Sensors and Detectors Part 2

Transcription

1 Revision 1 December 2014 Sensors and Detectors Part 2 Instructor Guide Reviewed by: Cassandra Bitler 11/3/2014 Project Manager, OGF Date Approved by: Robert Coovert 11/3/2014 Manager, INPO Learning Development Date Approved by: Kevin Kowalik 11/3/2014 Chairperson, Industry OGF Working Group Date NOTE: Signature also satisfies approval of associated student guide and PowerPoint presentation. GENERAL DISTRIBUTION

2 GENERAL DISTRIBUTION: Copyright 2014 by the National Academy for Nuclear Training. Not for sale or for commercial use. This document may be used or reproduced by Academy members and participants. Not for public distribution, delivery to, or reproduction by any third party without the prior agreement of the Academy. All other rights reserved. NOTICE: This information was prepared in connection with work sponsored by the Institute of Nuclear Power Operations (INPO). Neither INPO, INPO members, INPO participants, nor any person acting on behalf of them (a) makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this document, or that the use of any information, apparatus, method, or process disclosed in this document may not infringe on privately owned rights, or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this document. ii

3 Table of Contents INTRODUCTION... 1 TLO 1 RADIATION DETECTORS... 2 Overview... 2 ELO 1.1 Detector Concepts... 3 ELO 1.2 Detector Theory of Operation... 9 ELO 1.3 Proportional Counter Theory ELO 1.4 Proportional Counter Circuit ELO 1.5 Ion Chamber ELO 1.6 Gamma Compensation ELO 1.7 Geiger-Mueller Tube Detector ELO 1.8 Scintillation Detector TLO 1 Summary TLO 2 PERSONNEL RADIATION MONITORING Overview ELO 2.1 Portable Radiation Monitoring Instruments ELO 2.2 Neutron Detection ELO 2.3 Dosimetry and Types of Radiation Detected ELO 2.4 Thermoluminescent Dosimeter Operation ELO 2.5 Self-Reading Pocket Dosimeter ELO 2.6 Electronic Dosimeter ELO 2.7 Film Badge TLO 2 Summary TLO 3 NEUTRON DETECTORS Overview ELO 3.1 Nuclear Instrument Terms ELO 3.2 Detector Types ELO 3.3 Nuclear Instruments Components ELO 3.4 Core Voiding and Loading Effects TLO 3 Summary SENSORS AND DETECTORS PART 2 SUMMARY iii

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5 Sensors and Detectors Part 2 Revision History Revision Date Version Number Purpose for Revision Performed By 11/3/ New Module OGF Team 12/11/ Added signature of OGF Working Group Chair OGF Team Duration Logistics 5 hours 30 minutes Ensure that the presentation space is properly equipped with the following: Projector Internet access, if needed Whiteboard or equivalent Space for notes, parking lot, mockups, or materials Sufficient space for all students Ensure that the following course materials are prepared and staged: All student materials Instructor materials Media, photos, and illustrations Props, lab equipment, or simulator time, as applicable Ensure that all students have fulfilled the course prerequisites, if applicable. Instructor preparation: Review the course material prior to beginning the class. Review the NRC exam bank and as many new exams as are available prior to the class to ensure that you are prepared to address those items. Ensure that all students have access to the training material for selfstudy purposes. Introduction Proper operation of an industrial plant, such as a nuclear power generating station, requires the measurement of many plant parameters. Operator and automatic actions rely on accurate information provided by sensors and detectors installed within plant systems for controlling plant parameters. Rev 1 1

6 Nuclear facility operators are also required to monitor key parameters that can affect plant operation and public safety on a regular schedule, and analyze the parameters for trends and abnormal conditions. Sensors, detectors, and their associated circuitry measure and indicate parameters including temperature, pressure, level, flow, position, radiation, and reactor power level. 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. Objectives Logistics Use PowerPoint slides 1 3 and the instructor guide (IG) to introduce the Sensors and Detectors Part 2 module. 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 or higher on the following Terminal Learning Objectives (TLOs): 1. Describe the operation of radiation detectors and conditions that effect their accuracy and reliability. 2. Describe the operation of personal radiation monitoring instruments and conditions that effect their accuracy and reliability. 3. Describe the operation of neutron detectors and conditions that effect their accuracy and reliability. TLO 1 Radiation Detectors Duration 3 hours 45 minutes Logistics Use PowerPoint slides 4 7 and the IG to introduce TLO 1. Overview Radiation detectors sense the presence of the various types of radiation that occur in the normal operation of a nuclear power reactor. Radiation results from the nuclear reaction taking place through fission and activation of particles exposed to the neutron flux of the reactor. Radiation detection includes detecting both the presence and the level of radiation. Radiation detection provides indication, alarms, and input for automatic functions. Radiation detection is important because of the significant effect that radiation has on personnel and equipment. In addition, the ability to detect radiation is necessary to determine the power level of the reactor. Objectives Upon completion of this lesson, you will be able to do the following: 1. Describe the following radiation detection concepts and terms: a. Electron-ion pair b. Specific ionization c. Stopping power d. Alpha (α) 2 Rev 1

7 e. Beta (β) f. Gamma (γ) g. Neutron (n) 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 3. Describe the operation of a proportional counter to include: a. Radiation detection b. Quenching c. Voltage variations 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 5. Describe the operation of an ionization chamber to include: a. Radiation detection b. Voltage variations c. Gamma sensitivity reduction 6. Describe how a compensated ion chamber compensates for gamma radiation. 7. Describe the operation of a Geiger-Mueller (GM) detector to include: a. Radiation detection b. Quenching c. Positive ion sheath 8. Describe the operation of a scintillation counter to include: a. Radiation detection b.three classes of phosphors c. Photomultiplier tube operation ELO 1.1 Detector Concepts Introduction Radiation detection relies on general principles and characteristics of all types of radiation as well as specific behavior of radiation types. These radiation types include: Alpha Beta Gamma Neutron Duration 40 minutes Logistics Use PowerPoint slides 8 22 and the IG to present ELO 1.1. Use radiation primer video to illustrate penetrating power of different types of radiation. Rev 1 3

8 Radiation Detection Terminology Electron-Ion Pair Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This results in the loss of units of negative charge by the affected atom. The atom becomes electrically positive (a positive ion). An electron-ion pair comprises the products of a single ionizing event. Specific Ionization Specific ionization is the number of ion pairs formed per unit path length for a given type of radiation travel through matter. Specific ionization is dependent on the mass, charge, energy of the particle, and the electron density of matter. The greater the mass of a particle, the more interactions it produces in a given distance. A larger number of interactions results in the production of more ion pairs and a higher specific ionization. A particle s charge has the greatest effect on specific ionization. A higher charge increases the number of interactions that occur in a given distance. Increasing the number of interactions produces more ion pairs, therefore increasing the specific ionization. As the energy of a particle decreases, it produces more ion pairs for the same amount of distance traveled. Think of the particle as a magnet. As a magnet passes over a large pile of paper clips, the magnet attracts some of the clips. Maintain the same distance from the pile and vary the speed of the magnet. Notice that the slower the magnet passes over the pile of paper clips, the more clips attach to the magnet. The same is true of a particle passing by a group of atoms a given distance away. The slower a particle travels the more atoms it affects. Stopping Power Stopping power or linear energy transfer (LET) is the energy lost per unit path length, which depends on the type and energy of the particle and on the properties of the material it passes. Radiation types Alpha Particle (α) Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, often written as He 2+. They result from the radioactive decay of heavy metals and certain nuclear reactions. Since they are a highly ionizing form of particle radiation, they have low penetration power. The alpha particle is a large nuclear particle relative to the other radiation types, and has a resultant charge of +2. Radioactive nuclei such as uranium, thorium, actinium, or radium emit alpha particles in a process known as alpha decay. Alpha decay often occurs among nuclei that have a favorable neutron/proton ratio, but contain 4 Rev 1

