Radiation Dosimetric Principles introduction

1. RADIATION DOSIMETRY
PRINCIPLES
SABARI KUMAR P
M.Sc Radiation Physics

2. Outline
INTRODUCTION
GENERAL PROPERTIES OF DOSIMETERS
Types:
GAS FILLED DOSIMETRY
PHOTO EMULSION DOSIMETRY
CHEMICAL DOSIMETRY
SCINTILLATION DOSIMETRY
CALORIMETER DOSIMETRY
THERMO LUMINESCENCE DOSIMETRY
 SEMICONDUCTOR DOSIMETRY

3. INTRODUCTION
Radiation dosimetry deals with the measurement of the exposure or
absorbed dose resulting from the interaction of ionizing radiation with
matter
Device used for radiation detection named as Radiation Dosimeter
defined as capable of reading (R) that is measure of absorbed dose (D)
deposited in its sensitive Volume V by ionizing radiation
Ideally R  D but practically R  dD/dt
Dosimeter = Radiation Detector + Reader + Auxiliary equipment
 Basic required characteristics for Radiation dosimetry system are:
1.Accuracy, Precision
2.Linearity of signal with dose
3.Large dynamic range of measurement
4.Small dependence of signal on dose rate, beam quality direction
5.compact size and convenience of use
6.Adequate spatial resolution

4. General Properties of Radiation Dosimeters
Modes of Operations:
Many radiation detectors produce an electrical signal after each
interaction of radiation. This electrical signal processed by external
electronic circuit.
Based on the signal processed by external circuit, two main
classifications made such as
1. Pulse Mode : Signal from each
interaction process separately
Ex: Proportional counters , GM counters, Scintillators
2. Current Mode: Signals from each
interaction are averaged together
and forms net current signal
Ex: Ion chamber detectors, Scintillators
3. Mean Square Mode: Provides additional signal processing and
computes the time average of the squared amplitude of signal
Ex: Neutron Detectors in Reactor Instrumentation

5. Efficiency:
Efficiency of a detector is defined as measure of its ability to detect
radiation.
Efficiency = No. of particles detected/No. of particles emitted
= No. of particles detected x No. of particles reached
No. of Particles reached No. of particles emitted
Efficiency = Geometric Efficiency X Intrinsic Efficiency
 Geometric Efficiency: Ratio of reaching and emitting particles
 Geometric Relation b/w Source and Detector
 Intrinsic Efficiency: Ratio of Detecting and reaching particles
Energy, Z , Density and Thickness of detector

6. Pulse Height Spectra:
When operating a radiation detector
in pulse mode, each individual pulse
amplitude carries important
information regarding the charge
generated by that particular
radiation interaction in the detector.
Mainly two types such as
1. Differential Pulse Height Distribution
2. Integral Pulse Height Distribution
Energy Resolution:
The radiation detector should measures
The energy distribution of the incident
Radiation.

7. Counting curves & Plateaus:
 It is often desirable to establish an operating point that will provide
maximum stability over long periods of time.
In integral distribution spectra, the regions of minimum slope are
called Counting Plateaus.
This is the region where the system shows optimum operation with
less sensitivity.

8. Dead Time:
 The minimum amount of time that must separate two true events is
called Dead time.
Two models of dead time behavior counting systems are came:
Paralyzable: True events that occur during the dead period assumed
as the extension of dead period. Doesn’t consider as next event.
Nonparalyzable: True events that occur during the dead period
ignored.

