D OSIMETRY & RADIATION DETECTORS - PowerPoint Presentation

1. DOSIMETRY & RADIATION DETECTORSAASIF MAJEED LONE ASSISTANT PROFESSORCENTURION UNIVERSITY

2. DosimetryRadiation Dosimetry is a study in physics which deals with the measurement of radiation which may include Exposure, Absorbed dose etc., Dosimetry is extensively used for radiation protection and is routinely applied to radiation workers, where irradiation is expected but regulatory levels must not be exceeded. Dosimetry contains required quantitative methods which are used to determine the dose of radiation, which helps in, a. The need of protection against ionizing radiation, b. Application of radiation in medicine.

3. Dosimetric quantities 1. Activity 2. Exposure 3. Absorbed dose Radioactivity: Spontaneous emission of certain unstable atomic nuclei with the emission of certain radiations is called 'Radioactivity'.

4. Activity :Activity refers to amount of unstable nuclei that gains stability through radio disintegration per unit time. AWhere dN is the number of nuclear transformation (decay) in unit time dt.  

5. Exposure: Exposure is a measure of ionization produced in air by photons (X rays or Gamma rays).  Exposure is given by, where dQ is the absolute value of total charge of ions of one sign produced in air when all electrons (negatron's or positrons) liberated by photons in air of mass dm are completely stopped by air. SI unit of exposure is C/Kg. Special Unit of exposure is Roentgen. It is applicable only for: Photon energies below 3 MeV Interaction is only between photons and air.  

6. Absorbed dose: The Absorbed dose (D), is the energy absorbed per unit mass. This quantity is defined for all ionizing radiation (not only for EM radiation, as in the case of the exposure) and for any material.where,dE is the energy imparted to matter of mass dm. The unit of absorbed dose is Gray. Energy imparted =(Energy incident)- (Energy leaving the mass)- (Energy released in nuclear transformations) 

7. Types of radiation There are two types of radiation 1.Non Ionizing Radiation: Radiation that does not have sufficient energy to eject the orbital electrons from the medium. E.g. Microwaves, ultraviolet light, lasers, radio waves and infrared light.2.Ionizing Radiation: Radiation that has sufficient energy to eject orbital electrons from the medium it is passing through.E.g. Alpha particles, neutrons, gamma rays and Xrays.

8. Radiation Dosimeters Radiation Dosimeters are the devices used for detection of the radiation which directly or indirectly measures Exposure, Kerma, Absorbed dose, Equivalent dose or other quantities. The dosimeter along with its reader is referred to as a Dosimetry System. Two parts of Radiation measuring system are: 1. A detector 2. A measuring apparatus(electrometer) The interaction of radiation with the system takes place in the detector. The measuring apparatus takes the output of the detector and performs the function required to accomplish the measurements.

9. Properties of an useful dosimeterProperties of an useful dosimeter are as follows : 1. High accuracy and precision 2. Linearity of signal with dose over a wide range 3. Small dose and dose rate dependence 4. Flat Energy response (Quality dependence) 5. Small directional dependence 6. High spatial resolution

10. DETECTORSPrinciple of Gas filled detectorsGas Multiplication: Gas Multiplication is a consequence of increasing the electric field within the gas to a sufficiently high value. At low value of field the ions & electrons produced/created by the incident radiation simply drift to their respective collecting electrodes. During migration of these charges many collisions occur with the natural gas molecules. Because of low mobility ,positive or negative ions achieve very little average energy between collisions. Free electrons are easily accelerated by the applied field and may have the significant K.E when undergoing such a collision. If the energy is greater than ionization energy of the neutral gas molecule there may be an additional ion pair created in the collision as the average energy of the electron between the collision increases with increasing the electric field, there is a threshold value of the field above which the secondary ionization occurs.

11. Regions of Detector operations Graph of α vs applied voltage(within detector) is plotted. At very low voltage the field is insufficient to prevent the recombination of the ion pairs and collected charge is less than that of represented by the original ion pairs. Recombination is suppressed as the voltage is raised, saturation region is achieved. Ionization chamber works in this mode of operation. Gas multiplication begins as the voltage applied crosses the threshold field. Collected charge begins to multiply and α increases. Over some region the gas multiplication will be linear and collected charge will be proportional to number of original ion pairs created by incident radiation. This region is true proportionality region and represents conventional proportional counter mode of operation.

