2. Interaction of radiation with matter..pptx

2.Interactio
n of
radiation
with matter.

The ionizing radiation interaction is divided in to two categories :
1.Particles
1.1 Neutral particles : neutron
1.2 charged particles : alpha and - /+ beta particles
2.Electromagnetic waves
Interaction of radiation with
matter
The interaction depends on :
1.Type of the radiation.
2. Energy of the radiation.
3. Type of interaction medium.

Introduction
• “The interaction of charged particles with matter” concerns the transfer of energy from the
charged particles to the material through which they travel.
• The “charged particles” considered here are:
- Alpha particles (+2 charge)
- Beta particles (+ or -1 charge) or electrons
• Photons and neutrons, which have no charge, interact very differently.
• Charged particles passing through matter continuously interact with the electrons and
nuclei of the surrounding atoms.
• In other words, alpha and beta particles are continually slowing down as they travel
through matter.
• The interactions involve the electromagnetic forces of attraction or repulsion between the
alpha or beta particles and the surrounding electrons and nuclei.

Quantitative Measures of Energy Loss
The four most common measures of energy loss by charged particles:
1. W Value
2. Specific Ionization
3. Stopping power or Linear Energy Transfer
4. Mass Stopping Power

• Beta particles lose an average of 34 eV per ion pair produced in air.
• Alpha particles lose an average of 36 eV per ion pair produced in air.
• Alpha particles and beta particles (or electrons) lose an average of approximately
22 eV per ion pair produced in water (Turner p. 140, 161)
W Value
• It depends on:
• W is the average energy lost by a charged particle per ion pair produced.
the type of charged particle
material that is being ionized
• W doesn’t change much with the energy of the particle, but it does increase at low energies (< 0.2
MeV) for protons and alpha particles.

• It depends on:
• Specific ionization is the average number of ion pairs produced per unit distance traveled in a
material by a charged particle.
the type of charged particle,
the energy of the charged particle the
material through which it travels.
• Alpha particles produce 20,000 to 60,000 ion pairs per centimeter (cm) in air.
• Beta particles might produce 100 ion pairs per cm in air.
Specific Ionization

• There is no practical difference between stopping power and linear energy
transfer.
• The stopping power or the linear energy transfer is the average energy lost by a
charged particle per unit distance traveled.
• Typical units: MeV/cm or eV/um
• When a distinction is made:
 Stopping power is used to describe the total energy lost by the charged particle.
 Linear energy transfer (LET) is used to describe the energy lost by the charged particle that is
locally absorbed in the material the particle is traveling through.
In this sense, stopping power is akin to kerma while LET is akin to absorbed dose
Stopping Power and Linear Energy Transfer

• LET is sometimes referred to as the restricted
stopping power.
It describes the energy lost by charged particles in
low energy interactions.
The assumption is that the secondary electrons
produced in these low energy interactions don’t travel
outside the volume of interest and deposit their energy
locally.
This would exclude interactions producing delta
rays or bremsstrahlung.
• The maximum energy that can be transferred
in these interactions is sometimes indicated
with a subscript, e.g.,
LET1 keV, LET5 keV
• The greater the energy cutoff, the
larger the LET, e.g., LET5 keV >
LET1 keV
• If no restriction is placed on the energy of
the interactions, the unrestricted LET is
indicated as LET∞
• LET∞ is the same as stopping power.
Stopping Power and Linear Energy Transfer

Mass Stopping Power
• The mass stopping power can be more convenient to use than the stopping power.
• It is the stopping power (e.g., MeV/cm) divided by the density (g/cm3) of the material.
• The units of the mass stopping power are usually MeV cm2 g-1 (MeV per g/cm2)
• It is the average energy lost by a charged particle per unit distance traveled where the distance
is expressed as an aerial density (g/cm2)

