Operator Generic Fundamentals Components - Sensors and Detectors 2

Operator Generic Fundamentals Components - Sensors and Detectors 2

Sensors and Detectors Introduction
Part 2 of sensor and detectors focuses on personnel and process radiation monitoring, as well as nuclear instrument detectors used for monitoring reactor power level. Operator and automatic actions rely on accurate information provided by sensors and detectors Operators use sensors and detectors to Monitor key parameters that can affect plant operation and public safety Analyze the parameters for trends and abnormal conditions Sensors, detectors, and their associated circuitry measure and indicate different kinds of parameters Including radiation and reactor power level Related KAs Sensors and Detectors (CFR 41.7) Nuclear Instrumentation K1.17 Effects of core voiding on neutron detection Portable and Personal Radiation Detection K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors K1.19 Use of portable and personal radiation monitoring instruments K1.20 Theory and operation of failed-fuel detectors Intro

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): Describe the operation of radiation detectors and conditions which effect their accuracy and reliability. Describe the operation of personal radiation monitoring instruments and conditions which effect their accuracy and reliability. Describe the operation of neutron detectors and conditions which effect their accuracy and reliability. Intro

Radiation Detectors TLO 1 – Describe the operation of radiation detectors and conditions that effect their accuracy and reliability. 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 It is important to have an understanding of how these sensors and detectors measure plant parameters and how they are prone to failure. Recognizing the indications associated with failed sensors and detectors is an essential skill for plant operators. Familiarity with instrument failure modes will ensure proper interpretation of plant parameters during abnormal operating events, allowing operators to take appropriate mitigating actions. TLO 1

Enabling Learning Objectives for TLO 1
Describe the following radiation detection concepts and terms: Electron-ion pair Specific ionization Stopping power Alpha (α) Beta (β) Gamma (γ) Neutron (n) 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 Describe the difference between alpha and beta curves TLO 1

Describe the operation of a proportional counter to include: Radiation detection Quenching Voltage variations 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 Timer TLO 1

Describe the operation of an ionization chamber to include: Radiation detection Voltage variations Gamma sensitivity reduction Describe how a compensated ion chamber compensates for gamma radiation. Describe the operation of a Geiger-Mueller (GM) detector to include: Quenching Positive ion sheath Describe the operation of a scintillation counter to include: Three classes of phosphors Photomultiplier tube operation TLO 1

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). 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 Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.1

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 of a particle, the more interactions it produces in a given distance Charge – has the greatest effect on specific ionization Higher charge increases number of interactions which occur in a given distance 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 produces more ion pairs ELO 1.1

Stopping Power Energy lost per unit path length depends on type and energy of particle and on properties of material it passes Or linear energy transfer (LET) ELO 1.1

Radiation Primer Video
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 Link to Video: Play video and request feedback at the end of the video, hold questions until the end of the ELO. Radiation Primer Video ELO 1.1

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 ELO 1.1

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 ELO 1.1

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 Most predominant with low- energy gammas Figure: Photoelectric Effect ELO 1.1

Compton Scattering Elastic collision between an electron and a photon Photon has more energy than is required to eject the electron from orbit, or it cannot give up all of its energy in a collision with a free electron All energy from photon cannot be transferred, photon must be scattered Result is ionization of the atom, a high energy beta, and a gamma at a lower energy level than the original Figure: Compton Scattering ELO 1.1

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 Figure: Pair Production ELO 1.1

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 size of an alpha particle Source is primarily nuclear reactions Fission Decay of radioactive elements ELO 1.1

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 ELO 1.1

Inelastic scattering causes some of the neutron’s kinetic energy to be transferred to target nucleus in form of kinetic energy and some internal energy This transfer of energy slows neutron, but leaves the nucleus in an excited state Excitation energy is emitted as a gamma ray photon Interaction between neutron and nucleus is best described by the compound nucleus mode Neutron is captured Re-emitted from the nucleus along with a gamma ray photon Re-emission is considered threshold phenomenon ELO 1.1

Elastic scattering is most likely interaction between fast neutrons and low atomic mass number absorbers Sometimes referred to as the "billiard ball effect" The neutron shares its kinetic energy with the target nucleus without exciting the nucleus Radiative capture takes place when a neutron is absorbed to produce an excited nucleus Excited nucleus regains stability by emitting a gamma ray ELO 1.1

Fission process for uranium (U235 or U238) is a nuclear reaction A neutron is absorbed by the uranium nucleus to form the intermediate (compound) uranium nucleus (U236 or U239) Compound nucleus fissions into two nuclei (fission fragments) with the simultaneous emission of one to several neutrons Fission fragments produced have a combined kinetic energy of about 168 MeV for U235 and 200 MeV for U238, which is dissipated, causing ionization Fission reaction can occur with either fast or thermal neutrons ELO 1.1

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.) Neutron Gamma Beta Alpha Correct answer in D. Correct answer is D. ELO 1.1

Detector Theory of Operation
ELO 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 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 Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.2

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 Difference in mass is negated once detector voltage is increased to Region IV since the detector completely discharges with each initiating event ELO 1.2

Detector Voltage Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Recombination Region (Region I)
Voltage increases to V1 Pulse height increases until saturation value At V1, field strength between cathode and anode is sufficient for collection of all ions produced within detector At voltages less than V1 Ions move slowly toward electrodes Ions tend to recombine to form neutral atoms or molecules Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Ionization Region (Region II) Voltage increased
No appreciable increase in pulse height Field strength more than adequate to ensure collection of all ions produced Figure: Gas Amplification Curve ELO 1.2

