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Radiation Detection

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Title: Radiation Detection


1
Radiation Detection Measurement II
  • Pulse height spectroscopy
  • Nonimaging detector applications
  • Counting statistics

2
Pulse height analyzers
  • Many radiation detectors produce electrical
    pulses whose amplitudes are proportional to the
    energies deposited in the detector by individual
    interactions
  • PHAs are electronic systems that may be used with
    these detectors to perform pulse height
    spectroscopy and energy-selective counting
  • In energy-selective counting, only interactions
    that deposit energies within a certain energy
    range are counted

3
PHAs (cont.)
  • Energy-selective counting can be used to
  • Reduce the effects of background radiation
  • Reduce the effects of scatter
  • Separate events caused by different radionuclides
    in a mixed radionuclide sample
  • Two types of PHAs single-channel analyzers
    (SCAs) and multichannel analyzers (MCAs)
  • Pulse height discrimination circuits incorporated
    in scintillation cameras and other nuclear
    medicine imaging devices to reduce effects of
    scatter

4
Single-channel analyzer systems
  • High-voltage power supply typically provides 800
    to 1,200 volts to the PMT
  • Raising voltage increases magnitude of voltage
    pulses from PMT
  • Preamp connected to PMT using very short cable
  • Amplifies voltage pulses to minimize distortion
    and attenuation of signal during transmission to
    remainder of system

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6
SCA systems (cont.)
  • Amplifier further amplifies the pulses and
    modifies their shapes gain typically adjustable
  • SCA allows user to set two voltage levels, a
    lower level and an upper level
  • If input pulse has voltage within this range,
    output from SCA is a single logic pulse (fixed
    amplitude and duration)
  • Counter counts the logic pulses from the SCA for
    a time interval set by the timer

7
Energy discrimination occurs by rejection of
pulses above or below the energy window set by
the operator
8
SCA energy modes
  • LL/UL mode one knob directly sets the lower
    level and the other sets the upper level
  • Window mode one knob (often labeled E) sets the
    midpoint of the range of acceptable pulse heights
    and the other knob (often labeled ?E or window)
    sets a range of voltages around this value.
  • Lower-level voltage is E - ?E/2 and upper-level
    voltage is E ?E/2

9
Example of a single-channel analyzer
10
Plotting a spectrum using a SCA
  • The SCA is placed in window mode, the E setting
    is set to zero, and a small window (?E) is
    selected
  • A series of counts is taken for a fixed length of
    time per count, with the E setting increased
    before each count but without changing the window
    setting
  • Each count is plotted on graph paper as a
    function of baseline (E) setting

11
Energy calibration of SCA
  • On most SCAs, each of the two knobs permits
    values from 0 to 1,000 to be selected
  • By adjusting the amplification of the pulses
    reaching the SCA either by changing the voltage
    applied to the PMT or by changing the amplifier
    gain the system can be calibrated so that these
    knob settings directly indicate keV
  • A Cs-137 source, which emits 662-keV gamma rays,
    is often used for calibration

12
Multichannel analyzer systems
  • An MCA system permits an energy spectrum to be
    automatically acquired much more quickly and
    easily than does a SCA system
  • The detector, HV power supply, preamp, and
    amplifier are the same as for SCA systems
  • The MCA consists of an analog-to-digital
    converter, a memory containing many storage
    locations called channels, control circuitry, a
    timer, and a display

13
Modern, computer-based multichannel analyzer
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15
After the analog pulses are digitized by the ADC,
they are sorted into bins (channels) by height,
forming an energy spectrum.
16
Interactions of photons with a spectrometer
  • An incident photon can deposit its full energy
    by
  • A photoelectric interaction (A)
  • One or more Compton scatters followed by a
    photoelectric interaction (B)
  • A photon will deposit only a fraction of its
    energy if it interacts by Compton scattering and
    the scattered photon escapes the detector (C)
  • Energy deposited depends on scattering angle,
    with larger angle scatters depositing larger
    energies