• It is the number of ion pairs produced per unit path length of the charged particle (IP/Cm).
• The value of (SI) depends on the type and energy of the particle as well as the type of material.
Quantities
Specific ionization
 The (dE/dx) amount of energy lost per unit length along the track ofthe particle which is called “Stopping power” .
This (dE/dx) excludes the binding energy of electron.
Stopping power (S.P) = dE/dx Units : MeV/cm
 If ρ is the density of the medium, then the ratio of energy loss anddensity is called “Mass Stopping Power”
 Mass Stopping Power (S.P)m =(S.P)/ρ = 1/ρ (dE/dx) Units : MeV.cm2/g
 (S.P) µ 1/b2è(S.P) µ C2/ν2
Stopping Power
As velocity of particle decreases,
the energy lost rate increases and
does more ionization.

The capacity of particulate radiation to ionize atoms depends on its
mass, velocity and charge. The rate of loss of energy from a particle
as it moves along its track through matter.
LET(linear energy transfer)
The greater the physical size of the particle, the higher its charge and
lower the velocity, the greater its LET.
Alpha particle
High mass
High charge
Low velocity
Densely ionizing
Beta particle
Light mass
Lower charge
Less ionizing
(high
LET)
(low
LET)
LET is measured by the ionization density (e.g., ion pairs/cm of
tissue) along the path of the radiation.
Higher LET causes greater biological impact and is assigned a higher
Quality Factor(QF).
Example QF values: X, gamma, and beta have QF = 1; alpha
QF=20; thermal neutrons QF=3; "fast" neutrons (>10 KeV) QF
= 10; fission fragments QF>20.
Quantities

• The force associated with these interactions can be
described by Coulomb’s equation:
k is a constant = 9 x 109 N-m2/C2.
q1 is the charge on the incident particle in Coulombs. q2 is
the charge on the “struck” particle.
r is the distance between the particles in meters.
Things to notice about the equation:
• The force increases as the charge increases
• The force increases as the distance
decreases (it quadruples if the distance is
cut in half)
• The force can be positive or negative
(attractive or repulsive)
Force of the Interaction

 Ionization(alphas and betas)
 Excitation (alphas and betas)
 Bremsstrahlung (primarily betas)
 Cerenkov radiation (primarily betas)
Ionization is almost always the primary mechanism of energy loss.
Charged Particle
Interactions
The four types of interactions are:

Ionization
• A charged particle (alpha or beta particle) exerts
sufficient force of attraction or repulsion to completely
remove one or more electrons from an atom.
• The energy imparted to the electron must exceed the
binding energy of the electron.
• Ionization is most likely to involve atoms near
the charged particle's trajectory.
• Each ionization event reduces the charged particle's
velocity, i.e., the alpha or beta particles loses kinetic
energy.
Ion Pairs
• Ionization turns a neutral atom into an ion pair.
• The electron stripped away from the atom is the
negative member of the ion pair.
• It is known as a secondary electron.
• The secondary electron has some, but not much,
kinetic energy -usually less than 100 eV.
• Sometimes it has enough energy to ionize additional
atoms. Then it is referred to as a delta ray.
• The atom , now with a vacancy in one of its electron
shells, is the positive member of the ion pair.

Excitation
• The charged particle (alpha or beta particle) exerts just enough force
to promote one of the atom’s electrons to a higher energy state
(shell).
Insufficient energy was transferred to ionize the atom.
• Excitation usually occurs farther away from the charged
particle's trajectory than ionization.
• The excited atom will de-excite and emit a low energy ultraviolet
photon.
• Each excitation event reduces the charged particle's velocity.