9 too many nucleons for stability. This sometimes leaves the nucleus in an excited state, with the emission of a gamma ray removing the excess energy. Beta Particle (β) The beta particle is an ordinary electron or positron ejected from the nucleus of a beta-unstable radioactive atom. The beta particle has a single negative or positive electrical charge and a very small mass. Beta particles are highenergy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. Gamma Ray (γ) The gamma ray is a photon of electromagnetic radiation with a very short wavelength and high energy. An unstable atomic nucleus emits a gamma ray, which has high penetrating power. Neutron (n) Neutrons have no electrical charge and have nearly the same mass as a proton (a hydrogen atom nucleus). A neutron is hundreds of times larger than an electron, but one quarter the size of an alpha particle. The primary source of neutrons is nuclear reactions, such as fission; decay of radioactive elements is a secondary source of neutrons. Because of its size and lack of charge, the neutron is difficult to stop, and has relatively high penetrating power. Alpha Beta Reactions Alpha Particles Alpha particles exhibit the least penetrating radiation energy. The major energy loss for alpha particles is due to electrical excitation and ionization. As an alpha particle passes through air or soft tissue, it loses, on the average, 35 ev per ion pair created. Due to its highly charged state, large mass, and low velocity, the specific ionization of an alpha particle is very high. The specific ionization of an alpha particle is tens of thousands of ion pairs per centimeter in air. An alpha particle travels a relatively straight path over a short distance. Beta Particles The interaction of a beta particle and an orbital electron leads to electrical excitation and ionization of the orbital electron. These interactions cause the beta particle to lose energy in overcoming the electrical forces of the orbital electron. The electrical forces act over long distances; therefore, the two particles do not have to come into direct contact for ionization to occur. The amount of energy lost by the beta particle depends upon both its distance of approach to the electron and its kinetic energy. Rev 1 5

10 Beta particles and orbital electrons have the same small mass; therefore, collisions deflect them easily. Because of the numerous deflections, the beta particle follows a tortuous path as it passes through absorbing material. The specific ionization of a beta particle is low due to its small mass, small charge, and relatively high speed of travel. The production of beta particles is termed beta decay. There are two forms of beta decay, β, and β+, which respectively give rise to the electron and the positron. Gamma Reactions Gamma Rays There are three methods of attenuating (reducing the energy level of) gamma rays: Photoelectric effect Compton scattering Pair production Gamma Interaction The photoelectric effect occurs when a low energy gamma strikes an orbital electron. The gamma expends all its energy in ejecting the electron from its orbit. The result is ionization of the atom and expulsion of a high-energy electron. The photoelectric effect is most predominant with low energy gammas and rarely occurs with gamma rays having energy above 1 MeV (million electron volts). Compton scattering is an elastic collision between an electron and a photon, as shown. In this case, the 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. Since the photon cannot transfer all its energy, the photon must be scattered; the scattered photon must have less energy, or a longer wavelength. The result is ionization of the atom, a high- 6 Rev 1

11 energy beta, and a gamma at a lower energy level than the original. Compton scattering is most predominant with gamma rays with an energy level in the 1.0 to 2.0 MeV range. At higher energy levels, pair production is the predominant interaction. When a high-energy gamma passes close enough to a heavy nucleus, the gamma disappears, and its energy reappears in the form of an electron and a positron (same mass as an electron, but has a positive charge). This transformation of energy into mass must take place near a particle, such as a nucleus, to conserve momentum. The kinetic energy of the recoiling nucleus is very small; therefore, all of the photon's excess energy over that needed to supply the mass of the pair appears as kinetic energy of the pair. For this reaction to take place, the original gamma ray must have at least 1.02 MeV energy. Neutrons Neutrons may collide with nuclei causing one of the following reactions: Inelastic scattering Elastic scattering Radiative capture Fission Inelastic scattering causes transfer of some of the neutron's kinetic energy to the target nucleus in two forms: some kinetic energy and some internal energy. This transfer of energy slows the neutron, and leaves the nucleus in an excited state. The excitation energy results in a gamma ray photon emission. The compound nucleus mode describes the interaction between the neutron and the nucleus; the nucleus captures the neutron, the nucleus re-emits the Rev 1 7

12 neutron along with a gamma ray photon. The threshold phenomenon is the term for this re-emission. The neutron threshold energy varies from infinity for hydrogen, (inelastic scatter cannot occur) to about 6 MeV for oxygen, to less than 1 MeV for uranium. Elastic scattering is the most likely interaction between fast neutrons and low atomic mass number absorbers. The phrase "billiard ball effect" describes this interaction, when the neutron shares its kinetic energy with the target nucleus without exciting the nucleus. Radiative capture (n, γ) takes place when a neutron is absorbed to produce an excited nucleus. The excited nucleus regains stability by emitting a gamma ray. The fission process for uranium (U 235 or U 238 ) is a nuclear reaction in which a neutron is absorbed by the uranium nucleus to form the intermediate (compound) uranium nucleus (U 236 or U 239 ). The compound nucleus undergoes fissions into two nuclei (fission fragments), with the simultaneous emission of one or several neutrons. The fission fragments produced have a combined kinetic energy of about 168 MeV for U 235 and 200 MeV for U 238, which is dissipated, causing ionization. The fission reaction can occur with either fast or thermal neutrons. The distance that a fast neutron will travel, between its introduction into the slowing-down medium (moderator) and thermalization, depends on the number of collisions and the distance between collisions. Though the actual path of the slowing neutron is tortuous because of collisions, the average straight-line distance can be determined; this distance is the fast diffusion length or slowing-down length. The distance traveled, once thermalized, until the neutron is absorbed, is the thermal diffusion length. Fast neutrons rapidly degrade in energy by elastic collisions when they interact with low atomic number materials. As neutrons reach thermal energy, or near thermal energies, the likelihood of their capture increases. In present day reactor facilities, the thermalized neutron continues to scatter elastically with the moderator until fuel or non-fuel material absorbs it, or until it leaks from the core. Secondary ionization caused by the capture of neutrons is important in the detection of neutrons. Neutrons will interact with B -10 to produce Li -7 and He -4. The lithium and alpha particles share the energy and produce easily detectable "secondary ionizations." 8 Rev 1

13 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 ELO 1.2 Detector Theory of Operation Introduction The instruments used to measure radiation provide either a dose or a dose rate. Both are useful data when working in a radiological environment. The dose is a total accumulated exposure while the dose rate is the amount of exposure per unit of time. Radiation detectors usually sense a specific type or energy range of radiation. Duration 50 minutes Logistics Use PowerPoint slides and the IG to present ELO 1.2. Within a nuclear plant, several types of radiation detectors sense the various types of radiation and contamination present. Detectors sense alpha particles, beta particles, gamma rays, and neutrons. The detectors use ionization and the electron pairs produced to measure the radiation energy. Differences in detector designs allow detectors to sense different particles. Detector Voltage The relationship between the applied voltage and pulse height in a detector is very complex. Pulse height and the number of ion pairs collected are directly related. The figure below illustrates the number of ion pairs collected vs. applied voltage. Rev 1 9

14 Figure: Gas Amplification Curve The figure includes two curves: one curve for alpha particles and one curve for beta particles. Each curve passes through several voltage regions, labeled Regions I through Region VI. The alpha curve is higher than the beta curve from Region I through part of Region IV due to the larger number of ion pairs produced by the initial reaction of the incident radiation. At low detector voltages, an alpha particle will create more ion pairs than a beta since the alpha has a much greater mass. Once the detector voltage reaches the middle of Region IV, the detector completely discharges with each initiating event, negating the mass difference. Recombination Region The recombination region (Region I) refers to a range of applied voltages from zero to V 1. As the applied voltage increases to V 1, the pulse height increases until it reaches a saturation value. At V 1, the field strength between the cathode and anode is sufficient for collection of all ions produced within the detector. At voltages less than V 1, ions move slowly toward the electrodes, and the ions tend to recombine to form neutral atoms or molecules. Also, the pulse height is less than it would have been if all the ions that originally formed reached the electrodes. Therefore, there is no operation of gas ionization instruments in this region of response. Ionization Region As voltage increases from V 1 to V 2 in the ionization region (Region II), there is no appreciable increase in the pulse height. The field strength is more than adequate to ensure collection of all ions produced; however, it is insufficient to cause any increase in ion pairs due to gas amplification. This region is the ionization region. 10 Rev 1

15 Proportional Region As voltage increases from V 2 to V 3 in the proportional region (Region III), the pulse height increases smoothly. The voltage is sufficient to produce a large potential gradient near the anode, and it imparts a very high velocity to the electrons produced through ionization of the gas by charged radiation particles. The velocity of these electrons is sufficient to cause ionization of other atoms or molecules in the gas, called gas amplification. This gas amplification is termed a Townsend avalanche. The gas amplification factor (A) varies from 10 3 to Because the gas amplification factor (A) is proportional to applied voltage, this region is termed the proportional region. Limited Proportional Region In the limited proportional region (Region IV), as voltage increases from V 3 to V 4, additional processes will occur, leading to increased ionization. The strong field causes increased electron velocity, which results in excited states of higher energies capable of releasing more electrons from the cathode. These events cause the Townsend avalanche to spread along the anode. The positive ions remain at their origination location and reduce the electric field to a point where further avalanches are impossible. For this reason, Region IV is termed the limited proportional region, and there is no detector operation in this region. Geiger-Mueller Region The pulse height in the Geiger-Mueller (GM) region (Region V) is independent of the type of radiation causing the initial ionizations. The pulse height obtained is about several volts. The field strength is so great that the discharge, once ignited, continues to spread until amplification cannot occur due to a dense positive ion sheath surrounding the central wire (anode). The lowest voltage in Region V is termed the threshold voltage (V 4 ). This is where the number of ion pairs level off and remain relatively independent of the applied voltage. This leveling off is called the Geiger plateau; the plateau extends from an applied voltage of about 1,000 volts to about 1,300 volts. The threshold is normally about 1,000 volts. In the GM region, the gas amplification factor (A) depends on the specific ionization of the target radiation. Continuous Discharge Region In the continuous discharge region (Region VI), a steady discharge current flows. The applied voltage (V5 and above) is so high that, once ionization takes place in the gas, there is a continuous discharge of electricity, so there is no detector operation in this region. Radiation detectors normally target a certain type of radiation. Since the detector response can be sensitive to both energy and intensity of the Rev 1 11

