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Purpose

The purpose of this document is to describe the setup, measurement, and reporting 

procedures for absorbed-dose-to-water calibrations. 

Scope

The measurement service described in this document is listed as NIST service 

code 46110C. The procedure covers the absorbed-dose-to-water calibration of 

ionization chambers in a 60Co beam1that is traceable to the NIST water 

calorimeter2. All chambers must be waterproof or able to be made waterproof as 

measurements are made with chambers submersed in a water phantom. Appendix 

D of this document also includes a description of the associated absorbed dose to 

water proficiency test (from gamma-ray beams) listed as NIST service 46050S. 

Definition

Absorbed dose to water is defined as the energy from ionizing radiation absorbed 

by a given mass of water, 1 J/kg = 1 Gy.

Equipment

• Computer equipped with an absorbed dose to water calibration program based 

on the LabVIEW programming language that allows automating the data 

acquisition process 

• Electrometer for the measurement of the ionization current and electrical 

charge collection

• Temperature probes for continuous temperature monitoring of both the air in 

the vicinity of the chamber and the water in the phantom

• Pressure transducer

• Co-60 irradiator source that has been calibrated in terms of absorbed dose to 

water using the NIST primary standard instruments

• Water phantom with chamber mounting apparatus

• An aluminum meter scale with 1 mm precision

• An in-house secondary standard ionization chamber

The computer is interfaced to all devices used for calibration including an 

electrometer, thermometer and pressure transducer. The temperature probes, 

pressure transducers and electrometers are auxiliary equipment used for the 

calibration service. The temperature probes are calibrated against a reference 

standard thermometer. The pressure transducers are calibrated against a reference 

standard barometer. The electrometers are calibrated against reference class air 

capacitors. The calibration of the reference standards are provided by the NIST 

Measurement Services Division. A NIST check chamber is used to decide if this
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auxiliary equipment needs to be checked against reference standards calibrated by 

the Process and Measurements Division.

Reference Scale

A metallic scale is used to measure the distance between the source and the detector. 

Other Equipment

Other equipment used during a typical calibration include a telemicroscope and a 

chamber clamping mount on a 2D translational stage for positioning the chamber at a 

fixed distance from the source and centered with the beam centerline. Also, a laser is 

used for positioning the detectors along the beam-centerline.

Support Equipment

The thermometers, barometers and electrometers referred to in this section are considered 

essential support equipment to the calibration of cavity ionization chambers. A list of all 

the thermometers, barometers and electrometers is maintained in an excel sheet that can 

be accessed by the facility computer from L:\internal\846.02\Gamma-Chamber-Cal. The 

Excel sheet includes: the identity of the item (and its software when applicable), 

manufacturer name, model number, serial number and use location. It also includes the 

calibration of all thermometers and barometers used in the facilities against the 

Dosimetry Group reference standard Thermometer and Barometer. 

It is important to note the quality of the ionization chamber calibration is not monitored 

by the calibration of the support equipment but rather by the routine quality assurance 

check performed using the NIST reference standard chambers (aka NIST check chambers 

or NIST QA chambers) as discussed in the sections ahead. The quality check of the 

calibration of all customer ionization chambers is determined by the reproducibility of the 

calibration coefficients of the NIST check chamber. Therefore, if a calibration coefficient 

of the NIST check chamber does not reproduce, the performance and calibration of the 

support equipment would be investigated. There are no specified calibration intervals for 

the critical support equipment because the equipment is calibrated using the in-house 

reference standards if there is a question of reproducibility of the NIST check chamber. 

Since the QA procedure for all gamma-ray calibrations requires the calibration of a NIST 

reference standard ionization chamber, any change in the reproducibility above 0.5 % to 

1 % of a typical chamber response depending on the chamber type and history, may 

require an investigation into the support equipment used for the calibration. If any of the 

critical support equipment is found to be out of calibration or damaged, it would be 

disconnected, and its condition clearly marked on the instrument. A calibrated, identical 

model replacement instrument would be used for calibrations to continue. 
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Safety

Radiation safety

Rooms containing 60Co sources commonly used for absorbed-dose-to-water

calibrations have been designated as High Radiation Areas and therefore are 

interlocked with the source opening shutter control. Specific requirements for 

entry and exit from the rooms are contained in these procedures and are posted on 

entry doors and walls. Radiation safety training and assessment services are 

provided by the NIST Gaithersburg Radiation Safety Division (GRSD).

