GAMMA RAYS INTERACTION WITH MATTER
© M. Ragheb
4/21/2009
1. INTRODUCTION
Gamma rays interaction with matter is important from the perspective of shielding
against their effect on biological matter.
They are considered as ionizing radiation whose
scattering by electrons and nuclei leads to the creation of a radiation field containing negative
electrons and positive ions.
The main modes of interaction of gamma rays with matter are the photo effect both in its
photoelectric and photonuclear forms, Compton scattering and electron positron pair production.
To a minor extent photofission, Rayleigh scattering and Thomson scattering also occur.
Each of these processes occurs in different forms.
Different types of scattering can occur
depending on the quantum mechanical properties of the gamma photons.
Electron positron pairs
can be formed in the field of a nucleus and in that of an electron.
The photoelectric effect can
knock out atomic electrons, whereas the photonuclear reaction would knock out elementary
particles from the nucleus.
Gamma rays are emitted in the decay process of radioactive isotopes.
On a cosmic scale,
Gamma Ray Bursts (GRBs) or magnetars generate intense gamma radiation fields that could
affect space travel and exploration.
In addition, bursts of Terrestrial Gamma Ray Flashes, TGFs
occur relatively high in the atmosphere as a result of thunderstorms and are not from the same
sources of gamma rays seen on the ground.
About 15 to 20 such events are observed per month.
2. GAMMA PHOTONS ENERGY
A particle of zero rest mass such as a neutrino or a gamma photon will have a kinetic
energy given by:
ν
γ
h
E
=
,
(1)
where:
h is Planck’s constant = 6.62x10
-27
[erg.sec],
λ
ν
c
=
is the frequency of the gamma photon,
c = 3x10
10
[cm/sec] is the speed of light,
λ
is the wave length of the electromagnetic radiation [cm].
The momentum carried by the gamma photon is a vector quantity given by:
ˆ
ˆ
E
h
p
i
i
c
c
γ
γ
ν
=
=
(2)
1

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Gamma rays interaction with matter causes the generation of other charged particles such
as positrons and electrons at relativistic speeds.
If we consider the ratio of the particle speed to
the speed of light as:
v
c
β
=
,
and its rest mass as m
0
, then the particle’s relativistic parameters become:
2
/
1
2
0
)
1
(
β
−
=
=
m
m
Mass
(3)
2
/
1
2
0
2
/
1
2
0
)
1
(
)
1
(
β
β
β
−
=
−
=
=
=
c
m
v
m
mv
p
Momentum
(4)
2
0
0
2
1/2
1
1
(1
)
Kinetic energy
T
m c
mc
m c
β
⎛
⎞
=
=
−
=
−
⎜
⎟
−
⎝
⎠
2
2
(5)
2
2
0
2
1/2
(1
)
m c
Total energy
E
mc
β
=
=
=
−
(6)
Squaring and rearranging Eqn. 3, we can obtain a relationship between the total energy E
and momentum p:
2
2
0
2
2
2
2
2
0
2
2
2
2
2
2
0
2
2
2
2
2
0
1
m
m
m
m
m
m c
v m
m c
m c
p
m c
β
β
=
−
−
=
−
=
−
=
Dividing into m
2
0
c
2
, we get:
2
2
0
0
2
0
2
2
0
1
.
1
.
1
p
mc
m c
m c
mc c
m c c
E
m c
⎛
⎞
⎛
⎞
=
−
⎜
⎟
⎜
⎟
⎝
⎠
⎝
⎠
⎛
⎞
=
−
⎜
⎟
⎝
⎠
⎛
⎞
=
−
⎜
⎟
⎝
⎠
2