Photon Beam Attenuation

Half value layer (HVL)

Example:
Lead, 100keV HVL= 0.012cm
Lead, 500keV HVL= 0.42cm
Water, 100keV HVL= 4.15cm
Water, 500keV HVL= 7.15cm
What is the thickness of aluminum required to reduce a 200keV x-ray beam to 10% of its intensity?
Assume HVL of Al for 200keV x-ray is 2.14cm.
 Mass Attenuation Coefficient
➢ Mass attenuation coefficient
➢ Linear attenuation coefficient divided by density of the material.
𝝁
➢
𝝆

Independent of density of material.

Unit: g-1.cm2

➢ Mass Attenuation Coefficient (MAC) provides a measure of the

fractional attenuation per unit mass of material encountered.
 X-Ray Beam Attenuation

 reduction in beam intensity by Lower Higher

Energy
 absorption (photoelectric) Energy
 deflection (scattering)
 Attenuation alters beam
 quantity
 quality
 higher fraction of low energy photons
removed
 Beam Hardening
Half Value Layer
 Poly-energetic Attenuation

 Curved line on semi-log graph

 line straightens with increasing attenuation
 slope approaches that of monochromatic
beam at peak energy
 Mean energy increases with attenuation
 beam hardening
 Factors Affecting Attenuation

• Energy of radiation / beam quality

• higher energy
• more penetration
• less attenuation
• Matter
• density
• atomic number
• electrons per gram
higher density, atomic number, or electrons per gram
increases attenuation
Radiation Measurement Units
 Radiation Exposure
➢Radiation exposure is a measure of the ionization of air due to ionizing
radiation from high-energy photons (i.e. X-rays and gamma rays).

➢Unit: Roentgen (R)

➢1 R means the amount of X-rays radiation that is required to liberate positive
and negative charges of one electrostatic unit of charge (esu) in 1 cc of dry air at
standard temperature and pressure (STP). [charges of only one sign is
considered]

• 1 esu = 3.33564×10−10 C
• Air weighs 0.0012929 gram
𝑬𝒙𝒑𝒐𝒔𝒖𝒓𝒆 𝒊𝒏 𝑹 per cubic centimeter
𝑬𝒙𝒑𝒐𝒔𝒖𝒓𝒆 𝑹𝒂𝒕𝒆 =
𝑻𝒊𝒎𝒆
 Absorbed dose (D)
• To measure the interaction of all types of radiation with any kind of
material, the term absorbed dose is used. Energy absorbed per unit mass
of a material is defined as the Absorbed Dose.
• The units of measure for absorbed dose in the SI system is J/kg or gray
(Gy).
1 𝑗𝑜𝑢𝑙𝑒
• 1𝐺𝑦 =
1 𝐾𝑔 𝑜𝑓 𝑡𝑖𝑠𝑠𝑢𝑒
• A radiation field that deposits 1 Joule of energy in 1 kg of material has an
absorbed dose of 1 Gy.
• The old unit of measure for absorbed dose is rad (short for “radiation
absorbed dose”).
• 1 rad = 0.01 Gy 1 Gy =100 rad
 Equivalent dose (H)

• The most important meaning of dose is connected with radiation hazards of

biological tissues.
• The risk of damage is different for different types of radiation.
• Equivalent dose is that dose which gives the same risk of damage to health
whatever the type of radiation.
• Unit: Sievert (Sv)
1 𝐽𝑜𝑢𝑙𝑒
• 1 𝑆𝑣 = × 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝐾𝑔 𝑡𝑖𝑠𝑠𝑢𝑒
• H = 𝐷 × 𝑊𝑅 in Sv, where WR is the weighting factor for type of radiation
 Equivalent dose (H)
• H = 𝐷 × 𝑊𝑅 in Sv, where WR is the weighting factor for type of radiation

1 𝑟𝑒𝑚 = 0.01 𝑆𝑣 1 𝑆𝑣 = 100𝑟𝑒𝑚

**A radiation worker received an X-ray dose of 25mGy over a course of one year.
What is the equivalent dose?
 Effective dose (È)
• The risk of damage is different for different organ or tissue.
• When different organs and tissues are irradiated, the effect on
whole body is calculated as effective dose.
E = 𝐻 × 𝑊𝑇 in Sv, where WT is the weighting factor for type of tissue

❖ During an X-ray exposure, the lungs and

stomach of a patient received doses of
1.0mGy and 0.8mGy respectively. What is
the total effective dose?
 Radiation Detectors
❑ Any material that exhibits measurable radiation related changes can be used as detector for
ionizing radiation.

