Dosimetry concepts and dosimeters

DOSIMETRY CONCEPTS AND
DOSIMETERS
SAILAKSHMI.P
MSc. Radiation Physics
University of Calicut

DOSIMETRY
 Deals with the measurement of the absorbed dose or dose rate
resulting from the interaction of ionizing radiation with matter.
 It also refers to the determination of radiologically relevant quantities
such as:
 Exposure
 Kerma
 Fluence etc.

DOSIMETRY CONCEPTS
 AAPM (American Association of Physicists in
Medicine)
Task Group-21 (1983)
TG-51 (1999)
 IAEA (International Atomic Energy Agency)
Technical Report Series -277(1997)
TRS-398 (2000)

TG -51
 The TG-51 protocol is based on “absorbed dose to water” calibration.
 Conceptually easier to understand and simpler to implement
Co
wD,Q
Q
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60
NMkD 
rawpolelecTPion MPPPPM 

TG-51
 Suitable nominal energy
• photon beams: 60Co-50MV
• electron beams: 4-50MeV
 Ion chambers calibrated
• in terms of absorbed dose to water in a 60Co beam.
 Purpose
• to ensure uniformity of reference dosimetry in external
beam radiation therapy with high-energy photons and
electrons.

General Formalism
 In a Co-60 beam:
 In any other photon beam:
(only cylindrical chamber allowed at present)
 In any electron beam:
(both cylindrical and parallel-plate chambers allowed)
)(
6060
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Corrected meter reading :
 Polarity corrections, Ppol :
 Electrometer correction factor, Pelec
rawpolelecTPion MPPPPM 
raw
rawraw
pol
2M
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P


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Temperature Pressure Correction:
 Standard environmental condition:
To=22oC
Po=101.33 kPa
humidity between 20%~80% (variation 0.15%)
 PTP corrects charge or meter readings to standard environmental condition.

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P
101.33
22273.2
T273.2
PTP

 Corrections for ion-chamber collection inefficiency, Pion :
 if Pion 1.05, another ion chamber should be used
 Voltages should not be increased above normal operating voltages. (~300V
or less)
 60Co
 pulse/swept beam

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Point of Measurement &
Effective Point of Measurement
point of
measurement
Effective point
of
measurement
cylindrical parallel plate
rcav
r
Photon: r = 0.6 rcav
electron: r = 0.5 rcav

Beam Quality Specification (Photons)
For this protocol, the photon beam quality is specified by %dd(10)x, the
percent depth-dose at 10 cm depth in water due to the photon component
only, that is, excluding contaminated electrons.
For low energy photons (<10 MV with %dd(10) < 75%)
%dd(10)x = %dd(10) (contaminated electron is negligible)
For high energy photons (>10 MV with 75%<%dd(10)<89%)
%dd(10)x  1.267%dd(10) – 20.0
A more accurate method requires the use of a 1-mm thick lead foil placed
about 50 cm from the surface. (505cm or 301cm)
%dd(10)x = [0.8905+0.00150%dd(10)pb] %dd(10)pb
[foil at 50 cm, %dd(10)pb>73%]
Co
wD,Q
Q
w
60
NMkD 

Beam Quality Specification (electrons)
Percent depth ionization to be measured at SSD = 100 cm for field size  1010 cm2
(or 2020 cm2 for E>20 MeV).
Parallel-plate chamber: measured curve II.
Cylindrical chamber: measured curve I,
needs to be shifted by 0.5 rcav to get curve II.
Curve II is the percent ionization curve.
R50 = 1.029I50 – 0.06 (cm) for 2I50 10 cm
R50 = 1.059I50 – 0.37 (cm) for I50 >10 cm
Co
wD,ecal
'
R
Q
gr
Q
w
60
50
NkkMPD 

Reference Depth:
photon dref = 10 cm e- dref = 0.6 R50 - 0.1 cm
photon source

Photon Beam Dosimetry
Co
wDQ
Q
w NkMD
60
,
 M fully corrected chamber reading
 kQ quality conversion factor
 absorbed dose to water calibration factorCo
wDN
60
,

