Friday, May 30, 2008
Monday, May 19, 2008
Wednesday, May 14, 2008
Travelling vs Standing waves
· Accelerate electrons using a microwave using either a traveling wave, which absorbs the residual power with a dummy to prevent “backward reflected waves”, or a stationary wave accelerator which reflects the waves from each end towards each other to produce “standing waves”.
Mnemonic, when one is travelling they don't look backward (i.e. prevent backward reflecting waves)
When one is stationary or standing they can reflect back on their life (reflect waves to from each end)
Mnemonic, when one is travelling they don't look backward (i.e. prevent backward reflecting waves)
When one is stationary or standing they can reflect back on their life (reflect waves to from each end)
Sunday, May 11, 2008
Why use dref for electrons in TG-51
TG-51 gives a reference for the reference depth for electron beam, which is Ref.#17.Quote: For electron beam reference dosimetry in radiotherapy, it is shown that by choosingthe reference depth as dref=0.6R50 -0.1 cm, ..., the Spencer-Attix water-to-airstopping-power ratioat dref is given by (L/p)(water to air) = 1.2534-0.1487(R50)exp(0.2144).This is the magic.
REF YAHOO STUDY GROUP 4/23/08
REF YAHOO STUDY GROUP 4/23/08
Photon Interactions Part III
As far as the photon fate after the interaction with an
atom is concerned there are two possible outcomes:
• Photon disappears (i.e., is absorbed completely) and a portion
of its energy is transferred to light charged particles (electrons
and positrons in the absorbing medium).
• Photon is scattered and two outcomes are possible:
• The resulting photon has the same energy as the incident photon and no
light charged particles are released in the interaction.
• The resulting scattered photon has a lower energy than the incident photon
and the energy excess is transferred to a light charged particle (electron).
atom is concerned there are two possible outcomes:
• Photon disappears (i.e., is absorbed completely) and a portion
of its energy is transferred to light charged particles (electrons
and positrons in the absorbing medium).
• Photon is scattered and two outcomes are possible:
• The resulting photon has the same energy as the incident photon and no
light charged particles are released in the interaction.
• The resulting scattered photon has a lower energy than the incident photon
and the energy excess is transferred to a light charged particle (electron).
Photon Interaction (Tightly Bound)
1.4.1 Slide 5 (125/194)
1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
A tightly bound electron is an electron whose binding
energy is comparable to, larger than, or slightly smaller
than the photon energy .
• For a photon interaction to occur with a tightly bound electron, the
binding energy of the electron must be of the order of, but
slightly smaller, than the photon energy
• An interaction between a photon and a tightly bound electron is
considered an interaction between photon and the atom as a
whole.
Photoelectric effect (tightly bound)
1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
A tightly bound electron is an electron whose binding
energy is comparable to, larger than, or slightly smaller
than the photon energy .
• For a photon interaction to occur with a tightly bound electron, the
binding energy of the electron must be of the order of, but
slightly smaller, than the photon energy
• An interaction between a photon and a tightly bound electron is
considered an interaction between photon and the atom as a
whole.
Photoelectric effect (tightly bound)
Loosely bound electrons (Compton)
1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
A loosely bound electron is an electron whose binding
energy to the nucleus is small compared to the
photon energy
An interaction between a photon and a loosely bound
electron is considered to be an interaction between a
photon and a free (unbound) electron.
1.4.1 Types of indirectly ionizing photon irradiations
A loosely bound electron is an electron whose binding
energy to the nucleus is small compared to the
photon energy
An interaction between a photon and a loosely bound
electron is considered to be an interaction between a
photon and a free (unbound) electron.
Photon Interactions part 2
1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
Interactions of photons with nuclei may be:
• Direct photon-nucleus interactions (photodisintegration)
or
• Interactions between the photon and the electrostatic field of the
nucleus (pair production).
Photon-orbital electron interactions are characterized as
interactions between the photon and either
• A loosely bound electron (Compton effect, triplet production)
or
• A tightly bound electron (photoelectric effect).
1.4.1 Types of indirectly ionizing photon irradiations
Interactions of photons with nuclei may be:
• Direct photon-nucleus interactions (photodisintegration)
or
• Interactions between the photon and the electrostatic field of the
nucleus (pair production).
