Type of Detector

8/3/2019 Type of Detector

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SUPERVISOR Dr . YAHIA ALRMADEEN

BY

AYESH ALDIRIFIRST SEMESTER 2011-2012

ID : 8090338

[email protected]


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Types of detectorsA wide range of detectors has developed since 1910.

How things advance:

Rutherford: Detector is student in dark room

counting flashes on a zinc sulphide screen. So, student decided to make life easier. . .

Physics develops. . .


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Types of detectors Gas-filled detectors consist of a volume of gas

between two electrodes

In scintillation detectors, the interaction ofionizing radiation produces UV and/or visible light

Semiconductor detectors are especially purecrystals of silicon, germanium, or other materials

to which trace amounts of impurity atoms havebeen added so that they act as diodes


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Types of detectors (cont.) Detectors may also be classified by the type of

information produced: Detectors, such as Geiger-Mueller (GM) detectors, that

indicate the number of interactions occurring in thedetector are called counters

Detectors that yield information about the energydistribution of the incident radiation, such as NaI

scintillation detectors, are called spectrometers Detectors that indicate the net amount of energy

deposited in the detector by multiple interactions arecalled dosimeters


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Modes of operation Inpulse mode, the signal from each interaction is

processed individually

In current mode, the electrical signals from individualinteractions are averaged together, forming a netcurrent signal


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Interaction rate Main problem with detectors in pulse mode is that

two interactions must be separated by a finite

amount of time if they are to produce distinctsignals

This interval is called the dead time of the system

If a second interaction occurs in this interval, its

signal will be lost; if it occurs close enough to thefirst interaction, it may distort the signal from thefirst interaction


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Dead time Dead time of a detector system largely determined

by the component in the series with the longest

dead time Detector has longest dead time in GM counter systems

In multichannel analyzer systems the analog-to-digitalconverter often has the longest dead time

GM counters have dead times ranging from tens tohundreds of microseconds, most other systemshave dead times of less than a few microseconds


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Paralyzable or nonparalyzable In aparalyzable system, an interaction that occurs

during the dead time after a previous interactionextends the dead time

In a nonparalyzable system, it does notAt very high interaction rates, a paralyzable system will

be unable to detect any interactions after the first,causing the detector to indicate a count rate of zero


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Current mode operation In current mode, all information regarding

individual interactions is lost

If the amount of electrical charge collected fromeach interaction is proportional to the energydeposited by that interaction, then the net currentis proportional to the dose rate in the detector

material Used for detectors subjected to very high

interaction rates


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Spectroscopy Most spectrometers operated in pulse modeAmplitude of each pulse is proportional to the

energy deposited in the detector by the interaction

causing that pulse The energy deposited by an interaction is not

always the total energy of the incident particle orphoton

Apulse height spectrum is usually depicted as agraph of the number of interactions depositing aparticular amount of energy in the spectrometer asa function of energy


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Detection efficiency The efficiency (sensitivity) of a detector is a measure of

its ability to detect radiation

Efficiency of a detection system operated in pulsemode is defined as the probability that a particle orphoton emitted by a source will be detected


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efficiencyIntrinsicefficiencyGeometricEfficiency

detectorreachingNumber

detectedNumber

emittedNumber

detectorreachingNumber

Efficiency

emittedNumber

detectedNumberEfficiency


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Intrinsic efficiency Often called the quantum detection efficiency or

QDE

Determined by the energy of the photons and theatomic number, density, and thickness of thedetector

For a a parallel beam of monoenergetic photons

incident on a detector of uniform thickness:

x

e

-1efficiencyIntrinsic


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Gas-filled detectorsA gas-filled detector consists of a volume of gas

between two electrodes, with an electricalpotential difference (voltage) applied between the

electrodes Ionizing radiation produces ion pairs in the gas Positive ions (cations) attracted to negative

electrode (cathode); electrons or anions attracted

to positive electrode (anode) In most detectors, cathode is the wall of the

container that holds the gas and anode is a wireinside the container


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Types of gas-filled detectors Three types of gas-filled detectors in common use:

Ionization chambers

Proportional counters

Geiger-Mueller (GM) counters, Rutherfords student Geiger,and his student Muller, developed the Geiger-Muller tube.

Type determined primarily by the voltage appliedbetween the two electrodes

Ionization chambers have wider range of physicalshape (parallel plates, concentric cylinders, etc.)

