Transcript Gas-Filled Detector

Operator Generic Fundamentals
Components - Sensors and Detectors 2
© Copyright 2016
Operator Generic Fundamentals
2
Terminal Learning Objectives
At the completion of this training session, the trainee will demonstrate
mastery of this topic by passing a written exam with a grade of ≥ 80
percent on the following Terminal Learning Objectives (TLOs):
1. Describe the operation of radiation detectors and conditions
which effect their accuracy and reliability.
2. Describe the operation of personal radiation monitoring
instruments and conditions which effect their accuracy and
reliability.
3. Describe the operation of neutron detectors and conditions which
effect their accuracy and reliability.
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TLOs
Operator Generic Fundamentals
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Radiation Detectors
TLO 1 – Describe the operation of radiation detectors and conditions that
effect their accuracy and reliability.
1.1 Describe the following radiation detection concepts and terms:
a. Electron-ion pair
b. Specific ionization
c. Stopping power
d. Alpha (α)
e. Beta (β)
f. Gamma (γ)
g. Neutron (n)
1.2 Describe the theory of operation of a gas-filled detector to include:
a. How electric field affects ion pairs
b. How gas amplification occurs
c. Name the regions of the gas amplification curve
d. Describe the interactions taking place within the gas of the detector
e. Describe the difference between alpha and beta curves
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TLO 1
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Enabling Learning Objectives for TLO 1
1.3 Describe the operation of a proportional counter to include:
a. Radiation detection
b. Quenching
c. Voltage variations
1.4 Given a block diagram of a proportional counter circuit, state the
purpose of the following major blocks:
a. Proportional counter
b. Preamplifier/amplifier
c. Single channel analyzer/discriminator
d. Scaler
e. Timer
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TLO 1
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Enabling Learning Objectives for TLO 1
1.5 Describe the operation of an ionization chamber to include:
a. Radiation detection
b. Voltage variations
c. Gamma sensitivity reduction
1.6 Describe how a compensated ion chamber compensates for
gamma radiation.
1.7 Describe the operation of a Geiger-Mueller (GM) detector to
include:
a. Radiation detection
b. Quenching
c. Positive ion sheath
1.8 Describe the operation of a scintillation counter to include:
a. Radiation detection
b. Three classes of phosphors
c. Photomultiplier tube operation
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TLO 1
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Radiation Detection Concepts
ELO 1.1 – Describe the following radiation detection concepts and terms:
electron-ion pair, specific ionization, stopping power, alpha (α), beta (β),
gamma (γ), and neutron (n).
• Radiation detectors sense the presence and level of radiation
– Also determine power level of the reactor
• Radiation results from
– Fission
– Activation of particles exposed to the neutron flux of the reactor
• Provides indication, alarms, and input for automatic functions
• Radiation detection is important because of the effect that radiation
has on personnel and equipment
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Radiation Detection Concepts
Electron-Ion Pair
• Ionization is process of converting an atom or molecule into an ion by
adding or removing electrons or other ions
• Results in loss of units of negative charge by affected atom
• Atom becomes electrically positive (a positive ion)
• Products of a single ionizing event are called an electron-ion pair
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Radiation Detection Concepts
Specific Ionization
• Number of ion pairs formed by a given type of radiation as it travels
through matter, dependent on
‒ Mass – the greater the mass, the more interactions per given
distance
‒ Charge – has the greatest effect on specific ionization
o Higher charge increases number of interactions per given
distance
o Increasing number of interactions produces more ion pairs
‒ Energy of the particle – as energy of a particle decreases, it
produces more ion pairs for the same amount of distance traveled
‒ Electron density of matter – increased density increases the
number of interactions
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Radiation Detection Concepts
Stopping Power
• Energy lost per unit path length depends on type and energy of
particle and on properties of material it passes
• The more ion pairs produced per unit of distance travelled
o Quicker the stopping power
– Alphas (++) travel shorter distances than betas (-)
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Radiation Types
Alpha Particle
• Consists of two protons and two neutrons bound together into a
particle identical to a helium nucleus
• Highly ionizing form of particle radiation with low penetration
• Produced from radioactive decay of heavy metals and some nuclear
reactions
• Specific ionization of an alpha particle is high
– Tens of thousands of ion pairs per centimeter in air
• Travels a relatively straight path over a short distance
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Radiation Types
Beta Particle
• Electron or positron ejected from the nucleus of a beta-unstable
radioactive atom
• Single negative or positive electrical charge and a very small mass
• High-energy, high-speed electrons or positrons emitted by certain
types of radioactive nuclei
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Radiation Types
Gamma Ray
• Photon of electromagnetic radiation with a very short wavelength and
high energy
• Emitted from an unstable atomic nucleus with high penetrating power
• Three methods of attenuating gamma-rays:
– Photoelectric effect
– Compton scattering
– Pair production
Radiation Detector Video
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Radiation Types - Gamma Attenuation
Photoelectric Effect
• Occurs when a low energy
gamma strikes an orbital
electron
• Total energy of gamma is
expended in ejecting electron
from its orbit
• Result is ionization of atom and
expulsion of a high-energy
electron
Figure: Photoelectric Effect
• Most predominant with lowenergy gammas
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Radiation Types - Gamma Attenuation
Compton Scattering
• Elastic collision between an
electron and a photon
• Photon has more energy than is
required to eject the electron
from orbit
• All energy from photon cannot
be transferred, photon must be
scattered
Figure: Compton Scattering
• Result is
– a high energy beta
– a gamma of lower energy
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Radiation Types - Gamma Attenuation
Pair Production
• High energy gamma passes
close enough to a heavy
nucleus
• Gamma disappears, energy
reappears in form of electron
and positron
• Transformation of energy into
mass must take place near a
particle to conserve momentum
• Kinetic energy of recoiling
nucleus is very small
– All of photon’s energy in
excess of that needed to
supply mass of pair appears
as kinetic energy of pair
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Figure: Pair Production
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Radiation Types – Neutron
• Have no electrical charge
• Nearly the same mass as a proton
• Hundreds of times larger than an electron, but one quarter the mass
of an alpha particle
• Source is primarily nuclear reactions
– Fission
– Decay of radioactive elements
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Radiation Types – Neutron
Neutron
• Difficult to stop with relatively
high penetrating power
• May collide with nuclei causing
one of the following reactions:
– Inelastic scattering
– Elastic scattering
– Radiative capture or fission
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Radiation Detection Concepts
Knowledge Check
Which one of the following types of radiation will produce the greatest
number of ions while passing through 1 centimeter of air? (Assume the
same kinetic energy for each.)
