6th Training Course on Radiation Protection for Radiation Workers and RCOs of BAEC, Medical Facilities & Industries Training Institute, AERE, Savar, BAEC 24 - 29 October 2021

1. Dr. Md. Mosharraf Hossain BHUIYAN
CSO, HPRWMU, INST,AERE, Savar
Calibration and limitation of Instruments
6thTraining Course on Radiation Protection for Radiation Workers and RCOs of
BAEC, Medical Facilities & Industries
Training Institute, AERE, Savar, BAEC

2. CONTENTS
 Radiation Detection Instrument & Limitations
 Calibration of Radiation Measuring Equipment
 Basic Concept of Radiation
Training Course on Radiation Protection for Radiation Workers and
RCOs of BAEC, Medical Facilities & Industries, 24 – 29 October 2021

3. Basic Concept of Radiation

4. Energy in the form of particles or waves
Radiation
Released in the form of electromagnetic waves or particles
Emitted by the nucleus of an atom or orbital electron or
Coulomb Field of the nucleus

5.  Radiation is a fact of life. Generally, it is an emission and
transmission of energy through space in the form of
charged and uncharged particles as well as electromagnetic
radiation.
Types of Radiation
(i) Ionizing Concern to Health Physics professionals
(i) Non-ionizing Industrial safety concern
 The origin of the radiation:
 Natural radiation
 Artificial (man-made) radiation
5
Radiation

6. Major Types of Ionizing Radiation
 Alpha
 Beta
 Gamma
 Neutron
Classification of Radiation
Ionizing Radiation: Radiation is energy transmitted as particles
or waves. Ionizing radiation has sufficient energy to dislodge
orbital electrons, thereby producing ions.
Examples: alpha, beta, gamma, neutron, and x-rays
 Non-Ionizing Radiation: Radiation that does not have sufficient
energy to dislodge orbital electrons.
Examples: visible light, infra-red , micro-waves, radio-waves

7. Radiation Protection
Importance of radiation protection: Generally, any exposure to
ionizing radiation carries a risk of causing cancer.
Although, this risk is extremely small. However, as the risk is
proportional to the level of exposure, it's important to ensure
exposures are kept as low as possible. This is the ALARA principle
(as low as reasonably achievable).
Radiation protection instruments: All instruments used for
radiation protection purposes should be calibrated regularly
(usually every year) by a qualified expert at SSDL.
Radiation protection regulation: As per BAER Act 2012, the
authorization holder shall be responsible for radiation protection.
The conditions regarding the radiation protection related to nuclear
installation, radiation facility, radioactive material, radioactive waste
and other facilities shall be prescribed by regulations.

8. Radiation Measuring Detectors
Common Types of Radiation Detectors:
 Alpha detector  Alpha Radiation
 Beta detector  Beta Radiation
 Gamma detector  Gamma Radiation
 Neutron detector  Neutron Radiation

9. Calibration of Radiation Measuring
Equipment

10. Calibration Requirement
 The instrument manufacturer sometimes cannot calibrate the
instrument over the complete dose equivalent range with a
reference radiation quality.
 Periodic calibration and standardization of radiation protection
survey instruments need to be done to insure correct/valid
radiation readings.
 Radiation monitoring instruments used for quantitative radiation
measurements are required by regulation, and need to be
calibrated at intervals not to exceed 12 months.
 Survey instruments that have never been calibrated or are out-of-
calibration cannot be used.
 It is also a regulatory requirement for radiation workers to use only
operable and pre-calibrated survey instruments in their work with
radioactive materials.

11. Purpose of Calibration
The primary objectives of calibration are:
 To ensure that an instrument is working properly and
hence will be suitable for its intended monitoring purpose.
 To determine (under a controlled set of standard
conditions) the indication of an instrument response as a
function of the measurand value (the quantity intended to
be measured). This should be done over the complete
range of indication of the instrument.
 To adjust the instrument response (if possible) in
calibration, so that the overall measurement accuracy of
the instrument is optimized.

