Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 1 of 11 Procedure08v440

Procedures for SP250 47020C (Calibration of 125I,

103Pd, or 131Cs Brachytherapy 

Seeds) and 47021C (Each Additional 125I,

103Pd, or 131Cs Seed of Same Design)

Purpose

This procedure describes the calibration of brachytherapy seeds containing the 

radionuclides 125I,

103Pd, or 131Cs in terms of air-kerma strength. The national air-kerma 

strength standard for low energy (< 50 keV) photon-emitting encapsulated sources is the 

Wide-Angle Free-Air Chamber (WAFAC).

Scope

125I, 103Pd, or 131Cs seeds submitted for calibration must have air-kerma strengths within 

the range of 0.5 Gy m2/h to 100 Gy m2/h.

Definitions

Air Kerma is the sum of the initial kinetic energies of all charged particles (e.g., 

electrons) liberated by uncharged particles (e.g., photons) in a mass of air. The SI unit of 

air kerma is the Gray (Gy), where 1 Gy = 1 J / kg.

Air-Kerma Strength is the product of the air-kerma rate, in vacuo, at a distance d and the 

square of this distance. Air-kerma strength is typically expressed in units of Gy m2/h, 

also represented by “U”. 

Brachytherapy is a type of radiation therapy in which an encapsulated radioactive source 

is placed in or near a tumor or lesion.

Wide-Angle Free-Air Chamber (WAFAC) is a cylindrical, variable-volume ionization 

chamber used to directly realize the quantity air kerma for low energy (< 50 keV) photonemitting radioactive sources1

.

Equipment

• WAFAC system hardware – includes ionization chamber with two stepper motors 

for volume and position adjustment (Vexta, Model PK266-02B), motorized filter 

wheel (includes lead plug for leakage measurements and aluminum filters of 

various thicknesses), seed-mounting post with stepper motor for rotation of source 

about its axis (Compumotor, Model PK3/110V), and a cathetometer for 

measuring the distance between the source and entrance aperture of the ionization 

chamber (e.g., Gaertner Scientific, S/N 1796A). Detailed specifications for

WAFAC hardware are found in the “NIST Irradiator Control System Operation 

and Maintenance Manual” located in room 245/H115. (A procedure for 

construction of new Mylar electrodes is kept in a red loose-leaf notebook located 

beside the WAFAC control computer in room 245/H115.) 

• High-voltage power supply (Bertan, Model 225, S/N 4207 or equivalent) to bias 

the ionization chamber.
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 2 of 11 Procedure08v440

• Electrometer (Keithley, Model 642, S/N 58461 or equivalent) and capacitor (e.g., 

General Radio Co., Model 1403-D, S/N 6265, calibrated by the NIST Electricity 

Division) to collect and measure liberated charge.

• Temperature (Hart, Model 1504, S/N 97168 or equivalent) and pressure (Setra, 

Model Datum 2000) gauges to allow correction to reference conditions (22 oC and 

760 mm Hg).

• Computer with interface cards for data acquisition and instrument control, 

LabVIEW software, and WAFAC system control program (WAFAC_20.vi).

•

241Am source (R.S.# 00-0038) for periodic constancy check on ionizationchamber response.

Health and Safety Precautions

Radiation Safety – Sources shall always be handled with tongs behind a leaded-plastic 

shield containing a tray to contain the source in case it is accidentally dropped. An 

audible survey meter must be kept within reach to ensure that the location of the source is 

known at all times. A radiation dosimetry (TLD or similar) badge must be worn when 

working in the facility. Finger dosimeters shall be worn when manipulating a source. 

Great care shall be used when handling a source, as excessive force could damage the 

encapsulation and cause leakage of radioactive material. When measurements are in 

progress, the door to the laboratory shall be locked. A sign that reads “Caution: 

Radiation Area” shall be displayed on the leaded-glass wall at the entrance to the area 

where the source is mounted. When a source is not in use, it shall be placed in its lead 

pig and stored in the locked, lead-lined safe inside the locked WAFAC laboratory.

