Radiation Physics Division 46010C, 46050S RPD-P-04

CALIBRATION OF RADIATION DETECTORS IN TERMS OF AIR-KERMA USING 

GAMMA-RAY BEAMS

Version Date Author Approval Pages Filename

7.00 4/25/2023 RM AKT 1 of 33 Procedure04v700

Purpose

The purpose of this document is to describe the setup, measurement and procedures for calibration of 

instruments in terms of air kerma using gamma-ray beams from 137Cs and 60Co irradiator sources. 

Scope

The measurement service described in this document is listed as NIST service code 46010C. Appendix 

C of this document also includes a description of the associated air kerma proficiency test (from gammaray beams) listed as NIST service 46050S. The document starts by describing the physical quantities air

kerma and exposure and provides a brief background describing the rationale behind the calibration 

process. It later describes the calibration systems used and the procedures that are typically followed in 

performing a calibration, analyzing the data, and reporting the results of the calibration. The appendix 

includes a copy of a sample of the current calibration reports used for the different types of instruments 

submitted for calibration. 

Definitions and Background

Description of Service

The Dosimetry Group of the NIST Radiation Physics Division receives a variety of instruments for 

calibration in gamma-ray beams. Calibration coefficients or calibration factors are provided for the 

radiation detectors sent to NIST for calibration. Calibrations are performed in terms of the physical 

quantities air kerma and exposure.

The Quantities Air Kerma and Exposure

The quantity kerma characterizes a beam of photons or neutrons in terms of the energy transferred to 

any material. For the calibration service described in this document, consideration is limited to photon 

beams in air. Air kerma is the total energy per unit mass transferred from a photon beam to air. Air 

kerma, Kair, is the quotient of dEtr by dm, where dEtr is the sum of the initial kinetic energies of all 

electrons liberated by photons in a volume element of air and dm is the mass of air in that volume 

element. Then

 

dm

dE K

tr

air =
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CALIBRATION OF RADIATION DETECTORS IN TERMS OF AIR-KERMA USING 

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The SI unit of air kerma is the gray (Gy), which equals one joule per kilogram; the old unit of air kerma,

the rad, equals 0.01 Gy.

The quantity exposure characterizes an x-ray or gamma-ray beam in terms of the electric charge 

liberated through the ionization of air. Exposure is defined as the total charge per unit mass liberated in 

air by a photon beam and is represented by the equation:

 

dm

dQ X =

where dQ is the sum of the electrical charges on all the ions of one sign produced in air when all the 

electrons liberated by photons in a volume element of air, whose mass is dm, are completely stopped in 

air. The SI unit of exposure is the coulomb per kilogram (C/kg); the special unit of exposure, the 

roentgen (R), is equal to exactly 2.58x10-4 C/kg. The ionization arising from the absorption of 

bremsstrahlung emitted by the secondary electrons is not included in dQ. Except for this small 

difference, significant only at high energies, the exposure as defined above is the ionization equivalent 

of air kerma. The relationship between air kerma and exposure can be expressed as a simple equation:

 

















−













= 

−

e g

W K X

1

1

2.58 10 4

where W/e is the mean energy per unit charge expended in air by electrons, and g is the fraction of the 

initial kinetic energy of secondary electrons dissipated in air through radiative processes. The current

value accepted by the NIST for W/e is 33.97 J/C. The currently accepted g values for 60Co and 137Cs 

beams are 0.32 % and 0.16 %, respectively. 

Characterization of the NIST Gamma-Ray Beams in Terms of Air Kerma

There are a total of seven gamma-ray irradiator sources that produce the 137Cs and 60Co gamma-ray 

beams that are used for calibrating instruments in terms of air kerma and exposure. The air kerma rates

and exposure rates in these facilities are realized by using the NIST primary standard instruments, a

suite of six graphite-wall, air ionization, Bragg-Gray cavity chambers developed at NIST.
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CALIBRATION OF RADIATION DETECTORS IN TERMS OF AIR-KERMA USING 

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The reference air kerma rate that is realized with the primary standard instrument is later decay corrected 

to provide the value of the air kerma rate at a given distance from the source for any given date and time 

of the year.