12. Traceability of Reference Instrument
 Each reference instrument used for calibration purposes has itself
been calibrated against a primary reference instrument of higher
quality, i.e., up to the quality level at which the instrument is
accepted for national standard.
 The frequency of such calibration (depending on type, quality,
stability, usage and environment), needs to be such as to establish
a reasonable confidence that instrument’s value will not move
outside the limits of its specification between successive
calibrations.
 The calibration of any instrument against a reference instrument is
valid in exact terms only at the time of calibration.
The reference instrument Measures the radiation characteristics
Used for instrument calibration Need to be traceable to an
appropriate international / national standard laboratory
This means

13. Calibration Essentials
Calibration is a required process to verify the relationship
between the observed response of the calibrating instrument
and the measured standard value of the operational quantity
under well-defined reference conditions
Calibration ensures that radiation protection/measuring
instruments are working properly thus suitable for their
intended purpose
The radiation monitoring instruments have to be calibrated once
in a year, as per National Regulatory Act (e.g., BAER Act 2012)
and Rule (e.g., NSRCD rule 97), to ensure good operational
performance and high reliability
NEED TO MEASURE THE OPERATIONAL QUANTITIES
 Ambient DE: H*(d)
 Directional DE: H'(d,Ω)
 Personal DE: Hp(d)

14. PROTECTION QUANTITIES
 Organ absorbed dose: DT
 Equivalent dose: HT
 Effective dose: E
OPERATIONAL QUANTITIES
 Ambient DE: H*(d)
 Directional DE: H'(d,Ω)
 Personal DE: Hp(d)
Calculated using Q(L) and
simple phantoms
(sphere or slab)
Calculated using wR, wT,
and anthropomorphic
phantoms
Compared by
measurements and
calculations
Monitored quantities:
Instrument response
Related by Calibration
and Calculation
Dose Limits
PHYSICAL QUANTITIES
 Fluence: F
 Kerma: K
 Absorbed dose: D
Radiation Quantities

15. Operational Quantities for Calibration
For the purpose of calibration or standard measurement the
following operational quantities are need to be estimated:
 Ambient dose equivalent H*(d)
 Directional dose equivalent H(d, )
 Personal dose equivalent Hp(d)
External radiation Limiting quantity
Operational quantity for
Area
monitoring
Individual
monitoring
Strongly
penetrating radiation
Effective dose H* (10) Hp (10)
Weakly
penetrating radiation
Skin dose H (0.07, ) Hp (0.07)
Dose to the lens
of the eye
H (3, ) Hp (3)
Summary of Operational Quantities

16. Operational Quantities for Calibration
Ambient Dose Equivalent H*(10)
 The ambient dose equivalent H*(10), at a point, is the dose
equivalent that would be produced by the aligned field, in the
ICRU sphere at a depth 10 mm.
 Ambient dose equivalent is important for area monitoring of
high penetrating radiation.
 The definition of H*(10) indicates that any instrument
designed for measurement must require backscattering
factor.
The conversion coefficient (Gy/Sv) for H*(10): Conversion of
Air Kerma (Gy) provides the calibration standard in-terms of
ambient dose equivalent (Sv) for dosimeters.

17.  The directional dose equivalent H (d, Ω) is the dose
equivalent, produced at a point by the aligned and expanded
radiation field in the ICRU sphere at a depth 0.07 mm on a
radius in a specified direction Ω.
 Directional dose equivalent is important for area monitoring
of low penetrating radiation.
 Directional dose equivalent strongly depends on Ω i.e. on how
ICRU sphere is oriented in the expanded field, and has a
particular use in the assessment of dose to the skin or eye
lens
Directional Dose Equivalent H(0.07)
Operational Quantities for Calibration

