Regional Basic Professional Training Course in Korea

❙ 113 ❙
2. Radiation Protection
When the thickness t of the barrier is chosen such that the original number of photons N0
is reduced to one‐half of its value, N=1/2 N0 then t is written as t1/2 and is referred to as
the HVL:
N =
1
2
N
0
= N
0
e
-μ
t
1/2
The HVL, or as it is sometimes called, the half‐value thickness (HVL), is a measure of
the quality or penetrating power of the beam. The higher the energy of the photons, the
thicker will be the HVL.
Dividing both sides of Eq. (2.6) by N0, a useful relation can be derived between μ and t 1/2:
which gives:
Since:
we obtain:
1 -μ
= e
2
Multiplying both sides by minus one gives:
t
1/2
ln 1 ‐ ln 2 = ‐μ t1/2
ln 1 = 0
‐ln 2 = ‐μ t 1/2
This result leads to two relations. (1) Divide both sides of Eq. (2.7) by μ:
ln
2
(2.6)
ln 2 = μ t 1/2 (2.7)
=
t
1 / 2
μ (2.8)