Topic 6 - Radiological Monitoring and Method - Detectors MAC2022.ppt

RADIOLOGICAL MONITORING AND METHOD: DETECTOR Pusat Latihan Nuklear Malaysia / Nuclear Malaysia Training Centre Module Module Objectives Objectives • To understand what is radiological monitoring and its purpose • To become familiar with the different types of radiation detectors • To become famil...

RADIOLOGICAL MONITORING AND METHOD: DETECTOR Pusat Latihan Nuklear Malaysia / Nuclear Malaysia Training Centre Module Module Objectives Objectives • To understand what is radiological monitoring and its purpose • To become familiar with the different types of radiation detectors • To become familiar with the different types of radiological monitoring equipment Contents Contents • • • • • • Introduction Objectives of Radiological Monitoring Classification of Monitoring Equipment Types of Radiation Detectors Types of Monitoring Instrument Summary Introduction Introduction • Radiation is a hazard that cannot be sensed directly by any of the five senses. • Working with radiation sources must be carried out with due consideration given on proper selection and use of measuring instruments and an effective monitoring programme. Introduction Introduction The programme covers both radiation and contamination and may consists of the following components: Radiological Monitoring Personnel Main Aim To control occupational exposure of working personnel. Workplace More toward controlling public Environmental exposure. Objectives Objectives of of Radiological Radiological Monitoring Monitoring As prescribed in the Basic Safety Radiation Protection, 2010:  To assess the radiation exposure situation in compliance with regulatory requirements.  To verify the effectiveness of radiation protective measures provided at work places.  To identify occurrence of any abnormal radiation exposure situation in work places.  To keep a constant surveillance over the working environment and to detect the quantity and extent of contamination. Classification of Monitoring Instruments MONITORING INSTRUMENTS RADIATION INDIVIDUAL CONTAMINATION AREA INDIVIDUAL AREA RADIATION MONITORING INSTRUMENTS RADIATION AREA INDIVIDUAL 1. Thermo Luminescence Dosimeter (TLD) 2. Optically Stimulated Luminescence Dosimeter (OSLD) 3. Radiophoto Luminescence Dosimeter (RPLD) 4. Pocket dosimeter 5. Integrating dosimeter Beta 1. Survey meter, type ionization chamber 2. TLD 3. OSLD 4. RPLD Gamma & X-ray 1. Survey meter, type Geiger Muller (GM) 2. Dose rate meter 3. Fixed installed monitor 4. TLD 5. OSLD 6. RPLD Neutron 1. Dose Equivalent Meter RADIATION MONITORING TLD badge and ring Film badge Pocket dosimeter Individual Monitoring Portable survey meter Fixed installed monitor Area Monitoring TLD CONTAMINATION MONITORING CONTAMINATION INDIVIDUAL 1. 2. Air sampler Bioassay technique using whole body counter, liquid scintillation counter and alpha spectrometer AREA Surface Contamination Survey Meter 1. 2. 3. Smears GM Probes Dual Phosphor Probes Surface contamination meter (hand, foot & clothing) Airborne Contamination Continuous 1. 2. 3. Sample Gamma spectrometer Liquid scintillation counter Alpha spectrometer CONTAMINATION MONITORING Contamination meter Thyroid Counter Frisking Technique Whole body counter Hand & Foot monitor Types Types of of Radiation Radiation Detector Detector • The basic interaction of radiation with matter is the excitation or ionization of an atom or a molecule. • All detectors of ionizing radiation make use of ionization and excitation process. • There are direct or indirect measurements of ionization. • Selection of a specific measuring device depends on several factors including:  Relative intensity of the radiation.  Required measurement accuracy. Types Types of of Radiation Radiation Detector Detector Classified according to the medium of interactions of the radiation: 1. 2. 3. 4. 5. 6. Gas-filled Detector Scintillation Detector Semiconductor Detector Photographic Emulsion Luminescence Dosimeter Neutron Detector 1. 1. Gas-filled Gas-filled Detectors Detectors Used for charged particles producing ionization in gas filled chamber. Most common gas-filled detectors are: 1.1 Ionization Chambers 1.2 Proportional Counters 1.3 Geiger-Muller Counters 1. 