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CALIBRATION OF X-RAY RADIATION DETECTORS

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Purpose

The purpose of this procedure is to describe all steps involved in the measurement of air kerma using the 

three, primary x-ray standard free-air ionization chambers required for the calibration service listed as 

46011C [1]. The details of the proficiency test service (46050S) are included in Appendix C.

Scope

The calibration and irradiation of instruments that measure x rays are performed in terms of the physical 

quantity air kerma. The process for establishing calibration coefficients (or factors) for radiation 

detectors is explained in this procedure. Calibrations are performed by comparing the instrument to a 

NIST primary standard, which includes three free-air chambers for x rays. 

Referenced documents

International Organization for Standardization

ISO/IS 4037-1:1996 X and gamma reference radiations for calibrating dosimeters and dose rate 

meters and for determining their responses as a function of photon energy--Part 1.: Radiation 

characteristics and production methods

Consultative Committee for Ionizing Radiation (CCRI)

BIPM, Qualités de rayonnement, Consultative Committee for Ionizing Radiation (CCEMRI) (Section I), 

1972, 2, R15.

National Institute of Standards and Technology

NBS Special Publication 250-16 Calibration of X-ray and Gamma-ray Measuring Instruments

NIST Special Publication 250-58 Calibration of X-ray and Gamma-ray Measuring Instruments

NIST Calibration Services Users Guide 1998

NBS Handbook 64 Design of Free-Air Ionization Chambers

NBS Handbook 78 Report of the International Commission on Radiological Units and 

Measurements

NIST Special Publication 811 Guide for the Use of the International System of Units (SI)

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

Measurements

Records

Laboratory data books

Binders

Electronic files
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Definitions 

air kerma - 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. The 

SI unit of air kerma is the gray (Gy).

beam quality - used to refer to a specific x-ray beam with a characteristic half-value layer and produced 

by a constant potential kilovoltage.

calibration - the process whereby the response of a dosimeter or measuring instrument is characterized 

through comparison with an appropriate national standard.

calibration coefficient - the quotient of the air kerma in the absence of the chamber and the charge 

generated by that radiation in the ionization chamber, expressed in units of Gy/C.

calibration factor - the quotient (dimensionless) of the air kerma or exposure in the absence of the 

chamber and the electrometer reading with the ionization chamber.

effective energy - the energy of a monoenergetic x-ray beam that has the same half-value layer as the 

spectrum in question.

exposure - exposure (X) is the quotient of dQ by dm, where dQ is the sum of the electrical charges on all 

the ions of one sign produced in air when all the electrons are completely stopped in air of mass dm. The 

SI unit of exposure is the coulomb per kilogram (C/kg); the special unit of exposure, the roentgen (R), is 

equal to exactly 2.58E-4 C/kg.

half-value layer - (HVL) the thickness of the specified material added as a beam attenuator that reduces 

the air-kerma rate by one half of the unattenuated-beam air-kerma-rate value.

homogeneity coefficient - (HC) the ratio of the first to the second half-value layer.

monitor instrument - an instrument used to monitor the stability of the air-kerma rate during an 

irradiation.

QMS – quality management system

quarter-value layer - (QVL) the thickness of the specified material added as a beam attenuator that 

reduces the air-kerma rate to one quarter of the unattenuated-beam air-kerma-rate value.

second half-value layer - the difference between the quarter-value layer and the half-value layer.

x-ray unit - system comprised of a high-voltage generator, an x-ray tube and an x-ray controller. 
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Key words

air kerma; calibration; exposure; free-air chamber; half-value layer; ionization chambers; mammography 

chamber calibrations; primary standard; standard; uncertainty estimate; x rays.

Background information

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

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

photon beams in air. A complete description of the determination of air kerma and the traceability of the

standards is found in sections 4.1, 6.2 and 6.8 of the NIST Special Publication 250-58.

Requirements of instruments to be calibrated

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.

Explanation of calibration service offered

An x-ray tube produces bremsstrahlung spectra, inhomogeneous beams with photon energies from very 

low values to a high-energy cutoff given by the maximum potential applied to the x-ray tube. These 

beams are customarily filtered with a high purity metal to reduce the unwanted low-energy x-rays. Four

x-ray calibration systems are used for the calibration services. Two of the systems contain x-ray tubes 

with tungsten anodes. The x-ray beams from these anodes are filtered with aluminum, copper, tin and/or 

lead. Two anode types are offered for the mammography calibration service, molybdenum (Mo) and 

rhodium (Rh). The Mo-generated x-ray beams are filtered with Mo, Al, or Rh foils, while the Rh beams 

are filtered with Rh and Al foils. It is conventional to characterize the "quality" of the filtered x-ray 

beam in terms of the thickness of aluminum or copper required to reduce the air kerma rate to 50 % and 

to 25 % of its original value. These thicknesses are called the half-value layer (HVL) and the quartervalue layer (QVL). The HVL and QVL measurements must be made using good-geometry attenuation 

in order to obtain accurate and reproducible numbers. The second HVL is the difference between the 

QVL and HVL. The homogeneity coefficient (HC) is the ratio of the first to the second HVL, often 

expressed as a percent. A HC value near 1 (or 100 %) indicates that the filtration has produced an 

approximately homogeneous beam that is approaching monoenergetic conditions.

