EURAMET Project Form “Report”
Status:
1.
progress report
final report
Ref. No.:1175
2. Subject Field:Ionizing Radiation
3.
Type of collaboration:
Comparison of measurement standards
3A.
In the case of a comparison:
Registered as Key comparison (KC) or Supplementary Comparison (SC) in the KCDB:
no
yes
If yes: No. of KC/SC:EURAMET.RI(I)-S8
4.
Participating Partners:
4A
EURAMET members or associates (Institute’s standard acronym with country code in
brackets)
IRCL/GAEC-EIM (GR), LNE-LNHB (FR)
4B
Institutes not being EURAMET members or associates (Institute’s full name and name
of the country)
5.
Title:
Bilateral comparison (IRCL/GAEC-EIM and LNE-LNHB) of calibration of brachytherapy well type
ionization chamber in terms of iridium 192 reference air kerma rate
6.
Progress:
Scope
A HDR brachytherapy well type ionization chamber of type Standard Imaging HDR 1000+ with a
Standard Imaging CDX 2000B electrometer will be calibrated at the IRCL/GAEC-EIM and at the
LNE-LNHB in terms of HDR iridium 192 reference air kerma rate. The calibration coefficients
with their associate uncertainty will be compared. The degree of equivalence (DoE) of the
IRCL/GAEC-EIM will be deduced.
Dosimetry equipment being calibrated
The well type chamber system being calibrated consisted of:
Well type chamber : Standard imaging HDR1000 Plus, REF 90008 s/n A080951
Electrometer : Standard imaging CDX-2000B, REF 90001 s/n J081235
Insert Source Holder : HDR Iridium Source Holder
A. CALIBRATION AT THE IRCL/GAEC-EIM
A.1 Calibration Method description
The calibration was performed at a clinical remote after-loading iridium 192 brachytherapy
system operated in a hospital.
The substitution method was followed.
The reference air kerma rate (RAKR), of the iridium 192 source was determined using the
reference transfer dosimetry equipment of the IRCL/GAEC-EIM.
The well type ionization chamber being calibrated was exposed to the same source. The
calibration coefficient NKR was obtained by comparing the ḰR and the well type ionization
chamber reading. Appropriate correction factors for the influence quantities were considered, as
presented below.
Stability checks with a Cesium 137 source were performed before and after the transport of the
instrument to the hospital.
A.2 Reference - transfer dosimetry equipment
The Reference - transfer well type chamber system consisted of:
Well type chamber: Standard imaging HDR1000 Plus, s/n A011002
Electrometer: Standard imaging CDX-2000B s/n J011085
Insert Source Holder: HDR Iridium Source Holder
Traceability: LNE-LNHB (FR)
Calibration certificate: 101 482 / 08-04-2008
A.3 Beam used during calibration
Source: Iridium 192 microselectron V2, REF 105.002 (DRN 07736)
s/n NLF 01 D36C6723, LOT 85077/05
Brachytherapy system: NUCLETRON HDR microSelectron remote after loading
iridium 192 brachytherapy system
Operated at: “ALEXANDRA” hospital, brachyterapy section, Athens.
Calibration performed by: Costas J. Hourdakis, Scient. Resp. of IRCL/GAEC-EIM
HDR system operator (hospital staff): Alexandra Roussi, Medical Physicist
Calibration report completed by: Costas J. Hourdakis
A.4 Determination of the Reference Air Kerma Rate (RAKR)
The point of maximum response of the reference – transfer well-type chamber was determined.
This point corresponds to the distance between the source (center) and the end of the
applicator - nylon cable which was fixed in the source holder.
Determination of the RAKR, KR
Date : 15 February 2011 - Time : 12:10
Polarizing voltage: 302 V positive to collecting electrode (negative charge collected)
Temperature: 22.0 oC Pressure : 992.0 hPa
Source position: 1243 position corresponding to the point of maximum well-type chamber
response.
