EFFECT DUE TO NEUTRON CAPTUREDOE-HDBK-1017/2-93Plant MaterialsRadioactive capture, or thermal neutron capture, produces many gamma rays (sometimes calledphotons) in the 5 MeV to 10 MeV energy range. When a gamma-ray photon is emitted by theexcited compound nucleus formed by neutron capture, the residual atom suffers recoil(sometimes referred to as the shotgun effect). This recoil energy is often large enough todisplace the atom from its equilibrium position and produce a cascade of displacements, orFrenkel defects, with a resultant property change of the material. The (n,g) reaction withthermal neutrons can displace the atom since the gamma photon has momentum (), whichEgcmeans that the nucleus must have an equal and opposite momentum (conservation ofmomentum). Egis the gamma-ray (photon) energy, and c is the velocity of light. If the recoilatom has mass A, it will recoil with a velocity u such that=Au(5-1)Egcwhere all quantities are expressed in SI units. The recoil energy Er is equal to 1/2 Au2,sothatEr=.(5-2)E2g2Ac2Upon converting the energies into MeV and A into atomic mass (or weight) units, theresult isEr=5.4 x 10-4.(5-3)E2gAThe maximum energy of a gamma ray accompanying a (n,g) reaction is in the range between6 MeV and 8 MeV. For an element of low atomic mass (about 10), the recoil energy could be2 keV to 3 keV, which is much greater than the 25 eV necessary to displace an atom.In a thermal reactor, in which the thermal neutron flux generally exceeds the fast neutron flux,the radiation damage caused by recoil from (n,g) reactions may be of the same order as (orgreater than) that due to the fast neutrons in a material having an appreciable radioactive capturecross section for thermal neutrons. Other neutron reactions (for example, (n,p), (n,g)) will alsoproduce recoil atoms, but these reactions are of little significance in thermal reactors. Thermalneutron capture effects are generally confined to the surface of the material because mostcaptures occur there, but fast-neutron damage is likely to extend through most of the material.MS-05Page 38Rev. 0
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