Neutrons, Fission, and Criticality - Atoms, Radiation, and-Radiation Protection - page 223

neutron-energy spectrum, and other factors that it does not provide a useful basis
for even a crude estimate of dose. However, the fact that an exposure occurred (or
did not) can thus be established.

197 Au(n, γ ) 198 Au
This isotope, which is 100% abundant, has a thermal-neutron capture cross section
of 98.8 barns. Although not as sensitive as indium, its longer half-life of 2.70 days
permits monitoring at later times after exposure.

235 U(n, f)
Fission (f ) is discussed in Section 9.10. Because of the large release of energy
(∼ 200 MeV), the fission process provides a distinct signature for detecting thermal

neutrons, even in high backgrounds of other types of radiation (cf. Section 10.7).

9.8
Energetics of Threshold Reactions

As mentioned in the last section in connection with the reaction 3 16 S(n, p) 3 15 P ,the
activation of different nuclides through reactions with different threshold energies
provides information on the spectrum of neutrons to which they are exposed. In
this section we show how threshold energies can be calculated.
An endothermic reaction, by definition, requires the addition of energy in or-
der to take place. The reaction thus converts energy into mass. (In the notation of
Section 3.2, Q <0 .) Such a reaction can be brought about by one particle striking
another at rest, provided the incident particle has sufficient energy. In Section 8.6
we considered threshold energies for photonuclear reactions. In this instance, the
reaction occurs when the photon has an energy h ν ≥

- Q needed to provide the in-
crease in mass. The condition for the threshold energy for a neutron reaction is
different. The neutron must have enough energy to supply both the increase in
mass, - Q , and also the continued motion of the center of mass of the colliding
particles after the collision. For photons, the latter is negligible.
To calculate threshold energies we consider a head-on collision. A particle with
mass M 1 strikes a particle with mass M 2 , initially at rest. The identity of the parti-
cles is changed by the reaction, and so there will generally be different masses, M 3
and M 4 , after the encounter. The collision is shown schematically in Fig. 9.9. The

Fig. 9.9 Schematic representation of a head-on collision
producing a nuclear reaction in which the identity of the
particles can change.