TLD Diagrams

Khan pg 147 Add detail on this

Magnetron

Klystron diagram

Isocentric treatment technique

Isocentric treatment technique assures

Reproducibility of treatment setup.

Phantom scatter factor, Sp

Phantom scatter factor gives the

1) output factor measured in phantom
2) influence of head and phantom scatter on beam output measured in phantom with increasing collimator field size.

Collimator scatter factor

Collimator scatter Sc provideds the
1) output factor measured in air
2) influence of head scatter on beam output w ith increasing collimator field size

TMR depends on

TMR depends on
depth
field size
beam quality

PDD depends on all of these plus distance to the surface (SSD). TMR was invented to simplify matters so they didn't have to reposition the patient as they rotate around as they did in SSD treatments with PDD.

TAR

TAR concept is usually not preferred with accelerator photon beams because the dose in free air is not well defined for these beams

TAR can be derived from PDD data

At dmax TAR and PSF are practically the same quantity.

Scatter and energy of the beam

At a phantom depth of 10 cm, on the central axis, scatter contribution to the dose is more for a Co-60 beam than an accelerator beam.

This is because scatter towards the central axis decreases as the beam quality increases; hence the decrease in the field size dependence for higher energy photon beams.

SUMMARIZED

At any given depth, the scatter contribution to the total dose decreases with an increase in incident photon energy.

Off axis ratio

The off axis ratio is the dose at any point in a plane perpendicular to the central axis to that of the dose on the central axis in that plane.

Exit dose and the patient

Exit dose calculated using standard PDD tables will not give the correct exit dose. The actual dose will be less due to a lack of backscatter thickness at the patient exit point.

Output of a kV or a Co-60 unit

The output of a kV or a C0-60 unit can be measured in terms of air kerma rate.

TAR decreases with field asymmetry

TAR decreases with increased field asymmetry due to decrease in side scatter.

TAR and field size

TAR increases with increase in field size

BSF and energy

At low energies (< about 0.5 mm Cu) BSF increases with an increase in energy
Due to increased penetration of back scattered radiation

At orthovoltage beam qualities (> about 0.5 mm Cu HVL) BSF actually decreases and forward scatter increases.

TAR

TAR can be measured directly or derived from PDD data.

TAR at dmax and PSF are the same quantity.

In general TAR increases with an increase in depth.

At shallow depths, TAR is greater for low beam qualities (kV x-ray region) because of increased side scatter.

At larger depths TAR is larger for higher energy photons compared to low beam qualities due to increased penetration.

TAR concept

TAR concept involves equilibrium does in air "free space dose" as defined in Khan 2003, which becomes more and more ambiguous and also difficult to evaluate.

For further study see Attix 1999. Note look for where the difficulty in the evaluation of this quantity for Co60 beams are discussed. So the TMR concept replaces this quantiy by a phantom defined quantity that is less ambiguous.

Isodose and PDD data

Isodose and PDD data can't be used directly for a patient, without any modification. Often they have to be corrected for body curvature and internal heterogeneities since the beam data are acquired for a water phantom (homogeneous medium) for normal incidence.

Isocentric vs Constant SSD methods

In the case of isocentric treament, compared to constant SSD methods,

a) setup time decreases
b) setup error decreases
c) treatment accuracy increases

Disadvantage of using a physical wedge

Disadvantages of wedges oh there be a plenty:

1) hardening of the beam, which also depends on wedge thickness
2) attenuation of the beam and an increase in treatment tme
3) inconvenience in handling heavy wedges. Gropes and complaining from therapists who haven't seen the inside of a gym since about the time Jane Fonda released her first fitness video on VHS tape.

Independent movement of jaws

Independent movement of jaws is necessary for
a) creating of blocked fields
b) avoiding beam divergence at one edge when necessary
c) creating dynamic wedge fields.

