Dosimetry Hierarchy of Dosimetric regulation Radiation Dosimeters General properties of Radiation Dosimeters Gas filled detectors Free Air ionization chamber Thimble Chamber ,Farmer and Parallel plate ionization chambers Plane Parallel chamber Extrapolation chamber Proportional counter Gieger-muller counter Scintillation detector Solid state detectors: Thermo luminescent Detector Personnel monitoring: TLD Badge Diode dosimetry Semicondutor diode Pocket dosimeter Film dosimetry Radiographic film Radiochromic film.

1. Basic Dosimetric principles
and Dosimeters
Vinay Desai
M.Sc Radiation Physics
KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY
Bengaluru

2. Contents
1. Dosimetry
2. Hierarchy of Dosimetric regulation
3. Radiation Dosimeters
4. General properties of Radiation Dosimeters
5. Gas filled detectors
6. Free Air ionization chamber
7. Thimble ,Farmer and Parallel plate ionization chambers
8. Plane Parallel chamber
9. Extrapolation chamber
10. Proportional counter
11. Gieger-muller counter
12. Scintillation detector
13. Solid state detectors:
14. Thermo luminescent Detector
15. Personnel monitoring:
16. TLD Badge
17. Diode dosimetry
18. Semicondutor diode
19. Pocket dosimeter
20. Film dosimetry
21. Radiographic film
22. Radiochromic film.

3. Dosimetry
• Radiation Dosimetry is a study in physics which deals with the
measurement of radiation which may include Exposure,
Absorbed dose etc.,
• Dosimetry is extensively used for radiation protection and is
routinely applied to radiation workers, where irradiation is
expected but regulatory levels must not be exceeded.
• Dosimetry contains required quantitative methods which are
used to determine the dose of radiation, which helps in,
a. The need of protection against ionizing radiation,
b. Application of radiation in medicine.

4. Dosimetric quantities
1. Activity
2. Exposure
3. Absorbed dose
Radioactivity:
Spontaneous emission of certain unstable atomic nuclei with the emission of
certain radiations is called 'Radioactivity'.

5. PSDL
IAEA
SSDL
USER 1 USER 2
SSDL
USER 3 USER 4
Hierarchy of Dosimetric regulation
IAEA-International Atomic Energy Agency
PSDL-Primary Standard Dosimetric Laboratory
SSDL-Secondary Standard Dosimetric Laboratory
Users- Hospitals, Research purpose users of radioactive materials etc.,

6.  Activity :Activity refers to amount of unstable nuclei that gains
stability through radio disintegration per unit time.
-Where dN is the number of nuclear transformation (decay) in unit
time dt.
dT
dN
A 

7.  Exposure: Exposure is a measure of ionization produced in air by
photons (X rays or Gamma rays).
 Exposure is given by,
where dQ is the absolute value of total charge of ions of one sign produced in air
when all electrons (negatron's or positrons) liberated by photons in air of mass dm are
completely stopped by air.
• SI unit of exposure is C/Kg.
• Special Unit of exposure is Roentgen.
• It is applicable only for:
• Photon energies below 3 MeV
• Interaction is only between photons and air.
dm
dQ
X 

8. Absorbed dose:
The Absorbed dose (D), is the energy absorbed per unit mass. This
quantity is defined for all ionizing radiation (not only for EM radiation,
as in the case of the exposure) and for any material.
where,dE is the energy imparted to matter of mass dm.
The unit of absorbed dose is Gray.
Energy imparted =(Energy incident)- (Energy leaving the mass)- (Energy released in nuclear transformations)
dm
dE
D 

9. Types of radiation
 There are two types of radiation,
1.Non Ionizing Radiation:
• Radiation that does not have sufficient energy to
eject the orbital electrons from the medium.
E.g. Microwaves, ultraviolet light, lasers, radio waves
and infrared light.
2.Ionizing Radiation:
• Radiation that has sufficient energy to eject
orbital electrons from the medium it is passing
through.
E.g. Alpha particles, neutrons, gamma rays and
Xrays.