The RAKR is obtained from : RAKR=Qav∙kPT∙NKR∙ks∙kpol∙kdec
where
Qav : the mean value of the accumulated charge (nC) over 30 sec (10 successive
measurements). Qav = 2398.28 nC (over 30 sec), i.e. 4796.60 nC for 1 min integration
kPT : the air density correction, kPT = 1.0284
NKR the calibration coefficient of the whole system (well-type chamber and
electrometer) NKR = 7.800E-03 mGy h-1 (1m) per nC for 1 min integration
ks : the ion recombination correction factor as determined (measured) by the twovoltage method, ks = 1.0008
kpol : the polarity correction factor, kpol = 0.9996 as measured.
kdec : the 192Ir decay correction factor due to the different time (tint) of measurements
with the reference-transfer chamber and the chamber being calibrated. The tint = 1 h 10 min,
t1/2 = 1771.8 h (for 192Ir) resulting in kdec = 0.99998
Leakage current was negligible (0.00 nC/30 sec)
The corresponding RAKR on 15-02-2011 at 13:20 was ( = 38.4911 mGy h-1 (1m) ),
RAKR = 38.5 ± 0.7 mGy h-1 (1m), (k=2)
Uncertainties for the RAKR
Influence quantity
Type A (%)
NKR
Dosimetry system stability
0.06
Electrometer accuracy
Scale reading / resolution
Difference in 192Ir spectra (LNHB & HDR)
Source position at the same point (as by LNHB & IRCL)
kPT correction factor
0.17
Temperature difference at two places
Humidity
Electrometer Built-In timer
NR
Leakage current
Recombination correction factor
Polarity correction factor
Decay correction factor
Stability of charge, Q measurements
0.05
Combined standard uncertainties (Type A & B)
Combined standard uncertainty
0.85 %
Expanded uncertainty (k=2)
1.69 %
0.18
Type B (%)
0.65
0.00
0.00
0.29
0.03
0.08
0.03
0.00
NR
0.00
0.29
0.29
0.06
0.83
A.5 Calibration of the well-type chamber
The point of maximum response of the well-type chamber was determined.
Readings of the well-type system
Date: 15 February 2011 - Time: 13:20
Polarizing voltage: 302 V positive to collecting electrode (negative charge collected)
Temperature: 22.0 °C Pressure: 992.0 hPa
Source position: 1243 position corresponding to the point of maximum well-type chamber
response.
The corrected for air density reading (Qav,cor) of the well-type system was
Q(av,cor)=Qav∙kPT∙ks
where
Qav : the mean value of the accumulated charge (nC) over 30 sec of 10 successive
measurements. Qav = 2430.84 nC (over 30 sec), i.e. 4861.68 nC for 1 min integration
kPT : the air density correction, kPT = 1.0284
ks : the ion recombination correction factor as determined (measured) by the twovoltage method, ks = 1.0005
Leakage current was negligible (0.00 nC/30 sec)
The correction factor for polarity was measured as kpol = 1.0004, but it is not included
in the Qav,cor. This factor may be applied by the user if opposite bias voltage (negative to
collecting electrode) is used.
The calibration coefficient was deduced by: NKR=RAKR/Q(av,cor)
NKR = 7.69482 • 10-3 mGy h-1 (1m) per (nC collected over 1 min)
Uncertainties for the Calibration coefficient, NKR
Influence quantity
Reference air kerma rate
Scale reading / resolution
Difference in 192Ir spectra
Source position at the point of max response
kPT correction factor
Temperature difference at two places
Humidity
Electrometer Built-In timer
Leakage current
Recombination correction factor
Stability of charge, Q measurement
Combined standard uncertainties (Type A & B)
Combined standard uncertainty
0.92 %
Expanded uncertainty (k=2)
1.83 %
Type A (%)
0.17
NR
0.17
Type B (%)
0.85
0.00
0.00
0.03
0.08
0.03
0.00
NR
0.00
0.29
0.03
0.90
Final calibration result
Well type chamber : Standard imaging HDR1000 Plus, REF 90008, s/n A080951
Electrometer : Standard imaging CDX-2000B, REF 90001, s/n J081235
Insert Source Holder : HDR Iridium Source Holder
Polarizing voltage : 302 V positive to collecting electrode
Source : Iridium 192
NKR = (7.69 ± 0.14) • 10-3 mGy • h-1 (1m) • nC-1 for 1 min integration
•
The above calibration coefficient refers to 20oC (293.15oK) temperature, 1013.25 hPa
pressure and 50 % relative humidity.
•
The uncertainty refers to the expanded uncertainty with coverage factor of 2 (k=2).
•
The calibration coefficient applies to the point of maximum response of the well-type
chamber. The user must always determine the point of maximum response.