Bremsstrahlung
• Bremsstrahlung radiation is electromagnetic
radiation that is produced when charged
particles are deflected (decelerated) while
traveling near an atomic nucleus.
• Bremsstrahlung is almost exclusively
associated with electrons (beta particles)
because the latter are easily deflected.
• Large particles (e.g., alpha particles) do not
produce significant bremsstrahlung because they
travel in straight lines. Since they aren’t
deflected to any real extent, bremsstrahlung
production is inconsequential.
• Bremsstrahlung photons may have any
energy up to the energy of the incident
particle.
For example, the bremsstrahlung photons
produced by P-32 betas have a range of
energies up to 1.7 MeV, the maximum
energy of the P-32 alphas.
• Bremmstrahlung is most intense when:
- The beta particles or electrons have
high energies
- The material has a high atomic number

Bremsstrahlung
Intensity of Bremsstrahlung – Monoenergetic Electrons
Z is the atomic number of the material
E is the kinetic energy of the electron (MeV)
According to Evans, the fraction of the energy of monoenergetic
electrons that is converted to bremsstrahlung (f) can be calculated as
follows

Bremsstrahlung
Intensity of Bremsstrahlung – Monoenergetic Electrons
• Turner gives slightly different equation for the fraction of
the energy of monoenergetic electrons that is converted to
bremsstrahlung:
E is the kinetic energy of the electrons (MeV)

Bremsstrahlung
Intensity of Bremsstrahlung – Beta Particles
• The following equation (Evans) estimates the fraction of
beta particle energy converted to bremsstrahlung (f).
Beta particles are emitted with a range of energies up to some
maximum value (Emax).
Emax is the maximum energy of the beta particles(MeV)

Bremsstrahlun
g
Intensity of Bremsstrahlung – Beta Particles
• The beta energy rate (MeV/s) is the activity of the beta
emitter multiplied by the average energy of the beta
particles:
Beta energy rate = Activity x Average beta energy
(MeV/s) (dps) (MeV)
• This is multiplied by the fraction (f) to determine the
bremsstrahlung energy emission rate in MeV/s.
Bremsstrahlung energy rate = Beta energyrate x f
(MeV/s) (MeV/s)

Bremsstrahlun
g
Bremsstrahlung Spectra
• The following discussion tries to explain the
shape of a bremsstrahlung spectrum (e.g., that
produced in the target of an x-ray tube)
• Bremsstrahlung photons have a range of energies
up to the maximum energy of the electrons/beta
particles.
• When monoenergetic electrons lose energy in an
extremely thin target, the bremsstrahlung
spectrum is flat up to the maximum energy of
the electrons.
Imaginary targets like this are not found in
the real world.
• Monoenergetic electrons losing energy in a thick
(real world) target can be considered to interact
in a series of thin sections (targets).
• The deeper into the target a given section is, the
lower the energy of the electrons, and the lower
the maximum energy of the bremsstrahlung
produced there.
The bremsstrahlung produced in the deeper
sections by the lower energy electrons contributes
to the low energy end of the overall
bremsstrahlung spectrum:
• Bremsstrahlung produced in the shallow sections of
the target where the electron energies are higher
contributes to the high energy portion of the
spectrum.

Bremsstrahlung
Bremsstrahlung Spectra
• There is always some shielding/filtration between the
source of the bremsstrahlung and the point of interest.
• For example, the glass wall of an x-ray tube will shield the
bremsstrahlung generated in the target (anode) as would a
filter intentionally placed in front of the tube.
• This shielding primarily reduces the intensity of the low
energy bremsstrahlung.
• As such, a “real world” bremsstrahlung spectrum looks
more like that on the next slide.

Cerenkov Radiation Production
• Cerenkov radiation is the blue light emitted by charged particles that travel
through a transparent medium (e.g., water) faster than the speed of light in that
medium.
• Just as a plane going faster than sound produces a cone of sound (a sonic boom),
a charged particle going faster than light produces a cone of light (Cerenkov
radiation)
• The production of Cerenkov radiation is essentially limited to high energy (i.e.,
fast) beta particles and electrons.
• Cerenkov radiation is often associated with reactor fuel pools or nuclear criticality
accidents.
• It is possible to quantify beta emitters by measuring the intensity of their
Cerenkov radiation (Cerenkov counting)

 & 
interactions
o Alpha particles ionize by attracting an electron from
an atom.
o Beta particles ionize by repelling an electron from an
atom.
o Charged particles lose energy in to medium mainly
through the effect of ionization and excitation .
o If the transferred energy to the atomic electrons is
enough to remove them from the atom , electrons
are ejected, and the atom is said to be ionized.
o If the transferred energy to the atomic electrons is
not enough to remove them from the atom ,
electrons are excited to higher energy levels .

