Page 156 - Radiochemistry and nuclear chemistry

Absorption of Nuclear Radiation 141

scattering. This process is known as backscattering, since the backing (or support) for
radioactive samples may cause scattering of a certain fraction of the particles through as
much as 180 ~ The fraction of back-scattered/3-radiation depends on the geometry of the
measuring system, the energy of the/~-particles, and the thickness and electron density of
the backing material. In Figure 6.14 the percent backscattering as a function of the atomic
number of the backing material is shown for four/~-energies (Emax); the radioactive sample
is considered infinitely thin (i.e. no self-absorption). From the curve for 32p on platinum
(Z = 78) we see that about 40 % of the measured radiation is due to back-scattered radiation
(0.8/(1.0 + 0.8) = 0.4). Backscattering increases with the thickness of the backing material
up to a saturation value which is reached when the thickness of the backing is about one-
fifth of the extrapolated range of the/3-particles in that material.

6.5. Absorption of v-radiation

The absence of charge and rest mass for 7-rays results in little interaction with the
absorbing atoms and in long ranges. The number of ion pairs produeexl in a given path
length by 7-rays is only 1-10% of that produced by ~-particles of the same energy (Fig.
6.5); e.g. a 1 MeV 7-ray produces only about one ion pair per centimeter of air. As a
consequence of this low specific ionization of 7-rays, the ionization is almost completely
secondary in nature resulting from the action of a few high energy primary ion pairs.

6.5.1. Attenuation coefficient

Unlike heavy particles and electrons which lose their energy as a result of many
collisions, 7-rays are completely stopped in one or, at most, a few interactions. For thin
absorbers the attenuation of 7-rays follows relation (6.7), where ~ is the number of photons
m -2 s-1. The proportionality factor # is called the (total) attenuation coefficient. When it
has the dimension of m- 1 and the thickness x is expressed in meters, # is referred to as the
linear attenuation coefficient. The attenuation coefficient can be expressed in other ways:

Pm = P/P = era NA/M = a e ZN A/M (6.19)

where p is the density, M the average atomic weight, and Z the average atomic number of
the absorber. NAp/M can be replaced by N v, by which we can define a macroscopic
absorption cross-section F, (cf. w 14.1). E- 1 is the mean free path or relaxation length of the
radiation in the absorbing material. #m (in cm 2 g-1 when x is in centimeters) is the mass
attenuation coefficient; o a is the probability of reaction between a 7-ray and the electron
cloud of the absorber atom (atomic reaction cross-section, m 2 atom- 1); oe is the probability
of the reaction of a -y-ray with a single electron of the absorber (electron reaction
cross-section, m 2 electron- 1). oa and o e are analogous to the nuclear reaction cross-sections
discussed later. In Table 6.1 only the equivalent nuclear and atomic reaction cross-sections
are given.
Since a 7-ray may be removed from the beam in the first few Angstr6ms of its entrance
into the absorber or may travel several centimeters with no interaction at all and then be