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Pakistan Institute of Engineering & Applied Sciences(PIEAS)
Lectures onRadiation Detection
Delivered in Professional Training Course onRadiation Safety & RWM at PNRA (April, 2008)
Dr. Nasir M Mirza
Deputy Chief Scientist,
Department of Physics & Applied mathematics,
PIEAS, P.O. Nilore, 45650, Islamabad.
Email: [email protected]
Ph: +92 51 9290273 (ext: 3059)
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Other Types of Radiation Detectors
Recommended Text Books
1. Glenn F Knoll sRadiation Detection & Measurement (recentedition).
Lecture 5
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Personal Monitoring
Individual monitoring is needed external radiation measurements: The worker wears a dosimeter from the followings
Ion-chambers and Pocket dosimeter
Stray radiation chambers
TLD (thermo luminescent dosimeter)
Electronic dosimeter
Film Badges
Photographic films
MOSFETs
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Detection of Ionization in Air
Adapted
from Collins
2001
Ion chamber
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Ionization Chambers
Thimble chambers
600cc chamber
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Ionization Chambers
Farmer 0.6 cc chamber
and electrometer
Most important chamber
in radiotherapydosimetry
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Electrometer
From the chamber
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Parallel plate chambers
From Metcalfe et al 1996
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Parallel Plate Ionization Chambers
Used for
low energy X Rays (< 60 KV)
Electrons of any energy but rated as the preferred method for
energies < 10 MeV and essential for energies < 5 MeV
Many types available in different materials and sizes
Often sold in combination with a suitable slab phantom
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Parallel Plate Ionization Chambers -
Markus chamber
small
designed for electrons
Holt chamber
robust embedded in polystyrene
slab
examples
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Well type ionization chamber
For calibration of
brachytherapy sources
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Ionization chamber type survey meters
not as sensitive as G-M devices but not affected by
pulsed beams such as occur with accelerators
this is the
preferred
device around
high energy
radiotherapy
accelerators
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Pocket dosimeter
Pocket dosimeter
Charger
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Pocket Dosimeters
Advantage of direct reading dosimeter
No need to recharge after being read.
Exposure range 0 200 mR 15% of true exposure
Energy range 50 keV 2 MeV
Uses
Personal monitoring: worn by persons exposed to xor rays
Area monitoring: put at particular points one ormore dosimeter for al least for 1 week
Medical and dental exposure measuring
Auxiliary charger is required with dosimeter
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Pocket Dosimeters
Air wall chamber
Air wall chamber is made on the basisof operational definition of X-unit orRoentgen
Used for exposure measurements
Air wall chamber as a pocketdosimeter is widely used for personalmonitoring
Works on the principle ofQ = C Q
Two types of pocket dosimeter
Indirect reading (condenser type)
Direct reading (quartz fiber type)
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Pocket Dosimetersconti.
I ndirect reading dosimeter (condenser type)
The condenser is charged with a battery
Exposure is measured by an auxiliary device
The device is an electrostatic voltmeter that iscalibrated in roentgen, is called charge reader.
integrated X or ray exposure is measured up to 200mR 15% for E: 0.05 2 MeV energy, outside thisrange correction factors applied.
Dosimeter also respond to
- particles
Neutrons if coated internally with boron
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Condenser type dosimeter
The dosimeter measures x and rays within 15% from 30keV to 12 MeV in the range 0200 mR
Condenser typedosimeter and itscharge reader.
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Pocket Dosimeters cont
Direct reading dosimeter
Operate on the principle of gold-leaf electroscope
Quartz fiber is displaced by charging it to a potential
of ~ 200 V.
The image is brought to a fixed zero position
Exposure to radiation: Discharge in chamber,
shifting of fiber towards original position.
The amount of discharge or change is position of the
fiber is proportional to radiation exposure
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Pocket Dosimeters
Standard type is for x and ray measurements
Calibrated with Radium, 60Co, 137CS
Limitations:
Discharge even when not in radiation field of interest If leakage is more than 5% of full scale reading per day
should not be used
Two dosimeters need to be worn
Lower reading is taken into account
Every day recharging is required
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Pocket Dosimeter
Simplified cross section of a
direct reading quartz fiber
electroscope type pocket
dosimeter
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Stray Radiation Chambers
Used for area monitoring More sensitive than pocket dosimeters
Designed to be used with a charge reader
Charge readers are similar to those for condenser typepock dosimeters
Especially useful in monitoring scattered radiation from
medical and dental x ray exposures
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Stray Radiation Chambers
The smaller chamber has
range of 010 mR, while
the larger one, which is
more sensitive, has arange 01 mR.
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Various TLD types
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Simplified scheme of the TLD process
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TLD glow curves
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The role of different dopants
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Dose response ofLiF:Mg,Ti:
wide dosimetric range
watch supralinearity
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Variation of TLD response with radiation quality
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Materials & choices
LiF:Mg,Ti (the gold standard)
CaF2 (all natural, or with Mn, Dy
or Tm)
CaSO4
BeO
Al2O3 :C (record sensitivity 1
micro-Gy)
LiF:Mg,Cu,P (the new star?)
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What can one expect...
Reproducibility: single chip
2% (0.1Gy, 1SD)
Accuracy(4 chips standard, 2
chips measurement) 3%
(0.1Gy, 95% confidence)
about 30 minutes per
measurement...
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TLD Readers
TLD Measuring Systems
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