Panasonic PT-DZ12000U Operating Instructions Manual page 71

Neutron Half-Value Layers in centimeters
Energy in MeV
Polyethylene
W ater
Concrete
Dam p soil
exam ple:
How m any half-value layers of polyethylene are needed
to attenuate a 100 m Rem /hr 5 MeV neutron source to 5
m Rem /hr? How thick does the polyehylene need to be?
I = I x 0.5
0
I = 5 m Rem /hr
I = 100 m Rem /hr
0
n = the num ber of half-value layers
n
I/I = 0.5
0
5/100 = 0.05
ln 0.05 = n x ln 0.5
ln 0.05/ln 0.5
-2.996/-0.693
4.32 = n
It will take 4.32 half-value layers of polyethylene to
reduce attenuate the neutron source.
4.32 half-value layers is 4.32 x 6.1 cm = 26.4 cm
1
3.7
4.3
6.8
8.8
n
n
= 0.5
= n
= n
5
6.1
6.9
11
14.3
60
10 15
7.7 8.8
8.8 10.1
14 16
18.2 20.8