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 Townsend avalanche (electron burst) Gas amplification factor is proportional to applied voltage Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Limited Proportionality Region (Region IV)
As voltage increases, additional processes occur leading to increased ionization Strong field causes increased electron velocity Results in excited states of higher energies capable of releasing more electrons from cathode Causes Townsend avalanche to spread along anode Positive ions remain near where they were originated and reduce electric field to where further avalanches are impossible Not used for detector operation Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Geiger-Mueller Region (Region V)
Pulse height is independent of type of radiation Pulse height obtained is several volts Field strength is so great that the discharge continues to spread until amplification cannot occur Due to a dense positive ion sheath surrounding the central wire (anode) Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Geiger-Mueller Region (Region V)
V4 is the threshold voltage Where number of ion pairs level off and remain relatively independent of the applied voltage This is called the Geiger plateau which extends over a region of 200 to 300 volts Threshold normally ~1000 volts Gas amplification factor depends on specific ionization of radiation to be detected Figure: Gas Amplification Curve ELO 1.2

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 ELO 1.2

Gas-Filled Detector Gas-filled cylinder with two electrodes
Cylinder itself may act as one electrode Thin taut 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 ELO 1.2

Gas-Filled Detector Central electrode, or anode, collects negative charges Anode is insulated from chamber walls and cathode which collects positive charges Voltage is applied to the anode and chamber walls Resistor in circuit is shunted by a capacitor in parallel Anode is at a positive voltage with respect to detector wall Figure: Gas-Filled Detector Diagram ELO 1.2

Gas-Filled Detector Charged particle passing through gas-filled chamber Ionizes some of gas along its path of travel Positive anode attracts electrons, or negative particles Detector wall, or cathode, attracts the positive charges Collection of these charges reduces voltage across capacitor Causing pulse across resistor that is recorded by an electronic circuit Voltage applied to anode and cathode determines electric field and its strength Figure: Gas-Filled Detector Diagram ELO 1.2

Gas-Filled Detector As detector voltage is increased, electric field has more influence upon electrons produced Sufficient voltage causes a cascade effect that releases more electrons from cathode 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 Figure: Gas-Filled Detector Diagram ELO 1.2

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 Resultant electrons cause further ionizations Multiplication of electrons is gas amplification Figure: Gas-Filled Detector Diagram ELO 1.2

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, reducing the electric field strength, thereby 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 ELO 1.2

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. charged particle; displaces neutral particle; ionizes charged particle; ionizes neutral particle; displaces Correct answer in C. Correct answer is C. ELO 1.2

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. decrease; less decrease; more remain the same; less remain the same; more Correct answer in A. Question P5606 in bank. Correct answer is A. ELO 1.2

Proportional Counter Theory
ELO Describe the operation of a proportional counter to include: a. Radiation detection b. Quenching c. Voltage variations Uses a slightly higher voltage between anode and cathode Charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas Electrons produced in these secondary ion pairs, along with the primary electrons, continue to gain energy as they move towards anode Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors Figure: Proportional Counter ELO 1.3

Proportional Counter The number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle Each pulse corresponds to one gamma ray or neutron interaction Number of electrons produced is proportional to energy of incident particle Figure: Proportional Counter ELO 1.3

Proportional Counter Linear relationship between the number of ion pairs collected and the applied voltage. Charge amplification of 104 is possible in the proportional region Can detect alpha, beta, gamma, or neutron radiation in mixed radiation fields; fill gas will determine what type of radiation will be detected Argon and helium are the most frequently used fill gases and allow for the detection of alpha, beta, and gamma radiation When detection of neutrons is necessary, the detectors use boron trifluoride gas ELO 1.3

Proportional Counter Figure shows the three regions of ionization as a function of applied voltage When single gamma interacts with gas, it produces ~ 10,000 secondary electrons The 10,000 electrons produced by the gamma ray increase to 40,000 by gas amplification High amplification factor of the proportional counter is its major advantage over the ionization chamber Positively charged gas ions produced are quenched by ~ 10 percent organic gas added to detector Figure: Gas Ionization Curve ELO 1.3

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 __________________. recombination attenuation gas amplification gas quenching Correct answer is C. gas amplification Correct answer is C. ELO 1.3

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 b. Preamplifier/amplifier c. Single channel analyzer/discriminator d. Scaler e. 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 Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.4

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 ELO 1.4

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 common 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 ELO 1.4

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 ELO 1.4

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 ELO 1.4

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? 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. In a proportional counter, neutron-induced pulses are significantly larger than gamma pulses. The detector instrumentation filters out the smaller gamma pulses. 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. The BF3 proportional counter measures neutron flux of sufficient intensity that the gamma signal is insignificant compared to the neutron signal. Correct answer is B. Bank Question P16 Correct answer is B. ELO 1.4

Ionization Chamber ELO 1.5 – Describe the operation of an ionization chamber to include: a. Radiation detection b. Voltage variations c. 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 Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.5

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 ELO 1.5

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 ELO 1.5

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 ELO 1.5

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 ELO 1.5

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: Neutron is captured by a boron atom and an energetic alpha particle is emitted Alpha particle causes ionization within the chamber and ejected electrons cause further secondary ionizations ELO 1.5

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 Discrimination can eliminate gamma Reducing sensitive volume of chamber without reducing boron coated area, reduces sensitivity to gammas ELO 1.5

Ionization Chamber Knowledge Check
Ionization chambers have two distinct disadvantages when compared to proportional counters: they are ______________ and they have a________ response time. less sensitive; faster less sensitive; slower more sensitive; slower more sensitive; faster Correct answer is B. Correct answer is B. ELO 1.5

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 Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.6

Figure: Compensated Ion Chamber ELO 1.6

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 If both chambers are identical and both batteries are of the same voltage, the net current flow is exactly zero Compensating chamber cancels current due to gamma rays ELO 1.6

Chambers are never truly identical Chambers are normally constructed as concentric cylinders Advantage is that both chambers are exposed to nearly the same radiation field Figure: Compensated Ion Chamber with Concentric Cylinders ELO 1.6

Proper compensation is extremely important during reactor startup and shutdown When compensating voltage is set too high (overcompensated) Some neutron current and all of the gamma current is blocked Indicated power is lower than actual core power When compensating voltage is set too low (undercompensated) Gamma current is only partially blocked Indicated power is higher than actual core power Figure: Typical Compensation Curve ELO 1.6