12. Further increase in the voltage results in non-linear effects(mainly +ve ions are created by secondary ionization). Unlike the electrons the +ve ions move slower towards the electrode and require more time to reach towards the electrode. Therefore each pulse within the counter creates a cloud of +ve ions which are slow to disperse as it drifts towards the cathode. If the concentration of these ions are sufficiently high then they represents a ‘Space charge effect’. The voltage is made sufficiently high so that the space charge created by the ion pairs dominantly determine the subsequent history of the pulse. Avalanche proceeds until sufficient no.Of ion pairs are created up to the limit of applied voltage. This is a self limiting process. Finally the ion collection of the detector is no longer reflects any properties of incident radiation. This region is called ‘Gieger muller’ region.

13. Free Air ionization chamberFree Air ionization chamber The free-air, or standard, ionization chamber is an instrument used in the measurement of exposure in roentgens. Generally, such a primary standard is used only for the calibration of secondary instruments designed for field use. The free-air chamber installations are thus confined principally to some of the national standards laboratories. An x-ray beam, originating from a focal spot S, is defined by the diaphragm D, and passes centrally between a pair of parallel plates. A high voltage (field strength of the order of 100V/cm) is applied between the plates to collect ions produced in the air between the plates.

14. The ionization is measured for a length L defined by the limiting lines of force to the edges of the collection plate C. The lines of force are made straight and perpendicular to the collector by a guard ring G. Electrons produced by the photon beam in the specified volume must spend all their energy by ionization of air between the plates. Such a condition can exist only if the range of the electrons liberated by the incident photons is less than the distance between each plate and the specified volume. In addition, for electronic equilibrium to exist, the beam intensity must remain constant across the length of the specified volume, and the separation between the diaphragm and the ion collecting region must exceed the electron range in air.

15. Proportional counter Proportional counter Proportional counter is a type of gas filled detector always used in pulsed mode and rely on the gas multiplication phenomenon. Gas Multiplication is a consequence of increasing the electric field within the gas to a sufficiently high value. Primary ions produced undergo further ionization producing secondary ions and the process continues. The total charge collected is then measured and is given by, Where , Q = M.No.eQ- Total charge generated NO is Original no. Of ion pairs. E- electron charge. M-Multiplication factor of Gas.

16. Geiger-Muller CounterGeiger-Muller Counter GM counter is an instrument used for detection of ionizing radiation. It cant determine the type or energy of the radiation. GM counter uses the ‘Townsend avalanche’ phenomenon to produce an easily detectable electronic pulse. Under proper condition a situation is created in which avalanche itself trigger an second avalanche at a different position in the GM tube. • GM tube is a sensing part of the GM counter is an cylindrical metal envelope filled with inert gas (Helium ,Neon, argon) at low pressure(0.1 atm).

17. Wall of the tube is stainless steel inside coated with metal or graphite to form a cathode. Anode is a tungsten wire about 0.03-0.04 inch dia. Stretched along the centre of cylinder.High potential difference is applied between electrodes (500V-1500V). When ionizing radiation strikes on the tube, gas molecule inside get ionized and positive ions accelerated towards the cathode and electrons towards the anode. Due to high voltage applied the accelerated ions produce further ionization with the gas molecules. As a result of rapid multiplication of ion creates a large no. of electron avalanche which is spread along the anode. One single avalanche per original ionizing event will produce 106 to 108 excited molecules.

18. Scintillation detectorScintillation detectorScintillation detector works on principle of detection of ionizing radiation by measuring the scintillation light produced by certain materials like NaI, CsI, Cadmium tungstate ,amorphous silicon etc., When the crystals absorb the energy the e- jump from ground state to conduction band. The excited e- return to the valance band with emission of photon. When photon interacts with the crystal photoelectrons are produced within the crystals. These photoelectrons travel through the crystal & they ionize the atoms of crystal, as a result Flashes of UV or visible light are produced (Luminescence). Only small part of energy imparted is converted into light rest is dissipated into form of heat.