Radiation safety training and assessment services are provided by the Gaithersburg 

Radiation Safety Division (GRSD).

Electrical/Mechanical Safety – The operator shall heed the “Danger High Voltage” 

warning sign (attached directly to the WAFAC) and not touch the WAFAC to avoid 

possible electric shock when high voltage (-1674 V DC or -450 V DC) is applied.

As the stepper motors controlling the ionization-chamber volume and position, filterwheel position, and seed-mounting post are controlled by the computer, they could move 

without warning. The operator shall therefore avoid placing hands or objects near the 

motorized components of the system while they are turned on.

Emergency Procedures - If a source is accidentally dropped and cannot be immediately 

located visually, the operator shall move away from the last known position of the source 

(but remain in the room) and use the audible survey meter to ensure that the source is not 

attached to them. If the source is somehow attached to the operator, remove the source 

with tongs and call GRSD (x5800) to notify them of the accidental exposure. If not, use 

the survey meter to locate and secure the source. If a source is accidentally damaged, 

escape of radioactive material is possible. The operator shall move away from the source 

(but remain in the room) and call GRSD (x5800) to notify them of the accident. 
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 3 of 11 Procedure08v440

Procedures

Acceptance of Sources

1. Calibrations must be scheduled prior to shipment of the sources to NIST. The 

customer must provide the activity and encapsulated radionuclide of each source

so that a NIST 364 form may be filled out and given to GRSD (electronically 

submitted). 

2. Sources must be shipped directly to GRSD for a contamination check upon 

arrival. (GRSD must have a copy of the source manufacturer’s radioactive 

materials license.) Sources showing evidence of leakage or shipping containers 

having detectable removable contamination in any manner will not be accepted 

for calibration. 

3. A Report of Calibration Number (DG) shall be obtained from the Dosimetry 

Group office (456/B106-C) and entered into the appropriate raw-data analysis 

Excel spreadsheet (Pd103WAFACraw.xls for 103Pd seeds, I125WAFACraw.xls 

for 125I seeds, or Cs131WAFACraw.xls for 131Cs seeds) prior to beginning the 

calibration of a source. Note that these spreadsheet template files should be saved 

under a different name, specific to the seeds being calibrated.

Environmental Conditions – Prior to taking any measurements, the temperature in the 

calibration laboratory (245/H115-1) is recorded. In order to proceed with the calibration, 

the temperature must be within the range (22 ± 3) oC.

Calibration Set-up

1. If the LabVIEW program WAFAC_20.vi is not already open from the previous 

calibration, load it from the path c:\WAFAC2006\WAFAC_20.vi.

2. Using tongs and a leaded-plastic shield, remove the seed to be calibrated from its 

lead pig and place it vertically on the seed-mounting post with the aid of the seedmounting jig.

3. Remove the shield, then measure the distance between the center of the seedmounting post and the front face of the WAFAC aperture using the cathetometer.

4. Enter the measurement parameters (number of measurement cycles, number and 

duration of leakage and exposure measurements, radionuclide, filter number) in 

the set-up screen of WAFAC_20.vi. Note that actual the parameters used depend 

on knowledge of approximate source strength supplied by the manufacturer and 

prior experience – use laboratory notebook for previous calibrations of similar 

sources as a guide.

5. Turn on source rotation (1 rpm) with the program GEMINI_1rpmStart_LV7.vi, 

and visually verify that the seed is actually rotating.

Calibration Sequence

1. Run the program WAFAC_20.vi. Three measurement cycles shall be completed.
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 4 of 11 Procedure08v440

2. After stopping the seed rotation with the program GEMINI_1rpmStop_LV7.vi, 

remove the seed from the post and holding it with spring-loaded tongs, rotate it 

radians about the perpendicular bisector of the seed, flipping it end-for-end.

3. Remount the seed by placing it vertically on the seed mounting post with the aid 

of the seed-mounting jig, then turn on source rotation (1 rpm) with the program 

GEMINI_1rpmStart_LV7.vi, and visually verify that the seed is actually rotating.

