1
Hµnéi , 20-2.2006

NguyÔn TriÖu Tó
8261730
0904505414
2
M.Planck
Einstein
N.Bohr M.Curie
3
cell
Atom
Atomic nucleus
Radius of Earth
Radius of observable universe
15000.000.000.light -years
4
Processing
Wide Range of

Radiation
Energy and Intensity
Environment
Industrial
application
Medical
diagnosis
Energy
GeV
keV

The measurement of nuclear radiation is based on its
interaction with the detector.
1* The function of nuclear radiation detectors
2* The absorption phenomena in
the measurement of the radiation
3* With respect to radiation protection.
 We shall deal with the most important mechanisms of the interaction
between nuclear radiation and matter in their basic features.
In order to understand:
6
To organize the discussions that
follow, it is convenient to arrange
the four major categories of
radiations into the following matrix:
Heavy charged particles
(characteristic distance ≅10
-5
m)
Fast electrons
(characteristic distance ≅10
-3
m)
Neutrons
(characteristic length ≅10
-1
m)
X-rays and γ rays
(characteristic length ≅10
-1
m)

hν
k
e
T
e
= E
γ
- I
i
Photoelectric effect cannot take place for a free electron (not associated
with an atom).
Photoelectric
effect
K
L
γ-ray
M
X
A.E
e-
Photoelectric effect
A.E :Auger electron
X-ray
K
L
M
11
12
The law of conservation of energy:


2222
=−−−=−−=
βββββ
γ
orcmE
e
(1 — β)
2
= 1 — β
2
β = 0 and β = 1
Photoelectric effect is therefore possible only for bound
electrons.
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the photoelectric effect cross section
1/

With decreasing E
γ
(increasing ratio of the electron binding to the
photon energy I
K
/E
γ
), the cross section increases first as 1/E
γ
, and later
(as E
γ
approaches I

for E
γ
> I
K
.
* Photoeffect is especially signifficant for heavy materials where
the probability is considerable even for high energies of γ-quanta.
* In light materials, this effect becomes significant only for relatively
low energies of γ-quanta.
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θ
ϕ
hν
hν ’
−
e
E
In Compton scattering the incoming gamma-ray photon is
deflect through an angle
θ
with respect to its original
direction. The photon transfers a portion of its energy to
the electron (assumed to be initially at rest), which is
then known as a recoil electron.
(Free electron)
incoming γ -ray
scattered γ -ray

( recoil electron).
15

c
2
Secondary electron :
E
e
= h ν - h ν’
Compton scattering
e-
θ
φ
E=hν
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







−
−
+=+=
1
1
1
''
2
2
β

22
2422
hhhhcm
e
−+=−
( ) ( ) ( )( )
θνν
cos1//'/
−=−
cmhcc
e
( ) ( )
,2/sin2cos1'
2
θθλλλ
Λ=−Λ=−=∆
Λ= h/m
e
c = 2.42 x 10
-10
cm: The Compton wavelength for electron
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1.The wavelength λ of the displaced line increases
with the scattering angle θ in such a way that:
πθλ
πθλ
θλ
=Λ=∆
=Λ=∆
==∆

)cos1(
2
2
'
θ
υ
θ
υ
υυυ
−+
−
=−=
−
cm
h
cm
h
hhhE
e
e
e
2
2
21
2
cm
h
cm
h
hE

2
2
2
2
11
+
+
−
−
−
+
−
=
e
e
e
e
cmcm
E
ββ
γ
(1)
+−
+=
ee
PPP
γ
(2)
It follows from formula (1) that:
+−

P
2222
1111
β
β
β
β
ββ
γ
γ
+−
+>
ee
PPP
γ
However, this inequality cannot be true,
since in accordance with formula
(2) these vectors form a triangle. 23
→ The threshold energy :
MeVcmE
e
02.12
2
0
=≅
(3)
in the Coulomb field of a nucleus

c
2
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Pair production
Positron combines with an
electron nearby, after
losing kinetic energy, then
the electron and positron
pair annihilates and emits
two photons ( annihilation
photon; m
0
c
2
=511keV).
This process is called
positron annihilation.