EFFECT DUE TO NEUTRON CAPTURE
DOE-HDBK-1017/2-93
Plant Materials
Radioactive capture, or thermal neutron capture, produces many gamma rays (sometimes called
photons) in the 5 MeV to 10 MeV energy range. When a gamma-ray photon is emitted by the
excited 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 to
displace the atom from its equilibrium position and produce a cascade of displacements, or
Frenkel defects, with a resultant property change of the material. The (n,g) reaction with
thermal neutrons can displace the atom since the gamma photon has momentum (
), which
Eg
c
means that the nucleus must have an equal and opposite momentum (conservation of
momentum). E
g
is the gamma-ray (photon) energy, and c is the velocity of light. If the recoil
atom has mass A, it will recoil with a velocity u such that
=
Au
(5-1)
Eg
c
where all quantities are expressed in SI units. The recoil energy Er is equal to 1/2 Au2,
s
o
that
Er
=
.
(5-2)
E2g
2Ac2
Upon converting the energies into MeV and A into atomic mass (or weight) units, the
result is
Er
=
5.4 x 10-4
.
(5-3)
E2
g
A
The maximum energy of a gamma ray accompanying a (n,g) reaction is in the range between
6 MeV and 8 MeV. For an element of low atomic mass (about 10), the recoil energy could be
2 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 (or
greater than) that due to the fast neutrons in a material having an appreciable radioactive capture
cross section for thermal neutrons. Other neutron reactions (for example, (n,p), (n,g)) will also
produce recoil atoms, but these reactions are of little significance in thermal reactors. Thermal
neutron capture effects are generally confined to the surface of the material because most
captures occur there, but fast-neutron damage is likely to extend through most of the material.
MS-05
Page 38
Rev. 0