4. Run the program WAFAC_20.vi. Three measurement cycles shall be completed.

Analysis and Reporting of Results

1. All measured data is automatically appended to the file c:\WAFAC2006\Data 

Files\data.txt. The file data.txt should be backed up after calibration of each set of 

seeds by saving a copy to a dedicated external hard drive (or equivalent device), 

which is in turn backed up by transfer to a computer in room 456/A105-E.

2. To analyze the results of each measurement cycle, measurement data from the file 

data.txt is copied into the appropriate raw-data analysis Excel spreadsheet 

(Pd103WAFACraw.xls for 103Pd seeds, I125WAFACraw.xls for 125I seeds, or 

Cs131WAFACraw.xls for 131Cs seeds). Note that these spreadsheet template files 

should be saved under a different name, specific to the seeds being calibrated, and 

the spreadsheet for each measurement cycle should be printed out.

3. To combine the results of all six measurements on a single source, the data from 

the measurement cycle printouts (see # 2 above) is in-put by hand into the 

appropriate Excel analysis spreadsheet (Pd103WAFAC.xls for 103Pd seeds,

I125WAFAC.xls for 125I seeds, or Cs131WAFAC.xls for 131Cs seeds). Given that 

the poor condition of building 245 adversely affects the environmental control 

and electrical power, it is recommended that more than six measurement cycles be 

completed for each seed (this process will be continued in the H-wing until the 

new facility conditions dictate otherwise). If an outlier in the data is suspected, 

the raw data file is checked for evidence of anomalies in environmental 

conditions, erroneous charge readings, and incorrect electrode positions. If such 

an anomaly is discovered, the data for that particular measurement cycle is 

excluded. If no anomaly exists, then the last three measurement cycles for each 

orientation of the seed are used in the analysis. Note that these spreadsheet 

template files should be saved under a different name, specific to the seeds being 

calibrated, and the spreadsheet for each seed should be printed out. The 

spreadsheet calculates an average value of air-kerma strength for the source, 

including uncertainty. This value is entered into the official calibration report, an 

example of which is given in Appendix A. (Note that a CD containing copies of 

these data analysis spreadsheet template files, a copy of the WAFAC control 

program WAFAC_20.vi, and the 241Am quality assurance measurement 

spreadsheet files Am241WAFACraw.xls, Am241WAFAC.xls, and 

Am241WAFACsum.xls are located beside the WAFAC control computer in room 

245/ H115, with additional backup copies stored in room 456/A105-E on the 

dedicated external hard drive or equivalent device specified above in 1.) 
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 5 of 11 Procedure08v440

4. After review and approval, the official calibration report is mailed to the 

customer. The lead pigs containing the calibrated sources are packaged in a 

cardboard box with foam insert and shipped either back to the customer or to an 

Accredited Dosimetry Calibration Laboratory (as specified by the customer).

WAFAC Quality Assurance

1. To verify constancy of the WAFAC over time, its response to an 241Am source is 

periodically measured (approximately once every 6 months) using the above 

procedures. Measurement data from individual cycles in the file data.txt is copied 

into the raw-data analysis Excel spreadsheet Am241WAFACraw.xls, then the 

results from all cycles are typed by hand into the spreadsheet Am241WAFAC.xls. 

Note that these spreadsheet template files should be saved under a different name, 

specific to the date of the measurement, and the spreadsheet for each 

measurement cycle should be printed out. The overall measurement result is

entered by keyboard into an Excel spreadsheet (Am241WAFACsum.xls) and 

compared to the history of such measurements to verify that there are no changes 

in the sensitivity of the WAFAC over time. Deviations greater than 1 % from the 

average of previous measurements should be investigated by repeating the 

measurement several times, noting any unusual behavior of the measurement 

system. If after repeated measurements of the 241Am source the > 1 % deviation 

continues to exist, the electrometer, thermometer, and barometer should be recalibrated using the procedures given below.

2. To track the calibration results of a given seed design over time, the response of a 

well-ionization chamber to each WAFAC-calibrated seed is measured and a 

response coefficient equal to the quotient of the well-chamber current and the airkerma strength is calculated. Variations of up to ± 2 % in the response coefficient 

are not uncommon due to source manufacturing variability and uncertainties in 

the WAFAC and well-chamber measurements themselves. Deviations greater 

than 2 % from the average of previous measurements should be investigated. 