18. Personal Dose Equivalent Hp(d)
 For calibration or calculating the dose equivalent to an
individual, the values of Hp(10) and Hp (0.07) are obtained
in a standard phantom.
 If the phantom is ICRU sphere the relevant quantities are
H’(10) for Hp(10) and H’(0.07) for Hp (0.07)
 The operational quantity for the individual monitoring is the
personal dose equivalent Hp(d).
 The personal dose equivalent is the dose equivalent in soft
tissue at an appropriate depth (d) below a specified point on
the body. Hp(d) can be measured with a detector, which is
worn at the surface of the body and the covered with an
appropriate thickness of tissue equivalent material.
Operational Quantities for Calibration

19.  The instrument that are commonly used for area
monitoring are calibrated free in air.
 The survey meters are placed with its reference point
coinciding with the point of test.
 Most of the survey meters and area monitors are
calibrated in terms of ambient dose equivalent H* (10).
 The secondary standard chamber is used to determine
the reference ambient dose equivalent based on the air
kerma rate at the point of test.
Calibration procedures for -Survey meters
Recommended Processes:

20. Calibration arrangement: Schematic diagram
Calibration arrangement of the radiation measuring instrument
with reference beams
Source
Calibrator
Calibration stand rail
Central beam axis
Laser beam alignment
Source to chamber
distance
188
mm
85 cm
Survey meter under calibration
f

21. Computer Operated GAMMA/ X-RAY Calibrator System
Secondary Standard Dosimetry Laboratory (SSDL)
OB-85 calibrator :
Co-60 Source
OB-34 calibrator :
Cs-137 and Co-60 Source
X-ray
machine
G-10 Calibrator
Reference instrument
for radiation field
standardization

22. Calibration Principles
 Physical quantities, such as air kerma or exposure rate or
particle fluence, of which usually primary standard exists is
determined by the reference instruments at a reference
point with reference radiation quality.
 Conversion coefficient is applied to relate the physical
quantities to the radiation protection quantity.
 A device under calibration is placed at the reference point in
order to determine the response in terms of the protection
quantity, i.e., ambient, personal or directional dose equivalent.
Principle of Calibration
Calibration is a prerequisite to determine the correction of
an instrument response relative to a series of known
standard values over the range of the instrument

23. Determination of Calibration Factor
Calibration Factor (N1) of an instrument is directly related to
the dose equivalent quantity H *(10) as:
NI = H*(10) /MI  NI = NR .h.MR /MI Here, H*(10) = NR .h.MR
 NI : calibration factor of the instrument under calibration (under
reference condition)
 MI : measured value of the instrument under calibration, corrected for
reference condition i.e., indication multiplied with the applicable
correction factors (e.g. for differences in air density)
 NR : calibration factor of the reference instrument (under reference
condition)
 MR : measured value of the reference instrument corrected for reference
condition
 h : conversion coefficient for the reference instrument to the dose
equivalent quantity associated with instrument under calibration.
 Dose equivalent quantity H needs to be estimated
using reference instrument under reference condition

24. Dose Equivalent Quantity H*(d)
 To characterize the radiation field of gamma calibrators
(e. g., 137Cs and 60Co), H*(d) needs to be estimated using
the standard procedure recommended by IAEA in its
publication SRS-16, or ISO-4037
 Consequently, various survey meters such as GM, NaI (Tl),
ionization chambers are calibrated in terms of the
standardized radiation field with H*(d) to quantify the
radiation level for radiation safety.
 The radiation field needs to be characterized in terms of
ambient dose equivalent rate, using the standard
measuring instrument, for the calibration of a -ray survey
meter
Application of H*(d): Radiation Field Characterization

25. Determination of Physical Quantity for
characterization of radiation field:
Air Kerma or exposure rate
Steps-1: Calculation of correction factors: Ktp, Kpol, Ks
Steps-2: Calculation of Air Kerma rate, Ka ( Gy/h)
Steps-3: Calculation of Ambient Dose Equivalent, H*(10) (Sv/h)
Steps-4: Calibration Factor Determination in Terms of H*(10)
Reference Dose Measurement :
Standard Gamma Calibration Field