9. GAS FILLED DOSIMETRY
This dosimetry mainly deals with production of Ion pairs, when
radiation passes through the detector
Based on response of the systems these are classified as
Ion chamber
Proportional counter
GM Counter
BASIC STRUCTURE AND RESPONSE
CURVE

10. At zero voltage  No electric field to attract ion pairs ,
no current flows
At low voltages  No sufficient electric field to collect all ion
pairs, so collected charge is less than originally
produced ion pairs. (Recombination Region)
At moderate voltages  Electric Field is sufficient to collect all ion pairs
ion collection saturates(Ion saturation Region)
At high voltages  Gas multiplication starts that each ion can
produce at least one secondary ion pair, and
is proportional to original ion pairs.
(Proportional Region)
At very high voltages  Nonlinear effects occur in proportional region
due positive ions cloud which alters electric
field significantly ( Limited Proportional Region)
At very very voltages  Electric field dominates positive ion cloud and
gas multiplication spreads through out the
detector. No reflections of incident radiation
properties ( Geiger Mueller Region)

11. Free Air Ion Chamber:
The basic design idea underlying in Free Air Ion chamber is to produce
perfect CPE in the defined collecting volume for a particular photon
beam and hence to measure directly exposure.
The walls of the chamber doesn’t play any role in its response.

12. If Q is the charge collected and ρ is the density of air, Then exposure
at point P (centre of the specified volume)
X(P) =Q/ M
Where M is the irradiated air mass , Which can derived as
M = ρ Ap L
Ap = cross section area of the beam at point P
L = Length of the collecting volume
==> X(P) =Q/ ρ Ap L
But it is difficult to measure X(P) accurately at point P. The law of
divergence obeyed by the photon beam can apply to overcome this issue
X(P) α 1/d2 and A(P) α d2
==> X(P) .Ap = constant
==> X(P).Ap = X(D).AD ==> Ap = [X(D).AD]/X(P)
Finally,  X(D) = Q/ ρ AD L
Here AD = Area of cross section of the Diaphragm at point D
X(D) = exposure at point D
This indicates that the point measurement becomes centre point of
Diaphragm of Chamber

13. From above equation one can conclude that Exposure is independent
of W
The defined volume M must be surrounded by equilibrium thickness
of air, to establish CPE at point P
Equilibrium Thickness is defined as the Max. thickness for providing
CPE is the range of the max. energy electron produced by the photons in
the buildup region.
As energy increases, the equilibrium thickness also increases.
This causes increase of Free Air Ion chamber size results non uniform
electric field between plates .
To avoid this situation, Pressured Air chambers introduced.
But main draw back in this chambers is the density of air, air
attenuation, photon scatter and reduction of ion collection
The pressurized air density is three times greater than normal air.
The Cavity Air chambers are introduced to over come all above
problems.

14. Cavity chambers consists of solid envelop surrounding a gas filled
cavity in which electric field establish to collect ions formed by exposure
In order for the cavity chamber to be equivalent to a free air chamber,
few considerations have to made such as:
1. The chamber wall should be air equivalent.
2. Cavity volume accurately know
3. Chamber wall thickness is sufficient to provide CPE
4. Minimum Stem Leakage

15. Generally, wall material is Graphite and central electrode is Aluminum
 Effective atomic number of Graphite < atomic number of Air
This results less ionization in air cavity compared to Free air chamber.
This effect is compensated by high Z central electrode, its dimensions
and geometrical placement in air cavity.
Calibration of Chamber:
 It is impractical to construct such a air cavity chamber like free air
chamber.
For this reasons, Air cavity chambers are always calibrate against a
free air chamber.
Practically, chamber produces some photon attenuation
If less wall thickness that required for CPE==> low response
If high wall thickness that required for CPE==>low response
due high attenuation.

16. The true Exposure is defined as the max. chamber response for zero
wall thickness.
To obtain true exposure with air cavity chamber, a calibration factor is
introduced which converts the measured exposure into true exposure in
free air.
Stem Leakage: The ionization produced anywhere rather than
sensitive volume of chamber.