Electrical safety

To avoid possible electrical shock, one must not touch any chamber or connector 

once voltage has been applied to the chamber. Certain chambers (for example, the 

Exradin A12) can carry the voltage on the outside of the chamber. When these 

chambers are submerged in the water tank, the water and any metal part coming 

in contact with the water become a potential shock hazard. To ensure safety 

during operation, the high voltage must be turned off before making a height 

adjustment.

Procedures

Communication with the Customer 

Customers can request calibration services in a variety of ways. Typically, a new or firsttime customer will establish contact with the Dosimetry Group by telephone and/or email requesting information regarding techniques offered, charges, backlog time, 

turnaround time, and shipping/mailing information. At this stage, there is generally an 

opportunity to discuss with the prospective customer appropriate technical and/or 

logistics aspects of the calibration. Technical aspects may include discussions on the 

beam qualities, dose rates, conditions under which the instruments are used, etc... 

Logistics aspects may include discussions about how to submit purchase orders through 

the NIST storefront, how to ship instruments to NIST, any accessories that the customer 

may need to include in the shipment depending on the type of instrument being 

calibrated, etc... The customer is informed that a purchase order must be received at 

NIST before an official calibration is performed. The customer is instructed to make use 

of the NIST storefront which can be found on the NIST calibration service webpage. 

Through the storefront the customer will shop for the desired calibration service and will 

also be able to upload all necessary documentation which includes a type of payment 

such as a purchase order. The purchase order should include a detailed description of the 

calibration request, including instrument model and serial numbers, and the name and 

telephone number of a technical contact. If an incomplete purchase order is received, 

every effort is made to get a detailed description of the service requested. 
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Initiation of Paperwork and Inspection of Instruments sent to NIST for Calibration

Once an order for a calibration service is placed on the NIST storefront and both a 

purchase order has been submitted and the instruments have been received at NIST, every 

effort is made to start the calibration process as soon as possible. This process consists of 

two stages: 

• Handling of the administrative portion of the calibration

• Handling of the instruments and their calibration. 

Regarding the administrative portion of the calibration, after an order is received at NIST 

through the NIST storefront, a NIST order number consisting of 10 digits is generated

with the following format: O-0000000000. The NIST technical contact (person 

performing the calibration) is then notified by email. The technical contact is able to 

review the order placed through the NIST calibration e-commerce platform called 

Salesforce. Information submitted by the customer through Salesforce includes the model 

and serial number of the instruments to be calibrated and the purchase order from the 

customer. Once this information is reviewed by the technical contact, if any information 

is missing from the customer, every effort is made to get the information needed by 

contacting the customer. All documentation associated with the calibration is stored 

under the NIST order number assigned to the calibration being requested. This includes 

the summary sheet on Salesforce, a copy of the purchase order submitted by the 

customer, the shipping document, copies of the calibration reports and all calibration data

and corresponding data analysis sheets. A unique identifier named the Dosimetry Group 

(DG) number is used to identify the services for a given instrument received. This 

identifier is used by several members of the Dosimetry Group, and it consists of a 5-digit 

number followed by the fiscal year as DGN: 00000-00. Both the NIST order number and 

the DG number are included on the cover of the calibration report. 

Regarding the handling of the instruments, instruments arriving for calibration are 

unpacked and inspected for damage. Special attention is given to the condition and type 

of connector. If an adapter is sent with the chamber, this should be noted on the 

inventory list along with the description of the chamber. Shipping damage is reported to 

the NIST shipping department. When an instrument arrives in a state of disrepair that is 

obvious by visual inspection, the customer is notified, and a decision is made whether to 

return the instrument to the customer, or if the repair is minor, have NIST personnel 

perform the repair. 

