Niab 2016
- 3. In The Name Of ALLAH The Most Merciful & Benevolent Beyond Reckoning
- 7. 7 The Electromagnetic Spectrum Waveform of Radiation NONIONIZING IONIZING Radio Microwaves Infrared Visible light Ultraviolet X-rays Gamma rays
- 8. Radiation is the emission and transmission of energy through space or through a material medium. Radiation can be in the form of sub-atomic particles (protons, neutrons and electrons) or electromagnetic waves
- 9. 9 What is Radiation? Form of energy Emitted by nucleus of atom or orbital electron Released in form of electromagnetic waves or particles
- 10. Ionizing Radiation (IR) causes ions to be produced when radiation is absorbed in matter Non-Ionizing radiation (NIR) refers to radiation energy that, instead of producing charged ions, when passing through matter, has sufficient energy only for excitation.
- 15. Why We Measure Personal Dosimetry vs Regulatory requirements Classification of safe and dangerous zones Safety vs Protective measures Quality control and Quality assurance Data consistancy and standardization
- 16. Importance of Radiation in Daily Life Medicine: Radiology, Radiation Therapy Power Production and Space Exploration In Agriculture to improve variety & yield production Weapons of mass destruction Industrial applications: Gauges, Radiography, Mineral exploration
- 17. Why is Radiation Detection Difficult? Can’t see it Can’t smell it Can’t hear it Can’t feel it Can’t taste it We take advantage of the fact that radiation produces ionized pairs to try to create an electrical signal
- 18. How Do I Protect Myself? Reducing the dose from any source radiation exposure involves the use of three protective measures: – TIME – DISTANCE – SHIELDING
- 19. Time − The amount of exposure an individual accumulates is directly proportional to the time of exposure − Keep handling time to a minimum
- 20. Distance − The relationship between distance and exposure follows the inverse square law. The intensity of the radiation exposure decreases in proportion to the inverse of the distance squared − Dose2 = Dose1 x (d1/d2)2
- 21. Shielding − To shield against beta emissions, use plexiglass to decrease the production of bremsstrahlung radiation. − If necessary, supplement with lead after the plexiglass − To shield against gamma and x-rays, use lead, leaded glass or leaded plastic
- 22. Self Protection Measures 1st Avoid exposure and contamination Detect radiation exposure Back away when too high 2nd Use Personal Protective Equipment (PPE) 3rd Decontaminate yourself
- 23. Simplified Radiation PPE Protect your respiratory tract Respirator, surgical mask, etc. Protect your skin Gloves! Outer clothing Chemical suit is not always needed
- 24. Self Decontamination Wash it off Hand washing Tepid water, mild soap No scrubbing If showering, use shampoo Remove and launder clothing Monitor after decon
- 25. 25 Purple and yellow Radiation Symbol
- 26. ALARA Goal of exposure management: keep radiation exposures to a level As Low As Reasonably Achievable (ALARA)
- 27. 27 Radiation Units Units of radiation include • Curie - measures activity as the number of atoms that decay in 1 second. • rad (radiation absorbed dose) - measures the radiation absorbed by the tissues of the body. • rem (radiation equivalent) - measures the biological damage caused
- 28. 28 Units of Radiation Measurement
- 29. RADIATION DETECTORS • Instruments used in the practice of health & Medical physics serve a wide variety of purposes • one finds instruments designed specifically for the measurement of a certain type of radiation, such as Alpha Particles, Beta Particles low-energy X-rays, high-energy gamma rays. fast neutrons, and so on
- 30. • The basic requirement of any such instrument is that its detector interact with the radiation in such a manner that the magnitude of the instrument's response like development of current or voltage pulse, Charging and Discharging of Capacitors is proportional to the radiation effect or radiation property being measured
- 31. Ionizing Radiation 31 Overall View Ionizing radiation interacts with matter in various ways: ionization (photoelectric effect), excitation, braking radiation, Compton effect, pair production, annihilation etc. Mechanisms of interaction are utilized for the detection of ionizing radiation. Function and principles of electroscope, ionization chambers, proportional chambers, Geiger-Muller counters, solid-state detectors, and scintillation counters, depends on those ways of interaction. and cloud chambers have been describe.