• Gas filled detectors

• Electric charge • ionisation chambers • Other detectors
• proportional counters • Semi conductor detectors
• Emission of visible light
• Geiger Müller (GM) - • Film
• Chemical changes
tubes • Thermoluminescense
• Scintillation detectors detectors (TLD)
• solid
• liquid
 Gas filled detectors
The most widely used radiation detectors are devices that respond to ionizing radiation by producing
electrical pulses
Gas filled detectors
Gas filled detectors
As the electric field in an ion chamber system is increased the freed e- are accelerated and achieve
sufficient kinetic energy to cause additional ionizations within the detector

Gas amplification factor --- upto104

Initial pair

High voltage
Gas filled detectors
 Gas
amplification
factor ---
109 to 1010
Townsend Avalanche
Gas filled detectors
Gas filled detectors

Current (I)
 GM Counters

❑When the electric field strength across a

proportional counter is increased (> 106 V/m), the
device enters a GM region of operation.
❑ GM counter is gas-ionization device in which, the
ionization effect creates a response which can be
converted to an electrical output.
❑It is a gas-filled detector designed for maximum
gas amplification effect.
 Dead Time

➢ The electrons that are produced in the resulting avalanche are accelerated to the
anode and collected in a short period of time.
➢ However, the positive ions are more massive and make their way slowly to the
cylindrical cathode. If their average transient time is T, the GM tube is busy during T.
➢ If another ionizing particle enters the GM tube during T, it will not be counted. This
time (T) is called the dead time (or resolving time) of the GM tube.
➢ Dead time: Between 100 and 300µs
 Scintillation detectors

Scintillator (NaI (TI): sodium iodide, ZnS: Zinc Sulfide)

 Scintillation detectors

➢ A Scintillator is a material that converts energy lost by ionizing radiation into light.
➢ Ionizing radiation interacts with a scintillator which produces a pulse of light

➢ This light interacts (photoelectric interaction) with a photocathode which results in

the production of an electron

➢ The electron is multiplied in a photomultiplier tube that has a series of focused

dynodes with increasing potential voltage which results in an electrical signal
 Scintillation detectors

➢ The number of counts is dependent on the activity that is

present

➢ The energy of the electron, and consequently the associated

current is proportional to the incident energy of the ionizing
radiation
 Thermolumniscent Dosimeter (TLD)

The TLD badge is a personnel monitoring device with special chemical compounds (e.g., lithium fluoride) that
retain deposited energy from radiation exposure.

Thermoluminescence Mechanism:
• Thermoluminescence is the emission of light from a crystal on heating, after
removal of excitation (i.e. ionizing radiation).
• Radiation dose causes the electrons in the crystal to move from low energy
states to higher energy states.
• Impurities in crystals create energy trap, providing metastable state for the
electrons.
• Some of these excited electrons are trapped in metastable states
• These photons can be collected with a photomultiplier tube.
• By proper calibration, the dose delivered to the crystal can be measured.
Thermolumniscent Dosimeter (TLD)
Thermoluminescent Dosimeter (TLD)
 Semiconductor Detectors
Principle of Operation of Semiconductor Detectors

•Ionizing radiation enters the sensitive volume of the detector and

interacts with the semiconductor material.
•Particle passing through the detector ionizes the atoms of semiconductor,
producing the electron-hole pairs. The number of electron-hole pairs is
proportional to the energy of the radiation to the semiconductor. As a
result, a number of electrons are transferred from the valence band to the
conduction band, and an equal number of holes are created in the valence
band.
•The energy per pair is ~ 3 eV, which is smaller than w-values of gas-filled
detectors by a factor 10.
Semiconductor Detectors

Principle of Operation of Semiconductor Detectors

➢ A charge depleted region occurs at the interface of the n and p type
regions. This depleted region is created as the result of both electron
diffusion from n type material into p type and hole diffusion from p
type to n type material.
➢ The performance of the p-n junction as a practically used radiation
detector is improved by applying an external voltage that leads the
junction to be reversed biased. As the applied voltage raises, both the
width of the depletion region (i.e., the sensitive volume) increases.
•Under the influence of an electric field, electrons and holes travel to the
electrodes, where they result in a pulse that can be measured in an outer
circuit,
•This pulse carries information about the energy of the original incident
radiation. The number of such pulses per unit time also gives information
about the intensity of the radiation.
 Film badge

➢ Film badges are composed of chemically coated film (mostly

a silver bromide emulsion), covered by light tight paper in a
compact plastic container.
➢ Radiation causes a blackening (silver) of the film material.
➢ The degree of film darkening, known as optical density,
represents the radiation dose.
➢ The badge incorporates a series of filters to determine the
quality of the radiation.
Film badge