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  1. 1. INTRODUCTION  We cannot detect or measure ionizing radiation with our senses  To detect or measure radiation we must need instruments  Design, construction, type and use of detectors depends upon the properties and nature of radiation
  2. 2. RADIATION DETECTION  Suitable device is required to detect and measure the amount and energy of radiation  These devices consists 1. detector in which interaction takes place 2. measuring device to record the interaction
  3. 3.  For measuring the activity  (curie, becquerel)  For measuring the rate of radiation  (mR/h, muSv/h)  for personal monitoring  (mrem, mSv)
  5. 5. IONIZATION  This effect consists of removing electrons from originally neutral atoms or molecules, giving rise to positive ions and negative ions  Gaseous and solid media are used for ionization  1. Ionization chambers  2. Proportional chambers  3. Geiger muller counters  4. Semiconductor detectors
  6. 6. LUMINESCENCE  Process in which radiation excites the atom of a material and its energy is converted in to visible light flash  Using light sensitive photomultipliers , the light flashes are converted in to electrical pulses Scintillators (scintillator coupled to a photomultiplier forms scintillation detector)
  7. 7. PHOTOGRAPHIC EFFECT  Film exposed by x rays or gamma rays, will form a latent image with black metallic silver  The degree of blackness can be measured by means of optical density, which is proportional to radiation exposure
  8. 8. THERMOLUMINISCENCE  Radiation can impart energies to certain crystalline materials (lithium fluoride,calcium sulfate ) or certain glasses, which can store these energies for long time  The energy stored can be later released in the form of light or luminiscence by heating these materials  The quantity of light released can be measured and correlated to radiation dose  TL DOSIMETERS
  9. 9. CHEMICAL EFFECTS Radiation can cause chemical changes (oxidation of ferrous sulfate to ferric sulfate)  Radiation can also cause change of colouration in certain plastics  These changes can be measured and correlated to radiation dose
  10. 10. BIOLOGICAL EFFECTS  Radiation exposure to the body can be measured by biological methods  Eg; analysis of blood for chromosomal aberrations in persons exposed to radiation dose of above 10-1000rem
  11. 11. DETECTORS  Radiation interacts with detector materials and deposits energy by ionization and excitation  The energy deposited by single interaction is very small and hence all detectors need signal amplification  In addition, the detector system performs signal processing and signal storage with the help of electronic circuit
  12. 12. SIGNAL PROCESSING  It can be done by pulse mode or current mode Pulse mode: the signal from each interaction is processed individually Current mode: the electrical signals from individual interactions are averaged together and give net current signal
  13. 13. PULSE MODE  Two interactions must be seperated by a time interval, so that it can produce two distinct signals  This time interval is called dead time of the detector  Dead time depends upon the components in the detector system  (eg. Multichannel annalyser in GM counter detector)  Dead times of different detectors vary widely
  14. 14. CURRENT MODE  The electrical signal of each interaction is integrated to give net electric signal  Hence current mode operation is suitable for high interaction rates to avoid dead time losses
  15. 15. DETECTOR EFFICIENCY  It is a measure of its ability to detect radiation  It is a product of geometric efficiency and intrinsic efficiency
  16. 16. DETECTOR EFFICIENCY GEOMETRIC EFFICIENCY- fraction of emitted photon that reaches detector no of photons reaching the detector no of photons emitted by source INTRINSIC EFFICIENCY-fraction of particular photons that are detected no of photons detected no of photons reaching the detector ( intrinsic efficiency is often called quantum detection efficiency(QDE) which is determined by energy of photon, detector thickness, atomic number and density)
  17. 17. DETECTOR EFFICIENCY  The probability of detector efficiency varies from 0 to 1  It increases when the source is closer to the detector  It is 0.5 for a point source placed under a flat surface detector  It is 1 for well type detector system
  19. 19. GAS FILLED DETECTORS  These detectors has volume of gas between two electrodes in which voltage is applied  When exposed to radiation, the gas is ionized and ion pairs are formed  Positive ions move towards negative electrode and negative ions move towards positive electrode  The electron travels through the circuit and reaches cathode and recombines with positive ions. This forms an electric current
  21. 21. IONIZATION CHAMBER  It consists of an outer cylinder coated inside with graphite and central electrode  They are used in current mode  They are used as survey meters and dosimeters in radiotherapy  High atomic number gases Argon (Z=18) or xenon(Z=54) can be used to increase sensitivity towards x and gamma rays. Used in isotope calibrator and CT scans