26. (i) Air density Correction factors (𝑲𝒕𝒑) is obtained by the following equation:
𝑲𝒕𝒑 =
𝟐𝟕𝟑.𝟏𝟓+𝑻
𝟐𝟗𝟑.𝟏𝟓
×
𝟏𝟎𝟏.𝟑𝟐𝟓
𝑷
𝐾𝑡𝑝 : Air-Density correction factor; 𝑃 and 𝑇 : Temperature and pressure at
laboratory conditions; 𝑃0 𝑎𝑛𝑑 𝑇0: Standard pressure (101.325 kPa) and
temperature (200C).
Reference Dose Measurement of Standard
Gamma-Ray Calibration Field
Step 1
(ii) Polarity correction factor is calculated as: 𝑲𝒑𝒐𝒍 =
𝑴+ − 𝑴−
𝟐𝑴
(iii) Recombination correction factor is calculated as: 𝑲𝒔 =
𝑽𝟏
𝑽𝟐
𝟐
− 𝟏
𝑽𝟏
𝑽𝟐
𝟐
−
𝑴𝟏
𝑴𝟐
𝟐
M1 and M2 are the reading at two voltages V1 and V2
V1 is the normally used voltage, and V2 is the reduced voltage by a factor of 3
Determination of Correction factors

27. Calculation of the Air-Kerma rate 𝑲𝒂𝒊𝒓 : at each position from the
indicated value of charge rate 𝑀𝑈(nC), multiplied with chamber
calibration factor (𝑁𝑘 𝑆𝑆𝐷𝐿)in (Gy/nC), Air density Correction factor
(𝐾𝑡𝑝), Polarity correction factor (𝐾𝑝𝑜𝑙) and Recombination correction
factor (𝐾𝑠), using the following equation:
𝑲𝒂𝒊𝒓= 𝑴𝑼 × 𝑵𝒌 𝑺𝑺𝑫𝑳 × 𝑲𝒕𝒑 × 𝑲𝒑𝒐𝒍 × 𝑲𝒔
𝑀𝑈: Charge measured by the standard electrometer
𝑁𝑘 𝑆𝑆𝐷𝐿: Coversiton factor for charge to Air Kerma (Gy/nC)
Reference Dose Measurement of Gamma-Ray
Standard Calibration Field
Step 2

28. Calculation of the ambient dose equivalent rate 𝐻∗(10) by
multiplying the conversion coefficient (𝑆𝑣/𝐺𝑦) with 𝐾𝑎𝑖𝑟 as shown in
the following equation:
𝑯∗ 𝟏𝟎
𝝁𝑺𝒗
𝒉𝒓
= 𝑲𝒂𝒊𝒓 × 𝑪𝑭 𝑺𝒗/𝑮𝒚
𝐻∗ 10 ∶ Ambient dose equivalent in μSv/hr
𝐶𝐹𝑆𝑣/𝐺𝑦: Conversion Factor for Air Kerma to𝐻∗ 10
Reference Dose Measurement of Gamma-Ray
Standard Calibration Field
Step 3

29. Determination of Calibration Factor
Determination of Calibration Factor of an Instrument:
𝑁𝐼 =
𝐻∗
(10)
𝑀𝐼
Where
 NI is the calibration factor of the instrument under calibration
(under reference condition).
 H*(10) is the measured ambient dose equivalent of the
calibration radiation quality.
 MI is the measured value of the instrument under reference
condition.
Step 4
Calibration Factor:

30. Gamma Radiation Qualities for Calibration
Gamma
irradiator
Nuclide Activity range No. of
source
Air kerma rate
Covered range
Source
distance [m]
OB-34 137Cs 7.4 MBq -7.4 GBq 4 1698 – 4.19 Gy/h 0.5 – 1.0
60Co 3.7 MBq-370
MBq
3 56.0 – 1.05 Gy/h 0.5 – 1.0
OB-85 137Cs 740 GBq 1 38.14 – 4.05 mGy/h 0.5 – 3.0
60Co 37 GBq 1 1.54 mGy/h – 165
Gy/h
0.5 – 3.0
OB-2 137Cs 74 GBq 1 0.22 – 14.56 mGy/h 0.5 – 4.0
241Am 11.1 GBq 1 5.08 – 119.92 Gy/h 0.4 – 2.0
GC- 60 137Cs 7.4 MBq -7.4 GBq 4 1698 – 4.19 Gy/h 0.5 – 1.0
60Co 3.7 MBq - 370
MBq
3 56.0 – 1.05 Gy/h 0.5 – 1.0
G -10 137Cs 740 GBq 1 47.17 – 4.05 mGy/h 0.5 - 4.5
60Co 38 GBq 1 3.8 mGy/h - 759
Gy/h
0.5 - 4.5
OB 26 241Am 11.1 GBq 1 5.67 – 123.98 Gy/h 0.4 – 2.0

