fht 3511
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FHT 3511. Calibration of a PET-stack monitor based on coincidence technology Dr. Michael Iwatschenko November 2005. Content. the idea the requirements the concept the hardware the user interface Summary. PET Specifics. Production. Application. - PowerPoint PPT PresentationTRANSCRIPT
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FHT 3511
Calibration of a PET-stack monitorbased on coincidence technology
Dr. Michael IwatschenkoNovember 2005
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Content
the idea
the requirements
the concept
the hardware
the user interface
Summary
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PET Specifics
Half-life of PET-isotopes: 2 - 110 minutes
Multiple production facilities required!
Production Application
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Requirements: Where and Why
Where:
Measurement of the activity release in stack during production
Why:
Despite short half life time of nuclides- daily production = ongoing exposure of neighborhood
- 2 hard gammas per decay - facility in down town locations - initially no filters - release varies strongly depending on handling of process
- regulator’s request
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511 keV
ß+
ß+-decay in the gas flow
Conventional Monitoring Problems: ß+ : Fragile window, specific calibration (geometry, isotope)γ: Specific calibration (geometry); long range cross talk
γ detection in e. g. NaI(Tl)-detector
ß+ detection in proportional counters
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Isotopes
nuclide t 1/2 ß+ energy / range 511 keV other gamma
PET-isotopes:C-11 20.4 m 1.0 MeV / 3 m 200 % no
N-13 10.0 m 1.2 MeV / 4 m 200 % no
O-15 2.0 m 1.7 MeV / 6 m 200 % no
F-18 110 m 0.6 MeV / 1.3 m 194 % no
Suitable gamma check sources
Na-22 2.6 y 0.5 MeV 180 % 1274 keV (100 %)
Ge/Ga-68 271 d 1.9 MeV 178 % 1077 keV (3 %)
Conclusions:
- Different calibration factors for beta+ detection monitors
- Same calibration factor for gamma based monitors
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Confined volume – virtual shielding
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Some more detailsSome more details
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Impact of Coincidence MeasurementImpact of Coincidence Measurement
Typical Data (12 inch dia.)
efficiency background limit of detection(t = 600 s)
• gross : 60 % 5000 cps 0.6 kBq/m3• ROI: 20 % 1500 cps 1.0 kBq/m3• CCM: 2 % 10 cps 0,9 kBq/m3
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Online Subtraction of Background
PET activity as per the CCM method
R pet = R coi - R stat - R comp
R pet count rate due to coincident -annihilation quanta
R coi total coincident count rate of all detectors
R stat statistical coincidence count rate
R comp count rate due to Compton scattering of natural highenergy gamma nuclides (Tl-208 at 2.615 MeV of thenatural Th-232 decay chain) and of cosmic rays
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Arbitrary CoincidencesArbitrary Coincidences
Statistical coincidence count rate
T gate gate time of the coincidence circuit (typically 75 ns)
R w,i, R w,j gross count rates in the PET windows of the detectors j, i
n number of detectors
i, j index
Example:
1
1,,
1
n
ijwiw
n
ijgatestat RRTR
)75(1
950;6)1(2
1
12
nsTsR
sRnRTnn
R
gatestat
wwgatestat
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Double Detection of High Energy Events
False Coincidences
Double detection of - natural gamma rays > 1,5 MeV
- high energy radiation from cyclotron
Depending on: - setting of energy window - weather - operation of cyclotron
High energy (> 1 MeV) counts can be used for compensation!
Observation:The same compensation factor for Rn-washout and cyclotron can be applied!
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Details of Compensation
Dynamic compensation of Compton scattering
R comp = Factor · R comp gross - Offset cosmic
R comp gross high energy gamma (app. 1000 - 4000 keV)
gross count rate (Tl-208 at 2.615 MeV of thenatural Th-232 decay chain and cosmic rays)
Factor proportionality factor between high energy gross gamma and CCM channel count rates
Offset cosmic contribution of cosmic rays only
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Long Term Observation of Rn-Impact
High Energy
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Details of ROI-SettingsDetails of ROI-Settings
Setting of Windows
1. Gross gamma window (app. 50 - 1000 keV)
2. High energy (Compton) (app. 1000 - 4000 keV)
3. CCM window (app. 250 - 600 keV)
4. CCM window pulses multi coincidence circuit coincidence count rate Rcoi for all detectors
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Concept: Concept: the confined volumethe confined volume
Confined measurement of 511 keV gamma radiation
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Calibration FactorsCalibration Factors
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PET Stack Monitor 16" - compensated coincidence rates
0
500
1000
1500
2000
2500
3000
3500
4000
4500
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0 20 40 60 80 100
120
140
160
180
200
cm
CC
M [
cps]
0°22,5°45°Pos.A
Concept: Concept: verification of axial symmetryverification of axial symmetry
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Concept: Concept: Check source positionCheck source position
1
2
3
4
In detector array –
outside of the stack
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RCC (Recurrent Calibration Check)
812-25-2
Point Check
Source –No hot
Chemistry
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System OverviewSystem Overview
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Hardware: Hardware: Installed systemInstalled system
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Hardware: Hardware: Installed systemInstalled system
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Data Display in Operation Mode
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Advantages of Coincidence Monitoring
- calibration with point check source – no hot chemistry
- direct measurement around existing stack no by-pass required *** no thin, large mylar entrance window
- as local as a beta measurement
- full external 511 keV background rejection calibration factor independent of facility design no cross talk from neighboring ducts
- calibration factor independent of positron energy
- no sealing problems
- appealing software package
*** F-18 gets attached to by-pass walls – great underestimation of true release!