  22. 22. IONIZATION CHAMBER ADVANTAGES 1. Walls can be made tissue equivalent 2. All types of radiation can be measured 3. Can be calibrated to any energy DISADVANTAGES Small signal current which require high amplification and has restricted sensitivity
  23. 23. PROPORTIONAL COUNTER  Are designated with specific gas medium, operated with higher voltages.  In general, Nitrogen and Argon are used  Krypton and Xenon are used at higher energies with higher efficiencies  The higher electric field accelerates the electrons to high kinetic energy, producing secondary ionization  They provide large surface area and serve as detector in CT scans
  24. 24. GEIGER COUNTER  The Geiger Muller(GM) counter consists of a cylindrical cathode with a fine wire anode along its axis  It has high efficiency for charged particles and record every particle seperately  They are inefficient towards X rays and Gamma rays  Voltage pulse size is independent of radiation energy eg: 1Kev and 5Kev radiation give same pulse size. Hence they cannot be used as spectrometers and dose rate meters
  25. 25. SCINTILLATION DETECTOR  It emits visible or ultraviolet light when exposed to radiation  However, signal needs to be amplified, hence all scintillation crystals are provided with photomultiplier tubes  Scintillation material includes organic compounds and inorganic crystals  They have higher average atomic number and higher density and widely used in radiology
  27. 27. SCINTILLATION DETECTOR  When a photon interacts with the crystal, electrons are raised to excited state. The excited electrons return back to low energy with the emission of visible and UV light. This is called luminiscence  Operated at pulse mode and the electronic circuit can identify individual interactions  Commonly used in gamma cameras and CT scanners
  28. 28. RADIOLOGICAL SCINTILLATOR  Sodium iodide(NaI) is used in all nuclear medice applications  It is used in gamma camera, thyroid probe and gamma well counter under pulse mode operation  It has high density (Iodine Z=53) and provides high photoelectric absorption probability for X and Gamma rays
  29. 29. RADIOLOGICAL SCINTILLATOR  Bismuth germinate is used as detector in PET scanners (Bismuth Z=83)  Cesium iodide with Thallium activator is used in thin film transister technology in digital radiography  In CT scans, scintillator coupled with photodiodes are used  High resolution CT scans require crystals with less after glow. Cadmium tungstate and Gadolinium ceramics are commonly employed as scintillators
  30. 30. PHOTOMULTIPLIER TUBE  It converts visible and UV light in to an electric signal and does signal amplification of order of millions  It mainly consists of an evacuated glass tube containing and 10 to 12 dynodes and an anode  The electron emitted by the photocathode falls on first dynode and gets accelerated.
  31. 31.  Additional electrons ( 5 electrons per one incident electron) are produced in the dynode and similar process continues in the second dynode and so on.  The total amplication is about 10^5 for a 10 minute dynode pmt system
  32. 32. PHOTO DIODE  It is a semiconductor device that converts light in to electrical signal  When it is exposed to light, an electrical current is generated that is proportional to amount of incident light  Photodiodes with CdWO4 is used in CT scan as detector and also used in digital radiography with thin film transisters  They are smaller in size and cheaper
  33. 33. THERMOLUMINISCENT DOSIMETER  Organic scintillators used in personal monitoring and patient dose estimation  Amount of light emitted is proportional to the amount of energy absorbed by TL material  The peak light intensity is proportional to the radiation dose received  Measurement of peak light intensity forms the basis for thermoluminescent dosimetry(TLD)
  34. 34. THERMOLUMINISCENT DOSIMETER  Lithium fluoride is a useful TLD material with little fading. Its effective atomic number is close to that of tissue  Available in the form of powder,rods,discs or chips made of Teflon(PTFE) impregnated with lithium fluoride  It can be worn by a person or inserted in to a body cavity or pasted on a equipment  Heating and measurement of peak light intensity done in a special device called TLD reader after radiation exposure
  35. 35. PHOTOSTIMULABLE PHOSPHORS  These are Scintillators used in imaging plate technology  When exposed to radiation, fraction of excited electrons is trapped as absorbed energy  These electrons are released by scanning the plate by lazer beam (700 nm)  The laser light stimulates the trapped electrons and release visible light
  36. 36. PHOTOSTIMULABLE PHOSPHORS  The light may be collected by means of fiberoptic guide tube and passed on a photomultiplier tube. The PMT produces an electronic signal.  This is the used in computed tomography  Phosphors such as GD2O2S & BaFBr & BaFI can be used as photostimulable phosphors in CR  The imaging plate can be reused by erasing the entire trapped electrons
