an energy in the MeV range. This en

an energy in the MeV range. This energy is usually released in the form
of photons (gamma-rays). Prompt gamma-rays are in general higher in
energy than delayed gamma-rays (resulting from the long-term decay of
the product nucleus), since for most nuclei the binding energy released
by capturing a neutron is about 8 MeV or so. Not all the binding energy
is liberated as prompt gamma-rays, as part of the energy usually
remains within the formed compound nucleus, bringing it to an excited
state. The compound nucleus can promptly emit photons at various energies,
as it is “de-excited”. Therefore, a complex spectrum of prompt
gamma-rays can be emitted, with a maximum photon energy equal to
that equivalent to the mass defect of the reaction. In the above discussed
example of the reaction, more than 50 prompt gamma-rays
have been reported [247]. However, the primary (maximum) prompt
gamma-ray is usually monitored.
Since the mass defect in the formation of a compound nucleus is always
positive, prompt gamma-rays can be produced from almost any element.
However, the probability of formation depends on the thermal capture
cross-section. Table 8.3 provides a list of the prompt gamma-rays produced
from some common elements and the neutron cross-section needed
for their formation. Reference [250] lists 1915 prompt gamma-rays emitted
in the thermal-neutron activation of 84 elements. Obviously, elements
with very low cross-sections are difficult to activate, as they will
require a strong neutron source to produce measurable prompt activation
photons.
Measurement Model. Dropping the time-dependent terms in
Eq. (8.4), the measurement model for the intensity of prompt gammaradiation,
emitted at energy is expressed as:
where the terms are as defined in Eq. (8.4) and the yield factor, Y,
defined in Table 8.5, is introduced, since as mentioned earlier, not all
photons are necessarily emitted at energy As Table 8.3 shows, the
photon energy is in the MeV range, thus, the attenuation-coefficient of
photons, is likely to be low for most materials. Moreover, since most
activation will tend to occur near the surface of the object, due to the low
value of the value of y tends also to be small. These two factors lead
to a value of close to unity. Nevertheless, the attenuation factor
of neutrons, can still affect the measured activity. Thus,
the measured gamma activity is not a direct indication of the atomicdensity,
N, of the activated nuclei. Therefore, some a priori knowledge,