Electron Beam Dosimetry
Co
wDecalR
Q
gr
Q
w NkkPMD
60
50 ,
'

 M fully corrected chamber reading,
 correction factor that accounts for the ionization
gradient at the point of measurement
(for cylindrical chamber only)
 electron quality conversion factor.
Kecal photon to electron conversion factor, fixed for a given
chamber model
 absorbed dose to water chamber calibration factor
Co
wDN
60
,
Q
grP
'
50Rk

TRS - 398
 This protocols use different beam-
quality index TPR20,10 .
 To be specified as TPR20,10 = ratio
of dose at isocenter with 20cm
attenuation to the same with
10cm attenuation.
 SAD set up
 Field size = (10x10)cm

Absorbed Dose
 The energy absorbed per unit mass of any material
D = dE/dm
 where dE is the mean energy imparted by ionizing radiation
of mass dm.
 Old unit rad (radiation absorbed dose)
 1 rad = 10 -2 J/Kg = 1cGy
 SI unit of absorbed dose is Gray
1Gy = 1 J/Kg

Kerma
 Kinetic energy released in the medium per unit mass.
 K = dEtr /dm
 dEtr is the sum of the initial kinetic energies of all the charged
particles liberated by uncharged particles in a material of
mass dm.
 Unit = J/Kg
 K= K col+ K rad
 KERMA is used as an approximation to absorbed dose
when radiative losses are negligible.

Exposure
 Measure of ionization produced in air by photons.
X = dQ/dm
 dQ is the absolute value of the total charge of the ions of one
sign produced in air when all the electrons liberated by
photons in air of mass dm are completely stopped in air.
 SI Unit = C/Kg Special Unit = Roentgen(R)
 1R = 2.58 x 10 -4

Relating Absorbed Dose to Exposure
CPE
Dair = (Kc) air = X * (W/e)air
J/Kg J/Kg C/Kg 33.97 J/C
Dair = (Kc) air = 0.876 X
rad rad R

DOSIMETERS
 A dosimeter can be defined generally as any device that is capable of
providing a reading r that is a measure of the absorbed dose D, deposited
in its sensitive volume V by ionizing radiation.
 Dosimeter is a device that measures directly or indirectly
• Absorbed dose
• Exposure
• Kerma
• Equivalent dose
• Or other related quantities.

Dosimeters Divided Into Two:
 Absolute dosimeters
here the dose is determined without reference to another
dosimeter.
Eg: free air ionization chamber, specially designed spherical
chambers of known volume, calorimeter, Fricke dosimeter .
 Secondary dosimeters
these dosimeters requires calibration against a primary
standard.
Eg: thimble chambers, plane parallel ion chambers , TLD’s ,
Diodes and Films.

IONIZATION CHAMBERS
 The ion chamber consists of a cylindrical
chamber containing air at atmospheric
pressure.
 A moderate voltage (100 volts) is applied
between two electrodes, the anode and
cathode.
 The interaction of radiation in a gas
results in the production of ion pairs
consisting of a negative ion (electron)
and a positive ion.
 The negative ions are attracted to the
positive electrode (anode) and the
positive ions to the negative electrode
(cathode).

 This flow of ions produces a small
electric current which is a measure of
the radiation dose rate, ie , ionisation
produced per second.
 The current produced in the ion
chamber is very small ( ~ 10-12amps)
and therefore very sensitive
amplification electronics is required
making this type of monitor very
expensive.

Free Air Ionization Chamber
 The free-air, or standard, ionization
chamber is an instrument used in the
measurement of the roentgen
according to its definition.
 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 is applied between
the plates to collect ions produced in
the air between the plates.
 The ionization is measured for a
length L defined by the limiting lines
of force to the edges of the collection
plate C.