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. coated; uncoated compensated; uncoated uncoated; coated compensated; coated Correct answer is A. Correct answer is A. ELO 1.6

Geiger-Mueller Detector
ELO Describe the operation of a Geiger-Mueller (GM) detector to include: a. Radiation detection b. Quenching c. 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 Counting systems that use GM detectors are not as complex as those using ion chambers or proportional counters Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.7

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 ELO 1.7

Gamma produces an electron Electron moves rapidly toward positively charged central wire As electron nears wire, its velocity increases Velocity great enough to cause additional ionizations Additional ionizations produce a larger number of electrons in vicinity of central wire Figure: GM Detector ELO 1.7

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 Figure: GM Detector ELO 1.7

Positive ion sheath makes the central wire appear much thicker and reduces field strength 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 Figure: GM Detector ELO 1.7

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 ELO 1.7

Knowledge Check – NRC Bank Which one of the following describes the reason for the high sensitivity of a Geiger-Mueller tube radiation detector? Geiger-Mueller tubes are operated at relatively low detector voltages, allowing detection of low energy radiation. Changes in applied detector voltage have little effect on detector output. Any incident radiation event causing primary ionization results in ionization of the entire detector gas volume. Geiger-Mueller tubes are thinner than other radiation detector types. Correct answer is C. Correct answer is C. ELO 1.7

Knowledge Check – NRC Bank Which one of the following describes a characteristic of a Geiger- Mueller radiation detector? Radiation types can be identified by pulse height and duration. Specific radionuclides can be identified with the use of gamma spectrometry. Small variations in applied voltage will result in large changes in detector output. Any type of radiation that ionizes the detector gas will produce the same magnitude detector output pulse. Correct answer is D. Correct answer is D. Bank P215 ELO 1.7

Scintillation Detector
ELO 1.8 – Describe the operation of a scintillation counter to include: a. Radiation detection b. Three classes of phosphors c. Photomultiplier tube operation Uses a scintillation crystal (phosphor) to detect radiation and produce light pulses Radiation interacts in scintillation crystal energy transferred to bound electrons of the crystal’s atoms If energy is greater than ionization energy, electron enters conduction band and is free from binding forces of parent atom Leaves a vacancy in valence band and is termed a hole If energy is less than binding energy, electron remains attached Exists in an excited energy state Once again, a hole is created in the valence band Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.8

Adding impurities during growth of crystal produces activator centers with energy levels located within the forbidden energy gap Activator center can trap a mobile electron, raising activator center from ground state to an excited state When center de-excites, a photon is emitted Activator centers are referred to as luminescence centers Emitted photons are in visible region of electromagnetic spectrum ELO 1.8

A photomultiplier tube senses flashes and converts them into an electrical signal Constructed by coupling a suitable scintillation phosphor to a light-sensitive photomultiplier tube Figure: Scintillation Detector ELO 1.8

Photomultiplier is a vacuum tube containing A photocathode Series of electrodes called dynodes Light from a scintillation phosphor liberates electrons from photocathode by photoelectric effect Electrons are not of sufficient number or energy to be detected reliably by conventional electronics In photomultiplier tube, attracted by a voltage drop of about 50 volts to the nearest dynode Figure: Photomultiplier ELO 1.8

Photoelectrons strike first dynode with sufficient energy to liberate several new electrons for each photoelectron Second-generation electrons are attracted to second dynode Third-generation group of electrons is emitted Amplification continues through 10 to 12 stages Last dynode sufficient electrons are available to form a current pulse suitable for further amplification by transistor circuits Figure: Photomultiplier ELO 1.8

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 ELO 1.8

Knowledge Check – NRC bank Scintillation detectors convert radiation energy into light by a process known as... space charge effect. gas amplification. luminescence photoionization. Correct answer is C. Correct answer is C. ELO 1.8

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? Geiger-Mueller and scintillation Geiger-Mueller and ion chamber Ion chamber and scintillation Ion chamber and proportional Correct answer is A. Correct answer is A. Bank P4906 ELO 1.8

TLO 1 Summary Radiation Detection Terms
Electron-ion pair - the product of a single ionizing event Specific ionization - ion pairs produced per centimeter travel through matter Stopping power - energy lost per unit path length Alpha Particles Helium nucleus produced from radioactive decay of heavy metals and some nuclear reactions High positive charge of an alpha particle causes electrical excitation and ionization of surrounding atoms Beta Particles Electron or positron ejected from nucleus of beta-unstable radioactive atom Interaction of a beta particle and an orbital electron leads to electrical excitation and ionization of the orbital electron Gamma Rays Photon of electromagnetic radiation with short wavelength and high energy Three methods of attenuating gamma-rays are: photoelectric effect, compton scattering, and pair production TLO 1

TLO 1 Summary Neutrons Have no electrical charge
Nearly same mass as a proton (a hydrogen atom nucleus) Collide with nuclei, causing one of the following reactions: inelastic scattering, elastic scattering, radiative capture, or fission Gas-Filled Detectors Central electrode, or anode, attracts and collects the electron of ion-pair Chamber walls attract and collect the positive ion When applied voltage is high enough, ion pairs initially formed accelerate to a high enough velocity to cause secondary ionizations Resultant ions cause further ionizations Multiplication of electrons called gas amplification TLO 1