10.  Radiation Dosimeters are the devices used for detection of the
radiation which directly or indirectly measures Exposure, Kerma,
Absorbed dose, Equivalent dose or other quantities.
 The dosimeter along with its reader is referred to as a Dosimetry
System.
• Two parts of Radiation measuring system are:
1. A detector
2. A measuring apparatus(electrometer)
• The interaction of radiation with the system takes place in the
detector.
• The measuring apparatus takes the output of the detector and
performs the function required to accomplish the measurements.
Radiation Dosimeters

11.  Properties of an useful dosimeter are as follows :
1. High accuracy and precision
2. Linearity of signal with dose over a wide range
3. Small dose and dose rate dependence
4. Flat Energy response(Quality dependence)
5. Small directional dependence
6. High spatial resolution

12. General properties of Radiation Dosimeters
 Modes of operation:
• Many radiation detectors posses/produce electrical signal after
each interaction of radiation.
• This electrical signal is processed by external circuit.
• Based on two main classifications made,
1) Pulse mode: Signal from each interaction process separately.
Eg., GM Counters Proportional counter, Scintillation detectors.
2) Current mode: Signal from each interaction are averaged together
and form a current signal.
Eg., Ionization chamber

13. • Disadvantage of ‘Pulsed mode system’ is that two
interactions must be separated by certain amount
of time.
• If the signal is produced in the time gap between
first and second interaction then that signal will be
lost and not counted.
• This time gap is called ‘Dead time’.
• In ‘Current mode system’ ,all the information
regarding the individual information is lost i.e.,
neither interaction rate nor the energy deposited
by an individual can be determined .
Dead time
Averaged interaction

14. Types of Detectors
1) Gas Filled Detectors:
- ionization chambers
- proportional chambers
- Geiger – Mueller (GM) Counter
2) Scintillation detectors
- solid and liquid scintillator
3) Solid State Detectors
-Semiconductor detectors
- Thermo luminescent detectors
- Diode detector
-Pocket dosimeter
4) Photographic emulsions
5) Chemical dosimeters
6) Calorimetric dosimeters

15. Gas Filled Detectors

16. Gas filled
Detectors
Ionisation
chamber
Free air ion
chamber
Cavity
chamber
Cylindrical
chambers
Parallel
chambers
Extrapolation
chambers
Proportional
counters GM counters

17.  Principle of Gas filled detector
Basic structure of Response curve
• Gas Multiplication:
• Gas Multiplication is a consequence of increasing the electric field
within the gas to a sufficiently high value.
• At low value of field the ions & electrons produced/created by the
incident radiation simply drift to their respective collecting electrodes.
• During migration of these charges many collisions occur with the
natural gas molecules.
• Because of low mobility ,positive or negative ions achieve very little
average energy between collisions.
• Free electrons are easily accelerated by the applied field and may have
the significant K.E when undergoing such a collision.
• If the energy is greater than ionization energy of the neutral gas
molecule there may be an additional ion pair created in the collision
as the average energy of the electron between the collision increases
with increasing the electric field, there is a threshold value of the field
above which the secondary ionization occurs.

18. • The electron liberated by this secondary ionization also will be
accelerated by the electric field.
• During this subsequent drift it undergoes collisions with other
neutral gas molecule and thus can create an additional ionization.
• The gas multiplication thus takes the form of ‘Cascade’ known as
‘Townsend Avalanche’ in which each free electrons created in such
collision potentially create more free electrons by the same process.
• Fractional increase in no. Of electrons per unit path length is
governed by townsend equation,
• Where α is townsend co-eff for gas.
• Value of α is zero for electric field values below threshold & increases with increasing field
strength above this minimum.
Threshold to
Gas multiplication
Electric field strength
Townsend Avalanche
Pulse
dx
n
dn
.

