radiation detection & measurement i
TRANSCRIPT
-
8/11/2019 Radiation Detection & Measurement I
1/35
Radiation Detection &
Measurement I
Types of detectors
-
8/11/2019 Radiation Detection & Measurement I
2/35
Types of detectors
Gas-filled detectorsconsist of a volume of gas
between two electrodes
Inscintillation detectors, the interaction ofionizing radiation produces UV and/or visible
light
Semiconductor detectorsare especially pure
crystals of silicon, germanium, or other materials
to which trace amounts of impurity atoms have
been added so that they act as diodes
-
8/11/2019 Radiation Detection & Measurement I
3/35
Types of detectors (cont.)
Detectors may also be classified by the type ofinformation produced:
Detectors, such as Geiger-Mueller (GM) detectors, thatindicate the number of interactionsoccurring in thedetector are called counters
Detectors that yield information about the energydistribution of the incident radiation, such as NaI
scintillation detectors, are calledspectrometers Detectors that indicate the net amount of energy
deposited in the detectorby multiple interactions arecalled dosimeters
-
8/11/2019 Radiation Detection & Measurement I
4/35
Modes of operation
In pulse mode, the signal from each
interaction is processed individually
In current mode, the electrical signals from
individual interactions are averaged
together, forming a net current signal
-
8/11/2019 Radiation Detection & Measurement I
5/35
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 timeof the system
If a second interaction occurs in this interval, its
signal will be lost; if it occurs close enough to the
first interaction, it may distort the signal from the
first interaction
-
8/11/2019 Radiation Detection & Measurement I
6/35
Dead time
Dead time of a detector system largely determinedby the component in the series with the longestdead time
Detector has longest dead time in GM countersystems
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
-
8/11/2019 Radiation Detection & Measurement I
7/35
Paralyzable or nonparalyzable
In aparalyzablesystem, an interaction that
occurs during the dead time after a previous
interaction extends the dead time
In a nonparalyzable system, it does not
-
8/11/2019 Radiation Detection & Measurement I
8/35
-
8/11/2019 Radiation Detection & Measurement I
9/35
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 energy
deposited by that interaction, then the net current
is proportional to the dose rate in the detector
material
Used for detectors subjected to very high
interaction rates.
-
8/11/2019 Radiation Detection & Measurement I
10/35
Spectroscopy
Most spectrometers operated in pulse mode
Amplitude of each pulse is proportional to the
energy deposited in the detector by the interactioncausing that pulse
The energy deposited by an interaction is notalways the total energy of the incident particle or
photon Apulse height spectrumis usually depicted as a
graph of the number of interactions depositing aparticular amount of energy in the spectrometer as
a function of energy
-
8/11/2019 Radiation Detection & Measurement I
11/35
-
8/11/2019 Radiation Detection & Measurement I
12/35
Detection efficiency
The efficiency (sensitivity) of a detector is a
measure of its ability to detect radiation
Efficiency of a detection system operated in
pulse mode is defined as the probability that
a particle or photon emitted by a source will
be detected
-
8/11/2019 Radiation Detection & Measurement I
13/35
efficiencyIntrinsicefficiencyGeometricEfficiency
detectorreachingNumber
detectedNumber
emittedNumber
detectorreachingNumber
Efficiency
emittedNumber
detectedNumberEfficiency
-
8/11/2019 Radiation Detection & Measurement I
14/35
-
8/11/2019 Radiation Detection & Measurement I
15/35
Intrinsic efficiency
Often called the quantum detection efficiencyor
QDE
Determined by the energy of the photons and theatomic number, density, and thickness of the
detector
For a a parallel beam of monoenergetic photons
incident on a detector of uniform thickness:
x
e
-1efficiencyIntrinsic
-
8/11/2019 Radiation Detection & Measurement I
16/35
Gas-filled detectors
A gas-filled detector consists of a volume of gasbetween two electrodes, with an electricalpotential difference (voltage) applied between theelectrodes
Ionizing radiation produces ion pairs in the gas
Positive ions (cations) attracted to negative
electrode (cathode); electrons or anions attractedto positive electrode (anode)
In most detectors, cathode is the wall of thecontainer that holds the gas and anode is a wire
inside the container
-
8/11/2019 Radiation Detection & Measurement I
17/35
-
8/11/2019 Radiation Detection & Measurement I
18/35
Types of gas-filled detectors
Three types of gas-filled detectors in common use:
Ionization chambers
Proportional counters
Geiger-Mueller (GM) counters
Type determined primarily by the voltageapplied between the two electrodes
Ionization chambers have wider range of physicalshape (parallel plates, concentric cylinders, etc.)