Calibration of Electrometer – The following procedure should be used to re-calibrate the 

electrometer in the case of out-of-tolerance 241Am source measurement results.

1. Connect a capacitor that has been calibrated by the NIST Electromagnetics 

Division (e.g., NBS B-15) between the Fluke Model 343A DC voltage calibrator 

(S/N 2195014 or equivalent) and the electrometer to be calibrated. 

2. Allow both the electrometer and voltage calibrator to warm up for at least 2 hours.

3. Based on which coulomb scales of the electrometer are used when performing 

calibrations, select a series of test voltages to be used to calibrate the electrometer, 

taking into account the capacitance value of the NIST-calibrated capacitor. (A 

minimum of 5 data points per electrometer coulomb scale should be acquired.)

4. Select a voltage on the voltage calibrator and measure the accumulated charge on 

the capacitor with the electrometer.
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 6 of 11 Procedure08v440

5. Repeat step 4 until data is acquired for all relevant coulomb scales.

6. Calculate the calibration factor for each coulomb scale by taking the average of 

all ratios of the known accumulated charge to the charge indicated by the 

electrometer.

Calibration of Thermometer - The following procedure should be used to re-calibrate the 

thermometer in the case of out-of-tolerance 241Am source measurement results.

1. Place the Hart Model 1504 thermometer probe and a thermometer that has been 

calibrated by the NIST Measurement Services Division (e.g., Taylor S/N 

3738041) in an insulated box (cardboard/Styrofoam).

2. Record the temperatures obtained from both thermometers over a period of 

several hours until a minimum of 5 data points are acquired.

3. Calculate the calibration factor for the Hart thermometer by taking the average of 

all ratios of the known temperature to the temperature indicated by the Hart 

thermometer.

Calibration of Barometer - The following procedure should be used to re-calibrate the 

barometer in the case of out-of-tolerance 241Am source measurement results.

1. Place the Setra Model Datum 2000 barometer in close proximity to a barometer 

that has been calibrated by the NIST Process Measurements Division (e.g.,

Wallace & Tiernan S/N XX11242).

2. Record the pressures obtained from both barometers over a period of several 

hours until a minimum of 5 data points are acquired.

3. Calculate the calibration factor for the Setra barometer by taking the average of 

all ratios of the known pressure to the pressure indicated by the Setra barometer.

Evaluation of Measurement Uncertainties

WAFAC measurement uncertainties are determined based on the Guidelines for 

Evaluating and Expressing the Uncertainty of NIST Measurement Results2. The Type A 

component of uncertainty is equal to the standard deviation of the mean of replicate 

measurements. The Type B components are detailed in Appendix B. The combined 

standard uncertainty of the air-kerma strength calibration is equal to the square root of the 

quadratic sum of the Type A and Type B uncertainties, with a final reported expanded 

uncertainty obtained by multiplying the combined standard uncertainty by a coverage 

factor of two (k=2), representing an interval having a level of confidence of 

approximately 95 %.

Traceability of Measurements

The SI unit of air kerma (Kair) is the Gray (Gy), which is related to the quantity exposure 

(X) by multiplicative constants, Kair = X (W/e) / (1-g), where W/e is the mean energy per 

ion pair formed by electrons completely slowing down in air, and g is the mean fraction 

of the energy of the secondary electrons that is lost to bremsstrahlung. (For the lowenergy photons considered here, g is very small and set equal to zero.) Exposure is the 
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 7 of 11 Procedure08v440

total charge of ions of one sign per unit mass of air produced when all the electrons and 

positrons liberated or created by photons in air are stopped in the air (SI units of C/kg), 

and is directly realized by free-air chamber measurements, in this case by the WAFAC1. 

More detailed information concerning traceability and uncertainty analyses is 

summarized in reference 1 and in SP250-19, available using the following hyperlinks: 

http://nvlpubs.nist.gov/nistpubs/jres/108/5/j85sel1.pdf

and https://dx.doi.org/10.6028/NBS.SP.250-19.

