NPL Secondary standard ion chamber
Direct calibration with NPL primary standards: - Photon emitting radionuclides
- High energy beta emitting isotopes
Excellent stability over several decades (stdev: 0.1%)
- Accurate weighing
- Dilution
- Same reference time
NPL Calibration factors (pA/BMq):
Defined for specific: - Radionuclide (Individually derived from NPL primary standards)
- Dedicated holder
- Glass containers (vials, ampoules)
- Volume of liquid
Direct correlation between: Sample Activity (MBq) Current output (pA)
NPL Secondary standard ion chamber
Volume correction Factors: Radionuclide and Container dependent
- Minimal volume (mass) of active solution + Activity Assay
- Gradual top up with inactive carrier + Activity assay at each stage
Calibrator Response vs. Sample Volume: - graph (normalise to nominal volume)
- estimate % difference
Volume correction: I0 / Im = a2(m- m0)2 + a1(m- m0) + 1
where: I0 = current expected at the nominal mass of “m0”
Im = measured current at an individual mass “m”
NPL Secondary standard ion chamber
11.5
11.55
11.6
11.65
11.7
11.75
11.8
11.85
11.9
1.4 2.4 3.4 4.4 5.4 6.4 7.4 8.4
I m (
pA
)
Mass (g)
I123 10R Schott vial
y = -0.000299x2 + 0.004444x + 1.000026 R² = 0.998322
0.984
0.989
0.994
0.999
1.004
1.009
1.014
1.019
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Io/Im
(m-4)
I123 10R Schott vial
Fidelis Secondary standard radionuclide calibrator:
(Vinten, Isocal IV, NPL-CRC)
NPL-designed ion chamber: - same as the NPL SS ion chamber
- tested by NPL
Electrometer and user interface unit
Fidelis Calibration factors and volume correction factors:
- Transferable from the NPL SS ion chamber
- Calibrator independent
- Direct traceability to NPL standards
- Continuously updated: www.npl.co.uk/fidelis
Fidelis Calibration Factors
Calibrated directly using NIST standards of 60Co and 57Co
Expressed as: Calibration setting number (Dial factor)
Defined for specific: - Radionuclide: Relative to 60Co dial factor
- Container: “glass ampoule with 0.6mm wall thickness”
good approximation to syringes - corrections given
- Volume of liquid: 5 ml
Linear relation: Sample Activity (MBq) Dial factor <60Co Dial Factor <ICh response
Capintec Dial Setting numbers
123I UK comparison:
Container type/size:
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
0% 30% 60% 90% 120%
Sample volume as % of total container volume
Ca
pin
tec
ac
tiv
ity/N
PL
ac
tiv
ity
5 ml syringe (A)
1 ml syringe
5 ml syringe (B)
10 ml syringe
2 ml BS ampoule
5 ml BS ampoule
P6 vial
5 ml NBS ampoule0
5
1015
20
25
3035
40
45
5055
60
65
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6
Reported value / NPL value
Number of
resultsReported P6 vial value/NPL value
Reported syringe value/NPL value
Why recalibrate?
10R Schott vial
(1.0 mm)
P6 Vial
( 1.2mm)
Difference
(0.2mm)
123I 1.721 pA/MBq 1.685 pA/MBq 2 %
90Y 0.0734 pA/MBq 0.0682 pA/MBq 7 %
99mTc 1.240 pA/MBq 1.227 pA/MBq 1 %
Confirm the accuracy of the calibration factors
Recalibrate/derive new factors for the preferred measurement format:
- radionuclide
- container type: vials, syringes
- sample volume
- shielding
- attenuating holders (99Mo breakthrough kit, 123I and 90Y Copper insert)
Why recalibrate?
Methods of calibration:
Standardised source
NPL calibration of hospital-supplied sources (comparison exercise)
Transfer of calibration factors/dial settings to other containers
Theoretically derived factors from photon energy response curves
(needs validated by other means)
- Standard MBq
Assay standard source: - Measured MBq
Fidelis: Increase/decrease calibration factor by % difference between
Standard MBq and Measured MBq => NEW Calibration Factor (pA/MBq)
Capintec: Gradually increase/decrease Dial setting until
Measured MBq = Standard MBq => NEW Dial Factor
Issues?