19. Regions of Detector operations
• Graph of α vs applied voltage(within detector) is
plotted.
• At very low voltage the field is insufficient to
prevent the recombination of the ion pairs and
collected charge is less than that of represented
by the original ion pairs.
• Recombination is suppressed as the voltage is
raised, saturation region is achieved.
• Ionization chamber works in this mode of
operation.
• Gas multiplication begins as the voltage applied
crosses the threshold field.
• Collected charge begins to multiply and α
increases.
• Over some region the gas multiplication will be
linear and collected charge will be proportional
to number of original ion pairs created by
incident radiation.
• This region is true proportionality region and
represents conventional proportional counter
mode of operation.

20. • Further increase in the voltage results in non-linear
effects(mainly +ve ions are created by secondary
ionization).
• Unlike the electrons the +ve ions move slower
towards the electrode and require more time to
reach towards the electrode.
• Therefore each pulse within the counter creates a
cloud of +ve ions which are slow to disperse as it
drifts towards the cathode.
• If the concentration of these ions are sufficiently
high then they represents a ‘Space charge effect’.
• The voltage is made sufficiently high so that the
space charge created by the ion pairs dominantly
determine the subsequent history of the pulse.
• Avalanche proceeds until sufficient no.Of ion pairs
are created up to the limit of applied voltage.
• This is a self limiting process. Finally the ion
collection of the detector is no longer reflects any
properties of incident radiation.
• This region is called ‘Gieger muller’ region.
Graph of α vs applied voltage

21. Free Air ionization chamber

22. Free Air ionization chamber
• The free-air, or standard, ionization chamber is an instrument used
in the measurement of exposure in roentgens.
• Generally, such a primary standard is used only for the calibration of
secondary instruments designed for field use.
• The free-air chamber installations are thus confined principally to
some of the national standards laboratories.
• An x-ray beam, originating from a focal spot S, is defined by the
diaphragm D, and passes centrally between a pair of parallel plates.
• A high voltage (field strength of the order of 100V/cm) is applied
between the plates to collect ions produced in the air between the
plates.

23. • The ionization is measured for a length L defined by the limiting
lines of force to the edges of the collection plate C.
• The lines of force are made straight and perpendicular to the
collector by a guard ring G.
• Electrons produced by the photon beam in the specified volume
must spend all their energy by ionization of air between the plates.
• Such a condition can exist only if the range of the electrons
liberated by the incident photons is less than the distance between
each plate and the specified volume.
• In addition, for electronic equilibrium to exist, the beam intensity
must remain constant across the length of the specified volume,
and the separation between the diaphragm and the ion collecting
region must exceed the electron range in air.

24. • If ΔQ is the charge collected in Coulombs and ρ is the density
(kg/m3 ) of air, then the exposure XP is given by at the center of the
specified volume (point P) is given by,
• where AP is the cross-sectional area of the beam at point P
• L is the length of the collecting volume.
• Corrections applied during measurement include
a. correction for air attenuation;
b. correction for recombination of ions;
c. correction for the effects of temperature, pressure, and humidity on the
density of air;
d. correction for ionization produced by scattered photons.

25. Disadvantages of Free air ion chamber
• Free air ionization chambers are not used above 3MeV
photon measurements. As (recombination > after 3MeV).
• Reduction in plate separation causes high air attenuation
and reduces efficient ion collection.
• Delicate and Bulky.
• Only used for Calibration of secondary instruments.

26. Thimble ,Farmer and Parallel plate
ionization chambers

27. Thimble,Farmer and Parallel plate ionization chambers
• Also called as cavity chambers, are small air enclosed chambers,
highly precise design determined to be accurate,
• Consists of high sensitive air volume, Wall and central electrode
materials must be close to homogeneous in order for correction
factor to be known.
• 2 types of cavity chambers are:
1. Cylindrical chamber (Thimble /Farmer chamber).
2. Parallel plate chambers

28. Thimble chamber

29. Principle of Thimble chamber:
• Consider an air shell consisting of air cavity exist at the centre of a
spherical volume.
• If the sphere of air is irradiated uniformly with a photon beams also
if that the distance between the outer sphere and the inner cavity is
equal to the maximum range of electrons generated in air.
• And if the number of electrons entering the cavity is the same as
that leaving the cavity, electronic equilibrium exists .
• if we are able to measure the ionization charge produced in the
cavity by the electrons liberated in the air surrounding the cavity.
• charge per unit mass or the beam exposure at the center of the
cavity can be calculated if the volume or mass of air inside the
cavity are known.

