INTERACTION BETWEEN X-RAYS
AND MATTER
Coherent Scattering
Photoelectric effect
Compton Scattering
COHERENT SCATTERING
8%
Classic scattering
Incident
X-ray photon
Scattered
X-ray photon
Low energy photons
only scattering, no Ionization and only little energy loss
COHERENT SCATTERING
– Rayleigh (coherent) scatter interaction with entire atom
. Photon changes direction (though usually by small angle)
. “Elastic” scattering: Photon energy unchanged
. Most important for “soft” (low energy) . and x rays (< 10
keV)
. Adds very little to film fog
COMPTON SCATTERING
Compton effect or Compton scattering (C) incoherent
scattering:
Directly
proportional to
electron density
Can occur with
very low atomic
weight targets
even at
relatively low Xray energies
62 %
COMPTON SCATTERING
62 %

An x-ray photon of lower
energy is scattered from the
atom

Proportional to electron
density of absorber

Interaction with outer
electron

Changes direction

Adds to film fog
COMPTON SCATTERING
 Incident X-ray changes direction and loses energy,
imparting that energy to the electron (Compton
electron)
 Compton electron interacts with other atoms
producing secondary ionizations
 Possess relatively low energy, produce low energy xrays
 Energies of 100 keV -- 10 MeV – absorption of
radiation mainly due to the Compton effect
PHOTOELECTRIC EFFECT
Interaction with bound atomic electron
. Incident photon disappears
. Photon energy absorbed by electron
. Momentum absorbed by atom
. Probability of PE absorption increases for:
» Low incident photon energy
» High electron density in medium (mass density x Z)
. Following absorption, photoelectrons, and “characteristic”
x rays are emitted
PHOTOELECTRIC EFFECT
• Critical indiagnostic X
rays
• Ionization occurs
• Binding energy is small
for biologic tissues
• Usually with - K shell
electron
• Recoil electron travels
only short distance
30 %
DIFFERENTIAL ABSORPTION
Different tissues - different amount of absorption
Makes radiographs possible
Hard tissue
Soft tissue -
- mostly absorbed
heterogeneous beam
Mainly by photoelectric effect - varies with third power of Z
Of absorber
6.5 times more in bone than water
SECONDARY ELECTRONS
Electrons ejected by photoelectric effect or
Compton effect
Energy given up by:
 Collisional
ionization or excitation
 Radiative
Bremsstrahlung radiation
low energy
BEAM ATTENUATION
Reduction in intensity while passing through
mattter
Either by photo electric effect or Compton
scattering
Half Value Layer - thickness of an absorber needed
to reduce the number of x ray
photon by half
RADIATION DOSE
 Radioactivity
 Exposure
 Absorbed Dose
 Dose Equivalent
 Effective dose
The International
System (SI) unit
RADIATION DOSE
Activity:
rate at which the isotope decays. Radioactivity may be
thought of as the volume of radiation produced in a given
amount of time.
International System (SI) unit - Becquerel (Bq) (curie (Ci)
Exposure:
measure of the strength of a radiation field at some point in
air. measure made by a survey meter. Gray Gy (Roentgen (R)
Effective dose:
Sum of product of equivqlent dose to each organ or tissue
Sivert Si
RADIATION DOSE
Absorbed Dose:amount of energy that ionizing radiation
imparts to a given mass of matter. SI unit gray (Gy), “rad”
(Radiation Absorbed Dose) 1 rad - 0.01 Gy
Different materials that receive the same exposure may
absorb different energy
Dose Equivalent: relates the absorbed dose to the biological
effect of that dose
absorbed dose of specific radiation X "quality factor" = dose
equivalent (SI unit - sievert (SV), (rem)" One rem = 0.01 SV
X- or Gamma radiation - the quality factor is 1
Radiation quantities and units
QUANTITY
SI UNIT
TRADITIONAL
UNIT
Exposure
Coulomb / kg
Roentgen (R)
Absorbed dose
Gray (Gy)
rad
Effective dose
Sievert (Sv)
rem
radioactivity
Becquerel (Bq)
Curie (Ci)
DOSIMETRY
Quantity of radiation exposure
Radiation dosimeters are devices which
are capable of measuring an accumulated
absorbed dose of ionizing radiation
issued four times a year
- rate measuring instruments and
- personal dose measuring instruments
•
•
Direct
Indirect.
DOSIMETERS
Ring Dosimeters
Film Badges
DOSIMETERS
Pocket Dosimeter - Direct Read Pocket
Dosimeter
Digital Electronic
Dosimeter
Ionization Counters
Scintillation Counters
Particle Track Devices
The Cloud Chamber
Geiger Muller Counter
THERMOLUMINESCENT DOSIMETERS (TLDS)
15% for low doses
3% for high doses
TLD - a phosphor, such as lithium fluoride (LiF) or calcium
fluoride (CaF), in a solid crystal structure
exposed to ionizing radiation
ionization
trapping of free electrons
Heating the crystal
1 millirem
radiation interaction
Release of energy as light
Thermoluminescence dosimeter
T L material
T L D reader
EXPOSURE
T L heated
Emitted light
proportonal to
absorbed dose
Equipment used
to measure
Parts of TLD reader
• HEATER
• PHOTOMULTIPLIER TUBE
–HEAT FILTER ALOW THE LIGHT TO PASS
–MEASURE THE EMMITED LIGHT
• ELECTRONIC SYSTEM
(POWER SUPPLY)
AVAILABILITY
• TLD AVAILABLE IN
– SOLID
– POWDER
• APPLICATION OF TLD
– RADIOTHERAPY
– RADIODIAGNOSIS
– PERSONAL MONITORING
P M TUBE
FILTER
TRAY
TLD
READER
PROCEDURE
HEATER