A. Neutron
B. Gamma
C. Beta
D. Alpha
Correct answer is D.
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Detector Theory of Operation
ELO 1.2 – Describe the theory of operation of a gas-filled detector to
include: how electric field affects ion pairs, how gas amplification occurs,
name the regions of the gas amplification curve, describe the interactions
taking place within the gas of the detector, and describe the difference
between alpha and beta curves.
• Instruments measuring radiation provide a measurement of dose or
dose rate
– Dose is a total accumulated exposure
– Dose rate is the amount of exposure per unit of time
• Radiation detectors detect a specific type(s) or energy range
• Detectors use ionization and electron pairs produced to measure the
radiation energy
• Most detector designs have
– Positive probe – collects electrons (negative ions)
– Negative can – collects positive ion
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Detector Voltage
Applied Voltage
• Pulse height and number of ion pairs collected are directly related
• Ion pairs collected versus applied voltage
– Two curves are shown: one curve for alpha particles and one
curve for beta particles; each curve is divided into several voltage
regions
• Alpha curve is higher than the beta curve from Region I to part of
Region IV due to the larger number of ion pairs produced by the initial
reaction of the incident radiation
• Alpha particle will create more ion pairs than a beta since alpha has a
much greater mass
– Similar concept to neutron pulse greater than gamma pulse
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Detector Voltage
Recombination Region (Region I)
• At voltages less than V1
– Ions move slowly toward
electrodes
– Ions tend to recombine to
form neutral atoms or
molecules
– Not all “primary” ionizations
collected
• Few detectors used in this region
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Figure: Gas Amplification Curve
Operator Generic Fundamentals
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Detector Voltage
Ionization Region (Region II)
• Voltage relatively constant from
V1 to V2
• Voltage high enough to collect
all “primary” ionizations
• Least sensitive region
• Most accurate region
• Used for
– radiation detectors
Figure: Gas Amplification Curve
– Intermediate and Power
Range neutron detectors
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Detector Voltage
Proportional Region (Region III)
• Ion pairs collected increases
linearly as voltage increases
• Increased voltage imparts high
velocity to electrons
– High velocity electrons cause
gas amplification
o Secondary ionizations
• Gas amplification factor is
proportional to applied voltage
• Subject to positive space charge
effect
– Positive ions hang out near
positive probe and capture
negative ions
o Tends to lower signal slightly
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Figure: Gas Amplification Curve
Operator Generic Fundamentals
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Detector Voltage
Limited Proportionality Region
(Region IV)
• Additional secondary
ionizations due to higher
voltage
– Change not proportional to
voltage
– Causes Townsend
avalanche to spread along
anode
Figure: Gas Amplification Curve
– Not used for detector
operation
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Detector Voltage
Geiger-Mueller Region (Region V)
• Pulse height is independent of
type of radiation
• Cannot distinguish between types
of particles
• Most sensitive region
• Least accurate
– Entire can ionizes when
particle is detected
Figure: Gas Amplification Curve
– Some dead time while quench
gas clears detector
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Detector Voltage
Continuous Discharge Region
(Region VI)
• Steady discharge current flows
• Applied voltage is so high that
once ionization takes place in
the gas, there is a continuous
discharge of electricity
• Detector cannot be used for
radiation detection
Figure: Gas Amplification Curve
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Gas-Filled Detector
• Gas-filled cylinder with two electrodes
– Cylinder itself may act as one electrode
– Thin wire along axis of cylinder acts as other electrode
• Gases used since their ionized particles can travel more freely than
those of a liquid or a solid
• Typical gases used are:
– Argon
– Helium
– Boron-tri-fluoride used to measure neutrons
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Gas-Filled Detector
• Central electrode, or anode,
collects negative charges
• Anode is insulated from
chamber walls and cathode
which collects positive charges
C
• Voltage is applied to the anode
and chamber walls
Figure: Gas-Filled Detector Diagram
• Charged particle passing
through gas-filled chamber
– Ionizes some of gas along
its path of travel
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Gas-Filled Detector
• Positive anode attracts
electrons, or negative particles
• Detector wall, or cathode,
attracts the positive charges
C
• Collection of these charges
reduces voltage across
capacitor
Figure: Gas-Filled Detector Diagram
– Causing pulse across
resistor that is recorded by
an electronic circuit
• Voltage applied to anode and
cathode determines electric
field and its strength
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Gas-Filled Detector
• As detector voltage is
increased, electric field has
more influence upon electrons
produced
C
• Sufficient voltage causes a
cascade effect that releases
more electrons from cathode
Figure: Gas-Filled Detector Diagram
• Forces on electron are greater
and its mean-free path between
collisions is reduced at this
threshold
• Total number of electrons
collected by anode determines
change in charge of the
capacitor
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Gas-Filled Detector
• Change in charge is directly
related to total ionizing events
which occur in gas
• Ion pairs initially formed by
incident radiation attain a great
enough velocity to cause
secondary ionization of other
atoms or molecules in gas
C
Figure: Gas-Filled Detector Diagram
• Resultant electrons cause
further ionizations
• Multiplication of electrons is gas
amplification
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Gas-Filled Detector
Proportional Counter – Positive Space Charge
• Gas-filled detectors operating at high voltages within the proportional
region have effect called the positive space charge
– Pulse amplitude from an ionizing event is reduced because
positive ions form a cloud around the positive electrode
o reducing the electric field strength
o limiting secondary ionizations
• Occurs as the detector voltage is increased to the high end of the
proportional region
– Prevents collection of both gamma- and neutron-induced pulses
– Yields less accurate neutron count rate
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Detector Theory of Operation
Knowledge Check
In a gas filled detector, as a __________ passes through the gas-filled
chamber, it __________ some of the gas along its path of travel.