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18
Interactions (cont.)
  • Even if the incident photon interacts by the
    photoelectric effect, less than its total energy
    will be deposited if the inner-shell electron
    vacancy created by the interaction results in
    emission of a characteristic x-ray that escapes
    the detector (D)

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Interactions (cont.)
  • Detectors normally shielded to reduce effects of
    natural background radiation and nearby radiation
    sources
  • An x-ray or gamma-ray may interact in the shield
    of the detector and deposit energy in the
    detector
  • Compton scatter in the shield, with the scattered
    photon striking the detector (E)
  • A characteristic x-ray from the shield may
    interact with the detector (F)

21
Spectrum of Cesium-137
  • Cs-137 decays by beta particle emission to
    Ba-137m, leaving the Ba-137m nucleus in an
    excited state
  • The Ba-137m nucleus attains its ground state by
    the emission of a 662-keV gamma ray 90 of the
    time
  • In 10 of decays, a conversion electron is
    emitted instead, followed by a 32-keV K-shell
    characteristic x-ray

22
Energy spectrum
Pulse height spectrum
23
Reasons for differences in spectra
  • First, there are a number of mechanisms by which
    an x-ray or gamma-ray can deposit energy in the
    detector, several of which deposit only a
    fraction of the incident photon energy
  • Second, there are random variations in the
    processes by which the energy deposited in the
    detector is converted into an electrical signal

24
NaI(Tl) crystal/PMT
  • Random variations in
  • The fraction of deposited energy converted into
    light
  • The fraction of the light that reaches the
    photocathode of the PMT
  • The number of electrons ejected from the back of
    the photocathode per unit energy deposited by the
    light
  • Cause random variations in the size of the
    voltage pulses produced by the detector, even
    when the incident x-rays or gamma rays deposit
    exactly the same energy

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Pulse height spectrum of Cs-137
  • Photopeak corresponding to interactions in which
    the energy of an incident 662-keV photon is
    entirely absorbed in the crystal
  • Compton continuum caused by 662-keV photons that
    scatter in the crystal, with the scattered photon
    escaping the crystal
  • The Compton edge is the upper limit of the
    Compton continuum

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28
Pulse height spectrum (cont.)
  • Backscatter peak caused by 662-keV photons that
    scatter from the shielding around the detector
    into the detector
  • Barium x-ray photopeak caused by absorption of
    barium K-shell x-rays (31 to 37 keV)
  • Photopeak caused by lead K-shell x-rays (72 to 88
    keV) from the shield

29
Spectrum of Technetium-99m
  • Tc-99m is an isomer of Tc-99 that decays by
    isomeric transition to its ground state, with the
    emission of a 140.5-keV gamma ray
  • In 11 of the transitions, a conversion electron
    is emitted instead of a gamma ray

30
Decay scheme of Tc-99m and pulse height spectrum
31
Tc-99m (cont.)
  • Photopeak caused by total absorption of the
    140-keV gamma rays
  • Escape peak caused by 140-keV gamma rays that
    interact with the crystal by photoelectric effect
    but with resultant iodine K-shell x-rays (28 to
    33 keV) escaping the crystal
  • Photopeak caused by absorption of lead K-shell
    x-rays from the shield
  • Compton continuum is quite small because the
    photoelectric effect predominates in iodine at
    140 keV

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33
Energy resolution
  • Energy resolution of a spectrometer is a measure
    of its ability to differentiate between particles
    or photons of different energies
  • Determined by irradiating detector with
    monoenergetic particles or photons and measuring
    width of resulting peak in the pulse height
    spectrum
  • Statistical effects in the detection process
    cause the amplitudes of pulses from detector to
    randomly vary about the mean pulse height, giving
    the peak a Gaussian shape

34
Energy resolution (cont.)
  • Width is usually measured at half the maximal
    height of the peak called the full width at
    half-maximum (FWHM)