  37. 37. SEMICONDUCTOR DETECTORS A semiconductor diode with reverse bias voltage supply can be used to detect visible and UV light When the diode is exposed to light photons, the low energy electrons (valence bond) are excited in the depletion region and raised to higher energy state( conduction band) The hole move towards the p-type semiconductor and electron move towards n-type semiconductor This produces a momentary current flow in the circuit and forms the voltage signal
  38. 38. SEMICONDUCTOR DETECTORS  The amount of energy required is 3ev to create a electron hole pair compared to that of 34ev in ion chambers  The voltage pulse is larger than ion chamber and the pulse raise time is shorter, since the electron hole moves rapidly. Hence they can be used in spectrometers  The intrinsic noise is higher due to semiconductor resistance and thermal energy produced
  39. 39. SEMICONDUCTOR DETECTORS  Silicon based P-N junction diode reduce noise significantly than germanium at room temperature  The pulse is narrower than ion chambers and hence energy resolution is better compared to ion chambers and scintillation detectors  These are used in kvp meter, digital pocket meter(silicon), gamma ray spectrometry  They can be used as photodetectors in flat panel detectors
  40. 40. PRACTICAL DOSIMETERS  FREE AIR IONIZATION CHAMBER  The whole chamber is provided with lead lining to prevent the entry of external radiation in to chamber  The beam size is controlled by diaphragm D  The ionization is measured for a length L of the collector plate C  The lines are made straight & perpendicular to the collector by a guard ring G
  41. 41. Temperature and pressure correlation  The response of ion chamber is affected by air temperature and pressure, since the density of air depends on temperature and pressure  The density or mass of air in the chamber volume will increase as the temperature decreases or pressure increases.  As a result, the chamber reading for a given exposure will increase  The chambers are usually calibrated under standard atmospheric conditions (760 mm hg, 22C)
  42. 42.  THIMBLE IONIZATION CHAMBER  It is basically an ionization chamber with small volume of air  Generally bakelite or plastic are used as wall material  The inner surface of the wall is coated by a conducting material like graphite  The graphite acts as an outer electrode and participates in charge collection  The central cathode is made up of thin aluminium
  43. 43.  When radiation passes through the chamber, ion pairs are produced in the air cavity as well as the walls of the chamber  These ion pairs are collected by the electrodes and it is measured in terms of ionization charge Q  It is very small in size and very much suitable for routine measurements in hospitals, for calibrating X- ray, telecobalt units and linear accelerators  They needs to be calibrated once in 3 yrs
  44. 44. POCKET DOSIMETER  It is an ion chamber with a quartz fiber suspended with in an air filled chamber on a wire frame  The movement of quartz fiber is proportional to radiation exposure, which is measured in Roentgen  The dosimeter are available in different ranges 0- 200mR, 0-500mR, 0-5R, 0-20R, 0-200R, 0-600R for measurement of X & gamma rays  For personal monitoring, smallest range (0-200mR) should be used
  45. 45. POCKET DOSIMETER  The main advantage of lies in its ability to provide instant on the spot check of radiation dose received by the personnal  Film and TLD will not show accumulated exposure immediately  This is very useful in non routine work in which radiation levels vary considerably & may be quite hazardous eg: cardiac catheterization laboratory  Small in size and easy to use & do not provide permanent record
  46. 46. POCKET DOSIMETERS  Now a days, this are avaiable with easy display of instant radiation measurements  Presently, semiconductor diode based pocket dosimeters digital display are also available  They have good energy & polar response, with reliable readings, matching to TLD badges. This make loud beep sounds for every 15 to 30 min.  The sound becomes more frequent as dose rate increases and becomes continuous sound at high radiation fields
  47. 47. DOSE-AREA PRODUCT METER  It is a flat radiolucent air chamber, fitted over the collimater of the X-ray unit.  Since air is used as medium, the attenuation is very little  It measures air dose and radiation field area  It indicates how much patient area is exposed with a given radiation dose. So that assessment of radiation hazards and associated biological effect is made easy
  48. 48. DOSE-AREA PRODUCT METER  It is used in fluoroscopic (angiogram) or cardiac catheterization laboratory examinations, where the procedure is long  It is also useful in pediatric imaging to assess pt dose  It is also called as Roentgen-area product (RAP) meter
