Physics Lecture 7

Front 1

Attenuation

Back 1

The process by which a beam of radiation is reduced in intensity when passing through some material of matter.

Front 2

Range

Back 2

Maximum distance between a beta particle can travel in a given medium.
- Type of medium determines the range
air vs. plastic

Front 3

For beta particles

Back 3

the range is strongly dependent on the number of absorber electrons in the particles path.
Atomic number of the absorber does not effect absorption much.
** This is only important with gamma rays.

Front 4

Density Thickness

Back 4

approximately proportional to the areal density of electrons

density x thickness
g/cm^2

Front 5

Energy Loss

Back 5

Beta Particles may interact with other electrons through inelastic collisions.

Front 6

Inelastic collisions

Back 6

Kinetic energy is not conserved. Some of the energy of the incident Beta particle is used to break the binding energy of orbital electrons it collides with.

Front 7

Elastic collisions

Back 7

A collision between particles where all of the energy of the incident particle is transfered to the other particle.

Balls on a billiard

Front 8

Specific Ionization

Back 8

The number of ion pairs produced per unit length traveled by the particle.
(Known as LET in radiobiology)

Front 9

Stopping Power

Back 9

Energy lost per distance traveled in a given medium. A given medium can be described by its stopping power for a particle of a certain energy. Can be used in place of range.

Front 10

**Bremsstrahlung

Back 10

breaking radiation, electrons traveling close to a dense nucleus are attracted and accelerated. As they do so, they loose energy in the form of an X-ray.
**beta emiter = don't shield with lead

Front 11

Bremsstrahlung

Back 11

Energy slows down and then emitted as an X-Ray = diagram

Front 12

Bremsstrahlung is the main

Back 12

method by which x-rays are produced in radiography.

Front 13

Alpha

Back 13

Aloha particles absorption is essentially flat because of the mono-energetic nature of alphas.
-Alpha particles emitted all have the same energy.

Front 14

Alphas interact with electrons in the absorbing medium:

Back 14

Excitation and ionization

Front 15

Alpha particles don't

Back 15

travel to far.

Front 16

Alpha Particles Trials

Back 16

don't travel in a straight line.

Front 17

Gamma Rays

Back 17

Different than for beta and alpha radiations. Charged particles have a definite range in matter where as X and gamma radiations can be reduced in intensity but theoretically never completely absorbed.

Front 18

Energy of gamma rays

Back 18

matters here.
- Atomic Number matters here as well.

Front 19

Linear Attenuation coefficient has units of cm^-1

Back 19

The value is dependent on the energy of the radiation and the material used for shielding

Front 20

Problems q

Back 20

See P.P and STUDY!!!

Front 21

Energy of gamma rays

Back 21

matters here.
- Atomic Number matters here as well.

Front 22

Linear Attenuation coefficient has units of cm^-1

Back 22

The value is dependent on the energy of the radiation and the material used for shielding

Front 23

Problems q

Back 23

See P.P and STUDY!!!

Front 24

Half Value Life

Back 24

The thickness of a particular substance which reduces the initial beam of radiation to half of its original value.
- Useful for shielding calculations
- Dependent on energy and the material used for attenuation.

Front 25

Half Value Life formula

Back 25

every one will reduce intensity by half

Front 26

Problems

Back 26

See P.P STUDY!!!

Front 27

Interaction Mechanisms

Pair Production

Back 27

Photon Energy must be greater than 1.02 MeV. It is a phenomenon that when a photon in this energy range passes near a nucleus, the photon disappears and reappears as a positron and electron pair. Energy appears as mass
**Only happens if you have a photon energy greater than 1.02MeV.

Front 28

Pair Production

Back 28

More likely to occur with high atomic number absorbers. Positron and electron are projected forward. They loose energy through excitation, ionization and Bremmstrahlung. Finally, the positron combines with an electron to produce two 0.51MeV photons.
(annihilation photons)

Front 29

Compton Scattering

Back 29

A collision between a photon and a free electron. Results in transference of energy to the electron and scattering of the incident photon. Scattered photon then is lower energy than the incident photon.
( Scattered radiation results from compton scattering from outer shell electron.)

Front 30

Ejected electron is known as the

Back 30

recoil electron

Front 31

In low Z material, all electrons have a relatively low binding energy and can be considered free electrons.

Back 31

Probability of comton scattering occurring is therefore, more likely, with low Z material.

Front 32

So more Compton scattering occurs in

Back 32

soft tissue than bone.

Front 33

A range of energies is possible for the

Back 33

scattered photon and the ejected electron,.

Front 34

Photoelectric Absorption

Back 34

Interaction between a relatively low energy photon and a tightly bound electron.

Front 35

Favored by high

Back 35

Z materials. This makes lead a good shielding material.

Front 36

Incident photon interacts with an inner orbit electron (K,L,M shells)

Back 36

resulting in an ejection.

Front 37

Binding energy of the orbital electron must be equal or less that of the

Back 37

incident photon.

Front 38

E=hf- 0
where
e is the

Back 38

energy of the photoelectron, what is ejected.

Front 39

hf is the

Back 39

energy of the incident photon

Front 40

o is the

Back 40

binding energy of the orbital electron

Front 41

Or the
The incident photon energy =

Back 41

electron binding energy + electron kinetic energy

Front 42

Photoelectric Absorption is

Back 42

inelastic collision

Front 43

The photon is absorbed by the atom;

Back 43

energy goes to free an electron.

Front 44

Important for patient dosimetry, since the energy of the photoelectron is usually dependent in the patient

Back 44

through ionization and excitation.
- Preferred mechanisms for interaction with gamma camera because all the energy id deposited in the crystal without scattering.

Front 45

You want PE

Back 45

effect here in crystal

Front 46

Coherent Scattering

Back 46

An electron absorbs all energy of a photon, and emits all the energy in a new direction. Occurs in less then 5% of photon interactions.

Front 47

Neutrons

Back 47

Have no useful naturally occurring neutron emitters.
- Neutrons have no current use in NM; other than the creation of useful isotopes through activation in a nuclear reactor.
-Fission

Front 48

STUDY GRAPH

Back 48

FINAL

Front 49

Summary of Radiation Interactions with Matter

Back 49

GO!

Front 50

Particulate Radiations

Back 50

Elastic & Inelastic collisions
Excitations
Ionizations/Branshten

Front 51

Photons

Back 51

Photoelectric effect
Compton Effect
Pair Production
Classical Scattering, Coherent Scattering, Rayleigh Scattering = mean all of the same thing just different names.