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IAEA International Atomic Energy Agency Radiation Detection & Measurements - 1 Day 3 – Lecture 3.

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Presentation on theme: "IAEA International Atomic Energy Agency Radiation Detection & Measurements - 1 Day 3 – Lecture 3."— Presentation transcript:

1 IAEA International Atomic Energy Agency Radiation Detection & Measurements - 1 Day 3 – Lecture 3

2 IAEA 2 Objective To learn about different types of radiation detectors used in radiation protection

3 IAEA 3 Contents Detector Material Detector Principles Detector Types

4 IAEA 4 Detectors The detector is a fundamental base in all practice with ionizing radiation Knowledge of the instruments potential as well as their limitation is essential for proper interpretation of the measurements

5 IAEA 5 Detector Material Any material that exhibits measurable radiation related changes can be used as detector for ionizing radiation. Change of colors Chemical changes Emission of visible light Electric charge Active detectors: immediate measurement of the change. Passive detectors: processing before reading

6 IAEA 6 Detector Material Any material that exhibits measurable radiation related changes can be used as detector for ionizing radiation. Change of colors Chemical changes Emission of visible light Electric charge Active detectors: immediate measurement of the change. Passive detectors: processing before reading

7 IAEA 7 Detector Principles Gas filled detectors ionisation chambers proportional counters Geiger Müller (GM) - tubes Scintillation detectors solid liquid Other detectors Semi conductor detectors Film Thermoluminescense detectors (TLD)

8 IAEA 8 Detector Types 1) Counters Gas filled detectors Scintillation detectors 2) Spectrometers Scintillation detectors Solid state detectors 3) Dosimeters Gas filled detectors Solid state detectors Scintillation detectors Thermoluminiscent detectors Film

9 IAEA 9 EffectType of InstrumentDetector Electrical 1.Ionizing Chamber 2.Proportional Counter 3.GM Tube 4.Solid State Detector 1.Gas 2.Gas 3.Gas 4.Semiconductor Chemical 1.Film 2.Chemical Dosimeter 1.Photographic Emulsion 2.Solid or Liquid Light1.Scintillation counter1.Crystal or Liquid Thermo- luminescense 1.Thermo - luminescense dosimeter 1.Crystal Heat1.Calorimeter1.Solid or Liquid Detector Types

10 IAEA 10 Gas Filled Radiation Detectors These detectors consist of: a gas filled tube a positive electrode (anode) and negative electrode (cathode)

11 IAEA 11 Regions Of Operation For Gas-filled Detectors

12 IAEA 12 Ionization Chamber  Simplest of all gas filled radiation detectors  An electric field (10 4 V/m) is used to collect all the ionizations produced by the incident radiation in the gas volume  In most ionization chambers, the gas between the electrodes is air.  The chamber may or may not be sealed from the atmosphere.  Many different designs for the electrodes in an ionization chamber, but usually they consist of a wire inside of a cylinder, or a pair of concentric cylinders.

13 IAEA 13 Ionization Chamber HV + - Negative ion Positive ion 1234 Electrometer The response is proportional to ionization rate (activity, exposure rate) General Properties Of Ionisation Chambers  High accuracy  Stable  Relatively low sensitivity

14 IAEA 14 Examples Of Ion Chamber

15 IAEA 15 Applications of Ion Chambers  Current Mode  Radiation Survey  Radiation Source Calibrator  Radioactive Gases Measurement  Pulse Mode  Counting  Alpha Spectroscopy

16 IAEA 16 General Properties of Ionisation Chambers High accuracy Stable Relatively low sensitivity High accuracy Stable Relatively low sensitivity

17 IAEA 17 Problems With Ion-chambers A basic problem with ionization chambers is that they are quite inefficient as detectors for x and gamma-rays. Only a very small percentage (less than 1percent) of X- or gamma rays passing through the chamber actually interact with and cause ionization of air molecules. for x and gamma- rays, their response changes with photon energy because photon absorption in the gas volume detection efficiency and relative penetration of photons through the chamber walls both are energy-dependent processes

18 IAEA 18 Proportional Counter  Proportional counter are operated at an electric field strength 10 6 V/m for Gases at STP causing Avalanches  Applications  Low Energy X-Radiations  Neutron Detection  Spectroscopy

19 IAEA 19 Gas Multiplication and Avalanche in Proportional Detector anode cathode an electron The avalanche will stop after the electric field reduced to a threshold caused by the space charge of accumulated positive ions in the gas.

