Radiation detection and measurement.pptx

This presentation is prepared for students of medical radiation physics.

1. Radiation detection and measurement Ram D att Joshi M. . S c . MIT Final year IOM, Kathmandu, Nepal

2. Introduction The detection and measurement of ionizing radiation are the basis for the majority of diagnostic imaging and radiotherapy . All detectors of ionizing radiation require the interaction of the radiation with matter.

3. Radiation detection Most energy deposited by ionizing radiation is ultimately converted into thermal energy. For e.g., a 140-keV gamma ray deposits 2.24 × 10 −14 Joules if completely absorbed. To raise the temperature of 1 g of water by 1˚C (i.e., 1 calorie) would require the complete absorption of 187 trillions ( 187 × 10 12 ) photons.

4. Types of detectors Detection method Gas filled Scintillators Semiconductors Types of information Counters Spectrometers D osimeters

5. Basic modes of operation There are two basic modes of operation of detectors, Pulse mode Current mode

6. Basic modes of operation Pulse mode The signal from each interaction is processed individually. Two interactions must be separated by a finite amount of time to produce distinct signals. This interval is called the dead time of the system. If a second interaction is close enough in time to the first interaction; it may even distort the signal from the 1 st interaction. e.g . GM counters.

7. Effect of i nteraction on detectors o perated in pulse m ode Dead time The fraction of counts lost from dead-time effects is smallest at low interaction rates and vice-versa. The dead times of different types of systems vary widely. e.g.GM counters have dead times ranging from tens to hundreds of microseconds, whereas most other systems have dead times of less than a few microseconds.

8. Behavior of detector systems operated in pulse mode Paralyzable system An interaction that occurs during the dead time after a previous interaction extends the dead time. Non- paralyzable system I t does not happen.

9. Basic modes of operation Current mode The electrical signals from individual interactions are averaged together, forming a net current signal. The information regarding individual interaction is lost. Neither the interaction rate nor the energies deposited by individual interactions can be determined. Detectors subject to very high interaction rates are often operated in current mode to avoid dead-time information losses, e.g . II tubes , DR, CT scanner, Ion chambers and dose calibrators in NM.

10. Detection efficiency The efficiency (sensitivity) of a detector is a measure of its ability to detect radiation. Efficiency = no. detected / no. emitted Efficiency = geometric efficiency x intrinsic efficiency

11. Gas filled detectors- basic principle A gas-filled detector consists of a volume of gas between two electrodes, with an electric potential difference (voltage) applied between the electrodes.

12. Gas filled detectors- basic principle The cathode is the wall of the container that holds the gas or a conductive coating on the inside of the wall, and the anode is a wire inside the container

13. Gas filled detectors There are three types of gas-filled detectors in common use:- Ionization chambers, Proportional counters, and GM counters The type of detector is determined by the voltage applied between the electrodes.

14. Gas filled detectors Recombination region-the current increases as the voltage is raised. Ionization chamber region- as the voltage is increased further, a plateau is reached in the curve. T he applied electric field is sufficiently strong to collect almost all ion pairs.

15. Gas filled detectors Proportional region- beyond the ionization region, the collected current again increases as the applied voltage is raised. The amount of electrical charge collected from each interaction in proportional region is proportional to the amount of energy deposited in the gas of the detector by the interaction.

16. Gas filled detectors Geiger- muller region- in which the amount of charge collected from each event is the same, regardless of the amount of energy deposited by the interaction. Gas-filled detectors cannot be operated at voltages beyond the GM region because they continuously discharge.

17. Ionization chamber Operated in current mode. Because gas multiplication does not occur at the relatively low voltages applied to ionization chambers. Advantage to operating them in current mode is the almost complete freedom from dead-time effects, even in very intense radiation fields.

18. Ionization chamber Almost any gas can be used to fill the chamber. If the gas is air and the walls of the chamber are of a material whose effective atomic number is similar to air, the amount of current produced is proportional to the exposure rate. The sensitivity of ion chambers to x-rays and gamma rays can be enhanced by filling them with a gas that has a high atomic number, such as argon (Z=18) or xenon (Z=54), and pressurizing the gas to increase its density.

19. Ionization chamber Uses : In portable survey meters and it can accurately indicate exposure rates from less than 1 mR /h to tens or hundreds of roentgens per hour. For performing QA test of diagnostic and therapeutic x-ray machines.

20. Ionization chamber Well type ion chambers called dose calibrators are used in nuclear medicine to assay the activities of dosages of radiopharmaceuticals to be administered to patients (pressurized with Ar ).

21. Thimble ionization chamber The chamber wall is shaped like sewing thimble. The high voltage, usually 200-500V is applied to the thimble wall with the central electrode connected to the electrometer input at or near the ground potential. The guard ring encircling the insulator is also at ground potential.

22. Thimble ionization chamber The wall of the thimble ionization chamber should be air equivalent. The most commonly used wall materials are graphite (carbon ), bakelite or a plastic coated on the inside. The graphite coated inner surface of the thimble wall acts as one electrode and the other is a rod of Al held in the centre of the thimble but electrically insulated from it. Polystyrene, polyethylene, Nylon, Mylar and Teflon are used as insulator materials.

23. Thimble ionization chamber

24. Proportional counter The proportional counter is a type of gaseous ionization detector device used to count particles of ionizing radiation. A key feature is its ability to measure the energy of incident radiation. W idely used where discrimination between radiation types is required, such as between alpha and beta particles.