Free Air Ionization Chamber
 Free-air ionization chambers are too delicate and bulky for routine use.
 Their main function is in the standardizing laboratories where they can
be used to calibrate field instruments such as a thimble chamber.

THIMBLE CHAMBERS
 It works based on the Bragg –Gray Cavity theory
 A spherical volume of air is shown with an air cavity
at the center.
 This sphere of air is irradiated uniformly with a
photon beam.
 And the distance between the outer sphere and the
inner cavity is equal to the maximum range of
electrons generated in air.
 Then the number of electrons entering the cavity is
the same as that leaving the cavity, ie, electronic
equilibrium exists.
 If the air wall is compressed into a solid shell, we get
a thimble chamber. Then the thicknesses required for
the thimble chamber are considerably reduced
Air cavity
Air shell
Solid Air Shell
Air Cavity

THIMBLE CHAMBERS
 The wall is shaped like a sewing thimble—hence the name.
 The inner surface of the thimble wall is coated by a special material to make it
electrically conducting. This forms one electrode.
 The other electrode is a rod of low-atomic-number material such as graphite or
aluminum held in the center of the thimble but electrically insulated from it.
 A suitable voltage is applied between the two electrodes to collect the ions
produced in the air cavity.
 The system as a whole behaves like a free-air chamber.
 Most commonly used wall materials are made either of graphite (carbon), Bakelite,
or a plastic coated on the inside by a conducting layer of graphite or of a
conducting mixture of Bakelite and graphite.

FARMER CHAMBER
 The thimble wall is made of pure graphite and the central electrode is of pure
aluminum.
 The insulator consists of polytrichlorofluorethylene.
 The collecting volume of the chamber is nominally 0.6 cm3.
 The thimble is at ground potential and the guard is kept at the same potential as
the collector.
 Most often the collector is operated with a positive voltage to collect negative
charge, although either polarity should collect the same magnitude of ionization
charge if the chamber is designed with minimal polarity effects

FARMER CHAMBER
 Cylindrical (thimble) ionization chamber
– Most popular design
– Independent of radial beam direction
– Typical volume between
0.05 -1.00 cm3
– Typical radius ~2-7 mm
– Length~ 4-25 mm
– Thin walls: ~0.1 g/cm2
– Used for: electron, photon, proton or ion beams.
 The guard electrode serves two different purposes:
One is to prevent leakage current from the high-voltage electrode
(the collector)
and the other is to define the ion-collecting volume.

The Extrapolation Chamber
(variable volume)
• Extrapolation chambers are parallel-plate
chambers with a variable electrode
separation.
• They can be used in absolute radiation
dosimetry (when embedded into a tissue
equivalent phantom).
• Cavity perturbation for electrons can be
eliminated by:
– Making measurements as a function of the
cavity thickness
– Extrapolating electrode separation to zero.
• Using this chamber, the cavity
perturbation for parallel plate chambers
of finite thickness can be estimated.

PARALLEL PLATE CHAMBERS
 Parallel Plate/Plane parallel chamber is recommended for:
– Dosimetry of electron beams with energies below 10 MeV.
– Depth dose measurements in photon and electron beams.
– Surface dose measurements of photon beams.
Depth dose measurements in the build-up region of megavoltage
photon beams.

PLANE PARALLEL CHAMBERS
 They have a fixed electrode spacing (1-
2mm).
 Sensitive volume = 0.35 cm3
 In a plane-parallel ion chamber, the plane-
collecting electrode is surrounded by a wide
margin of guard ring to prevent undue
curvature of the electric field over the
collector.
 In such a chamber, when graphite coatings
are used as collecting surfaces on an
insulator, the collector can be separated
from the guard ring by a scratch through
the graphite coating.