Records

Descriptions of all measurements performed are recorded in an official NIST laboratory 

notebook that is stored in a filing cabinet in 245/H115. Printouts of the WAFAC Excel 

spreadsheets that contain the results of each measurement are kept in a folder (stored in a 

file cabinet in room 245/H115) labeled with the source manufacturer’s name, 

radionuclide, and month/year of calibration.

References

1. Seltzer, Stephen M., Lamperti, Paul L., Loevinger, Robert, Mitch, Michael G.,

Weaver, James T., and Coursey, Bert M., New National Air-Kerma-Strength 

Standards for 125I and 103Pd Brachytherapy Seeds, J. Res. Natl. Inst. Stand. 

Technol. 108, 337-358 (2003).

2. Taylor, Barry N., and Kuyatt, Chris E., Guidelines for Evaluating and Expressing 

the Uncertainty of NIST Measurement Results, National Institute of Standards 

and Technology Technical Note 1297, 24 pages (Sep. 1994).

Filing and Retention

The RPD Quality Manager shall maintain the original and all past versions of this RPD

Procedure.
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 8 of 11 Procedure08v440

Appendix A – WAFAC Calibration Report

National Institute of Standards and Technology

REPORT OF AIR-KERMA STRENGTH

MEASUREMENT

FOR

Customer Name

Address

City, State, Country

Seed Identification: Model XXX

Arrival Date: XX Month 20XX

SP250 Service ID # 47020C, 47021C

Measurements performed by Jason Walia

Report reviewed by Ronaldo Minniti

Report approved by Michael G. Mitch, Leader

Dosimetry Group

For the Director

National Institute of Standards and Technology

by

James M. Adams, Chief

Radiation Physics Division

Physical Measurement Laboratory

Information on technical aspects of this report may be obtained from Jason Walia, 

National Institute of Standards and Technology, 100 Bureau Drive Stop 8460,

Gaithersburg, MD 20899-8460, 301-975-5592. Report format revised 11/19.

 DG: XXXXX/XX

 Order #: 682.02/O-XXXXXX-XX

 Report Date: XX Month 20XX

 Page 1 of 3
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 9 of 11 Procedure08v440

National Institute of Standards and Technology

REPORT OF AIR-KERMA STRENGTH

MEASUREMENT

FOR

Customer Name

Address

City, State, Country

Seed Identification: Model XXX

Arrival Date: XX Month 20XX

SP250 Service ID # 47020C, 47021C

Description of seed provided by customer:

Construction:

Diameter (mm):

Length (mm): 

Half-Life (d): 

Radionuclide:

Purity rating:

NIST Reference time and date: 00:00:01 EST, XX Month 20XX

Temperature range during measurements: XX K to XX K

Pressure range during measurements: XXX kPa to XXX kPa

Measurement Results

Source

ID No.

Number of 

Measurements

Air-Kerma Strength

(μGy m2/h)

at 295.15 K (22 oC) and

101.325 kPa (1 Atm)

Reproducibilitya

(%)

Expanded Combined

Relative Uncertaintyb

(%)

a Obtained from the replicate measurements as the standard deviation of the mean.

b See page 3 for note on uncertainty.

DG: XXXXX/XX

Order #: 682.02/O-XXXXXX-XX

Report Date: XX Month 20XX

Page 2 of 3
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 10 of 11 Procedure08v440

Explanation of Terms Used in the Calibration Procedures and Tables

Air-Kerma Strength: The realization of the radiation quantity air-kerma strength done 

at NIST establishes the National Standard. This can in turn be transferred to other 

measurement facilities through a suitable measuring instrument, thus enabling traceability 

to the National Standard. The air-kerma strength is the product of the air-kerma rate and 

the square of the distance to the reference point assumed in vacuum, in a direction 

perpendicular to the long axis of the cylindrical encapsulated brachytherapy source. For 

more details see Specification of Brachytherapy Source Strength, Report 21 of the 

American Association of Physicists in Medicine, Am. Inst. of Phys., MD, June 1987.