Only ionization chambers known to be stable and reproducible are accepted for 

calibration in this program. Institutions submitting ionization chambers for calibration 

are strongly urged to perform stability checks involving redundant measurements in 

highly reproducible radiation fields before sending their instruments to NIST, and to 

repeat those checks after NIST calibration, and again at suitable intervals. Instruments 
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submitted for calibration, and material submitted for irradiation, must be shipped in 

reusable containers.

Detector Setup and calibration

The instruments to be calibrated using gamma-ray beams are calibrated by using a 

previously determined value of the absorbed dose to water rate obtained by the decay of 

the initial value to the date and time of the calibration being made. The value of the 

absorbed dose to water rate for a given distance from the source at a given date and time 

is displayed by the data-acquisition program. 

For all customer calibrations, a NIST reference-class transfer ionization chamber is 

calibrated for quality assurance. Generally, the NIST chamber selected is similar in 

design or collection volume to the customer chamber being calibrated and has a previous 

calibration history in the reference radiation qualities which were selected by the 

customer. 

Follow the procedures for the setup and calibration sections below for one of the NIST 

reference-class transfer ionization chambers and then again for the customer chamber.

Setup

1. Set the water phantom on the moveable cart of the 60Co platform and fill 

approximately 2/3 full of water. Allow water to come to temperature 

equilibrium, which is usually 1.0 oC to 1.5 oC lower than room temperature. 

This can take up to 24 hours.

2. Turn on the source control panel. Set the timer to 99999 and press the 

initialize button.

3. Enter name, date, and other pertinent information in the appropriate source 

logbook. Test lights and interlocks during this time using manual source 

controls.

4. Turn on the computer and click on the “absorbed dose calibration” icon to 

launch the LabVIEW program (see Appendix C for the entries to the 

program).

5. The following environmental conditions should be present when performing 

calibrations. The temperature of the room should be stable within one single 

measurement and ideally around 22 oC. If the temperature is not stable during 

a single measurement, calibrations should be postponed. Also, the 

temperature should not exceed 25 oC and should not be lower than 19 oC. If 

the temperature falls out of this range, the calibration should be postponed 

until the temperature is back within the working range. The preferred 
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humidity conditions are between 20 % and 50 % RH, but calibrations still can 

be performed if humidity levels fall out of this range. It is preferable to 

calibrate instruments on days that the pressure is around 101.3 kPa (760 mm 

Hg), but calibrations can still be performed if the atmospheric pressure 

deviates from this value. Calibrations should be postponed, however, if the 

pressure is not stable during a single measurement.

Calibration

1. Ensure that the chambers to be calibrated are properly waterproofed. When 

applicable, ensure that air vents extend beyond the phantom to prevent water 

from entering the chamber.

2. Thread the waterproof temperature probe from the bottom of the mounting 

apparatus through the hole in the horizontal mounting bracket where the 

chamber is to sit, then lay it along the groove.

3. Mount the chamber onto the bracket in such a way that allows the temperature 

probe to lie along the underside of the chamber. Lower the chamber (by 

rotating the height adjustment dial) into the water to approximately the desired

depth, or at least to a depth such that the entire mounting block is under the 

water. Connect the signal and high voltage cables to the chamber. 

4. On the 60Co housing, find the two dials each controlling a pair of collimator 

jaws, east/west and north/south, respectively. Locate the plumb bob attached 

to a 50.8 mm x 50.8 mm aluminum block. Insert the block into the source 

collimator opening. This will define the beam center line. Translate the water 

phantom so that the front face of the phantom (south side) touches the beam 

center line. This procedure places the front water surface at the focal plane of 

the telemicroscope. 

5. Fully open both pairs of jaws, then close each of them to 10.75 on the dial. 

Confirm that the jaws close tightly against the jig made for this dial setting. 

Do not touch the trimmer adjustment knobs.

