Page 157 - Radiochemistry and nuclear chemistry

142 Radiochemistry and Nuclear Chemistry

removed, it is not possible to apply the range concept to 3~-rays in the way that it is applied
to charged particles. However, it is experimentally easy to measure the thickness of
absorber necessary to remove half of the initial 3~-rays (half thickness value) from a beam,
or reduce it to 1/10, 1/100, etc. Figure 6.15 shows the required thickness of concrete and
lead necessary to reduce 3~-rays of different energies by factors of 10 (shielding thicknesses
are further discussed in w The half-thickness value is

x: h = In(2)/# (6.20)

and the 1 / 10 value

Xl/lO -- In(lO)/# (6.21)

6.5.2. Partial absorption processes

Gamma-ray absorption occurs as illustrated in Figure 6.16 by four different processes:
coherent scattering, photoelectric effect, Compton effect, and pair production. For each of
these processes, a partial coefficient can be expressed:

# = #coh + /Zphot + #Comp + #pair (6.22)

100

LEAD
f
25 s 250
s~oO
A ,i
E 20 CONCRETE --~,., 2oo
v~
v)
LU
Z s
U ., %4
~r 15 ,s 150
I- ,p
h,
Ill
m
m 10 100
102
9 9 ~ ~ |
5 50

0 0
0 1 2 3 4 6 6 7' 8 9 10
GAMMA RAY ENERGY (MeV)
FIG. 6.15. Thickness of lead and concrete reducing 7-ray flux by different powers of 10,
as function of 7-ray energy. The curves are for thick absorbers and include build-up factors.