Biomedical Engineering Reference
In-Depth Information
ner surface of an ionization chamber. A disadvantage of fissionable materials is
that they are alpha emitters, and one must sometimes contend with the pileup of
alpha-particle pulses.
Activation Foils
Slow neutrons captured by nuclei induce radioactivity in a number of elements,
which can be made into foils for neutron detection. The amount of induced activity
will depend on a number of factors—the element chosen, the mass of the foil, the
neutron energy spectrum, the capture cross section, and the time of irradiation.
Examples of thermal-neutron activation-foil materials include Mn, Co, Cu, Ag, In,
Dy, and Au.
Intermediate and Fast Neutrons
Nuclear reactions are also important for measurements with intermediate and fast
neutrons. In addition, neutrons at these speeds can, by elastic scattering, transfer
detectable kinetic energies to nuclei, especially hydrogen. Elastic recoil energies are
negligible for slow neutrons. Detector systems can be conveniently discussed in
four groups—those based on neutron moderation, nuclear reactions, elastic scat-
tering alone, and foil activation. Recent developments also include bubble detec-
tors.
Neutron Moderation
Two principal systems in this category have been developed: the long counter and
moderating spheres enclosing a small thermal-neutron detector. A cross section of
the cylindrical long counter is shown in Fig. 10.43. This detector, which is one of
the oldest still in use, can be constructed to give nearly the same response from a
neutron of any energy from about 10 keV to 5 MeV. The long counter contains a
BF
3
tube surrounded by an inner paraffin moderator, as shown. The instrument
is sensitive to neutrons incident from the right. Those from other directions are
either reflected or thermalized by the outer paraffin jacket and then absorbed in the
B
2
O
3
layer. Neutrons that enter from the right are slowed down in the inner paraf-
fin moderator, high-energy neutrons reaching greater depths on the average than
low-energy ones. With this arrangement, the probability that a moderated neutron
will enter the BF
3
tube and be registered does not depend strongly on the initial
energy with which it entered the counter. Holes on the front face make it eas-
ier for neutrons with energies <1 MeV to penetrate past the surface, from which
they might otherwise be reflected. The long counter does not measure neutron
spectra.
Neutron spectral information can be inferred by the use of polyethylene mod-
erating spheres (Bonner spheres) of different diameters with small lithium iodide
scintillators at their centers. A series of five or more spheres, ranging in diame-
ter from 2 to 12 in., is typically used. The different sizes provide varying degrees
of moderation for neutrons of different energies. The response of each sphere is
calibrated for monoenergetic neutrons from thermal energy to 10 MeV or more.
