EXPERIMENT G
Keele University Physics/Astrophysics Laboratory
School of Physical and Geographical Sciences Experimental Scripts
53
Attenuation of X-rays
1. Introduction
X-ray radiation is penetrating and can be, of course, harmful to human tissue. It is clearly very
important from a safety point of view to understand how x-ray radiation interacts with matter so that
it can be safely used in medical imaging applications.
2. X-ray interactions with matter
If a beam of X-rays of a given energy is incident on an absorber, the reduction in intensity of the beam is
given by the following equation:
()
xII m
μ
−= exp
0
(1)
Where
I
0
is the intensity of X-rays hitting the absorber (over a fixed time), and
I
is the intensity of X-
rays which pass through without interacting at all.
x
is the thickness of the absorber, measured as
mass per unit area (e.g. in units of
g cm
-2
)
and
μ
m
is called the
mass attenuation coefficient (e.g.
in units of
cm
2
g
-1
)
. Clearly the larger the values of
x
and
μ
m
, then the greater the absorption of the
radiation. The mass attenuation coefficient depends on the density of the material and the wavelength
of the X-ray. One can also define a
half thickness
,
x
1/2
, defined as the mass per unit area required to
reduce the intensity of the radiation by a factor of two.
1
m
x
μ
693.0
2
1=
(2)
In this experiment you will measure the mass attenuation coefficient and the half-value thickness of x-
rays, by measuring count rates with a varying thickness of aluminium placed in between the source
and the detector.
1
Note that equations 1 and 2 are almost identical to the equations for radioactive decay rates:
()
tAA
λ
−= exp
0
and
λ
693.0
21 =T
.