30. Principle of Thimble chamber:
• If the thimble wall is compressed into solid shell we get a thimble
chamber ,where wall is air equivalent.
• The thickness of the wall is such that the electronic equilibrium
exists within the air cavity.
• As density of solid-air equivalent is much greater than that of free
air, The Thickness of chamber wall is reduced such that the
electronic equilibrium exists within the cavity.
• Eg.,
1) 100-250 Kvp X-ray range thimble wall thickness is 1mm.
2) 1.25 MeV 60CO γ range thimble wall thickness is 5mm.
• ‘Build-up caps’ are used during measurement in air.

31. Construction & Working:
• Inner wall is coated with special material to make it electrically
conducting. This forms one electrode.
• Another electrode is a rod of low atomic no. material such as
graphite or aluminium rod held in the centre of thimble by an
insulating material.
• Suitable voltage is applied between two electrodes to collect the
ions produced in air cavity.
Chamber wall
Central electrode
P.D.
Insulator

32. • When the radiation passes through the chamber the ions (-ve &
+ve) are produced in that air cavity.
• These ion are collected by the electrodes and it is measured by the
electrometer in terms of ‘ionization charge’ (Q) .
• Volume (V) of air must be known to calculate charge per unit mass.
• The Exposure (X) is given by,
• ρ is density of air.
)( V
Q
X



Ions (-ve & +ve)

33. Chamber characteristics
• There should be minimal variation in sensitivity or exposure
calibration factor over a wide range of photon energies.
• There should be suitable volume to allow measurements to the
expected range of exposure.
• There should be minimal variation in sensitivity with the direction
of incident radiation.
• Minimal stem leakage.
• Chamber should have been calibrated for exposure against a
standard instrument for all radiation qualities for which exposure is
to be measured.
• Minimal recombination loss.

34. Farmer chamber

35. Farmer chamber
Components and characteristics of Farmer chamber:
• Chamber wall: Wall is made-up of Graphite or plastic such as
PMMA (Acrylic) nylon.
• Wall thickness varies between different makes and models. The
approximate range is 0.04 to 0.09 g/cm2.
• Outer Electrode: It is the thimble wall and the inner surface of the
thimble wall coated with a conducting material.
• Central Electrode: It consists of a thin aluminum rod of 1mm
diameter. It is the collector electrode that delivers the ionization
current to electrometer.
• Guard Electrode: Cylindrical conductor that wraps around the
insulator surrounding the central electrode in the stem of the
chamber.

36. • Chamber Volume: The cavity volume determines the mass of air in
the cavity and also the sensitivity of the chamber.
• Farmer-type chambers have a cylindrical cavity with a nominal
volume of 0.6mL. The cavity radius is approximately 0.3cm.
• Energy dependence: Energy dependence of an ion chamber
depends on the composition and thickness of the wall material.
• Stem effect: The stem effect arises out of radiation-induced signal
in the chamber stem and the cable, if exposed. The stem effect
originating in the stem is directly related to the length of the
unguarded stem.
• The amount of stem effect is a function of energy as well as type of
beam. Fully guarded Farmer-type chambers have almost
immeasurable stem effect.
Energy response of the Chamber

37. Extrapolation chamber

38. Extrapolation chamber
• Extrapolation ionization chamber is used for measuring surface
dose in an irradiated phantom
• The beam enters through a thin foil window that is carbon coated
on the inside to form the upper electrode.
• The lower or the collecting electrode is a small coin-shaped region
surrounded by a guard ring and is connected to an electrometer.
• The electrode spacing can be varied accurately by micrometer
screws.
• By measuring the ionization per unit volume as a function of
electrode spacing, one can estimate the incident dose by
extrapolating the ionization curves to zero electrode spacing.