•
A correction factor ks=1.0005 was applied to the calibration coefficient determination for
the influence of the ion recombination effect. The user should measure and apply the ks factor
for its own source / system conditions.
•
A polarity factor kpol = 1.0004 may be applied by the user if opposite bias voltage
(negative to collecting electrode) is used. However, it is advised the user to determine the kpol
at its own measurement conditions
A.8 Stability of the HDR well type chamber being calibrated
The HDR dosimetry system (well type chamber with its electrometer) being calibrated in this
project, has been checked for its stability before and after its transport to / from LNE-LNHB.
A cesium 137 (AEA Technology, CDCSJ5, VZ-1271/4) brachytherapy source was used.
The % difference of the charge measurement with the cesium 137 check source before and
after the calibrations at the LNE-LNHB was 0.21 %. The charge measurements have been
corrected for all influence quantities (air density, source decay, etc).
Furthermore, this check source is being used periodically for the evaluation of the long term
stability of the dosimetry system. According to the quality control checks, the standard
uncertainty of the measurements from November 2008 until April 2011 was 0.06 %.
B. CALIBRATION AT THE LNE-LNHB
B.1 Calibration Method description
The RAKR was determined using an indirect method for the determination of the iridium 192
source reference air kerma rate. A cavity ionization chamber is used, the calibration coefficient
of which for iridium 192 spectrum is determined indirectly by interpolation from X-rays (250 kV)
and cesium 137 calibrations.
B.2 Reference dosimetry equipment
The Reference cavity ionization chamber: NE2571 N°3169
Cable: TNC type with a 10 m length
Polarization voltage: + 300 V (on external electrode)
Traceability of the calibration of the NE2571 cavity ionization chamber using the Cesium 137
beam: calibration certificate n°101580 delivered by the LNE-LNHB (FR).
Traceability of the calibration of the NE2571 cavity ionization chamber using the X-rays (250
kV) beam: calibration certificate n°101578 delivered by the LNE-LNHB (FR).
The device used to measure the current is associated to atmospheric probes and to a
micrometric displacement set up. The all instrumentation is controlled by a computer equipped
with dedicated software.
Measurement current set:
Electrometer: Keithley Model 642
Calibrated capacity of 1000pF/air
High voltage supplying: Stanford Research Systems PS310
Function generator: Stanford Research Systems DS345
Numerical voltmeter: Keithley Model 2001
Numerical voltmeter: Keithley Model 2700 + Keithley Model 7706 card
Reference clock: Tekelec Epsilon Clock NTP
Atmospheric probes set:
Temperature probes: probe Pt100 Convergie 2601B and thermocouple probe Lab Facility Type K
Atmospheric pressure probe: Druck RPT301
Thermo-humidity probe: Gefran Coreci Transmicor T241
Micrometric motorized displacement set: Newport MM4006, Newport M-ILS250 PP and Newport
RV160 PP.
B.3 Beam calibrated
RADIOACTIVE SOURCE: Source of Iridium192 provided by Mallinckrodt Medical B.V., Type:
Microselectron V2 ; Reference : 105.002, n°NLF01 D36C6313
HDR BRACHYTHERAPY SOURCE AFTERLOADER : Nucletron MicroSelectron HDR V2 n°31359
SOURCE HOLDER SET UP: rigid homemade catheter made of cross-linked polystyrene (Length:
25 cm, Internal diameter: 2 mm)
B.4 Determination of the Reference Air Kerma Rate (RAKR)
The reference air kerma rate of the LNE-LNHB, at the reference date that is 25/03/2011 11:47
(UTC), is: 20.47 mGy.h-1 (1m) ± 1.2 % (k=2).
Uncertainty budget (k=1) for the reference air kerma rate
Quantity
Type A (%)
Temperature correction
Pressure correction
Humidity correction
Decay (during measurements)
Position
Calibration coefficient of the thimble
Attenuation correction
Diffusion correction
Axial field non uniformity response of the thimble chamber
Current measurement
1.10-3
Type B (%)
0.05
0.04
0.01
negligible
0.11
0.34
0.13
<0.07
0.2
0.16
Combined standard uncertainty ≤ 0.6 %
B.5 Calibration of the well-type chamber
Calibration method
•
The point of measurement mentioned in this certificate is measured between the sourcecenter and the bottom of the chamber insert; it is the point of maximum response of the welltype ionization chamber: (I-IPM)/IPM=f(D)
With: I: current delivered by the ionization chamber; IPM: current delivered by the ionization
chamber at the point of measurement; D: distance from the bottom of the chamber insert.
