Transcript Chapter 3 General Properties of

HP SURVEY INSTRUMENT
CALIBRATION AND SELECTION
CHAPTER 3
General Properties of Radiation Detectors
January 13 – 15, 2016
TECHNICAL MANAGEMENT SERVICES
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Chapter 3 –
General Properties of Radiation Detectors
Gas-Filled Detectors
Ionization Chambers
Gas Proportional Detectors
Geiger-Muller Detectors
Scintillation Detectors
Photomultiplier Tubes and Photodiodes
Semiconductor Detectors
Neutron Detectors
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• Detector - produces an observable signal
when interacting with radiation.
• Sensor – monitors the detector and
converts detector signal to an electrical signal.
Sensor and detector often are same device.
• Electronics assembly – Supplies operating
voltage, processes signal from sensor then
sends to readout unit for display.
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• Readout unit – displays instrument reading
in rate mode (cps, dpm, mrem/h, etc.. ) and/or
scaler mode (counts, mrem, uSv, etc …). Can
be an analog meter and/or digital display. Can
be integrated with probe or a separate device
for use with multiple probes.
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Gas, liquid and solid media are utilized as
radiation detectors
Energy deposited in detector is converted
directly or indirectly to an electrical signal
Instrument calibration converts electrical signal
to a quantity of interest e.g. dose rate
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Basic Electrical Quantities
Current–A measure of the movement or flow of
electrons past a point in a circuit
–1 amp = 6.24 x 1018 electrons/second
=1 coulomb/second
Voltage
–A measure of electrical potential energy force
that causes electron flow
–Measured in volts
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Basic Radiation Measurement System
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ALMOST EVERY TYPE OF RADIATION DETECTOR
RESPONDS TO
ALMOST EVERY TYPE OF RADIATION
THERE ARE NO BAD INSTRUMENTS
ONLY MISUNDERSTOOD INSTRUMENTS
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Ion Chamber in Current Mode
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Ion Chamber in Pulse Mode
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Detector Medium
/ Radiation Type Detected
Ionization Chamber /
Alpha, Beta, Gamma, Neutron, Tritium
LRAD /
Alpha, Beta, Gamma, Neutron, Tritium
Tritium in Air (Ion Chamber) /
Alpha, Beta, Gamma, Neutron, Tritium
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Eberline RO-3
Ion Chamber / Alpha, Beta, Gamma, Neutron
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Eberline RO- 20
Ion Chamber / Alpha, Beta, Gamma, Neutron
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RO-20 Beta Dose Response
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Long Range Alpha Detector – LRAD
Ion Chamber / Alpha, Beta, Gamma, Neutron ??
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The Eberline LRAD (Long Range Alpha Detector)
measures the ion pairs created in air from ionizing
radiation and then displays the result in terms of
activity (dpm).
For a 5 MeV alpha particle which is totally
attenuated within a few centimeters of air, about
150,000 ion pairs are generated.
The LRAD collects the ion pairs generated by
ionizing radiation and uses a current-to-frequency
converter to provide a numerical output which is
converted to dpm.
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LRAD Counting Chamber: 13” H x 22” W x 16” D
Exterior Size:
23” H x 38” W x 20” D
Weight:
180 lbs
Power 120V, 60 Hz
Battery requirements
Electrometer: 5 C-cells, 1,200 hours
Detector/Static Precipitator: 90 V lithium
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Sartrex Tritium in Air Monitor
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The Model 209 is capable of measuring down
to 10μCi/m3. Its single range allows readings
of concentrations up to 20,000μCi/m3.
The detector is an 80 cm3 ionization chamber.
Gamma radiation cancellation is provided by
an adjacent sealed 80 cm3 ionization
chamber.
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QUESTIONS
How many DPM of Tritium does the Model 209
detect ?
What is the current in femto-amps at the stated
detection level of the Model 209 ?
What is the leakage current in femto-amps of the
Model 209 ?
How does this leakage current compare to the other
ion chamber instruments we have discussed ?
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Ion pair production
-Created when ionizing radiation interacts with the
detector gas.
Ion pair collection
-When a voltage potential is established across the
two electrodes the ion pairs will be attracted to the
respective electrode with the opposite charge.
Analysis
-The amount of current flow is representative of the
energy and number of radiation events that caused
ionization. The readout circuitry analyzes this current
and provides an indication of the amount of radiation
that has been detected.