1. Gas-filled Gas-filled Detectors Detectors Main differences among the three types of gasfilled detectors lie in:  Gas used.  Pressure at which the gas is maintained within the chamber.  Voltage level that is maintained between the central electrode and walls of the chamber. 1. 1. Gas-filled Gas-filled Detectors Detectors • The different region of operation of gas filled detector for two different doserate. High Doserate High Dose rate Low Doserate Low Dose rate • An electric field is used to collect all the ionizations produced by the incident radiation in the gas volume 1. 1. Gas-filled Gas-filled Detectors Detectors 1.1 Ionization Chamber • Ion chambers are the simplest of all gas filled detectors. • Best used as photon measuring instruments but can be modified to monitor for alpha, beta, and even neutron radiation. • Less sensitivity compared to G-M counter but can be used in high counting rate situations. • Have good energy dependence characteristics. • For most applications, this measurement gives a good approximation of the effective dose rate to our bodies 1. 1. Gas-filled Gas-filled Detectors Detectors 1.1 Ionization Chamber – free air ionization chamber The current (flow of free electrons) is directly related to the amount of radiation incident on the chamber Free-air ionization chamber for xray between 7.5 –100 kV Principle of ion-chamber • Ion pairs (electron and positive ion) are created when gas between the electrodes is ionized by incident ionizing radiation. • If a voltage were applied between two electrodes, the positive ions and electrons move to the electrodes of the opposite polarity under the influence of the electric field. • An electrical current is then generated by the movement of electrons to the anode which is measured by an electrometer circuit. • The accumulated charge is proportional to the number of ion pairs created, and hence the radiation dose. 1. 1. Gas-filled Gas-filled Detectors Detectors 1.1 Ionization Chamber - medical Radcal 2026: 6cc, 6m, 60 cc dan 180cc 1. 1. Gas-filled Gas-filled Detectors Detectors 1.1 Ionization Chamber - area monitoring Eberline RO2 Victoreen 450 & 450P 1. 1. Gas-filled Gas-filled Detectors Detectors 1.2 Proportional Counters  Proportional tubes are almost always operated in pulse mode.  Rely on the phenomenon of gas multiplication to amplify greatly the charge (electron) represented by the original ion pairs created within the gas because of a strong applied electric field. The amplified charge produce large voltage pulse and proportional to radiation dose reading.  One important application is the detection and spectroscopy of low energy X-radiation.  Widely applied in the detection of neutrons and quantify alpha & beta activity. 1. 1. Gas-filled Gas-filled Detectors Detectors 1.2 Proportional Counter Widely applied in the detection of neutrons. Air proportional alpha Windowless 100 cm2 gas proportional detector for tritium detection BF3 proportional detector with moderator for neutron; End window GM detector for alpha, beta and gamma. 1. 1. Gas-filled Gas-filled Detectors Detectors 1.3 Geiger-Muller (GM) Counter • The tube is sealed and filled at low pressure with an inert gas such as argon, helium or neon with some gases added. • Useful for monitoring low-level beta and gamma radiation. • High sensitivity. • Meters of choice for monitoring contamination and searching of lost radiation sources. • Relatively long dead time and makes them unsuitable for accurate counting at high counting rates. 1. 1. Gas-filled Gas-filled Detectors Detectors 1.3 Geiger-Muller Tube Gas amplification • Electrons receive enough energy to ionize • Avalanche of secondary produce near the anode • Current multiplication 1. 1. Gas-filled Gas-filled Detectors Detectors 1.3 Geiger-Muller Tube Types of GM detector • The end-window detector employs a thin wall (Mica) at its end to allow most alpha and beta radiations to enter the detector without being stopped. This detector can also measure gamma/x-ray radiation. • The pancake detector also has an end-window, with a wider diameter which permits faster detection. • A side-wall tube can detect beta and gamma or x-ray radiation using an aluminum or stainless steel outer wall that can slides or rotates to selectively expose the actual detector to the radiation. 