Download - Proportional Counters
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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.