A beam flattening filter is used with

A beam flattening filter is used with a

PHOTON BEAM

Now who is smart than a 5th grader. Most 5th grade medical physicists would know this answer.

Followup
How is it retracted for e-beams?

I know it's on a carousel.

Low energy electrons and scattering

Low energy electrons are more easily scattered compared to high energy electrons.

Yes remember Scattering is Zsquared/Esquared

SCIENTIFIC NOTATION!!!!!

For $%$%$sake, why doesn't blogger allow scientific notation. Has our society devolved to where all the blogs revolve around mere words, the tabloids, gossip and political bullshit?

What happened to people who want to use this for their mathematical equations? Am i the only one out there.

Buehler, Buehler.....

Electron beams and their range

Electron beams have a finite range in the medium.

The range of electons of energy E (MeV) in water is roughly E/2 cm.

NOTE: The range of electrons in lead is E/20 cm (per WMK and Khan e- chapter)

energy fluence of an accelerator electron beam.

Energy fluence of an accelerator electron beam and dose fall off.

IT DOSE NOT FALL OFF ACCORDING THE ISL. The dose measured at dmax point in a phantom, by varying the SSD, roughly follows (to better than 2%) the ISL for small changes in distance (say within about 5 cm), but not with respect ot the target position, but with respect to a point much closer than the target. This apparent distance, which is also a function of field size, must be experimentally determined for every electron beam quality of the machine (See Khan 2003 for more details)

Megavoltage x-ray vs kV

kV imaging utilizes a different method of filtration to attenuate the beam. The filter may be something like aluminum, ex 200 kVp x-ray beam filtered by 1 mm thick aluminum filter. This distribution includes the glass envelope of the x-ray tube, the surrounding oil, and the exit window of the tube housing as well. This so called inherent filtration is equivalent to approximately 1 mm Al in most x-ray tubes. Can also be a combination filter such as a Thoraeus filter TCA (tin copper aluminum)

For a megavoltage x-ray beam however, the beam is hardened by the inherent filtration of the transmission target as well as by transmission through the flattening filter.

Buildup in electron beams

Buildup in electron beams is due to electron scattering and the finite range of the delta rays produced.

BONUS NOTE
The tail of the electron beam depth dose distribution is due to dose produced by the brehmmstrahlung radiation or the x-ray background (photon contamination)

TMR vs TPR

TMR is a special case of TPR where dref=dmax.

Scatter and PDD

Scatter generally increases PDD. Since it generally increases dose on the central axis.

Beam divergence and beam attenuation

Beam divergence and beam attenuation in a phantom decrease with PDD.

Yese because both parameters reduce the dose at a given depth.

Surface buildup and extrapolation chamber

The entry window thickness becomes the thickness of the dead layer of the skin and the chamber would measure the dose just below the dead layer of the skin.

Field size dependence and electron contamination

The field size dependence of the PDD of an accelerator photon beam is very much influenced by the electron contamination of the beam.

This is becausee of electron and scatter photon contamination of the beam.

AAPM TG-51 protocol

In the AAPM TG-51 protocol for electrons, Electron beam quality is specified by the beam penetration parameter R50,d, the depth where the electron beam depth dose drops to 50% of the peak value. This is more accurate than the parameter Eo, the mean energy of the electron beam used in the earlier protocols (as derived from R50 using an empirical equation).

AAPM TG-51 and photons

In the AAPM TG-51 protocol, beam quality for photons is specified by PDD (10)x.

Photons and Inverse Square Law (ISL)

The energy fluence of the photons fall off according to the Inverse square law of distance with the source or the target position as the origin.

The dose measured at the dmax point in a phantom by varying the SSD, roughly follows (to better than 2%) the inverse square law, for moderate changes in SSD (say within about 20 cm). This however must be verified for the treatment machine before putting it into clinical use.

SIDE NOTE:
Note that photons do not have a finite range in the patient.