31. X-ray Radiation Beam Qualities for Calibration
Beam
code
Tube
voltage
(kV)
1st HVL in mm of Cu HC Effective
photon
energy
(keV)
Air kerma
rate
( mGy/h)
ISO 4037
reference
Experimental
N40 40 0.84 0.88 0.957 32.21 37.12
N60 60 0.24 0.25 0.961 46.85 79.94
N80 80 0.58 0.62 1.016 65.49 14.28
N100 100 1.11 1.18 1.044 84.66 7.08
N120 120 1.71 1.775 0.947 102.00 10.45
N150 150 2.36 2.31 0.843 116.61 58.14
N200 200 3.99 4.33 0.947 176.544 22.20

32. Beam
Quality
Effective
Energy in
keV
HVL in
mm
Conversion Coefficients for H*(10) ( Sv/Gy)
Cu ISO BCRU NPL PTB NRPB AERE NAMAS
N40 32.21 0.088 1.192 1.194 1.134 1.180 1.164 1.145 1.148
N60 46.85 0.250 1.615 1.618 1.547 1.579 1.579 1.528 1.558
N80 65.49 0.620 1.741 1.748 1.733 1.743 1.742 1.743 1.732
N100 84.66 1.180 1.713 1.704 1.706 1.706 1.705 1.706 1.706
N120 102.00 1.775 1.647 1.637 1.635 1.646 1.645 1.647 1.635
N150 116.61 2.305 1.599 1.585 1.587 1.597 1.697 1.598 1.588
N200 176.544 4.330 1.449 1.437 1.431 1.442 1.442 1.420 1.423
Calculated conversion coefficient for
ambient dose equivalent H*(10)

33. Beam
Quality
Effective
Energy in
keV
HVL in
mm
Conversion Coefficients for H (0.07) ( Sv/Gy)
Cu ISO BCRU NPL PTB AERE NAMAS
N40 32.21 0.088 1.263 1.266 1.238 1.258 1.220 1.243
N60 46.85 0.250 1.497 1.499 1.465 1.478 1.456 1.470
N80 65.49 0.620 1.600 1.609 1.602 1.592 1.582 1.602
N100 84.66 1.180 1.603 1.596 1.597 1.588 1.578 1.598
N120 102.00 1.775 1.546 1.547 1.546 1.547 1.536 1.545
N150 116.61 2.305 1.507 1.502 1.506 1.506 1.495 1.496
N200 176.544 4.330 1.369 1.371 1.369 1.377 1.366 1.369
Calculated conversion coefficient for
directional dose equivalent H(0.07)

34. Beam
Quality
Effective
Energy in
keV
HVL in
mm Cu
Conversion Coefficients for
Hp(10) ( Sv/Gy)
Conversion Coefficients for
Hp(0.07) ( Sv/Gy)
ICRU
Tissue
Slab
PMMA
Slab
ISO water
slab
phantom
ICRU
Tissue
Slab
PMMA
Slab
ISO water
N40 32.21 0.088 1.139 1.0551 1.1388 1.253 1.1607 1.2528
N60 46.85 0.250 1.623 1.5131 1.6511 1.531 1.4273 1.5575
N80 65.49 0.620 1.885 1.7925 1.9315 1.723 1.6384 1.7655
N100 84.66 1.180 1.877 1.8239 1.9217 1.718 1.6694 1.7589
N120 102.00 1.775 1.805 1.7651 1.8473 1.665 1.6282 1.7040
N150 116.61 2.305 1.740 1.7085 1.7820 1.617 1.5877 1.6560
N200 176.544 4.330 1.542 1.5323 1.5739 1.469 1.4598 1.4994
Calculated conversion coefficient for
personal dose equivalent Hp(10) and Hp(0.07)