6. Allow the water to settle such that the surface is stable. Adjust the 

telemicroscope such that the horizontal crosshair centers on the bottom most 

line of the water surface. Due to surface tension and optical effects, there are 

multiple lines visible in the scope. Always align with the line that is closest to 

the bottom. Once the telemicroscope is aligned to the water surface, push the 

phantom away from the beam center line.

7. Insert the meter scale onto the holders under the source opening such that it is 

suspended under the source. Adjust the height of the entire platform using the 

wall mount control unit so that the horizontal crosshair of the telemicroscope 
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centers on the 53.8 cm mark of the meter scale. In the weaker intensity 

vertical beam this position corresponds to a distance of 95 cm from the source. 

Turn off the wall mount control unit. This is important to do to prevent 

possible drifts in the motion of the platform while the unit remains on. Lower 

the telemicroscope by 5.00 cm from the 53.8 cm position to 58.8 cm as seen 

on the scale. At this scale position, the telemicroscope provides a reference 

distance of 100 cm from the source. Remove the scale.

8. Slide the phantom back under the source, this time using the laser to align the 

chamber in the center of the beam. Adjust the height of the chamber so that 

its center aligns with the horizontal crosshair of the telemicroscope. 

9. Activate the interlock and exit the room following the Safety Procedure for 

the Vertical Beam Facility (a copy of the procedure is kept inside the source 

logbook).

10. Apply voltage at the polarity and maximum allowable value as specified by 

the customer. Allow time for the chamber to “warm up” – this can take up to 

one hour, depending on the chamber. A periodic check measurement can 

determine if the current has stabilized. Perform pre irradiations of the 

chamber by opening the shutter at this time if necessary. 

11. Complete the calibration following the prompts on the computer program (see 

Appendix C for the program main panel). 

12. Adjust the high voltage on the chamber to half of its initial value. Allow at 

least 15 minutes before repeating the calibration.

13. Turn off the high voltage to the chamber. Remove the chamber from the 

water. Compare calibration results to the chamber history if available.

Analysis and Report

The average current, in units of A, or C/s, measured in the above procedures is 

normalized to reference environmental conditions (22 °C and 101.3 kPa). The decay 

corrected absorbed dose rate (traceable to the NIST primary standard), in the unit of Gy/s, 

is divided by the average current (normalized to the reference environmental conditions) 

to obtain a calibration coefficient in the units of Gy/C. This is the value reported to the 

customer. The ratio of current at full voltage to that at half voltage is also reported. See 

Appendix B for a report template.
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Quality Control

A minimum of 5 total measurements should be made for each calibration point. The 

standard deviation within these 5 or more measurements is expected to be no greater than 

0.5 % for the NIST reference-class chambers. If it is greater, the cause of the value being 

larger needs to be understood. For example, in some cases this could be caused by the 

chamber needing more time to equilibrate thermally. In other cases, it could be that an 

additional pre-irradiation time is required.

Results for customer chambers with histories should agree to within 2 % of any previous 

calibration. If this criterion is not met, check for trends in the data. It may also be 

advisable to contact the customer to see if they have noticed any unusual behavior with 

the chamber.

For all NIST reference-class chambers, a record is maintained of all calibrations, 

calibration results, calibration dates, and the previous calibrations are compared with the 

current calibration to detect any trend or measurement discrepancy. The calibration 

history and record for all NIST reference chambers can be accessed by the facility 

computer from L:\internal\846.02\Gamma-Chamber-Cal.

