Lec 5 Other Detectors-1

7/28/2019 Lec 5 Other Detectors-1

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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)
mailto:[email protected]:[email protected]


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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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