19. Scintillation detector consists of, 1. Luminescent scintillation material. 2. Optical device to facilitate the collection of light(Optical interface) 3. Photomultiplier tube(PMT) 4. Electrical counter

20. The low energy photons falls on the photocathode of PMT and ejects no. of photoelectrons. Photoelectrons are accelerated by P.D. Applied between Cathode & Dynodes of the tube. On striking the 1st Dynode Photoelectrons ejects several electrons by secondary emission and further multiplies with rest of dynode interactions. After multiplication of 105 – 109 the electron avalanche, electrons arrive at collector plate. It produces a voltage pulse on O/P condenser, which is coupled to external Pulse amplifying circuit. Thus initial energy of single ionizing particle is transformed into single voltage pulse.

21. Thermo luminescent DetectorThermo luminescence is a process in which A thermo luminescence crystal is irradiated, a very minute fraction of absorbed energy is stored in crystal lattice. The same energy can be recovered later as visible light and measured.The amount of energy of light can be calculated and is proportional to amount of irradiated energy. Some of commonly used TL phosphors are, 1) LiF 2) CaSo4 3) Al2O3 4) LiB4 O7 5) Dy (2Lithium tetra Borate) These phosphors provides electron holes due to their crystalline lattice imperfection and they, a. Traps the electrons holds for long period of time.b. Emits light when electrons jump to ground state from excited state.

22. Principle and Working When these phosphors are irradiated the electrons in the valance band get excited and they rises to conduction band( e- free to move). There exists an electron trap inside (due to crystalline lattice imperfection) where e- gets trapped. Vacancy is created in the conduction band. When heated the trapped electrons get sufficient energy to escape from the trap and jump back to conduction band radiatively. Luminescence is produced as a result of emission of energy. The intensity of emission of light is directly proportional to Rate of electron escape.

23. Personnel monitoringTLD Badge: TLD Card is made up of high impact plastic. There are 3 filters in cassette corresponding to each disc namely Cu+Al , perspex & open window. 1st disc is sandwiched between pair of filter combination of 1mm Al and 0.9mm Cu. 2nd disc is sandwiched between pair of 1.5mm thick Plastic filters (180 mg/cm2). 3rd disc is positioned under circular open window. Metallic filter- Gamma rays Plastic filter- Beta rays Open window- Alpha rays

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25. Film BadgesA special emulsion photographic film in a light- tight wrapper enclosed in a case or holder with windows, with appropriate filters, is known as a film badge  The badge holder creates a distinctive pattern on the film indicating the type and energy of the radiation received.  Consists of three parts: - Plastic film holder Metal filters Film packet

26. Advantages And Disadvantages Of The Film BadgeLightweight, durable, portableOnly records exposure where it’s worn Cost efficient Not effective if not wornPermanent legal recordCan be affected by heat and humidityCan differentiate between scatter and primary beam Sensitivity is decreased above and below 50 keV  Can discriminate between x, gamma, and beta radiationExposure cannot be determined on day of exposure Can indicate direction from where radiation came fromAccuracy limited to + or - 20%Control badge can indicate if exposed in transit

27. Pocket dosimeterBasically its an ionization chamber which is charged to a suitable voltage and the discharge in the electrostatic charge on a metal conductor due to ionization of air in chamber by radiation. Dosimeter must be periodically recharged and read before it gets charged and dose is logged into charts one’s exposure level. Magnifying lens and illumination allows one to directly read the dose at any time by aiming the illumination lens at light source. Device is mainly sensitive to gamma rays and X-rays but also detects the β-radiation Advantages:1. Immediate reading. 2. Energy independent of wide range of gamma rays. Disadvantages: 1. Low accuracy, Reading errors(manually) small dynamic range. 2. Susceptible to moisture.

