Introduction to Health Physics Chapter 9 Health Physics Instrumentation

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Introduction to Health PhysicsChapter 9Health
Physics Instrumentation
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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

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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

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Radiation Measurement Principles
Signal ?? Physical ?? Chemical ?? Biological ??
Amplification ??
Reader ??
Detector ???
Assessment ??
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RADIATION DETECTORS
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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

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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

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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

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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

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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

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Gas-Filled Particle Counters

• Ionization Chamber Counter
• disadvantages the relatively feeble output pulse

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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

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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

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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

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Geiger-Muller Counter
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Gas-Filled Particle Counters

• Geiger Counter
• avalanche

ionization
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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

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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

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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

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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

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Resolving Time

• dead time, recovery time, resolving time

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Resolving Time

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

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Scintillation Counters

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

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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

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Scintillation Counters
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Scintillation Counters
Photomultiplier Tube ???
scintillator ???
PM Tube
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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

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Semiconductor Detectors
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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 cm2/s. In the second
  case, the photon energy is 2- MeV and the flux is
  100 photons per cm2/s. The energy absorption
  coefficient for muscle for 0.1-MeV gamma
  radiation is 0.0252 cm2/g for 2-MeV gamma the
  energy absorption coefficient is 0.0257 cm2/g.
  The dose rates for the two radiation fields are
  given by

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Ionization Chamber Dosimeter
Personal Pen Dosimeter ???????
Diagnostic IC ??????
Therapeutic IC ??????
Survey meter ?????
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Film Dosimeters
OD
Dose (log)
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Thermoluminescent Dosimeters
Glow curve (????)
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Energy Dependence
Film
TLD
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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

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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

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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

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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

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NEUTRON MEASUREMENTS

• Neutron Dosimetry
• Commercially available neutron dose-equivalent
  meters

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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.

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• ?????? (Mutual Recognition Arrangement, MRA)

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• ??????Traceability of Measurement

??????(BIPM) International Bureau of Weights
Measures
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?? NIM
?? NPL
?? PTB
?? NIST
?? ARPANSA
?? ETL
???? ???????? ??? NRSL
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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

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APPLICATIONS OF STATISTICAL MODELS
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Application B Estimation of the Precision of a
Single Measurement
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COUNTING STATISTICS

• The Binomial Distribution
• n is the number of trials
• for which each trial has a success probability p,
  then
• the predicted probability of counting exactly x
  successes

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COUNTING STATISTICS

• The Binomial Distribution
• Exp. dice throw
• 3 ones in 3 consecutive throws. n3, p1/6, x3
• P(3)3!/3!(1/6)31/216
• 2 ones in 3 consecutive throws. n3, p1/6, x2
• P(2)3!/(1!2!)(1/6)2(5/6)5/72
• 1 ones in 3 consecutive throws. n3, p1/6, x1
• P(1)3!/2!(1/6)(5/6)225/72

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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

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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

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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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COUNTING STATISTICS

• The Poisson distributions
• 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
• Under these conditions, p ltlt 1, the binomial
  distribution approaches the Poisson distribution

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COUNTING STATISTICS

• The Poisson distributions
• standard deviation
• coefficient of variation CV
• Variance s 2

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COUNTING STATISTICS

• The Poisson distributions
• sum or difference

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COUNTING STATISTICS

• The Poisson distributions
• Exp. A) 10000 counts in 10-min, s 100 per 10
  min
• 1000 ? 10 cpm, CV
  (10/1000)1001
• Exp. B) 1000 counts in 1-min, s 32 per 1-min
• 1000 ? 32 cpm, CV
  (32/1000)1003.2
• When two quantities, each of which has its own
  variance

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COUNTING STATISTICS

• The Poisson distributions
• Example 9.6
• 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?

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COUNTING STATISTICS

• The Poisson distributions
• product or a quotient

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COUNTING STATISTICS

• The Poisson distributions
• Example 9.7

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COUNTING STATISTICS

• The Poisson distributions
• Example 9.9

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PROBLEMS

• 9.1, 9.2, 9.4, 9.8, 9.18, 9.19, 9.20, 9.23, 9.24,
  9.27, 9.33