Photon-orbital electron interactions are characterized as
interactions between the photon and either
• A loosely bound electron (Compton effect, triplet production)
or
• A tightly bound electron (photoelectric effect).
Photon Interactions
1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
In penetrating an absorbing medium, photons may
experience various interactions with the atoms of the
medium, involving:
• Absorbing atom as a whole
• Nuclei of the absorbing medium
• Orbital electrons of the absorbing medium.
1.4.1 Types of indirectly ionizing photon irradiations
In penetrating an absorbing medium, photons may
experience various interactions with the atoms of the
medium, involving:
• Absorbing atom as a whole
• Nuclei of the absorbing medium
• Orbital electrons of the absorbing medium.
Electron Interactions and Stopping Power
Electrons traversing an absorber lose their kinetic energy
through ionization collisions and radiation collisions.
The rate of energy loss per gram and per cm2 is called the
mass stopping power and it is a sum of two components:
• Mass collision stopping power
• Mass radiation stopping power
The rate of energy loss for a therapy electron beam in
water and water-like tissues, averaged over the electron’s
range, is about 2 MeV/cm.
through ionization collisions and radiation collisions.
The rate of energy loss per gram and per cm2 is called the
mass stopping power and it is a sum of two components:
• Mass collision stopping power
• Mass radiation stopping power
The rate of energy loss for a therapy electron beam in
water and water-like tissues, averaged over the electron’s
range, is about 2 MeV/cm.
Activity (Basics)
Activity represents the total number of disintegrations
(decays) of parent nuclei per unit time.
The SI unit of activity is the becquerel (1 Bq = 1 s-1).
Both the becquerel and the hertz correspond to s-1, however, hertz
expresses frequency of periodic motion, while becquerel expresses
activity.
The older unit of activity is the curie ,
originally defined as the activity of 1 g of radium-226.
Currently, the activity of 1 g of radium-226 is 0.988 Ci.
(1 Ci = 3.7 1010 s1)
(decays) of parent nuclei per unit time.
The SI unit of activity is the becquerel (1 Bq = 1 s-1).
Both the becquerel and the hertz correspond to s-1, however, hertz
expresses frequency of periodic motion, while becquerel expresses
activity.
The older unit of activity is the curie ,
originally defined as the activity of 1 g of radium-226.
Currently, the activity of 1 g of radium-226 is 0.988 Ci.
(1 Ci = 3.7 1010 s1)
Review of the basics Ch 1 Atomic Physics
The constituent particles forming an atom are:
• Proton
• Neutron
• Electron
Protons and neutrons are known as nucleons and they form the
nucleus.
Atomic number Z
Number of protons and number of electrons in an atom.
Atomic mass number A
Number of nucleons in an atom,
where
• Z is the number of protons (atomic number) in an atom.
• N is the number of neutrons in an atom.
• Proton
• Neutron
• Electron
Protons and neutrons are known as nucleons and they form the
nucleus.
Atomic number Z
Number of protons and number of electrons in an atom.
Atomic mass number A
Number of nucleons in an atom,
where
• Z is the number of protons (atomic number) in an atom.
• N is the number of neutrons in an atom.
Radiation fundamentals
Exposure (X), 1 R= 2.58 x10-4 C/kg air
Dose, 1Gy=100 rad
Equivalent Dose, 1 Sv=100 Rem (note weighting factors are applied)
Activity 1 Bq= 1Ci/3.7x10 to the power 10
Dose, 1Gy=100 rad
Equivalent Dose, 1 Sv=100 Rem (note weighting factors are applied)
Activity 1 Bq= 1Ci/3.7x10 to the power 10
Categories of ionizing radiation
Ionizing photon radiation is classified into four categories:
Characteristic x ray
Results from electronic transitions between atomic shells.
Bremsstrahlung
Results mainly from electron-nucleus Coulomb interactions.
Gamma ray
Results from nuclear transitions.
Annihilation quantum (annihilation radiation)
Results from positron-electron annihilation.
Courtesy Pgorsak Ch1
Characteristic x ray
Results from electronic transitions between atomic shells.
Bremsstrahlung
Results mainly from electron-nucleus Coulomb interactions.
Gamma ray
Results from nuclear transitions.
Annihilation quantum (annihilation radiation)
Results from positron-electron annihilation.
Courtesy Pgorsak Ch1
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