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Introduction to Health Physics Chapter 9 Health Physics Instrumentation

Introduction to Health Physics Chapter 9 Health Physics Instrumentation. RADIATION DETECTORS. Instruments used in the practice of health physics serve a wide variety of purposes

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Introduction to Health Physics Chapter 9 Health Physics Instrumentation

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  1. Introduction to Health PhysicsChapter 9Health Physics Instrumentation

  2. RADIATION DETECTORS • Instruments used in the practice of health physics serve a wide variety of purposes • one finds instruments designed specifically for the measurement of a certain type of radiation, such as low-energy X-rays, high-energy gamma rays. fast neutrons, and so on

  3. RADIATION DETECTORS • The basic requirement of any such instrument is that its detector interact with the radiation in such a manner that the magnitude of the instrument's response is proportional to the radiation effect or radiation property being measured

  4. Radiation Measurement Principles Calibration 校正 Signal 信號 Physical 物理 Chemical 化學 Biological 生物 Amplification 放大 Reader 計讀 Detector 偵測器 Assessment 評估

  5. RADIATION DETECTORS

  6. PARTICLE-COUNTING INSTRUMENTS • Gas-Filled Particle Counters • variable voltage source V, high-valued resistor R • a gas-filled counting chamber D, which has two coaxial electrodes that are very well insulated from each other • All the capacitance associated with the circuit is indicated by the capacitor C

  7. PARTICLE-COUNTING INSTRUMENTS • Gas-Filled Particle Counters • If the time constant RC of the detector circuit is much greater than the time required for the collection of all the ions resulting from the passage of a single particle through the detector, then a voltage pulse of magnitude

  8. PARTICLE-COUNTING INSTRUMENTS • Gas-Filled Particle Counters • A broad-output pulse would make it difficult to separate successive pulses. • if the time constant of the detector circuit is made much smaller than the time required to collect all the ions. This pulse, allows individual pulses to be separated and counted

  9. Gas-Filled Particle Counters • Ionization Chamber Counter • the range of voltage great enough to collect the ions before a significant fraction of them can recombine yet not great enough to accelerate the ions sufficiently to produce secondary ionization by collision • The exact value of this voltage is a function of the type of gas, the gas pressure, and the size and geometric arrangement of the electrodes

  10. Gas-Filled Particle Counters • Ionization Chamber Counter • the number of electrons collected by the anode will be equal to the number produced by the primary ionizing particle • the gas amplification factor is equal to one • The pulse size from a counter depends on the number of ions produced in the chamber makes it possible to use this instrument to distinguish between radiations of different specific ionization

  11. Gas-Filled Particle Counters • Ionization Chamber Counter • disadvantages : the relatively feeble output pulse

  12. Gas-Filled Particle Counters • Proportional Counter • As the voltage across the counter is increased beyond the ionization chamber region, a point is reached where secondary electrons are produced by collision. This multiplication of ions in the gas, which is called an avalanche • The output voltage pulse is proportional to the high voltage across the detector • The pulse size dependence on ionization for the purpose of distinguishing between radiations

  13. Gas-Filled Particle Counters • Proportional Counter • The gas amplification factor is greater than one • to use a very stable high-voltage power supply • the gas amplification depends on • the diameter of the collecting electrode • the gas pressure

  14. Gas-Filled Particle Counters • Geiger Counter • increase the high voltage beyond the proportional region will eventually cause the avalanche to extend along the entire length of the anode • the size of all pulses - regardless of the nature of the primary ionizing particle- is the same • When operated in the Geiger region, therefore, a counter cannot distinguish among the several types of radiations

  15. Geiger-Muller Counter

  16. Gas-Filled Particle Counters • Geiger Counter • avalanche ionization

  17. Gas-Filled Particle Counters • Quenching a Geiger Counter • After the primary Geiger discharge is terminated, the positive ions slowly drift away from the anode wire and ultimately arrive at the cathode or outer wall of the counter. Here they are neutralized by combining with an electron from the cathode surface. In this process, an amount of energy equal to the ionization energy of the gas minus the energy required to extract the electron from the cathode surface (the work function) is liberated. If this liberated energy also exceeds the cathode work function, it is energetically possible for another free electron to emerge from the cathode surface---and thereby produce a spurious count

  18. Gas-Filled Particle Counters • Quenching a Geiger Counter • Prevention of such spurious counts is called quenching • External quenching • electronically, by lowering the anode voltage after a pulse until all the positive ions have been collected • Internal quenching • chemically, by using a self-quenching gas

  19. Resolving Time • The negative ions, being electrons, move very rapidly and are soon collected, while the massive positive ions are relatively slow-moving and therefore travel for a relatively long period of time before being collected • These slow-moving positive ions form a sheath around the positively charged anode, thereby greatly decreasing the electric field intensity around the anode and making it impossible to initiate an avalanche by another ionizing particle. As the positive ion sheath moves toward the cathode, the electric field intensity increases, until a point is reached when another avalanche could be started

  20. Resolving Time • dead time • The time required to attain this electric field intensity • recovery time • the time interval between the dead time and the time of full recovery • resolving time • The sum of the dead time and the recovery time

  21. Resolving Time • dead time, recovery time, resolving time

  22. Resolving Time • Measurement of Resolving Time • the "true“ counting rate • the observed counting rate of a sample is R0

  23. Scintillation Counters • A scintillation detector is a transducer that changes the kinetic energy of an ionizing particle into a flash of light

  24. Scintillation Counters • Whereas the inherent detection efficiency of gas-filled counters is close to 100% for those alphas or betas that enter the counter, their detection efficiency for gamma rays is very low-usually less than 1% • Solid scintillating crystals have high detection efficiencies for gamma rays