The rates are readily available on the console of the data acquisition system used to perform the 

calibration of instruments.

Generalities of the Calibration of an Instrument in Terms of Air Kerma and Exposure

Instruments that are sent to the NIST are calibrated in terms of air kerma and exposure. The goal of the 

calibration is to determine either a calibration coefficient or a calibration factor depending on the type of 

instrument to be calibrated. Determination of these parameters requires the measurement of an 

ionization current, or the direct measurement of a radiation dose quantity obtained from the display 

reading of an electrometer. In addition, the temperature and the pressure of the air surrounding the 

detector must be measured for the case of ionization chambers that are open to the atmosphere.

Calibration Coefficient: The calibration coefficient is defined as the quotient of the air kerma and the 

charge generated by the radiation in the ionization chamber. This parameter is determined for current

type measuring instruments.

Calibration Factor: The calibration factor is defined as a dimensionless ratio of air kerma (or exposure) 

and the electrometer reading with a given ionization chamber or detector. This parameter is determined 

for cable-connected-type instruments consisting of an electrometer and probe combination.

Pressure and Temperature Correction: The average charge used to compute the calibration coefficient is 

based on measurements with the wall of the ionization chamber at the stated polarity and potential. With 

the assumption that the chamber is open to the atmosphere, the measurements are normalized to a 

pressure of one standard atmosphere (101.325 kPa) and a temperature of 295.15 K (22 oC). Use of the 

chamber at other pressures and temperatures requires normalization of the ion currents to these reference 

conditions using the normalizing factor F. The normalizing factor F is computed from the following 

expression: 

F = (273.15+ T) (295.15H)

where T is the temperature in degrees Celsius, and H is the pressure expressed as a fraction of a standard 

atmosphere. (1 standard atmosphere = 101.325 kilopascals = 1013.25 millibars = 760 millimeters of 

mercury).
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CALIBRATION OF RADIATION DETECTORS IN TERMS OF AIR-KERMA USING 

GAMMA-RAY BEAMS

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Equipment 

Gamma-Ray Irradiator Sources

All NIST irradiators generate a collimated beam of gamma-rays. The orientation of each of these beams 

is listed in the table below. 

Radionuclide

Nominal 

Rates as of 

2021-01-01

(Gy/s)

Orientation

60Co 1.5 x 10-2 Vertical

60Co 5.0 x 10-4 Vertical

60Co 4.5 x 10-8 Horizontal

60Co 5.0 x 10-9 Horizontal

137Cs 4.0 x 10-4 Vertical

137Cs 8.2 x 10-6 Horizontal

137Cs 8.5 x 10-7 Horizontal

Console

In each of the radiation facilities there is a separate control unit for each source. The control unit 

operates the shutter in front of the radioactive source to either expose or shield the radiation source 

contained in the irradiator unit. These control units are interfaced to a computer containing the dataacquisition software.
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Data-Acquisition System

There are two data acquisition systems (DASs) used in these services. One is a mobile measurement 

console referred to as the portable system. The home location of the portable system is in the horizontal 

beam range; however, it can be transported to other locations and used in all the gamma-ray facilities to 

perform calibration of instruments. 

The portable system consists of all instrumentation required for measurement and standardization of 

ionization currents. This data acquisition system is a Visual Basic interfacing system, which can 

automatically acquire all or some of the calibration data for cable-connected instruments and passive or 

other types of cable-connected instruments, such as those with their own readout. The mobile console 

contains a Keithley Model 616 electrometer, a Setra Model 350A digital barometer and a Digitec Model 

5810 digital thermometer. Each cable connected instrument has an analog output signal. The feedback 

elements for the electrometer selector switch in the "Volts" position are capacitors mounted in a 

capacitor-selector chassis. The equations for computing temperature are dependent on the thermistor 

used and, for measurements in control room, the signals are taken from YSI readouts mounted in the

source-control consoles. The equation used for computing atmospheric pressure from the Setra device, 

and the data for converting the analog signals from the thermistor probes to air temperatures and from 

the pressure transducer to atmospheric pressure are stored in the computer program for each calibration 

range.