Dynamic Wedge

A dynamic wedge is created by moving one jaw of the collimator towards the other, which creates the wedged field profile.

One of the advantages of a dynamic wedge over a physical wedge for treatment is that there is no change in beam quality across the field size. (No beam hardening for a dynamic wedge like there is when a physical wedge differentially attenuates the photon beam).

UNIVERSAL WEDGE

A universal wedge is an internally mounted wedge that is usable for many different field sizes.

Scatter contribution

At any given depth, the scatter contribution to total dose decreases with increase in photon energy.

CORRECT (SEE EARLIER POST)

Flattening filter

C0-60 machines do not have a flattening filter because the photon emission from a Co-60 source is more isotropic compared to high energy photon beams, which are peaked in the direction of a central axis. So a flattening filger is not necessary.

Scatter contribution to dose

At a phantom depth of 10 cm, on the central axis, scatter contribution to the dose is more for an accelerator photon beam compared to a Co-60 beam.

NO, Scatter towards the central axis decreases as the beam quality (energy) increases; hence the decrease in the field size dependence seen for higher energy photon beams.

SO SCATTER CONTRIBUTION TO DOSE IS LESS FOR LINACS THAN A CO-60 Machine.

Off-axis ratio

The off-axis ratio (OAR) is the dose at any point in a plane perpendicular to the central axis to that of the dose on the central axis in that plane.

Off-axis ratios can be used to determine dose at off axis points.

Output of kV or Co-60 units

The output of a KV or a Co-6o unit can be measured in terms of air kerma rate.

BSF (Back Scatter Factor)

At low energies (< about 0.5 mm Cu) BSF increases with an increase in energy.

Yes, this occurs due to increased penetration of backscattered radiation.

At orthovoltage beam qualities (> about 0.5 mm Cu HVL)
BSF actually decreases and forward scatter increases.

TAR (tissue air ratio)

TAR concept is usually not used with the accelerator photon beams because the dose in free air is not well defined due to conceptual difficulties.

TAR concept involves equilibrium dose in air ("free space dose") as defined in Khan (2003), which becomes more and more ambiguous and also difficult to evaluate.

For further study (see Attix 1999) Note look for where the difficulty in the evaluation of this quantity for Co-60 beams is discussed. So the TMR concept replaces this quantity by a phantom defined quantity that is less ambiguous.

TAR can be measured directly or derived from the PDD data.

TAR at dmax and PSF (peak scatter factor) are the same quantity.

In general TAR decreases with an increase in depth.

At shallow depths TAR is greater for low beam qualities (kV x-ray region) because of increased side scatter.

At larger depths TAR is larger for high energy photons compared to low beam energies, due to increased penetration.

TAR increases with an increase in field size.

TAR decreases with increasing field asymmetry. This occurrs due to decrease in side scatter.

Isodose distribution and proper phantom size

To measure the isodose distribution for a 10cmx10cm field size, it is not necessary to use a 40x40 water phantom.

The requirement is that the field must be surrounded by 5 cm of water on all sizes. So for smaller field sizes, smaller phantoms can be made use of.

SSD vs SAD techniques

In SSD techniques, the beams are weighted at dmax points

In SAD techniques, teh beams are weighted at the target center.

Gy, cgy, rads

1 Gy= 100 cGy= 100 rads

Therefore, 1 rad = 1 cGy

Exposure of 1 roentgen

An exposure of 1 roentgen corresponds to a charge release of

1 esu
also 2.58x10 -4 C/kg

Effective Z of air

The effective Z of air in the photoelectric region is about 7.6

SI Unit of Kerma

The SI unit of kerma is
J/kg

J/kg is given the special name of GRAY

Farmer chamber requirement

A Farmer type chamber used for exposure or air kerma measurements in a hospital must have a tissue equivalent wall.

