Page 145 - Radiochemistry and nuclear chemistry

130 Radiochemistry and Nuclear Chemistry

Whereas it is possible to specify maximum ranges for charged particles, this is not
possible for neutral particles such as neutrons and v-quanta. If the absorber is not too thick,
these particles undergo only one collision, or at the most a few, before they are absorbed.
As a result the absorption curve has an exponential form.

r = ~b 0 e- zx (6.7)

where tt is the total attenuation coefficient. Thus for n and ~ we have

~bab s (x) = e -px (6.8)
The reduction in intensity of a beam can occur by two mechanisms. One involves the
deflection or scattering of the particles from the direct line of path between the source and
the detector and is described by the scattering coefficient tts. The second mode of reduction
is the complete transfer of the projectile energy to the absorbing material (the particles are
"captured" ) and is designated by the (energy) absorption coefficient #a" The (total)
attenuation coefficient in (6.7) is the sum of both these modes.

# = #s + #a (6.9)

Both tts and tt a can be measured independently. The (total) attenuation coefficient is of
primary interest in radiation shielding, while the (energy) absorption coefficient is important
in considering radiation effects on matter.

6.3. Absorption of protons and heavier ions
The mode of interaction of protons and heavier charged particles with the atoms of the
absorbing material can be illustrated by considering the absorption of a-particles. With rare
exception, a-particles emitted by radioactive nuclides have energies between 4 and 9 MeV.
Since the a-particles are so much heavier than electrons, they are deflected very slightly
when their Coulomb fields interact with atoms or molecules to form ion pairs. As a result,
c~-particles travel in a straight line as they pass through matter, which explains the straight
paths observed for a-particles in cloud chamber photographs (Fig. 6.5). This is in contrast
to the very curved or irregular paths of the secondary electrons emitted in the formation of
the ion pair. For a 5 MeV a-particle the maximum energy of the secondary electrons is 2.7
keV. However only a small fraction of the secondary electrons actually receive this much
energy; the average energy of the secondary electrons is closer to 100 eV. The ionization
caused by more energetic secondary electrons is usually referred to as 6-tracks (cf. {}7.2).
In solids and liquids the total path length for a-particles from radioactive decay is quite
short. However, in gases at standard temperature and pressure the paths are several
centimeters long (Table 6.2). The range in air for c~-particles with an initial energy Ec~ MeV
can be calculated by the empirical equation (Pair = 1.293 kg m-3):

J~air = 0.31Ea 3/2 (cm) = 0.40 Ea 3/2 (nag cm -2) (6.10)

The range/~z in other materials can be approximated roughly by