- 33. How a Radiation Detector Works The radiation we are interested in detecting all interact with materials by ionizing atoms While it is difficult (sometime impossible) to directly detect radiation, it is relatively easy to detect (measure) the ionization of atoms in the detector material. Measure the amount of charge created in a detector electron-ion pairs, electron-hole pairs Use ionization products to cause a secondary reaction use free, energized electrons to produce light photons Scintillators We can measure or detect these interactions in many different ways to get a multitude of information
- 34. General Detector Properties Characteristics of an “ideal” radiation detector High probability that radiation will interact with the detector material Large amount of charge created in the interaction process average energy required for creation of ionization pair (W) Charge must be separated an collected by electrodes Opposite charges attract, “recombination” must be avoided Initial Generated charge in detector (Q) is very small (e.g., 10-13C) Signal in detector must be amplified Internal Amplification (multiplication in detector) External Amplification (electronics) Want to maximize V C Q V
- 35. Detection and measurement includes the following components: Detector Preamplifier Amplifier Single channel analyzer Multi-channel analyzer Scalar-Timer
- 36. a detector produces a signal for every particle entering in it. Every detector works by using some interaction of particles with matter. Use characteristic effects from interaction of radiation with matter to detect, identify and/or measure properties of radiation. Respond to radiation by producing various physical effects
- 37. Detection Processes Ionization: Gas/Liquid chambers and Semiconductor Detectors Scintillation: Scintillation counters and TLDs Sparking: Sparking chamber Blackening of photographic film: Nuclear Emulsion detector and Film dosimetry Bubbling/Clouding of supersaturated liquids/vapors: Cloud Chambers and Bubble Chambers
- 38. Detection Processes Physical Changes: NMR Dosimetry and SSNTDs Themodynamical Changes: Calorimetric Dosimetry Activation: Neutron Activation Detectors Biological Changes: ESR Detectors and Biosensors
- 39. Following is the list of most common types of detectors: Gas-filled counters ionization chamber Proportional Counter Geiger-Muller counter Scintillation detectors Semiconductor detectors
- 40. Gas Detectors Most common form of radiation detector Relatively simple construction Suspended wire or electrode plates in a container Can be made in very large volumes (m3) Mainly used to detect b-particles and neutrons Ease of use Mainly used for counting purposes only High value for W (20-40 eV / ion pair) Can give you some energy information Inert fill gases (Ar, Xe, He) Low efficiency of detection Can increase pressure to increase efficiency g-rays are virtually invisible
- 41. Ionization Chambers Two electric plates surrounded by a metal case Electric Field (E=V/D) is applied across electrodes Electric Field is low only original ion pairs created by radiation are collected Signal is very small Can get some energy information Resolution is poor due to statistics, electronic noise, and microphonics Good for detecting heavy charged particles, betas
- 42. Proportional Counters Wire suspended in a tube Can obtain much higher electric field E 1/r Near wire, E is high Electrons are energized to the point that they can ionize other atoms Detector signal is much larger than ion chamber Can still measure energy Same resolution limits as ion chamber Used to detect alphas, betas, and neutrons
- 43. Scintillator Detectors Voltage is not applied to these types of detectors Radiation interactions result in the creation of light photons Goal is to measure the amount of light created Light created is proportion to radiation energy To measure energy, need to convert light to electrical signal Photomultiplier tube Photodiode Two general types Organic Inorganic } light electrons
- 44. Ionizing Radiation 44 Scintillation Counters The Key Components of a Typical Scintillation Counter High voltage supplier and multi-channel analyzer / computer system Photomultiply tube Photo- cathode Na(Tl)I crystal Thin Al window X- or g rays
- 46. Ionizing Radiation 46 Fluorescence Screens Fluorescence materials absorb invisible energy and emit visible light. J.J. Thomson used fluorescence screens to see electron tracks in cathode ray tubes. Electrons strike fluorescence screens on computer monitors and TV sets give dots of visible light. Röntgen saw the shadow of his skeleton on fluorescence screens. Rutherford observed alpha particle on scintillation material zinc sulfide. Fluorescence screens are used to photograph X-ray images using films sensitive visible light.