16 radiation, each type of detector has defined operating limits based on the characteristics of the target radiation. Many detectors can detect alpha and beta particles, gamma rays, or neutrons. Some types of detectors are capable of distinguishing between the types of radiation; others are not. Some detectors only count the number of particles that enter the detector, while others can determine both the number and energy of the incident particles. Most detectors have one thing in common: they respond only to electrons produced in the detector. In order to detect the different types of incident particles, the detector must convert the targeted particle's energy to electrons in the detector. Gas-filled detectors measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and GM regions with an arrangement most sensitive to the target radiation. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF 3 counters, fission counters, and proton recoil counters are examples of neutron detectors. Gas Filled Detector Gas filled detectors consists of a gas filled cylinder with two electrodes. Sometimes, the cylinder itself acts as one electrode, and a needle or thin, taut wire along the axis of the cylinder acts as the other electrode. The pulsed operation of the gas-filled detector illustrates the principles of basic radiation detection. Radiation detectors contain gases because their ionized particles can travel more freely than ionized particles of a liquid or a solid can. Most detectors use argon and helium gas; detectors designed to measure neutrons use boron trifluoride. The figure below shows a schematic diagram of a gas-filled chamber with a central electrode (anode). Figure: Gas-Filled Detector Diagram The central electrode, or anode, collects negative charges and the cathode (the chamber wall) collects positive charges. Insulation separates the anode from the cathode. A voltage acts on the anode and the chamber walls. A capacitor shunts the resistor in the circuit, so that the anode is at a positive voltage with respect to the chamber wall. 12 Rev 1

17 As a charged particle passes through the gas-filled chamber, it ionizes some of the gas (air) along its path of travel. The positive anode attracts the electrons, or negative particles. The detector wall, or cathode, attracts the positive charges. The collection of these charges reduces the voltage across the capacitor, causing a pulse across the resistor that an electronic circuit records. The voltage applied to the anode and cathode determines the electric field and its strength. As detector voltage is increased, the electric field has more influence upon electrons produced. Sufficient voltage causes a cascade effect that releases more electrons from the cathode. Forces on the electron are greater and at this threshold produce a reduction in the mean-free path between collisions. The total number of electrons collected by the anode determines the change in the charge of the capacitor. The change in charge is proportional to the total ionizing events that occur in the gas. The ion pairs initially formed by the incident radiation attain a high enough velocity to cause secondary ionization of other atoms or molecules in the gas. The resultant electrons cause further ionizations. This multiplication of electrons is termed gas amplification. Knowing the capacitance, detector characteristics, and radiation allows computation of the pulse height. Knowing the detector size and specific ionization, or range of the charged particle allows calculation of the number of ionizing events. The only variable is the gas amplification factor that is dependent on applied voltage. One effect on the operation of gas-filled detectors operating at high voltages within the proportional region is the positive space charge. This is where the pulse amplitude from an ionizing event reduces because positive ions form a cloud around the positive electrode, reducing the electric field strength, thereby limiting secondary ionizations. This effect occurs as the detector voltage increases to the high end of the proportional region, preventing collection of both gamma and neutroninduced pulses, yielding a less accurate neutron count rate. 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 Rev 1 13

18 Duration 10 minutes Logistics Use PowerPoint slides and the IG to present ELO 1.3. ELO 1.3 Proportional Counter Theory Proportional Counters A proportional counter is a detector that operates in the proportional voltage region. The figure below illustrates a simplified proportional counter circuit. Proportional counter detectors use a slightly higher voltage between the anode and cathode. The strong electrical field accelerates the charges produced in the initial ionization fast enough to ionize other electrons in the gas. To be able to detect a single particle, there must be an increase in the number of ions produced. The electrons produced in these secondary ion pairs, along with the primary electrons, continue to gain energy as they move towards the anode; as they do, they produce more and more ionizations. The result is that each electron from a primary ion pair produces a cascade of ion pairs; the primary ions acquire enough energy to cause secondary ionizations and increase the charge collected. The terms gas multiplication or amplification describe this effect. In this voltage regime, the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle. Hence, these gas ionization detectors are termed proportional counters. In the proportional voltage region, there is a linear relationship between the number of ion pairs collected and the applied voltage. It is possible to attain a charge amplification of 10 4 in the proportional region. By proper functional arrangements, modifications, and biasing, the proportional counter can detect alpha, beta, gamma, or neutron radiation in mixed radiation fields. Figure: Proportional Counter To a limited degree, the fill gas will determine what type of radiation the proportional counter will be able to detect. Argon and helium are the most frequently used fill gases and allow for the detection of alpha, beta, and 14 Rev 1

19 gamma radiation. When detection of neutrons is necessary, the detectors use boron trifluoride gas. The simplified circuit, illustrated above, shows that the detector wall acts as one electrode, while the other electrode is a fine wire in the center of the chamber with a positive voltage applied. The figure below shows the three regions of ionization as a function of applied voltage. When a single gamma ray interacts with the gas in the chamber, it produces a rapidly moving electron that produces secondary electrons. Depending on the gas used in the chamber, the gamma ray forms about 10,000 secondary electrons. As the applied voltage increases, it reaches a voltage level where the amount of recombination becomes very low. Further voltage increases above this point do not appreciably increase the number of electrons collected. The region in which all 10,000 electrons are collected is the ionization region. Figure: Gas Ionization Curve As the applied voltage increases above 1,000 V, the number of electrons becomes greater than the initial 10,000. The additional electrons that are collected are due to gas amplification. As voltage increases, the velocity of the 10,000 electrons produced increases. Voltages above a certain threshold accelerate the 10,000 electrons to such speeds that they have enough energy to cause more ionization. Gas amplification is the term for this phenomenon. As an example, if the 10,000 electrons produced by the gamma ray increase to 40,000 by gas amplification, the amplification factor equals 4. Gas amplification factors can range from unity in the ionization region to 10 3 or 10 4 in the proportional region. The high amplification factor of the proportional counter is its major advantage over the ionization chamber. The internal amplification of the proportional counter is such that it registers low energy particles (< 10 KeV), whereas the ion chamber is limited by amplifier noise to particles of > 10 KeV energy. Rev 1 15

20 Proportional counters are extremely sensitive, and the voltages are large enough so that all of the electrons are collected within a few tenths of a microsecond. Each pulse corresponds to one gamma ray or neutron interaction. The amount of charge in each pulse is proportional to the number of original electrons produced. The proportionality factor in this case is the gas amplification factor. The number of electrons produced is proportional to the energy of the incident particle. For each electron collected in the chamber, there is a positively charged gas ion left over. These gas ions are heavy and move much more slowly than electrons. Eventually, these positive ions move away from the positively charged central wire to the negatively charged wall and become neutral by gaining an electron. In the process, these positive ions release some energy, which causes additional ionization of the gas atoms. The positively charged central wire attracts the electrons produced by this ionization; the electrons move toward the central wire and multiply en route. The resulting pulse of charge is unrelated to the target radiation and can set off a series of pulses. For the detector to sense the true value of the target radiation, eliminating or "quenching" these pulses is necessary. One method for quenching these discharges is to add a small amount (~10 percent) of an organic gas, such as methane, in the chamber. The quenching gas molecules have a weaker affinity for electrons than the chamber gas does; therefore, the ionized atoms of the chamber gas readily take electrons from the quenching gas molecules. Thus, the ionized molecules of quenching gas reach the chamber wall instead of the chamber gas. The ionized molecules of the quenching gas become neutral by gaining an electron, and the energy liberated does not cause further ionization, but causes dissociation of the molecule. This dissociation quenches multiple discharges. The quenching process consumes the quenching gas molecules, thus limiting the proportional counter lifetime. Some proportional counters compensate for the quenching gas consumption by constantly replenishing the gas, making their lifetime indefinite. These counters are gas flow counters. 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 16 Rev 1