A. charged particle; displaces
B. neutral particle; ionizes
C. charged particle; ionizes
D. neutral particle; displaces
Correct answer is C.
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Proportional Counter Theory
ELO 1.3 - Describe the operation of a proportional counter to include:
Radiation detection, Quenching, Voltage variations.
• Uses a slightly higher voltage
between anode and cathode
• Primary ionizations cause
secondary ionizations
• Electrons move towards positive
anode
• Positive ions move towards
negative cathode (can)
• Each pulse corresponds to one
gamma ray or neutron interaction
• Number of electrons produced is
proportional to energy of incident
particle
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Figure: Proportional Counter
Instrument
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Proportional Counter
• Linear relationship between the number of ion pairs collected and the
applied voltage.
• Can detect:
– alpha, beta, gamma, or neutron radiation in mixed fields
– fill gas will determine what type of radiation will be detected
• Argon and helium are the most frequently used fill gases
– allow for the detection of alpha, beta, and gamma radiation
• When detection of neutrons is necessary
– detectors use boron trifluoride gas
o Source range NI
– Uses pulse height discriminator
• Distinguish between gamma and neutron pulses
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Proportional Counter Theory
Knowledge Check
In a proportional counter, each electron from a primary ion pair
produces a cascade of ion pairs. This effect is known as
__________________.
A. recombination
B. attenuation
C. gas amplification
D. gas quenching
Correct answer is C.
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Detector Theory of Operation
Knowledge Check – NRC Bank
A proportional detector with pulse height discrimination circuitry is being
used in a constant field of neutron and gamma radiation to provide
source range neutron count rate indication. Assume the pulse height
discrimination value does not change.
If the detector voltage is decreased significantly, but maintained within
the proportional region, the detector count rate indication will
__________ and the detector will become __________ susceptible to
the positive space charge effect.
A. decrease; less
B. decrease; more
C. remain the same; less
D. remain the same; more
Correct Answer: A
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Proportional Counter Circuit
ELO 1.4 – Given a block diagram of a proportional counter circuit, state
the purpose of the following major blocks: proportional counter,
preamplifier/amplifier, single channel analyzer/discriminator, scaler, and
timer.
• Proportional counters measure charge produced by each particle of
radiation
• To make full use of the counter’s capabilities, it is necessary to
measure the number of pulses and the charge in each pulse
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Proportional Counter Circuitry
• Capacitor converts charge pulse to a voltage pulse
– Voltage is equal to amount of charge divided by capacitance of
capacitor
• Preamplifier amplifies voltage pulse
• Amplifier circuit further amplifies signal
• Single channel analyzer determines pulse size
Figure: Proportional Counter Circuit
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Proportional Counter
• Output passes to a scaler that counts number of pulses it receives
– A timer gates the scaler so it counts pulses for a predetermined
length of time
• Knowing number of counts per a given time interval allows
calculation of the count rate (number of counts per unit time)
• Proportional counters can also count neutrons by introducing boron
into the chamber
– Most commonly by combining it with tri-fluoride gas to form boron
tri-fluoride
• When a neutron interacts with a boron atom, an alpha particle is
emitted
• Counter can be made sensitive to neutrons and not to gamma rays
with the discriminator
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Proportional Counter
• Gamma rays can be eliminated because neutron-induced alpha
particles produce more ionizations than gamma rays produce
• Gamma ray-induced electrons have a much longer range than
dimensions of the chamber
– Alpha particle energy is greater than gamma rays produced in a
reactor
• Neutron pulses are much larger than gamma ray-produced pulses
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Proportional Counter
• Using a discriminator, the scaler can be set to read only larger pulses
produced by a neutron
• A discriminator is basically a single channel analyzer with only one
setting
Figure: Discriminator Characteristics
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Proportional Counter Circuit
Knowledge Check – NRC Bank
A BF3 proportional counter is being used to measure neutron level during a
reactor startup. Which of the following describes the method used to ensure
that neutron indication is not being affected by gamma reactions in the
detector?
A. Two counters are used, one sensitive to neutron and gamma and the
other sensitive to gamma only. The outputs are electrically opposed to
cancel the gamma-induced currents.
B. The BF3 proportional counter measures neutron flux of sufficient
intensity that the gamma signal is insignificant compared to the neutron
signal.