The instrument used to obtain the results given in this report, the Wide-Angle Free-Air 

Chamber (WAFAC), directly realizes the quantity air kerma.

The measured air-kerma strength is obtained from:

SK =(W/e)Inetd

2 Πiki/(ρair Veff)

where

W/e = 33.97 J/C

ρair = air density

Inet is the net current (background and leakage subtracted)

d is the source-to-aperture distance

Veff is the effective detector volume

Πiki is the product of the correction factors to be applied to the measurement

The air-kerma strength(s) given in this report can be used to determine a well-chamber

calibration coefficient for the identified source model.

Measurement Geometry: The measurements were performed with a wide-angle free-air 

chamber whose aperture, 8 cm in diameter, is located at a nominal distance of 30 cm 

from the seed axis. This arrangement thus effectively averages the air-kerma rate within a 

cone whose half-angle is approximately 8. Any contribution to the measurement from 

titanium x rays, produced in the encapsulation, has been eliminated by the use of an 

appropriate absorber. To mitigate possible geometric anomalies and source nonuniformity, each seed was rotated about its long axis during a measurement, and reversed 

end-for-end for each successive measurement.

Uncertainty: The combined standard uncertainty assigned to these results has been 

evaluated as the square root of the quadratic sum of the component standard 

uncertainties, including those evaluated by statistical means (Type A) and those evaluated 

by other means (Type B). The expanded uncertainty has been obtained by multiplying the 

combined standard uncertainty by a coverage factor of two, to represent an interval 

having a level of confidence of approximately 95 %.
Radiation Physics Division 47020C and 47021C RPD-P-08

CALIBRATION OF 125I,

103Pd, or 131Cs BRACHYTHERAPY SEEDS

Version Date Author Approval Pages Filename

4.40 6/9/21 JSW JMA 11 of 11 Procedure08v440

DG: XXXXX/XX

Order #: 682.02/O-XXXXXX-XX

Report Date: XX Month 20XX

Page 3 of 3

Appendix B – Relative Standard Uncertainties in the Determination of Air-Kerma 

Strength SK for 125I,

103Pd, and 131Cs Brachytherapy Seeds Using the Wide-Angle 

Free-Air Chamber

Estimated relative uncertainties (k = 1) are given in percent, and include the Type A 

uncertainty, si, estimated by statistical methods, and the Type B uncertainty, uj, estimated 

by other means.

 

125I 103Pd 131Cs

si(%) uj(%) si(%) uj(%) si(%) uj(%)

net current, Inet,diff sI 0.06 sI 0.06 sI 0.06

W/e - 0.15 - 0.15 - 0.15

air density, ρair - 0.03 - 0.03 - 0.03

aperture distance, d - 0.24 - 0.24 - 0.24

effective chamber volume, Veff 0.11 0.01 0.11 0.01 0.11 0.01

decay correction, k1 - 0.02a - 0.08a- 0.18a

recombination, k2 - 0.05 - 0.05 - 0.05

attenuation in filter, k3 - 0.61b - 0.51 - 0.64

air attenuation in WAFAC, k4 - 0.08b - 0.10 - 0.06

source-aperture attenuation, k5 - 0.24b - 0.31 - 0.22

inverse-square correction, k6 - 0.01 - 0.01 - 0.01

humidity, k7 - 0.07 - 0.07 - 0.07

in-chamber photon scatter, k8 - 0.07 - 0.07 - 0.07

source-holder scatter, k9 - 0.05 - 0.05 - 0.05

electron loss, k10 - 0.05 - 0.05 - 0.05

aperture penetration, k11 - 0.02 - 0.08 - 0.03

external photon scatter, k12 - 0.17 - 0.19 - 0.16

Combined standard (s

2

I + 0.7622

)

1/2 (s2

I + 0.7282

)

1/2 (s2

I + 0.797

2

)

1/2

uncertainty

a)

 Assuming time from reference date is no more than ~15 days.

b)

 Including spectral variations due to possible Ag K x rays.