Neutron emission Positron emission Proton emission Spontaneous fission

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