C. In a proportional counter, gamma-induced pulses are of insufficient
duration to generate a significant log-level amplifier output. Only
neutron pulses have sufficient duration to be counted by the detector
instrumentation.
D. In a proportional counter, neutron-induced pulses are significantly
larger than gamma pulses. The detector instrumentation filters out the
smaller gamma pulses.
Correct answer is D.
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Ionization Chamber
ELO 1.5 – Describe the operation of an ionization chamber to include:
radiation detection, voltage variations, and gamma sensitivity reduction.
• Detect radiation when voltage is adjusted to ionization region
• Charge obtained is result of collecting ions produced by radiation
• This charge will depend on type of radiation being detected
• Two distinct disadvantages compared to proportional counters
– Less sensitive
– Slower response time
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Ionization Chamber
• Flat plates or concentric cylinders may be used in an ionization
chamber
• Flat plates preferred due to:
– Well-defined active volume
– Ensures ions will not collect on insulators causing distortion of
electric field
• Ionization chamber construction allows for integration of pulses
produced by incident radiation
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Ionization Chamber
• Use relatively low voltage between anode and cathode
– Only charges produced in initial ionization event collected
• Weak output signal corresponds to number of ionization events
• Higher energies and intensities of radiation will produce more
ionization
– Results in a stronger output voltage
Figure: Simple Ionization Circuit
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Ionization Chamber
• Beta particles will pass between plates and strike atoms in air
• Sufficient energy beta particles cause an electron ejection from air
– A beta particle may eject 40 to 50 electrons for each cm traveled
• Ejected electrons often have enough energy to eject more electrons
from the air
• Total number of electrons produced is dependent
– Energy of beta particle
– Energy of gas between plates
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Ionization Chamber
• Can be used to detect gamma rays
• Ammeter only sensitive to electrons; gamma rays must interact with
the atoms in air between the plates to release electrons
– Compton scattering
– Photoelectric effect
– Pair production
• Energy of incident gamma converted into kinetic energy of ejected
electrons
• Ejected electrons move at very high speeds and cause other
electrons to be ejected from their atoms
• Electrons collected by positively charged plate and measured by
ammeter
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Ionization Chamber
• Can be used to detect neutrons
– Neutrons have no charge, therefore cause no ionizations
• Inner surface of ionization chamber is covered with a thin coat of
boron
• The following reaction can takes place:
10
5
B  01n37Li     24He    5e 
• Neutron is captured by a boron atom and an energetic alpha particle
is emitted
• Detectors in this region are not affected by small changes in voltage
– Constant output for varying voltage
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Ionization Chamber
• Neutrons may also be detected using gas boron tri-fluoride (BF3)
instead of air in the ion chamber
• Neutrons react with boron to produce alpha particles
• When detecting neutrons
– Beta particles shielded by detector walls
– Gamma rays cannot be shielded
o Discrimination can eliminate gamma
o Reducing sensitive volume of chamber without reducing boron
coated area, reduces sensitivity to gammas
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Compensated Ion Chamber
ELO 1.6 – Describe how a compensated ion chamber compensates for
gamma radiation.
• Consists of two separate chambers
– One chamber is coated with boron
– Other chamber is not coated
• Coated chamber is sensitive to both gamma rays and neutrons
• Uncoated chamber is sensitive only to gamma rays
• Net output of both detectors is read on a single ammeter
– Polarities arranged so currents oppose one another
– Reading indicates difference between the two currents
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Compensated Ion Chamber
Figure: Compensated Ion Chamber
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Compensated Ion Chamber
• Boron coated chamber is the working chamber
• Uncoated chamber is the compensating chamber
• When exposed to a gamma source:
– Working chamber battery sets up current flow that deflects meter
in one direction
– Compensating chamber battery sets up current flow that deflects
meter in opposite direction
– Compensating chamber cancels current due to gamma rays
• Compensation required for Intermediate Range NIs
– Neutron population relatively low
• When operating in Power range
– Neutron to gamma ratio so high, no compensation required
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Gamma Compensation
Knowledge Check
Compensated ionization chambers consist of two separate chambers;
one chamber is coated with boron, and one chamber is not. The
___________ chamber is sensitive to both gamma rays and neutrons,
while the ___________ chamber is sensitive only to gamma rays.
A. coated; uncoated
B. compensated; uncoated
C. uncoated; coated
D. compensated; coated
Correct answer is A.
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Geiger-Mueller Detector
ELO 1.7 - Describe the operation of a Geiger-Mueller (GM) detector to
include: radiation detection, quenching, and positive ion sheath.
