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Ionisation chamber

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Air-filled ion chambers are used in portablesurvey meters, for performing QA testing ofdiagnostic and therapeutic x-ray machines,and are the detectors in most x-ray machinephototimers

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GM COUNTERS

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appl

Often used for inexpensive survey meters

In general, GM survey meters are inefficientdetectors of x-rays and gamma rays

GM detectors suffer from extremely long deadtimes seldom used when accurate measurementsare required of count rates greater than a fewhundred counts per second

Portable GM survey meter may become paralyzedin a very high radiation field should always useionization chamber instruments for measuring such

fields

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Proportional counters

Operates at voltages beyond ionisation chamber

region ,but below that of geiger region. o/p pulse from the chamber starts to increase

with the increase in field strength at the central

electrode. The pulse is amplified by 1000 or more. o/p

pulse is of few mv.

Design of the counter and applied voltages are sochosen that a high voltage exists close to the

anode.

Preamp is a voltage follower- high i/p imp

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Followed by low noise linear amp having gain in the

range of 500-1000. non overloading type

Preamp shd be placed near the detector. Pulse height analyser-counts a pulse only if its amp

falls within specific limits.

Scaling unit counts down the pulses so that they aredigitally displayed.

App:

Commonly used in standards laboratories, healthphysics laboratories, and for physics research

Seldom used in medical centers

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Scintillation counter Scintillator is a crystalline sub which produces minute

flashes of light in the vis or uv range when it absorbsionising rad.

Light flashes are of very short duration and detected byPMT, which produces a pulse for each particle.

Scint + pmt=scintillation counter

NaI with 0.5% of thallium iodide- r ray detector.

Anthracene beta

Scintillators are used in conventional film-screenradiography, many digital radiographic receptors,fluoroscopy, scintillation cameras, most CT scanners,

and PET scanners

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PMTs perform two functions:

Conversion of ultraviolet and visible light photons

into an electrical signal

Signal amplification, on the order of millions to

billions

Consists of an evacuated glass tube containing a

photocathode, typically 10 to 12 electrodes called

dynodes, and an anode

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Semiconductor detectors

Silicon & Germanium crystalsalpha and beta

Func in a similar manner to that of the gas

ionisation chamber.