Proportional counters and GM counters must havethin wire anode


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Ionization in a gas

The loss of energy of a particle by ionization of a gas isthe basis of a range of detectors:

Ionization chamber: Signal from primary ionization.

Proportional counter: Signal from avalanche

(secondary ionization) multiplication in high electricfield: gain.

GM Tube: Saturated avalanche, high gain, noproportionality.


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Ionization chambers If gas is air and walls of chamber are of a material

whose effective atomic number is similar to air, theamount of current produced is proportional to the

exposure rateAir-filled ion chambers are used in portable survey

meters, for performing QA testing of diagnosticand therapeutic x-ray machines, and are the

detectors in most x-ray machine phototimers Low intrinsic efficiencies because of low densities

of gases and low atomic numbers of most gases


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Proportional counters Must contain a gas with specific properties

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

Seldom used in medical centers


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Detecting neutrons Use proportional counter filled with BF3 (10%10B) to

detect reaction products

Thermal neutron capture: moderate neutrons with e.g.wax.


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GM counters GM counters also must contain gases with specific

properties Gas amplification produces billions of ion pairs

after an interaction signal from detector requireslittle amplification Often used for inexpensive survey meters In general, GM survey meters are inefficient

detectors of x-rays and gamma rays Over-response to low energy x-rays partially

corrected by placing a thin layer of higher atomicnumber material around the detector


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GM counters (cont.) GM detectors suffer from extremely long dead

times 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 use

ionization chamber instruments for measuringsuch fields


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Scintillation detector

Scintillation photons

from recombinationof e-h pairs (electronhole pair).


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Scintillation detectors Scintillators are used in conventional film-screen

radiography, many digital radiographic receptors,fluoroscopy, scintillation cameras, most CTscanners, and PET scanners

Scintillation detectors consist of a scintillator and adevice, such as a PMT, that converts the light into

an electrical signal


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Scintillators Desirable properties:

High conversion efficiency

Decay times of excited states should be short

Material transparent to its own emissions Color of emitted light should match spectral sensitivity

of the light receptor

For x-ray and gamma-ray detectors, should be large

high detection efficiencies Rugged, unaffected by moisture, and inexpensive to

manufacture


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Scintillators (cont.)Amount of light emitted after an interaction increases

with energy deposited by the interaction

May be operated in pulse mode as spectrometers

High conversion efficiency produces superior energyresolution


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Materials Sodium iodide activated with thallium [NaI(Tl)],

coupled to PMTs and operated in pulse mode, is usedfor most nuclear medicine applications

Fragile and hygroscopic

Bismuth germanate (BGO) is coupled to PMTs andused in pulse mode as detectors in most PET scanners


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Photomultiplier tubes PMTs perform two functions:

Conversion of ultraviolet and visible light photons intoan electrical signal

Signal amplification, on the order of millions to billions

Consists of an evacuated glass tube containing aphotocathode, typically 10 to 12 electrodes calleddynodes, and an anode


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Photomultiplier tubes

Charged particles produce e-h pairs. e-h pairs combine to make light; light produces

electrons from

Photocathode electrons shower in photomultiplier

current pulse at anode.


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Dynodes Electrons emitted by the photocathode are

attracted to the first dynode and are accelerated tokinetic energies equal to the potential differencebetween the photocathode and the first dynode

When these electrons strike the first dynode,about 5 electrons are ejected from the dynode for

each electron hitting it These electrons are attracted to the second

dynode, and so on, finally reaching the anode


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PMT amplification Total amplification of the PMT is the product of

the individual amplifications at each dynode

If a PMT has ten dynodes and the amplification ateach stage is 5, the total amplification will beapproximately 10,000,000

Amplification can be adjusted by changing the

voltage applied to the PMT


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Semiconductor detector

Solid state ionization chamber. Basically a silicondiode.


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Semiconductor detectorSemiconductor detector: SiSmall size.

Good resolution.

Good timing properties.Limited in energy.

Semiconductor detector: Ge

Used for gammas.

Expensive.Excellent resolution.

Requires LN2 for cooling.


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Cerenkov detector

Particle travels faster than speed of light in mediumc=n.


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References

K. S. Krane, Introductory nuclear physics, Wiley, 1988.

John Lilley, Nuclear Physics, Wiley, 2001.

Glenn F. Knoll, Radiation Detection andMeasurement, 3rd ed., Wiley, (New York), 2000.

William R. Leo, Techniques for Nuclear and ParticlePhysics Experiments, 2nd ed., Springer, (Berlin), 1994.


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Type of Detector

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