Biomedical Engineering Reference
In-Depth Information
Fig. 9.1
Oak Ridge Bulk Shielding Reactor, an early
swimming-pool type in which water served as coolant,
moderator, and shield. Glow around reactor core is blue light
emitted as Cerenkov radiation (Section 10.4) by electrons that
travel faster than light in the water. (Photo courtesy Oak Ridge
National Laboratory, operated by Martin Marietta Energy
Systems, Inc., for the Department of Energy.)
and has a peak thermal-neutron fluence rate of
2.6 × 10
15
cm
-2
s
-1
. Its neutrons are
used to explore the structure and behavior of irradiated materials such as met-
als, polymers, high-temperature superconductors, and biological samples. Unique
studies of materials after welding and other stresses as well as neutron-activation
analyses are conducted. The HFIR produces some 35 primary radioisotopes for
medical and industrial purposes. It is the Western World's sole producer of
252
Cf.
Particle accelerators are used to generate neutron beams by means of a number
of nuclear reactions. For example, accelerated deuterons that strike a tritium target
produce neutrons via the
3
H(d,n)
4
He reaction, that is,
2
1
H+
1
H
4
2
He +
0
n.
(9.1)
→
To obtain monoenergetic neutrons with an accelerator, excited states of the product
nucleus are undesirable. Therefore, light materials are commonly used as targets
for a proton or deuteron beam. Table 9.1 lists some important reactions that are
used to obtain monoenergetic neutrons. The first two are exothermic and can be
used with ions of a few hundred keV energy in relatively inexpensive accelerators.