17. PROPORTIONAL COUNTERS
One type of gas filled detectors always used in pulsed mode and rely
on the Gas Multiplication Phenomena
Gas multiplication is a consequence of increasing the electric field
within the gas to sufficiently high value.
Free electron mobility increases which causes additional ionization
and so on.
The gas multiplication process
becomes cascade and knows as
Townsend Avalanche.
The fractional increase in the
no of electrons per unit path length
is governed by:
dn/n = α dx
α – Townsend co-efficient for the gas

18. Gas Multiplication requires large values of the electric field.
For uniform multiplication for all ion pairs formed by the original
radiation interaction, gas multiplication region must be confined to a
very small volume compared with the total volume of the gas.
All primary ion pairs formed outside the region drifts to multiplying
region before multiplication
Finally, each electron undergoes the same multiplication process
regardless of its original position and the multiplication factor will be
same for all original ion pairs.
Gas is main criteria in Proportional counters such that they don’t
exhibit an appreciable electron attachment co-efficient.
Noble gases either pure or binary mixtures can be useful and chosen
for below 100 gas multiplication factor
90% Argon + 10% Methane  P – 10 gas generally used

19. The total charge generated by original ion pairs becomes
Q = M n0 e
Where Q – Total charge generated
n0 – original nu. Of ion pairs produced
e – electron charge
M – Gas Multiplication factor
M can be calculated by,
V – Applied Voltage
a – Anode Radius
b – Cathode Radius (Inner)
P – Gas pressure
K – Constant








 Kln
)a/bln(.a.P
V
ln
V
2ln
)abln(
V
Mln

20. GEIGER MUELLER COUNTERS
These are third general category to Gas Filled Detectors based on
ionization.
In G M tube, higher electric fields are created that enhance the
intensity of each avalanche
Under proper condition, one avalanche can itself trigger a second
avalanche in different position with in the tube which results self
propagating chain reaction.
 once this Geiger discharge reaches a certain size, all individual
avalanches come into play and ultimately terminate the chain reaction.
Each Geiger discharge is terminated after developing about the same
total charge, regardless of the number or original ion pairs created by
the incident radiation.
All pulses from G M tube are often same amplitude regardless of
original ion pairs so wont provide the properties of the incident
radiation. That’s why G M counters are simple counter of radiation
induced events.

21. In a Townsend avalanche, original electron excites the gas molecules.
This excited molecules results emission of photon with UV or Visible
light after de-excitation.
This photons are the key element in propagation of chain reaction
that make Geiger Discharge.
Normally Noble Gases are used to G M tubes. Additionally a second
component “Quenching Gas” also used
The multiple pulsing is potentially much severe in GM tube.
To prevent this excessive multiple pulses, Quenching Method is used.
Can possible by two ways : External Quenching
Internal Quenching

22. In External,  By reducing voltage after fixed time of each pulse
 choosing High Resistance value at outside the circuit
which causes delayed discharge
 In Internal  Adding secondary gas with primary gas. This helps that
absorbing the UV light photons
General Quenching Gases are: Ethyl Alcohol & format
Dead Time:
The Build-up of the positive ion space charge that terminates the
Geiger discharge ensures that a considerable amount of time must pass
before a second Geiger discharge can be generated in the tube.
After first Geiger discharge, the electric field has been reduced below
the critical point by the positive space charge.
If another ionization event occur under these conditions, a second
pulse will not be observed because gas multiplication is prevented.
During the time the tube is therefore “Dead” and any radiation
interactions that occur in the tube during this time will be lost.

23. The dead time of GM tube is defined as the period between the initial
pulse and the time at which a second Geiger discharge, regardless of its
size, can be developed
Mostly, this time is of order 50-100μ Sec.
Usually, the dead time is used to describe the combined behavior of
the detector-counting system.
The recovery is the time interval required for the tube to return to its
original state and become capable of producing a second pulse of full
amplitude.

24. FILM DOSIMETRY
This dosimetry is specially use Photo emulsions for Qualitative
measurements of ionizing radiation beams.
The photo emulsions such as Silver Bromide (AgBr) dispersed in
Gelatin layer on cellulose base material
some of the grains will be "sensitized" through interaction of the
radiation with electrons of the silver halide molecules.
The process forms semi-stable clusters consisting of a few neutral
atoms of silver at the surface of some of the grains.
 The sensitized grains remain in this state indefinitely, thereby storing
a latent image of the track of the ionizing particle through the emulsion.
In the subsequent development process, the entire sensitized grain is
converted to metallic silver, that the developed grain is visible.
The emulsion is fixed by dissolving away the undeveloped silver
halide grains, and a final washing step removes the processing solutions
from the developed emulsion.