35. Calibration Procedures of Personal Dosimeter
 Calibration of personal dosimeters as well as measurements of their
response as a function of energy and the direction of incidence should
be carried out on the ISO water phantom of size 30cm  30cm  15cm
(water filled slab made with PMMA).
 A set of conversion factor recommended by ICRU can be applied to
convert air kerma to personal dose equivalent.
 Prior to calibrate the personal dosimeters, the standard true value of
the personal dose equivalent needs to be estimated on an ICRU tissue
slab phantom.
Personal Dosimeter
 In calibration, dose rate and calibration time must be controlled in
order to achieve the dose equivalent value of interest.

36. Calibration Procedures of Personal Dosimeter
During calibration, the personal dosimeter (i.e., TLD) is placed on
the front face of the phantom so that the reference direction of
dosimeter coincides with the normal to phantom face.
(a) (b) (c)
Phantoms for calibrating personal dosimeters:
(a) ISO water slab phantom (b) ISO water pillar
Phantom (c) ISO PMMA rod phantom
Personal Dosimeter Calibration
Arrangement with Tissue
Equivalent Phantom at SSDL

37. Radiation Detection Instruments &
Limitations
37

38. Categoryof radiation detectors
o Gas-filled detectors
oIonization chamber
oProportional counters
oGeiger-Müller counters
o Scintillation counters
o NaI(Tl), liquid, plastic, ZnS
o Semiconductor detectors
oGe, Si, Ge(Li), Si(Li)
o Photographic emulsions
o Solid state track detectors
oBF3 counter
o Gas-filled detectors
oIonization chamber
oProportional counters
oGeiger-Müller counters
o The main difference among
three types of gas-filled
detectors is applied voltage.
o Ionization chambers have wider
range of physical shape. (parallel
plates, concentric cylinders, etc.)
o Proportional counters and GM
counters are cylindrical shape
and have thin wire anode.

39. Radiation Measuring Instruments
Features
The basic requirement of an instrument for
measurement: Radiation should interact with detector
in such a manner that the magnitude of the
instrument’s response is proportional to radiation effect
or radiation property being measured
A standard radiation instruments should have the
following properties -
 Energy independent over a wide range
 Minimal effect of temp, pressure and humidity
 High reliability
 High efficiency
 Easy to handle, and cost effective
Properties
39

40.  The Operating voltage of the Ionization chamber, Proportional
counter and GM detector is classified into three zones.
 GM counter indicates a distinct plateau curve in comparison of the
proportional counter.
 In a proportional counter, the number of ion-pairs created within
the gas proportionally increase with the applied voltage.
Detector’s Operational Voltage Range

41. Portable Radiation Instruments
Geiger-Müller Survey Meter
A radiation survey meter equipped with a GM detector, used for
measurement of gamma radiation field, as well as beta radiation by
using sliding window.
Detector Used : Thin Walled GM Tube enclosed in brass Cylinder.
Energy Response : 600 KeV - 2 MeV
Range :
(i) 0 - 0.2 mR/hr (0 - 0.002 mGy/hr)
(ii) 0-2 mR/hr (0 - 0.02mGy/hr)
(iii) 0-20mR/hr (0 - 0.2mGy/hr)
Window Thickness : 30 - 35 mg/cm2
Operating Voltage : 1000 V
Gas Filled : Argon 41

42. Limitations
 A further limitation is the inability to measure high radiation
rates due to the "dead time" of the tube. This is an insensitive
period after each ionization of the gas during which any
further incident radiation will not result in a count.
 The output pulse from a Geiger-Muller tube is always the
same magnitude regardless of the energy of the incident
radiation, the tube cannot differentiate between radiation
types i.e., does not give energy information.