Uncertainty Analysis

The uncertainty analysis of the absorbed-dose-to-water calibration of ion chambers is 

outlined in the NIST Special Publication 250-741. In brief, the various sources of 

uncertainty of the calibration outlined in the NIST SP250-74 reference are: 

1. The determination of the reference absorbed-dose-to-water rate2

2. Co-60 decay constant 

3. Charge variability – this value is obtained from the standard deviation of 

replicate measurements incorporating several reference class ionization chambers 

(for working standards this value will be higher)

4. Correction due to recombination loss

5. Time – shutter control reproducibility, a Type B uncertainty

6. Air density – the Type A portion of the uncertainty is determined from the 

standard deviation of replicate measurements. The manufacturers of the 

temperature probe and the pressure gauge provide the Type B uncertainty
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7. Humidity – no correction is made for the effect of water vapor on the 

instrument being calibrated. It is assumed that both the calibration and the use of 

that instrument take place in air with a relative humidity between 10 % and 70 %, 

where the humidity correction is nearly constant 

8. Variation in response due to positioning uncertainty at 5 cm depth in water,

0.05 %

9. Variation in response due to positioning uncertainty at 1 m from the source,

0.02 %

Documentation/Calibration Reports/Storage

After the instrument has been calibrated the calibration report is electronically generated. 

Currently, the reports are generated in the most recent version of Microsoft Word. 

Templates are available to simplify this procedure and to ensure consistency in the 

reporting format. A sample report can be found in Appendix B of this report. All results 

reported relate only to the instruments or items tested and/or calibrated during the 

calibration as indicated in the calibration certificate (this statement is also included in 

the calibration report).

All reports of Calibration and reports of Special Test are signed by personnel explicitly 

authorized to do so. The review and signing process can be summarized as follows: the

calibration report is reviewed and initialed by the preparer and then sent electronically for 

review to a Dosimetry Group calibration staff member. This first reviewer may or may 

not find any issues with the report. In the case any issues are found, they are discussed 

with the person conducting the calibration to clarify and resolve the issue prior to the 

reviewer’s approval. Once any potential issues are resolved the reviewer approves the 

final report and it is sent electronically to the remaining reviewers which are the Group 

Leader and Division Chief. Once all reviewers have approved and initialed/signed the 

report, the report is sent by the technical contact to the customer electronically. If the 

report is completed prior to the instruments being shipped, a hard copy of the report is 

also included in the box with the instruments.

After all requested calibration work is completed, the technical contact informs the 

accounting office that the calibration work has been completed and that the instruments 

need to be returned to the customer. The instruments are packed either in the original 

container or in a more suitable one if necessary. A shipping document is available on 

Salesforce which is printed and attached to the box of instruments for shipping. After 

shipment, all documents associated with the calibration are kept in the facility computer 

and a backup is kept in L:\internal\846.02\Gamma-Chamber-Cal. The calibration is 

identified by the NIST order number, and each instrument sent for calibration is 

identified by the DG number described previously. 
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Traceability

The SI unit of absorbed dose is the gray (Gy), which is defined as the energy from 

ionizing radiation absorbed by a given mass of water; 1 J/kg = 1 Gy, directly 

determined by the NIST water calorimeter in a 60Co beam. More detailed 

information concerning traceability and uncertainty analyses is summarized in 

references 1 and 2.

International Comparisons

International comparisons have been made with other National Metrology 

Institutes around the world. During these international comparisons, a reference 

class chamber is calibrated at both facilities and the values of the calibration 

coefficients obtained at both institutions are compared. The reference section lists 

comparisons made in the last few years using the same 60Co gamma-ray beam that 

is used for the calibration service described in this procedure. 

Filing and Retention

The Radiation Physics Division (RPD) Quality Manager shall maintain the 

original and all past versions of this RPD Procedure.

References

1. Minniti R., Shobe J. Seltzer S. M., Chen-Mayer H. and Domen, S.R., 

Absorbed Dose to Water Calibration of Ionization Chambers in a 60Co 

Gamma-Ray Beam, NIST Special Publication 250-74, September 2006. 

2. Domen, S.R., A sealed water calorimeter for measuring absorbed dose, J. Res. 

Natl. Inst. Stand. Technol., 99, pp. 121 – 141, 1994.

3. P. J. Allisy-Roberts and J. Shobe, “Comparison of the standards of absorbed 

dose to water of the NIST and the BIPM for 60Co gamma rays,” Rapport 

BIPM 98/5 (1998).