21 ELO 1.4 Proportional Counter Circuit Introduction Proportional counters measure the 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. The figure below shows the pulses and associated block diagram of a typical circuit used to make such measurements. Duration 15 minutes Logistics Use PowerPoint slides and the IG to present ELO 1.4. Figure: Proportional Counter Circuit The following bullets expand on the steps shown in the figure above. The capacitor converts the charge pulse to a voltage pulse. The voltage is equal to the amount of charge divided by the capacitance of the capacitor. The preamplifier amplifies the voltage pulse. The amplifier further amplifies (typically about 10 volts maximum). A single channel analyzer then determines the pulse size. Single Channel Analyzer The figure below shows the operation of a single channel analyzer (SCA). Figure: Single Channel Analyzer Operation The single channel analyzer has two dial settings: Level dial Rev 1 17

22 Window dial For example, when the level is set at 2 volts, and the window at 0.2 volts, the analyzer will give an output pulse only when the input pulse is between 2 and 2.2 volts. The output pulse is usually a standardized height and width logic pulse, as shown in the figure below. Figure: Single Channel Analyzer Output Since the single channel analyzer can be set so that only a certain pulse size produces an output, it provides for the counting of one specific radiation type in a mixed radiation field. This output passes to a scaler that counts the number of pulses it receives. A timer gates the scaler so that the scaler counts the pulses for a predetermined length of time. Knowing the 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. The most common means of introducing boron is by combining it with tri-fluoride gas to form boron trifluoride (BF 3 ). When a neutron interacts with a boron atom, the atom emits an alpha particle. It is possible to sensitize a BF 3 counter to neutrons but not to gamma rays. The neutroninduced alpha particles produce more ionizations than gamma rays, effectively eliminating gamma ray contributions. This is due mainly to the fact that gamma ray-induced electrons have a much longer range than the dimensions of the chamber; the alpha particle energy is, in most cases, greater than gamma rays produced in a reactor. Therefore, neutron pulses are much larger than gamma ray-produced pulses. By using a discriminator, the scaler can be set to read only the larger pulses produced by the neutron. A discriminator is a single channel analyzer with only one setting. The figure below illustrates the operation of a discriminator. If the discriminator is set at 2 volts, then any input pulse > 2 volts causes an output pulse. 18 Rev 1

23 BF 3 Proportional Counter Figure: Discriminator Characteristics The BF 3 proportional counter monitors low power levels in a nuclear reactor. The "startup" or "source range" channels use proportional counters. Proportional counters are inappropriate for high power levels because they are pulse-type detectors. Typically, it takes 10 to 20 microseconds for each pulse to go from 10 percent of its peak, to its peak, and back to 10 percent. If another neutron interacts in the chamber during this time, the two pulses are superimposed. The voltage output would never drop to zero between the two pulses, and electron production causes the chamber to draw a steady current. The figure below shows a block diagram of a BF 3 proportional counter. Knowledge Check Figure: BF 3 Proportional Counter A BF 3 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- Rev 1 19

24 induced currents. B. In a proportional counter, neutron-induced pulses are significantly larger than gamma pulses. The detector instrumentation filters out the smaller gamma pulses. 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. The BF 3 proportional counter measures neutron flux of sufficient intensity that the gamma signal is insignificant compared to the neutron signal. Duration 10 minutes Logistics Use PowerPoint slides and the IG to present ELO 1.5. ELO 1.5 Ion Chamber Introduction Ionization chambers are electrical devices that detect radiation when the voltage level corresponds to the ionization region. The charge obtained is the result of collecting the ions produced by radiation. This charge will depend on the type of radiation detected. Ionization chambers have two distinct disadvantages when compared to proportional counters: they are less sensitive and they have a slower response time. There are two types of ionization chambers: the pulse counting ionization chamber and the integrating ionization chamber. The pulse counting ionization chamber detects pulses caused by particles traversing the chamber. The integrating chamber sums the pulses, and measures the integrated total of the ionizations produced over a predetermined period. Both the pulse counting and the integrating functions may use the same type of ionization chamber. However, the integrating type ionization chamber is most commonly used. Ion Chambers An ionization chamber, also known as an ion chamber, is comprised of flat plates or concentric cylinders. The flat plate design is preferred because it has a well-defined active volume and ensures that ions will not collect on the insulators and cause a distortion of the electric field. The concentric cylinder design has a less well-defined active volume because the electric field varies nearer the insulator. Ionization chamber construction differs from the proportional counter (flat plates or concentric cylinders versus a cylinder and central electrode) to allow for the integration of pulses produced by the incident radiation. The proportional counter would require such exact control of the electric field between the electrodes that it would not be practical. Ion chambers have a 20 Rev 1

25 relatively low voltage between the anode and cathode, which results in a collection of only the charges produced in the initial ionization event. This type of detector produces a weak output signal that corresponds to the number of ionization events. Higher energies and intensities of radiation will produce more ionization, which will result in a stronger output voltage. The figure below illustrates a simple ionization circuit consisting of two parallel plates of metal with an air space between them. The plates connect to a battery and a highly sensitive ammeter, wired in series. Figure: Simple Ionization Circuit If a detector becomes near a radioactive source that is an emitter of beta particles, the beta particles will pass between the plates and strike atoms in the air. With sufficient energy, the beta particle causes an atom in the air to eject an electron. A single beta particle may eject 40 to 50 electrons for each centimeter of path length traveled. The electrons ejected by the beta particle often have enough energy to eject more electrons from other atoms in air. The total number of electrons produced is dependent on the energy of the beta particle and the gas between the plates of the ionization chamber. In general, a 1 MeV beta particle will eject approximately 50 electrons per centimeter of travel, while a 0.05 MeV beta particle will eject approximately 300 electrons. The lower energy beta ejects more electrons because it has more collisions. Each electron produced by the beta particle, while traveling through air, will produce thousands of electrons. A current of 1 microampere consists of about electrons per second. If applying 1 volt to the plates of the ionization chamber, some of the free electrons will be attracted to the positive plate of the detector. This attraction is not strong because 1 volt does not create a strong electric field between the two plates. The free electrons will tend to drift toward the positive plate, causing a current to flow that the ammeter indicates. Not all of the free electrons will make it to the positive plate because the positively charged atoms that result from electron ejection may capture them. Therefore, the ammeter will register only a fraction of the number of free electrons between the plates. When the voltage is increased, the free electrons are more strongly attracted to the positive plate. They will move toward the positive plate more quickly and will have less opportunity to Rev 1 21

26 recombine with the positive ions. The figure below shows a plot of the number of electrons measured by the ammeter as a function of applied voltage. Figure: Recombination and Ionization Region At zero voltage, no attraction of electrons between the plates occurs. The electrons will eventually recombine, so there is no current flow. As the applied voltage is increased, the positive plate will begin to attract free electrons more strongly, and a higher percentage will reach the positive plate. Further voltage increases will reach a point where all of the electrons produced in the chamber will reach the positive plate. Any further increase in voltage past this point will have no effect on the number of electrons collected. The simple ionization chamber shown in the figure below can detect gamma rays. Since the ammeter is sensitive only to electrons, gamma rays must interact with the atoms in air between the plates to release electrons. The gamma rays interact by Compton scattering, photoelectric effect, or pair production. Each of these interactions causes conversion of some, or all of the energy of the incident gamma rays into the kinetic energy of the ejected electrons. The ejected electrons move at very high speeds and cause other air atoms to eject electrons. The positively charged plate collects all of these electrons and the ammeter measures resultant current. Figure: Ionization Chamber Metal normally encloses the plates in an ionization chamber. This metal chamber shields the plates from outside electric fields and contains the air 22 Rev 1