1. 1. Gas-filled Gas-filled Detectors Detectors 1.3 Geiger-Muller Tube (a) End window GM probe, energy uncompensated, for alpha, beta and gamma surveys (b) Internal GM & external probe for alpha, beta, X or gamma count rates (c) Internal GM & external probe, can detects gamma underwater (d) Energy uncompensated &compensated GM (option) (a) (b) (c) (d) 2. 2. Scintillation Scintillation Counter Counter • Scintillation can be produced using solid medium and this is the underlying mechanism for the sodium iodide thallium-activated detector, NaI(Tl). • Measures the light released by a crystal after an interaction with radiation. • Consists of crystal NaI(Tl) & Photomultiplier tubes CRYSTAL NaI (Tl) RADIATION LIGHT ELECTRONS 2. 2. Scintillation Scintillation Counter Counter • Alpha particles may be detected using Zinc Sulfide (ZnS) • Photons interactions are detected using sodium iodide (NaI) Portal Monitor I125 and X-ray survey Scintillation survey meter 3. 3. Semiconductor Semiconductor Detector Detector  Maximize ionizing radiation capture.  Use of devices employing semiconductors.  Electron-hole pairs created along the path taken by the charged particle (primary radiation or secondary particle) through the detector.  Motion in an applied electric field generates the basic electrical signal from the detector. 3. 3. Semiconductor Semiconductor Detector Detector Schematic of Semiconductor Detector 3. 3. Semiconductor Semiconductor Detector Detector 3. 3. Semiconductor Semiconductor Detector Detector • Radiation is measured by means of the number of charge carriers set free in the detector, which is arranged between two electrodes. • Ionising radiation produces free electrons and holes. The number of electron-hole pairs depends on the energy transmitted (intensity) by the radiation to the semiconductor. • As a result, a certain number of electrons are transferred from the valence band to the conduction band, and an equivalent number of holes are created in the valence band. •Under the influence of an electric field, electrons as well as holes travel to the electrodes, where they give rise to a pulse that can be measured in an outer circuit. 3. 3. Semiconductor Semiconductor Detector Detector Germanium Detector 3. 3. Semiconductor Semiconductor Detector Detector • Silicon (Si) Detector    • Atomic number 14. Extremely low noise, which results from the use of highresistivity Si substrates. A low-leakage-current fabrication process. Germanium (Ge) Detector  Excellent energy resolution, potentially high spatial resolution, large active volumes leading to high detector efficiencies, simplified fabrication, and enabling unique detector geometries and detection schemes. 3. 3. Semiconductor Semiconductor Detector Detector Advantages:  Long life expectancy.  Improved reliability - the readings more stable than other detector.  Improved maintainability -semiconductor detectors operate at a fixed.  Low voltage, the need for work in adjusting the detectors is greatly reduced.  Miniaturization - semiconductor detectors and equipment are small – easily carried & saving space.  Compact design. 3. 3. Semiconductor Semiconductor Detector Detector Disadvantages: • Lower sensitivity. • Poor energy resolution, scatter rejection. • Poor spectral performance. • Voltage supplied must large enough. • Too high energy photon. 4. 4. Photographic Photographic Emulsion/Film Emulsion/Film  Film dosimeters consist of a piece of photographic film in a holder  The holder is fitted with a range of filters which allows to distinguish between beta, X-ray, gamma and thermal neutron radiations.  Degree of blackening (optical density) on the developed film can be compared with calibrated films that have been exposed to known doses for determining both the total dose received by the wearer and also the contribution to total dose by each type of radiation  Film badges are useful for measuring doses to individuals as information on both the type and energy of the radiation received can also be determined 4. 