Transcript Health Physics Course 16.739 Instrumentation I

Instrumentation and Detection
Harry M. Johnson, PhD
CancerCare
Winnipeg, Manitoba, Canada
Instrumentation and Detection
• Purpose: To discuss the detection of
radiation from the point of view of the
classical instrumentation.
Instrumentation and Detection: Outline
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The Basic Gas-Filled Detector
Classes of Gas-Filled Detectors
The Geiger Tube as an Exposure Meter
Choosing a Radiation meter
Scintillation Detectors
Calibrating a Contamination Meter
Neutron Meter
Thermoluminescent Personnel Dosimetry
The Gas-Filled Detector - 1
• Gas-filled counting chamber
• Use air as the gas for initial
testing
• sealed or unsealed
• coaxial electrodes, well insulated
• variable voltage, defined anode
and cathode
• high resistance resistor
• capacitance C
• radiation enters chamber and
ionizes the gas
• ions drift towards the electrodes
• Voltage pulse is detected
The Gas-Filled Detector - 2
• Apply low voltage
• a few ion pairs drift to
electrodes
• time constant RC is
long
• Voltage V= Q/C is
detected at output
• Pulse curve has
elongated shape
• Difficult to detect
successive pulses
The Gas-Filled Detector - 3
• Maintain the same voltage
• “drifting” of ion pairs drift to
electrodes is the mechanism of
charge transport from ion pair
production
• shorten the time constant RC
• Voltage V= Q/C continues to be
detected at output
• Pulse curve has “clipped” shape
• Successive pulses are detectable
• Now possible to count the
individual pulse
• This is the preferred chamber
design
• Possible to calculate maximum
pulse rate
Gas-Filled Detectors
Incident
Ionizing
Radiation
Voltage Source
+
+ + + +
- - -
Anode +
Cathode Air or Other Fill Gas
Electrical
Current
Measuring
Device
Ion Chamber Instrument
• Example of Ion
Chamber
• 6 cc Chamber
• 180 cc Chamber
• Readout unit is
located remote
from the detector
The Gas-Filled Detector
• The first plateau
region is the “ion
chamber” mode
• Typically 300 volts
applied voltage
• Pulse size is
independent of LET
• No secondary
ionizations - which
would amplify the
pulse height
• Pulse size distinction
is a disadvantage
The Ionization Chamber
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Radiation of constant flux is applied to chamber
Vary the voltage in the few-hundreds of volts region
ionizations in the air of the chamber create ion pairs
a plateau region is noted - the operating plateau for the
ion chamber
V is just high enough to collect all ions from ionizations
Ions are not being accelerated - still “drifting” to
electrodes
Pulses are distinguishable - each pulse is a single
ionization event
Pules size is independent of voltage
a beta particle produces 1000 ion pairs, output pulse
voltage is low millivolts
Low output voltage is disadvantageous
The Proportional Counter
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Adjust the voltage upward beyond the ion chamber plateau
Ions from the initial ion pairs are accelerated
electron acceleration is more important than positive ions
Secondary electrons are produced by collisions by the
primary ion-pair products
Change the gas in the chamber
These new design features create a gas amplification factor
(>1) - called an “avalanche”
By increasing the tube voltage the avalanche spreads along
the anode wire
Pulse size is proportional to the chamber voltage
Pulse size depends on the electric field gradient
Anode (central) diameter is important
The Proportional Counter
• This is the “Proportional Counter” Region
• Note slight upward slope of Voltage-Pulse curve in this
region
• Pulse size is dependent on LET - giving the ability to
discriminate between type of radiation
• Anode wire size is typically 0.02 to 0.1 mm diameter
• Decreasing the gas pressure increases the multiplication,
although the tube may be at atmospheric pressure
• High-voltage supply must be very stable
The Geiger Tube - 1
• Basic Readout
module
• Two Geiger tubes:
“Pancake” type and
the “End-Window”
type.
• Entrance windows
are very thin
The Geiger Tube - 2
• Continue to raise the voltage past the “proportional
counter” plateau to a new plateau.