39. Plane Parallel chamber

40. Plane Parallel chamber:
• It consists of a thin wall or window made-up of thick Mylar ,
polystyrene or mica (0.01-0.03mm) .
• It allows the measurement at the surface of the phantom without
wall attenuation.
• It has two plane walls ,one serving as a entry window and polarising
electrode and other as the back wall acts as collecting electrode
(usually block of conducting plastic or non conducting material with
graphite layer).
• It also contains guard ring system.
• Small electrode spacing of approx 2mm minimizes cavity
perturbations in the radiation field.
• This feature is especially important in the dosimetry of electron
beams (energy <10MeV) where cylindrical chambers may produce
significant perturbations in the electron fluence due to the
presence of their large air cavity.

41. Proportional counter

42. Proportional counter
• Proportional counter is a type of gas filled detector always used in
pulsed mode and rely on the gas multiplication phenomenon.
• Gas Multiplication is a consequence of increasing the electric field
within the gas to a sufficiently high value.
• Primary ions produced undergo further ionization producing
secondary ions and the process continues.
• The total charge collected is then measured and is given by,
• Where ,
• Q- Total charge generated NO is Original no. Of ion pairs.
• E- electron charge. M-Multiplication factor of Gas.
eNoMQ 

43. Geiger-Muller Counter

44. Geiger-Muller Counter
• GM counter is an instrument used for detection of ionizing
radiation.
• It cant determine the type or energy of the radiation.
• GM counter uses the ‘Townsend avalanche’ phenomenon to
produce an easily detectable electronic pulse.
• Under proper condition a situation is created in which avalanche
itself trigger an second avalanche at a different position in the GM
tube.
• GM tube is a sensing part of the GM counter is an cylindrical metal
envelope filled with inert gas (Helium ,Neon, argon) at low
pressure(0.1 atm).

45. • Wall of the tube is stainless steel inside coated with
metal or graphite to form a cathode.
• Anode is a tungsten wire about 0.03-0.04 inch dia.
Stretched along the centre of cylinder.
• High potential difference is applied between
electrodes (500V-1500V).
• When ionizing radiation strikes on the tube, gas
molecule inside get ionized and positive ions
accelerated towards the cathode and electrons
towards the anode.
• Due to high voltage applied the accelerated ions
produce further ionization with the gas molecules.
• As a result of rapid multiplication of ion creates a
large no. of electron avalanche which is spread along
the anode.
• One single avalanche per original ionizing event will
produce 106 to 108 excited molecules.

46. Geiger Discharge
• In a Townsend avalanche many secondary electrons are produced.
• Thus a dense sheath of ionization propagates along the central wire
in both directions, away from the region of initial excitation,
producing what is termed a Geiger-Mueller discharge.
• When excited molecules return to their ground state,emission of
photons with UV or visible light wavelength occurs.
• The temporary presence of a positive space charge surrounding the
central anode terminates production of additional avalanches by
reducing the field gradient near the center wire below the
avalanche threshold.
• If ions reach the cathode with sufficient energy they can liberate
new electrons, starting the process all over again, producing an
endless continuous discharge that would render the detector
useless.
• For preventing this external circuitry is used to "quench" the tube.

47. • Quenching:
1. External Quenching
2. Internal Quenching
• External Quenching: It is done by reducing voltage after fixed time
of each pulse by choosing high resistance value at outside the
circuit which causes delayed discharge.
• Internal Quenching: Adding secondary gas (poly-atomic vapours
such as butane or ethanol).When ions move towards cathode they
collide with the quenching molecules and transfer the charge and
some energy to them.
• Quencher atom gets de-excited by dissociation into neutral
quencher atom.
•

48. Dead time:
• space charge close to the anode reduces the intense electric field
sufficiently that approaching electrons do not gain sufficient energy
to start new avalanches.
• The detector is then inoperative (dead) for the time required for the
ion sheath to migrate outward far enough for the field gradient to
recover above the avalanche threshold.
• The time required for recovery to a value high enough for a new
pulse to be generated and counted is called the "dead time" and is
of the order of 100 microseconds.
• GM counter significantly underestimate radiation level when used
to count radiation around pulsed machine such as accelerator.