27 or other gas. Gamma rays easily penetrate the metal walls of the chamber. The metal case stops beta particles and alpha particles from entering. For detection of alpha and beta particles, some means are necessary to provide a thin wall or "window." This window must be thin enough for the alpha and beta particles to penetrate. However, a window of almost any thickness will prevent an alpha particle from entering the chamber. It is also possible to detect neutrons with an ionization chamber. Neutrons are uncharged; therefore, they cause no ionizations themselves. If a thin layer of boron coats the inner surface of the ionization chamber, the following reaction can take place: A boron atom captures a neutron, resulting in emission of an energetic alpha particle. The alpha particle causes ionization within the chamber and ejected electrons cause further secondary ionizations. Another method for detecting neutrons using an ionization chamber is to use the gas boron trifluoride (BF 3 ) instead of air in the chamber. The incoming neutrons produce alpha particles when they react with the boron atoms in the detector gas. Either method will detect neutrons in nuclear reactor neutron detectors. When using an ionization chamber to detect neutrons, it is possible to prevent beta particles from entering the chamber with chamber walls thick enough to shield out all of the beta particles. Gamma rays will penetrate the detector shielding; therefore, they always contribute to the total current read by the ammeter. This effect is undesirable because the detector responds not only to neutrons, but also to gamma rays. Several ways are available to minimize this problem. Discrimination is possible because the ionizations produced by the alpha particles differ in energy levels from those produced by gamma rays. A 1 MeV alpha particle moving through the gas loses all of its energy in a few centimeters. Therefore, all of the secondary electrons result along a path of only a few centimeters. A 1 MeV gamma ray produces a 1 MeV electron, and this electron has a long range and loses its energy over the entire length of its range. If we make the sensitive volume of the chamber smaller without reducing the area of the coated boron, we reduce the sensitivity to gamma rays. Knowledge Check Ionization chambers have two distinct disadvantages when compared to proportional counters: they are, and they have a response time. A. less sensitive; faster B. less sensitive; slower Rev 1 23

28 C. more sensitive; slower D. more sensitive; faster Duration 10 minutes Logistics Use PowerPoint slides and the IG to present ELO 1.6. ELO 1.6 Gamma Compensation Introduction In some instances, it is necessary to discriminate between types of radiation. For example, an intermediate power range neutron monitor will experience a large gamma field not proportional to the actual power. These gammas would ionize the detector gas and appear as a neutron event giving a false indication of power level. Compensating for the response to gamma rays extends the useful range of the ionization chamber. Compensated ionization chambers consist of two separate chambers; one chamber has a boron coating, and one chamber has no coating. The coated chamber is sensitive to both gamma rays and neutrons, while the uncoated chamber is sensitive only to gamma rays. Instead of having two separate ammeters and subtracting the currents, a certain circuit orientation allows a single ammeter to measure the net output of both detectors. Arranging the polarities so that the two chambers' currents oppose one another will allow the ammeter to measure the difference between the two currents. One plate of the compensated ion chamber is common to both chambers; one side has a boron coating, while the other side has no coating. The figure below shows the basic circuitry for a compensated ion chamber. Figure: Compensated Ion Chamber The boron-coated chamber is the working chamber; the uncoated chamber is the compensating chamber. When exposed to a gamma source, the working chamber battery will set up a current flow that deflects the ammeter in one direction. The compensating chamber battery will set up a current flow that deflects the ammeter in the opposite direction. If both chambers are identical, and both batteries are the same voltage, the net current flow is exactly zero. Therefore, the compensating chamber cancels the current due to gamma rays. Since the compensating chamber 24 Rev 1

29 cancels the current due to gamma rays, the remaining current is due to neutrons. Compensated Ion Chambers The two chambers of a compensated ion chamber are never truly identical; often, their design includes different shapes on purpose. Typically, chambers use a concentric cylinder orientation, as illustrated in the figure below. Figure: Compensated Ion Chamber with Concentric Cylinders The design incorporating concentric cylinders has an advantage because both chambers experience nearly the same radiation field. Even though the chambers are different, proper selection of the operating voltage eliminates the gamma current. Working chamber operating voltage is given by the manufacturer and is selected to cause operation on the flat portion of the response curve where very little recombination occurs If working chamber voltage increases to operating voltage, and compensating voltage remains zero, the measured current will reflect gammas only in the working chamber. For this reason, compensating voltage is set while the reactor is shutdown (a minimum number of neutrons are present). As the compensating chamber voltage increases, the measured current will decrease as more of the current from the working chamber offsets the opposite current from the compensating chamber. Eventually, the voltage becomes large enough so that the two currents cancel. One hundred percent compensated refers to a chamber where the voltage has become large enough that the currents have canceled and the measured current is zero. At 100 percent compensation, the detector will respond to neutrons alone. The compensating chamber usually has a slightly larger sensitive volume than the working chamber. If the compensating current increases to a value greater than the working chamber current, the ammeter measures a net negative current. The term overcompensated refers to a chamber in which the compensating current is greater than the working current, and the ammeter measures a net negative current. The compensating chamber cancels too much current from the working chamber, and the ammeter reads Rev 1 25

30 low. In this case, the compensating chamber cancels out all of the gamma current and some of the neutron current. Percent compensation of a compensated ion chamber gives the percentage of the canceled out gamma rays. Measured current provides the basis for calculating percent compensation, on the condition that the detector exposure is to gamma rays only. The ionization chamber compensation curve represented in the figure below is a plot of the percent compensation versus compensating voltage. Figure: Typical Compensation Curve It is important to develop and plot this curve before using a compensated ion chamber. In ideal situations, compensated ion chambers operate at 100 percent compensation, and indicated current is due to neutrons only. Small changes in compensating voltage change the percent compensation. There are serious consequences of operating with either an overcompensated or an undercompensated chamber. The purpose of nuclear instrumentation is to detect and measure neutron level, which is the direct measure of core power. If the compensating voltage is set too high or overcompensated, some neutron current as well as the entire gamma current is blocked and indicated power is lower than actual core power. If compensating voltage is set too low, or undercompensated, the gamma current is only partially blocked, and indicated power is higher than actual core power. At high power, gamma flux is significantly smaller than neutron flux and neutron flux may mask the effects of improper compensation. Proper compensation is extremely important during reactor startup and shutdown. 26 Rev 1

31 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 ELO 1.7 Geiger-Mueller Tube Detector Geiger-Mueller Detector The Geiger-Mueller or GM detector is a radiation detector that operates in Region V, or the GM region. GM detectors produce larger pulses than other types of detectors. However, discrimination is not possible, since the 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. Duration 15 minutes Logistics Use PowerPoint slides and the IG to present ELO 1.7. The number of electrons collected by a gas-filled detector varies as applied voltage is increased. Once the voltage increases beyond the proportional region, the curve reaches a second flat portion of the curve. This portion is termed the Geiger-Mueller region. The Geiger-Mueller region has two important characteristics: The number of electrons produced is independent of applied voltage. The number of electrons produced is independent of the number of electrons produced by the initial radiation. This means that the radiation producing one electron will have the same size pulse as radiation producing hundreds or thousands of electrons. This characteristic depends on the way in which electrons are collected. The figure below shows key components of a GM detector. Rev 1 27

32 Figure: GM Detector When a gamma produces an electron, the electron moves rapidly toward the positively charged central wire. As the electron nears the wire, its velocity increases. At some point, its velocity is great enough to cause additional ionizations. As the electrons approach the central wire, the additional ionizations produce a larger number of electrons near the central wire. For each electron produced, there is a positive ion produced. As the applied voltage increases, the number of positive ions near the central wire increases and a positively charged cloud (called a positive ion sheath) forms around the central wire. The positive ion sheath reduces the field strength of the central wire and prevents further electrons from reaching the wire. It might appear that a positive ion sheath would increase the effect of the positive central wire, but this is not true; the positive potential acts only on the very thin central wire that makes the strength of the electric field very high. The positive ion sheath makes the central wire appear much thicker and reduces the field strength, referred to as the detector's space charge phenomenon. The positive ions will migrate toward the negative chamber picking up electrons. As in a proportional counter, this transfer of electrons can release energy, causing ionization and the liberation of an electron. In order to prevent this secondary pulse, a quenching gas is used, usually an organic compound. The GM counter produces many more electrons than does a proportional counter; therefore, it is a much more sensitive device. Its sensitivity makes it ideal for detecting low-level gamma rays and beta particles. A GM tube collects electrons produced very rapidly, usually within a fraction of a microsecond. The output of the GM detector is a pulse charge and is often large enough to drive a meter without additional amplification. Because the counter produces the same size pulse regardless of the amount of initial ionization, the GM counter cannot distinguish radiation of different energies or types. For this reason, GM counters are not adaptable for use as neutron detectors. Because of its sensitivity, simple counting circuit, and ability to detect low-level radiation, portable instrumentation applications primarily use the GM detector. 28 Rev 1