Beta Particles
• Beta particles (or electrons) interact by
all of the following mechanisms:
– Ionization
– Excitation
– Bremsstrahlung
– Cerenkov radiation (relatively
unimportant)
For betas above 150 eV, roughly 95% of the
particle’s energy loss in water is due to
ionization.
• Not as intensely ionizing as alphas (because they have
higher velocities and one half the charge).
• Low specific ionization (ca. 100 ion pairs per cm in
air)
• Low stopping powers (low LET radiation)
• Betas might produce (the specific ionization) in air.

Beta Particles
Range
• Much greater range than alphas (except for the lowest
energy betas):
- Approximately 3 meters in air for a 1 MeV beta
- A few millimeters in tissue (water)
• The atomic number of the material is not a major factor. In fact,
the range of beta particles under 20 MeV is greater in lead than
in water!
• The next slide shows two empirical equations relating the
range of a beta particle to its energy.

Beta
Particles
Range (as a density thickness)
• The range of a beta particle can be determined if the energy is known:
• The energy of a beta particle can be determined if the range is known:
R is the range in mg/cm2
E is the maximum beta energy in MeV

Beta Particles
Range (as a density thickness)
• The easiest way to determine the range
of a beta particle is to use a curve
similar to the graph.

Beta Particles
Range and Penetration
• Beta particles (and electrons) travel in convoluted
paths.
• They do not travel in a straight line.
• The “range” of a beta particle usually refers to the
total path length.
• The range is greater than the distance between
the beginning and the end of the path followed
by the particle (the penetration thickness).
In other words, the range of a beta particle is greater
than the thickness of a material that can be penetrated.
• The following image shows the predicted paths of 800
keV beta particles in water.
The average penetration thickness: 1500 um
The average range (path length): 3500 um.
Beta particle tracks are convoluted.
From Turner, James. Atoms, Radiation, and Radiation Protection,2nd
edition. 1995, pg. 151.

Beta
Particles
Bremsstrahlung
• Bremsstrahlung is most significant for high energy
beta emitters such as P-32 and Sr-90.
• The presence of bremsstrahlung is often interpreted as
a indication that high energy beta emitters are present.
• Nevertheless, bremsstrahlung can be detected when
low energy beta emitters (e.g., tritium) are present in
high enough activities (e.g., a tritium exit sign).
• To minimize the production of unwanted
bremsstrahlung, beta sources should be
shielded with a low atomic number
material.
For example, high energy beta emitters
are commonly shielded with plastic.

Beta
Particles
Bremsstrahlung
• Shielding a high energy beta source with lead could
increase the production of bremsstrahlung.
Nevertheless, if the lead is thick enough, it will also stop the
bremsstrahlung.
• Sometimes a beta shield has two layers:
- plastic nearest the source to stop any betas
- lead outside the plastic to stop any bremsstrahlung.

Beta Particles
Cerenkov Radiation
• The emission of Cerenkov radiation is an
interesting, but relatively unimportant, type of
beta particle (or electron) interaction.
• Cerenkov radiation is the blue glow that often
seen in a reactor’s spent fuel pool.
• The Cerenkov radiation primarily is due to
the high energy Compton scattered
electrons produced by gamma emissions
from the fuel. Fuel assemblies being removed from the
reactor vessel at TMI’s operating unit.

