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Radiation Detectors, Fall 2007 (Miyaoka)
Radiation Detectors
Radiation Detectors, Fall 2007 (Miyaoka)
Why do we need radiationdetectors?
•Personal safety•Cannot see radiation•Survey work areas•Measure exposure•Measure patient doses•Diagnostic counting/imaging
Radiation Detectors, Fall 2007 (Miyaoka)
What are we trying todetect?
•X-rays•High energy photons•Beta particles•Alpha particles
Radiation Detectors, Fall 2007 (Miyaoka)
Radiation Detector Systems•What do you want to know
–How much?–Energy?–Position?
•Important properties–Energy resolution–Spatial resolution–Sensitivity–Count rate capability
Radiation Detectors, Fall 2007 (Miyaoka)
Radiation Detectors
•Current or integrating mode
•Pulse or counting mode
Radiation Detectors, Fall 2007 (Miyaoka)
Current mode
Dose calibrator
•Measures average rate ofphoton flux
•Avoids dead-time losses•Can be used in very intenseradiation fields
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Radiation Detectors, Fall 2007 (Miyaoka)
Pulse mode
Dose calibrator
•Detects individual photons•Required for individual energyinformation
•Required for most imagingapplications
•Can have significant dead-timelosses
Radiation Detectors, Fall 2007 (Miyaoka)
System Dead-time Models
From: The Essential Physics of Medical Imaging (Bushberg, et al)
paralyzable
non-paralyzable
Radiation Detectors, Fall 2007 (Miyaoka)
Count rate and dead-time
Dose calibratorFrom: The Essential Physics of Medical Imaging (Bushberg, et al) Radiation Detectors, Fall 2007 (Miyaoka)
Types of Radiation detectors•Counters
–Number of interactions–Pulse mode
•Spectrometers–Number and energy of interactions–Pulse mode
•Dosimeters–Net amount of energy deposited–Current mode
•Imaging Systems–PET (counting mode) / CT (current mode)–NM (counting mode)
Radiation Detectors, Fall 2007 (Miyaoka)
Types of Radiation Detectors
Dose calibrator
•Gas-filled detectors•Solid-state (semiconductor)detectors
•Organic liquid scintillators•Film•Inorganic scintillators
Radiation Detectors, Fall 2007 (Miyaoka)
Gas-filled Detectors
Dose calibrator
Ionizing event in airrequires about 34 eV
From: Physics in Nuclear Medicine (Cherry, Sorenson and Phelps)
3
Radiation Detectors, Fall 2007 (Miyaoka)
Ion Chamber Response Curve
Dose calibratorFrom: Physics in Nuclear Medicine (Cherry, Sorenson and Phelps) Radiation Detectors, Fall 2007 (Miyaoka)
Dose Calibrator
Dose calibratorFrom: Physics in Nuclear Medicine (Cherry, Sorenson and Phelps)
ATOMLAB 200 Dose Calibrator
No amplificationNo dead-time
Radiation Detectors, Fall 2007 (Miyaoka)
Geiger-Muller Counter
Dose calibratorFrom: Physics in Nuclear Medicine (Sorenson and Phelps)
Quenching gas is used to prevent pulsating discharges.
Radiation Detectors, Fall 2007 (Miyaoka)
Organic Liquid Scintillators
Dose calibratorLiquid scintillator cocktail
Radiation Detectors, Fall 2007 (Miyaoka)
Liquid Scintillator Cocktail
• Organic solvent• Primary scintillator (p-terphenyl and
PPO)• Secondary solute (wave-shifter)• Additives (e.g., solubilizers) to
reduce quenching (i.e., chemical,color, dilution)
Radiation Detectors, Fall 2007 (Miyaoka)
Semiconductor Detectors
Dose calibrator
•Same principle as gas-filleddetectors
•2000-5000 times denser than gases•Only ~3 eV required for ionization•Susceptible to thermal noise•Room temperature operation
4
Radiation Detectors, Fall 2007 (Miyaoka)
Cadmium Zinc Telluride
Dose calibratorRadiation Detectors, Fall 2007 (Miyaoka)
Dosimeter - Film Badge
Dose calibratorFrom: The Essential Physics of Medical Imaging (Bushberg, et al)
A) Film packB) Black (opaque) envelopeC) FilmD) Plastic film badgeF) Teflon filterG) Lead filterH) Copper filterI) Aluminum filterJ) “Open window”
Radiation Detectors, Fall 2007 (Miyaoka)
Pocket Dosimeter
Dose calibratorFrom: The Essential Physics of Medical Imaging (Bushberg, et al) Radiation Detectors, Fall 2007 (Miyaoka)
Inorgranic Scintillators•crystalline solids•scintillate because of characteristics ofcrystal structure•impurities often required for scintillationproperties
From: Society of Nuclear Medicine: Basic Science of Nuclear Medicine CD
Radiation Detectors, Fall 2007 (Miyaoka)
Inorganic scintillatorsNaI(Tl) BGO LSO(Ce) GSO(Ce)
Density (gm/cm3) 3.67 7.13 7.4 6.71
EffectiveAtomic Number 51 75 66 59
AttenuationCoefficient(@ 511 keV, cm-1 ) 0.34 0.955 0.833 0.674
Light Output(photons/Mev) 40K ~8K ~30K ~20K