- 47. Semiconductor Detector A semiconductor detector acts as a solid-state ionization chamber The operation of a semiconductor radiation detector depends on its having either an excess of electrons or an excess of holes. A semiconductor with an excess of electrons is called an n-type semiconductor, while one with an excess of holes is called a p-type semiconductor
- 49. Ideal Detector for Detection of Radiation Radiation Ideal Detector Thin Semiconductor Detectors Proportional Counters b Organic Scintillators Geiger Counters Proportional Counters g Inorganic Scintillators Thick Semiconductor Detectors neutrons Plastic Scintillators Proportional Counters (He, BF3) Lithium Glass Scintillators
- 50. one of the oldest devices used to detect and measure ionizing (nuclear) radiation Named for Hans Geiger who invented the device in 1908, and Walther Müller who collaborated with Geiger in developing it further in 1928 one of the most sensitive, especially for the low radiation levels typically found in most situations.
- 51. Following is the assembly of the components of GM counter: Power-supply GM tube Discriminator Scaler/timer
- 52. GM Counter Assembly Variable voltage source Gas-filled counting chamber Two coaxial electrodes well insulated from each other Electron-pairs produced by radiation in fill gas move under influence of electric field produce measurable current on electrodes, or transformed into pulse
- 53. GM Tube In Action wall fill gas R Output Aor Anode (+) Cathode (-) End window Or wall
- 54. Indirect Ionization Process wall Incident gamma photon
- 55. Direct Ionization Process wall Incident charged particle e - e - e - e - e - e -e - e - beta (β-)
- 56. Competing Processes - recombination R Outpute - e - + +
- 57. Voltage versus Ions Collected Voltage Number of Ion Pairs collected Ionization region Saturation Voltage 100 % of initial ions are collected Recombination region
- 58. The characteristics curve depends on three factors Plateau: the part of the curve where the number of counts per second is (almost) independent of the voltage. Threshold voltage: always lies in the plateau region and is a function of the gas pressure and the anode diameter Figure of merit: is always less than 1%
- 59. A process causing the discharge to terminate Two methods used for quenching are: External quenching Internal quenching
- 60. Used to restore the counter to its quiescent state after the passage of ionizing radiation An RC circuit is used for reducing the high voltage applied to the tube, for a fixed time after each pulse, to a value that is too low to support further gas multiplication The voltage must be reduced for a few hundred secs which is greater than the transit time of the positive ions A counter with 98% pure argon is used
- 61. The advantage of external quenching is that it gives long life time to GM tube The disadvantage is that it has long recovery time
- 62. The quenching agent gas in the Geiger counter stops the flow of electrical current after a few microseconds. the quenching gas is of low ionization potential (halogens or organic vapors) Halogens are preferably used because it increases the life of GM tube Organic quenched tubes usually have a flatter plateau than halogen quenched tubes
- 63. The purpose of the quenching additive to the gas is to effectively absorb UV-photons emitted from the electrodes when the ions produced in the multiplication process impact on the electrodes. Such photons otherwise liberate secondary electrons (via the photo-electric effect) which may initiate the avalanche process all over again, thereby leading to catastrophic breakdown of the tube (i.e. a spark).