A second data-acquisition system, permanently located in the vertical beam facilities, is used to perform 

calibration of instruments in this facility. This system consists of a computer containing the appropriate 

boards that interface a Hart Scientific temperature readout, a Setra pressure transducer and two Keithley 

617 electrometers used to collect the charge. The software is developed in LabVIEW and is used to 

perform calibrations of ion chambers in terms of air-kerma rate and absorbed-dose-to-water.

Temperature probes

In each of the calibration facilities, a temperature probe is located near the chamber. All of the 

temperature probes are interfaced to the DAS computer that records the value of the temperature during 

the data acquisition. 
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CALIBRATION OF RADIATION DETECTORS IN TERMS OF AIR-KERMA USING 

GAMMA-RAY BEAMS

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Pressure Transducer 

A pressure transducer, located in the control room at approximately the same height above sea level as 

the height of the ionization chambers positioned for calibration, is interfaced to the DAS-computer and 

is used to measure the atmospheric pressure during irradiation time. 

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 calibrations of the reference standards are provided 

by the NIST Measurement Services Division. A NIST check chamber is used to decide if the calibration 

of the equipment used for calibration needs to be checked against reference standards calibrated by the 

Process Measurements Division. Further discussions on the use of the NIST check chamber are 

provided in the sections ahead. 

Reference Scale

In each room is a metallic scale that is used to measure the distance between the source and the detector. 

Other Equipment

Other equipment used during a typical calibration include a telemicroscope, a movable cart and a 

chamber stand for positioning the chamber at a fixed distance from the source. 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 
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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. 

Procedure

Communication with the Customer 

Customers can request calibration services in a variety of ways. Typically, a new or first-time customer 

will establish contact with the Dosimetry Group by telephone and/or e-mail 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, air kerma 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 beam quality codes, 

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: O0000000000. 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 numbers of the instruments to be calibrated, the radiation beam qualities (Cs-137 or 

Co-60) to which each instrument needs 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, a copy 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. 
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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 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 air-kerma rate obtained by the decay of the initial value to the date and time of 

the calibration being made. The value of the air-kerma 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. 

The following environmental conditions should exist when performing calibrations. The temperature of 

the room should be stable during one single measurement, 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. Preferred 

humidity conditions are between 20 % and 50 %, but calibrations can still 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. 

The procedure followed in all the gamma ranges for positioning the ion chambers and detectors is 

basically the same in all rooms. There are some minor differences regarding the positioning of detectors 

in the vertical orientation that mainly have to do with the order of the steps followed (steps 1 through 5). 

The procedure described below assumes a setup being made in any of the four horizontal calibration 

ranges. 

1. Previous to setting up an instrument for calibration, a choice must be made for the appropriate sourceto-detector distance taking into account the appropriate exposure rate and beam size for that particular 

detector. The beam size is compared to the largest dimension of the active volume. The general 
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practice is to use a beam size that is only a few centimeters larger than the active volume size so as to 

minimize irradiation of inappropriate volumes in the probe stem. The beam size at a given distance 

from the source is shown by the data acquisition software for any distance from the source. 

2. In all the gamma-ray facilities, a metallic scale is used to set the source-to-detector-distance. The 

source-to-detector distance is set by sighting the telemicroscope on the appropriate scale distance.

3. Set the detector in the holder and connect all cables.

4. The probe to be calibrated is adjusted to the beam center-line using the laser beam associated with 

each source. 

5. The probe is then centered in the telemicroscope scale-reticle. An exception to this technique is when 

the probe is larger than 10 cm. The technique for set-up then involves measuring the probe in the 

direction of the beam using metric calipers and determining the radius. The probe is then placed in the 

beam, aligned as above, and adjusted so that the front or back of it is tangent to the telemicroscope cross

hairs.

6. In the case of ionization-type detectors, apply the appropriate collection potential requested by the 

customer. The collecting voltage is verified at the chamber. This ensures that the voltage connection 

has been made. It is also important to minimize the exposure of all connections to the radiation beam. 