4. I. Csete, AG. Leiton, V. Sochor, A. Lapenas, J.-E Grindborg , I. Jokelainen, H. 

Bjerke, J. Dobrovodsky, A. Megzifene, H. J. Costas, R. Ivanov, B. Vekic, J. , 

Kokocinski, J. Cardoso, L.Buermann, W. Tiefenboeck , G. Stucki, E. van 

Dijk, M. P. Toni, R. Minniti, J. P. McCaffrey, C. N. M. Silva, I. Kharitonov, 

D Webb, M. Saravi, F. Delaunay, “Comparison of air kerma and absorbed 

dose to water measurements of 60Co radiation beams for radiotherapy, Report 

on EUROMET project no. 813, identifiers in the BIPM key comparison 

database (KCDB) are EUROMET.RI(I)-K1 and EUROMET.RI(I)-K4,” 

Metrologia Vol. 47, Tech. Suppl 06012, April 2010.
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Appendix A

Sample Uncertainty Analysis

Type A (%) Type B (%)

Absorbed dose rate at 5 cm in water10.16 0.39

Charge 0.10 0.10

Time 0.05

Air density 0.03

Positioning at 5 cm depth in water 0.03

Positioning at 1 m source distance 0.05

Measurement reproducibility 0.05

Recombination 0.01 0.05

Humidity 0.06

60Co decay constant5

0.05

quadratic summation 0.19 0.43

Relative combined standard uncertainty 0.47

Relative expanded uncertainty, k = 2 0.94
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Appendix B

Sample Calibration Report 
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Appendix C

Calibration Program Control Panel

Notes:

• A field with on the left is a “counter” which accepts either manual input or 

incremental input from the counter.

• A field with “t” on the right is a “pull down” menu containing pre-entered values for 

selection.

• The two internal reference fields, upon program execution, will automatically provide 

for the user the decay corrected air kerma rate (irrelevant to this calibration 

procedure) and absorbed dose rate used in the calibration.

• Other fields accept manual input.

Table

Operator

Measurement type

Room

Source

Distance

Aperture

Air Kerma rate

Absorbed dose rate

Measurement method

Electrometer owner

Electrometer make

Electrometer model

Electrometer serial #

Electrometer address

t

absorbed dose t

B036 t

Co-60 t

1 meter t

10.75 x 10.75 t

 (internal reference)

(internal reference)

Computer t

 NIST

t

t

t

 27

Monday, October, 20, 2003 4:35:55 PM

Absorbed Dose Calibration Program

t

t

t

 300 Volts

Negative t

 Inherent t

white line t

Perpendicular t

 mmHg

deg C

Chamber owner

Chamber make

Chamber model

Seial #

Applied voltage

Polarity

Time HV set

Equil/waterproffing

Rotation

Orientation

Pressure

Temperature

Humidity

DG Number

TF Number
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APPENDIX D: Service 46050S, Proficiency Test for absorbed dose to water 

General

The proficiency test for this measurement service complies with the NIST QMS ISO/IEC 17025:2017

Appendix F for proficiency testing. This sub-level quality document provides the details of the additional 

requirements of ISO 17043:2010 for this measurement service proficiency test program.

Design of proficiency testing schemes

The protocol for each proficiency test will include the following details, taken from QMI, Appendix F, but 

repeated here for thoroughness. Each participant will receive a personalized proficiency test protocol. A 

sample protocol is provided in this procedure with the required elements listed below

a) the name and location of the NIST measurement service;

b) identification of the proficiency testing program manager and other personnel involved in the design 

and operation of the proficiency testing scheme;

c) the activities to be conducted by NIST collaborators and the names and addresses of NIST 

collaborators involved in the operation of the proficiency testing scheme; collaborations do not exist 

for this proficiency test.