28. WorkingIt consists of ‘Lauristen electroscope’ and has sealed air filled ionization chamber. An metal electrode strip is attached to terminal on the end of pen for charging. Other end is delicate Gold pated quartz fibre attached to it which at rest is parallel to electrode. Ends of chamber are transparent and microscope is focused on fibre. 150-200 V is applied during charge. When the ionizing radiation passes through the chamber it collides with air molecules and ionize the air. Reduced charge of the electrode reduces the force on the fibre causing it to move back towards electrode. Position of fibre can be read through the microscope with scale behind fibre.

29. Percentage Depth Dose (PDD)The ratio (in percent) of the dose absorbed at a predefined depth (Dx) to Dmax (the dose maximum) for a predefined SSD and field size is termed the percentage depth dose (PDD or DD%).DD% is also defined as the dose at a specific depth as a function of distance, field and energy in a water phantom.The percentage depth dose curve provides information on the quality of the radiation and its energy.The depth at dose maximum can be calculated.The most probable energy at the surface of the phantom can be found by calculating the range of the electrons. This can give information on X-ray contamination.

30. Isodose CurvesIsodose curves are prepared by combining the points in the phantom or target tissue that receive the same dose. They are calculated by various dosimetric measurements, and the highest dose is considered 100%. The curves are placed in percentage order, and then used to create the dose distribution graphics for the target tissue and the energy of interestBy the using the isodose curves during treatment planning, the dose distribution of the radiation delivered to the target tissue and neighboring structures can be seen from different angles.In a plot of isodose curves, the y-axis shows the depth below the surface of the skin, while the x-axis shows the range of the field.

31. PenumbraThe penumbra is defined as the region of steep dose rate decrease at the edge of radiation beam, noting that the dose rate decreases as a function of the distance from the central axisTypes of PenumbraThe physical penumbra is the penumbra measured in the dose profile. It is the distance between the points at which the 20 and 80% isodose curves cross the x-axis at Dmax.There are several components to the physical penumbra:Geometrical penumbra: This occurs due to the size of the source; large sources have larger geometrical penumbras.Transmission penumbra: This occurs due to the beam emerging from the edges of blocks or collimators. It can be decreased by making sure that the shapes of the focalized blocks take into account the beam divergence.

32. Backscatter Factor (BSF)In a phantom, the ratio of the dose maximum to the dose in air at the same depth is called the backscatter factor (BSF).BSFIncreases as the energy increases (gets closer to 1)Increases as the field size increases (gets closer to 1)Is independent of SSDSince the energy of the scattering photon increases as the energy increases, BSF increases.At >2 MV, the BSF approaches 1The depth at which the BSF is measured depends on the energyThe BSF measurement depth at energies below that of Co-60 is the surface, since Dmax is close to the surface

33. Tissue to Air Ratio (TAR)The ratio of the dose at depth d (Dd) in a phantom to the dose at the same depth in air (Dair) for the distance used in SAD is defined as the tissue to air ratio (TAR).The BSF is only defined at Dmax, whereas TAR can be defined at any depth. When d = Dmax, TAR = BSFTARIncreases as the energy increasesIncreases as the field size increasesIs independent of SSD at low megavoltage energiesIs dependent on SSD at high megavoltage energies (due to electron contamination)The BSF includes primary radiation and scattered radiation; the TAR only includes scattered and absorbed radiationIf Dd = Dmax in the TAR formula → peak scatter factor (PSF).

34. Tissue Maximum Ratio (TMR)The ratio of the dose measured at a depth d (Dd) to Dmax in a phantom is defined as the tissue maximum ratio (TMR) :It is defined by performing two measurements in the phantom (i.e., Dd and Dmax are measured).The TMR is normalized to Dmax, in contrast to the TAR.TMR1. Increases as energy increases2. Increases as the field size increases3. Is independent of SSD at low megavoltage energies4.Is dependent on SSD at high megavoltage energies (due to electron contamination)

35. Scatter Air Ratio (SAR)The ratio of dose measured at d depth (Dd-phantom) in phantom to the dose measured at the same depth in air (Dd-air) is defined as SAR.It is used for the calculation of mean scattered dose.It is independent on SSD as TAR, but dependent parameter on energy, depth, and field size.

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