Beta Particles
Air
(D=0.001293 g/cm3)
Water
(D=1 g/cm3)
W (eV/ip) 34 22
3.3 x 10-3 1.89
Approximate Data for 1 MeV Beta Particles
88
Stopping Power/LET
(MeV/cm)
Mass Stopping Power
(MeV cm2 g-1)
2.6 1.89
(Ion
Specific Ionization
pairs per cm)
100 86,000
Range (g/cm2) 0.4 0.5
Range (cm) 300 0.5

 interactions
the beta-particle counting rate decreases rapidly at first, and
then, as the absorber thickness increases, it decreases slowly.
Eventually, a thickness of absorber is reached that stops all the
beta particles; the Geiger counter then registers only
background counts due to environmental radiation.
The end point in the absorption curve, where no further
decrease in the counting rate is observed, is called the range of
the beta in the material of which the absorbers are made
the absorber half-thickness (that
thickness of absorber which stops
one-half of the beta particles) is
about one-eighth the range of the
beta.

• Beta particles follow tortuous paths in matter
as the result of multiple scattering events
• Ionization track is sparse and no uniform.
• Larger mass alpha particles of heavy charged
particle results in dense and usually linear
ionization track.
Path length is actual distance particle travels; range is actual depth of penetration in matter
 interactions

Beta + particle , Positron
o Physically, it is an electron but may be positively or negatively
charged Emitted with a wide range of energies up to some
maximum value ( with a continuous spectrum of energies for a
given radionuclide.
o Its range in matter depends on the energy and the type of the
material.
o More penetrating than alpha particles. For example:(0.5) MeV
of beta particle has a range of 2 meters in air, 3 mm in tissue.
o Beta particle does not produce as many ion pairs per unit
length of path as alpha particles, and thus, have a greater
range in matter.
o Betas interact (loss of its energy) with the
material by:
• For Beta with low energy (up to 0.5 MeV) ,
the main interaction is by Ionization and
excitation with the orbital electrons of the
atoms.
• For Beta with high energy (> 1 MeV) , the
main interaction is by emitting x-rays
(Bremsstrahlung) (braking radiation) as a
result of changing b’s direction when it
becomes close to the nucleus.

Bremsstrahlung(Radiative losses)
• Probability of bremsstrahlung production per atom is
proportional to the square of Z of the absorber.
• Energy emission via bremsstrahlung varies inversely
with the square of the mass of the incident particle
(Z/M)2.
• Protons and alpha particles produce less than
one-millionth the amount of bremsstrahlung
radiation as electrons of the same energy.
The energy percentage f of beta particles, which is
lost via the emission of bremstrahlung radiation as a
function of both beta particles maximum energy
Emax and the atomic number Z is determined as:
f = 0.035 Emax Z %

A very small source (physically) of 3.7 × 1010 Bq (1 Ci) of 32P is inside a lead shield just thick enough to prevent any
beta particles from emerging. What is the bremsstrahlung energy flux at a distance of 10 cm from the source
(neglect attenuation of the bremsstrahlung by the beta shield)?

Alpha Particles
• The principal types of interactions for alpha
particles are:
- Ionization
- Excitation
• Usually have energies from 4 to 8 MeV
• High specific ionization
(because of their +2 charge and low velocity)
• High LET radiation - lose their energy very
quickly as they travel through matter.
• Easy to shield – can be stopped by a piece of paper
• Not an external hazard – cannot penetrate the dead
layer of skin on the surface of the body
• Potential internal hazard – the large radiation
weighting factor for alpha particles (20) means
that the consequence of a given alpha particle
dose is greater than that for other types of
radiation.

