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Radiation Detectors and their Applications.ppt
- 1.
- 2. Objective • Principle ofradiation detection as a consequence of the interactions of radiation with matter • Radiation measurement is simply to quantify the observable changes that resulted from these interactions
- 3. The detector isa fundametal base in all practice with ionising radiation Knowledge of the instruments potential as well as their limitation is essential for proper interpretation of the measurements Absolute Need For Detectors
- 4. Detector material Any materialthat exhibits measurable radiation related changes can be used as detector for ionising radiation. •Change of colours •Chemical changes •Emission of visible light •Electric charge Active detectors: immediate measurement of the change. Passive detectors: processing before reading
- 5. Types of Detector •Active Detectors Immediate measurement of the change • Passive Detectors Processing before reading
- 6. Radiation Detection Principles EffectInstrument Detector medium Electrical Ionization chamber 1. Gas Proportional counter 2. Gas Geiger counter 3. Gas Solid State 4. Semiconductor Chemical Film 1. Photographic emulsion Chemical Dosimeter 2. Solid or liquid Light Scintillation counter 1. Crystal or liquid Cerenkov counter 2. Crystal or liquid Thermoluminescence TL dosimeter Crystal Heat Calorimeter Solid and liquid
- 7. DETECTOR TYPES 1) Counters Gasfilled detectors Scintillation detectors 2) Spectrometers Scintillation detectors Solid state detectors 3) Dosimeters Gas filled detectors Solid state detectors Scintillation detectors Thermoluminiscent detectors Film
- 8. Ionization • Ions andElectrons Leaves a track of ionized atoms – ION TRACK In a field-free region the electrons and ions rapidly neutralize- RECOMBINATION Number ions pairs depends on energy Knowledge of energy to create ion pair Charge separation and charge collection we measure energy
- 9. The basic detectionsystem • Three main components Sensitive Material Some form of amplification Some form of display system
- 10. GAS-FILLED DETECTORS: • Activevolume is gas • Ions and electrons separated by E • +ve eletrode is the anode collects -ve ions • -ve eletrode is the cathode collects +ve ions
- 11. GAS-FILLED DETECTORS: • Thereare three flavours • Ion Chamber – Medium E • Proportional Counter – High E • Geiger Muller Counter Very High E
- 12. GAS-FILLED DETECTORS: IonizationChamber HV + - Negative ion Positive ion 1234 Electrometer The response is proportional to ionization rate (activity, exposure rate)
- 13. GAS-FILLED DETECTORS: IonizationChamber • Collection of exactly all charges • Eliminate recombination – charge loss • Avoid electron multiplication – charge creation • Multiplication can be avoided by keeping E below the point at which it occurs
- 14. Ionisation chambers Applications innuclear medicine •Activity meter •Monitoring instruments •Accurate and precise dose measurement; calibration
- 15. General properties ofionisation chambers •High accuracy •Stable •Relatively low sensitivity
- 16. Regions of operationfor gas- filled detectors Knoll
- 17. Proportional counter Electrons producedby the primary ionization acquire sufficiently high energy to cause secondary ionization before reaching the anode and an avalanche of ionization occurs
- 18. GAS-FILLED DETECTORS: Proportional Counter •As electron multiplication kicks in we enter the proportional region • Each electron created is a source of avalanche • Large reproducible no. of electron reaches the anode • Output pulse charge always proportional to the radiation energy
- 19. Proportional counter Applications innuclear medicine •Monitoring instruments
- 20. Properties of proportionalcounters as monitor •A little higher sensitivity than the ionisation chamber •Used for particles and low energy photons
- 21. Geiger Müller-tube principle Knoll - + - Ifthe voltage is increased ( 1.5KV typically), the secondary avalanche hitherto limited to a small region on the anode, will extend across the entire length of the anode. A single incident particle cause full ionization
- 22. Geiger Müller -tube Applications in nuclear medicine •Contamination monitor •Dosemeter (if calibrated)
- 23. General properties of GeigerMüller - tubes •High sensitivity •Lower accuracy
- 24. A common SurveyMeter
- 25.
- 26. Scintillation detectors • Radiationto be detected is first converted to visible light • Light is then converted to electrical signal • Materials similar to that of TV screen
- 27. Scintillation detectors • Threecomponents Scintillation medium Light collector Light detector
- 28. Scintillation detector Scintillation detector Detector Photocathode cathodd Dynodes Anode Amplifier PHA Scaler Scintillationdetectors are based on the detection of the fluorescent radiation, usually visible light, emitted when a electron returns from an excited state to the valence band following radiation absorption
- 29. Pulse height analyzer UL LL Time Pulseheight (V) The pulse height analyzer allows only pulses of a certain height (energy) to be counted. counted not counted
- 30. Pulse-height distribution Nal(Tl) Pulse-heightdistribution Nal(Tl)
- 31. PM PM Sample mixed withscintillation solution Liquid scintillation detector
- 32. Scintillation detectors Scintillation detectors Applicationsin nuclear medicine Applications in nuclear medicine •Sample counters •Single- and multi-probe systems •Gamma camera •Monitoring instruments
- 33.
- 34. Semi-conductor detector • Employsthe use of solid medium for ionization interaction • The range of radiation is much smaller in solids • Easier control of properties and long term stability • Highly ionized tracks are also formed • Total charge created depends on energy of incident radiation • Specific ionization is 3-4 ev
- 35. Semi-conductor detector • Problemis charge separation • Electrons or ions will not move in insulators • E can’t be maintained to separate the charges in conductors • The answer is to use semiconductor junctions
- 36. Semi-conductor detector • Radiationthrough semiconductor does not create electron-ion pairs • Electrons are given enough energy to move into the conduction band • Leaving a hole in the valence band • Electron-hole pairs are thus produced
- 37. Semi-conductor detector as spectrometer •Solid Germanium or Ge(Li) detectors • Principle: electron - hole pairs (analogous to ion-pairs in gas-filled detectors) • Excellent energy resolution
- 38. Comparison of spectrum froma Na(I) scintillation detector and a Ge(Li) semi- conductor detector Knoll
- 39. Semi-conductor detector Applications innuclear medicine •Identification of nuclides •Control of radionuclide purity
- 40. Film Principle: As normalphotographic film Silver halide grains, via changes due to irradiation and development to metallic silver Application in nuclear medicine: Personal dosemeter
- 41. Thermoluminescence (TLD) principle Tl material heatingfilament emitted light PMT
- 42.
- 43. TLD Applications in Nuclearmedicine •personal dosemeters (body, fingers…) •special measurements
- 44. TLD Advantages: Small size High sensitivity Integrating Tissueequivalent Disadvantages: Time consuming No permanent record
- 45. Summary • Even thoughionizing radiation produces undesirable effects, these effects are the bases of radiation detection • A radiation detector is therefore medium in which radiation will interact, create observable and measurable effects
- 46.
Editor's Notes
- #16 The figure illustrates that the number of ionpairs and hence the size of the signal generated in a gas filled detector is dependent on the applied voltage
- #17 There is a multiplication by secondary ionization events which is proportional to the voltage as well as the number of primary ionizations caused by a particle or photon.
- #21 In a GM-tube a very high electrical filed is achieved near the thin central electrode. The primary ionization is causing an avalanche of secondary ionizations. In this process there will be emission of UV-photons with energy high enough to cause new ionizations. The result will be a total discharge of the detector. The result will be an electrical pulse that can be measured without any further amplification.