Introduction to Health Physics Chapter 9 Health Physics Instrumentation RADIATION DETECTORS вЂў Instruments used in the practice of health physics serve a wide variety of purposes вЂў one finds instruments designed specifically for the measurement of a certain type of radiation, such as low-energy X-rays, highenergy gamma rays. fast neutrons, and so on RADIATION DETECTORS вЂў 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 Radiation Measurement Principles Signal дїЎи™џ Amplification Physical з‰©зђ† Chemical еЊ–её Biological з”џз‰© ж”ѕе¤§ Reader иЁ€и®Ђ Calibration ж ЎжЈ и©• дј° Assessment Detector еЃµжё¬е™Ё RADIATION DETECTORS PARTICLE-COUNTING INSTRUMENTS вЂў Gas-Filled Particle Counters вЂ“ variable voltage source V, high-valued resistor R вЂ“ a gas-filled counting chamber D, which has two coaxial electrodes that are very well insulated from each other вЂ“ All the capacitance associated with the circuit is indicated by the capacitor C PARTICLE-COUNTING INSTRUMENTS вЂў Gas-Filled Particle Counters вЂ“ 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 PARTICLE-COUNTING INSTRUMENTS вЂў Gas-Filled Particle Counters вЂ“ A broad-output pulse would make it difficult to separate successive pulses. вЂ“ 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 Gas-Filled Particle Counters вЂў Ionization Chamber Counter вЂ“ 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 вЂ“ 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 Gas-Filled Particle Counters вЂў Ionization Chamber Counter вЂ“ the number of electrons collected by the anode will be equal to the number produced by the primary ionizing particle вЂ“ the gas amplification factor is equal to one вЂ“ 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 Gas-Filled Particle Counters вЂў Ionization Chamber Counter вЂ“ disadvantages : the relatively feeble output pulse Gas-Filled Particle Counters вЂў Proportional Counter вЂ“ 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 вЂ“ The output voltage pulse is proportional to the high voltage across the detector вЂ“ The pulse size dependence on ionization for the purpose of distinguishing between radiations Gas-Filled Particle Counters вЂў Proportional Counter вЂ“ The gas amplification factor is greater than one вЂ“ to use a very stable high-voltage power supply вЂ“ the gas amplification depends on вЂў the diameter of the collecting electrode вЂў the gas pressure Gas-Filled Particle Counters вЂў Geiger Counter вЂ“ increase the high voltage beyond the proportional region will eventually cause the avalanche to extend along the entire length of the anode вЂ“ the size of all pulses - regardless of the nature of the primary ionizing particle- is the same вЂ“ When operated in the Geiger region, therefore, a counter cannot distinguish among the several types of radiations Geiger-Muller Counter Gas-Filled Particle Counters вЂў Geiger Counter вЂ“ avalanche ionization Gas-Filled Particle Counters вЂў Quenching a Geiger Counter вЂ“ 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 Gas-Filled Particle Counters вЂў Quenching a Geiger Counter вЂ“ Prevention of such spurious counts is called quenching вЂ“ External quenching вЂў electronically, by lowering the anode voltage after a pulse until all the positive ions have been collected вЂ“ Internal quenching вЂў chemically, by using a self-quenching gas Resolving Time вЂў 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 вЂў 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 Resolving Time вЂў dead time вЂ“ The time required to attain this electric field intensity вЂў recovery time вЂ“ the time interval between the dead time and the time of full recovery вЂў resolving time вЂ“ The sum of the dead time and the recovery time Resolving