Neutron emission

Neutron emitters to left of lower dashed line (see also: Table of nuclides)

Z →	0	1	2
n ↓	n	H	He	3	4
0		1H		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	16B	17C	18N	19O	20F	21Ne	22Na	23Mg	24Al	25Si
						12	18C	19N	20O	21F	22Ne	23Na	24Mg	25Al	26Si
							13	20N	21O	22F	23Ne	24Na	25Mg	26Al	27Si
								14	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. 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.

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.

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

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.

See also

• Neutron radiation
• Neutron source
• Neutron drip line
• Proton emission

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