• Slight upward slope of Voltage-Pulse curve in this region.
• Counting rate is independent of voltage.
• Change the gas to Argon ( = 15.7 eV; W = 26.4 eV) or
Methane ( = 15.2 eV; W = 27.0 eV).
• Add 10% ethyl alcohol to eliminate uv .
• Tube is sealed.
• Chamber pressure may be 10% of atmospheric pressure
• Output pulse (without amplification) height:approx 1 volt
• Little need for amplification
• Output pulses occur independent of size of initiating
ionization - no discrimination re LET of the radiation.
The Geiger Tube - 3
• This is Geiger (Geiger-Mueller) tube region (1908).
• Voltage is sufficiently high that both ions of the initiating
ion pair are accelerated.
• Accelerated ions cause additional ionizations (avalanche).
• Accelerate +ve ions strike cathode (tube shell), cause
excitations of cathode molecules, yielding UV production.
• UV is additional source of gas ionization/excitation.
• When intense ionization occurs in the tube the E-Field
along the anode wire drops to zero. This causes dead time.
• GM tube can go into continuous discharge when this
occurs. Add alcohol to argon gas to quench the discharge.
• Alcohol reduces dead time to 100 micro seconds. (Resolve
up to 10,000 pulses/sec).
• GM tube is easy to build, simple electronics, cheap.
The Geiger Tube - 4
• Choose operating voltage for G-M region at 1/3 to 1/2 way
up the plateau.
• Alcohol is present to quench the ionizations and absorb the
UV produced when accelerated +ve ions strike shell.
• Alcohol molecules dissociate in this process.
• Lifetime of the tube is limited by the alcohol - total lifetime
is 108 - 109 ionizations.
• Ifd voltage is raised above the Geiger region, the avalanche
spreads and continuous discharge occurs. Gas tube cannot
operate as a detector above the Geiger region.
Resolving Time and Dead Time
• Two ionizations in G-M
tube in rapid succession
may not be resolved.
• The first ionization
causes a Dead Time
when no new pulse can
be detected
• Followed by Recovery
Time when a new pulse
may not be identifiable
• Resolving Time is sum of
Dead and Recovery
times
Resolving Time and Dead Time - 2
• Avalanche starts near the anode wire, spreads along
anode due to the High Voltage in G-M region
• Electrons move more quickly than +ve ions
• Rate-limiting step is the transit time of +ve ions to the
cathode. This transit time defines the collection time
• Resolving time can be defined
• Tube resets itself after recovery
• Collection time may be a few hundred microseconds
• If CT is 250 s, what is limiting rate of detectable
ionizations - how many photos per sec is max input?
Advantages/Disadvantages of Gas
Detection Tubes
• Ion Chamber: simple, accurate, wide range,
sensitivity is function of chamber size, no dead time
• Proportional Counter: discriminate hi/lo LET,
higher sensitivity than ion chamber
• GM Tube: cheap, little/no amplification, thin
window for low energy; limited life
Geiger Tube as Exposure Meter
• “Exposure” is the parameter measuring the
ionization of air.
• Geiger tube measures ionization pulses per
second - a “count rate”.
• The number of ionizations in the Geiger
tube is a constant for a particular energy but
is energy dependent.