Limited availability – (Half life)
High cost
Calibration with standard source
Prior to calibration: Measured MBq
From Calibration: NPL MBq % difference: Measured MBq and NPL MBq
Post calibration:
Same matrix source Activity assay: Measured MBq
Fidelis: Increase/decrease Calibration factor by % difference from NPL calibration
=> NEW Calibration Factor (pA/MBq)
Capintec: Adjust Dial factor until Measured MBq is increased/decreased by
% difference from NPL calibration => NEW Dial Factor
Or: Prior to calibration: Measured Activities over a range of Dial factors
Post calibration: Select Dial factor for which Measured MBq = NPL MBq
NPL calibration of hospital-supplied sources
Calibration transfer to other containers
EXTENSION OF CALIBRATION FACTORS
TO OTHER CONTAINERS
Stock Solution
Ionisation Ionisation
Chamber Chamber precalibrated uncalibrated
geometry geometry
MBq/g pA/g
Calibration Figure
pA/MBq (well-defined geometry)
Accurate
weighing
Alternative method without weighing
Calibration transfer to other containers
Vial active
solution full: (known calibration factor)
Measure: Vial MBq
Transfer
active
solution
Vial residue
active solution: (refill with carrier)
Measure: Vial res MBq
Syringe activity = Vial full activity – Vial residue activity
Advantages:
•No weighing involved
•Not dependent of the type of syringe/needle or volume of solution in the syringe
Limitations:
- The accuracy of the “vial residue activity” measurement (beta emitters)
Syringe
active solution
Why?
- Correct Activity Assay prior to administration
- Improved diagnostic and treatment
- Waste disposal (123I)
Where from?
- Impurity check – gamma spectrometry
- Information available from the supplier
Impurities correction
How?
- Relative decay rate of impurities to that of the main radionuclide and/or
- Relative response of impurities to that of the main radionuclide
NPL ion chamber
Half life
(days)
Cal. Factor
(pA/MBq)
123I 0.55098 1.685
121Te 19.16 5.932
125I 59.388 0.3706
NPL ion chamber
Half life
(days)
Cal. Factor
(pA/MBq)
99mTc 0.25028 1.227
99Mo 2.7479 2.700
Impurities correction
89Sr 85Sr Ro/Ri % Impurity Corr
Half-lives: 50.57 days 64.850 days
NPL Chamber (P6): 0.0279 pA/MBq 5.258 pA/MBq 186 18 %
Capintec: 40.0 5 %
Xi (at measurement time) 0.12 %
89Sr correction
for 85Sr impurity
GPG93
89Sr Activity = Corr. factor A * Indicated Activity
“NPL Report DQL-RN 012
Comparison of Strontium-89
Solution Sources in UK Hospitals, 2003”
Available from:
www.npl.co.uk/rcuf see “publications”
Impurities correction
99Mo breakthrough kit
99Mo kit – dimensions
Lead walls thickness: 0.7 cm
Lead Density: 11.34 g cm-3
Height: 9.4 cm
Inner diameter: 4 cm
Calibrator
type
99mTc
Attenuation factor
99Mo
Attenuation factor
Lead wall thickness
effect
NPL
ion chamber
Measured: 4.5 GBq -100%
Theoretical: 4 GBq – 100%
Measured: 4.43
Theoretical: 4.5
± 0.05 cm:
± 6% response variation
Capintec 4.5 GBq -100% 8.04
Title of Presentation
Name of Speaker
Date
The National Measurement System is the UK’s national infrastructure of measurement
Laboratories, which deliver world-class measurement science and technology through four
National Measurement Institutes (NMIs): LGC, NPL the National Physical Laboratory, TUV NEL
The former National Engineering Laboratory, and the National Measurement Office (NMO).
The National Measurement System delivers world-class
measurement science & technology through these organisations