- 64. Device serving as pulse height selector able to make selection from output analogical pulses, rejecting the impulses with voltage amplitude inferior to a certain threshold voltage Threshold voltage should neither too low nor too high to avoid noise and data-loss respectively Its function is double: To eliminate the noise To provide a standard shaped pulse to scaler
- 66. Scalar: counts the number of pulses Timer: measures the length of counting time in a given measurement Collectively used to: Make measurement of pulses for a preset length of time (set on timer) recording the number of counts by the scaler Determine the count-rate by measuring duration with timer for a preset number of counts
- 68. Resolving Time The negative ions, being electrons, move very rapidly and are soon collected, while the massive positive ions are relatively slow-moving and therefore travel for a relatively long period of time before being collected These slow-moving positive ions form a sheath around the positively charged anode, thereby greatly decreasing the electric field intensity around the anode and making it impossible to initiate an avalanche by another ionizing particle. As the positive ion sheath moves toward the cathode, the electric field intensity increases, until a point is reached when another avalanche could be started
- 69. Resolving Time dead time The time required to attain this electric field intensity recovery time the time interval between the dead time and the time of full recovery resolving time The sum of the dead time and the recovery time
- 70. Resolving Time dead time, recovery time, resolving time
- 71. Resolving Time Measurement of Resolving Time the "true“ counting rate the observed counting rate of a sample is R0
- 72. Objective: to extract the amplitude or timing information the electrical signal is coupled to an amplifier, sent through gain and filtering stages, and finally digitized to allow data storage and analysis. amplitude or timing information include the different characteristics of the radiation, such as the type, the intensity and energy of the radiation The signal can be either processed entirely through analog circuit or can be converted into digital form
- 73. The signal can be a continuously varying signal a sequence of pulses, occurring periodically at known times randomly All of these affect the choice of signal processing techniques.
- 74. First steps in signal processing: Formation of the signal in the detector (sensor) Coupling the sensor to the amplifier Detectors use either direct detection or indirect detection
- 75. The detector pulse has a very low amplitude & time duration i.e. narrow band width To extract any kind of information requires amplification of detector signal Preamplifier is a simple and efficient amplifier directly connected to detector
- 76. A preamplifier, in effect, acts as a capacitance terminator thus preventing deterioration of detector. Matches the high electric impedance of detector with low impedance of the coaxial cable connected to subsequent signal processing circuit Basically plays a role as an impedance matcher between the detector and the rest of the circuit
- 77. The function is to amplify the pulses from detector via a preamplifier Also used to shape a pulse for further detection
- 78. High-voltage power supply typically provides 800 to 1,200 volts to the PMT Raising voltage increases magnitude of voltage pulses from PMT Preamp connected to PMT using very short cable Amplifies voltage pulses to minimize distortion and attenuation of signal during transmission to remainder of system
- 80. SCA Contd. Amplifier further amplifies the pulses and modifies their shapes – gain typically adjustable SCA allows user to set two voltage levels, a lower level and an upper level If input pulse has voltage within this range, output from SCA is a single logic pulse (fixed amplitude and duration) Counter counts the logic pulses from the SCA for a time interval set by the timer
- 82. SCA energy modes LL/UL mode – one knob directly sets the lower level and the other sets the upper level Window mode – one knob (often labeled E) sets the midpoint of the range of acceptable pulse heights and the other knob (often labeled E or window) sets a range of voltages around this value. Lower-level voltage is E - E/2 and upper-level voltage is E + E/2
- 84. An MCA system permits an energy spectrum to be automatically acquired much more quickly and easily than does a SCA system The detector, HV power supply, preamp, and amplifier are the same as for SCA systems The MCA consists of an analog-to-digital converter, a memory containing many storage locations called channels, control circuitry, a timer, and a display
- 88. G-M tube: Advantages Variety of sizes and shapes Inexpensive The slightest radiation event strong enough to cause primary ionization results in ionization of the entire gas volume Thus detector is highly sensitive, even in lowest intensity radiation fields Only simple electronic amplification of the detector signal is required Hardware lasts longer Requires less power Strong output signal means G-M needs less electrical noise insulation than other detectors NET 13088
- 89. G-M tube: Disadvantages Incapable of discerning between type and energy of the radiation event Only counts events and yields output in events per unit time or dose rate A beta particle or gamma ray, high or low energy, represents one event counted Only capable of detecting fields to some upper limit of intensity Limited to lower intensity fields due to detector dead time NET 13089
- 90. Personnel Monitor Devices The most common monitor devices to determine the personal exposure history are: Radiation Film Badges Pocket Dosimeter
- 91. Radiation film badges are composed of two pieces of film, covered by light tight paper in a compact plastic container. Various filters in the badge holder allow areas to be restricted to X-ray, g-ray, b- rays only. Radiation causes a blackening (silver) of the film material (mostly a silver bromide emulsion) The sensitivity of the film material is limited For g-radiation the sensitivity is in the range of 10 - 1800 mrem. For b-radiation the sensitivity is in the range of 50 - 1000 mrem. Special film material is used for neutron monitoring. The badge is usually not sensitive for radiation because the -particles are absorbed in the light-tight paper.
