Reactor Theory (Neutron Characteristics)DOE-HDBK-1019/1-93NEUTRON SOURCESRev. 0Page 3NP-02There is an abundant supply of high energy gammas in a reactor that has been operated becausemany of the fission products are gamma emitters. All water-cooled reactors have somedeuterium present in the coolant in the reactor core because a small fraction of natural hydrogenis the isotope deuterium. The atom percentage of deuterium in the water ranges from close tothe naturally occurring value (0.015%) for light water reactors to above 90% deuterium for heavywater reactors. Therefore, the required conditions for production of photoneutrons exist.The supply of gamma rays decreases with time after shutdown as the gamma emitters decay;therefore, the photoneutron production rate also decreases. In a few particular reactors,additional D O (heavy water) may be added to the reactor to increase the production of2photoneutrons following a long shutdown period.InstalledNeutronSourcesBecause intrinsic neutron sources can be relatively weak or dependent upon the recent powerhistory of the reactor, many reactors have artificial sources of neutrons installed. These neutronsources ensure that shutdown neutron levels are high enough to be detected by the nuclearinstruments at all times. This provides a true picture of reactor conditions and any change inthese conditions. An installed neutron source is an assembly placed in or near the reactor forthe sole purpose of producing source neutrons.One strong source of neutrons is the artificial nuclide californium-252, which emits neutrons atthe rate of about 2 x 10 neutrons per second per gram as the result of spontaneous fission.12Important drawbacks for some applications may be its high cost and its short half-life (2.65years).Many installed neutron sources use the (,n) reaction with beryllium. These sources arecomposed of a mixture of metallic beryllium (100% beryllium-9) with a small quantity of analpha particle emitter, such as a compound of radium, polonium, or plutonium. The reactionthat occurs is shown below.The beryllium is intimately (homogeneously) mixed with the alpha emitter and is usuallyenclosed in a stainless steel capsule.Another type of installed neutron source that is widely used is a photoneutron source thatemploys the (,n) reaction with beryllium. Beryllium is used for photoneutron sources becauseits stable isotope beryllium-9 has a weakly attached last neutron with a binding energy of only1.66 MeV. Thus, a gamma ray with greater energy than 1.66 MeV can cause neutrons to beejected by the (,n) reaction as shown below.
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