49. Scintillation detector

50. Scintillation detector
• Scintillation detector works on principle of detection of ionizing
radiation by measuring the scintillation light produced by certain
materials like NaI, CsI, Cadmium tungstate ,amorphous silicon etc.,
• When the crystals absorb the energy the e- jump from ground state
to conduction band. The excited e- return to the valance band with
emission of photon.
• When photon interacts with the crystal photoelectrons are
produced within the crystals. These photoelectrons travel through
the crystal & they ionize the atoms of crystal, as a result Flashes of
UV or visible light are produced (Luminescence).
• Only small part of energy imparted is converted into light rest is
dissipated into form of heat.

51. Working of Scintillation detector
• Scintillation detector consists of,
1. Luminescent scintillation material.
2. Optical device to facilitate the collection of light(Optical interface)
3. Photomultiplier tube(PMT)
4. Electrical counter

52. • The low energy photons falls on the photocathode of PMT and
ejects no. of photoelectrons.
• Photoelectrons are accelerated by P.D. Applied between Cathode &
Dynodes of the tube.
• On striking the 1st Dynode Photoelectrons ejects several electrons
by secondary emission and further multiplies with rest of dynode
interactions.
• After multiplication of 105 – 109 the electron avalanche, electrons
arrive at collector plate.
• It produces a voltage pulse on O/P condenser, which is coupled to
external Pulse amplifying circuit.
• Thus initial energy of single ionizing particle is transformed into
single voltage pulse.

53. • The no. of electrons ejected is directly proportional to the amount of light
reaches it.
• The amount of light absorbed is directly proportional to the Energy
absorbed from photon by crystal.
• Thus the size of pulsed energy is directly proportional to the energy of
incident photons in crystals.
No. of electrons
ejected
Amount of Light
reaches it
Energy absorbed
by Photons by
Crystals
Energy of incident
Photons in
Crystals

54. Solid state detectors:
Thermo luminescent Detector

55. Solid state detectors:
Thermo luminescent Detector
• Thermo luminescence is a process in which A thermo luminescence crystal
is irradiated, a very minute fraction of absorbed energy is stored in crystal
lattice.
• The same energy can be recovered later as visible light and measured.
• The amount of energy of light can be calculated and is proportional to
amount of irradiated energy.
• Some of commonly used TL phosphors are,
• 1) LiF 2) CaSo4 3) Al2O3 4) LiB4 O7 5) Dy (2Lithium tetra Borate)
• These phosphors provides electron holes due to their crystalline lattice
imperfection and they,
a. Traps the electrons holds for long period of time.
b. Emits light when electrons jump to ground state from excited state.

56. Principle and Working
• When these phosphors are irradiated the electrons in the valance
band get excited and they rises to conduction band( e- free to
move).
• There exists an electron trap inside (due to crystalline lattice
imperfection) where e- gets trapped. Vacancy is created in the
conduction band.
• When heated the trapped electrons get sufficient energy to escape
from the trap and jump back to conduction band radiatively.
• Luminescence is produced as a result of emission of energy.
• The intensity of emission of light is directly proportional to Rate of
electron escape.