Farmer chambers and buildup cap

Farmer chambers are usually calibrated with a buildup cap at Co-60 energy to

PROVIDE EQUILIBRIUM WALL THICKNESS

Equilibrium wall thickness of chambers

KV region and chambers
The wall thickness of the chamber used for beam output in the kV range is about 60 to 90 mg/cc

The equilibrium wall thickness required for the chamber used for beam output measurements at Co-60 energy is about 500 mg/cc.

Plane parallel chambers (and perturbation)

Some plane parallel chambers exhibit significant perturbation at low electron energies.

TRUE.

For example, the Markus chamber does.

The Roos chamber designed by PTW Germany is a modification over the Markus Chamber and does not exhibit fluence perturbation at low electron energies. It's stability however depends on the reliability of the aquaduc coating on it.

Farmer type chambers

Farmer type chambers can be used for the dosimetry of accelerator produced photon and electron beams.

TRUE (except for incident electron beam energies with Eo<10 MeV).

So why do we use these for e- measurements? Relative is OK but what about absolute?

Chamber calibration

Hospital chambers must be calibrated in terms of exposure or air kerma or absorbed dose to water in an Accredited Dosimetry Calibration Laboratory, which is traceable to the Primary Standards Laboratory.

Graphite cavity ionization chambers

Graphite cavity ionization chambers are the primary standard of absorbed dose for therapy machines.

For kv x-ray beams, free air ionization chambers are the primary standard of exposure or air kerma.

Primary Standards of Exposure

Primary Standards of Exposure, Air Kerma or Water Absorbed Dose have been established by some Primary Standards Dosimetry Laboratories (PSDL's) for a Co-60 beam.

Hospital chambers used for beam calibration must be traceable to these standards either directly or indirectly.

Ionization curves to depth dose curves.

How do you convert ionization curves to % depth dose curves?

Ref p 303 Khan
Depth ionization curves obtained with air ionization chambers can be converted into depth dose curves by making corrections for change in stopping power ratio of water to air with depth.

TG-51 Effective point of measurement of parallel plate chamber.

Where is the effective point of measurement of parallel plate chambers?

For plane parallel chambers, the center of the front (upstream) face of the chamber air cavity is the point of measurement.

Purpose of the guard ring in the plane parallel chamber is to

The purpose of the guard ring in the plane parallel chamber is to:
(DEFINE THE COLLECTION VOLUME SEEMED TO BE THE ONLY REASONABLE ANSWER, CHECK THIS.)

A guard ring prevents leakage. It creates a homogeneous electric field between the two electrodes. It prevents secondary electrons scattered from the wall being counted in the chamber.

Add picture from Pgorsak.

TG-51 Cross calibrating parallel plate chamber

To cross calibrate a parallel plate chamber what would one use?
OPTIONS: Co60, high energy photons, high energy electrons, low energy electrons.

Per tG-51 pg 1860.

Since Co-60 cal factors of plane parallel chambers are sensitive to their construction,
calibrate against calibrated cylindrical chambers in a high energy electron beam.

How many TVL's in a linac head

At 1 m from the source, nonshielded vs shielded you get 0.1% (leakage through the head).

To get that think of half values 1/2=50 .......1/16=6.25.........1/1024=10 HVL'S

So 10 HVL's are required to get it to 0.1%

We know 1 TVL=3.3 HVL's so, so to get 10 HVL's we need about 3 TVL's.

The answer is then 3 TVL's.

Regulations

Received a link to this website today. It's quite useful for linking you to various regulations

http://www.physics.isu.edu/radinf/rsotoolbox.htm

Absorbed Dose to Water

Therapy chambers can be calibrated in terms of exposure, air kerma or "Absorbed Dose to Water". Accredited laboratories offer such calibrations.

Farmer ionization chamber

A Farmer type ioniziation chamber must be used for radiation therapy beam calibration.

Volume about 0.6 cc cubed.