- 92. Pocket dosimeter The pocket dosimeter or pen dosimeter is a common small sized ion chamber which measures the originated charge by direct collection on a quartz fiber electroscope. The U-shaped fiber is close to a U-shaped wire. If the fiber is charged it will be deflected away from the wire. The position of deflection is a measure of the accumulated radiation dose.
- 93. The dosimeter records total exposure from the initial charging to the time of reading. It is an active device as the radiation exposure can be read immediately as opposed to the passive film badge which is only read after approximately six months.
- 94. Dosimeters, which are also available in high or low ranges, can be in the form of a badge, pen/tube type, or even a digital readout and all measure exposure or the total accumulated amount of radiation to which you were exposed. (The Civil Defense pen/tube tube would show a reading like below when looking through it.) It's also similar to the odometer of a car; where both measure an accumulation of units. The dosimeter will indicate a certain total number of R or mR exposure received, just as the car odometer will register a certain number of miles traveled.
- 95. Thermoluminescence (TL) is the ability to convert energy from radiation to a radiation of a different wavelength, normally in the visible light range. Two categories Fluorescence - emission of light during or immediately after irradiation Not a particularly useful reaction for TLD use Phosphorescence - emission of light after the irradiation period. Delay can be seconds to months. TLDs use phosphorescence to detect radiation.
- 96. Thermoluminescence Radiation moves electrons into “traps” Heating moves them out Energy released is proportional to radiation Response is ~ linear High energy trap data is stored in TLD for a long time
- 97. TL Process Valence Band (outermost electron shell) Conduction Band (unfilled shell) Phosphor atom Incident radiation Electron trap (meta stable state) -
- 98. TL Process, continued Valence Band (outermost electron shell) Conduction Band Phosphor atom Thermo luminescent photon Heat Applied-
- 99. Output – Glow Curves A glow curve is obtained from heating Light output from TL is not easily interpreted Multiple peaks result from electrons in "shallow" traps Peak results as traps are emptied. Light output drops off as these traps are depleted. Heating continues Electrons in deeper traps are released. Highest peak is typically used to calculate dose Area under represents the radiation energy deposited in the TLD
- 100. Trap Depths - Equate to Long Term Stability of Information Time or temperature
- 101. TLD Reader Construction Power Supply PMT DC Amp Filter Heated Cup TL material To High Voltage To ground Recorder or meter
- 103. NET 130 Module 5 - Radiation Detection Principles and Instruments 103 Crystal retains this energy until heat is applied. The “trapped” energy is then released in the form of light, as the atoms of the crystal return to their “ground state” The light emitted is then correlated to dose received Once the TLD has been “read”, memory is cleared TLD is then available for re-use NET 130 Module 5 - Radiation Detection Principles and Instruments 103 Thermoluminescent Dosimeter
- 106. THANK YOU !!!