TLO 1 Summary Gas Amplification Regions Recombination Region
Voltage is such a low value that recombination takes place before most of the negative ions are collected by electrode Ionization Region Voltage is sufficient to ensure all ion pairs produced by incident radiation are collected No gas amplification takes place Proportional Region Amount of gas amplification is proportional to the applied voltage Limited Proportionality Region As voltage increases, additional processes occur, leading to increased ionizations Since positive ions remain near their point of origin, further avalanches are impossible Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary Geiger-Mueller Region
Ion pair production is independent of radiation, causing initial ionization Field strength is so great that discharge continues to spread until amplification cannot occur due to a dense positive ion sheath surrounding central wire Continuous Discharge Region Applied voltage is so high that, once ionization takes place, there is a continuous discharge of electricity Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary Proportional Counters Summary
Radiation enters a proportional counter Detector gas, at the point of incident radiation, becomes ionized Detector voltage set so that electrons cause secondary ionizations as they accelerate toward electrode Electrons produced from secondary ionizations cause additional ionizations Multiplication of electrons called gas amplification Varying detector voltage within proportional region increases or decreases the gas amplification factor Quenching gas is added to give up electrons to the chamber gas so that inaccuracies are NOT introduced due to ionizations caused by the positive ion Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary Proportional Counter Circuitry Summary
Measures charge produced by each particle of radiation Preamplifier/amplifier amplifies the voltage pulse to a usable size Single channel analyzer/discriminator produces an output only when input is a certain pulse size Scaler counts pulses received during a predetermined length of time Timer provides the gating signal to scaler Ionization Chamber Summary When radiation enters an ionization chamber, the detector gas at the point of incident radiation becomes ionized Some electrons have sufficient energy to cause additional ionizations Voltage potential set up on detector attracts electrons to the electrode If the voltage is set high enough, all of the electrons will reach the electrode before recombination takes place To reduce gamma sensitivity reduction, reduce the amount of chamber gas or increase the boron coated surface area Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary Compensated Ion Chamber Summary
Two concentric cylinders: boron-coated chamber and uncoated chamber Both gammas and neutrons interact in boron-coated chamber Only gammas interact in uncoated chamber Voltages to each chamber are set so current from gammas in boron- coated chamber cancel current from gammas in uncoated chamber GM Detector Summary Voltage set so that any incident radiation produces same electrons As long as voltage remains in GM region, electron production is independent of operating voltage and the initial number of electrons produced by the incident radiation Operation voltage causes a large number of ionizations to occur near the central electrode as the electrons approach The large number of positive ions form a positive ion sheath that prevents additional electrons from reaching the electrode Quenching gas prevents secondary pulse due to ionization by positive ions Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary Scintillation Counter Summary
Radiation interactions with crystal center cause electrons to be raised to excited state When center de-excites, crystal emits photon in the visible light range Three classes of phosphors are used: Inorganic crystals Organic crystals Plastic phosphors Photon, emitted from phosphor, interacts with photocathode of photomultiplier tube, releasing electrons Using a voltage potential, electrons are attracted and strike nearest dynode with enough energy to release additional electrons Electrons subsequently multiplied through series of dynodes, developing sufficient electrons to form current pulse suitable for further amplification by transistor circuits Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary Now that you have completed this objective, you should be able to do the following: Describe the operation of radiation detectors and conditions which effect their accuracy and reliability. Describe the following radiation detection concepts and terms: Electron-ion pair Specific ionization Stopping power Alpha (α) Beta (β) Gamma (γ) Neutron (n) Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 1

TLO 1 Summary 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 Describe the difference between alpha and beta curves Describe the operation of a proportional counter to include: Radiation detection Quenching Voltage variations TLO 1

TLO 1 Summary 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 Timer Describe the operation of an ionization chamber to include: Radiation detection Voltage variations Gamma sensitivity reduction Describe how a compensated ion chamber compensates for gamma radiation. Describe the operation of a Geiger-Mueller (GM) detector to include: Quenching Positive ion sheath TLO 1

TLO 1 Summary Describe the operation of a scintillation counter to include: Radiation detection Three classes of phosphors Photomultiplier tube operation TLO 1

Personnel Radiation Monitoring
TLO 2 – Describe the operation of personal radiation monitoring instruments and conditions which effect their accuracy and reliability. Important to conduct radiation surveys to Determine the hazards to personnel Assure safe movement of radioactive material Radiation surveys monitor conditions throughout a nuclear power plant using portable radiation detectors Radiation levels change based on changing plant conditions resulting from power level changes and operating systems that interface with the primary systems. TLO 2

Describe the use of portable personnel radiation monitoring instruments. Describe how the following detect neutrons: Self-powered neutron detector Wide-range fission chamber Flux wire State the various types of radiation detected by the following dosimeters: Thermoluminescent dosimeter Direct reading pocket dosimeter Electronic dosimeter Film badge TLO 2

Describe the operation of a thermoluminescent dosimeter and advantages and disadvantages as compared to other devices. Describe how the direct reading pocket dosimeter measures ionizing radiation and advantages and disadvantages as compared to other devices. Describe how the electronic dosimeter measures ionizing radiation and advantages and disadvantages as compared to other devices. Describe how the film badge measures ionizing radiation and advantages and disadvantages as compared to other devices. TLO 2

Portable Radiation Monitoring
ELO 2.1 – Describe the use of portable personnel radiation monitoring instruments. Most monitoring stations have posted instructions for monitoring use; ensure you have read and understand the instructions Prior to using portable meter, it must be verified to be working properly Each survey instrument is required to be calibrated Calibration due date should be indicated on the instrument calibration sticker If calibration has expired, inform health physics personnel and shift supervisor and do not use the instrument Related KA - K1.19 Use of portable and personal radiation monitoring instruments ELO 2.1

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 and properly situated to commence monitoring Indicating scale is visible and 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 ELO 2.1

Portable Detector Use Instrument source check or response check ensures that the instrument will operate properly when exposed to a known source Most meters are source checked by health physics to ensure proper operation and indication within a specified range A sticker on the detector may indicate the last source check Response check just verifies detector responds when placed on a known source ELO 2.1

Knowledge Check Which of the following should be performed prior to each use of a portable radiation detector? (Select all that apply.) Battery strength verified Calibration date check Source check Visual inspection for damage or defects Correct answers are A, B, and D. Correct answer is A, B and D. ELO 2.1

Neutron Detection Self-Powered Neutron Detector
ELO 2.2 – Describe how the following detect neutrons: self-powered neutron detector, wide-range fission chamber, and flux wire. Self-Powered Neutron Detector Monitor neutron flux in various portions of core on a continuous basis. Central wire is made from a material that absorbs a neutron and undergoes radioactive decay by emitting an electron (beta decay) Figure: Self-Powered Neutron Detector ELO 2.2

