Page 185 - Radiochemistry and nuclear chemistry

Radiation Effects on Matter 169

The relationship between LET, which refers to the energy absorbed in matter, and the
stopping power, which refers to the energy loss of the particle, is

dElo~/dx = dE~b~/dx + E x (7.6)

The difference E x in these two energy terms is related to the energy loss by electromagnetic
radiation (mainly bremsstraldung).

7.1.2. Uncharged radiation

When neutrons or photons having the incident particle energy Ein are absorbed, a certain
fraction of energy Etr is transferred into kinetic energy of charged particles when traversing
the distance dx. We can define an energy transfer coefficient as

P'tr = Ein -I dEtr/dx (re'l) (7.7a)

If we neglect the bremsstrahhmg associated with the absorption of the secondary charged
particles formed in the initial absorption processes, Etr is the energy absorbed (designated
as Eabs), and we can write (7.7a) as

pa = Ein -1 dEabs/dX (m -1) (7.7b)

7.2. Radiation tracks

The energy lost when a high energy charged particle is slowed in matter gives rise to a
trail (or, more commonly, track) of ionized and excited molecules along the path of the
particle; Figure 7.1 schematically depicts a track. Figure 6.5 shows cloud chamber
photographs of such tracks. A photon imparts a large fraction of its energy to a single
electron which subsequently ionizes and excites many other molecules along its path. The
absorption of any type of ionizing radiation by matter thus leads to the formation of tracks
of ionized and excited species. Whereas these species generally are the same in a particular
sample of matter regardless of the type of radiation, the tracks may be sparsely or densely
populated by these species. Expressions such as linear energy transfer (LET) and stopping
power (S) are based on the implicit assumption of a continuous slowing down process and
thus gives quantitative information on the average energy loss but only qualitative
information on the densities of reactive species.
Electrons are liberated in the ionization process with greatly varying kinetic energy. If the
energy of these secondary electrons is relatively low ( < 100 eV) their range in liquids and
solids is short and the ionizations and excitations caused by these electrons take place close
to the primary ionizations leading to the formation of small spurs containing ionize~ and
excited species. Secondary electrons with high kinetic energy form tracks of their own bran-
ching from the primary track. Such electrons are called ~-rays. For high energy electrons
spurs are formed at well separated intervals along the track whereas for densely ionizing
radiation such as c~ particles, protons and recoil atomic ions the spurs overlap and form