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TABLE 1 Neutron Production by Spontaneous Fission
Neutron Sources Summary

Nuclear Physics and Reactor Theory Volume 1 of 2
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Reactor Theory (Neutron Characteristics) DOE-HDBK-1019/1-93 NEUTRON SOURCES Rev. 0 Page 3 NP-02 There is an abundant supply of high energy gammas in a reactor that has been operated because many  of  the  fission  products  are  gamma  emitters.        All  water-cooled  reactors  have  some deuterium present in the coolant in the reactor core because a small fraction of natural hydrogen is the isotope deuterium.    The atom percentage of deuterium in the water ranges from close to the naturally occurring value (0.015%) for light water reactors to above 90% deuterium for heavy water  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  of 2 photoneutrons following a long shutdown period. Installed Neutron Sources Because intrinsic neutron sources can be relatively weak or dependent upon the recent power history of the reactor, many reactors have artificial sources of neutrons installed.  These neutron sources  ensure  that  shutdown  neutron  levels  are  high  enough  to  be  detected  by  the  nuclear instruments  at  all times.  This provides a true picture of reactor conditions and any change in these  conditions.  An  installed  neutron source is an assembly placed in or near the reactor for the sole purpose of producing source neutrons. One  strong source of neutrons is the artificial nuclide californium-252, which emits neutrons at the rate of about 2 x 10    neutrons per second per gram as the result of spontaneous fission. 12 Important  drawbacks  for some applications may be its high cost and its short half-life (2.65 years). Many  installed  neutron  sources  use  the  (    ,n)  reaction  with  beryllium.      These  sources  are composed of a mixture of metallic beryllium  (100%  beryllium-9)  with a small quantity of an alpha  particle  emitter,  such as a compound of radium, polonium, or plutonium.   The reaction that occurs is shown below. The  beryllium  is  intimately  (homogeneously) mixed with the alpha emitter and is usually enclosed in a stainless steel capsule. Another type of  installed  neutron source that is widely used is a photoneutron source that employs the (    ,n) reaction with beryllium.  Beryllium is used for photoneutron sources because its  stable isotope beryllium-9 has a weakly attached last neutron with a binding energy of only 1.66 MeV.  Thus, a gamma ray with greater energy than 1.66 MeV can cause neutrons to be ejected by the (    ,n) reaction as shown below.







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