The NIST tungsten x-ray beam qualities are divided into three groups according to filtration, i.e., light 

(L), moderate (M), and heavy (H) filtration. The beam codes consist of a letter L, M, or H, followed by 

the generating constant potential in kilovolts. For example, M100 indicates moderate filtration and 100 

kV constant potential. The special (S) series beam codes, S60 and S75, have characteristics that are not 

consistent with those of the L, M, and H groups. The qualities for each group were chosen so that 
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relatively smooth curves result for the graph of tube potential versus HVL. Table 1 gives a complete 

listing of the available NIST beam codes. Depending on the energy response and design of the 

ionization chamber, the calibration coefficients for a specific ionization chamber often fall on smooth 

curves when plotted against HVL. In this case, all calibration points have been chosen from a single 

group, L, M, or H. If calibration points are chosen from more than one group, discontinuities will occur, 

hence no attempt should be made to interpolate between such calibration factors.

The mammography beam qualities offered at NIST were chosen to cover the range of HVLs of x-ray 

beams found in clinical settings. The beam codes that name the beam qualities are a combination of the 

chemical symbol of the anode and the filter respectively, followed by the constant potential in kilovolts. 

The letter "x" ends the beam codes that name the exit beam qualities. The exit beam qualities, which 

represent the transmission of the x-rays through the breast, are generated by an additional filtration of 

2.0 mm of Al. The mammography beam qualities offered are listed in Table 2. 

NIST maintains reference beam qualities from two international organizations, the ISO and the 

CCEMRI (now known as the CCRI). These beams are generally used for international traceability and 

measurement comparisons. A list of all ISO beams offered at NIST is found in Table 3a, using the ISO 

4037 1996 version; the changes to the reference beams from the updated ISO4037 have not been 

implemented at NIST. The NIST H group of qualities agrees with the ISO narrow spectrum (NS) 

qualities recommended by the ISO document 4037. The NIST M group of qualities is in agreement with 

the recommendation for radiation therapy calibration in IEC Publication 731. NIST supports the 

reference CCRI beam qualities, 25 kV and above, described in the CCEMRI/CCRI (1972) document for 

traceability with the BIPM. The NIST techniques based on these beams are listed in Table 3b.

The selection of beam qualities for instrument calibration depends on the situation of interest. The H 

qualities are usually used for calibration of radiation-protection instrumentation, as these beams have the 

narrowest spectrum at each generating potential, and probably most nearly approximate radiation that 

has penetrated a protective barrier. The M qualities are usually used for calibration of radiation-therapy 

instruments. The L qualities are predominately for calibration of instruments used for measurement of 

unfiltered or lightly filtered beams that give high exposure rates, as is often the case in radiation biology 

and Grenz-ray therapy. The Mo and Rh beam qualities are offered to simulate the clinical 

mammography beams.

Design philosophy and theory

X-ray calibrations are performed by using the substitution method. Using this method, the air kerma or 

air-kerma rate is determined at some point in space by a free-air chamber. The instrument to be 

calibrated is then placed at the same point in space as the standard and the response of the instrument is 

determined. The calibration coefficient is the quotient of the air kerma, Kair in the absence of the 

chamber, and the charge, Q, generated by that radiation in the ionization chamber:

Q

K CalibrationCoefficient=

air
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The calibration factor is the quotient of the air kerma or exposure, in the absence of the chamber, and the 

electrometer reading with the ionization chamber:

electrometer reading

K Calibration Factor=

air

electrometer reading

X

Calibration Factor=

X-ray facility features

Four x-ray calibration systems are available for instrument calibrations; the x-ray calibrators and the 

primary standards are shown in Figure 1, which shows H127-3, the NIST X-Ray Calibration Range and

the linear positioning system. One system is used for tungsten x rays generated at constant potentials of 

10 kV to 100 kV, another is used for tungsten x rays generated at potentials of 50 kV to 300 kV. The 

mammography x rays are generated at constant potentials of 23 kV to 40 kV either using the Mo or the 

Rh system. Standardization of x-ray beams for the quantity air kerma or exposure is carried out at NIST 

by means of three free-air ionization chamber standards. These standards are discussed in Refs. [3, 4, 5 

and 6]. Features critical to the proper use of the standards are listed in Table 4. All the standards are 

shown in Figure 2 and Figure 3, except for the Lamperti chamber. The Lamperti primary free-air 

chamber has been established as a primary standard for low energy x-rays so it is included, as an 

available standard and offers redundancy. Currently it is dedicated for use in the Electronic 

Brachytherapy facility. 
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Figure 1 The NIST X-Ray Calibration Facility showing primary standards and the linear positioning 

system.