Exposure is the ionization equivalent of collision kerma in air

Exposure is the ionization equivalent of collision kerma in air. In the figure above (ADD), exposure at
point P= X(P)= Kc(P)/We) (J/kg) (J/c)
To obtain exposure in roentgens we must use the conversion factor 2.58 x10 -4 (C/kg)/R

Collision kerma Kc

That part of kerma (i.e. electron kinetic energy) that leads to subsequent collision interactions and hence energy deposition is known as collision kerma (Kc).

Collision part of kerma is NOT ALWAYS equal to absorbed dose. But when charged particle equilbrium exists, for example, at P in the above figure, electron energy escaping from at a point P will be compensated by electron energy entering at P (ENTER DIAGRAM LATER). In this case Kc(P) can be equated to D(P) with a small correction that accounts for the very small attenuation of photons over electron ranges.

Since any dosimetric system, whether it is a Primary Standard or a therapy dosimetry responds to energy absorbed in the sensitive volume, kerma (or exposure) of indirectly ionizing particles can be measured only under electronic equilibrium conditions.

Kerma and energy deposition

Kerma is only defined for photons which interact with matter and convert to electron energy.

Not all of kerma ends up as energy deposition in the medium. Part of kerma (electron kinetic energy produced) is converted into bremmstrahlung which escapes from the region of interest, so that only that part of kerma called the collision part of kerma is energy that is dissipated in the medium by ionization and excitation events.

Kerma

Kerma, K, at a particular point is defined as the kinetic energy released (ex in the form of charged particle energies) per unitmass, in an infinitesmal volume, around that point by IDIR (indirectly ionizing radiation)

Ionizing vs nonionizing radiation

Radiation that has enough energy to ionize the atoms and thus dissipate energy in matter is referred to as ionizing radiation.

Indirectly ionizing radation is commonly stated as not directly producing ionization of atoms in the medium. BUT THEY CAN, they can cause ionization in single interactions producing charged
particles (e.g. electrons or positrons), which are the agents for causing futher ionizations and excitations in the medium. For instance, if we assume that in a Compton interaction a photon produces an electron of 1 MeV in air, this amounts to one ionization in air by the photon.

However, the 1 MeV electron goes on to produce (1,000,000/33.97) ionizations in air. So photons or neutrons are indirectly ionizing radiation.

Errors in dose due to the TPS

Errors can arise in the dose computed by the treatment planning system because of

1) inaccuracies in algorithms
2) inaccuracies in the software implementation of algorithms
3) operator inputting using wrong parameters (ex beam data) or parameters incorrectly
4) limitations of the algorithms while applying in a specific case

Advantages of 3-D treatment planning

The advantages of 3-D treatment planning are
1) more accurate inhomogeniety and patient contour corrections
2) possibility of non-coplanar treatment plans
3) better plan optimization using DVH's

TPS and treatment planning

In order to use a TPS for accurate treatment planning, any TPS installed in a radiation therapy department must be:

1) acceptance testing before commissioning
2) provided with machine beam data input
3) provided with patient specific data input
4) verified for the accuracy of the dose computation

Accuracy of the dose computed by the TPS depends on
1) accuracy of the beam calibration
2) dose calculation algorithm

Aspect ratio for a CT image

For a CT image, aspect ratio is a calibration for the width and height of an image on the monitor.

It can be given by 2:3.

Pixel pitch

In a CT image, pixel pitch is a number that characterizes the actual dimensions of a pixel in millimeters. If the x pixel pitch is 0.72 mm and the y pixel pitch is 0.72 mm, the actual area of a 256x256 image is (256x256 mm squared)

CT analog to digital conversion

CT has an analog to digital convertor.

Matrix size and CT images

As the matrix size of an image increases, the following also occurs:

1) the relative size of each pixel decreases
2) the image quality improves
3) The spatial resolution increases.

Best absorbing material for neutrons?

Best absorbing material for neutrons is polyethylene.

