Biomedical Engineering Reference
In-Depth Information
Fig. 10.46
Arrangement of proton-recoil telescope for measuring spectrum of a neutron beam.
Neutron spectra can also be inferred from the observed range distribution of
recoil protons in nuclear track emulsions. Neutrons with at least several hundred
keV of energy are needed to produce protons with recognizable tracks.
Threshold Foil Activation
Like low-energy neutrons, intermediate and fast neutrons can be detected by the
radioactivity they induce in various elements. With many nuclides, a threshold en-
ergy exists for the required nuclear reaction. When foils of several nuclides are
simultaneously exposed to a neutron field, differences in the induced activity be-
tween them can be used to obtain information about the neutron energy spectrum
as well as the fluence.
As described at the end of Section 9.8, the activity induced in a foil or other target
is a combined result of (1) the neutron fluence (and fluence rate) at energies above
threshold and (2) the energy-dependent cross section for the reaction. Activation
provides an estimate of neutron fluence at an effective threshold energy above the
minimum given by Eq. (9.29). The effective threshold energy is thus only an ap-
proximate concept; for a given material, different specific values can be found in
the literature. Table 10.3 lists some reactions and their effective threshold energies
used for fast-neutron detection. As an example, if an exposed aluminum foil shows
induced activity from
27
Mg and a simultaneously exposed cobalt foil shows no in-
duced activity from
56
Mn, then one can infer that neutrons with energies 3.8 MeV
<
T
<5.2
MeV were present. To obtain accurate spectral data from threshold-detector
systems, one must take into account such factors as the masses of the particular
isotopes in the foils, their neutron cross sections as functions of energy, the expo-
sure history of the foils, and the half-lives of the induced radioisotopes.
Bubble Detectors
The popular bubble detector is a unique and important personal neutron dosime-
ter. Figure 10.47 shows a pair of detectors, one exposed to neutrons and the other
unexposed. The basic dosimeter consists of 8 cm
3
of a clear polymer in which tens