GIEGER MULLER COUNTER
 If the voltage in an ionisation system is
increased beyond a certain point, an effect
known as gas amplification occurs.
 The negative ions are now accelerated
towards the anode and are of sufficient
energy to cause further ionisation themselves
before reaching the anode.
 If the voltage is increased further, the gas
amplification or avalanche effect is so great
that a single ionising particle produces a large
pulse of current.
 The size of the pulse is the same regardless of
the energy of the incident radiation.

GIEGER MULLER COUNTER
 The Geiger counter is normally constructed in a
tubular form with the metal outer casing acting as the
cathode and a thin wire running through the centre
acting as the anode.
 There is a thin end window usually constructed of
mica to allow soft beta particles to enter.
 Inside the tube the counter gas (normally 90% argon
and 10% methane) is held at less than 1 atmosphere.
 The methane is there as a quenching agent to “mop”
up the positive ions which would otherwise strike the
cathode, releasing further electrons which would
cause the counter to go into continuous discharge.
 Modern counters now use halogen as a quenching
agent as methane has a finite lifetime and halogen
does not.

GM COUNTER
Area Gamma Monitor GA-720
 Useful for monitoring Gamma dose
rate levels in working areas of
radioisotope laboratories, in oncology
departments near cobalt therapy
machines or Brachytherapy machines
or at other similar medical systems &
also in a medical cyclotron facility, or
at other medical & industrial
radiological installations.
 Radiation detected: X-rays and Gamma
Radiation.
 Range : 0.1mR/hr - 100mR/hr

GM COUNTER
Contamination Monitot: CM710P
 Radiation Detected : Beta and
Gamma
 Radiation Detector : Halogen
quenched G.M. Detector a. Pan
Cake - LND7311
 Doserate: (0 - 200) mR/hr of
Gamma for End Window
 Operating voltage: +900V

SOLID STATE DOSIMETERS
 The term solid state detectors refers to
certain classes of crystalline substances
which exhibit measurable effects when
exposed to ionising radiation.
 In these substances electrons exist
discrete energy bands separated by
forbidden bands.
 The highest energy band in which
electrons normally exist is the valence
band.
 The transfer of energy from a photon or
charged particle to a valence may raise it
to through the forbidden band into the
exciton band or the conduction band.

SOLID STATE DOSIMETERS
 The vacancy left behind by the electron is known as a hole .
 The three states shown above may be permanent or only exist for a short
time depending on the material and temperature.
 In returning to the valence band the difference in energy is emitted as
fluorescent radiation, normally a light photon.

Thermo Luminescent Dosimeters
 These detectors utilise the electron
trapping process.
 One of the most common materials is
lithium fluoride and dysprosium doped
calcium sulphate(CaSo4:Dy ) which is
selected because after irradiation
electrons in the crystal matrix are raised
to a metastable excited state.
 Under normal temperatures these
electrons remain in this state, but heating
the material to over 3000C releases them
from the traps and they rapidly return to
the valence band with the emission of a
light photon.

BARC TLD
 There are 3 disc in TLD
 1st disc consist of Al and Cu
combination filter window, which cut
of the beta radiation and gives the TL
due to the X and Gamma radiation.
 2nd disc consist of the plastic window
which cut of the soft beta radiations
and records X-rays, Gamma rays and
hard Beta radiations.
 3rd disc has no filter records all the
radiation.
 It can measure doses ranging from
100 µSv to 10 Sv

ALBEDO DOSIMETER
 This dosimeters are sensitive for the neutrons which are scattered back from the
body.
 Since the human body consists of hydrogen atoms, a significant fraction of fast and
intermediate neutrons are slowed down to epithermal energies and back scattered.
 These back scattered neutrons are called Albedo neutrons which interact with TL
material.
 Neutron detector mechanism involves an Li-6 (n, ) H3 reaction. The alpha and
triton are absorbed by the LiF detector with a deposition of energy that can be
detected by common TLD read out technique.
 Natural lithium TLD’s are sensitive to thermal neutrons and the sensitivity can be
increased by making the TLD out of lithium enriched with Li6
 Li6 and Li7 enriched TLD’ s are used in pairs because to subtract gamma rays. ie,
TLD-600 response to both gamma and neutron radiation and the other TLD-700
respond only to gamma radiation. So the difference
in reading give the neutron dose.