•
The calibration consists in irradiating the well-type chamber at a known value of
Reference Air-Kerma-Rate. The source is projected at the point of measurement. The signal
value is recorded with and without the radioactive source (DKCI5 Method).
The calibration coefficient NKR, in terms of Reference Air-Kerma-Rate, is given by the equation:
NKR=RAKRx(t/(LB-LMP))x(1/kT)x(1/kp)x(1/kdec)x(1/krec)
Where:
- NKR is the calibration coefficient per “unit of reading” for 1 minute integration. It is expressed
in mGy.h-1(1m)
- RAKR is the value of the Reference Air-Kerma-Rate obtained by source calibration. It is
expressed in mGy.h-1 (1m).
- t is the integration time (min).
- LB is the raw signal delivered by the electrometer after integration time t.
- LMP is the raw signal delivered by the electrometer after integration time t (without any
radioactive source).
- kT is the atmospheric air temperature correction (T) to the reference condition (T0) ,
kT=T/T0 where T is expressed in K.
- kp is the atmospheric air pressure correction (p) to the reference condition p0, kp=p0/p
where p is expressed in hPa.
- kdec is the radioactive decay correction between the source calibration date and the chamber
calibration date (T1/2 = 73.827 j ) .
- krec is the recombination correction factor.
•
Due to the high ionization current and the large collection volume, the volume
recombination might be significant. The recombination correction factor krec is applied for the
determination of the calibration coefficient.
It is given for information, and it is estimated by : krec=1/[(4/3-Q1/(3xQ2)]
Where:
Q1 and Q2 represent the charge measured for voltages V1 and V2 where V1=2xV2
Calibration coefficient
Reference air density condition: T0 = 293.15 K, p0 = 1013.25 hPa, HR0 = 50 %
NKR = 7.648 × 10-3 mGy.h-1(1m). nC-1 for 1min integration
uncertainty U(NKR)/NKR = 1.3% (k=2)
The calibration coefficient is given for the reference atmospheric conditions. The user must
apply the proper air density correction for its own measurement conditions.
The usual humidity correction is not applied in this certificate. The user must not apply it.
The volume recombination correction is obtained by measurements at two different bias
voltages: nominal voltage and half-value of nominal voltage. For this calibration, the obtained
correction factor is krec = 1.0005 ± 0.0003. The user must estimate the correction for its own
measurement conditions.
The bias polarity correction is not applied in this certificate. The calibration coefficient is given
for the required polarity. If the opposite voltage is used, the calibration coefficient must be
multiplied by 1.0003.
B.6 Correction factors
The charge and associated current values are normalised for temperature, pressure and decay
(T1/2(192Ir) = 73.827 (13) days, data taken from the LNHB radionuclide table).
The reference date for all measurements is 27/03/2011 11:47 (UTC).
Range of environmental conditions during measurements
Temperature: 19.3 - 19.4 °C
Pressure: 997.8 – 1001.04 hPa
Relative humidity: 47 - 49 %
The measured background
The maximum measured background is: 80 fA.
The instrument range is HIGH (as for the measurements with the source).
Ion recombination
The recombination correction is: 1.0005 ± 0.0003.
B.7 The well type chamber calibration factor
The calibration coefficient NKR, is: NKR= 7.648 × 10-3 mGy.h-1(1m).nC-1 (1 min)
NKR= 4.589 × 108 mGy.h-1(1m).A-1.
Uncertainty budget (k=1) of the calibration factor of the well type chamber
Quantity
Type A (%)
Type B (%)
Reference air kerma rate
0.6
Temperature correction
0.05
Pressure correction
0.04
Humidity correction
0.1
Decay correction (from the date of the source calibration to the date of well type chamber
calibration and from the measurement time)
0.07
Reading resolution on electrometer
0.03
Standard deviation on the current measurement
0.2
Recombination correction
0.03
Combined standard uncertainty:
0.65 %
The relative uncertainty on NKR is thus 1.3 % (at k=2).
C. Analysis of the comparison
The result of the comparison is expressed in the form
R(IRCL)=(NKR_IRCL)/(NKR_LNHB )
The R(IRCL) value can be used to validate the calibration procedure used by the IRCL/GEACEIM for the calibration of the brachytherapy well type chambers.