Neutron Detection Self-Powered Neutron Detector
Very little instrumentation is required; only a millivolt meter or an electrometer Emitter material has a much greater lifetime than boron or U235 lining (used in wide-range fission chambers) Figure: Self-Powered Neutron Detector ELO 2.2

Neutron Detection Wide-Range Fission Chamber
Fission chambers use neutron-induced fission to detect neutrons Similar in construction to that of an ionization chamber Coating material is highly enriched U235 The neutrons interact with the U235, causing fission One of the two fission fragments enters the chamber ELO 2.2

Neutron Detection Activation Foils and Flux Wires
Measure reactor neutron flux profiles Section of wire or foil is inserted directly into the reactor core After activation, the flux wire or foil is rapidly removed from the reactor core and the activity counted Activated foils can also discriminate energy levels by placing a cover over the foil to filter out (absorb) certain energy level neutrons ELO 2.2

Neutron Detection Knowledge Check
Which of the following are types of neutron detectors? (Select all that apply.) Self-powered neutron detector Activation wire Self-powered Geiger-Mueller tube Fission chamber Correct answers are A, B, and D. Correct answer is A, B and D. ELO 2.2

Dosimetry and Types of Radiation Detected
ELO 2.3 – State the various types of radiation detected by the following dosimeters: a. Thermoluminescent dosimeter b. Self-reading pocket dosimeter c. Electronic dosimeter d. Film badge Thermoluminescent dosimeter used to detect beta and gamma radiation accumulated doses Self-reading pocket dosimeter used to detect gamma radiation Electronic dosimeter used to detect gamma and x-ray radiation Film badges used to measure and record gamma rays, x-rays, and beta particles Related KA - K1.19 Use of portable and personal radiation monitoring instruments ELO 2.3

Dosimetry and Types of Radiation Detected
Knowledge Check Which of the following are used in personnel dosimetry? (Select all that apply.) Fission chamber Film badge Self-reading dosimeter Thermoluminescent dosimeter Correct answers are B, C, and D. Correct answer is B, C and D. ELO 2.3

Thermoluminescent Dosimeter
ELO 2.4 – Describe the operation of a thermoluminescent dosimeter and advantages and disadvantages as compared to other devices. Two types of dosimeter of legal record (DLR) currently used Thermoluminescent dosimeter (TLD) Optically stimulated luminescent dosimeter (OSLD) Note: Many plants have switched to or are using OSLDs, which use aluminum oxide to absorb the radiation energy and a laser rather than heat to release the stored energy and measure the amount of ionizing radiation received (remaining material will focus on TLDs) Related KA - K1.19 Use of portable and personal radiation monitoring instruments ELO 2.4

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 ELO 2.4

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 ELO 2.4

Measure doses as low as 1 millirem 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 ELO 2.4

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 ELO 2.4

Knowledge Check A _____________ measures ionizing radiation exposure by measuring the amount of visible light emitted from a _________ when the detector is heated. direct reading dosimeter; crystal thermoluminescent dosimeter; crystal thermoluminescent dosimeter; wire direct reading dosimeter; wire Correct answer is B. Correct answer is B. ELO 2.4

Self-Reading Pocket Dosimeter
ELO 2.5 – Describe how the direct reading pocket dosimeter measures ionizing radiation and advantages and disadvantages as compared to other devices. 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 Related KA - K1.19 Use of portable and personal radiation monitoring instruments ELO 2.5

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 ELO 2.5

Point a light source to read position of the fiber Fiber is viewed on a translucent scale which is graduated in units of exposure Have a full scale reading of 200 milliroentgens Some designs higher scale Figure: Pocket Dosimeter Internals ELO 2.5

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 If dropped, exit area immediately and inform health physics Recall last good reading and time ELO 2.5

Disadvantages (continued) Must be recharged and recorded at the start of each working shift Charge leakage, or drift, can also affect accuracy Leakage - gradual loss of repulsion charge on moveable fiber No greater than 2 percent of full scale in a 24-hour period ELO 2.5

Knowledge Check The following are advantages of a self-reading dosimeter except... Reusable Reliable, even if dropped Primarily sensitive to gamma radiation Immediate reading of dose Correct answer is B. Correct answer is B. ELO 2.5

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? Alpha Beta Gamma Neutron Correct answer is C. Correct answer is C. Bank Question P714 ELO 2.5

Electronic Dosimeter ELO 2.6 – Describe how the electronic dosimeter measures ionizing radiation and advantages and disadvantages as compared to other devices. Another type of pocket dosimeter Records dose information and dose rate Constructed using a Geiger-Mueller counter that measures Gamma X-ray Related KA - K1.19 Use of portable and personal radiation monitoring instruments ELO 2.6

Electronic Dosimeter Output of detector is collected
Predetermined exposure reached collected charge is discharged to trigger an electronic counter Digital counter displays Accumulated exposure Dose rate ELO 2.6

Electronic Dosimeter Can include an audible alarm feature
Can provide a continuous audible signal when a preset exposure has been reached Minimizes reading errors associated with direct reading pocket dosimeters Allows higher maximum readout before resetting is necessary Advantages Reliability Ability to indicate accumulated dose as well as dose rate Audible response ELO 2.6

Electronic Dosimeter Knowledge Check
A Geiger-Mueller electronic dosimeter measures what type of radiation? (Select all that apply.) X-rays Beta Gamma Neutron Correct answers are A and C. Correct answer is A and C. ELO 2.6

Film Badges ELO 2.7 – Describe how the film badge measures ionizing radiation and advantages and disadvantages as compared to other devices. Film badges are no longer commonly used for personnel monitoring devises in commercial power plants. Used to measure and record radiation exposure or accumulated dose due to: Gamma rays X-rays Beta Contains a piece of radiation-sensitive film Film packaged in a light proof, vapor proof envelope Prevents light, moisture or chemical vapors from affecting film Related KA - K1.19 Use of portable and personal radiation monitoring instruments Figure: Film Badge ELO 2.7