• GM detectors produce larger pulses than other types of detectors
• Discrimination is not possible
– Pulse height is independent of the type of radiation
• Geiger-Mueller region has two important characteristics:
– Number of electrons produced is independent of applied voltage
– Number of electrons produced is independent of the number of
electrons produced by the initial radiation
• Radiation producing one electron will have same size pulse as
radiation producing hundreds or thousands of electrons
– Reason for this characteristic is related to the way in which
electrons are collected
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Geiger-Mueller Detector
• Gamma produces an electron
• Electron moves rapidly toward
positively charged central wire
– As electron nears wire, its
velocity increases
C
• Velocity great enough to cause
secondary ionizations
– Larger pulse
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Figure: GM Detector
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Geiger-Mueller Detector
• As applied voltage is increased,
number of positive ions near
central wire increases
– Positively charged cloud
(called a positive ion sheath)
forms around central wire
• Positive ion sheath reduces
field strength of central wire,
preventing further electrons
from reaching wire
C
Figure: GM Detector
• Positive ion sheath makes the
central wire appear much
thicker and reduces field
strength
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Geiger-Mueller Detector
• Phenomenon is the detector’s
space charge
• Positive ions migrate toward
negative chamber picking up
electrons
• As in a proportional counter,
transfer of electrons can release
energy
– Causing ionization and
liberation of an electron
• To prevent secondary pulse,
quenching gas is used
– Usually an organic
compound
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ELO 1.7
C
Figure: GM Detector
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Geiger-Mueller Detector Summary
• GM counter produces many more electrons than proportional counter
– More sensitive device
• Often used to detect low-level gamma rays and beta particles
• Electrons produced collected rapidly, usually within fraction of
microsecond
• Output is pulse charge large enough to drive meter without
amplification
• Cannot distinguish radiation of different energies or types
– Same size pulse is produced regardless of amount of initial
ionization
– Not adaptable for neutron detectors
• Used for portable instrumentation due to:
– Sensitivity
– Simple counting circuit
– Ability to detect low-level radiation
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Geiger-Mueller Detector
Knowledge Check – NRC Bank
Which one of the following describes the reason for the high sensitivity
of a Geiger-Mueller tube radiation detector?
A.
Changes in applied detector voltage have little effect on
detector output.
B.
Geiger-Mueller tubes are thinner than other radiation detector
types.
C.
Any incident radiation event causing primary ionization results
in ionization of the entire detector gas volume.
D.
Geiger-Mueller tubes are operated at relatively low detector
voltages, allowing detection of low energy radiation.
Correct answer is C.
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Scintillation Detector
ELO 1.8 – Describe the operation of a scintillation detector.
• Uses a scintillation crystal (phosphor) to detect radiation and produce
light pulses
– Process called “luminescence”
• Radiation interacts in scintillation crystal
– energy transferred to bound electrons of the crystal’s atoms
• If a photon or beta particle hits the crystal, it produces visible light
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Scintillation Detector
• A photomultiplier tube senses flashes
– converts them into an electrical signal
• Constructed by coupling a suitable scintillation phosphor to a lightsensitive photomultiplier tube
Figure: Scintillation Detector
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Scintillation Detector
• Photomultiplier is a vacuum tube
containing
– A photocathode
– Series of electrodes called
dynodes
• Light from a scintillation phosphor
liberates electrons
• Photoelectrons strike first dynode
and liberate several new
electrons
• Second-generation electrons are
attracted to second dynode
• Amplification continues through
10 to 12 stages
– Large enough pulse to
measure
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ELO 1.8
Figure: Photomultiplier
Operator Generic Fundamentals
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Scintillation Detector
• Scintillation detector advantages
– Efficiency
– High precision
– High counting rates
• Can be used to determine the energy, as well as the number of the
exciting particles (or gamma photons)
• Photomultiplier tube output is very useful in radiation spectrometry
– Determination of incident radiation energy levels
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ELO 1.8
Operator Generic Fundamentals
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Scintillation Detector
Knowledge Check – NRC bank
Scintillation detectors convert radiation energy into light by a process
known as...
A. gas amplification.
B. space charge effect.
C. luminescence.
D. photoionization.
Correct answer is C.
© Copyright 2016
ELO 1.8
Operator Generic Fundamentals
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Scintillation Detector
Knowledge Check – NRC Bank
Which one of the following contains the pair of radiation detector types
that are the most sensitive to low-energy beta and/or gamma radiation?
A. Geiger-Mueller and scintillation
B. Geiger-Mueller and ion chamber
C. Ion chamber and scintillation
D. Ion chamber and proportional
Correct answer is A.
© Copyright 2016
ELO 1.8
Operator Generic Fundamentals
67
Personnel Radiation Monitoring
TLO 2 – Describe the operation of personal radiation monitoring
instruments and conditions which effect their accuracy and reliability.
2.1 Describe the use of portable personnel radiation monitoring
instruments.
2.2 Describe the operation of the following personnel radiation
detection devices, including advantages and disadvantages of
each:
a. Thermoluminescent dosimeter (TLD)
b. Self-reading pocket dosimeter (SRPD)
c.
Electronic dosimeter
d. Film badge
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TLO 2
Operator Generic Fundamentals
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Portable Radiation Monitoring
ELO 2.1 – Describe the use of portable personnel radiation monitoring
instruments.
• Ensure you are qualified to use portable personnel monitoring
instruments
• Prior to using, verify it is working properly
• Each survey instrument is required to be calibrated
– Check calibration due date on the instrument calibration sticker
– If calibration has expired
o inform health physics personnel and/or shift supervisor
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Portable Detector Use
• Instrument should be visually inspected for damage or defects
– Cords should be in good shape, not kinked or frayed
– Probe should be intact
– Indicating scale indicates a reasonable background reading
• Battery strength should be verified high enough for proper operation
– Accomplished by placing the meter in the battery check position
– If battery check not satisfactory, meter should not be used
– Battery check not required if instrument connected to AC power
source
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Portable Detector Use
• Verify source check current
– Most meters are source checked by health physics
o ensures proper operation and indication within a specified
range
– A sticker on the detector may indicate the last source check
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ELO 2.1
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Dosimetry and Types of Radiation
Detected
ELO 2.2 – Describe the operation of the following personnel radiation
detection devices, including advantages and disadvantages of each:
thermoluminescent dosimeter, self-reading pocket dosimeter, electronic
dosimeter, and film badge.