The associate standard uncertainty, uR of the RIRCL is deduced from
uR x uR=uIRCL x uIRCL + uLNHB x uLNHB + ustab x ustab
where uIRCL and uLNHB are the combined standard uncertainties of the calibration coefficients
derived by the IRCL/GAEC-EIM and LNE-LNHB respectively and ustab is the standard
uncertainty for the stability of the well type chamber being calibrated.
The Degree Of Equivalence D(IRCL) of the IRCL/GAEC-EIM is D(IRCL) = R(IRCL) – 1
and its associated standard uncertainty uD = uR and the expanded uncertainty UD = 2 uD
The analysis of the results showed that:
1. The measurements with the IRCL/GAEC-EIM reference – transfer well type chamber chamber
were performed at positive polarity (+300 V). The calibration certificate (No 101 482 / 08-042008 / LNE-LNHB) stated that “High voltage: 300 V” and “Collected charge: Negative”. The
IRCL/GAEC-EIM interpreted the calibration certificate as the “high voltage” was negative and as
a consequence, a polarity correction factor kpol (0.9996 instead of 1.000) was applied. This led
to underestimation of the RAKR and the calibration coefficient by 0.04%. This “error” is added
to the comparison result, RIRCL and to the Degree Of Equivalence, DIRCL of the
IRCL/GAEC/EIM
2. According to the formalism presented as above, R(IRCL) = 1.0061 and D(IRCL) = 0.61 %
Considering the polarity correction factor “error”, these values becomes R(IRCL) = 1.0065 and
D(IRCL) = 0.65 %.
Therefore, the final reporting values are: R(IRCL) = 1.007 and D(IRCL) = 0.65 %
3. The associate values for the standard uncertainties are uIRCL = 0.92% and uLNHB=0.65 %.
For the ustab, the percentage difference of the measurement with the check source before and
after the well type chamber transport to LNHB is used, being 0.21 % (section A.8). Assuming a
rectangular distribution, ustab = 0.12 %.
Therefore, the standard uncertainty of the R(IRCL) and D(IRCL) is uR = 1.2 % and the
expanded uncertainty UR = UD = 2.4 %
7.
Coordinator´s name:Costas J. HOURDAKIS
Address:
Greek Atomic Energy Commission, Ionizing radiation Calibration
Laboratory, P.O. Box 60092, Postal code 15310, Agia Paraskevi, Athens, Attiki, Greece
8.
Telephone: +30 210 6506765
E-mail:
khour@gaec.gr
Completion Date:
15/04/2011
Notes for completion of the form overleaf
Fax:
+30 210 6506748
9. Date
08/06/2011
NOTES FOR THE COMPLETION OF THE FORM
(numbers refer to boxes overleaf)
IMPORTANT:
Forms are to be send to the EURAMET Secretariat (secretariat@euramet.org) as word or pdf file by
TC Chair or coordinator of the project with copy to TC Chair.
1
Ref. No. The project reference number which is assigned by the EURAMET Secretariat and on
which progress is being reported.
2
Subject Field The field specified on the EURAMET Project Form “Proposal”
3
Type of collaboration The field specified on the EURAMET Project Form “Proposal”
4A
EURAMET members or associates Any institutes which have participated in the
collaboration should be indicated in alphabetical order using their standard acronyms and
country codes.
4B
Institutes not being EURAMET members or associates should be indicated with
full name in alphabetical order. See country codes in the last section of the directory. Every
effort should be made to establish potential partners prior to completing a Proposal Form.
5
Title The title given in the EURAMET Project Form “Proposal”.
6
Progress A brief description of the progress should be entered in the space provided.
Comments on the advantages of undertaking the work collaboratively through EURAMET
would be useful. Completion of this Report is not deemed as publication of the work.
Collaborators are encouraged to publish their work through normal channels, mentioning it
was undertaken as a EURAMET collaboration.
7
Coordinator The Coordinator is the person who is appointed as the contact point for the
project detailed overleaf. The name, full postal address, telephone and fax numbers and
e-mail address of the coordinator should be given.
8
Completion Date If the progress of a project is being reported on this form then an
estimate of the completion date should be made. If the project has now been completed then
the actual date of completion should be given. For permanent agreements (e.g. development
of primary standards) "ON-GOING" should be entered.
9
Date of transmission to EURAMET Secretariat.