| 词条 | Neutron emission |
| 释义 |
Neutrons are also produced in the spontaneous and induced fission of certain heavy nucleides. Spontaneous neutron emissionAs 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 mode 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 {{val|2.7|e=-21|ul=s}}) and helium-5 (helium-4, {{val|7|e=-22|u=s}}).[1] In tables of nuclear decay modes neutron emission is commonly denoted by the abbreviation n. Photoneutron emission{{Main|Photodisintegration}}Some nucleides, e.g. Be-9, can be induced to eject a neutron by gamma radiation. Beta-delayed neutron emissionNeutron 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 lifetime 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] Delayed neutrons in reactor control{{Main|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 fissionInduced fissionA 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 fissionSpontaneous fission happens when a nucleus splits into two smaller nuclei and generally one or more neutrons. See also
References1. ^{{cite web|url=http://education.jlab.org/glossary/neutron_emission.html |title=Neutron Emission|format=webpage |date= |accessdate=2014-10-30}} 2. ^{{cite web|url=http://newenergytimes.com/v2/library/2000/2000Li-Sub-BarrierFusion.pdf |title=Neutron emission lifetime and why |format=PDF |date= |accessdate=2012-09-17}} External links
3 : Neutron|Nuclear physics|Radioactivity |
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