  49. 49. AREA MONITORING  The assessment of radiation levels at different locations in the vicinity of radiation installation is known as area monitoring or radiation survey  The objective is to ensure radiation safety and minimize personal exposure  An ideal monitor should have uniform response to X and gamma radiation over the range of 15KeV to 3meV
  50. 50.  Instruments used for area monitoring are called radiation survey meters or area monitors  Any survey meter consist of two main parts  1) device which detect the radiation  2) display system to measure radiation
  51. 51.  The different types of meters used  1. Ionization type  2. Geiger muller type  3. Scintillation detector type
  52. 52. IONIZATION CHAMBER SURVEY METER  They are used to measure X-ray machine outputs, estimate radiation levels in brachytherapy, in monitoring radionucleide therapy pt and survey the radioactive material packages  They are capable of monitoring higher radiation exposure rate levels and used in different ranges
  53. 53. GM TYPE SURVEY METER  Very sensitive and useful for monitoring of low level radiation  The electronic circuit of GM is very simple and less costly  They are pulsed in nature. They should be used in X ray units, that emits continuous X rays. Not used in X ray units that emits pulsed X ray units(linear accelerators)
  54. 54. GM TYPE SURVEY METER  Used in nuclear medicine and suitable for radioactive contamination & low level radiation survey  They have long dead time (100 msec) and result in 20% loss. Hence ionization chamber survey meters are preferred for accurate radiation survey
  55. 55. PERSONAL MONITORING SYSTEMS 1) To monitor and control individual doses regularly 2) Report and investigate overexposures and recommend remedial measures urgently 3) Maintain lifetime cumulative dose records of the users of service
  56. 56. PERSONAL MONITORING DEVICES 1) Film badges 2) TLD badges 3) Pocket dosimeter
  57. 57. PERSONAL MONITORING DEVICES These devices provide 1) Occupational absorbed dose information 2) Assurance that dose limits are not exceeded 3) Trends in exposure to serve as check in working place In India, country wide personal monitoring service is offered by private agencies, accredited by BARC, mumbai
  58. 58.  Film badge has some disadvantages such as  -fading at high temperature and humidity  -high sensitivity to light, pressure and chemicals,  -complex dark room procedure and limited self life  Hence, TLD badges are currently used in india instead of film badges
  59. 59. THERMOLUMINISCENT DOSIMETER  It is based on the phenomenon of thermoluminiscence, the emission of light when certain materials are heated after radiation exposure  It is used to measure individual doses from X, beta and gamma radiations  It gives very reliable results since no fading is observed under extreme climatic conditions
  60. 60. THERMOLUMINISCENT DOSIMETER  The typical TLD badge consists of a plastic casette in which nickel coated aluminium card is placed  There are three filters in the cassette corresponding to each disk namely Cu+AI, perspex and open  The metallic filter is meant for gamma radiation, perspex is for beta radiation
  61. 61. THERMOLUMINISCENT DOSIMETER  When the TLD is exposed to radiation, the electrons in the crystal lattice are excited more from the valence band to conduction band.  They form a trap just below the conduction band  The number of electrons in the trap are proportional to radiation exposure and thus it stores the absorbed radiation energy in the crystal lattice
  62. 62. TLD READER  After radiation exposure, the dose measurements are made by using a TLD reader.  It has heater, photo multiplier tube(PMT), amplifier and a recorder  The TLD is placed in the heater cup, where it is heated. While heating, electrons return to their ground state with emission of light  This emitted light is measured by the PMT, which converts light in to electrical current  The PMT signal is the amplified & measured by a recorder
  63. 63.  The disks are reusable after proper annealing up to 300 times  The annealing process release the residual energy stored from earlier exposure  A typical annealing cycle consists of 400 degree Celsius for 1hr followed by 300 degree celsius for 3 hrs
  64. 64.  GUIDELINES FOR USING TLD BADGE  1. Worn at chest level, that is expected to receive maximum radiation exposure  2.Used only by persons directly working in radiation  3.Pregnant radiation workers should wear a second badge at waist level ( under lead apron) to assess radiation dose  4.The name, personal number, type of radiation, period of use, location on the body etc should be written legibily in block letters on the front side of badge
  65. 65.  5. A TLD badge once issued to a person should not be used by another person  6. Each institution must keep one TLD card loaded in TLD holder as control which is required for correct dose estimation  7. If lead apron is used, TLD badge should be worn under lead apron  8. while leaving the premises of the institute, the workers should deposit their badges in the place where control TLD is kept
  66. 66. THANK YOU