Alpha Particles
Range
• The range of an alpha particle is short:
- approximately 5 cm in air.
- 20 to 70 um in tissue (one, two or three cells)
• The survey instrument must be close (e.g., < 1 cm) to a contaminated surface if alpha emitting radionuclides are to
be detected.
It is best if the contaminated surface is dry and clean - dust or moisture could attenuate the alphas.
Range in Air
• Alphas with energies of 4 to 8 MeV (almost all alpha
emitters):
R (cm) = 1.24 E - 2.62
• Alphas with energies below 4 MeV:
R (cm) = 0.56 E
E is the alpha energy in MeV

Air
(D=0.001293 g/cm3)
Water
(D=1 g/cm3)
W (eV/ip) 36 22
1.23 950
Alpha Particles
Approximate Data for 5 MeV Alphas
71
Stopping Power/LET
(MeV/cm)
Mass Stopping Power
(MeV cm2 g-1)
950 950
Specific Ionization
(Ion pairs per cm)
34,000 4.3 x 107
Range (g/cm2) 5 x 10-3 3 x 10-3
Range (cm) 4 3 x 10-3 (30 mm)

 interactions
Increasing thickness of absorbers serves merely to
reduce the energy of the alphas that pass through the
absorbers; the number of alphas is not reduced until the
approximate range is reached. At this point, there is a
sharp decrease in the number of alphas that pass
through the absorber. Near the very end of the curve,
absorption rate decreases due to straggling, or the
combined effects of the statistical distribution of the
“average” energy loss per ion and the scattering by the
absorber nuclei. The mean range is the range most
accurately determined and corresponds to the range of
the “average” alpha particle. The extrapolated range is
obtained by extrapolating the absorption curve to zero
alpha particles transmitted.

 Heavy nuclear particle (a helium nucleus), doubly positively
charged,
relatively slow.
 Particles travel within the material in a straight line due to their
very large mass compared to the mass of electrons that interact with
them during their penetration of matter.
 loss their energy mainly by ionization (as they interact with orbital
electrons in the material, knocking them out of their atoms) and
by excitation (by pulling Inner orbital electrons to outerorbits).
 interactions
 The distance traveled by alpha particle before stopping is very
short It loses all of its energy in a very short distance it
creates many ion pairs in very short distance
Very very limited capability to penetrate material
 Example: If the energy of alpha is 5 MeV: It will travel about 4 cm in air, Normal paper is enough to stop it, Can
not penetrate the dead layer of human skin (0.07 mm), so it does not constitute an external biological hazard.

Summary
Types of Interactions
• Charged particles continuously interact as they travel
through matter - it is not a matter of probability.
• The major type of interactions: ionization
• The other types of interactions: excitation
bremsstrahlung
Cerenkov radiation
• Bremsstrahlung production is sometimes an important
concern with beta particles.
• Cerenkov radiation is interesting but rarely important.

Summary
Alpha Particles
• High specific ionization
• High LET (aka stopping power)
• Travel in straight lines
• Short range: a few cm in air
a couple of cells in the body
• Potential internal hazard but not an external hazard
Beta Particles
• Low specific ionization
• Low LET radiation (i.e., low stopping power)
• Convoluted path
• Large range: a few hundred cm in air
several mm in the body
• Penetration distance is less than the range

Summary
Beta Particles
• Produce bremsstrahlung photons when they
change direction.
• Maximum energy of the bremsstrahung
photons is the same as the maximum energy of
the beta particles.
• The higher the atomic number of the material, the
greater the fraction of the beta particle energy that
will be emitted as bremsstrahlung
• The higher the energy of the beta particle (or
electron) the greater the fraction of the energy
that will be emitted as bremsstrahlung.
• Bremsstrahlung complicates radiation
protection, sample counting, shielding,
and dosimetry.
• Bremsstrahlung production can be
minimized by shielding beta sources with
a low Z material such as plastic.

X and gamma rays interaction with matter through
below interactions :
Electromagnetic Radiation Interaction
Photoelectric effect. Low energy
dominant .
Compton scattering. Medium energy
dominant.
Pair production. High energy
dominant .

