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* '''Primary sources''', used for startup of a fresh reactor core; conventional [[neutron source]]s are used. The primary sources are removed from the reactor after the first fuel campaign, usually after few months, as [[neutron capture]] resulting from the thermal neutron flux in an operating reactor changes the composition of the isotopes used, and thus reduces their useful lifetime as neutron sources.
** [[Californium-252]] ([[spontaneous fission]])
** [[Plutonium-238]]
** [[americium-241]] & beryllium, (α,n) [[Nuclear reaction|reaction]]
** [[polonium]]-210 & beryllium, (α,n) [[Nuclear reaction|reaction]]
** [[radium]]-226 & beryllium, (α,n) [[Nuclear reaction|reaction]]
** [[Boron-11]] & [[nitrogen-14]], (α,n) [[Nuclear reaction|reaction]]
** [[Deuterium]] & Hydrogen-1, (γ,n) reaction<ref name="tpub" /> When [[plutonium-238]]/beryllium primary sources are utilized, they can be either affixed to [[control rod]]s which are removed from the reactor when it is powered, or clad in a [[cadmium]] alloy, which is opaque to thermal neutrons (reducing transmutation of the plutonium-238 by neutron capture) but transparent to [[fast neutron]]s produced by the source.<ref name="pat1" />▼
* '''Secondary sources''', originally inert, become radioactive and neutron-producing only after [[neutron activation]] in the reactor. Due to this, they tend to be less expensive. Exposure to thermal neutrons also serves to maintain the source activity (the radioactive isotopes are both burned and generated in neutron flux).
** [[Antimony|Sb]]-[[Beryllium|Be]] [[photoneutron]] source; antimony [[neutron activation|becomes radioactive]] in the reactor and its strong gamma emissions (1.7 MeV for <sup>124</sup>Sb) interact with [[beryllium-9]] by an (γ,n) reaction and provide [[photoneutron]]s. In a [[Pressurized water reactor|PWR reactor]] one neutron source rod contains 160 grams of antimony, and stay in the reactor for 5–7 years.<ref>{{cite book|url=https://books.google.com/books?id=SJOE00whg44C&pg=PA147&dq=neutron+startup+source&lr=&as_drrb_is=q&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=&num=50&as_brr=3&cd=22#v=onepage&q=neutron%20startup%20source&f=false |title=The radiochemistry of nuclear power plants with light water reactors|author=Karl-Heinz Neeb|page=147|publisher=Walter de Gruyter|year=1997 |isbn=3-11-013242-7}}</ref> The sources are often constructed as an antimony rod surrounded by beryllium layer and clad in [[stainless steel]].<ref name="tpub">{{cite web|author=Integrated Publishing |url=http://www.tpub.com/content/doe/h1019v1/css/h1019v1_108.htm |title=Neutron Sources Summary |publisher=Tpub.com |date= |accessdate=2010-03-28}}</ref><ref>{{cite web|url=http://www.lib.ncsu.edu/specialcollections/digital/text/engineering/reactor/murray/MurNBabneutron040953.html |title=Memorandum from Raymond L. Murray to Dr. Clifford K. Beck |publisher=Lib.ncsu.edu |date= |accessdate=2010-03-28}}</ref> Antimony-beryllium [[alloy]] can be also used.
▲When [[plutonium-238]]/beryllium primary sources are utilized, they can be either affixed to [[control rod]]s which are removed from the reactor when it is powered, or clad in a [[cadmium]] alloy, which is opaque to thermal neutrons (reducing transmutation of the plutonium-238 by neutron capture) but transparent to [[fast neutron]]s produced by the source.<ref name="pat1"/>
The chain reaction in the first critical reactor, CP-1, was initiated by neutron sources generated during its prior subcritical test runs. The very first of those could have been initiated by ambient cosmic-ray neutrons. Similarly, in modern reactors (after startup), delayed neutron emission from fission products suffices to sustain the amplification reaction while yielding controllable growth times. (In comparison, a bomb is based on immediate neutrons and grows exponentially in nanoseconds.)
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