• Thermoluminescent dosimeter
– detects beta and gamma radiation accumulated doses
• Self-reading pocket dosimeter
– detects gamma radiation
• Electronic dosimeter
– detects gamma and x-ray radiation
• Film badges
– measures and records gamma rays, x-rays, and beta particles
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
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Thermoluminescent Dosimeter
• Two types of dosimeter of legal record (DLR) currently used
– Thermoluminescent dosimeter (TLD)
– Optically stimulated luminescent dosimeter (OSLD)
• Measures ionizing radiation exposure by
– Heating a crystal in the detector
– Measuring the amount of visible light emitted from the crystal
• Amount of light emitted is dependent upon the radiation exposure
• Calcium fluoride crystal records gamma exposure
• Lithium fluoride crystal records gamma and neutron exposure
Figure: Typical TLD
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Operator Generic Fundamentals
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Thermoluminescent Dosimeter
• Radiation interacts with crystal
• Electrons in the crystal's atoms to jump to higher energy states
• Electrons trapped due to impurities in crystal
– Usually manganese or magnesium
• When crystal is heated, electrons give up stored energy
– Trapped electrons to drop back to their ground state
• The energy is released in a photon
– Equal to energy difference between trap state and ground state
• Released light is counted using photomultiplier tubes
– Number of photons counted is proportional to quantity (energy) of
radiation
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Thermoluminescent Dosimeter
• Used for
– Environmental monitoring
– Personnel exposure
monitoring
• Often worn for a period of time
(3 months or less)
• Worn in chest area on trunk of
the “whole body” for the body
dose
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Thermoluminescent Dosimeter
• Advantages
– Linearity of response to dose
– Relative energy independence
– Sensitivity to low doses
– Reusable
• Disadvantages
– No permanent record or re-readability is provided
– Immediate on spot reading not possible
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Self-Reading Pocket Dosimeter
• Measures dose, not dose rate
• Usually worn in conjunction with (and near) a TLD or OSLD
– Chest area to measure whole body dose
• Contains a small ionization chamber
– Central wire anode in ionization chamber
– Moveable quartz fiber attached to wire anode
• Anode charged to a positive potential
– Charge is distributed between the wire anode and quartz fiber
– Electrostatic repulsion deflects quartz fiber
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Self-Reading Pocket Dosimeter
• Gamma radiation in chamber produces ionization
– Alpha and beta particles cannot pass through metal casing
• Positively charged central anode attracts electrons produced by
ionization
• Electrons reduce net positive charge
• Moveable quartz fiber moves toward original position
– Movement is directly proportional to amount of ionization
Figure: Direct Reading Dosimeter and Charger
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Self-Reading Pocket Dosimeter
• Point a light source to read position of the fiber
• Fiber is viewed on a translucent scale
• Reads up to 200 milliroentgens
– Some designs higher scale
Figure: Pocket Dosimeter Internals
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Operator Generic Fundamentals
79
Self-Reading Pocket Dosimeter
• Advantages
– Provides immediate reading of radiation accumulated dose
– Reusable
• Disadvantages
– Limited range
– Does not provide a permanent record
– Can be discharged by dropping or bumping
– Charge leakage, or drift, can also affect accuracy
– If dropped, exit area immediately and inform health physics
o Recall last good reading and time
– Must be charged with DC voltage to “zero” the device prior to use
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
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Self-Reading Pocket Dosimeter
Knowledge Check – NRC Bank
A nuclear plant worker normally wears a thermoluminescent dosimeter
(TLD) or similar device for measuring radiation exposure. When a selfreading pocket dosimeter (SRPD) is also required, where will the SRPD
be worn and why?
A. Below the waist near the TLD to measure radiation from the
same source(s).
B. Below the waist away from the TLD to measure radiation from
different sources.
C. Above the waist near the TLD to measure radiation from the
same source(s).
D. Above the waist away from the TLD to measure radiation from
different sources.
Correct answer is C.
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
81
Self-Reading Pocket Dosimeter
Knowledge Check – NRC Bank
Which one of the following types of radiation is the major contributor to
the dose indication on a self-reading pocket dosimeter?
A. Alpha
B. Beta
C. Gamma
D. Neutron
Correct answer is C.
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
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Electronic Dosimeter
• Another type of pocket dosimeter (replaced SRPDs)
• Records dose information and dose rate
• Constructed using a Geiger-Mueller counter that measures
– Gamma
– X-ray
• Digital counter displays
– Accumulated exposure
– Dose rate
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Operator Generic Fundamentals
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Electronic Dosimeter
• Can include an audible alarm feature
• Can provide a continuous audible signal when a preset exposure has
been reached
• Allows higher maximum readout before resetting is necessary
• Advantages
– Minimizes reading errors associated with direct reading pocket
dosimeters
– Reliability
– Ability to indicate accumulated dose as well as dose rate
– Audible response
• Disadvantages
– Much higher cost than SRPDs
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Operator Generic Fundamentals
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Film Badges
• Film badges no longer commonly
used for personnel monitoring
• Used to measure and record
radiation exposure or
accumulated dose due to:
– Gamma rays
– X-rays
– Beta
• Contains a piece of radiationsensitive film
• Film packaged in a light proof,
vapor proof envelope
– Prevents light, moisture or
chemical vapors from
affecting film
© Copyright 2016
ELO 2.2
Figure: Film Badge
Operator Generic Fundamentals
85
Film Badges
• Special film used which is coated with two different emulsions
– One side coated with a large grain fast emulsion
o Sensitive to low levels of exposure
– Other side coated with a fine grain slow emulsion
o Less sensitive to exposure
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Film Badges
Advantages
• Provides a permanent record
• Able to distinguish between different energies of photons and can
measure doses due to different types of radiation
• Accurate for exposure greater than 100 millirem
Disadvantages
• Third party must develop it because a processor must read it
• Prolonged heat exposure can affect the film
• Exposures of less than 20 millirem of gamma radiation cannot be
accurately measured
• Can be very costly and inaccurate at lower doses
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
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Nuclear Instrument Detectors
TLO 3 – Describe the operation of neutron and failed fuel detectors and
conditions which effect their accuracy and reliability.