Power source to generate electronmagnetic waves in a linac

Which power source is used to generate electromagnetic waves for the accelerator guide in a linac?

A MAGNETRON.

Acceptable flatness for a linac

What is the clinically acceptable limit of flatness over the central 80% of an x-ray beam used in radiotherapy?

+/- 2%
Reference AAPM TG-40

Target used for high energy linacs

What type of target is used for high energy linacs?

Transmission type.

Geometric penumbra and Co60

The geometric penumbra of a Co-60 unit can be reduced by

1) using a source of smaller diameter
2) using a penumbra trimmer.

Source of electrons in an electron accelerator

The source of electrons in an electron accelerator is the electron gun.

Linac vs Co-60

Some important features of a linac compared to a Co-60 unit are

Higher beam outpu and sharper beam.

Shutter timer error

Shutter timer error for Co-60 arises due to finite travel time of the source to reach the treatment position.

It can be either positive or negative.

The length of a shutter timer error of a Co-60 unti is approximately 0.1 to 0.2 min.

It can be determined using an ionization chamber.

If the shutter timer error is positive to set the treatment time on the unit, it must be added to the computed "beam on " time. If negative obviously subtract it from the beam on time.

Focal spot of a linac

The focal spot of a clinical linac is about 3 mm

Advantages of a Co-60 unit in radiation therapy

The advantages of a Co-60 unit in radiation therapy are

1) High gamma energy
2) High specific activity
3) reasonably long half life.

Efficiency of x-ray production

The efficiency of x-ray production in the kv x-ray therapy tubes is about 1%. The other 99% is heat.

For theerapy (MV range) efficiency is about 50%

Effective energy of an x-ray beam

The effective energy of an x-ray beam is the energy of that monoenergetic beam that would give the same HVL as the x-ray beam in question.

Co-60 decay and other notes

In one month the output of a Co-60 machine would reduced by about 1%.

Other note: The diameter of a typical teletherapy source is about 2 cm.

The geometric penumbra of a Co-60 beam depends on
1) source diameter
2) diaphragm to patient distance
3) source to diaphragm distance

Flattening filter part II

The flattening filter position in the beam path is critical. Any error in the positioning of the flattening filter will affect

1)Beam flatness
2)Beam symmetry
3)Beam output

Flattening filter

A flattening filter is used in an accelerator to get a flat beam profile at the clinical depth.

The flattening filter used in a linac:

1) Hardens the beam more in the central region compared to the peripheral region
2) produces a horn in the cross-beam profile at shallower depths.
3) is cone shaped to attenuate more in the central region compared to the peripheral region

Co-60

Some facts about Co-60

It doesn't use a flattening filter to produce a uniform flat beam because the emission is not forward peaked and is more isotropic compared to a linac beam.

There is some radiation from a Co-60 machine when the machine is switched off due to leakage (unlike a linac)

A Co-60 source is produced by activating small pellets or thin discs and they are tightly packed into the source capsule to produce a Co-60 source. This avoids problems of self shielding which would result in nonuniform activity in the source.

One reason output has to be measured periodically is due to the source movement. If the source does not come to the same position every time the output can vary due to partical shielding.

Electron energy deposited in the x-ray target

Much of the electron energy deposited in the x-ray target appears as heat.

KV and therapy units

In a KV xray unit, the electrons are accelerated by a constant or pulsating dc potential.

In an accelerator, the electrons are accelerated by the electric field vectors of the microwave in a phase stable position.

Multiple scattering ofa pencil beam of electrons

Multiple scattering of a pencil beam of electrons in a scattering foil results in

a) it's angular spread
b) it's energy degradation
c) bremmstrahlung contamination

Energy loss/cm rule of thumb

The energy loss of electrons in water or tissue is roughly given (in MeV/cm) by

2 MeV/Cm

Compton scattering and energy of backscattered photon

The energy of the backscattered photon in MeV is 0.255

Electron traversing medium

Following events can occur when a electron traverses a medium

Complete conversion into a bremsstrahlung photon in a radiative collision

Photon interaction

The following events can occur:

No interaction
Complete absorption
Scatter

Mass attenuation coefficient due to Compton interactions

The mass attenuation coefficient due to Compton interactions:

decreases with energy
depends on electron density of the medium

Photoelectric cross section, Compton cross section

The photoelectric cross section is dependent on both the photon energy and the atomic energy of the medium.

