Neutron emission

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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.

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	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	21Al	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

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.

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]

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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.

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.

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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 happens when a nucleus splits into two (occasionally three) smaller nuclei and generally one or more neutrons.

• Neutron radiation
• Neutron source
• Proton emission

• "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