1. 1. RADIATION DETECTION AND MEASUREMENT Lecture 03 Shahid Younas
2. 2. Once upon a time.. Lecture 03 Gas Filled Detectors Detection Efficiency
3. 3. Gas Filled Detector- GM Counters Lecture 03 1. Radium Hen Detectors 2. Based on gas-amplification as that proportional counters. 3. Billions of ion-pairs; additional electronic amplification (increased voltage) is required. 4. Used as inexpensive survey meters.
4. 4. Gas Filled Detector- GM Counters Lecture 03 Flat, thin-window GM detectors. Pancake Type. Radioactive contamination.
5. 5. GM Detectors- Characteristic & Usage Lecture 03 1. Inefficient detectors of x-rays and gamma rays. 2. Size of voltage pulse is independent of the incident energy. 3. Cant be used as spectrometers and dose-rate meter.
6. 6. Gas Filled Detector- GM Counters Lecture 03 Do you know how the Proportional Counter differ from GM Counter? Chain of Avalanches
7. 7. Gas Filled Detector- GM Counters Lecture 03 In the proportional counter, each original electron leads to an avalanche which is basically independent of all other avalanches formed from other electrons associated with original ionizing event.
8. 8. Gas Filled Detector- GM Counters Lecture 03 In the GM counter, substantially higher electric fields are created that enhance the intensity of each avalanche.
9. 9. Gas Filled Detector- GM Counters Lecture 03 Under proper conditions, a situation is created in which one avalanche can itself trigger a second avalanche at a different position within the tube.
10. 10. Gas Filled Detectors Lecture 03 Do you know how many ion pair are produced once a radioactive tracer hits the GM tube. 109 to 1010
11. 11. Gas Filled Detector- GM Counters Lecture 03 1. GM survey meters can not truly measure exposure rates. 2. Their reading must be considered as crude approximation. 3. Calibration source of 662 keV. 4. Compensated detectors.
12. 12. Gas Filled Detector- GM Counters Lecture 03 GM Counters have the highest dead time amongst the detectors. Please someone guess the reason.
13. 13. Scintillation Detectors Lecture 03
14. 14. Scintillation Detectors Lecture 03 Scintillators are materials that emit visible or ultraviolet light after the interaction of ionizing radiation with the material.
15. 15. Scintillation Detectors Lecture 03 Roentgen discovered x-radiation. Do you know the material in which x-radiation were induced by Roentgen? Scintillator Detector Barium Plantino-cyandide
16. 16. Scintillation Detectors Lecture 03 Scintillators are used in conventional film-screen radiography, many digital radiographic receptors, fluoroscopy, scintillation cameras, most CT Scanners and PET Scanner. Scintillation detectors consist of a scintillator and a device, such as a PMT, that converts the light into an electrical signal.
17. 17. Scintillator Detectors Lecture 03 1. Amount of light emitted after an interaction increases with energy deposited by the interaction. 2. May be operated in pulse mode as spectrometers. 3. High conversion efficiency produces superior energy resolution.
18. 18. Development- Anger Gamma Camera Lecture 9: Nuclear Imaging-I What is the suitable photon energy range of usage of gamma camera? 100 to 200 keV
19. 19. Scintillator Detectors Lecture 03 Luminescence, Fluorescence and phosphorescence are synonyms of word scintillator. Do you know the difference between these terms? Luminescence is the emission of light after excitation Fluorescence is the prompt and phosphorescence is the delayed emission of light respectively.
20. 20. Scintillator Detectors- Ideal Properties Lecture 03 High conversion efficiency Decay times of excited states should be short Material transparent to its own emissions Color of emitted light should match spectral sensitivity of the light receptor. For x-ray and gamma ray detectors, should be large. Rugged , unaffected by moisture, and inexpensive to manufacture.
21. 21. Scintillation Detectors Lecture 03 For x-ray and gamma ray detectors, linear attenuation coefficient should be large. Do you know the advantage of large ?
22. 22. Scintillator Detectors- Materials Lecture 03 1. Sodium iodide activated with thallium [NaI(Tl)], coupled to PMTs and operated in pulse mode, is used for most nuclear medicine applications. 2. Fragile and hygroscopic. 3. Bismuth germinate (BGO) is coupled to PMTs and used in pulse mode as detectors in most PET scanners.
23. 23. Scintillator Detectors- Photomultiplier Tubes Lecture 03 PMTs perform two functions: Conversion of ultraviolet and visible light photons into an electrical signal. Signal amplification , on the order of millions to billions. Consists of an evacuated glass tube containing a photocathode, typically 10 to 12 electrodes called dynodes, and an anode.
24. 24. Scintillator Detectors- PM Tube Lecture 03
25. 25. Scintillator Detectors- Process in PM Tube Lecture 03
26. 26. Scintillation Detectors Lecture 03 Do you know how many PMTs and their diameter in modern Gamma Cameras? 60-120 PMTs with diameter ~ 2 inches
27. 27. Scintillator Detectors - Dynodes Lecture 03 Electrons emitted by the photo-cathode are attracted to the first dynode and are accelerated to K.E. equal to the potential difference between the photocathode and the first dynode.
28. 28. Scintillator Detectors - Dynodes Lecture 03 When these electrons strike the first dynode, about 5 electrons are ejected from the dynode for each electron hitting it. These electrons are attracted to the second dynode, and so on, finally reaching the anode.
29. 29. Scintillator Detectors PMT Amplification Lecture 03 Total amplification of the PMT is the product of the individual amplifications at each dynode. If a PMT has ten dynodes and the amplification at each stage is 5, the total amplification will be approximately, 5x5x5x5x5x5x5x5x5x5= 510= 10,000,000
30. 30. Scintillator Detectors PMT Amplification Lecture 03 Applied Voltage on PMT has significant role. Do you know its effect on amplification? Amplification can be adjusted by changing the voltage applied to the PMT.
31. 31. Scintillator Detectors with Trapping of Excited Electrons Lecture 03 Thermo luminescent Dosimeters Electrons become trapped in excited states after interaction with ionizing radiation. Heating & Reading Lithium fluoride (LiF) most useful for personal and area radiation monitoring.
32. 32. Scintillator Detectors PMT Amplification Lecture 02 LiF is most useful because its effective atomic number is close to tissue. Do you know the minimum threshold of exposure where it starts counting? 100 uSv/ hr
33. 33. Scintillator Detectors with Trapping of Excited Electrons Lecture 03 Photo-stimulable Phosphors (PSP) Alternative to TLDs. Reading is done by Laser Light Use in Radiology as image receptors instead of film-screen cassettes. Barium fluorohalide activated with europium.
34. 34. Scintillator Detectors PMT Amplification Lecture 03 Today we shall perform Detection Efficiency of Dose Calibrators in Injection Room at 4 pm.