to contain hydrogen, and generate t

to contain hydrogen, and generate their own gamma-background as the
neutrons are slowed-down and absorbed within the shielding material.
Nevertheless, proper arrangement of the system, by keeping the shielding
away from the gamma detector, can reduce this background component
in fast-neutron activation.
The low neutron-yield of radioisotopes limits their use in activation
analysis. Therefore, neutron generators or accelerators are often used for
this purpose. The neutron generators employed in such applications are
most often those that generate 14-MeV neutrons via the d-T reaction
(see section 2.3.1.3), due to their high-neutron yield. Irradiation by generators
tend also to produce a lower gamma-radiation background, since
the surrounding materials are not continuously exposed to neutrons,
and subjected to activation, at all time, but only when the generator is
turned on. Therefore, the discussion in this section is focused on activation
by 14-MeV neutrons. Delayed-activation is typically the result of
transmutation of the irradiated isotope to another isotope of the same
element, via the (n,2n) reaction , or due to the production of an isotope
of another element via the (n,p) or reactions. On the other hand,
prompt-activation of fast-neutrons results from the inelastic scattering,
of fast-neutrons, in which the nucleus absorbs the incident neutron,
becomes excited and immediately releases a neutron and a photon
carrying an energy characteristic of the parent nucleus. Both modes of
activation are discussed in the following subsections.
Delayed Fast-Neutron Activation
Oxygen. Although fast-neutron activation, in almost all cases, has
a lower activation cross-section than that of thermal-neutrons, there is
one notable exemption in the activation of oxygen. While the activation
cross-section of by thermal-neutrons is only 28 the corresponding
value for 14-MeV neutrons is much larger (42 mb), see Table 8.5.
The activation reaction by 14-MeV neutrons produces also
high-energy photons, 6.129 and 7.115 MeV, well above the background
gamma-ray energy resulting from the activation of other elements. The
high-energy photons are also less subject to attenuation by the material,
making it possible to examine large objects. Oxygen content can
even be measured at trace amounts, due to the lack of a competing
background component. It should be noted, however, that fluorine also
produces by the reaction, thus can interfere with the
accurate determination of oxygen content [251]. However, the concentration
of fluorine, if present, can be determined independently using
one of its other reactions, listed in Table 8.5, enabling correction for its
interference with the oxygen-produced signal. Oxygen is in general a dif-