20 IAEA 20 Properties of Proportional Counter  Can be applied to situations in which the number of ion pairs generated by the radiation is too small to permit satisfactory operation in pulse-type ion chambers.  A little higher sensitivity than the ionisation chamber  Used for particles and low energy photons

21 IAEA 21 GM Counters  When the electric field strength across a proportional counter is increased (> 10 6 V/m), the device enters a GM region of operation.  GM counter is gas-ionization device in which, the ionization effect creates a response which can be converted to an electrical output.  It is a gas-filled detector designed for maximum gas amplification effect.

22 IAEA 22 GM Tube Structure  The center wire (anode) is maintained at high positive voltage relative to the outer cylindrical electrode (cathode).  The outer electrode may be a metal cylinder or a metallic film layer on the inside of a glass or plastic tube.  Some GM counters have a thin radiation entrance window at one end of the tube.  The cylinder or tube is sealed and filled with a special gas mixture, typically argon plus a quenching gas.

23 IAEA 23 Fill Gases Gases used in a Geiger tube must meet some of the same requirements as for proportional counters. noble gases are widely used for the principal component of the fill gas in G-M tubes, with helium and argon the most popular choices. A second component is normally added to most Geiger gases for purposes of quenching, the electron avalanches.

24 IAEA 24 Uses of GM Tubes  Simple, low cost, easy to operate  Pulse type counter that records number of radiation events  All energy information is lost-no ability to do spectroscopy  Dead time greatly exceeds any other commonly used radiation detector  It has a high sensitivity but has a lower accuracy.

25 IAEA 25 Types of Geiger-Mueller (GM) Tubes

26 IAEA 26 Scintillation Detectors  Scintillation is a means of detecting the presence of ionizing radiation  Ionizing radiation interacts with a scintillator which produces a pulse of light  This light interacts with a photocathode which results in the production of an electron  The electron is multiplied in a photomultiplier tube that has a series of focused dynodes with increasing potential voltage which results in an electrical signal

27 IAEA 27 Scintillation Detectors  The number of counts is dependent on the activity that is present  The energy of the electron, and consequently the associated current is proportional to the incident energy of the ionizing radiation  By analyzing the energy and corresponding number of counts, the nuclide and activity may be determined

28 IAEA 28 Scintillation Detectors There are several types of Scintillator Detectors: scintillator NaI (sodium iodide): restricted to the detection of the gamma; plastic scintillator: solution of fluorescent compounds included in a transparent plastic material (gantry); scintillator ZnS (Zinc Sulfide): used for the detection of alpha radiation

29 IAEA 29 Scintillation Detector (alpha)

30 IAEA 30 Alpha Scintillation Detector The photomultiplier tube is located in the handle.

31 IAEA 31 Scintillation Detection (photon)

32 IAEA 32 Spectral Analysis Scintillation detectors, when used with a multichannel analyzer (MCA) provide information on the energy of a photon that has interacted with the detector as well as the activity present The spectra can be analyzed to determine which isotopes are present

33 IAEA 33 Thermolumniscent Dosimeter (TLD) Thermoluminescence Mechanism: Thermoluminescence is the emission of light from a crystal on heating, after removal of excitation (i.e. ionizing radiation). Radiation dose causes the electrons in the crystal to move from low energy states to higher energy states. Some of these excited electrons are trapped in metastable states These photons can be collected with a photomultiplier tube. By proper calibration, the dose delivered to the crystal can be measured.

34 IAEA 34 Simplified scheme of the TLD process

35 IAEA 35 Thermoluminescence TLD principle thermoluminescent material heating filament emitted light photomultiplier

36 IAEA 36 TLD glow curves

37 IAEA 37 TL Dosimeters LiF CaF 2 CaSO 4 Li 2 B 4 O 7 KBr Feldspars Quartz Topaz Diamond Following are few TL materials used as TL dosimeters.