25.  The Radiation effect is measured in terms of Light opacity of the film
Opacity = (I0/I) = eaN (From Beer’s Law)
where, I – Light intensity in exposed film
I0 – Light intensity in unexposed film
a - Cross sectional area for developed AgBr grain
N – No. of grains developed per cm2
Optical Density is defined as log of Opacity
O.D = log10 (I0/I) = log10(eaN)
O.D=0.4343 aN -------------(1)
If NAgBr – initial no. of undeveloped grains/cm2 and  - Energy Fluence
N/NAgBr = a  -------------(2)
From (1) & (2)  O.D = 0.4343 a2NAgBr 
From this equation  O.D  Emulsion thickness
 O.D  Square of Grain area
 O.D  Fluence
Finally one can conclude that  O.D  Dose

26. Characteristic Film Curve: This curve describes the Film performance in
the sense of Speed, Contrast, Latitude and Resolution of the film which
is commonly known as H-D curve (Hurter – Driffield)
This curve plots b/w O.D and log10(Dose)
Ideally Dose and O.D
Should be in linear but not
Possible in all the case.
Regions of H-D curve are:
A – Fog : Zero Exposures
B – Toe : Under Exposures
C – Linear: Intermediate Exposures
D – Shoulder : Over Exposure
E – Solarization : Film Properties
change

27.  Nuclear Emulsion: heavy thickness of emulsion for individual particle
tracks
 Radio chromic Emulsion: Grain less and dyed transparent emulsion
polymerized upon radiation exposure
Merits:
1. Ideal dosimetry for determining dose distribution for dynamic
beams and for studying the combination of stationary beams
2. High Spatial resolution, wide accessibility and flexibility to use
3. High reproducibility and reliability
4. Rapid and accurate analysis
Demerits:
1. Variation of response as a function of energy
2. High sensitivity to processing conditions and hostile environment
3. 3D dose distribution analysis is not possible
4. Blindness to low energy neutrons
5. Wet chemical processing requires

28. CHEMICAL DOSIMETRY
For complex dose distributions, sensitive gel dosimetry introduced
The dose is determined from Quantitative chemical change in an
appropriate medium.
When ionizing radiation absorbed by substance, they may change is
chemical properties and which can determine to measure radiation
Basic Principle:
When radiation interacts with water, primary products such as
Molecular products such as H2, H2O2 and Radicals produced in (10-10 sec)
and distribute heterogeneously close to charge particle tracks.
After 10-6 sec, the spatial distribution tends to homogeneous with
their chemical interactions with solutes.
This may change the properties of solution which is measured and
called as G-value ( Radiation Chemical Yield )
G(X) is defined as no. of chemical entities produced/destroyed/
changed per 100eV of radiation energy units: mol/J

29. G(x) = n(x)/E
Here n(x) – No. of entities changed/produced/destroyed
E = Energy imparted
Absorbed Dose calculation:
The chemical titration of irradiated and un-irradiated sample gives
the M of ions, whence the dose is obtained by,
Absorption spectroscopy is more convenient and sensitive method,
which can give values of transmitted intensities of irradiated and un-
irradiated samples and hence OD change
(OD) = L M
==> M = (OD)/ L M – Change in Molar concentration
From above equation, (OD) – OD change
 - Molar Extinction Coefficient
ρ – Density of Solution
L – Path Length of cell)x(LG
)OD(
D



)x(G
M
D




30. Cavity Theory considerations:
 It is impractical to make such a vessels to behave like Bragg – Gray
cavity.
If the diameter of vessels larger compared to range of charge
particles, the wall effects become negligible and CPE /TCPE can achieve
itself by solution vessels
Alternatively, the use of Polystyrene or Lucite provides close atomic
number which matches with water
 Solution must be air Saturated
Stirring or bubbling of solution need to avoid Local Oxygen depletion
in inhomogeneous radiation
Material μen/ρ (cm2/g) dE/ρ dX (MeV cm2/g) Ρ (g/cm3)
water 0.0296 2.355 1
Polystyrene 0.0288 2.305 1.04
Lucite 0.0288 2.292 1.18

