L4 interaction with matter

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5th BAERA Training Course on Radiation Protection for Radiation Control
Officers (RCOs) of Industrial Practices
Bangladesh Atomic Energy Regulatory Authority
Agargaon, Dhaka
06-09 November 2017
L4: Interactions of Radiation with
Matter

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Radiation means such
radiation, which, while
dispersing or propagating
through matter or space,
produces electromagnetic
induction or effects
Radiation

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Ionizing Radiation means such radiation as
is capable of producing ion pairs directly or
indirectly in a matter while passing through it;
When an individual particle causes an atom
or molecule to completely remove an electron
from its orbit, we call this Ionization
This energy release is Ionizing Radiation
Ionizing Radiation

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Atomic structure
proton
neutron
electron

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Atomic structure

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Introduction and Overview
 Interaction Types
Charged Particle Interactions
Photon Interactions
 Photon Interaction Rates
Attenuation
Linear Attenuation Coefficient
Mass Attenuation Coefficient

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 Charged particles :
Alpha & Beta particles, Protons---
-mass of these particles differ by many orders
of magnitude
-types of nuclear interaction are different
Photon Interaction:
X-ray and Gamma-ray
-Interactions are quite different
(photons are mass less and travel at the
speed of light)
Radiation interaction with matter

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Alpha & Beta particle decay,
Gamma Radiation
Alpha particle
charge +2
Beta particle
charge -1
Gamma radiation

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Radiation Mass Electric Charge Velocity
Alpha Particles relatively heavy double positive relatively slow
Beta Particles about 8,000 times lighter negative less than the velocity of light
Gamma Rays None None 3 x 108 m/s in free space
Alpha & Beta particle decay,
Gamma Radiation

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The charged particles ( and ) interact with matter
via electrostatic forces leaving behind an ionised atom or
molecule.
After each interaction the  or -particle loses energy
and with sufficient interactions will eventually be
‘stopped’.
Charged Particles

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 particles are relatively large, doubly charged and
travel at about 1/20 of the speed of light.
Because of its slow speed and high charge the 
particle will ionise virtually every molecule it encounters,
consequently they lose their energy in a short distance
and have a short range.
Alpha Particles

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- and + particles are much lighter than alphas, are singly
charged and travel at about the speed of light.
Consequently they interact less strongly than alphas and will
produce an ion pair once every thousand molecules encountered (air
or water).
When  particles pass close to an atom they can lose some of their
energy by a radiative process and emit bremsstrahlung radiation
(‘breaking radiation’) in the form of X-rays.
Most of the  energy is lost by ionisation and only a small amount
is lost to bremsstrahlung.
Beta Particles

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The range of an  or  particle is dependent on the
number of atoms the particle encounters when it travels
through a medium.
The best way to estimate the number of atoms in a
medium is using the concept of mass per unit area. If the
density of a material is given by  gcm-3, then the mass
per unit area of a sheet of thickness t is t gcm-2.
How can you measure the range of Alpha
and Beta particles?

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If we express the range in this form then we find that
the formula for the range of  and  particles:
2
2

 gcm
E
R 

2
1000

 gcm
E
R 

for  particles of maximum energy E (in MeV)
for  particles of maximum energy E (in MeV).
How can you measure the range of Alpha
and Beta particles?

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Example 1
If the maximum energy of a 32
P beta particle is 1.7 MeV
a) What is the range of this beta?
If the density of a sheet of perspex is 1.2 gcm-3
b) How far into the perspex would the beta particle penetrate?
Answer 1
a) Using formula the range of this beta is
2
7
.
1 = 0.85 gcm-2.


R
cm
7
.
0
2
.
1
85
.
0

b) Given the density of perspex, the penetration distance
is simply

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Example 2
How far will 5 MeV alpha particles penetrate in skin, given the
density of skin is 1 gcm-3?

1000
5
5 x 10-3 gcm-2, but density of skin,  = 1 gcm-3


R
= 5 x 10-3 cm = 50 m (the outer dead layer of skin  70 m)
Answer 2
Range R =
 penetration distance is

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 An alpha particle loses about 35.5 eV for each ion
pair formed
 Calculate the number of ion pairs produced by an
alpha particle with a kinetic energy of 5.5 MeV
Loss of energy
Ion pair production from alpha particle interaction

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An x-ray beam or gamma radiation passes through an
object, three possible fates await each photon, as shown
in below:
1. It can penetrate the section of matter without
interacting.
2. It can interact with the matter and be completely
absorbed by depositing its energy.
3. It can interact and be scattered or deflected from its
original direction and deposit part of its energy.
Photon Interactions

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Photons entering the human body will either Penetrate, be
absorbed, or produce Scattered Radiation
There are two kinds of
interactions through which
photons deposit their
energy; both are with
electrons. In one type of
interaction the photon
loses all its energy; in the
other, it loses a portion of
its energy, and the
remaining energy is
scattered. These two
interactions are shown
below.