7. The chamber is now ready to be calibrated. Follow the source setup procedure and then exit the 

room.

Source Setup for horizontal ranges

1. Sign in using the logbook for operating the source. This logbook is located in the control room for 

operating the relevant sources. Log-in the information requested in the logbook: date, operator 

name, time, shutter elapsed time, room, use, etc.

2. After filling in the logbook, get the key for unlocking the source’s mechanical safe-lock and unlock 

the source. ATTENTION: It is extremely important to unlock before operating the source. Failure 

to do so can damage the source. 

3. Turn on main power to console.
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4. Once the chamber has been aligned for calibration following the procedure described in the 

subsection entitled “Detector Setup,” enter the room to make sure it is vacant of people. Exit the 

room and close the door.

5. A check of the safety-interlock system and other visible indicators must be performed. The interlock 

system is checked by opening the source and later opening the door to the room containing the 

sources. The source must close immediately upon opening the door. This is verified only once at the 

start of the day. 

6. The source is opened by first pressing the “Reset” Interlock button, then initializing the timer by 

pressing the “Initialize” button, and finally pressing the “Open” button. In the open position, 

radiation is present in the room. By pressing the “Close” button, the source closes and there is no 

radiation present in the room.

7. When opening the source for the first time on the day of measurement, also verify that the buzzer 

sounds, indicating the detection of radiation in the room. 

8. After all safety checks outlined above have been performed, close the door to the room once more 

and reset the interlocks as explained above. At this point it is possible to start performing 

calibrations.

9. Upon completion of the work, shut down the power to the console. Remove the key from the source 

and place it in the drawer in the control room. 

10. Sign out in the logbook. Turn all lasers off. Turn off all voltages applied to ionization chambers.

Calibration of Instruments/Data Collection

1. Start the data-acquisition program. The name of the program is “Calibrator.”

2. On the first page, complete all information regarding the ion chamber to be calibrated. The steps 

described here apply to both the NIST chamber used for check purposes and to the customer ion 

chamber and/or radiation detector. Information entered includes items such as: customer name, 

calibration date, chamber make, model and serial number, voltage applied to chamber, calibration 

distance, reference used for alignment, number of scans, scan time, etc. 

3. Once the information is entered, data collection can start. The system is automated for current-type 

instruments. Typically, the scan times vary between 1 minute and 2 minutes, depending on the 

instrument being calibrated. The number of scans taken once the system has reached a stable regime 

must be no fewer than 5. Typically, between 5 and 10 scans are taken after a stable regime is reached;
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detectors typically take anywhere between 30 minutes to 3 hours to stabilize from the time the voltage is 

applied. Some ion chambers require a period of pre-irradiation (typically between 30 minutes and 60 

minutes).

4. Background measurements are taken prior to calibration of the instrument and after irradiation. If the 

background is a significant fraction of the expected exposure reading, this may be a sign of dirty 

insulators that, in most cases, can be fixed by cleaning the connector using canned dry gas. Since the gas 

is cold (due to expansion), some time must be allowed for the chamber to equilibrate with room 

temperature. If the cleaning procedure is not successful and the calibration system has been verified to 

be working correctly, then the chamber is not calibrated, and the customer is informed.

5. After data collection, a data sheet with the results is printed out. The data sheet contains the 

calibration coefficient or calibration factor obtained for that instrument. 

6. The current calibration results are compared with previous results to verify the quality of the 

calibration.

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 referenceclass 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.

Two methods are used to verify a calibration. The first is to calibrate a NIST chamber that has a 

calibration history and is similar to the customer's chamber described previously. The second check is 

an examination of previous calibrations of the customer's instrument at the same beam quality. If the 

discrepancy is significant, greater than 2.0 %, but dependent on the chamber type, an investigation is 

warranted. When there are several previous calibrations of the customer's instrument at any one beam 

quality, one can estimate the reproducibility and decide whether the current value is acceptable.