Decay Time 230 ns 300 ns 12 ns 60 ns40 ns
Wavelength 410 nm 480 nm 420 nm 430 nm
Index of Refraction 1.85 2.15 1.82 1.85
Hygroscopy yes no no no
Rugged no yes yes no
Sensitivity
Eng and spat res
Counting speed
Photo-sensor /Cost
Radiation Detectors, Fall 2007 (Miyaoka)
Photomultiplier Tube
From: Physics in Nuclear Medicine (Cherry, Sorenson and Phelps)
5
Radiation Detectors, Fall 2007 (Miyaoka)
Sample Spectroscopy System
From: The Essential Physics of Medical Imaging (Bushberg, et al) Radiation Detectors, Fall 2007 (Miyaoka)
Multichannel Analyzer
From: Physics in Nuclear Medicine (Sorenson and Phelps)
Radiation Detectors, Fall 2007 (Miyaoka)
Energy Resolution
From: Physics in Nuclear Medicine (Sorenson and Phelps) Radiation Detectors, Fall 2007 (Miyaoka)
Interactions of Photons with aSpectrometer
From: The Essential Physics of Medical Imaging (Bushberg, et al)
Radiation Detectors, Fall 2007 (Miyaoka)
Sample Spectrum (Cs-137)
A. PhotopeakB. Compton continuumC. Compton edge
D. Backscatter peakE. Barium x-ray photopeakF. Lead x-rays
Detection efficiency(32 keV vs. 662 keV)
From: The Essential Physics of Medical Imaging (Bushberg, et al) Radiation Detectors, Fall 2007 (Miyaoka)
Sample Spectrum (In-111)
source detector
From: Physics in Nuclear Medicine (Sorenson and Phelps)
6
Radiation Detectors, Fall 2007 (Miyaoka)
Effects of Pulse Pileup
From: Physics in Nuclear Medicine (Sorenson and Phelps) Radiation Detectors, Fall 2007 (Miyaoka)
Calibrations
Energy calibrationAdjust energy windows around a known photopeakOften done with Cs-137 and Co-57
Dose calibration (dose calibrator)Measure activity of know reference samples (e.g., Cs-
137 and Co-57)Linearity measured by repeated measurements of a
decaying source (e.g., Tc-99m)
Radiation Detectors, Fall 2007 (Miyaoka)
Pulse mode or current mode?
Dose calibratorATOMLAB 200 Dose Calibrator Radiation Detectors, Fall 2007 (Miyaoka)
Pulse mode or current mode?
Dose calibrator
Geiger-Mullercounter
Radiation Detectors, Fall 2007 (Miyaoka)
QuestionThe count rate for a 1 µCi source ismeasured as 25 kcps by a well counter.Assuming no corrections are applied, themeasured count rate for a 10 µCi sourcewill be:
a. 250 kcpsb. Less than 250 kcpsc. Greater than 250 kcps
Radiation Detectors, Fall 2007 (Miyaoka)
Questions
How many peaks would you expect fora 99m-Tc sample placed outside a wellcounter? What about inside a wellcounter? Is your answer dosedependent?
How many peaks would you expect fora 68-Ge sample placed outside a wellcounter? What about inside a wellcounter?
7
Radiation Detectors, Fall 2007 (Miyaoka)
Questions
Of the following, the most efficientdetector for x-rays is:
a. Geiger counterb. NaI(Tl) detectorc. Single channel analyzerd. Ionization chambere. Pocket (self-reading) dosimeter
From: The Essential Physics of Medical Imaging (Bushberg, et al) Radiation Detectors, Fall 2007 (Miyaoka)
Questions
Gas multiplication occurs in:
a. Geiger-Mueller countersb. Scintillation detectorsc. Semiconductor detectorsd. Ionization chamberse. Dose calibrators
From: The Essential Physics of Medical Imaging (Bushberg, et al)
Radiation Detectors, Fall 2007 (Miyaoka)
Questions (True or False)
In a photomultiplier tube, thephotocathode is at a positive voltagewith respect to the first dynode.
Small changes to the voltage applied toan ionization chamber have a large effectupon the charge collected from eachinteraction with ionizing radiation.
From: The Essential Physics of Medical Imaging (Bushberg, et al) Radiation Detectors, Fall 2007 (Miyaoka)
Questions (True or False)
A 1 MeV beta particle produces a pulseof the same amplitude in a G-M detectoras a 200 keV beta particle.
From: The Essential Physics of Medical Imaging (Bushberg, et al)
Radiation Detectors, Fall 2007 (Miyaoka)
Questions
Which detector system is most appropriateand accurate for the measurement of apure beta source:
a. Ionization chamberb. Geiger Muller tubec. NaI(Tl) well scintillation counterd. Thermoluminescent dosimetere. Liquid scintillation counter
From: Raphex Radiation Detectors, Fall 2007 (Miyaoka)
Questions
A pulse height analyzer (PHA) window canbe used to:
a. Identify the energy of a radionuclideb. Reject Compton scattered photonsc. Separate a mixture of radionuclidesd. Alter the sensitivity or resolution of the
systeme. All of the above
From: Raphex
8
Radiation Detectors, Fall 2007 (Miyaoka)
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