Radiation Detectors Why do we need radiationdepts.washington.edu › imreslab › from old SITE › pet...Index of Refraction 1.85 2.15 1.82 1.85 Hygroscopy yes no no no Rugged no

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Radiation Detectors, Fall 2007 (Miyaoka)

Radiation Detectors


Why do we need radiationdetectors?

•Personal safety•Cannot see radiation•Survey work areas•Measure exposure•Measure patient doses•Diagnostic counting/imaging


What are we trying todetect?

•X-rays•High energy photons•Beta particles•Alpha particles


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

•Current or integrating mode

•Pulse or counting mode


Current mode

Dose calibrator

•Measures average rate ofphoton flux

•Avoids dead-time losses•Can be used in very intenseradiation fields

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Pulse mode

Dose calibrator

•Detects individual photons•Required for individual energyinformation

•Required for most imagingapplications

•Can have significant dead-timelosses


System Dead-time Models

From: The Essential Physics of Medical Imaging (Bushberg, et al)

paralyzable

non-paralyzable


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)


Types of Radiation Detectors

Dose calibrator

•Gas-filled detectors•Solid-state (semiconductor)detectors

•Organic liquid scintillators•Film•Inorganic scintillators


Gas-filled Detectors

Dose calibrator

Ionizing event in airrequires about 34 eV

From: Physics in Nuclear Medicine (Cherry, Sorenson and Phelps)

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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


Geiger-Muller Counter

Dose calibratorFrom: Physics in Nuclear Medicine (Sorenson and Phelps)

Quenching gas is used to prevent pulsating discharges.


Organic Liquid Scintillators

Dose calibratorLiquid scintillator cocktail


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)


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

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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”


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


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


Photomultiplier Tube

From: Physics in Nuclear Medicine (Cherry, Sorenson and Phelps)

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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)


Energy Resolution

From: Physics in Nuclear Medicine (Sorenson and Phelps) Radiation Detectors, Fall 2007 (Miyaoka)

Interactions of Photons with aSpectrometer



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)


Sample Spectrum (In-111)

source detector

From: Physics in Nuclear Medicine (Sorenson and Phelps)

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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)


Pulse mode or current mode?

Dose calibratorATOMLAB 200 Dose Calibrator Radiation Detectors, Fall 2007 (Miyaoka)

Pulse mode or current mode?

Dose calibrator

Geiger-Mullercounter


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


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?

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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


Questions

Gas multiplication occurs in:

a. Geiger-Mueller countersb. Scintillation detectorsc. Semiconductor detectorsd. Ionization chamberse. Dose calibrators



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.


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.



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

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Documents

Radiation Detectors Why do we need radiationdepts.washington.edu › imreslab › from old SITE › pet...Index of Refraction 1.85 2.15 1.82 1.85 Hygroscopy yes no no no Rugged no