31. Many dosimeters found such as:
1.Ferrous Sulphate ( 4 – 400 Gy)
2. Ceric Sulphate ( 400–4x106 Gy)
3. Chlorinated Solutions ( 1K Gy)
4. Poly Acrylamide gels ( 2-30 Gy)
Basically the Ferrous sulphate
consists of 1m mol/L of Ferrous
Sulphate , 1m mol/L of NaCl ,
0.4m mol/L of Sulphuric Acid
and 96% water.
When irradiated, Fe2+ ions oxidize to Fe3+ and These Fe3+ ions
production is directly proportional to absorbed dose.
Fe2+ + OH -----------> Fe3+ +OH-
Fe2+ + HO2 ---------> Fe3+ + HO2
-
Fe2+ + H2O2 ----------> Fe3+ + OH + OH
-

32. Merits:
These solutions have effective Z and μen/ρ that are closed to water
 Can be irradiated in a container similar shape and volume of object
to be studied
Absolute dosimetry is possible
Wide range of measurements such as 10 to 1010 rad possible
Linear Response versus Dose found in some chemicals
Can measure the energy fluence of non-penetrating beams
Demerits:
Lack of storage stability
Temperature dependence and readout procedure dependence
Diffusion of ions through the gel leads to the loss of Penumbra effects
in matter which leads to immediate measurements require
Some what dose rate and LET dependence

33. SCINTILLATION DOSIMETRY
Scintillators emit visible light or UV light when irradiates
When irradiation happens, electrons in Scintillator raise to excited
state and fall back to ground state with emission of Visible or UV light
 Emitted Light is proportional to absorbed dose in the scintillation
material
Properties of Scintillation Detectors:
Conversion Efficiency should be high
Decay times of excited states should be short
Material should be transparent, rugged and unaffected by moisture
Attenuation Coefficient should be large, so that conversion efficiency
 Different types of Scintillation Detectors are available such as
1. Organic Scintillators
2. Inorganic Scintillators
3. Gas Scintillators

34. Scintillation Mechanism in Organics:
 Luminescence occurs in two different types in Scintillators :
Fluorescence – Prompt Emission of Light after excitation
Phosphorescence – Delayed emission of light after excitation
Fluorescence process due to transition between S1 to S0 states
Phosphorescence process due to transition between T1 to S0 states

35.  If τ is the fluorescence decay time for S10 level, then
Intensity (I) = I0 e-t/τ
Only small part of energy imparted is converted into light rest
dissipated in the form of Heat.
The scintillation efficiency is defined as the fraction of all incident
particle energy which is converted into light.
High efficiency is needed but it may
causes Quenching ( Radiation less
de-excitation processes)
Scintillation efficiency depends
on LET of charged particle
delivering energy.
High LET particles causes dense
ionization tracks which create damaged
molecules and hence Decrease in light output
 LET consideration must need while measuring
Different Radiation types at a time

36. While Scintillator operates in Current Mode, Fluorescence and
Phosphorescence can’t be separate
But in Pulse Mode, by using external electronic circuit, it can possible
to remove gradient increases and decreases from fluorescence
Mainly binary organic Scintillators are widely used
Small efficient Scintillator is dissolved in Bulky solvent
Sometimes, a third component called wave shifter adds, which
absorbs primary light and reemits at longer wavelength
This is for closer matching the spectral sensitivity of PM tube
The dependence of light yield of organic scintillator on the type of
particle is specially termed as Me Vee (MeV electron equivalent)
1Me Vee = 1Mev Fast electrons generated Light
If dL/dx – Fluorescence energy emitted per unit path length
dE/dx – Specific Energy lost of the charged particle
S – Scintillation efficiencydx
dE
S
dx
dL