Film Badges Special film used which is coated with two different emulsions One side coated with a large grain fast emulsion Sensitive to low levels of exposure Other side coated with a fine grain slow emulsion Less sensitive to exposure Two different emulsions are used to ensure an accurate dose record If radiation exposure causes fast emulsion in processed film to be darkened to a degree that it cannot be interpreted, fast emulsion is removed and dose is computed using slow emulsion ELO 2.7

Film Badges Film contained inside a film holder or badge
Badge incorporates a series of filters to determine radiation quality (energy) Radiation of a given energy is attenuated to a different extent by various types of absorbers The same quantity of radiation incident will produce a different degree of darkening under each filter The arrangement of filters in the holder may vary with different radiation monitoring services. Because the arrangement of filters varies from one service to another, accurate readings require no mixing of the films and holders of different services. The indicated areas of the dosimeter illustrated in the figure below are: W - window allows all radiation which can penetrate wrapper to reach the film N - thin plastic filter which attenuates beta radiation depending on its energy K - thick plastic filter which attenuates low energy photon radiations and absorbs all but the highest energy beta radiation A - aluminum filter used with area K to assess doses from photons with energies from 15 to 65 keV C - composite of cadmium and lead filters to assess doses from thermal neutrons which interact with the cadmium T - composite of tin and lead filters used with area C to assess doses from thermal neutrons E - edge shielding to prevent low energy photons entering around area T I - indium foil sometimes included to detect fast neutrons Figure: Film Badge Filters ELO 2.7

Film Badges Comparing results Radiation energy can be determined
Dose calculated knowing the film response for that energy Holder can also contain an open window to determine exposure due to beta Beta particles are effectively shielded by a thin amount of material ELO 2.7

Film Badges Advantages Disadvantages 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 ELO 2.7

Film Badges Knowledge Check
In a film badge, the film is contained inside a film holder or badge that incorporates a series of _________ to determine the _________ of the radiation. filters; dose rate films; dose rate films; quality filters; quality Correct answer is D. Correct answer is D. ELO 2.7

TLO 2 Summary TLDs detect both beta and gamma radiation doses
Main types of TLDs: calcium fluoride or lithium fluoride crystal TLD crystal is heated to release photon light Photomultiplier tube measures light pulses Self-reading pocket dosimeter is small ionization chamber Measures gamma Electrostatic repulsion deflects the quartz fiber Electrons produced by ionization are attracted to positively charged central anode, reduce net positive charge, and allow quartz fiber to return in direction of original position Position of fiber observed through system of built-in lenses and scale Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 2

TLO 2 Summary Electronic dosimeter is battery-operated Geiger-Mueller counter Measures gamma and x-ray Indicates accumulated dose and dose rate Film badges record radiation exposure due to gamma rays, X-rays, and beta particles Use radiation-sensitive film and filters Film is exposed to read; variations between films indicates exposure TLO 2

TLO 2 Summary Now that you have completed this objective, you should be able to do the following: Describe the use of portable personnel radiation monitoring instruments. Describe how the following detect neutrons: Self-powered neutron detector Wide-range fission chamber Flux wire State the various types of radiation detected by the following dosimeters: Thermoluminescent dosimeter Direct reading pocket dosimeter Electronic dosimeter Film badge TLO 2

TLO 2 Summary Describe the operation of a thermoluminescent dosimeter and advantages and disadvantages as compared to other devices. Describe how the direct reading pocket dosimeter measures ionizing radiation and advantages and disadvantages as compared to other devices. Describe how the electronic dosimeter measures ionizing radiation and advantages and disadvantages as compared to other devices. Describe how the film badge measures ionizing radiation and advantages and disadvantages as compared to other devices. TLO 2

Neutron Detectors TLO 3 – Describe the operation of neutron detectors and conditions which effect their accuracy and reliability. Thermal power produced in a reactor is proportional to neutron density or flux present in core Average number of neutrons per square meter per second is the common unit of neutron flux Some of the neutrons present in the core leak out The higher the neutron flux density in the core, the more neutrons leak out Amount of leakage neutrons leaving the reactor is proportional to reactor power Nuclear instrumentation is important to be able to provide power level indication in all ranges of power operations from start up to full power. Instruments also provide alarms and trips based on power measurements. TLO 3

Neutron Detectors A PWR generally has two types of neutron detection systems installed: In-core (Incore) NIs – located within core of reactor; used for flux mapping to determine hot channel factors, etc. Out-of-core (Excore) NIs – used for reactor power monitoring and reactor protection and control A BWR also uses incore and excore neutron detection systems: In-core (Incore) NIs – located within core of reactor; used for reactor power monitoring and reactor protection and control Out-of-core (Excore) NIs – used as accident monitoring located in the containment TLO 3

Define the following terms: Signal-to-noise ratio Discriminator Analog Logarithm Reactor period Decades per minute (DPM) Scalar List the type of detector used in each of the following nuclear instruments: Source range Intermediate range Power range TLO 3

Describe the purpose of the following nuclear instrumentation major components: Preamplifiers and amplifiers Discriminator Log count rate amplifier Period and startup rate meters State the effect core voiding, core loading pattern, and environmental effects could have on neutron detection and power indication. TLO 3

Nuclear Instrument Terms
ELO 3.1 – Define the following terms: signal-to-noise ratio, discriminator, analog, logarithm, reactor period, decades per minute (DPM), and scalar. Signal-to-noise ratio – the ratio of the electrical output signal to the electrical noise generated in the cable run or in the instrumentation Discriminator - process of distinguishing between different types of radiation on the basis of pulse height Discriminator circuit selects the minimum or maximum pulse height that is to be counted No related KAs ELO 3.1

