Page 188 - Radiochemistry and nuclear chemistry

172 Radiochemistry mad Nuclear Chemistry

according to Figure 7.2, and A is the source strength (Bq). E.g. 24Ne decays (Fig. 4.6) to
8% with two 7's (0.878 and 0.472 MeV) and to 92% with one 7 (0.472 MeV). Thus the
sum in (7.9) contains two terms, one for 0.878 MeV 7 (nl = 0.08) and one for 0.472 MeV
7 (n2 = 1). The ki-values are taken from Figure 7.2. The relaxation length #i x is
computed for each energy E i and the corresponding attenuation factors of the radiation
shield e-~ x (x m thick) are calculated using e.g. Figure 6.17. Build-up factors B i can be
estimated from Figure 6.20. It should be noted that (7.9) does not account for scattering
around the shield ('sky-shine'), which has to be estimated separately; see text books on
radiation shielding.
Closely related to the absorbed dose is the kerma (K),which is the kinetic energy released
per unit mass by uncharged particles and electromagnetic radiation (n and 7):

K = dEtr/dm = ~ E tttr O- 1 (7.10)

where E is the radiation energy, ~ is the particle fluence (particles/m 2) and/hr/p is the
mass energy transfer coefficient. The SI unit of the kerma is J/kg.
In the following we refer to all absorbed radiation energy as the radiation dose
independent of whether the incident radiation is charged or uncharged particles or photons.
Radiation chemical yieM is described in terms of G-values. Originally G(x) was defined
as the number of molecules of x transformed per 100 eV absorbed energy (a practical
notation as most reactions have G-values of < 10). In the SI system the symbol G(x) is the
same but the unit is mol/J. The conversion factor between the two units is 1 mol/J = 9.649
• 106 molecules per 100 eV absorbed.

7.4. Metals

Metals consist of a solid lattice of atoms whose valence electrons cannot be considered
to belong to any particular atom, but rather to a partially filled energy band (the conduction
band) established by the total lattice network. Interaction of radiation with the metal can
cause excitation of bound electrons in the atoms to the conduction band.
While irradiation by 7-rays and electrons has little influence on metallic properties, heavy
particles cause serious damage through their collision with atoms in the metal lattice
network. This results in displacements of the atoms from their lattice positions. The number
of displacements (ndisp) depends on the amount of energy transferred in the collision event
(Etr) to the recoiling (target) atom, and the energy required for moving this atom from its
lattice position. This so-called displacement energy (Edisp) is 10 - 30 eV for most metallic
materials. According to the Kinchin-Pease rule,

ndisp _< Etr/(2 Edisp ) (7.11)

The maximum energy transferred can be calculated assuming purely elastic collisions
between hard spheres (w Thus for a 1.5 MeV fission neutron, Etr(max) is 425 keV
in C, 104 keV in Fe, and 25 keV in U. With Edisp ~. 25 eV, up to 8500, 2080 and 500
displacements, respectively, occur in these metals due to the absorption of a fission neutron
(neglecting nuclear reactions). In practice the numbers are somewhat smaller, especially at