Time вЂў dead time, recovery time, resolving time Resolving Time вЂў Measurement of Resolving Time вЂў the "trueвЂњ counting rate вЂў the observed counting rate of a sample is R0 Scintillation Counters вЂў A scintillation detector is a transducer that changes the kinetic energy of an ionizing particle into a flash of light Scintillation Counters вЂў 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% вЂў Solid scintillating crystals have high detection efficiencies for gamma rays Scintillation Counters Scintillation Counters scintillator й–ѓз€Ќй«” Photomultiplier Tube й›»еўћз®Ў PM Tube Semiconductor Detector вЂў A semiconductor detector acts as a solid-state ionization chamber вЂў The operation of a semiconductor radiation detector depends on its having either an excess of electrons or an excess of holes. вЂў 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 Semiconductor Detectors DOSE-MEASURING INSTRUMENTS вЂў Radiation flux VS radiation dose rate вЂ“ Example 9.2 вЂў 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 2MeV 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: еёёз”ЁеЃµжё¬иј»е°„зљ„е„Ђе™Ё еЉ‘й‡Џз† й–Ђећ‹еЃµжЄўе™Ё з« ж‰‹жЏђеЃµжЄўе™Ё еЉ‘й‡Џй…Ќ Ionization Chamber Dosimeter Personal Pen Dosimeter дєєе“Ўз†ећ‹еЉ‘й‡ЏиЁ€ Diagnostic IC иЁєж–·з”Ёжёёй›ўи…” Therapeutic IC жІ»з™‚з”Ёжёёй›ўи…” Survey meter иј»е°„еЃµжЄўе™Ё OD пЂЅ log L0 , ODпјље…‰еЇ†еє¦(optical density ) OD Film Dosimeters Lt гЂЂгЂЂ 100%йЂЏе…‰зЋ‡пјљгЂЂ ( L0 пЂЅ Lt )гЂЂOD пЂЅ 0 гЂЂгЂЂ 10%йЂЏе…‰зЋ‡пјљгЂЂ ( L0 пЂЅ 10 Lt )гЂЂOD пЂЅ 1 гЂЂгЂЂ 1%йЂЏе…‰зЋ‡пјљгЂЂ ( L0 пЂЅ 100 Lt )гЂЂOD пЂЅ 2 Dose (log) Thermoluminescent Dosimeters Glow curve (з™је…‰ж›Із·љ) Energy Dependence Film TLD DOSE-MEASURING INSTRUMENTS вЂў Electronic Dosimeters вЂ“ employ solid-state semiconductors, silicon diodes, to detect beta and gamma radiation over a very wide range of dose rates and doses вЂ“ measure and display instantaneous dose rate and integrate over time NEUTRON MEASUREMENTS вЂў Detection Reactions вЂ“ Neutrons, like gamma rays, are not directly ionizing; they must react with another medium to produce a primary ionizing particle вЂ“ Because of the strong dependence of neutron reaction rate on the cross section for that particular reaction, вЂў use different detection media, depending on the energy of the neutrons that we are trying to measure, вЂў modify the neutron energy distribution so that it will be compatible with the detector NEUTRON MEASUREMENTS вЂў Detection Reactions вЂ“ 10B(n,a)7L вЂў either as BF3 gas or as a thin film on the inside surfaces of the detector tube вЂў The ionization due to the alpha particle and the 7Li recoil nucleus is counted вЂ“ Elastic scattering of high-energy neutrons by hydrogen atoms. (scattered proton) вЂ“ Nuclear fission: fissile material (n,f) fission fragments вЂ“ Neutron activation: threshold detectors NEUTRON MEASUREMENTS вЂў Neutron Dosimetry вЂ“ 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 вЂ“ Commercially available neutron dose-equivalent meters, utilize a thermal neutron detector surrounded by a spherical or semispherical moderator NEUTRON MEASUREMENTS вЂў Neutron Dosimetry вЂ“ Commercially available neutron dose-equivalent meters NEUTRON MEASUREMENTS вЂў Neutron Dosimetry вЂ“ Bubble Dosimeter вЂў completely unresponsive to gamma radiation вЂў allowing calibration and readout directly in microsieverts or in millirems of neutron dose вЂў The number of bubbles is directly proportional to the neutron-equivalent dose. вЂў з›ёдє’иЄЌеЏЇеЌ”и° (Mutual Recognition Arrangement, MRA) C IPM й—њ йЌµ жЇ” е°Ќ (key com parisons)и€‡ еЌЂ еџџ жЂ§ й—њ йЌµ жЇ” е°Ќ (regional key com parisons)гЂЃ й›™ й‚Љ й—њ йЌµ жЇ” е°Ќ (bilateral com parisons)зљ„ зµђ еђ€ ж–№ ејЏ пјљ еЏѓ еЉ C IPM й—њ йЌµ жЇ” е°Ќ зљ„ ењ‹ е®¶ еЇ¦ й©— е®¤ пјљеЏѓеЉ еЌЂеџџжЂ§жЇ”е°Ќзљ„ењ‹е®¶еЇ¦й©—е®¤ пјљ еЏѓ еЉ C IPM и€‡ еЌЂ еџџ жЂ§ жЇ” е°Ќ зљ„ ењ‹ е®¶ еЇ¦ й©— е®¤ пјље…¶д»–ењ‹е®¶еЇ¦й©—е®¤ пјљй›™й‚Љй—њйЌµжЇ”е°Ќ вЂў й‡Џжё¬иїЅжєЇй«”зі»Traceability of Measurement ењ‹йљ›еє¦й‡ЏиЎЎе±Ђ(BIPM) International Bureau of Weights & Measures еђ„ењ‹е®¶жЁ™жє–еЇ¦й©—е®¤ е¤§й™ё NIM и‹±ењ‹ NPL еѕ·ењ‹ PTB зўєдїќиј»е°„й‡Џзљ„й‡Џжё¬дёЂи‡ґжЂ§ дёиЏЇж°‘ењ‹ жёёй›ўиј»е°„ењ‹е®¶жЁ™жє– еЇ¦й©—е®¤ NRSL зѕЋењ‹ NIST жѕіжґІ ARPANSA жЄў ж Ў еЇ¦ й©— е®¤ жњЂзµ‚дЅїз”ЁиЂ… е…¶д»–иЎЊжҐ иЈЅйЂ е·Ґе» ж ёиѓЅй›»е» й†«з”ЁиЁєз™‚й™ў ж—Ґжњ¬ ETL COUNTING STATISTICS вЂў 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. вЂў When we make a measurement, we estimate the true average rate from the observed count rate вЂў The error of a determination is defined as the difference between the true average rate and the measured rate APPLICATIONS OF STATISTICAL MODELS Application B: Estimation of the Precision of a Single Measurement COUNTING STATISTICS вЂў The Binomial Distribution вЂ“ n is the number of trials вЂ“ for which each trial has a success probability p, then вЂ“ the predicted probability of counting exactly x successes COUNTING STATISTICS вЂў The Binomial Distribution вЂ“ Exp. dice throw : вЂў 3 ones in 3 consecutive throws. n=3, p=1/6, x=3 P(3)=[3!/3!]*(1/6)^3=1/216 вЂў 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 вЂў 1 ones in 3 consecutive throws. n=3, p=1/6, x=1 P(1)=[3!/2!]*(1/6)*(5/6)^2=25/72 COUNTING STATISTICS вЂў The normal distributions вЂ“ As n increases, the distribution curve becomes increasingly symmetrical around the center line вЂ“ For the case where n is infinite, we have the familiar bellshaped normal curve COUNTING STATISTICS вЂў The normal distributions вЂ“ 34% of the area lies between the mean and 1s above or below the mean. вЂ“ about 14% of the area is between 1s and 2s вЂ“ only about 2% of the total area lies beyond either + or - 2s from the mean COUNTING STATISTICS вЂў The normal distributions вЂ“ Since the curve is symmetrical about the mean, 68% of the area lies between п‚±1s вЂ“ 96% of the area is included between п‚±2s COUNTING STATISTICS вЂў The Poisson distributions вЂ“ Many categories of binary processes can be characterized by a constant and small probability of success for each individual trial. Included are most nuclear counting experiments in which the number of nuclei in the sample is large and the observation time is short compared with the half-life of the radioactive species вЂ“ Under these conditions, p << 1, the binomial distribution approaches the Poisson distribution COUNTING STATISTICS вЂў The Poisson distributions вЂ“ standard deviation: вЂ“ coefficient of variation CV вЂ“ Variance: s 2 COUNTING STATISTICS вЂў The Poisson distributions вЂ“ sum or difference: COUNTING STATISTICS вЂў The Poisson distributions вЂ“ Exp. A) 10000 counts in 10-min, s = 100 per 10 min 1000 п‚± 10 cpm, %CV = (10/1000)*100%=1% вЂ“ Exp. B) 1000 counts in 1-min, s = 32 per 1-min 1000 п‚± 32 cpm, %CV = (32/1000)*100%=3.2% вЂ“ When two quantities, each of which has its own variance COUNTING STATISTICS вЂў The Poisson distributions вЂ“ Example 9.6 вЂ“ A 5-min sample count gave 510 counts, while a 1-h background measurement yielded 2400 counts. What is the net sample counting rate and the standard deviation , of the net counting rate? COUNTING STATISTICS вЂў The Poisson distributions вЂ“ product or a quotient COUNTING STATISTICS вЂў The Poisson distributions вЂ“ Example 9.7 COUNTING STATISTICS вЂў The Poisson distributions вЂ“ Example 9.9 иЁ€ж•ёзµ±иЁ€ ( Counting statistic ) иЁ€ж•ёзµ±иЁ€ ( Counting statistic ) PROBLEMS вЂў 9.1, 9.2, 9.4, 9.8, 9.18, 9.19, 9.20, 9.23, 9.24, 9.27, 9.33

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