33 Knowledge Check Which one of the following describes the reason for the high sensitivity of a Geiger-Mueller tube radiation detector? A. Geiger-Mueller tubes are operated at relatively low detector voltages, allowing detection of low energy radiation. B. Changes in applied detector voltage have little effect on detector output. C. Any incident radiation event causing primary ionization results in ionization of the entire detector gas volume. D. Geiger-Mueller tubes are thinner than other radiation detector types. ELO 1.8 Scintillation Detector Introduction The scintillation detector or counter is a solid-state radiation detector that uses a scintillation crystal (phosphor) to detect radiation and produce light pulses. As radiation interacts in the scintillation crystal, energy transfers to bound electrons of the crystal s atoms. If the transferred energy is greater than the ionization energy, the electron enters the conduction band and is free from the binding forces of the parent atom. This leaves a vacancy in the valence band, termed a hole. Duration 15 minutes Logistics Use PowerPoint slides and the IG to present ELO 1.8. If the transferred energy is less than the binding energy, the electron remains attached, but exists in an excited energy state. Once again, a hole forms in the valence band. By adding impurities during the growth of the scintillation crystal, the manufacturer is able to produce activator centers with energy levels located within the forbidden energy gap. The activator center can trap a mobile electron, which raises the activator center from its ground state to an excited state. When the center de-excites, it emits a photon in a process called luminescence. The term luminescence refers to the activator centers in a scintillation crystal. The emitted photons are in the visible region of the electromagnetic spectrum. Scintillation Detectors A photomultiplier tube senses the flashes and converts them into an electrical signal. Scintillation counters require coupling a suitable scintillation phosphor to a light sensitive photomultiplier tube. The figure Rev 1 29

34 below illustrates an example of a scintillation counter using a thalliumactivated sodium iodide crystal. Figure: Scintillation Detector There are three classes of solid-state scintillation phosphors: organic crystals, inorganic crystals, and plastic phosphors. Inorganic crystals include lithium iodide (LiI), sodium iodide (NaI), cesium iodide (CsI), and zinc sulfide (ZnS). Inorganic crystals have high density, high atomic number, and pulse decay times of approximately 1 microsecond. Thus, they exhibit high efficiency for detection of gamma rays and are capable of handling high count rates. Organic scintillation phosphors include naphthalene, stilbene, and anthracene. The decay time of this type of phosphor is approximately 10 nanoseconds. This type of crystal is well suited for detecting beta particles. Plastic phosphors consist of scintillation chemicals added to a plastic matrix. The decay constant is the shortest of the three phosphor types, approaching 1 or 2 nanoseconds. The figure below shows a cross-section of a photomultiplier tube. The photomultiplier is a vacuum tube with a glass envelope containing a photocathode and a series of electrodes called dynodes. Light from a scintillation phosphor liberates electrons from the photocathode by the photoelectric effect. These electrons are too few in number or too low in energy for conventional electronics to detect them reliably. However, in the photomultiplier tube, a voltage drop of about 50 volts attracts them to the nearest dynode. 30 Rev 1

35 Figure: Photomultiplier The photoelectrons strike the first dynode with sufficient energy to liberate several new electrons for each photoelectron. A second dynode attracts the second-generation electrons where a larger third-generation group of electrons is emitted. This amplification continues through 10 to 12 stages. At the last dynode, sufficient electrons are available to form a current pulse suitable for further amplification by transistor circuits. A single external bias, approximately 1,000 volts DC, and network of external resistors to equalize the voltage drops establish the voltage drops between dynodes. The advantages of a scintillation counter are its efficiency, high precision, and high possible counting rates. These latter attributes are a consequence of the extremely short duration of the light flashes, from about 10-9 to 10-6 seconds. The intensity of the light flash and the amplitude of the output voltage pulse are proportional to the energy of the particle responsible for the flash. Consequently, scintillation counters can determine the energy, as well as the number, of the exciting particles (or gamma photons). The photomultiplier tube output is very useful in radiation spectrometry (determination of incident radiation energy levels). Knowledge Check Scintillation detectors convert radiation energy into light by a process known as... A. space charge effect. B. gas amplification. C. luminescence. D. photoionization. Rev 1 31

36 Duration 55 minutes Logistics Use PowerPoint slides and the IG to review TLO 1 material. Use directed and nondirected questions to students, check for understanding of ELO content, and review any material where student understanding of ELOs is inadequate. TLO 1 Summary Radiation detection terms: An electron-ion pair is the product of a single ionizing event. Specific ionization is the number of ion pairs produced per centimeter of travel through matter. Stopping power is the energy lost per unit of path length. Alpha particles: An alpha particle is a 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: A beta particle is an ordinary electron or positron ejected from nucleus of a 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: Neutrons: A gamma ray is a photon of electromagnetic radiation with a very short wavelength and high energy. Three methods of attenuating gamma rays are photoelectric effect, Compton scattering, and pair production. Neutrons have no electrical charge. They have nearly the same mass as a proton (a hydrogen atom nucleus). They collide with nuclei, causing one of the following reactions: inelastic scattering, elastic scattering, radiative capture, or fission. Gas-filled detectors: In a gas-filled detector, the central electrode, or anode, attracts and collects the electron of an ion-pair. Chamber walls attract and collect the positive ion. When applied voltage is high enough, the ion pairs initially formed accelerate to a high enough velocity to cause secondary ionizations. The resultant ions cause further ionizations. This multiplication of electrons is termed gas amplification. Gas amplification regions: 1. Recombination region: The voltage is such a low value that recombination takes place before the electrode collects most of the negative ions. 2. Ionization region: 32 Rev 1

37 The voltage is sufficient to ensure all ion pairs produced by incident radiation are collected. No gas amplification takes place. 3. Proportional region: The voltage is sufficient to ensure all ion pairs produced by incident radiation are collected. The amount of gas amplification is proportional to the applied voltage. 4. Limited proportional region: As voltage increases, additional processes occur, leading to increased ionizations. Since positive ions remain near their point of origin, further avalanches are impossible. 5. 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. 6. Continuous discharge region: Applied voltage is so high that, once ionization takes place, there is a continuous discharge of electricity. Proportional counters: Radiation enters a proportional counter and the detector gas, at the point of incident radiation, becomes ionized. Detector voltage is set so that electrons cause secondary ionizations as they accelerate toward electrode. Electrons produced from secondary ionizations cause additional ionizations. The multiplication of electrons is 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. Proportional counter circuitry: A proportional counter measures the charge produced by each particle of radiation. A preamplifier/amplifier amplifies the voltage pulse to a usable size. A single-channel analyzer/discriminator produces an output only when input is a certain pulse size. A scaler counts the number of pulses received during a predetermined length of time. A timer provides the gating signal to scaler. Rev 1 33

38 Ionization chamber: When radiation enters an ionization chamber, the detector gas at the point of incident radiation becomes ionized. Some of the electrons have sufficient energy to cause additional ionizations. The voltage potential set up on the 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. Compensated ion chamber: A compensated ion chamber has two concentric cylinders: a boroncoated chamber and an uncoated chamber. Both gammas and neutrons interact in the boron-coated chamber. Only gammas interact in the uncoated chamber. Voltages to each chamber is set so the current from gammas in the boron-coated chamber cancels the current from gammas in the uncoated chamber. GM detector: Voltage of a Geiger-Mueller (GM) detector is set so that any incident radiation produces the same number of electrons. As long as voltage remains in the 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 forms a positive ion sheath that prevents additional electrons from reaching the electrode. A quenching gas prevents a secondary pulse due to ionization by the positive ions. Scintillation counter: Radiation interactions with a crystal center raise electrons to an excited state. When the center de-excites, the crystal emits a photon in the visible light range. The following three classes of phosphors are used: Inorganic crystals Organic crystals Plastic phosphors A photon, emitted from phosphor, interacts with the photocathode of a photomultiplier tube, releasing electrons. Using a voltage potential, the electrons are attracted and strike the nearest dynode with enough energy to release additional electrons. 34 Rev 1

39 A series of dynodes subsequently multiplies the additional electrons as they pass through, developing sufficient electrons to form a current pulse suitable for further amplification by transistor circuits. Now that you have completed this lesson, you should be able to do the following: 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) 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 3. Describe the operation of a proportional counter to include: a. Radiation detection b. Quenching c. Voltage variations 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 5. Describe the operation of an ionization chamber to include: a. Radiation detection b. Voltage variations c. Gamma sensitivity reduction 6. Describe how a compensated ion chamber compensates for gamma radiation. 7. Describe the operation of a Geiger-Mueller (GM) detector to include: a. Radiation detection b. Quenching c. Positive ion sheath 8. Describe the operation of a scintillation counter to include: a. Radiation detection b. Three classes of phosphors c. Photomultiplier tube operation Rev 1 35

