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	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

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

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

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.