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Specific Ionization in Air at STP
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Gas-Proportional (Sealed) /
Alpha, Beta, Gamma, Neutron
Gas-Proportional (gas flow) /
Alpha, Beta, Gamma, Neutron, Tritium
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Ludlum Air Proportional Alpha Detector
L43-44-1
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WINDOW AREA: 154 cm² (23.9 in²) active;
100 cm² (15.5 in²) open
WINDOW: 0.4 mg/cm² aluminized mylar
OPERATING VOLTAGE: altitude sensitive
sea level: 2050 volts
609.6 m (2000 ft): 2000 volts
1524 m (5000 ft): 1925 volts
2134 m (7000 ft): 1875 volts
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COUNTER THRESHOLD SETTING: 1.5–2.0 mV
WORKING ENVIRONMENT: splashproof shields
and desiccant vented chamber for outdoor use
SIZE: 10.2 x 8.9 x 23.6 cm (4 x 3.5 x 9.3 in.) (H
x W x L)
WEIGHT: 0.7 kg (1.5 lb)
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HV Plateau using 345 Bq Pu-239 Source
Select 1960 as operating voltage.
54 cps with a 345 Bq source = 15.7% 4-pi eff.
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LND Model 431 Sealed Gas Proportional Detector
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GM (thin end window ~ 2.5 mg/cm2)
Alpha, Beta, Gamma, Neutron
GM Detector (> 7 mg/cm2)
Beta, Gamma, Neutron
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LND 7311 GM Detector
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Scintillator (ZnS) /
Alpha, Beta ?, Gamma ?, Neutron ?
Ludlum 43-92 Alpha Detector
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Ludlum 43-5 Alpha Detector
Alpha, Beta ?, Gamma ?, Neutron ?
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Scintillator (ZnS) /
Alpha ?, Beta, Gamma ?
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Scintillator (ZnS) /
Alpha, Beta, Gamma
Berthold LB124-SCINT Alpha and Beta-Gamma
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CROSSTALK
Discrimination
Crosstalk is a phenomenon that occurs on
proportional counting systems that employ electronic,
pulse-height discrimination, thereby allowing the
simultaneous analysis for alpha and beta-gamma
activity.
Discrimination is accomplished by establishing two
thresholds, or windows, which can be set in
accordance with the radiation energies of the
isotopes of concern.
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Recall that the pulses generated by alpha
radiation will be much larger than those
generated by beta or gamma.
This makes the discrimination between alpha
and beta-gamma possible.
This same method of pulse height discrimination
is also applicable to sandwich detectors such as
ZnS on a plastic scintillator.
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Numerous materials scintillate -liquids, solids,
and gases. A material which scintillates is
commonly called a phosphor or a fluor.
The scintillations are commonly detected by a
photomultiplier tube (PMT).
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There are four classes of phosphors of interest in
applications of scintillation:
–Organic crystals
–Organic liquids
–Inorganic crystals
–Inorganic powders
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Organic crystal phosphors are normally aromatic
hydrocarbons that contain benzene rings.
The most common organic crystal is anthracene.
Anthracene offers a high response to beta
radiation and is commonly used in beta
phosphors.
Some investigation has been done in applying
anthracene to a combination neutron/gamma
detector.
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Organic liquid phosphors, called fluors, are
comprised of organic material suspended in an
organic solvent.
The organic liquid phosphor material is called the
solute and is the scintillator.
The solvent absorbs the radiation and transfers
energy to the solute.
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Inorganic crystals are comprised of inorganic salts,
normally halides, which contain small quantities of
impurities, called activators. The most commonly
used inorganic crystal scintillator is sodium iodide,
activated with thallium -NaI(Tl).
NaI(Tl) crystals have a high density -3.7 g/cm3,
which provides good gamma detection efficiency.
NaI(Tl) has a high response to beta particles;
however, the need to hermetically seal a NaI(Tl)
crystal to prevent deterioration, limits the actual
beta response.
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Zinc Sulfide activated with Silver (ZnS(Ag)) is an
inorganic powder which is commonly used as a
phosphor in alpha scintillators.
ZnS(Ag) scintillators have a high density, 4.1
g/cm3, and a relatively high response to beta and
alpha radiation.
The scintillator response to beta and gamma is
typically minimized by the use of ZnS(Ag) as a
thin film which is within the alpha interaction
range, but too thin for that of beta or gamma.
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Scintillator(Plastic) /
Alpha, Beta, Gamma, Neutron
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Scintillator (NaI, CZT, etc.) /
Gamma
Scintillator (LSC) /
Alpha, Beta, Gamma, Neutron, Tritium
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PMTs and Photodiodes / Gamma
The purpose of the photomultiplier tube is to detect
the scintillations and to provide an output signal
proportional to the amount of scintillations. In doing
this, photomultiplier tubes can provide
amplifications of 1E6 and higher.
Construction details vary from design to design,
however, all photomultipliers have typical
components which are the photocathode, dynode
assembly, anode, and voltage divider.
The photocathode is made of a material that
converts the light photons to electrons.
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The PMT itself is a gamma (photon) detector. For
this reason ALL detectors that use a PMT to
detect light from a source such as a scintillator
have a response to external gamma (photon)
radiation.
