reference measurements using a “com

reference measurements using a “comparator” object, of geometry and
material content and distribution similar to the objects to be inspected,
is used.
Background Radiation. Background radiation is another complicating
factor in quantifying prompt-gamma emissions and in obtaining
measurements at a high level of statistical confidence (see Appendix G).
Thermal neutrons are absorbed by almost all elements, releasing either
prompt or delayed photons in many cases. Therefore, absorption of
neutrons in the surrounding shielding materials, source and detector
collimators, or even structure materials and floors or the detector’s material
itself, can result in the emission of gamma-rays at different levels.
Some of the emitted background photons would be low-energy photons
that are easy to discard by energy discrimination, but some may arise
at energies close to that emitted by the element(s) of interest. Neutron
interactions within a photon detector’s material can also damage it, if
the exposure is prolonged or is intense, and deteriorate the detector’s
performance. Higher photon energies may also interfere with the energy
of interest by the Compton continuum (see section 4.3) that they
produce within the detector, as they lose energy by Compton scattering
within the inspected object. The 0.511 MeV photons of pair-production
(see section 3.4), arising from the subsequent interaction of the emitted
photons and their scattering, can also contribute to the background signal
for measurements below 0.511 MeV. Background radiation can also
be emitted at high energy from naturally radioactive elements, such as
(which emits 1.46 MeV photons), but the activity of such elements
is usually low and can be ignored. In general though, the gamma-ray
background tends to be relatively high below 2 MeV.
Residual Activity. Prompt-activation can also result in long-lived
residual activities that can be of radiological concern in some cases. For
example, is activated by thermal-neutron absorption
to the latter emits 2.7541 keV photons, with a half-life of 15 hours.
Sodium is also present in NaI(Tl) gamma detectors, and in foodstuff,
and can result in some residual activity that can last for a few days
following activation. The natural iodine in a NaI(Tl) detector can
also be activated to which emits 442.9 keV photons
with 25 min half-life. The presence of possible residual activities in the
inspected object, and its radiological impact, must be assessed prior to
activating a material.