Proportional counters and GM counters must havethin wire anode
-
8/11/2019 Radiation Detection & Measurement I
19/35
-
8/11/2019 Radiation Detection & Measurement I
20/35
Ionization chambers
If gas is air and walls of chamber are of a materialwhose effective atomic number is similar to air,the amount of current produced is proportional tothe exposure rate
Air-filled ion chambers are used in portablesurvey meters, for performing QA testing ofdiagnostic and therapeutic x-ray machines, and arethe detectors in most x-ray machine phototimers
Low intrinsic efficiencies because of low densitiesof gases and low atomic numbers of most gases
-
8/11/2019 Radiation Detection & Measurement I
21/35
-
8/11/2019 Radiation Detection & Measurement I
22/35
Proportional counters
Must contain a gas with specific properties
Commonly used in standards laboratories,
health physics laboratories, and for physicsresearch
Seldom used in medical centers
-
8/11/2019 Radiation Detection & Measurement I
23/35
GM counters
GM counters also must contain gases with specificproperties
Gas amplification produces billions of ion pairsafter an interaction signal from detector requireslittle amplification
Often used for inexpensive survey meters
In general, GM survey meters are inefficientdetectors of x-rays and gamma rays
Over-response to low energy x-rays partiallycorrected by placing a thin layer of higher atomic
number material around the detector
-
8/11/2019 Radiation Detection & Measurement I
24/35
-
8/11/2019 Radiation Detection & Measurement I
25/35
-
8/11/2019 Radiation Detection & Measurement I
26/35
GM counters (cont.)
GM detectors suffer from extremely long dead
timesseldom used when accurate measurements
are required of count rates greater than a fewhundred counts per second
Portable GM survey meter may become
paralyzed in a very high radiation fieldshould
always use ionization chamber instruments formeasuring such fields
-
8/11/2019 Radiation Detection & Measurement I
27/35
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
a device, such as a PMT, that converts the light
into an electrical signal
-
8/11/2019 Radiation Detection & Measurement I
28/35
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 sensitivityof the light receptor
For x-ray and gamma-ray detectors, should be large
high detection efficiencies Rugged, unaffected by moisture, and inexpensive to
manufacture
-
8/11/2019 Radiation Detection & Measurement I
29/35
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 producessuperior energy resolution
-
8/11/2019 Radiation Detection & Measurement I
30/35
Materials
Sodium iodide activated with thallium
[NaI(Tl)], coupled to PMTs and operated in
pulse mode, is used for most nuclearmedicine applications
Fragile and hygroscopic
Bismuth germanate (BGO) is coupled toPMTs and used in pulse mode as detectors
in most PET scanners
-
8/11/2019 Radiation Detection & Measurement I
31/35
Photomultiplier tubes
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 tubecontaining a photocathode, typically 10 to
12 electrodes called dynodes, and an anode
-
8/11/2019 Radiation Detection & Measurement I
32/35
-
8/11/2019 Radiation Detection & Measurement I
33/35
-
8/11/2019 Radiation Detection & Measurement I
34/35
Dynodes
Electrons emitted by the photocathode are
attracted to the first dynode and are accelerated to
kinetic 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
-
8/11/2019 Radiation Detection & Measurement I
35/35
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 be
approximately 10,000,000
Amplification can be adjusted by changing the
voltage applied to the PMT