d) criteria to be met for participation if applicable;

e) the number and type of expected participants in the proficiency testing scheme;

f) selection of the measurand(s) or characteristic(s) of interest, including information on what the 

participants are to identify, measure, or test for in the specific proficiency testing round;

g) a description of the range of values or characteristics, or both, to be expected for the proficiency test 

items;

h) the potential major sources of errors involved in the area of proficiency testing offered;

i) requirements for the production, quality control, storage and distribution of proficiency test items;

j) reasonable precautions to prevent collusion between participants or falsification of results, and 

procedures to be employed if collusion or falsification of results is suspected;

k) a description of the information which is to be supplied to participants and the time schedule for the 

various phases of the proficiency testing scheme;

l) for continuous proficiency testing schemes, the frequency or dates upon which proficiency test items 

are to be distributed to participants, the deadlines for the return of results by participants and, where 

appropriate, the dates on which testing or measurement is to be carried out by participants;

m) any information on methods or procedures which participants need to use to prepare the test material 

and perform the tests or measurements;

n) procedures for the test or measurement methods to be used for the homogeneity and stability testing 

of proficiency test items and, where applicable, to determine their biological viability;

o) preparation of any standardized reporting formats to be used by participants;

p) a detailed description of the statistical analysis to be used;

q) the origin, metrological traceability and measurement uncertainty of any assigned values;

r) criteria for the evaluation of performance of participants;

s) a description of the data, interim reports or information to be returned to participants;

t) a description of the extent to which participant results, and the conclusions that will be based on the 

outcome of the proficiency testing scheme, are to be made public; and

u) actions to be taken in the case of lost or damaged proficiency test items.
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Preparation of proficiency test items

The NIST proficiency test chamber must meet the stated stability specifications of the 

manufacturer. The NIST reference class transfer ionization chambers have been determined 

appropriate for use through published comparison studies and history of use. The NIST proficiency 

testing program calibrates each using the calibration procedure associated with this service. The 

NIST proficiency testing program uses the same acquisition and storage procedures as the 

calibration service. The chambers procured for the proficiency testing quality assurance are 

dedicated to that purpose. The NIST proficiency testing program uses the same type chambers as 

are used for the calibration service. NIST will ship the chamber in a reusable shipping container 

which should be used for the return of the chamber to NIST.

Homogeneity and stability

The performance of the NIST testing chamber is evaluated at NIST according to the calibration 

procedure. The collected charge from the testing chamber is statistically analyzed in the same 

manner as stated in the calibration procedure. If the chamber completes the procedure at NIST with 

acceptable stability and is found to become unstable at the participant’s facility, NIST should be 

notified and the chamber should be excluded from the measurement comparison and a replacement 

chamber will be provided. NIST calibrates the ionization chamber once it is returned. If the 

proficiency test involves multiple participants, the testing is a star shaped design which means NIST 

recalibrates the chamber after it is used at each facility. This allows NIST to monitor the stability 

of the chamber throughout the comparison. 

Statistical design

The same statistical design is used for the proficiency test data as is used for the calibration service. Each 

chamber is measured at least three times and the average is used. The NIST value will be the average of 

the results before and after the chamber was sent to the customer. The standard deviation on the charge 

measurement is used in the uncertainty analysis. The proficiency test is a direct comparison of the 

measured calibration coefficients determined at NIST and the participating facility. The criteria for the 

comparison are established by the accreditation program, not by NIST. NIST does not provide the 

normalized error nor the Z-score. 

Assigned values

The calibration coefficient determined through the proficiency test process is clearly explained in the 

calibration service and the proficiency test report and repeated here for convenience: The absorbed-dose-towater rate at the NIST is realized at the calibration position by a water calorimeter. This realization of the 

radiation quantity of absorbed dose to water establishes the National standard, which can be transferred to 

other measurement facilities through a suitable measuring instrument, thus enabling traceability to the 

National standard. The 

60

Co gamma-ray rate at NIST is corrected to the date of calibration (from the 

previously measured value) by a decay correction based on a half-life of 5.27 years. The absorbed-dose-towater calibration coefficients, ND,w, are quotients of the absorbed-dose-to-water and the charge generated 

by the radiation in the ionization chamber. The average charge is based on measurements with the wall of 

the ionization chamber at the stated polarity and potential. 
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Choice of method or procedure

The participant must have a linking traceable measurement of air-kerma in order to participate in a 

proficiency test. The participant agrees to use the NIST chamber. Using the participant’s 

established measurement methods and procedures, the participant should provide NIST with the 

calibration coefficient(s) for the chamber found at their facility for each beam in units of Gy/C.