Photoelectric
effect
• All of the energy of the incoming photon is totally
transferred to the bound electron , Following interaction,
the photon ceases to exist.
• The incoming photon interacts with an orbital electron in
an inner shell – usually K.
• The orbital electron is dislodged.
• To dislodge the electron, the energy of the incoming
photon must be equal to, or greater than the electron’s
energy.
Probability of Occurrence
• A vacancy now exists in the inner shell.
• To fill this gap, an electron from an outer shell drops
down to fill the gap.
• Once the gap is filled, the electron releases its energy
in the form of a characteristic photon (x-ray).
• Depends on the following:
• It increases as the photon energy decreases, and the
atomic number of the irradiated object increases.
• The probability of photoelectric absorption, is
roughly proportional to
Z4 – Z5 .
• This type of interaction is prevalent in the diagnostic
range : 30 – 150 Kev.

Compton
scattering
• An incoming photon is partially absorbed in an outer shell electron
• The electron absorbs enough energy to break the binding energy, and is
ejected
• The ejected electron is now a Compton electron
• Not much energy is needed to eject an electron from an outer shell
• The incoming photon, continues a different path with less energy as
scattered radiation
Byproducts Of Compton Scatter
Compton scattered electron
Possesses kinetic energy and is capable of ionizing atoms
Finally recombines with an atom that has an electron
deficiency
Scattered x-ray photon with lower energy
Continues on its way, but in a different direction
It can interact with other atoms, either by photoelectric or
Compton scattering
It may emerge from the patient as scatter
Depends on the following:
• Increases as the incoming photon energy increases up
to certain limit then decreases as the photon energy
increases.
• depends linearly on Z of the matter.
• The Compton process is most important for energy
absorption for soft tissues in the range from 100 keV to
2MeV.

Compton
scattering
α = E0 / m0c2
Back scattering θ = 180o
Eelectron takes maximum energy from the
photon.
No electron scattering with angle more than 90o

Pair
Production
• Incoming photon must have an energy of at least 1.02 MeV.
• This process is a conversion of energy into matter and then matter back into energy.
• Two electrons are produced in this interaction.
1. An incoming photon of 1.02 MeV or greater interacts with the nucleus of an atom.
2. The incoming photon disappears
3. The transformation of energy results in the formation of two particles
4. Negatron
1. Possesses negative charge
5. Positron
1. Possesses a positive charge
 Increases with increasing photon energy
 Increases with atomic number approximately as Z2

A) very narrow beam consisting of parallel
monoenergetic photons.
b) A very small thickness x of the attenuator, so that,
multiple Compton scattering is negligible.
Attenuation of x and gamma
radiation
Valid only when
I = Io e
-  x
 = 0.693 / HVL
Lambert’s expression
X1/2 is the half value thickness (layer) of the shielding material.
linear attenuation coefficient, μ, is the probability of an interaction per unit distance traveled (unit cm-1).
μ depends on photon energy and on the type of material (Z) being traversed.

Attenuatio
n
 There is also another physical quantity
called mass attenuation coefficient (m)
used instead of linear attenuation
coefficient ().
 The mass attenuation coefficient is
equal to the linear attenuation
coefficient divided by the density of the
attenuation material.
 The unit of mass attenuation coefficient
is (cm2 / g).
 When using mass thickness (m) , it
should be used with (Xm) instead of
linear thickness where:
Xm = X  (g/cm2)

 "X1/2": The thickness of the material at which the
intensity of radiation entering it is reduced by one
half.
 Also there is the so-called tenth value thickness
(X1/10) . It is the thickness of the material at which
the intensity of radiation entering it is reduced by
one tenth.
 Example: If the number of photons entering
is No , the number of
photons that cross the tenth value layer without
interaction is 1/10.
 Both X1/2 and X1/10 depend on the energy of the
photons and atomic number.
Half Value Thickness
(Layer )
 