38 IAEA 38 TLD Advantages: Small size High sensitivity Integrating Tissue equivalent Disadvantages: Time consuming No permanent record

39 IAEA 39 BF 3 Neutron Detectors BF 3 Tube Construction Tube dimensions and geometry  Large size tubes at higher pressure of fill gas  Constructed of cylindrical geometry Cathode Al : low neutron absorption cross-section SS : preferred over Al because Al show alpha activity

40 IAEA 40 BF 3 Neutron Detectors Ageing effect Degradation in performance after operation of 10 10 - 10 11 registered counts Detection Efficiency Efficiency decreases abruptly with increase of neutron energies Dead spaces for charge collection reduce detection efficiency

41 IAEA 41 Lithium Containing Slow Neutron Detectors Neutron induced reaction is detected by lithium based scintillators LiI(Eu) scintillator function like NaI(Tl) detector Crystal size is greater than the range of reaction products, pulse height response is free of wall effect and a single is formed Scintillation efficiency is almost same for heavy charged particles and secondary electrons

42 IAEA 42 The 3 He Proportional Counter Design of 3 He Tube Diameter as large as possible Pressure of 3 He is increased to reduce range of charged particles Add a small amount of a heavier gas to increase stopping power

43 IAEA 43 Solid State Detectors Solid State detectors are also called Semiconductor detectors In these radiation detector, a semiconductor material such as a silicon (Si) or germanium (Ge) crystal constitutes the detecting medium. In the detecting medium electron-hole pairs are produced when a particle of ionizing radiation pass through it As a result a pulse of current generated is measured Operation of HPGe detectors require Liquid Nitrogen

44 IAEA 44 Solid State Detectors

45 IAEA 45 Using Solid as Detection Medium  In many radiation detection applications, the use of solid medium is of great advantage  For high energy electrons and gammas, solid state detectors are much smaller than gas filled detectors  Energy resolution can be improved by increasing number of charge carriers – possible in semiconductors

46 IAEA 46 Semiconductor Detectors Desirable features of – (semiconductor diode detectors) or solid state detectors Superior Energy Resolution Compact Size Fast Timing Characteristics Effective Thickness – Can be varied according to the requirement Semiconductor Materials Silicon – Used for charged particle spectroscopy Germanium - Used for gamma ray spectroscopy

47 IAEA 47 Semiconductor Detectors When a positive voltage is applied to the n-type material and negative voltage to the p-type material, the electrons are pulled further away from this region creating a much thicker depletion region The depletion region acts as the sensitive volume of the detector Ionizing radiation entering this region will create holes and excess electrons which migrate and cause an electrical pulse

48 IAEA 48 Reverse Bias Intrinsic/Depletion Region Cathode (-) Anode (+) + + - - Semiconductor Detectors

49 IAEA 49 Semiconductor Detectors Gamma rays transfer energy to electrons (principally by compton scattering) and these electrons traverse the intrinsic region of the detector e (+)(+)(+)(+) (-)(-)(-)(-)

50 IAEA 50 Film Badge Dosimeter Open Window 0.8 mm Pb filter Cu filters (0.05, 0.3 and 1.2 mm) Kodak Type 2 Radiographic Film

51 IAEA 51  Film dosimeters (commonly known as film badges) consist of a piece of photographic film in a holder  The holder is fitted with a range of filters which allows us to distinguish between beta, x-ray, gamma and thermal neutron radiations and also allows determination of the personal dose equivalent for H p (10), H p (0.07) and H p (3) Film Dosimeter

52 IAEA 52  By determining the degree of blackening (optical density) on the developed film and comparing it with calibrated films that have been exposed to known doses, it is possible to ascertain both the total dose received by the wearer and also the contribution to total dose by each type of radiation  The various filters used in film badges to ascertain whole body H p (10), skin H p (0.07) and eye H p (3) doses are shown in the following Figure and Table Film Dosimeter

53 IAEA 53 Filter MaterialApplication Open Windowbeta and very soft x-rays Plastic (50 mg cm -2 )  and x-ray dose and energy* Plastic (300 mg cm -2 )  and x-ray dose and energy* Dural (0.040”)  and x-ray dose and energy* Sn + Pb (0.028” 0.012”)  and x-ray dose and energy* Cd + Pb (0.028” 0.012”)slow neutrons** Lead (0.012”)edge shielding + Indium (0.4 g)neutron accident monitoring *quantitative determination of ** by gamma emitted after capture by cadmium + to prevent overlap of film blackening due to angled incident radiation Film Dosimeter

54 IAEA 54 Film Badge Dosimeter A A BB C C DD E E O Film Package A - Plastic filter B to E - Metallic filters O - Open window A A BB C C DD E E O Film Package A - Plastic filter B to E - Metallic filters O - Open window

55 IAEA 55 Film Badge Dosimeter The density on the film results from three basic sources:  Base+Fog  Exposure Black = exposed White = not exposed Al Filter Pb Filter

56 IAEA 56 Where to Get More Information  Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)  International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)

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