Operation of proficiency testing schemes - Instructions for participants

The proficiency test is requested by the participant or group of participants and the coordination of 

the test date is established on a mutually agreed upon schedule. The instructions for the test are 

provided in the test protocol.

Proficiency test items handling and storage

The NIST test chamber is stored in the laboratory where the test is conducted at NIST which is conducive 

to the manufacturer’s recommendations of the conditions for use. 

Packaging, labelling and distribution of proficiency test items

The chambers are securely packed with foam in reusable boxes. All chambers have unique serial 

numbers which are documented in the NIST reports.

Data analysis and evaluation of proficiency testing scheme results

The data analysis and records are handled according to the calibration service. Evaluation of performance is 

conducted by NIST staff. The evaluation is limited to a direct comparison as a percent difference. 

Generally, NIST commentary is not provided, since that is left to the accrediting body which would analyze 

the results applying the established criteria. NIST would include in the proficiency test report, 

commentary, where applicable, on an educational basis if the participant encounters complications.

Reports

The NIST proficiency testing report for each proficiency test will include the following details, taken from 

QMI, Appendix F, but repeated here for thoroughness. A sample NIST report is provided in this procedure. 

NIST proficiency testing reports include the following, unless it is not applicable or the NIST proficiency 

testing program has valid reasons for not doing so:

a) the name and contact details for the NIST proficiency testing program;

b) the name and contact details for the coordinator;

c) the name(s), function(s), and signature(s) or equivalent identification of person(s) authorizing the 

report;

d) an indication of which activities are performed by the participant(s);

e) the date of issue and status (e.g. preliminary, interim, or final) of the report;

f) page numbers and a clear indication of the end of the report;

g) a statement of the extent to which results are confidential;

h) the report number and clear identification of the proficiency testing scheme;

i) a clear description of the proficiency test items used, including necessary details of the proficiency 

test item's preparation and homogeneity and stability assessment;

j) the participants' results;
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k) statistical data and summaries, including assigned values and range of acceptable results and graphical 

displays;

l) procedures used to establish any assigned value;

m) details of the metrological traceability and measurement uncertainty of any assigned value;

n) procedures used to establish the standard deviation for proficiency assessment, or other criteria for 

evaluation;

o) assigned values and summary statistics for test methods/procedures used by each group of 

participants (if different methods are used by different groups of participants);

p) comments on participants' performance by the NIST proficiency testing program and technical 

advisers;

q) information about the design and implementation of the proficiency testing scheme;

r) procedures used to statistically analyze the data;

s) advice on the interpretation of the statistical analysis, only when applicable; and

t) comments or recommendations, based on the outcomes of the proficiency testing round, only when 

applicable.

When it is necessary to issue a new or amended report for a proficiency testing scheme, the report 

includes the following:

a) a unique identification;

b) a reference to the original report that it replaces or amends; and

c) a statement concerning the reason for the amendment or re-issue.

Communication with participants

The NIST calibration ordering system includes the option to request proficiency testing and 

provides the following details. 

a) documented eligibility criteria for participation;

b) confidentiality arrangements; and

c) details of how to apply.

If changes to the proficiency test scheme design or operation are required, the participant will be 

notified by email. If the results conclude in a performance that the participant needs to appeal, a 

retest will be offered at the expense of the participant, after an investigation by NIST and the 

participant. The participant must communicate the error of the previous test and the reason for the 

request to retest. If an error was made by NIST, the proficiency test will be repeated at no expense 

to the participant. If no reason is determined for poor performance, the test can be repeated at the 

expense of the participant. 

Sample Report

See below for a sample proficiency test report
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