3.1 Describe the purpose and operation of the following nuclear
instruments:
a. Source Range
b. Intermediate Range
c. Power Range
d. Fission chamber
3.2 State the effect core voiding, core loading pattern, and
environmental effects could have on neutron detection and power
indication.
3.3 Describe the theory and operation of failed fuel detectors.
© Copyright 2016
TLO 3
Operator Generic Fundamentals
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Nuclear Instrument Detectors
ELO 3.1 – Describe the purpose and operation of the following nuclear
instruments: Source Range, Intermediate Range, Power Range, Fission
chamber.
• A PWR generally has two types of neutron detection systems
installed:
– Out-of-core (Excore) NIs – used for reactor power monitoring and
reactor protection and control
o Source, Intermediate, and Power ranges
o Detect neutron leakage
o Secondary calorimetric used to calibrate from BOC to EOC
– In-core (Incore) NIs – used for flux mapping to determine hot
channel factors
o May be movable or stationary
o Fission chamber
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Nuclear Instrument Detectors
• Three ranges are used to monitor the power level of a reactor
throughout the full range of reactor operation
– Source range
– Intermediate range
– Power range
• Source range normally uses a proportional counter
• Intermediate and power ranges use ionization chambers
– Compensated ion chamber for intermediate range
– Uncompensated ion chamber for power range
o Neutron to gamma ratio so high that gammas are insignificant
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Operator Generic Fundamentals
90
Nuclear Instrument Detectors
• Ranges overlap
– Proper overlap monitored as
power increases
• Source Range
– Counts Per Second (CPS)
• Intermediate Range
– Amps (or percent power)
• Power Range
– Percent power
Figure: Three Range Overlap
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Operator Generic Fundamentals
91
Nuclear Instrument Detectors - SR
• Monitor/indicate when reactor is shutdown and initial phase of reactor
startup
– Neutron flux level
– Rate of change of neutron flux level
• Normally consists of two redundant count rate channels
– Composed of a high-sensitivity proportional counter
– Associated signal measuring equipment
• Typically used over a counting range of 0.1 to 106 counts per second
• Output displayed on meters in terms of the logarithm of the count rate
– Measures rate of change of the count rate as well
o Startup rate or reactor period
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Operator Generic Fundamentals
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Nuclear Instrument Detectors - SR
• Protective functions and interlocks
– Some plants have High Flux Trip on SR counts
o Allows blocking High Flux Trip on normal reactor startup
– When blocked might de-energize high voltage to detector
• extend detector life
© Copyright 2016
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Operator Generic Fundamentals
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Nuclear Instrument Detectors - SR
• B10 lined or BF3 gas-filled proportional counters are normally used as
source range detectors
• Proportional counter output is in form of one pulse for every ionizing
event
• Series of random pulses varying in magnitude representing neutron
and gamma ionizing events
Figure: Source Range Channel
© Copyright 2016
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Operator Generic Fundamentals
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Nuclear Instrument Detectors - SR
• Pulse height may only be a few millivolts
– Too low to be directly used without amplification
• Linear amplifier
– amplifies input signal by a factor of several thousand
• Discriminator
– excludes passage of pulses that are < a certain level
– excludes noise and gamma pulses
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Operator Generic Fundamentals
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Nuclear Instrument Detectors - IR
• Usually two redundant channels
– Each channel made up of
o Boron-lined or boron gas-filled compensated ion chamber
• Compensated ion chamber (CIC) detector
– Compensates for signals from gamma flux
• Provides a rate of change measure of neutron level
– Displayed in terms of startup rate in DPM
• High startup rate on either channel may initiate a protective action
– May be a control rod withdrawal inhibit and alarm or a high
startup rate reactor trip
© Copyright 2016
ELO 3.1
Operator Generic Fundamentals
96
Nuclear Instrument Detectors - IR
Figure: Intermediate Range Detector
© Copyright 2016
ELO 3.1
Operator Generic Fundamentals
97
Nuclear Instrument Detectors - IR
• Compensated ion chamber
output is an analog current
ranging from 10-11 to 10-3
amperes
– Instead of AMPS, can also
be calibrated to read %
power
• Log n amplifier is a
logarithmic current amplifier
that converts the detector
output to a signal proportional
to the logarithm of the
detector current
• Logarithmic output is
proportional to the logarithm
of the neutron level
© Copyright 2016
Figure: Intermediate Range Detector
ELO 3.1
Operator Generic Fundamentals
98
Nuclear Instrument Detectors - IR
• Differentiator measures rate change of logarithm of neutron level
– Measures reactor period or startup rate
• Startup rate in intermediate range is more stable because neutron
level signal is subject to less sudden large variations
– Often used as an input to the reactor protection system
• Reactor protective interface provides signals for protective actions
– Control rod withdrawal interlocks
– Startup rate reactor trips
© Copyright 2016
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Operator Generic Fundamentals
99
Nuclear Instrument Detectors - PR
• Normally consists of four identical linear power level channels which
originate in eight uncompensated ion chambers
– Usually 4 Upper and 4 Lower detectors
• Uncompensated ion chambers used because gamma compensation
is unnecessary
– Neutron-to-gamma flux ratio is high
– Number of gammas is insignificant compared to number of
neutrons
• Output of each channel is directly proportional to reactor power