43. Ion chamber survey meter
 Ion chamber instruments are used for high radiation rates.
 It provides accurate exposure measurements regardless of the
radioactive isotope being used.
 This monitor is also capable of measuring beta exposure rate
through a thin window by removing the front cap.
Detector Used : Ionization Chamber
Energy Response : 200KeV-3MeV
Range :
(i) 0 - 50 mR/hr
(ii) 0 - 500 mR/hr
(iii) 0 - 5 R/hr
43

44. Limitations
 Additional expense and care is required to make accurate
radiation exposure measurements over a range of radiation
energies for the Bremsstrahlung radiation produced by an
X-ray generator.
 Ion chamber survey meter tends to be less sensitive than a GM
survey meter (a slower response time), but is able to respond
more precisely in higher radiation fields.
 This type of detector produces a weak output signal that
corresponds to the number of ionization events.

45. Proportional Counter
 It is similar to a Geiger counter but operates at a lower potential
difference such that the magnitude of the discharge is directly
proportional to the number of gas molecules ionized by the
detected particle.
 The proportional counter is a type of gaseous
ionization detector device used to measure particles of ionizing
radiation.
A proportional counter uses a combination of the
mechanisms of a Geiger-Muller tube and
an ionisation chamber, and operates in an
intermediate voltage region between these.
 The key feature is its ability to measure the energy of incident
radiation, by producing a detector output that
is proportional to the radiation energy

46. Limitations
Quenching Problem
Excited ions emit UV photons  formation of new cascades due to
photo effect  Total cascade takes a long time  large dead time.
Solution: Absorption of UV in “quenching gas  P-10 gas (90% Ar
and 10% CH4)”
Ageing Problem
Plasma discharge  Destruction of gas
contaminants  formation of free radicals 
Polymarization  Polymers have high dipole
moment  attach to electrodes 
Reduction of pulse charges  Ageing effect

47. NaI(Tl) Scintillation Detector
 NaI(Tl) detector is the most efficient detector for X-rays.
 In this type of detector when a gamma ray interacting with a
scintillator (i.e., NaI crystal), it produces a pulse of light that is
converted to an electric pulse by a photomultiplier tube (PMT).
 The key feature is its ability to measure very low radiation field .
Sodium Iodide
(NaI) Crystal
 Detector characteristic
 These detectors may serve not only as radiation counters but also
as spectrometers.

48. Limitations
 Detector response depends on photon energy.
 A meter will show only half the actual dose rate
if the radiation comes from a Co-60 source, but
will over-respond by a factor of 6 or 7 if the
radiation is coming from the Am-241 in.
 Detector Resolution
 Detector Response
The resolution of a NaI detector gradually
decreases with energy, and hence in a radiation
field it cannot distinguish radionuclides.
The resolution (R) is usually given in terms of the full
width at half maximum (FWHM) of a peak. It is
calculated by dividing the value corresponding to the
peak centroid E of the full width (∆E) at FWHM of a
peak in a spectrum . Resolution (R) = ∆E/E
 In scintillation detector, it needs to make sure that either radiation is measuring of the
same nuclide to which it was calibrated or it needs to have a set of calibration factors
that will let us convert the meter reading to the correct dose rate.
 Calibration Factor

49. Response Limitation - Energy and Angular Dependency
 Energy dependency of GM pancake
indicates a significant over-response at
lower energies between approximately
20 to 160 keV (red line on the graph).
 Angular dependency of NaI type
Survey meter was evaluated based on
the variation in response for 137Cs.
Energy dependency Angular dependency
 However, an energy compensation filter
flattens the energy response of this type
GM pancake detector to facilitate
measuring Ambient Equivalent
Dose (Sieverts).
 Responses were found to be increased
with increasing the angle of incident
radiation from 00 to 900. After 900, the
angular responses are decreased with
increasing the angle of incident
radiation.

50. Summary