4. Photographic Photographic Emulsion/Film Emulsion/Film Black = exposed White = not exposed How was this film badge exposed? Al Filter Pb Filter No Filter 4. 4. Photographic Photographic Emulsion/Film Emulsion/Film  The presence of surface contamination on the holder can be ascertained by an irregular darkening of the film  Advantage: the films can be kept as a permanent record of an individual’s dose for reassessment at a later date if necessary  Disadvantage:  adverse effects to light and heat  They also require dark room facilities and significant manual handling during assessment  The films cannot be reused and, although they are cheap, they are in limited supply 5. 5. Luminescence Luminescence Dosimeter Dosimeter • Dose is determined by trapped electrons being freed by exposing the dosimeter to light (for optically and stimulated luminescence, OSL and Radiophotoluminescence, RPL) or heat (for thermoluminescent dosimeter, TLD) • When an electron is freed, it falls to a lower energy level and emits a photon of light • The number of photons emitted is proportional to the dose, (the number of trapped electrons) 5. 5. Luminescence Luminescence Dosimeter Dosimeter Electron promoted to conduction band moves to trap Electron falls to valence band causing luminescence Conduction Band e- Electron Trap Electron Trap e- BAND GAP e e Hole Trap + e e e e e e e e - - - - - - - Light Photon - Valence Band Hole Trap e- e- + e- e- e- e- e- e- 5.1 5.1 Thermoluminescence Thermoluminescence Dosimeter Dosimeter • There are 4 TLD chips on each badge. • They measure Hp(0.07) (skin dose); Hp(3) (lens of the eye); and Hp(10) (deep dose). • Two of the dosimeters are used to measure Hp(10) – one has a 6Li element which is sensitive to neutrons and 7Li with is not sensitive to neutrons. The difference between these two provides the neutron dose. 5.1 5.1 Thermoluminescence Thermoluminescence Dosimeter Dosimeter • TLD Characteristics • Measuring Radiation Types: – Photons > 5 keV; – Beta energies > 70 keV; and – Neutrons from thermal to 100 MeV; • Linear response from 10 Sv to 10 Sv; • Detection threshold of 1 Sv; • Reusable from 100 to 1000 times; • Fading of the signal of <20% over 3 months; • Composition is similar to tissue equivalent with LiF:Mg,Ti. 5.2 Optically stimulated luminescent dosimeter (OSLD) • The detector material used is Aluminum oxide doped with Carbon (Al2O3:C). • The OSLD is exposed to green light by OSLD reader and emitted blue light. • Amount of light is proportional to dose. OSL badge OSL holder OSLD 5.2 Optically stimulated luminescent dosimeter (OSLD) InLight OSLD Reader: -200 pieces per scan -< 1 hour to read 200 pieces of OSLD badge Annealing system: -Each OSLD is exposed with green light for 15 s to remove all residue signals. - 50 pieces per scan Radio photo luminescent glass dosimeter (RPLGD) • Detector material – A silver activated phosphate glass. • Store the energy from radiation until the glass is exposed to ultraviolet light, at which time the energy is released in the form of yellow light (560 nm). • A fluorimeter is used to measure the light output. 6. Neutron Detector Types of Neutron Detectors  BF3 neutron detectors  Boron lined neutron detectors  He3 neutron detectors  Fission chambers  Proton recoil counters  Radiative capture detectors 6. 6. Neutron Neutron Detector Detector Portable Neutron Monitor He Neutron Detectors 3 6. 6. Neutron Neutron Detector Detector He Vs. BF3 Detectors 3  Boron has a lower neutron cross section (3840 barns) compared with 3He (5400 barns), therefore boron counters are less sensitive than their helium counterparts.  The energy released per reaction is higher in 10B than 3He which enables BF3 counters to discriminate against gamma pulses.  BF3 counters function at much higher voltages than 3He counters (1-4 kV). For voltages above 2 kV it is recommended that a guard ring be used on the anode insulator. Guard rings prevent electrical leakage across the insulator and reduce noise from the counter signal. Thank You for your attention