25. Proportional counter A proportional counter uses a combination of the mechanisms of a Geiger-Muller tube and an ionization chamber, and operates in an intermediate voltage region between these. In this region, electrons approaching the anode are accelerated to such high kinetic energies that they cause additional ionization, called gas multiplication, which amplifies the collected current. T he amount of amplification increases as the applied voltage is raised.

26. Proportional counter In a proportional counter, the fill gas of the chamber is an inert gas which is ionized by incident radiation, and a quench gas to ensure each pulse discharge terminates. A common mixture is 90% argon, 10% methane, known as P-10.

27. GM counter- principle of operation A Geiger counter consists of a Geiger-Muller tube, the sensing element which detects the radiation and the processing electronics, which display the result. The Geiger Muller tube is filled with an inert gas such as helium, neon, or argon at low pressure, to which a high voltage is applied. The tube briefly conducts electrical charge when a particle or photon of incident radiation makes the gas conductive by ionization.

28. GM Counter The ionization is considerably amplified within the tube by the Townsend discharge e ffect to produce an easily measured detection pulse, which is fed to processing and display electronics.

29. Readout Two types of radiation readout; Counts:- the count display is the simplest reading method. -is the number of ionizing events displayed either as a count rate called counts per sec, or as a total over a set time period. - the count readout is normally used when alpha or beta particles are being detected.

30. Readout 2. Radiation dose :- it is dispalyed in a unit such as “ Seivert ” which is normally used for measuring gamma or X-ray dose rates.

31. GM Counter Applications : Hand held survey meters Finding radioactive contamination Particle detection Neutron detection : Boron trifluoride or Helium-3

32. GM Counter Advantages : They are relatively inexpensive. They are durable and easily portable. They can detect all types of radiation.

33. GM counter Disadvantages: They cannot differentiate which type of radiation is being detected. They cannot be used to determine the exact energy of the detected radiation. They have very low efficiency. They have extremely long dead time.

34. Scintillation detectors-basic principle Emit visible light or ultraviolet radiation after the interaction of ionizing radiation with the material. Oldest type of radiation detectors. Scintillators are used in conventional film-screen radiography, many direct digital radiographic image receptors, fluoroscopy, scintillation cameras, CT scanners, and positron emission tomography (PET) scanners.

35. Scintillation detectors- basic principle When ionizing radiation interacts with a scintillator, electrons are raised to an excited energy level. Ultimately, these electrons fall back to a lower energy state, with the emission of visible light or ultraviolet radiation. In all scintillators, the amount of light emitted after an interaction increases with the energy deposited by the interaction.

36. Scintillation detectors- basic principle Many organic and inorganic materials are there that can scintillate. Organic scintillators are not used for medical imaging because the low atomic numbers of their constituent elements and their low densities make them poor x-ray and gamma-ray detectors. In the inorganic materials, the scintillation is a property of crystalline structure.

37. Scintillation detectors Properties : The conversion efficiency , should be high. The decay times of excited states should be short. T he material should be transparent to its own emissions. T he frequency spectrum (color) of emitted light or UV radiation should match the spectral sensitivity of the light receptor (PMT, photodiode or film). I f used for x-ray and gamma-ray detection, the attenuation coefficient ( μ ) should be large. T he material should be rugged, unaffected by moisture, and inexpensive to manufacture.

38. Scintillation detectors in radiology

39. Conversion of light into an electrical signal Photomultiplier tube(PMT) - perform two functions; C onversion of ultraviolet and visible light photons into an electrical signal and signal amplification. When a scintillator is coupled to a PMT, an optical coupling material is placed between the two components to minimize reflection losses.

40. Conversion of light into an electrical signal Photodiodes - they are semiconductor diodes that convert light into electrical signals. Photodiodes are reverse biased. When the photodiode is exposed to light, an electrical current is generated that is proportional to the intensity of the light. Photodiodes produce more electrical noise than PMTs do, but they are smaller and less expensive. Do not amplify the signal, however, a type of photodiode called an avalanche photodiode does provide signal amplification.

41. Semiconductor detectors A crystal of a semiconductor material can be used as a radiation detector. The semiconductor crystal is “doped” with a trace amount of impurities so that it acts as a diode. A voltage is placed between two terminals on opposite sides of the crystal. When ionizing radiation interacts with the detector, electrons in the crystal are raised to an excited state, permitting an electrical current to flow.

42. Semiconductor detectors In semiconductors, valence-band electrons can be raised to the conduction band by ionizing radiation, leaving vacancy called hole.

43. Semiconductor detectors Can be : n- type (electron donor) or p- type (hole forming) A semiconductor diode consists of a crystal of semiconductor material with a region of n-type material that forms a junction with a region of p-type material.

44. Semiconductor detectors A reverse biased semiconductor diode can be used to detect visible light and UV radiation or ionizing radiation.

45. Semiconductor detectors Liquid nitrogen–cooled germanium detectors are widely used for the identification of individual gamma ray–emitting radionuclides in mixed radionuclide samples. Semiconductor detectors are seldom used for medical imaging devices because of high expense and low quantum detection efficiencies. Cadmium zinc telluride(CZT) is used as radiation detector in photon counting CT.

46. Thank you