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A photodiode is a semiconductor device that
converts light into current. The current is generated
when photons are absorbed in the photodiode. The
output current from a photodiode is typically much
less than from a PMT. In some applications a
photodiode can be used in place of a PMT by
optimizing the match between the light output
frequency of the scintillator
and the light frequency
efficiency of the photodiode.
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Semiconductors / Alpha, Beta, Gamma, Neutron
Semiconductor Detectors Semiconductor
detectors are solid-state devices that operate
essentially like ionization chambers. The charge
carriers in semiconductors are not electrons and
ions, as in the gas counters, but electrons and
"holes." Semiconductor detectors are made of
silicon, germanium, CdTe, Hgl, and other
materials.
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ORTEC Silicon Detectors
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Neutron detectors (BF3, 3He3, scintillators) /
Gamma, Neutron
neutron remmeters
Gas proportional counters
3He(n,p)T and 10B(n,α)7Li based instruments
Proton recoil counters
Activated foil-based instruments
Scintillator-based instruments
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10B
Uses BF3 gas usually enriched to ~95% 10B
Limited to gas pressures of about 2atm maximum
Excellent gamma rejection properties
Boron-coated counters also used with a counting
gas
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3He
Often used with high Z gas to promote energy
deposition
High pressure operation possible
Limited availability
Very good gamma discrimination
Chemically inert (unlike BF3)
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NE-213 Liquid Scintillator
Provides a neutron spectrum
Often used with a 10B detector to provide a
thermal neutron response
Good gamma discrimination down to about 2
MeV neutron energy
Light weight package
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Solid Plastic Scintillator
Could provide a neutron spectrum
Often used with a 10B detector to provide a
thermal neutron response
Good gamma discrimination to below1.5 MeV
neutron energy
Light weight package
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Instruments based on thermal neutron capture
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• Energy response of bare counter (10B or
3He) is not sufficient to produce a true rem
response
• Response is highest in region where dose per
neutron is lowest and vice versa
• Solution is to modify the incident spectrum to
improve the instrument response
• Adding a shell of PE gives a more tissue-like
response
• 9” diameter sphere is typically used
• Cd metal or a B-loaded PE internal shell often
used as well
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Pulsed field performance
Accelerator fields are frequently pulsed resulting
in high instantaneous neutron fluence and dose
rates.
Neutron rem meters using PE-moderators are
somewhat immune due to the time required to
thermalize fast neutrons (typically 50-100 s) –
effectively lengthening the pulse.
But there are limitations on the effectiveness of
the PE moderator and other means of neutron
dose measurements are required
E.g. ion chambers in current mode, Boron-lined
chambers or chambers with TE walls and gas
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The HPI 2080 (“Albatross”) is
designed to operate in pulsed (and
static) fields and uses the standard
PE-moderator approach
But uses two GM tubes, one
surrounded by Ag foil the other with
an equivalent mass thickness of tin.
The Ag foil is activated through TNA
Its GM tube counts subsequent beta
decays
The second GM tube compensates for
the gamma background
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Scintillator-based instruments – PRESCILA
Proton Recoil Scintillator Los Alamos
Developed at LANL and commercialized by Ludlum
Utilizes a dual detector approach (like the FujiNSN3)
for fast and thermal neutron detection.
Weight = 6lbs
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Fast detector is 5
EJ-410P scintillators
Recoil protons generated
generated in lucite
excite ZnS(Ag)
Light emission collected
by lucite lightguide and detected by central
PMT
Thermal channel is a 6LiF+ZnS(Ag) scintillator
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Other scintillators
Liquefied rare gases
6Li-based
glasses
Cs2LiYCl6:Ce (CLYC)
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Calibration of neutron rem meters
Require NIST-traceable source or instrument cal.
252Cf or 241AmBe most often used
Need to characterize neutron field (dose rate vs
distance from source)
Measurements and/or Monte Carlo
calculations
Instruments should be calibrated in a field which
gives the most conservative calibration factor
Function of instrument energy response and
the workplace field
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Points to consider in selecting a neutron rem
meter
Energy response
Angular response
Neutron sensitivity
Gamma rejection
Ergonomics
Calibration source
Reliability
Pulsed field performance
Ease of: use, maintenance/repair and
calibration
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ANSI N323AB Test and Calibration, Portable
Survey Instruments
ANSI N323D Installed Radiation Protection
Instrumentation
ANSI N42.33 Portable Radiation Detection
Instrumentation
IEEE N42.35 Evaluation and Performance of
Radiation Detection Portal Monitors
IEEE N42.43 Performance Criteria for Mobile
and Transportable Radiation Monitors
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End of Chapter 3
Questions ?
Comments ?
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