Post on 06-Apr-2018
8/3/2019 Type of Detector
1/47
SUPERVISOR Dr . YAHIA ALRMADEEN
BY
AYESH ALDIRIFIRST SEMESTER 2011-2012
ID : 8090338
Ayesh_diri@yahoo.com
8/3/2019 Type of Detector
2/47
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. . .
8/3/2019 Type of Detector
3/47
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
8/3/2019 Type of Detector
4/47
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
8/3/2019 Type of Detector
5/47
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
8/3/2019 Type of Detector
6/47
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
8/3/2019 Type of Detector
7/47
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
8/3/2019 Type of Detector
8/47
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
8/3/2019 Type of Detector
9/47
8/3/2019 Type of Detector
10/47
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
8/3/2019 Type of Detector
11/47
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
8/3/2019 Type of Detector
12/47
8/3/2019 Type of Detector
13/47
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
8/3/2019 Type of Detector
14/47
efficiencyIntrinsicefficiencyGeometricEfficiency
detectorreachingNumber
detectedNumber
emittedNumber
detectorreachingNumber
Efficiency
emittedNumber
detectedNumberEfficiency
8/3/2019 Type of Detector
15/47
8/3/2019 Type of Detector
16/47
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
8/3/2019 Type of Detector
17/47
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
8/3/2019 Type of Detector
18/47
8/3/2019 Type of Detector
19/47
8/3/2019 Type of Detector
20/47
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
8/3/2019 Type of Detector
21/47
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.
8/3/2019 Type of Detector
22/47
8/3/2019 Type of Detector
23/47
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
8/3/2019 Type of Detector
24/47
8/3/2019 Type of Detector
25/47
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
8/3/2019 Type of Detector
26/47
Detecting neutrons Use proportional counter filled with BF3 (10%10B) to
detect reaction products
Thermal neutron capture: moderate neutrons with e.g.wax.
8/3/2019 Type of Detector
27/47
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
8/3/2019 Type of Detector
28/47
8/3/2019 Type of Detector
29/47
8/3/2019 Type of Detector
30/47
8/3/2019 Type of Detector
31/47
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
8/3/2019 Type of Detector
32/47
Scintillation detector
Scintillation photons
from recombinationof e-h pairs (electronhole pair).
8/3/2019 Type of Detector
33/47
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
8/3/2019 Type of Detector
34/47
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
8/3/2019 Type of Detector
35/47
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
8/3/2019 Type of Detector
36/47
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
8/3/2019 Type of Detector
37/47
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
8/3/2019 Type of Detector
38/47
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.
8/3/2019 Type of Detector
39/47
8/3/2019 Type of Detector
40/47
8/3/2019 Type of Detector
41/47
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
8/3/2019 Type of Detector
42/47
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
8/3/2019 Type of Detector
43/47
Semiconductor detector
Solid state ionization chamber. Basically a silicondiode.
8/3/2019 Type of Detector
44/47
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.
8/3/2019 Type of Detector
45/47
Cerenkov detector
Particle travels faster than speed of light in mediumc=n.
8/3/2019 Type of Detector
46/47
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.
8/3/2019 Type of Detector
47/47