and disposal. In other cases, gamma

and disposal. In other cases, gamma-emissions from the activated nucleus
may be too weak in intensity, making it difficult to detect. For the
above reasons H, Li, B, C, N, F, Ne, Si, P, S and Ar are elements that
cannot be detected by delayed TNA, but may be analyzed by the promptgamma
emitted as the compound nucleus is de-excited [247]. Moreover,
prompt-gamma emission is more suited for on-line and in situ analysis,
or when it is not possible to remove the source, or the object, to halt
the activation process and perform analysis for delayed neutrons. With
the object continuously exposed to neutrons, it will also be difficult to
distinguish between gamma-rays produced at different time intervals by
the decay of the activated nuclei. Obviously, prompt activation has also
the advantage of providing immediate indications. However, neutron
activation performed outside the reactor is not as efficient as that performed
within the reactor, since a limited neutron flux can be produced
using neutron sources or generators. Also, in prompt analysis, neutrons
are absorbed in the surrounding shielding and construction materials,
and within the detector itself. This generates a background component
that interferes with the monitored signal. When it is possible to monitor
both prompt and delayed photon emissions, small concentrations
of almost all elements can be detected, except for oxygen [247]. Oxygen
is difficult to monitor with TNA due to its very low thermal-neutron
activation cross-section
Delayed Thermal-Neutron Activation
Delayed neutron-activation analysis requires removing the tested object
(sample) from neutron exposure, so that the emitted delayed
gamma-rays can be analyzed independently. The advantage of this approach
is that the sample can be analyzed in a low-background area,
and counting can be performed for as long as needed to provide an accurate
measurement. This enables accurate analysis of the interrogated
sample. However, it is neither desirable nor often practical to irradiate
a large volume of material, since thermal-neutrons tend to be absorbed
near the surface, due to their high cross-section. Moreover, the object
can remain radioactive for some time due to the delayed emission of
gamma-rays. Therefore, a small sample is commonly used in such analysis,
a milligram or less. Such a sample is inserted inside a reactor, or a
moderated-neutron assembly (see section 15.3). The efficiency of the activation
process can be enhanced by cyclic activation, i.e. repeating the
cycle of irradiation, delaying and counting, a few times to accumulate
an overall increase in the total measured activity [248]. This process is