Page 147 - Radiochemistry and nuclear chemistry

132 Radiochemistry and Nuclear Chemistry

TABLE 6.2. Range in water, and average linear energy transfer (LET) values for different
radiation

Upper half refers to monoenergetic (accelerated) particles. For B-decay E.b . = 1/3 Ems x

Maximum range Average LET
value in water
Radiation Energy (MeV) cm air mm water 0ceV/~,m)
Electron 1 405 4.1 0.24
3 1400 15 0.20
10 4200 52 0.19
Proton 1 2.3 0.023 43
3 14 0.014 21
10 115 1.2 8.3
Deuteron 1 1.7 - -
3 8.8 0.088 34
10 68 0.72 14
Helium 1 0.57 0.0053 190
3 1.7 0.017 180
10 10.5 0.11 92
Fiss. fragment 100 2.5 0.025 3300
Z2~a (cx) E, 4.80 3.3 0.033 145
21~ (o0 E,~ 5.30 3.8 0.039 136
222Rn (r E, 5.49 4.0 0.041 134
3H (/3) Ema x 0.018 0.65 0.0055 1.1
35S (t3) Ema x 0.167 31 0.32 0.17
~176 (/3) Ema x 0.544 185 1.8 0.10
32p (13) Ema x 1.71 770 7.9 0.07
~/' (~ Ema x 2.25 1020 11 0.07
137Cs (3') E v 0.66 x,/i = 8.1 cm H20 0.39
~~ (3') E, 1.20-1.30 x u = 11.1 cm H20 0.27

where A z is the atomic weight of the absorber. Figure 6.6 shows the range of various
charged particles in an aluminum absorber. The range of a 5 MeV c~ is 6 nag cm-2; thus
~m = 6 x 10-3/OAl cm = 0.002 ram. Alpha-particles from radioactive decay are easily
stopped even by the thickness of a sheet of paper.
When the absorber consists of a composite material, containing the weight fractions w 1,
w 2, w 3, etc of elements 1, 2, 3, etc with ranges ~l,/~2,/~3, etc, the range Rcomp in the
absorber is obtained from the relation

1/~comp = Wl/~ 1 + w2/~ 2 + w3/~ 3 + ... (6.12)

The number of ion pairs formed per millimeter of range for c~-particles, protons, and
electrons are shown in Figure 6.7a. The larger specific ionization of the a-particles
compared to the protons is related to the fact that the former are doubly charged. In general
the specific ionization increases with the ionic charge of the particle for the same kinetic
energy. Fission fragments that initially have very large energies also have very large ionic
charges leading to quite high specific ionization in their absorption in matter; their range
is 2-3 cm in air and 2-3 mg cm -2 in aluminum.