IAEA Interaction of radiation with matter - 1 Charged Particle Radiation Day 2 – Lecture 1 1.
-
Upload
ariel-knight -
Category
Documents
-
view
272 -
download
5
Transcript of IAEA Interaction of radiation with matter - 1 Charged Particle Radiation Day 2 – Lecture 1 1.
IAEA 2
• To understand the following interactions for particles: Energy transfer mechanisms Range energy relationships Bragg curve Stopping power Shielding
Objective
IAEA 3
Energy transfer mechanism
• Energy transfer from radioactive particles to other materials depends on: the type and energy of radiation the nature of the absorbing medium
• Radiation may interact with either or both the atomic nuclei or electrons
• The interaction results in excitation and ionisation of the absorber atoms
IAEA 4
When a charged particle interacts with an atom of the absorber, it may:
traverse in close proximity to the atom (called a “hard” collision)
traverse at a distance from the atom (called a “soft” collision)
A hard collision will impart more energy to the material
Particle Interactions
IAEA 5
The amount of energy deposited will be the sum of energy deposited from hard and soft collisions
The “stopping power,” S, is the sum of energy deposited for soft and hard collisions
Most of the energy deposited will be from soft collisions since it is less likely that a particle will interact with the nucleus
Stopping Power
IAEA 6
• The stopping power is a function of the charge of the particle, the energy of the particle, and the material in which the charged particle interacts
Stopping Power
IAEA 7
• Stopping power has units of MeV/cm – the amount of energy deposited per centimeter of material as a charged particle traverses the material
• It is the sum of energy deposited for both hard and soft collisions.
S = = +
Stopping Power
dEdx Tot
dEs
dxdEh
dx
IAEA 8
Mass Stopping Power
• Often the stopping power is divided by the density of the material,
• This is called the “mass stopping power”
• The dimensions for mass stopping power are MeV – cm2
g
IAEA 9
Stopping Power
Stopping power is used to determine dose from charged particles by the relationship:
D = in units of MeV/g, where
= the particle fluence, the number of particles striking an object over a specified time interval
dEdx
IAEA 10
Stopping Power
To convert to units of dose ..we do the following manipulation.
D = MeV/g = (1.6 x 10-10) Gy
dEdx
1ev = 1.6 X 10 -19 J
dEdx
IAEA 11
Bragg Curve – Alpha
Alpha Particle
Energy loss curve – increase initially and virtually no
energy deposited at the end of the track.
Bragg Curve - plot of specific energy loss ( ie rate of ionization ) along the track of a charged particle
A typical Bragg curve is depicted in this graphic for an alpha particle of several MeV of initial energy.
IAEA 12
Bragg Curve –Beta
Beta Path
Energy loss curve – virtually no energy deposited at the end of
the track.
The energy deposition of the electron increases more slowly with penetration depth due to the fact that its direction is changed so much more drastically
As the mass of the beta particle is the same as the orbital electrons they undergo several collisions … the torturous path
IAEA 13
Range – Beta particle
• Depends on the energy of the beta particles and the density of the absorber Beta particle energy reduces as density of the absorber increases
• Experimental analysis reveal that ability to absorb beta particle: Depends on the number of absorbing electrons (electrons per cm 2)in
the path of the beta ray – aerial density Lesser on the atomic number of the absorber
IAEA 14
Range - Energy relationship
• Attenuation of beta particles interposing layers of absorbers between beta
sourceThe number of beta particles
oreduce quickly at firstomore slowly as absorber thickness increasesocompletely stops after certain absorber thickness
Range of beta particle - the thickness of absorber material that stops all particles
IAEA 15
Range – Beta particle
• Aerial density is related to the density of the absorber
td g/cm2 = ρ (density of the absorber) g/cm3 X tl
(thickness of the absorber) cm
• beta shields are usually made from low Z materials
IAEA 16
Range – Beta particles
• Calculate density thickness for aluminium of thickness 1cm .
Note: (Density of Al = 2.7g/cm3)
IAEA 17
Range – Beta particles
• Calculate density thickness for aluminium of thickness 1cm .
td g/cm2 = ρ g/cm3 X tl cm
td = 2.7g/cm2
• A graph of beta energy VS density thickness is useful for shielding and identifying beta source
IAEA 18
Range – alpha particles
• Alpha particles least penetrating of the types of radiation• Alpha particles are mono-energetic. Therefore the
number of alpha particles not reduced until totally eliminated at particular thickness of the absorber.
• The thickness of absorber that totally stops alpha particles is the range of the alpha particle in the material.
• The most energetic alpha particle travels few cms in air, while in tissue only few microns.
IAEA 19
Linear Energy Transfer
LET is the rate of energy absorption by the medium
LET = keV per micron
DE = is the average energy imparted by the radiation of specific energy in traversing a distance of dx.
dEdx
IAEA 20
Linear Energy Transfer
• Specific ionisation is the number of ion pairs formed per unit distance travelled by the radiation particle and very useful concept in health physics
• Specific ionisation is very high for low energy beta particles and decreases as the energy increases.
• Specific ionisation is high for alpha particles. • Travelling through air or tissue alpha particle loses on average 35 eV per
ion pair it creates .• The high electrical charge and low velocity means tens of thousands of
ion pair per cm of air travelled.
IAEA 22
Absorbed dose is energy imparted per unit mass of material:
The unit of absorbed dose is the Gray (Gy)
(1 Gray = 1 joule/kg)
To calculate the dose from charged particles, we need to determine the amount of energy deposited per gram of material
Absorbed Dose
IAEA
Tissue Equivalent Stopping Power for Electrons
Energy (MeV) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Mass Stopping
Power, S/ (MeV-cm2)/g
4.2 2.8 2.4 2.2 2.0 2.0 1.9 1.9 1.8 1.8
IAEA 24
Stopping Power Example
Calculate the dose from a 37,000 Bq source of 32P spread over an area of 1 cm2 on the arm of an individual for 1 hour
D = (1.6 x 10-10) Gy
32P has a 0.690 MeV beta particle (average energy). Assume that 50% of the particles on the skin interact with the skin
dEdx
IAEA 25
= (½)(37,000 Bq)(1 dis/s/Bq)(1 hr)(3600 s/hr) = 6.67 x 107 dis
32P has a 0.690 MeV beta particle (average energy)
For tissue equivalent plastic and a beta particle with an energy of 0.690 MeV, the stopping power is 1.96 MeV-cm2/g
Stopping Power Example
IAEA 26
D = 6.67 x 107 X 1.96 X1.6 x 10-10 J/kg
D = 0.021 Gy
D = MeV-cm2/gdEdx
Stopping Power Example
IAEA 27
Where to Get More Information
Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)
International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)