  25. Scintillation Counters

  26. Scintillation Counters Photomultiplier Tube 電增管 scintillator 閃爍體 PM Tube

  27. Semiconductor Detector • A semiconductor detector acts as a solid-state ionization chamber • The operation of a semiconductor radiation detector depends on its having either an excess of electrons or an excess of holes. • A semiconductor with an excess of electrons is called an n-type semiconductor, while one with an excess of holes is called a p-type semiconductor

  28. Semiconductor Detectors

  29. DOSE-MEASURING INSTRUMENTS • Radiation flux VS radiation dose rate • Example 9.2 • Consider two radiation fields of equal energy density. In one case, we have 0.1-MeV photon flux of 2000 photons per cm^2/s. In the second case, the photon energy is 2- MeV and the flux is 100 photons per cm^2/s. The energy absorption coefficient for muscle for 0.1-MeV gamma radiation is 0.0252 cm^2/g; for 2-MeV gamma the energy absorption coefficient is 0.0257 cm^2/g. The dose rates for the two radiation fields are given by:

  30. 常用偵測輻射的儀器 劑量筆 手提偵檢器 門型偵檢器 劑量配章

  31. Ionization Chamber Dosimeter Personal Pen Dosimeter 人員筆型劑量計 Diagnostic IC 診斷用游離腔 Therapeutic IC 治療用游離腔 Survey meter 輻射偵檢器

  32. Film Dosimeters OD Dose (log)

  33. Thermoluminescent Dosimeters Glow curve (發光曲線)

  34. Energy Dependence Film TLD

  35. DOSE-MEASURING INSTRUMENTS • Electronic Dosimeters • employ solid-state semiconductors, silicon diodes, to detect beta and gamma radiation over a very wide range of dose rates and doses • measure and display instantaneous dose rate and integrate over time

  36. NEUTRON MEASUREMENTS • Detection Reactions • Neutrons, like gamma rays, are not directly ionizing; they must react with another medium to produce a primary ionizing particle • Because of the strong dependence of neutron reaction rate on the cross section for that particular reaction, • use different detection media, depending on the energy of the neutrons that we are trying to measure, • modify the neutron energy distribution so that it will be compatible with the detector

  37. NEUTRON MEASUREMENTS • Detection Reactions • 10B(n,a)7L • either as BF3 gas or as a thin film on the inside surfaces of the detector tube • The ionization due to the alpha particle and the 7Li recoil nucleus is counted • Elastic scattering of high-energy neutrons by hydrogen atoms. (scattered proton) • Nuclear fission: fissile material (n,f) fission fragments • Neutron activation: threshold detectors

  38. NEUTRON MEASUREMENTS • Neutron Dosimetry • The dose equivalent (DE) from neutrons depends strongly on the energy of the neutrons, We therefore cannot simply convert neutron flux into dose equivalent unless we know the energy spectral distribution of the neutrons • Commercially available neutron dose-equivalent meters, utilize a thermal neutron detector surrounded by a spherical or semispherical moderator

  39. NEUTRON MEASUREMENTS • Neutron Dosimetry • Commercially available neutron dose-equivalent meters

  40. NEUTRON MEASUREMENTS • Neutron Dosimetry • Bubble Dosimeter • completely unresponsive to gamma radiation • allowing calibration and readout directly in microsieverts or in millirems of neutron dose • The number of bubbles is directly proportional to the neutron-equivalent dose.

  41. 相互認可協議 (Mutual Recognition Arrangement, MRA)

  42. 國際度量衡局(BIPM) International Bureau of Weights & Measures • 量測追溯體系Traceability of Measurement 各國家標準實驗室 確保輻射量的量測一致性 大陸 NIM 英國 NPL 德國 PTB 美國 NIST 澳洲 ARPANSA 日本 ETL 中華民國 游離輻射國家標準 實驗室 NRSL 檢校實驗室 最終使用者 其他行業 製造工廠 核能電廠 醫用診療院

  43. COUNTING STATISTICS • Because of this fluctuating rate, it is not correct to speak of a true rate of transformation (which implies no statistical error in the measurement) but rather of a true average rate of transformation. • When we make a measurement, we estimate the true average rate from the observed count rate • The error of a determination is defined as the difference between the true average rate and the measured rate

  44. APPLICATIONS OF STATISTICAL MODELS

  45. Application B: Estimation of the Precision of a Single Measurement

  46. COUNTING STATISTICS • The Binomial Distribution • nis the number of trials • for which each trial has a success probability p, then • the predicted probability of counting exactly x successes

  47. COUNTING STATISTICS • The Binomial Distribution • Exp.dice throw : • 3 ones in 3 consecutive throws. n=3, p=1/6, x=3 P(3)=[3!/3!]*(1/6)^3=1/216 • 2 ones in 3 consecutive throws. n=3, p=1/6, x=2 P(2)=[3!/(1!*2!)]*(1/6)^2*(5/6)=5/72 • 1 ones in 3 consecutive throws. n=3, p=1/6, x=1 P(1)=[3!/2!]*(1/6)*(5/6)^2=25/72

  48. COUNTING STATISTICS • The normal distributions • As n increases, the distribution curve becomes increasingly symmetrical around the center line • For the case where n is infinite, we have the familiar bell-shaped normal curve

  49. COUNTING STATISTICS • The normal distributions • 34% of the area lies between the mean and 1s above or below the mean. • about 14% of the area is between 1s and 2s • only about 2% of the total area lies beyond either + or - 2s from the mean

  50. COUNTING STATISTICS • The normal distributions • Since the curve is symmetrical about the mean, 68% of the area lies between 1s • 96% of the area is included between 2s

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