SCINTILLATION DETECTOR
 Fluorescent radiation emitted when an electron returns from an excited state to the
valence band.
 Most monitors use sodium iodide (NaI) as the scintillator as it only takes about 1 s for
the electron to return to the valence band.
 The absorption of 1 MeV gamma photon results in about 10,000 excitations and the
same number of photons of light.
 These scintillations are detected by the front face of a photomultiplier tube via optical
coupling between the light tight can surrounding the NaI and the photo-cathode of the
PM tube.
 The photo-cathode detects these very faint light signals and converts them into
electrical pulses.

CONTAMINATION MONITOR
 Worked with unsealed radioactive materials
generates potential contamination of
surfaces.
 Surface contaminations are measured as
activity per unit area [Bq/cm²] for specified
Radionuclides.
 Surface contaminations are monitored
using a variety of methods and instruments.
 Surface contamination detector using
scintillation theory

RADIOGRAPHIC FILM
 A radiographic film consists of a transparent film base (cellulose acetate or
polyester resin) coated with 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.
 When 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.

RADIOGRAPHIC FILM
 The degree of blackening of the film is
measured by determining optical
density with a densitometer.
 The optical density, OD, is defined as:
log Io/It
 where I0 is the amount of light
collected without film and It is the
amount of light transmitted
through the film.
 A plot of net optical density as a
function of radiation exposure or
dose is termed the sensitometric
curve or H-D curve.
 Eg: Kodak EDR-2 and Kodak RPM-2

RADIOGRAPHIC FILM
 Film suffers from several potential errors such as :
changes in processing conditions,
 interfilm emulsion differences
 artifacts caused by air pockets adjacent to the film.
 For these reasons, absolute dosimetry with film is impractical.
 However, it is very useful for checking
radiation fields
 light-field coincidence
 field flatness
Symmetry
 and obtaining quick qualitative patterns of a radiation distribution.
 In the megavoltage range of photon energies, however, film has been
used to measure isodose curves with acceptable accuracy (±3%)

RADIOCHROMIC FILM
 The use of Radiochromic films for radiation dosimetry has been evolving since
the 1960s.
 With the recent improvement in technology associated with the production
of these films, their use has become increasingly popular, especially in
brachytherapy dosimetry.
 Major advantages of Radiochromic film dosimeters include:
 tissue equivalence
 high spatial resolution
 large dynamic range (10-2-106 Gy)
 relatively low spectral sensitivity variation (or energy dependence)
 insensitivity to visible light
 and no need for chemical processing(self developing).

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.
 Other varieties include thin layers of radiosensitive dye sandwiched between
two pieces of polyester base .
Polyester layer
Radiosensitive layer/
Active layer
28 µm
120 µm
120 µm
Gafchromic EBT3 Film
layer arrangement

Nuclear Track emulsion Type A (NTA)
Kodak Film
 Used for neutron dosimetry.
 The film consists of fine grain nuclear emulsions of thickness 30 micron coated
on one side of cellulose acetate base. It is wrapped in light tight black paper.
 Fast neutron undergo elastic scattering with hydrogen nuclei, which take
place in the emulsion itself or in the cellular acetate base or in the packaging
of the emulsion, in the film holder and for high energy neutron in the body of
the person who is wearing the badge.
 Recoil protons are produced which are recorded as photographic tracks in the
emulsion.
 The length of the track depends upon the proton energy.
 Recognition of tracks are possible only from neutron energy 0.5MeV
(threshold energy) and Above. And the tracks are counted on a microscope
after film processing.
 Greatest disadvantage of the NTA film is the fading. And there is a threshold
neutron energy.

Dosimetry concepts and dosimeters

Dosimetry concepts and dosimeters

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Dosimetry concepts and dosimeters