Biomedical Engineering Reference
In-Depth Information
14
N(n, p)
14
C
Since nitrogen is a major constituent of tissue, this reaction, like neutron capture
by hydrogen, contributes to neutron dose. The cross section for thermal neutrons
is 1.70 barns, and the
Q
value is 0.626 MeV. Since their ranges in tissue are small,
the energies of the proton and
14
C nucleus are deposited locally at the site where
the neutron was absorbed. Capture by hydrogen and by nitrogen are the only two
processes through which thermal neutrons deliver a significant dose to soft tissue.
23
Na(n,
γ
)
24
Na
Absorption of a neutron by
23
Na gives rise to the radioactive isotope
24
Na. The latter
has a half-life of 15.0 h and emits two gamma rays, having energies of 2.75 MeV
and 1.37 MeV, per disintegration. The thermal-neutron capture cross section is
0.534 barn. Since
23
Na is a normal constituent of blood, activation of blood sodium
can be used as a dosimetric tool when persons are exposed to relatively high doses
of neutrons, for example, in a criticality accident.
32
S(n, p)
32
P
For this reaction to occur, the neutron must have an energy of at least 0.957 MeV
[Eq. (9.32)]. It is an example of but one of many threshold reactions used for neu-
tron detection. As described in Section 10.7, the simultaneous activation of foils
made from a series of nuclides with different thresholds provides a means of es-
timating neutron spectra. The existence of
32
S in human hair has also been used
to help estimate high-energy (
∼
3.2
MeV) neutron doses to persons exposed in crit-
icality accidents. The product
32
P, a pure beta emitter with a maximum energy of
1.71 MeV and a half-life of 14.3 days, is easily counted.
113
Cd(n,
γ
)
114
Cd
Because of the large, 21,000-barn, thermal-neutron capture cross section of
113
Cd,
cadmium is used as a neutron shield and as a reactor control-rod material. The
relative abundance of the
113
Cd isotope is 12.3%. The absorption cross section of
113
Cd for neutrons is large from thermal energies up to
∼
0.2
eV. It drops off two
orders of magnitude between 0.2 eV and 0.6 eV. A method for measuring the ratio
of thermal to resonance neutrons consists of comparing the induced activities in
two identical foils (e.g., indium), one bare and the other covered with a cadmium
shield. The latter absorbs essentially all neutrons with energies below the so-called
cadmium cutoff of ∼
0.4 eV.
115
In(n,
γ
)
116m
In
The cross section for thermal-neutron capture by
115
In (95.7% abundant) is
157 barns, and the metastable
116m
In decays with a half-life of 54.2 min. The in-
duced activity in indium foils worn by persons suspected of having been exposed
to neutrons can be checked as a quick-sort method following a criticality accident.
In practical cases the method is sensitive enough to permit detection with an ion-
ization chamber as well as a GM or scintillation survey instrument. The degree
of foil activity depends so strongly on the orientation of the exposed person, the