40 Duration 1 hour 30 minutes Logistics Use PowerPoint slides and the IG to introduce TLO 2. TLO 2 Personnel Radiation Monitoring Overview Radiation surveys monitor conditions throughout a nuclear power plant using portable radiation detectors. Radiation levels change based on plant changing conditions resulting from power level changes and operating systems that interface with the primary systems. It is important to conduct radiation surveys in order to determine the hazards to personnel as well as to ensure safe movement of radioactive material. Objectives Upon completion of this lesson, you will be able to do the following: 1. Describe the use of portable personnel radiation monitoring instruments. 2. Describe how the following detect neutrons: a. Self-powered neutron detector b. Wide range fission chamber c. Flux wire 3. State the various types of radiation detected by the following dosimeters: a. Thermoluminescent dosimeter b. Direct reading pocket dosimeter c. Electronic dosimeter d. Film badge 4. Describe the operation of a thermoluminescent dosimeter, and advantages and disadvantages as compared to other devices. 5. Describe how the direct reading pocket dosimeter measures ionizing radiation, and advantages and disadvantages as compared to other devices. 6. Describe how the electronic dosimeter measures ionizing radiation, and advantages and disadvantages as compared to other devices. 7. Describe how the film badge measures ionizing radiation, and advantages and disadvantages as compared to other devices. Duration 10 minutes Logistics Use PowerPoint slides and the IG to present ELO 2.1. ELO 2.1 Portable Radiation Monitoring Instruments Introduction Routinely, nuclear plant operators require the use of various survey instruments. Due to the potential consequences of improper use of these instruments, it is imperative that all personnel follow prescribed procedures. 36 Rev 1

41 Detector Use It is necessary to verify that a portable meter is working properly before using it. Most monitoring stations have posted instructions for monitoring use; ensure you have read and understand the instructions. Periodic calibration is required for each survey instruments prior to use. The calibration due date should be indicated on the instrument calibration sticker. If the calibration has expired, inform radiation protection personnel, and do not use the out-of-calibration instrument. Visually inspect the instrument for damage or other defects. Cords should be in good shape and free of kinks or frays. The probe should be free of damage. The indicating scale should be visible, show movement, and should indicate a reasonable background reading. Verify the battery strength is high enough for proper operation. Place the meter in the battery check position; if the battery check shows inadequate voltage, do not use the meter. The battery check step is not required if the instrument is connected to an AC power source. The 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 the radiation protection department to ensure proper operation and indication within a specified range. A sticker on the detector may indicate the last source check date. The response check just verifies the detector response when checked with a known source. Knowledge Check Which of the following should be performed prior to each use of a portable radiation detector? (Select all that apply.) A. Battery strength should be verified B. Calibration date check C. Source check D. Visually inspected for damage or defects Rev 1 37

42 Duration 12 minutes Logistics Use PowerPoint slides and the IG to present ELO 2.2. ELO 2.2 Neutron Detection Introduction This section includes details of three types of detectors. They include the following: Self-powered neutron detectors Fission chambers Activation foils and wires Neutron Detectors Self-Powered Neutron Detector In very large reactor plants, the need exists to monitor neutron flux in various portions of the core continuously. Continuous monitoring allows for quick detection of instability in any section of the core. This need led to the development of the self-powered neutron detector that is small, inexpensive, and rugged enough to withstand the in-core environment. The self-powered neutron detector requires no voltage supply for operation, as shown in the figure below. Figure: Self-Powered Neutron Detector The central wire of a self-powered neutron detector is comprised of a material that absorbs a neutron and undergoes radioactive decay by emitting an electron (beta decay). Cobalt, cadmium, rhodium, and vanadium are the typical central wire materials. A good insulating material separates the central wire and the detector casing. Each time a neutron interacts with the central wire, it transforms one of the wire's atoms into a radioactive nucleus. The nucleus eventually decays by the emission of an electron. Because of these electron emissions, the wire becomes more and more positively charged. The positive potential of the wire causes a current to flow in the resistor, R. A millivolt meter measures the voltage drop across the resistor. Alternatively, an electrometer will measure the electron current from beta decay directly. There are two distinct advantages of the self-powered neutron detector: Very little instrumentation is required; only a millivolt meter or an electrometer 38 Rev 1

43 Emitter material has a much greater lifetime than boron or U 235 lining (used in wide range fission chambers) One disadvantage of the self-powered neutron detector is that the emitter material decays with a characteristic half-life. In the case of rhodium and vanadium, which are two of the most useful materials, the half-lives are 1 minute and 3.8 minutes, respectively. This means that the detector cannot respond immediately to a change in neutron flux, but takes as long as 3.8 minutes to reach 63 percent of steady-state value. Using cobalt or cadmium emitters, which emit their electrons within seconds after neutron capture, overcomes this disadvantage. The term prompt self-powered neutron detector refers to a self-powered neutron detector that uses cobalt or cadmium emitters. Wide-Range Fission Chamber Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U 235. The neutrons interact with the U 235, causing fission. One of the two fission fragments enters the chamber, while the other fission fragment embeds itself in the chamber wall. One advantage of using U 235 coating rather than boron is that the fission fragment has a much higher energy level than the alpha particle from a boron reaction. Neutron-induced fission fragments produce many more ionizations in the chamber per interaction than do the neutron-induced alpha particles. This allows the fission chambers to operate in higher gamma fields than an uncompensated ion chamber with boron lining. An advantage of fission chambers is that they may serve as a current indicating device and a pulse device simultaneously. They are especially useful as pulse chambers due to the very large pulse size difference between neutrons and gamma rays. Nuclear instrumentation systems often leverage the fission chamber's dual use capability in "wide range" channels. Fission chambers are also capable of operating over the source and intermediate ranges of neutron levels. Activation Foils and Flux Wires Whenever it is necessary to measure a neutron flux profile in a reactor, we insert a section of flux wire or foil directly into the reactor core. The flux wire or foil remains in the core for the length of time required for activation to the desired level. It is necessary to know the cross-section of the flux wire or foil to obtain an accurate flux profile. After the flux wire or foil reaches the desired activation level, we remove it from the reactor core and count the activity promptly. It is possible to configure activated foils to discriminate energy levels by placing a cover over the foil to filter out (absorb) certain energy level Rev 1 39

44 neutrons. A cadmium cover effectively filters out all of the thermal neutrons, and is a commonly used cover material. Knowledge Check Which of the following are types of neutron detectors? (Select all that apply.) A. Self-powered neutron detector B. Activation wire C. Self-powered GM tube D. Fission chamber ELO 2.3 Dosimetry and Types of Radiation Detected Introduction Dosimeters refer to a group of radiation detection devices that record a dose or dose rate. All the devices detect ionizing radiation, but some incorporate features to allow them to detect only certain types of radiation. For example, beta radiation does not travel far, and many materials will easily block it. Therefore, dosimeter design is important, as it will determine which type of radiation the dosimeter will detect. Duration 5 minutes Logistics Use PowerPoint slides and the IG to present ELO 2.3. Personnel radiation monitoring uses dosimeters of the following types: Thermoluminescent dosimeter Direct reading pocket dosimeter Electronic dosimeter Film badge Dosimeters Below are the different personal dosimetry devices and the types of radiation they detect: Normally, the thermoluminescent dosimeter, or TLD, is used to detect both beta and gamma radiation accumulated doses. A TLD measures ionizing radiation exposure by measuring the amount of visible light emitted from a crystal in the detector when the crystal is heated. The amount of light emitted is dependent upon the radiation exposure. A direct reading pocket ionization dosimeter or self-reading pocket dosimeter contains a small ionization chamber with a volume of 40 Rev 1

45 approximately two milliliters. Inside the ionization chamber is a central wire anode, and attached to this wire anode is a metal-coated quartz fiber. After charging the anode to a positive potential, the charge distributes between the wire anode and quartz fiber. Electrostatic repulsion deflects the quartz fiber, and the greater the charge, the greater the deflection of the quartz fiber. Gamma radiation incident on the chamber produces ionization inside the active volume of the chamber. This device does not detect alpha or beta radiation because alpha and beta particles cannot pass through the metal casing. The electronic dosimeter records dose information and dose rate. Frequently, the electronic dosimeter uses a Geiger-Mueller counter that measures gamma and x-ray radiation. Personnel dosimetry film badges are used to measure and record radiation exposure or accumulated dose due to gamma rays, x-rays, and beta particles. Knowledge Check Which of the following are used in personnel dosimetry? (Select all that apply.) A. Fission chamber B. Film badge C. Direct reading dosimeter D. Thermoluminescent dosimeter ELO 2.4 Thermoluminescent Dosimeter Operation Introduction There are two common dosimeters of legal record (DLR) used in the plants: a thermoluminescent dosimeter (TLD) and an optically stimulated luminescent dosimeter (OSLD). Some plants have switched or are using OSLDs, which are similar to the TLD, but uses 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. This section will focus on TLD operation. Duration 15 minutes Logistics Use PowerPoint slides and the IG to present ELO 2.4. Thermoluminescent dosimeters are widely used to measure personal radiation dose. They have widely replaced film badges for personnel monitoring. They are more durable and accurate. Understanding their operation will help more understand the interactions of ionizing radiation with matter. Thermoluminescent Dosimeter Rev 1 41