X 1
2 


ln 2

0.693 
ln10

2.3
1
10
1/10
X X = 3.32 X 1/2
Thickness of an absorber necessary to reduce the transmission of radiation to 50 percent
(HVL).
Radiation quality HVL (mm)
Concrete Lead
50 kV
100 kV
200 kV
500 kV
1 MV
2 MV
5 MV
10 MV
20 MV
4.3
10.6
25
36
44
64
96
119
137
0.06
0.27
0.52
3.6
7.9
12.5
16.5
16.6
16.3

Mass Attenuation
variation with
Energy and
dominant
interaction.
PE VS CS

If you have a lead shielding that is 5 cm thick and
the HVL for lead for 150 kev gamma rays is 2.5 cm
what will be the number of passing photons if you
know that the incident photons are 100 photon.

C
Neutron Interactions
Fusion
Scattering Absorption
Inelastic Fission
Capture
(n,)
Elastic
Charged
particle
N Mult
(n, xn)
Overview-Neutron Physics: Neutron
Reactions
Capture 
𝟏
𝟏𝑯: 2.2 MeV
𝟐𝟔
𝟓𝟔𝑭𝒆: 7.0 MeV
𝟏𝟕
𝟑𝟓𝑪𝒍: Several
𝟏𝟐
𝟏𝟔
Inelastic 
𝟔𝑪: 4.4 MeV
𝟖𝑶: 6.2 MeV

Overview-Neutron Physics: Neutron
Reactions
Cross Section  :
It is related to the probability of
interaction between two particles
In nuclear science.
𝑅 = Σ𝐼 = 𝜎𝑁𝑛v Interaction Rate (#/s.cm3)
𝐼 = 𝑛v
Σ = 𝜎𝑁
N, n, v
Beam Intensity (#/s.cm2)
Macroscopic Cross Section (cm2)
Number of target nuclei and beam particles per
unit volume and particle velocity

Neutrons are generally categorized by their energies in to 4 main categories :
Category
Thermal
Moderate
Fast
Relativistic
Energy
0.25-0.5 ev
0.5-10 ev
10 ev -10 Mev
> 10 Mev
Interaction
Absorption – Capture.
Elastic Scattering .
Elastic Scattering . & Inelastic Scattering.
Nuclear Reactions
All neutrons are initially Fast Neutrons which lose kinetic energy through interactions with their
environment until they become thermal neutrons which are captured by nuclei in matter.
Neutral particles :
neutron

Absorption – Capture
1n + 113Cd  114Cd + 
113Cd(n, ) 114Cd
This reaction is important in neutron shielding
and is also used as the principal reaction for
some neutron detectors.
1n + 10-B  7-Li + 4-He
10B(n,) 7Li
This is why boron controls are used in nuclear power reactors, since it
tends to reduce the number of neutrons present and therefore helps
control the fission process.

• Neutron collides with atomic nucleus
• Neutron deflected with loss of energy E
• E given to recoiling nucleus
• Energy of recoiling nucleus absorbed by medium.
The recoil nuclei quickly become ion pairs and loose energy through excitation and ionisation as
they pass through the biological material. This is the most important mechanism by which neutrons
produce damage in tissue.
• Struck atoms can also lose orbital electron
Neutron, E’
Recoiling
Nucleus
Incoming
Neutron, Eo
Nucleus
Total energy
unchanged
Elastic Scattering
2









m
M
m
M
E
E o

• Neutron momentarily captured by nucleus
• Neutron re-emitted with less energy
• Nucleus left in excited state
• Nucleus relaxes by emitting -rays or charged
particles
(adds to dose)
Inelastic
Scattering
Emitted
Neutron
-ray
Incoming
Neutron
Nucleus

• Fast neutrons lose their energy within
the human body through a elastic
collision reaction with hydrogen
nuclei, which is the dominant
component of the body.
Neutron Interaction within the Human Body
• The thermal neutron loses its
energy within the human body
through the radiative capture
interaction in Hydrogen.

2. Interaction of radiation with matter..pptx

2. Interaction of radiation with matter..pptx

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