35
Energy resolution of a pulse height spectrometer
36
Thyroid probe
  • Used for measuring
  • Uptake of I-123 or I-131 by the thyroid gland of
    patients
  • Monitoring activities of I-131 in the thyroids of
    staff members who handle large activities of
    I-131
  • Usually consists of a 5.1-cm diameter and 5.1-cm
    thick cylindrical NaI(Tl) crystal coupled to a
    PMT and preamp
  • Shielded on sides and back with lead and equipped
    with a collimator to detect photons from a
    limited portion of the patient

37
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38
Thyroid uptake measurements
  • May be performed using one or two capsules of
    I-123 or I-131 sodium iodide
  • A neck phantom, consisting of a Lucite cylinder
    of diameter similar to the neck and containing a
    hole parallel to its axis for a radioiodine
    capsule, is required
  • Each capsule is placed in the neck phantom and
    counted
  • One capsule is swallowed by the patient
  • The other capsule is called the standard

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40
Thyroid uptake (cont.)
  • Emissions from the patients neck are counted,
    typically at 4 to 6 hours after administration,
    and again at 24 hours after administration
  • Each time the patients thyroid is counted, the
    patients distal thigh is also counted for the
    same length of time, to approximate nonthyroidal
    activity in the neck, and a background count is
    obtained
  • All counts performed with NaI(Tl) crystal same
    distance (20 to 25 cm) from phantom, neck, or
    thigh

41
Thyroid uptake (cont.)
  • Single capsule technique
  • Avoids cost of second capsule and requires fewer
    measurements
  • More susceptible to instability of equipment,
    technologist error, and dead-time effects

42
Counting statistics
  • Sources of error
  • Characterization of data
  • Probability distribution functions for binary
    processes
  • Estimating the uncertainty of a single
    measurement
  • Propagation of error

43
Sources of error
  • Three types of errors in measurements
  • Systematic error measurements differ from the
    correct values in a systematic fashion
  • Random error caused by random fluctuations in
    whatever is being measured or in the measurement
    process itself
  • Blunder

44
Random error in radiation detection
  • Processes by which radiation is emitted and
    interacts with matter are random in nature
  • Whether a particular radioactive nucleus decays
    within a specified time interval
  • The direction of an x-ray emitted by an electron
    striking the target of an x-ray tube
  • Whether a particular x-ray passes through a
    patient to reach the film cassette of an x-ray
    machine
  • Whether a gamma ray incident upon a scintillation
    camera crystal is detected
  • Counting statistics enable judgments on the
    validity of measurements subject to random error

45
Accuracy and precision
  • If a measurement is close to the correct value,
    it is said to be accurate
  • If measurements are reproducible, they are said
    to be precise
  • Precision does not imply accuracy
  • If a set of measurements differs from the correct
    value in a systematic fashion, the data are said
    to be biased

46
Measures of central tendency
  • The mean (average) of a set of measurements is
    defined as follows
  • To obtain the median of a set of measurements,
    they must first be sorted by size
  • The median is the middlemost measurement if the
    number of measurements is odd
  • The median is the average of the two middlemost
    measurements in the number of measurements is even

47
Measures of variability
  • Variance and standard deviation are measures of
    the variability (spread) of a set of measurements

48
Estimated standard deviation
  • The standard deviation can be estimated, as
    previously described, by making several
    measurements
  • If the process being measured is a binary
    process, the standard deviation can be estimated
    from a single measurement
  • The single measurement is probably close to the
    mean the standard deviation is approximately the
    square-root of the mean also approximately the
    square-root of the single measurement

49
Confidence intervals
50
Propagation of error
  • In nuclear medicine, calculations are frequently
    performed using numbers that incorporate random
    error
  • It is often necessary to estimate the uncertainty
    in the results of these calculations
  • Propagation of error equations are used to obtain
    the standard deviation of the result

51
Propagation of error equations
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