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. Any 

discrepancy arising with a NIST check chamber greater than 0.5 % gives rise to a thorough 

investigation of the calibration procedure. Alignment, temperature indications, distance, etc., are to be 

checked again. If the discrepancy cannot be resolved, the complete calibration process is repeated.
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Assessment of Uncertainties

The method of uncertainty assessment follows the NIST policy of expressing uncertainty, as outlined in 

the NIST Technical Note 1297. Conventional statistical estimates are given as standard deviations of 

the mean, and are designated as “Type A”, which can be considered to be objective estimates. All other 

uncertainty estimates, which are designated “Type B”, are subjective estimates, based on extensive 

experience. The “Type B” uncertainties are estimated so as to correspond to approximately one standard 

deviation. The Type A and Type B estimates are combined according to the usual rule for combining 

standard deviations, by taking the square root of the sum of the squares (the quadratic sum). The 

quadratic sum of the two types of uncertainty is then considered to be the combined standard 

uncertainty, which is in turn multiplied by the coverage factor of two (k=2) to give an expanded 

uncertainty. The uncertainty is considered to have the approximate significance of a 95 % confidence 

limit. The appendix lists the details of the assessment of uncertainty in the air-kerma rates determined 

for the different gamma-ray beams. Also listed in the appendix are the details of the assessment of 

uncertainty in the calibration of a typical ionization chamber. 

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 the 

Appendix 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 first 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 
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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. 

Safety

The main safety consideration is radiation protection. As described below, every effort is made to avoid 

any possibility of radiation exposure, even though it would be highly unlikely that serious exposures 

could occur accidentally. Another safety consideration is exposure to high voltage, such as exists on 

ionization chambers and standard chambers during calibration. All radiation areas in the building are 

marked with striped tape and dosimeters must be worn by all personnel in these areas. Radiation safety 

training and assessment services are provided by the NIST Gaithersburg Radiation Safety Division.

Radiation Safety

All doors permitting access to the gamma-ray calibration ranges have interlocks as required by the 

Nuclear Regulatory Commission. The vertical-beam rooms have a time-delay device inside the room 

that must be actuated before leaving the radiation area. Automatic shielding doors protect occupants in 

the control area from the sources. In addition to the above safety features, a radiation detector with 

indicator lights and an audible signal is in each gamma calibration range. A second radiation detector 

located in the vertical-beam area, between the shielding door of the vertical beams and the outer door, 

alarms whenever the interlock is broken during an irradiation. At each entrance to a gamma-ray 

calibration range, a set of two red lights indicates a "beam on" condition.

High-Voltage Safety

The only danger that exists from high voltage comes from voltage that must be applied to the ionization 

chambers. To prevent dangerous electric shock, almost all power supplies contain current-limiting 

resistors in the high-voltage circuit. Common sense is dictated when working around ionization 

chambers that have exposed high-voltage electrodes. Appropriate warning signs are posted.

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 
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lists comparisons made in the last few years using the same 137Cs and 60Co gamma-ray beams that are 

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. Seltzer S. M., Bergstrom P., “Changes in the US Primary Standards for the Air Kerma from 

Gamma-Ray Beams,” in press.

2. Lamperti P.J., O’Brien M, “Calibration of X-ray and Gamma-ray Measuring Instruments,” NIST 

Special Publication 250-58, (2001). 

3. Loftus, T. P. and Weaver, J. T., Standardization of 60Co and 137Cs gamma-ray beams in terms of 

exposure, J. Res. Nat. Bur. Stand. (U.S.), 78A, 465-476 (1974). 

4. NIST Calibration Services Users Guide 1998.

5. NIST Technical Note 1297 Guidelines for Evaluating and Expressing the Uncertainty of NIST 

Measurements.

6. P.J. Allisy-Roberts and M. Boutillon, “Comparison of the standards of air kerma of the NIST and the 

BIPM for 60Co gamma rays,” Rapport BIPM 96/9 (1996).

7. A. Leitner, J. Witzani, M. Boutillon, P. Allisy-Roberts, F. Delaunay, E. Leroy, P. Lamperti, C. 

Strachotinsky and I. Csete, “ International comparisons of air kerma standards in 137Cs gamma 

radiation,” Metrologia 34, pp. 169-175 (1997).