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Interactions Between Photons and Electrons

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There are several processes by which the
electromagnetic fields of a photon interact with
an absorbing material.
The major interactions are :
(i) Photoelectric effect
(ii) Compton scattering
(iii) Pair production
Photon Interactions

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(i) Photoelectric Effect
The photoelectric effect is an absorption
process and usually occurs for low energy
photons (<0.1 MeV) such as X-rays. The
energy EX of the X-ray is transferred
(absorbed) to an inner electron, normally
a K-shell electron, and this gives the
electron sufficient energy to escape from
the atom. The atom is left positively
charged (ie, an ion) and in an excited
state due to the vacancy left in the inner
shell. This vacancy is quickly filled by
another electron dropping down from a
higher shell with the subsequent release
of a photon of frequency determined by
the two shells involved.

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(ii) Compton Scattering
The Compton Effect is essentially an elastic
collision process and generally occurs for
high-energy photons (>0.1 MeV) such as
gamma rays. An incoming photon of high
energy ( - ray) collides with an electron in
the valence band, ejecting the electron from
the atom. A photon of lower energy (and
hence different frequency) than the original is
produced that travels at an angle to the
direction of the incident photon, determined
by conservation of momentum. The energy of
the ejected or Compton electron can be
determined by knowledge of the energies of
the incoming and scattered photons.

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(iii) Pair Production
The third most important process
by which photons lose their energy
is electron-positron pair formation.
Al least 1.024 MeV of photon
energy are required for pair
production, because the energy
equivalent of the rest mass of the
electron and positron is 0.51 MeV
each
The available kinetic energy to be
shared by the electron and the
positron is the photon energy
minus 1.02 MeV, or that energy
needed to create the pair

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The relative importance of the three major types of gamma-ray interaction.
This lines show the values of Z and h for which the two neighboring
effects are just equal.


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Linear Attenuation Coefficient
Gamma rays interact with the atoms in
any material by means of the
photoelectric effect, pair production, or
Compton scattering. The gammas are
either absorbed or scattered away from
their original path. Let us consider a very
thin slice of a material slab, as in Fig 1,
with a thickness of x where I (x) is the
incident flux of gamma rays impinging
upon that slice and I (x + x)is the flux
of gamma rays emerging undisturbed
from that slice. So, the fraction which
has collided is

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Linear Attenuation Coefficient
 
x
I
I
fraction
collided



 
x
x
I
I




 
 
x
I
x
I
I



If x is sufficiently thin, say, the space between several atoms, the equation
(3.1) is proportional to x , as in
This can be reordered to
This proportionality relationship can be converted to an equation with a
proportionality constant, which in this case is designated as 
 
 
x
I
x
I
I





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Linear Attenuation Coefficient
This constant is called the linear attenuation coefficient. It can also be
thought of as a probability factor where the various gamma-atom
interactions have a probability of occurrence per unit of path length in the
material. The units of  are usually given as cm-1 i.e. inverse length. The
values for  vary for the energy of the gamma ray and the attenuating
material that is used. Other types of radiation have similar attenuation
coefficients, which also vary for the energy of the radiation and the type of
the material.
 
x
I
dx
dI



  x
e
I
x
I 

 0
Making the width of the slice, x ,
infinitesimally thin, equation above
becomes
Setting a boundary condition of x0
where x = 0, and integrating the
above, we get

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The attenuation coefficient or cross-sections give the
probabilities of removal of a photon from a beam under
conditions of good geometry, where it is assumed that
any of the possible interaction removes the photon from
the beam. The possible probability of a gamma ray
being absorbed is called the absorption coefficient.
The total attenuation coefficient, therefore, is the sum
of the coefficients for each of the three interactions,
which can be written as:
t
 = pp
cs
pe 

 

Attenuation coefficient

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t
 = pp
cs
pe 

 

pe
 = Photoelectric absorption coefficient
cs
 = Compton scattering coefficient
pp
 = Pair production attenuation coefficient
where,
Attenuation coefficient

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Linear Attenuation Coefficient

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Mass Attenuation Coefficient (µ/r) =
Linear Attenuation Coefficient (µ) / Density (r).
The total attenuation rate depends on the individual
rates associated with photoelectric and Compton
interactions. The respective attenuation coefficients are
related as follows:
µ(total) = µ(photoelectric) + µ(Compton).
Both types of interactions occur with electrons within the
material.
Relationship between the mass and
linear attenuation coefficients

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Mass Attenuation Coefficient

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In some situations it is more desirable to express the attenuation
rate in terms of the mass of the material
The quantity that affects attenuation rate is not the total mass of
an object but rather the area mass.
Area mass is the amount of material behind a 1-unit surface area,
The area mass is the product of material thickness and density:
Area Mass (g/cm2
) = Thickness (cm) x Density (g/cm3
).
The mass attenuation coefficient is the rate of photon interactions
per 1-unit (g/cm2
) area mass.
Mass Attenuation Coefficient

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Some examples of suitable shielding, alpha and beta
radiation can be stopped by shielding, X and gamma
radiation can only be attenuated