lec 6 other detectors-2

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Pakistan Institute of Engineering & Applied Sciences(PIEAS)

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)

Lectures onRadiation Detection

Delivered in Professional Training Course onRadiation Safety & RWM at PNRA (April, 2008)
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 6


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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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TL Response

Variation with gamma-ray energy of the TL response per roentgen


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TLDconti.

Oven for temperature annealing


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TLD Readers

TLD reader


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Personal monitoring with TLD

One or more small pieces of TL

material are placed in a holderthat is worn by a person beingmonitored for a prescribed time

After that TLD is heatedelectrically

Intensity of luminescence ismeasured with PMT

PMT output is read by digital

voltmeter in units of charge(nC)

The instrument is calibratedwith known dose of radiation


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Environmental monitoring with TLD

Environmental radiation doses

are much lower than the work

place doses

Require much more sensitive

material than for radiation

workers

Al2O3:C can measure does as

low as 0.1 SV (0.01m rem)

High degree of sensitivity

The best choice forenvironmental dosimetry

TLD reader


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Various Types of TL

Thermoluminescence materials are available in powder forms, and in

accurately sized discs, rods, chips, and cubes


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ELECTRONIC DOSIMETERS

Features

Smaller, more reliable, and moresophisticated instrument, lightweight (~100 g), accuratedosimeter

More useful for personal

dosimetery than film or TLD Measure and display

instantaneous dose rate andintegrate over time to obtain,store, display accumulated dose

Design to alarm at any desireddose rate or accumulated dose

Data can be stored in memorycan be down load into computerfor dose tracking or record

keeping


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Electronic dosimeter

Direct reading electronic

dosimeter for personal monitoring


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Electronic Dosimeters conti.

Material

Solid- state semiconductors, silicone diodes;

Detect and radiation over a wide range of dose rates

and doses.

137Cs: dosimeter with 10% accuracy can

Measure dose rate from 1 Sv/hr to 10 mSv/hr

Can store dose from 1 Sv to 10 Sv.


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Electronic Dosimeters conti.

Working Electronic Pocket dosimeter

Incorporate several detectors

Each one respond to radiation that has passed through

different filters Output of several detectors is added which is

proportional to the absorbed dose

Response, energy independent ( 25%) from 60 keV

1.5 MeV


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Geiger-Mueller Counter

It is Not a dosimeter It is just a

counter of radiation events;

Very sensitive;

Light weight and convenient to use

Suitable for miniaturization


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Geiger-Mueller (G-M) Devices

Useful for

Area monitoring

Room monitoring

Personnel monitoring

Care required in regions of high dose rate or pulsed beamsas reading may be inaccurate


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SURVEY METERS:

Response is proportional to absorbed energy

Have air equivalent wall measure exposure

Calibrated to read in units of mR/h, C/kg/h, Sv/h


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Film Badges

The sensitive element

is a strip of 16-mm film(bottom) that is sealed into a

light-tight envelope and is

placed inside a holder(top).

Energy dependence of the

film is overcome through the

differences in film darkening

under the open window and

under several different filtersthat are built into the holder.

FIGURE : Film badge dosimeter for X-rays, gamma rays, and beta rays.

Holder

Film


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Film Badges

FIGURE :

Relationship between

radiation exposure and optical

density.

CurveA is the response ofduPont type 555 and

CurveB is the response of

duPont type 834 dosimeter

film to 60Co gamma rays.


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Radiographic film

Reduction of silver halide to silver

Requires processing ---> problems with reproducibility

Two dimensional dosimeter

High spatial resolution

High atomic number ---> variations of response with

radiation quality


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Radiographic film

Cross section

Often prepacked

for ease of use


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Film: dose response

Evaluation of film via optical

density

OD = log (I0 / I)

Densitometers are commercially

available


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Radiographic film dosimetry in practice

Depends on excellent processor

QA

Commonly used for demonstration

of dose distributions

Problems with accuracy and

variations in response with X Ray

energy


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Radiochromic film

New development

No developing

Not (very) light sensitive

Better tissue equivalence

Expensive


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Semiconductor Devices

Diodes

MOSFET detectors

Diodes for water phantom

measurements


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Diodes

From Metcalfe et al. 1996

Mostly used like

a photocell generating

a voltage proportional

to the dose received.


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Metal Oxide SemiconductorField Effect Transistor

From Metcalfe et al. 1996

MOSFETs = extremely

small sensitive volume


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1. irradiation

2. Charge

carriers trapped

in Si substrate

3. Currentbetween source

and drain altered


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Diodes and other Solid State Devices

Advantages

direct reading

sensitive

small size

waterproofing possible

Disadvantages

temperature sensitive

sensitivity may change --> re-

calibration necessary

regular QA procedures need

to be followed


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Summary of lectures

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, notvery 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 andimportant

dosimetric

technique

New developments(MOSFETs) may

increase utility

Also used fordosimetric

intercomparisons

(audits)

New developments(radiochromic

film) may increase

utility

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