Page 183 - Radiochemistry and nuclear chemistry

Radiation Effects on Matter 167

elementary constituents and also combined into more complex polymeric products.
Radiation decomposition (radiolysis) of water caused evolution of hydrogen and oxygen gas
and formation of hydrogen peroxide. Conversely, it was shown that water could be
synthesized through irradiation of a mixture of H 2 and 0 2. In 1911 S. Lind found that 1
g of radium exposed to air resulted in the production of 0.7 g of ozone per hour. By
relating this radiation yieM to the number of ions produced by the amount of radiation,
Lind initiated the quantitative treatment of radiation-induced changes.

7.1. Energy transfer

The chemical effects of radiation depends on the composition of matter and the amount
of energy deposited by the radiation. In this section we consider only the energy transfer.
For this purpose it is practical to divide high energy radiation into (1) charged particles (e-,
e+, a, etc) and (2) uncharged particles (n) and electromagnetic radiation ('y). The latter
produce recoil atomic ions, products of nuclear reactions and electrons as charged
secondary ionizing particles. The terms direct and indirect ionizing radiation are often used
for (1) and (2) respectively.
The amount of energy imparted to matter in a given volume is

Eim p = Ein + ]~Q- Eou t (7.1)

where Ein is the energy (excluding mass energy) of the radiation entering the volume, Eou t
is the energy of the radiation leaving the volume, ]~Q is the sum of all Q-values for nuclear
transformation that have occurred in the volume. For a beam of charged particles Ein =
Elan; for "y-rays it is ET. If no nuclear transformations occur, ]~Q = 0. For neutrons which
are captured and for radionuclides which decay in the absorber, EQ > 0; in the case of
radionuclides already present in the absorber, Ein = 0.

7.1.1. Charged particles

We learned in the previous chapter that the energy of charged particles is absorbed mainly
through ionization and atomic excitation. For positrons the annihilation process (at Eld n ~.
0) must be considered. For electrons of high kinetic energy bremsstrahlung must be taken
into account. However, in the following we simplify by neglecting annihilation and brems-
strahlung processes. The bremsstrahlung correction can be made with the aid of Figure 6.9
which gives the average specific energy loss of electrons through ionization and
bremsstrahlung.
It has been found that the average energy w for the formation of an ion pair in gaseous
material by charged particles is between 25 and 40 eV. For the same absorbing material it
is fairly independent of the type of radiation and the energy. Table 7.1 lists values of w in
some gases. The ionization potentialsj of the gases are lower than the w-values and the rest
of the energy, w-j, must be used for excitation. Since the excitation energies per atom are
_< 5 eV, several excited atoms are formed for each ion pair formed. While it is easy to
measure w in a gas, it is more difficult to obtain reliable values for liquids and solids. They
also differ more widely; e.g. w is 1300 eV per ion pair in hexane (for high energy