Energy Compensation of Geiger Tube
Energy Response for Victoreen 489-4 GM Probe
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Ratio Output to Actual Exposure
Rate
• An energy
compensating
shield is
required to
smooth out the
energy
response
Bare Tube
Response
Response - Beta
Shield Closed
1
10
100
1000
0.1
Photon Energy in keV
10000
Scintillation Detector
• Readout
Module plus
Detector
• Photomultiplier
Tube (P)
• Scintillator
Chrystal (C)
P
C
Scintillation Detectors
Incident
Ionizing
Radiation
Light Photon Photomultiplier Tube
Pulse
Measuring
Device
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Sodium-Iodide
Crystal
Dynode
Photocathode
Optical Window
Anode
Scintillation Detectors
• Construction of
Crystal and PMT
assembly
• Design of basic
electronics
• Principle of
scintillation
• Principle of
operation
Scintillation Detection System
• Design of Basic Pulse
Height Analysis System
• Amplitudes of voltage
pulses are sorted by
PHA
• PHA counts number of
pulses for various
voltages (energies)
• Display is a histogram of
pulse heights
Scintillation Detector Materials - 1
Phosphor
Rel Pulse
Height
Decay Time
nanosec
NaI (Tl)
210
250
Cs I
55
1,100
Plastic
28-48
3-5
‘Liquid”
27-49
2-8
Scintillators - 2 (Derenzo et al 1992)
Phosphor
Na I (Tl)
Pulse Height
photon/MeV
38,000
Decay Time
nanosec
230
NE 102A
10,000
2.4
ZnWO4
10,000
5,000
CsI (pure)
59,000
800
CsI (Tl)
2,300
16
CdS(Te)
190
18
Analysis of Scintillator Peak - 1
• NaI (Tl) scintillation
peak for Cs-37: 662 keV
• Large crystal: 10x10 cm
• Only photons that lose
all energy (i.e. Compton
events + final
photoelectric event)
contribute to the “Total
Energy Peak”
Analysis of Scintillation Peak -2
• “Continuous Compton Distribution” arises from light
from Compton events and Secondary photons escape
from the crystal before the photoelectric event occurs.
• “Compton Edge” (478 keV) is the energy of the
maximum recoil electron for h = 662 keV.
• Tmax = 2 h /(2 + mc2/ h ) = 478 keV;
This occurs
when the Compton scattering angle (for the secondary
Compton photon) is 180 degrees.
Analysis of Scintillation Peak -3
• “Backscattered” photon has energy 662-478 = 184 keV.
The backscattered peak is visible in the spectrum.
• Relative area under the Total Energy Peak (photopeak)
depends on the size of the crystal.
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Ratio of areas under the “Total Energy” peak and the
“Compton Distribution” in a small detector is
approximately equal to ratio of photoelectric to
Compton cross-section in the crystal material.
Analysis of Scintillation Peak -4
• Escape Peak: when the detector is small, the Escape
Peak may be visible.
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• This peak arises when a K-shell vacancy occurs in
iodine (of the NaI material) following a photoelectric
event. A characteristic 28 keV x-ray is emitted.
Alpha Detectors
• Proportional Detectors: counting with
discrimination from beta-gamma ionizations.
• Scintillation Detector - Zinc Sulphide with
discrimination against beta-gamma ionizations
by pulse height control and by thin detector
efficiency.
ZnS Alpha Detector
• This alpha detector uses
a thin scintillator of zinc
sulphide on thin plastic,
aluminized to keep out
light.
• The detector is
connected to a rate
meter with pulse height
discriminator. It senses
only alpha radiation &
rejects beta and gamma.
Neutron Detectors - Choices
• The dose equivalent detector: a “rem meter”
• Activation foils: cadmium
• Bubble detectors
Neutron Rem Meter
• A gas detection tube
(BF3) is located at the
centre of a polyethylene
sphere with a thin
cadmium filter.
• Sphere moderates
neutrons to permit
detection by BF3 tube
• Energy range 0.025 eV
to 10 MeV
• Gamma radiation is
rejected
TLD Personnel Dosimetry
• Themoluminescent
crystals of LiF are well
suited to personnel
dosimetry.
• Ionizing radiation
creates electron
dislocation that remains
until heated.
• Light output on heating
is proportional to dose.
Choosing a Meter
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Contamination or Radiation?
X-ray, Gamma, Alpha or Neutron?
Energy Dependence?
Response Time: Fast or Slow?
Sensitivity: Low doses or high doses?
Fixed or Portable?
Calibration?
Summary
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Beta- Gamma Gas detectors
Contamination or Radiation
Scintillation Detectors
Analysis of PHA histogram of energy spectrum
Alpha Detectors
Neutron detectors
Personal Dosimetry Methods
Choosing a Meter