46 There are two main types of TLDs. Both consist of a small crystal, either calcium fluoride or lithium fluoride. Calcium fluoride records gamma exposure and lithium fluoride records gamma and neutron exposure. As the radiation interacts with the crystal, it causes electrons in the crystal's atoms to jump to higher energy states where they stay trapped due to impurities in the crystal, until heated. Heating the crystal causes the trapped electrons to drop back to their ground state, releasing a photon of energy equal to the energy difference between the trap state and the ground state. Photomultiplier tubes count the released light and the number of photons counted is proportional to the quantity of radiation striking the phosphor in the photomultiplier tube. The electrons also drop back to ground state after a long period; fading is the term for this effect, and it is dependent on the incident radiation energy and intrinsic properties of the TLD material. The figure below shows a typical TLD. Figure: Typical TLD The TLDs are versatile; they have applications for both environmental and staff monitoring in facilities involving radiation exposure, as well as others. Usually, personnel will wear a TLD for a period of time (usually 3 months or less) and then processing the TLD will determine the dose received, if any. When worn by personnel, the TLD is located in the chest area on the trunk to simulate the whole body for the body dose. If extremity monitoring is required, a TLD may be located on an extremity. The TLDs can measure doses as low as 1 millirem. The advantages of a TLD over other personnel monitors are its linearity of response to dose, its relative energy independence, and its sensitivity to low doses. It is also reusable, which is an advantage over film badges. However, TLDs provide no permanent record or re-readability, nor is an immediate, on the job readout possible. Knowledge Check A measures ionizing radiation exposure by measuring the amount of visible light emitted from a when the detector it is heated. A. direct reading dosimeter; crystal 42 Rev 1

47 B. thermoluminescent dosimeter; crystal C. thermoluminescent dosimeter; wire D. direct reading dosimeter; wire ELO 2.5 Self-Reading Pocket Dosimeter Introduction A direct reading pocket ionization dosimeter or self-reading pocket dosimeter contains a small ionization chamber. Inside the ionization chamber is a central wire anode, and attached to this wire anode is a metalcoated quartz fiber. After charging the anode to a positive potential, the charge distributes between the wire anode and quartz fiber. Electrostatic repulsion deflects the quartz fiber, and the greater the charge, the greater the deflection of the quartz fiber. The figure below shows a direct reading pocket dosimeter, a cut-a-way of the dosimeter, and the required charger. Duration 15 minutes Logistics Use PowerPoint slides and the IG to present ELO 2.5. Figure: Direct Reading Dosimeter and Charger Gamma radiation incident on the chamber produces ionization inside the active volume of the chamber. Alpha and beta particles cannot pass through the metal casing, and do not contribute to the reading. The positively charged central anode attracts electrons produced by ionization and collects the electrons. This collection of electrons reduces the net positive charge and allows the quartz fiber to return to its original position. The amount of movement is directly proportional to the amount of ionization that occurs. Self-Reading Pocket Dosimeter A system of built-in lenses enables observation of the fiber position, by pointing the instrument at a light source and viewing through the eyelens. To read the value, observe the fiber's location against a translucent scale, graduated in units of exposure (shown enlarged in the figure below). Typical pocket dosimeters have a full-scale reading of 200 milliroentgens but there are designs that will read higher amounts. While in a radiation Rev 1 43

48 area requiring the use of a pocket dosimeter, read the dosimeter frequently. The pocket dosimeter contains no recording capabilities, so record the measured exposure at the end of each shift. Figure: Pocket Dosimeter Internals The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of their accumulated radiation dose. It also has the advantage of being reusable. The limited range, inability to provide a permanent record, and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter. It is necessary to recharge and record the dosimeters at the start of each working shift. Charge leakage, or drift, can also affect the reading of a dosimeter. Leakage is the gradual loss of the repulsion charge on the moveable fiber. Leakage should be no greater than 2 percent of full scale in a 24-hour period. A direct reading pocket dosimeter is an example of an electroscope ionization chamber. Pocket dosimeters provide personnel with a means of monitoring their radiation exposure. The dosimeters are available in many ranges of gamma exposures from 0 through 200 milliroentgens to 0 through 1,000 roentgens. The sensitivity of the instrument is determined at the time of manufacture. Each instrument includes appropriate scale markings for its dose range. Knowledge Check The following are advantages of a self-reading dosimeter, except... A. reusable 44 Rev 1

49 B. are reliable, even if dropped C. primarily sensitive to gamma radiation D. immediate reading of dose ELO 2.6 Electronic Dosimeter Introduction The electronic dosimeter is another type of pocket dosimeter. The electronic dosimeter records dose information and dose rate. Frequently, the electronic dosimeter uses a Geiger-Mueller counter that measures gamma and x-ray radiation. The output of the radiation detector is collected and, upon reaching a predetermined exposure, the dosimeter discharges the collected charge to trigger an electronic counter. The counter then displays the accumulated exposure and dose rate digitally. Duration 5 minutes Logistics Use PowerPoint slides and the IG to present ELO 2.6. Electronic Dosimeter Some digital electronic dosimeters include an audible alarm feature that emits an audible signal or chirp with each recorded increment of exposure. Some models can also be set to provide a continuous audible signal upon reaching a preset exposure. The automatic and customizable features largely minimize the reading errors associated with direct reading pocket dosimeters. In addition, the instrument can achieve a higher maximum readout before resetting is necessary. The electronic dosimeter has commonly replaced the direct read pocket dosimeter due to electronic dosimeter reliability, and ability to indicate an accumulated dose as well as a dose rate. The figure below shows an example electronic dosimeter. Figure: Electronic Dosimeter Rev 1 45

50 Knowledge Check A Geiger-Mueller electronic dosimeter measures what type of radiation? (Select all that apply.) A. x-rays B. beta C. gamma D. neutron Duration 10 minutes Logistics Use PowerPoint slides and the IG to present ELO 2.7. ELO 2.7 Film Badge Introduction Film badges are no longer commonly used for personnel monitoring devices in commercial power plants. Film badges are used to measure and record radiation exposure or accumulated dose due to gamma rays, x-rays, and beta particles. As the name implies, the detector is a piece of radiation-sensitive film. A light proof, vapor proof envelope preventing light, moisture, or chemical vapors from affecting the film protects the film. The figure below shows a film badge holder opened to reveal the film retained by the holder. Figure: Film Badge Film badges use a special film that has coatings of two different emulsions. One side of the film has a coating of a large grain, fast emulsion that is sensitive to low levels of exposure. The other side of the film has a coating of a fine grain, slow emulsion that is less sensitive to exposure. If the radiation exposure causes darkening of the fast emulsion in the processed film to a degree that it cannot be interpreted, the fast emulsion is removed and the dose is computed using the slow emulsion. 46 Rev 1

51 Film Badge A film holder or badge contains the film inside (see figure below). The badge incorporates a series of filters (absorbers) to determine the quality (energy) of the radiation. The various types of absorbers attenuate radiation of different energies. Therefore, the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter. By comparing these results, the energy of the radiation can be determined; knowing the film response for that energy enables the dose calculation. The badge holder also contains an open window to determine radiation exposure due to beta particles. The badge needs a small open window because even the thin (usually plastic) film holder effectively shields beta particles. Figure: Film Badge Filters 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 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 Rev 1 47

52 I - indium foil sometimes included to detect fast neutrons Figure: Film Badge Internals The holder creates a distinctive pattern on the film (see figure above) indicating the type and energy of radiation to which it was exposed (discrimination). The badge monitoring service determines cumulative doses from beta, x-ray, gamma, and thermal neutron radiations by measuring the optical densities (darkness) of film under the filters and comparing the results with calibration films exposed to known doses. Films provide a permanent record; reanalysis at a future time is possible, if necessary. The major advantages of a film badge as a personnel-monitoring device are that it provides a permanent record, it is able to distinguish between different energies of photons, and it can measure doses due to different types of radiation. The film badge is very accurate for exposure greater than 100 millirem. The major disadvantages are: a third party must develop it, a processor must read it (which is time consuming), prolonged heat exposure can affect the film, and exposures of less than 20 millirem of gamma radiation cannot be accurately measured. The film badge can be very costly and is inaccurate at low levels of radiation. The inaccuracy can be as high as 50 percent; therefore, they are frequently used for extremity monitoring. It is important to position a film badge correctly on the wearer so that the dose the badge receives accurately represents the dose the wearer receives. Monitored personnel wear whole body badges on the body between the neck and the waist, often on the belt or a shirt pocket. Knowledge Check In a film badge, the film is contained inside a film holder or badge that incorporates a series of to 48 Rev 1