8. R. F. Laitano, P.J. Lamperti and M. P. Toni, “Comparison of the NIST and ENEA Air Kerma 

Standards,” Journal of Research of the NIST, Volume 103, Number 4, July-August 1998.

9. I. Csete, A. G. 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 
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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. 

10. C. Kessler, P.J. Allisy-Roberts, R. Minniti, “Comparisons of the radiation protection standards for air kerma 

of the NIST and the BIPM for 60Co and 137Cs gamma radiation”, Metrologia 51 (2014) Tech. Suppl. 0613. 

doi: 10.1088/0026-1394/51/1A/06013.
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Appendices

A. Example Uncertainty Analysis

B. Sample Calibration Reports

C. Proficiency Test: Service 46050S
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Appendix A: Example Uncertainty Analysis

Uncertainty analysis for the primary-standard measurement of air kerma rate, K



, in the vertical beam facility (60Co beams). 

K1



and 

K10



represent the air kerma rate measured with each one of the primary standard chambers with nominal volumes 

of 1 cm3and 10 cm3, respectively. Values shown are for the relative standard uncertainties in %.

Uncertainty component

K1



K10



Type A Type B Type A Type B

charge 0.10 0.10 0.06 0.10

time 0.05 0.05

volume 0.10 0.10 0.16 0.10

air density correction (temperature and pressure) 0.03 0.03

distance (axial) 0.02 0.02

, loss of ionization due to recombination 0.01 0.05 0.05 0.10

stem scatter 0.05 0.05

axial nonuniformity 0.02 0.05

radial nonuniformity 0.01 0.01

density of dry air at T = 0 oC and P =101.325 kPa 0.02 0.02

humidity correction 0.06 0.06

kwall, wall correction 0.17 0.17

ratio of mean photon mass energy-absorption coefficients, 

air/graphite 0.04 0.04

product of Wair/e and ratio of mean electron mass electronic 

stopping powers, graphite/air 0.11 0.11

(1− g)

, radiative-loss correction 0.03 0.03

quadratic sums 0.14 0.28 0.18 0.29

relative combined standard uncertainties of 

K1



and 

K10



0.31 0.34

relative combined standard uncertainty of 

K



0.31

relative expanded (

k = 2

) uncertainty of 

K



0.62
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Uncertainty analysis for the calibration of a reference-class ionization chamber in terms of air kerma. Values are for the 

relative standard uncertainties in %.

Uncertainty component Type A Type B

charge 0.10 0.10

time 0.05

air density correction (temperature and pressure) 0.03

distance 0.02

sat

'

k

, loss of ionization due to recombination 0.01 0.05

probe orientation 0.01

humidity 0.06

60Co decay1 0.01

quadratic sum 0.10 0.14

relative combined standard uncertainty of the chamber current I 0.17

relative combined standard uncertainty of 

K



0.31

relative combined standard uncertainty of the calibration coefficient 

NK

0.36

relative expanded (

k = 2

) uncertainty of the calibration coefficient, 

NK 

0.71 (→ 0.8)

1 The air-kerma rate, determined by the primary-standard instruments, of the 60Co source is decay corrected to the time of the 

calibration measurement. For this correction, NIST uses a half life of 1925.3 days.
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Appendix B: Sample Calibration Report
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APPENDIX C: Service 46050S, Proficiency Test for air kerma from gamma-ray beams

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

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 air kerma rate at NIST is realized at 

the calibration position by a free-air ionization chamber for x radiation and by graphite cavity ionization chambers 

for 60Co and 137Cs gamma radiation, and is expressed in units of grays per second (Gy/s). This realization of the 

radiation quantity of air kerma establishes the National standard, which can in turn be transferred to other 

measurement facilities through a suitable measuring instrument, thus enabling traceability to the National standard. 

The gamma-ray air kerma rates are corrected to the date of calibration (from previously measured values) by decay 
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corrections based on half-lives of 5.27 years for 60Co and 30 years for 137Cs. 

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;
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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;

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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