Circuitry and Circuit Elements
Analog - mechanism in which data is represented by continuously variable physical quantities As it applies to the intermediate range, the output of the intermediate range is an analog current Due to the wide range of the flux measured, use of logarithmic circuitry is required for indication on a single scale instrument Analog is used in contrast to digital to refer to circuits in which the magnitude of the signal carries the information Figure: Analog and Digital Displays ELO 3.1

Logarithm - exponent that indicates the power to which a number is raised to produce a given number (i.e., the logarithm of 100 to the base 10 is 2) In nuclear instrumentation refers to electronic circuitry of source and intermediate ranges These ranges utilize logarithms due to the wide range of measured flux and the necessity to measure that flux on a single meter scale Reactor Period - amount of time, normally in seconds, required for neutron flux (power) to change by a factor of e, or 2.718 ELO 3.1

Decades Per Minute (DPM) - units used for rate of power change Rate circuits are important in the source and intermediate ranges Rate information is displayed on a meter in DPM Meters indicate how fast reactor power is changing in decades (power of 10) in each minute Scalar - measurement or quantity that is capable of being represented on a scale Neutron flux on source range, intermediate range, and power range meters ELO 3.1

Nuclear Instrument Terms
Knowledge Check _________________ is the comparison of the electrical output signal to the electrical noise generated in the instrumentation. Signal-to-noise ratio Discriminator-to-noise ratio Analog-to-noise ratio Reactor-period-to-noise ratio Correct answer is A. Correct answer is A. ELO 3.1

Nuclear Instrument Detector Types
ELO 3.2 – List the type of detector used in each of the following nuclear instruments: source range, intermediate range, and power range. Source Range Nuclear Instrumentation 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 ELO 3.2

Source Range Nuclear Instrumentation Also measures rate of change of the count rate Rate of change is displayed on meters in terms of the startup rate from -1 to +10 DPM Protective functions are not normally associated with source range instrumentation Due to inherent limitations in this range Interlocks may be incorporated ELO 3.2

Source Range Nuclear Instrumentation Source range proportional counters may be placed in lead shielding to reduce gamma flux at the detectors Increases low-end sensitivity Adds to detector life High voltage power supply disabled and signal lead shorted when neutron flux has passed into the intermediate range Extends detector life Some reactor designs allow source range detectors to be moved from normal operating positions once the flux level increases above the source range ELO 3.2

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 ELO 3.2

Source Range Nuclear Instrumentation 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 to raise the pulse height to several volts Discriminator excludes passage of pulses that are less than a predetermined level Excludes noise and gamma pulses Pulses sent to pulse integrator where they are integrated to give a signal that is proportional to logarithm of count rate ELO 3.2

Source Range Nuclear Instrumentation Logarithmic count rate amplifier amplifies signal Varies directly with logarithm of pulse rate in detector Logarithmic count rate is then displayed on a meter with a logarithmic scale in counts per second Logarithmic count rate signal is differentiated to measure the rate of change in neutron flux ELO 3.2

Source Range Nuclear Instrumentation Differentiator output is proportional to reactor period Value of reactor period is inversely proportional to the actual rate of change of reactor power and relates to power changes by factors of e (approximately 2.718) Power rate change based on factors of 10, in DPM, is more meaningful to the reactor operator Differentiator output converted from reactor period to DPM through meter scale used ELO 3.2

Intermediate Range Nuclear Instrumentation Minimum of two redundant channels Each channel made up of Boron-lined or boron gas-filled compensated ion chamber Associated signal measuring equipment Output is a steady current produced by neutron flux ELO 3.2

Intermediate Range Nuclear Instrumentation Compensated ion chamber is used when current output is proportional to relatively stable neutron flux 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 ELO 3.2

Figure: Intermediate Range Detector ELO 3.2

Intermediate Range Nuclear Instrumentation Compensated ion chamber output is an analog current ranging from to 10-3 amperes 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 ELO 3.2

Intermediate Range Nuclear Instrumentation 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 ELO 3.2

Power Range Nuclear Instrumentation Normally consists of four identical linear power level channels which originate in eight uncompensated ion chambers Output is a steady current produced by the neutron flux 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 ELO 3.2

Power Range Nuclear Instrumentation Output of each power range 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 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 ELO 3.2

Power Range Nuclear Instrumentation 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 ELO 3.2

Power Range Nuclear Instrumentation 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 ELO 3.2

Knowledge Check In the figure below, where are gammas eliminated from the circuit? Discriminator Differentiator Pulse integrator Count rate meters Correct answer is A. Correct answer is are A ELO 3.2

Nuclear Instrument Components
ELO 3.3 – Describe the purpose of the following nuclear instrumentation major components: preamplifiers and amplifiers, discriminator, log count rate amplifier, and period and startup rate meters. 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 ELO 3.3

Components Three ranges overlap each other to allow for continuous reactor power monitoring under all conditions Figure: Three Range Overlap ELO 3.3

Figure: Amplifiers/Preamplifiers
Components Preamplifiers and amplifiers Detector output signals usually weak and require amplification Nature of input pulse and discriminator determines the characteristics that the preamplifier and amplifier must have Two stages of amplification used in most detection circuits to increase the signal-to-noise ratio Figure: Amplifiers/Preamplifiers ELO 3.3

Components Radiation detector is located some distance from the readout Shielded coaxial cable transmits the detector output to the amplifier Output signal of the detector may be as low as 0.01 volts Total gain of 1,000 is needed to increase this signal to 10 volts, which is a usable output pulse voltage There is always a pickup of noise in the long cable run; this noise can amount to volts ELO 3.3

Components Discriminator Circuit Selects minimum pulse height
When input pulse exceeds discriminator preset level, output pulse generated Discriminator input is normally amplified and shaped detector signal Signal is analog because amplitude is proportional to energy of the incident particle Biased diode circuit is simplest form of discriminator Figure: Discriminator Circuit ELO 3.3