The Compton scattering cross section depends on the gamma energy only.

Threshold energy for pair production

The threshold energy (in MeV) for pair production, in the Coulombic field of the nucleus is
1.02 MeV.

The threshold energy in MeV for pair production in vicinity of a nucleus is 2.04 MeV.

Energy losses by electrons

Energy losses by electrons are dependent on Z.

Energy losses are dependent on Z in collision interactions.

Stopping power is divided into collision and radiative interactions.

For radiative collisions, energy losses when electrons are stopping in a medium vary as Z to the 3rd power

Compton interaction

A Compton interaction involves a photon interacting with a free electron.

Photoelectric effect

Photoelectric effect involves a BOUND electron.

Probability of photoelectric effect

The probability of photoelectric effect in a medium roughly vaires as Z to the 4th (reported in some as Z to the 3rd power)

Pair Production

The probability of pair production (atomic cross section) in the interacting medium varies as Z squared (mnemonic Z squared , the two is a pair!)

In pair production after expending energy for the creation of the pair, the excess photon energy is shared by the electron and positron.

In pair production the positron at the end of it's range in the medium is annihilated resulting in two annihilation photons.

Port films and simulator films and cassettes

Source Van Dyk

The cassettes and films used to generate port films are quite different from those found in the simulator. Rather than a high atomic number fluorescent screen, the x-ray detector consists of a front metal plate and a rear plate made of either plastic or metal. The front metal plate acts as a buildup layer that generates high energy electrons, while the rear metal plate acts as an electron backscattering material and as a compression plate to ensure good contact between the film and the surrounding materials.

Most commercially available portal film cassettes use a 1 mm thick copper plate with a curved cassette lid.

Photoelectric vs Compton

In a photoelectric interaction, the photon loses all the energy

In a Compton interaction, the photon loses part of it's energy.

Photo-electric vs Compton effect

To interact by photoelectric effect, the interacting photon must have an energy equal to or just greater than the binding energy of the electron.

To undergo a Compton effect interaction the energy of the interacting photon must be very much larger than the electron binding energy. Thus explaining why the freed electron acts as almost a free electron.

Electron positron pairs (Pair production)

To produce an electron-positron pair in the vicinity of a nucleus, the interacting photon must have a minimum energy of 1.02 MeV.

High energy electrons in high Z materials

While high energy electrons scatter less than low energy due to (Z squared/ E squared) and the information in the previous post below this one in the blog, scattering of high energy electrons does increase in High Z materials.

High energy electron scattering < low energy electron scattering

High energy electron scattering increases in High Z materials!

Scatter and electrons

HIGH ENERGY ELECTRONS SCATTER LESS

High energy electrons are faster than low energy electrons and hence, interact for less time and hence scatter less than low energy electrons.

REMEMBER Scattering is approximately = Z squared/ E squared

Pencil beam of electrons

A pencil beam of electrons incident on a foil spreads into a beam of larger cross-section due to multiple Coulombic interactions within nuclei of atoms.

This is the principle behind the production of large field sizes for clinical treatment with electrons.

Compton interactions

Compton interactions are like billiard type collisions.

Because the binding energies are small compared to the photon energy, so the photon sees the electron as a "free" electron in a Compton interaction.

I always think of playing pool in Compton, California which is something i might not have the courage to do in real life!

Electrons and bremmstrahlung

As electrons interact with matter, they produce x-rays due to bremmstrahlung (or braking radiation)