57. • The irradiated material is placed in a heater cup or planchet, where it is
heated for a reproducible heating cycle.
• The emitted light is measured by a photomultiplier tube (PMT), which
converts light into an electrical current.
• The current is then amplified and measured by a recorder or a counter.
• As the temperature of the TL material exposed to radiation is increased,
the probability of releasing trapped electrons increases.
• The light emitted (TL) first increases, reaches a maximum value, and falls
again to zero.
• Because most phosphors contain a number of traps at various energy
levels in the forbidden band, the glow curve may consist of a number of
glow peaks.
• The different peaks correspond to different "trapped" energy levels.
The arrangement for measuring the TL output Glow curve

58. Personnel monitoring
• TLD Badge:
• TLD Card is made up of high impact plastic.
• There are 3 filters in cassette corresponding to each disc namely
Cu+Al , perspex & open window.
• 1st disc is sandwiched between pair of filter combination of 1mm Al
and 0.9mm Cu.
• 2nd disc is sandwiched between pair of 1.5mm thick Plastic filters
(180 mg/cm2).
• 3rd disc is positioned under circular open window.
• Metallic filter- Gamma rays
• Plastic filter- Beta rays
• Open window- Alpha rays

59. Characteristics of TLD
1. Used for Personal dosimetry monitoring.
2. Used for patient dosimetry.
3. Used for Measurement of Build-up region dose.
4. No need of processing.
5. Insensitive for room temperature, Pressure and humidity.
6. Energy independent.
7. Sensitivity ranges from 25KeV to 3MeV.
8. Exposure range is 1000mR to 10R.
9. Reusable & Linear response to dose.
10. Provides permanent record.
11. Small size and commercially efficient.
12. Demerits are Fading,Sensitive to light, lack of uniformity.

60. Semiconductor Diode

61. Diode dosimetry
• Silicon p-n junction diodes are used for relative dosimetry.
• They are highly sensitive, and response is instantaneous.
• Their small size, and ruggedness offer special advantages over
ionization chambers.
• They are particularly well suited for relative measurements in
electron beams, output constancy checks, and in vivo patient dose
monitoring.
• Their major limitations as dosimeters include
1. energy dependence in photon beams,
2. directional dependence,
3. thermal effects,
4. radiation-induced damage.

62. • Dosimetric diode consists of a silicon crystal doped with impurities
to make p- and n-type silicon.
• The p-type silicon is made by introducing a small amount of an
element(e.g., boron), making it into an electron receptor.
• When silicon is mixed with a material like phosphorus, it receives
atoms that are carriers of negative charge, thus making it into an
electron donor or n-type silicon.
• A p-n junction diode is designed with one part of a p-silicon disk
doped with an n-type material
• The p-region of the diode is deficient in electrons (or contains
"holes" ) , whereas the n-region has an excess of electrons.

63. • a small region called the depletion zone is present at the interface
between p- and n-type materials, this is created because of initial
diffusion of electrons from the n-region and holes from the p-
region across the junction, until equilibrium is established.
• The depletion zone develops an electric field, which opposes
further diffusion of majority carriers once equilibrium has been
achieved.
• When a diode is irradiated, electron-hole pairs are produced within
the depletion zone.
• They are immediately separated and swept out by the existing
electric field in the depletion zone. This gives rise to a radiation-
induced current.
• The current is further augmented by the diffusion of electrons and
holes produced outside the depletion zone within a diffusion
length.

64. Construction and working
• Diode detector, consists of a silicon p-n junction diode connected to a coaxial
cable and encased.
• The radiation beam to be incident perpendicularly at the long axis of the
detector.
• Collecting or sensitive volume (depletion zone) is approx of 0.2 to 0.3 mm3.
• It is located within a depth of 0.5 mm from the front surface of the detector,
unless electronic buildup is provided by encasing the diode in a buildup material.
• Diode is connected to an operational amplifier with a feedback loop to measure
radiation-induced current.
• There is no bias voltage applied. The circuit acts as a current-to-voltage
transducer, whereby the voltage readout at point B is directly proportional to
the radiation-induced current.
Clinical applications
1. dose profiles or output factors in a small field can be measured .
2. patient dose monitoring as no high voltage bias required.