• Typically covers a range from 0–125% of full power
• Output displayed on meter in terms of power level in percent of full
rated power
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Operator Generic Fundamentals
100
Nuclear Instrument Detectors - PR
• Gain of each instrument is adjustable, which provides a means for
calibrating the output
– Adjustments normally determined by using a plant heat balance
• Protective actions may be initiated by high power level on any two
channels
• Two detectors in each channel are functionally connected in parallel,
so measured signal is sum of the two detectors
• Output drives linear amplifier which amplifies signal to useful level
Figure: Power Range Channel
© Copyright 2016
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Operator Generic Fundamentals
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Nuclear Instrument Detectors - PR
• Reactor protective interface provides signals for protective actions
• Protective action signals provided
– Signal to reactor protection system at a selected value (normally
10% reactor power) to disable the high startup rate reactor trip
– Signal to protective systems when reactor power level exceeds
predetermined values
– Signal for use in the reactor control system
– Signal to the power-to-flow circuit
© Copyright 2016
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Operator Generic Fundamentals
102
Nuclear Instrument Detectors – Fission
Chamber
• Usually used for In-Core detector system
– May be moveable or fixed in-cores
o Moveable detectors take “snapshots” at various core heights
– Flux mapping
– Ensures core within thermal design limits (hot spot)
• Detector coated with highly enriched U-235 and an argon gas
– Neutrons in the core cause fissions in the detector
– Fission fragments ionize the argon gas
– Ionizations provide distinguishable pulses
© Copyright 2016
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Operator Generic Fundamentals
103
Core Voiding and Loading Effects
ELO 3.2 – State the effect core voiding, core loading pattern, and
environmental effects could have on neutron detection and power
indication.
• In PWR, neutron instrumentation used for power monitoring is
located external to core
– measures neutron leakage from core
• Under normal operations
– power level, coolant temperature, boron concentration, and core
enrichment loading can affect leakage
• If the flux at the core edge is affected
– neutron instrumentation may also be affected
• Voiding in the core will also impact reading
© Copyright 2016
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Operator Generic Fundamentals
104
Core Voiding
• If a nuclear reactor experiences a loss of coolant accident
– Less moderator to moderate neutrons
– Neutrons travel further
– More neutrons will leak out
– This loss of moderation also adds negative reactivity
o reduction in Keff
• Even though Keff is decreasing a higher percentage of neutrons are
leaking out
– SR NI readings would initially increase
• Eventually Keff decreases so far that SR counts start decreasing
© Copyright 2016
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Operator Generic Fundamentals
105
Core Voiding – Counts vs Voids
• As voiding increases from 0% to 100%
– Counts increase, then decrease
• As voiding decreases from 100% to 0%
– Counts still increase, then decrease
100
% void
0
Low
© Copyright 2016
Counts
ELO 3.2
High
Operator Generic Fundamentals
106
Core Voiding
• If core is refilled after voiding:
– Neutron count rate will initially increase
o more neutrons become available for fission due to restored
moderator
• As refilling continues
– neutron count rate will decrease due to more neutrons being
reflected into core
o Less leaking from core to interact with detectors
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
107
Core Voiding and Loading Effects
Knowledge Check – NRC Bank
A reactor is shut down at 100 cps in the source range when a loss of
coolant accident occurs.
Assuming the source neutron production rate remains constant, how
and why will excore source range detector outputs change as
homogeneous core voiding increases from 20 percent to 40 percent?
A. Increases, because more neutron leakage is occurring.
B. Decreases, because less neutron leakage is occurring.
C. Increases, because Keff is increasing.
D. Decreases, because Keff is decreasing.
Correct answer is A.
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
108
Failed Fuel Detectors
ELO 3.3 – Describe the theory and operation of failed fuel detectors.
• Proportional counters and/or fission chambers, located in RCS
letdown flow
• Determine the presence of
– fission product activity
– delayed neutrons
o Both can be indicative of failed fuel
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
109
Failed Fuel Detectors
• Examples of Failed-Fuel Detector Systems are:
– Gross Failed-Fuel Detector System
o Located in RCS hot leg and has about a 60-second delay from
core to detector
– Letdown Monitors
o could be isolated on low PZR level
• Types of activity monitored include gross gamma activity
– Cs-137 and iodine activity based upon dose equivalent I-131
– Basically, ANY fission product gases
o Recall fission product yield curve
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
110
Failed Fuel Detectors
Knowledge Check
During reactor power operation, a reactor coolant sample is taken and
analyzed. Which one of the following lists three radionuclides that are
all indicative of a fuel cladding failure if detected in elevated
concentrations in the reactor coolant sample?
A. Lithium-6, cobalt-60, and argon-41
B. Iodine-131, cesium-138, and strontium-89
C. Nitrogen-16, xenon-135, and manganese-56
D. Hydrogen-2, hydrogen-3, and oxygen-18
Correct answer is B.
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
111
NRC KA to ELO Tie
KA #
KA Statement
RO SRO
ELO
K1.17 Effects of core voiding on neutron detection
Theory and operation of ion chambers, G-M tubes and scintillation
K1.18 detectors
3.3
3.5
2.6
2.8
3.2
1.2, 1.7,
1.8
K1.19 Use of portable and personal radiation monitoring instruments
3.1
3.3
2.1, 2.2
K1.20 Theory and operation of failed-fuel detectors
2.5
2.7
3.3
© Copyright 2016
Operator Generic Fundamentals