Page 164 - Radiochemistry and nuclear chemistry

Absorption of Nuclear Radiation 149

= (n A/(47r r 2 ))e-m (6.26)

it is seen that the intensity from a point source of radiation can be decreased by increasing
either the distance from the source r or the thickness x of the absorber. Alternately, an
absorber with a higher absorption coefficient, #, can be chosen to reduce the thickness
required.
Equation (6.26) is valid only for point sources with ideal geometry, i.e. no back or
multiple scattering, etc. For -y-radiation the thicker the absorber the higher is the percentage
of radiation which may be scattered backwards through secondary (mainly Compton)
scattering. The effect of geometry and absorber thickness can be taken into account by
including a constant B in the absorption equation:

= B ~0 e'~ (6.27)

The "dose build-up'factor B not only takes into account multiple Compton and Rayleigh
scattering but also includes correction for positron formation at high ~,-energies and
subsequent annihilation. Since for thick radiation shielding and high -y-energies the factor
B may reach several powers of 10, it is quite important to be considered in designing
biological shielding for radiation. Calculation of B is difficult and empirical data are most
commonly used. Figure 6.20 gives B-values for a lead shield. The thickness of the shielding
is given in relaxation lengths p.r. This value is obtained from diagrams like Figure 6.17;
e.g. for a 3 MeV -t,/~m is found to be 0.046 cm 2 g-1. If the absorber is 0.1 m thick the
linear density is 100pb = 113 g cm -2 and the relaxation length becomes 5.2. With Figure
6.20 this gives (for the 3 MeV -y-line) a dose build-up factor B of 3. Thus the lead shield
transmits three times more radiation than is expected by the simple relation (6.26). The flux
reduction values for concrete and lead shielding in Figure 6.15 have been adjusted to take
the dose build-up into consideration.
Equation (6.27) is not directly applicable for neutrons. For an estimate of required
shielding we can use diagrams like that in Figure 6.21, which shows the attenuation of
neutrons of three different energies in concrete and water, the most common
neutron-shielding materials. It is necessary also to take into account the -y-rays emitted in
neutron capture, which increases the shielding thickness required.

6.8. Analytical applications of radiation absorption

In previous sections of this chapter we have shown how different particles omitted in
nuclear reactions are stopped in matter without causing nuclear reactions (nuclear reactions
are treated later in this book). If the property of the particle is well defined (e.g. mass,
charge, energy, otc) its interaction with the absorbing material can be well predicted,
provided the composition of the absorbing material also is well defined. Conversely, the
composition of the absorbing material (a "sample") can be determined from studying the
absorption process ( the "irradiationS). For example, atoms of the absorber (sample) may
be knockexi out and can be collected for analyses. When electrons are knocked out from the
atomic shells of the absorber (sample) atoms, either the energy of these electrons may be
analyzed, or the energy of the electromagnetic radiation emitted when the shells are refilled.