65. Pocket dosimeter

66. • Basically its an ionization chamber which is charged to a suitable
voltage and the discharge in the electrostatic charge on a metal
conductor due to ionization of air in chamber by radiation.
• Dosimeter must be periodically recharged and read before it gets
charged and dose is logged into charts one’s exposure level.
• Magnifying lens and illumination allows one to directly read the
dose at any time by aiming the illumination lens at light source.
• Device is mainly sensitive to gamma rays and X-rays but also detects
the β-radiation
• Advantages:
1. Immediate reading.
2. Energy independent of wide range of gamma rays.
3. Disadvantages:
4. Low accuracy, Reading errors(manually) small dynamic range.
5. Susceptible to moisture.
Pocket dosimeter

67. Working
• It consists of ‘Lauristen electroscope’ and has sealed air filled
ionization chamber.
• An metal electrode strip is attached to terminal on the end of pen
for charging. Other end is delicate Gold pated quartz fibre attached
to it which at rest is parallel to electrode.
• Ends of chamber are transparent and microscope is focused on
fibre.
• 150-200 V is applied during charge.
• When the ionizing radiation passes through the chamber it collides
with air molecules and ionize the air.
• Reduced charge of the electrode reduces the force on the fibre
causing it to move back towards electrode.
• Position of fibre can be read through the microscope with scale
behind fibre.

68. Film Dosimetry

69. Film Dosimetry
• Radiographic film is the dosimeter particularly used for qualitative
measurement of ionizing radiation beams.
• A radiographic film consists of a transparent film base (cellulose
acetate or polyester resin) coated an emulsion containing very small
crystals of silver bromide.
• When the film is exposed to ionizing radiation or visible light, a
chemical change takes place within the exposed crystals to form what
is referred to as a latent image.
• The film is developed, the affected crystals are reduced to small grains
of metallic silver. The film is then fixed.
• The unaffected granules are removed by the fixing solution, leaving a
clear film in their place.
• The metallic silver, which is not affected by the fixer, causes darkening
of the film.
• Thus, the degree of blackening of an area of the film depends on the
amount of free silver deposited and, consequently, on the radiation
energy absorbed.

70. • The degree of blackening of the film is measured
by determining optical density with a
densitometer.
• This instrument consists of a light source, a tiny
aperture through which the light is directed, and
a light detector (photocell) to measure the light
intensity transmitted through the film.
• The optical density is given by,
• Where,
• I0 - is the amount o f light collected without film
• It - amount of light transmitted through the film.
• absolute dosimetry with film is impractical.
• Hence it is used for checking radiation fields,
Light-field coincidence, field flatness, and
symmetry, and obtaining quick qualitative
patterns of a radiation distribution.
tI
I
OD

log
Densitometer
Composition of an
Radiographic film

71. Radiochromic film
• Radiochromic film consists of an ultrathin ( 7- to 23 -μm thick) , colorless,
radiosensitive leuco dye bonded onto a 100-μm thick Mylar base.
• Film contains special dye which is polymerised upon radiation exposure
(Colour changes to blue).
• No physical or chemical processing is required.
• Radiochromic films are almost tissue equivalent with effective Z of 6.0 to
6.5.
• Degree of colouring is using a spectrometer using a narrow spectral
wavelength (nominal to 610nm to 670nm).
• Laser scanner & Charge coupled devices (CCD) can also be used to scan
the films.
• Measurements are done with Sensitometers and expressed in OD,
• Where,
• I0 - is the amount o f light collected without film
• It - amount of light transmitted through the film
tI
I
OD

log

72. ADVANTAGES
1. Tissue equivalent with effective Z of 6.0 to 6.5.
2. High spatial resolution.
3. Large dynamic range (10-2 – 106 Gy)
4. Low energy dependent compared to radiographic film.
5. Insensitive to visible light.
6. No need for physical,Chemical or thermal processing.

73. E-mail:- vinaydesaimsc@gmail.com
Thank you.
Vinay Desai
M.Sc Radiation Physics
Radiation Physics Department
KIDWAI MEMORIAL INSTITUTE OF
ONCOLOGY
Bengaluru