37. By taking account of Quenching
Here KB – constant used to fit the expiremental value of scintillation
This is called Birk’s formula
From the Spectra, one can observe that little overlap b/w optical
absorption and emission spectra and little self absorption of the
fluorescence (Stokes Shift)
)
dx
dE(KB1
)
dx
dE(
.S
dx
dL



38. Scintillation Mechanism in Inorganic Crystals:
To enhance the probability of visible of light, small amount of
Activator add to the inorganic scintillator
Due to this Activator, formation of energy states in forbidden gap
Finally rise in probability of visible photon
Three possible outcomes in this process
1. Fluorescence Emission: Electron Transition from Activator excited
state to ground state
2. Phosphorescence Emission: Electron moves to activator ground state
then carry forwards to activator excited state and returns to ground
state
3. Quenching :Radiation less transition

39. Merits:
1. Plastic Scintillators are almost water equivalent in terms of electron
density and atomic composition
2. Very fast decay times (≈ 10-9 sec) makes excellent choice for
coincidence measurements with good time resolution
3. What ever shape, volume and size one wish can possible
4. Incase of plastic detectors, no directional dependence and no need
of ambient temperature and pressure corrections
Demerits:
1. Radiation damage due to prolonged periods of usage ( evidenced as
reduction in transparency caused by creation of color centre in
Scintillators)
2. Many reasons for damage such as dose rate, type of particles, energy
of particle, presence/absence of O2 or impurities.

40. CALORIMETRY
Calorimetry is a good approach for establishing absorbed dose
standards
It measures heat produced as a result of radiation absorption
Choice of Calorimeter Material:
Should not be any heat defect in material
Should be good conductor (No thermal gradient)
Easy conversion of Dmaterial to Dwater
 Water calorimeters are ideal choice but due to stability, convection
currents, heat defect, Graphite calorimeters became popular
Principle:
A small conductor disc can function as a calorimeter, if one can
accurately measures the temperature rise of this body as a result of
radiation absorption.

41. The equation of Calorimetry is Q= mS Tc
Q= C Tc
==> dQ = C dTc
==> dQ/dt = C. dTc/dt ---------->(1)
Where Tc = core temperature
C = mS is the thermal capacity of the material
This is equation satisfies only if core retains all the heat produced
But it is difficult due the ambient temperature can change by several
degrees since the core temperature rise in milli degrees
Heat flow  Temperature difference
If Ta – Ambient temperature
Tc > Ta ==> Heat flow from core to ambient
Tc<Ta ==> Heat flow from ambient to core
If dQ’ is the heat lost by core to ambient
dQ’/dt = -K (Tc – Ta) -------------> (2)

42. The Calorimetry equation becomes
Heat In = Heat retained + Heat lost
==>
Here P = dQ/dt
This is the fundamental equation for Calorimetry.
In other words,
Temperature increase T = E(1-δ)/hm
T = D (1-δ)/h
Where E = Absorbed Energy (J)
δ = Thermal Defect
h = Thermal capacity(j/Kg 0C)
m = Mass of the core (Kg)
D = Average absorbed dose (Gy)
)TT(K
dt
dT
C
dt
dQ
ac
c

 )TT(KP
C
1
dt
dT
ac
c


43. With the help of calorimeter one can measure
1. Absorbed dose in reference medium
2. Energy fluence in radiation beam
3. Power output of radio active source

44. Merits:
1. Inherently Dose rate independent
2. No LET dependence
3. Relatively stable against radiation damage
4. Any material can use as core in Calorimetry
Demerits:
1. Apparatus bulky and difficult to carry and setup ( Thermal insulation,
instrumentation for thermal control and measurement)
2. Endothermic and Exothermic reactions cause variation in integral
dose and energy occurs
3. Additional Uncertainties while conversion Dcore to Dwater

45. THERMO LUMINESCENCE DOSIMETRY
TLD is based on the ability of certain imperfect crystals to absorb and
store the energy of ionizing radiation upon heating in re-emitted in the
form of light.
The light is correlated to absorbed dose
Mechanism:

46. The simple first order kinetics for the escape of trapped charge
carriers at temperature (T) is
P = 1/τ = α e-E/KT
Where P – Probability of escaper per unit time(sec -1)
τ – Mean Life time in trap
α – Frequency Factor
E – Energy depth of the trap (eV)
K – Boltz man’s constant (8.62 x 10-5 eV/K)
From the relation one can observe that
T increases ==> P increases ==> τ decreases
As temperature increases, rate of escape of trapped electrons
gradually increases with temperature and reaches to one maximum
point (Tm) and follows a decrease as the supply of trapped electrons is
gradually exhausted.
The rate of escape of trapped electrons is directly proportional to
light emission.

47. The relation between light emission of TL and Temperature gives a
peaked curve which is called
Glow Peak.
The Process of Light emission
decreases with temperature
increases is called Thermal
Quenching.
Affecting Parameters:
1. Trap Stability: If traps are not stable at room temperature, changes
in radiation sensitivity and glow curve occurs
2. Trap Leakage : Inability of traps to hold charge carriers at ambient
temperature after irradiation is called Trap Leakage.
3. Intrinsic Efficiency : Only small part of energy deposited as absorbed
dose in TLD rest goes into heat production.
Intrinsic Efficiency = TL Light emission per unit mass/Absorbed Dose

48. Merits:
Wide useful dose range (few milli rad’s to 103 rad’s)
Dose rate independence (0 – 1011 rad/sec)
Small size, passive energy storage
Commercial availability
Reusability
Readout convenience (< 30 Sec for reading)
Economy, Accuracy and Precision
Availability with different sensitive's
Demerits:
Lack of uniformity : Individual dosimeter/batch calibration requires
Storage instability : Sensitivity may vary with time before irradiation
Fading : can’t retain 100% after Annealing
Light sensitivity : Leakage trap
Reader instability : consistency differs over long time periods
Spurious effect : Environmental effects

49. SEMI CONDUCTOR DOSIMETRY
A Silicon diode dosimeter is a p-n junction diode.
When a charged particle passes through a semiconductor, the overall
significant effect is the production of many electron-hole pairs along the
track of the particle.
The production may be direct or indirect.
If an electric field is presented throughout the active volume, both
charge carriers drift in opposite directions.
This motion of either electrons or holes constitutes an electrical
current which is proportional to energy deposited by charged particle.

50. The dominant advantage of semiconductor detectors lies in the
smallness of the ionization energy(for Si and Ge almost 3eV) when
compared to gas filled detectors (33.95eV)
Faithful measure of the energy deposited by the particle
Response time is very less when compared to Gas filled detectors.
In this dosimetry, there is an advantage in operating without any
external bias.
As the bias voltage is reduced to zero, the DC leakage current
decreases more rapidly than the radiation induced current.
 The density of Si is 2.3 g/cm3 that is 1800 times of air.
This results that production about 18,000 times as much as charge as
an ion chamber of the same volume, in the same X-ray field.
DEMERITS:
The sensitivity of diodes depends on their radiation history and hence
the calibration has to be repeated periodically.
Diodes show variation in dose response with temperature
Several correction factors need to apply for dose calculations.

51. Ion chambers Semiconductors TLDs Film
Advantages Well understood,
accurate, variety of
forms available
Small, robust Small, no cables
required
Two dimensional,
ease of use
Disadvantages Large, high voltage
required
Temperature
dependence
Delayed readout,
complex handling
Not tissue
equivalent, not
very reproducible
Common use Reference
dosimetry, beam
scanning
Beam scanning, in
vivo dosimetry
Dose verification,
in vivo dosimetry
QA, assessment of
dose distributions
Comment Most common and
important
dosimetric
technique
New developments
(MOSFETs) may
increase utility
Also used for
dosimetric
intercomparisons
(audits)
New developments
(radiochromic
film) may increase
utility
SUMMARY

52. Diode Detectors

53. Detector Arrays

54. Thank you