Components Diode D1 shown with cathode connected to a positive voltage source +V A diode cannot conduct unless the voltage across the anode is positive with respect to the cathode As long as anode voltage is less than cathode Diode D1 does not conduct No output At some point, anode voltage exceeds bias value +V and diode conducts Input signal is allowed to pass to output Figure: Discriminator Circuit ELO 3.3

Components Logarithmic meters
Ionization chamber current output may vary by 8 orders of magnitude Range may be from amps to 10-5 amps Most accurate method to display this range would be to utilize a linear current meter with several scales and capability to switch those scales Single scale (logarithmic) which covers entire range of values is used ELO 3.3

Components Logarithmic output meter must be provided with a signal which is proportional to the logarithm of the input signal Using a diode when the input signal is from an ionization chamber Voltage across diode equals the logarithm of the current through the diode Converts ionization chamber current to a voltage proportional to the logarithm of this current Figure: Logarithmic Meters ELO 3.3

Components Period Meters and Startup Rate
Essential to know the rate of change of power Normally increases or decreases exponentially with time Time constant for this change is referred to as the period Period of five seconds means that the value changes by a factor of e (2.718) in five seconds ELO 3.3

Nuclear Instrument Components
Knowledge Check The following component reduces if not eliminates cable noise in a source range instrument. Discriminator Amplifier Diode Preamplifier Correct answer is D. Correct answer is D. ELO 3.3

Core Voiding and Loading Effects
ELO 3.4 – 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 and 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; since nuclear instrumentation gives an indication of core power, the power readings may be erroneous Voiding in the core can have a similar effect Related KA - K1.17 Effects of core voiding on neutron detection ELO 3.4

Core Voiding Nuclear instruments detect neutron leakage from the reactor core If a nuclear reactor experiences a loss of coolant accident and the moderator becomes compromised, more neutrons will leak out The neutrons that would have been thermalized and remain in the core escape Nuclear instrument readings would increase initially because more neutron leakage occurs resulting in more neutrons reaching the detectors ELO 3.4

Core Voiding Important points:
As core begins to void, neutron count rate will initially increase (up to ≈50%) due to more neutrons escaping core and interacting with detectors As core voiding continues, neutron count rate will begin to decrease (above ≈50%) due to fewer neutrons available for subcritical multiplication (keff decreasing) If core is refilled after voiding: Neutron count rate will initially increase as 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 Less leaking from core to interact with detectors ELO 3.4

Core Voiding and Loading Effects
Knowledge Check Because nuclear instrumentation actually measures ____________ ___________, during a core voiding event, (< 50 percent core voids) indicated power level will ___________. neutron leakage; increase neutron leakage; decrease thermal power; increase thermal power; decrease Correct answer is A. Correct answer is A. ELO 3.4

Crossword Puzzle Across
4. A _________ counter can be set to read only pulses when using a discriminator circuit 8. Gas used in nuclear instruments to detect neutrons 9. A particle that has no electrical charge 10. Nuclear instrument range that uses compensated ion chamber detectors Down 1. Type of tube that senses flashes and converts them to an electrical signal 2. Single scale used for the indication of Source, Intermediate and Power Range instruments 3. Ion chamber that uses boron to detect neutrons 5. Detector type where light is emitted from a crystal component when it is heated 6. Particle identical to a helium nucleus 7. _________ reading dosimeter contains a detector where a moveable quartz fiber returns to its original position as the detector's Give students approximately 20 minutes to complete the puzzle. Review key with class after all students have completed. ELO 3.4

TLO 3 Summary Source Range Instrumentation
Source range uses a proportional counter Linear amplifier amplifies input signal by a factor of several thousand to raise pulse height to several volts Discriminator excludes passage of pulses less than a predetermined level Pulse integrator provides output signal proportional to logarithm of count rate Log count rate amplifier amplifies the signal for display on a meter Differentiator provides an output signal proportional to rate of power change Intermediate Range Instrumentation The log n amplifier converts detector output signal to a signal proportional to logarithm of detector current Differentiator provides an output proportional to rate of change of power Reactor protection interface provides signals for protective actions Intermediate range uses a compensated ion chamber Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 3

TLO 3 Summary Power Range Instrumentation
Power range uses an uncompensated ion chamber Gamma compensation is NOT required in the power range since neutrons outnumber gammas by such a large number that gammas are insignificant Linear amplifier amplifies the signal to a useful level Reactor protective interface provides signals for protective actions Core Voiding and Core Loading In a PWR, the neutron instrumentation is located external to the core and measures neutron leakage from the core If the flux at the core edge is affected, neutron instrumentation may also be affected; since nuclear instrumentation gives an indication of core power, power readings may be erroneous Voiding in the core can have a similar effect Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 3

TLO 3 Summary Now that you have completed this objective, you should be able to do the following: Describe the operation of neutron detectors instruments and conditions which effect their accuracy and reliability. Define the following terms: Signal-to-noise ratio Discriminator Analog Logarithm Reactor period Decades per minute (DPM) Scalar Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement TLO 3

TLO 3 Summary List the type of detector used in each of the following nuclear instruments: Source range Intermediate range Power range Describe the purpose of the following nuclear instrumentation major components: Preamplifiers and amplifiers Discriminator Log count rate amplifier Period and startup rate meters State the effect core voiding, core loading pattern, and environmental effects could have on neutron detection and power indication. TLO 3

Sensors and Detectors 2 Summary
Now that you have completed this module, you should be able to do the following: Describe the operation of radiation detectors and conditions which effect their accuracy and reliability. Describe the operation of personal radiation monitoring instruments and conditions which effect their accuracy and reliability. Describe the operation of neutron detectors and conditions which effect their accuracy and reliability. Review the objectives and conduct directed questioning to assure comprehension, review topic for any areas where retention needs improvement

Operator Generic Fundamentals Components - Sensors and Detectors 2

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Operator Generic Fundamentals Components - Sensors and Detectors 2

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Presentation on theme: "Operator Generic Fundamentals Components - Sensors and Detectors 2"— Presentation transcript:

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