Page 184 - Radiochemistry and nuclear chemistry

168 Radiochemistry and Nuclear Chemistry
Table 7.1. Ion pair formation energies for charged particles. All values in e V.

Absorber w j w-j
He(g) 43 24.5 18.5
H2(g ) 36 15.6 20.4
02(g ) 31.5 12.5 19
Air 34 15 19
H20(g or 1) 38 13 25
At(g) 5 MeV 0t 26.4
Ar(g) 340 MeV p 25.5
Ar(g) 10 keV e- 26.4
Ar(g) 1 MeV e- 25.5
Ar(g) average 26 15.7 10.3

electrons) while it is about 5 eV per ion pair in inorganic solids.
The specific energy loss of a particle in matter is called the stopping power r

,~ = dEloss/dx (J/m) (7.2)

where x is the distance traversed by the particle. To a good first approximation the stopping
power of a material is determined by its atomic composition and is almost independent of
the chemical binding of the atoms. Stopping power is a function of the particle velocity and
changes as the particle is slowed down.
The specific ionization J is the number of ion pairs produced per unit path length

J = d/~/dx (ion pairs/m) (7.3)

The value of J depends on the particle and its energy as seen from Figure 6.7. The relation
between ,~ and J is

,~ = w J (J/m) (7.4)
The mass stopping power, ~SIo, is commonly expressed in units of MeV/g cm 2.
Another important concept is the linear energy transfer (abbreviated as LED of charged
particles. It is defined as the energy absorbed in matter per unit path length traveled by a
charged particle

= d ,bJdX (7.5)

Values of LET in water are given for various particles and energies in Table 6.2. For the
same energy and the same absorbing material, the LET values increase in the order:
high energy electrons (also approximately v-rays)
~particles (also approximately soft: X-rays)
protons
deuterons
c~-partieles
heavy ions (ions of N, O, etc.)
fission fragments