Figure 2a Position of the device under test (DUT) from left to right: 300 kV FAC, T1, T2, 100 kV FAC, 

Mammo FAC, T3 and T4 .
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Figure 2b Position of the device under test (DUT) from right to left: 300 kV FAC, T1, T2, 100 kV FAC, 

Mammo FAC, T3 and T4 .
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Figure 3 The primary x-ray standards, showing the alignment points.

Reference alignment point

Wyckoff-Attix

Attix Ritz

Lamperti
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Features of the x-ray systems are listed in Table 5. The equipment room, H127-2, is shown in Figure 4 

and contains the x-ray generators, heat exchangers, voltage dividers, power disconnects, safety interlock 

controls, and junction control box. H127-2 is connected to the x-ray range and control area by custom 

shielding ports for the HV cable, oil and water-cooling lines. The Figure 4 inset pictures show the 

junction control interface and a generator control interface. Figure 4a shows the past customer 

calibration records which are stored in file cabinets at the end of the equipment room. Figure 4b shows 

the GE voltage dividers which are used for the 300 kV unit and a Pantak divider used for the 100 kV 

unit. The equipment room is shielded from ionizing radiation and accessible during the production of xrays. The Kimtron operating procedure, located in Appendix A, describes the use of the equipment.

Figure 4 The Equipment Room, showing location of junction control interface and interlocks.

 

 

Figure 4a Customer records Figure 4b Voltage dividers, GE on left and Pantak on right.
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The facility is equipped with a storage room for equipment, equipment manuals, quality equipment 

calibration records, shipping crates, and security for the vast array of secondary standards and high 

purity metals. This room has dedicated space for the inspection, storing and packing of the customer’s 

chambers. Figure 5, on the left, shows the NIST collection of secondary, reference class chambers and 

high purity metals. The bookcase seen at the end of the storage room has all quality management system 

(QMS) manuals and calibration support equipment records. Figure 5, on the right, shows NIST 

redundancy equipment storage as well as the shipping preparation area.

Figure 5 The storage room, showing NIST secondary standards on the left and calibration redundancy 

equipment on the right with customer shipping station.

The control area of the NIST x-ray calibration facility shows access to the three connecting rooms in 

Figure 6. The operator control center, laboratory benches, and workspace to facilitate measurement 

exchanges are contained in the large open area which was designed to allow movement of large 

equipment used during some measurement tests. The cable management network is shown in this picture 

connecting all rooms. The open control area allows for viewing of the calibration range through a lead 

glass window.

Figure 6 X-ray facility control area
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X-ray calibration data

Data are acquired for x-ray standardization by measurement of all conditions relevant to establishing the 

x-ray air-kerma rate at a particular distance from the x-ray-tube target. The data acquired consists of 

data for the computation of ionization currents, parameter data, and measurement-system test and 

information data. Data used to determine the ionization currents includes the electrometer charge 

measurements, the atmospheric pressure and pertinent air temperatures, and the shutter-open time 

interval. The parameter data includes the x-ray tube current and potential, the x-ray tube 

target-to-reference-point distance, and the beam-defining filter thickness and the diaphragm diameters. 

Test data includes the measurements of the collection potentials on the standard free-air chamber, the 

monitor chamber, and the chamber being calibrated, if appropriate. 

Ionization-chamber current-measurement techniques

Ionization currents in air-kerma-standardization measurements are produced by the irradiation of a gas 

in an ionization chamber. The ionization chamber may be a free-air chamber, such as one of the 

national standard chambers, or a cavity chamber, where the gas is surrounded by some wall material. 

Ionization chambers, regardless of type, consist of electrodes that are insulated from one another and are 

polarized in order to collect charge produced in the gas. The ions produced in the air by the beam are 

swept from the chamber volume by the electric field between the electrodes. Included in the 

measurement of these currents are currents not produced by the radiation of interest, but by background 

radiation and insulator leakage, which is referred to as background current. The magnitude and sign of 

these extraneous currents must be determined and the measured current corrected for their effects in 

order to determine the true ionization current. The importance of the correction for background and 

leakage is, of course, relative to the magnitude of the ionization current, but good measurement 

technique requires, prior to attempting radiation measurements, that the background currents be 

determined. As a rule of thumb, and without taking special precautions, the background current for a 

good-quality ionization chamber should be less than 5 fA. The measurement of background currents 

will also include currents due to background radiation, so the environment and special circumstances 

must be considered in evaluating data. For example, if tests are made on a large-volume ionization 

chamber in a background environment found suitable for small-volume chambers, the extra sensitivity 

of the large chamber requires separate evaluation of the background environment. Keithley 617, 6512 

and 6514 electrometers are used to measure the charge collected in the ionization chambers.
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Special operating features for the Attix Chamber

The Attix chamber, a variable-length, cylindrical free-air chamber, differs in design from the other three 

NIST conventional parallel-plate free-air ionization chambers. The differences contribute to its 

appropriateness as a standard for the measurement of exposure in the mammography energy region. 

This measurement procedure is based on a subtraction method [7], which involves finding the difference 

in collected charge for different electrode lengths. The chamber is composed of an aluminum cylinder 

with a fixed front plate, a variable-position back plate, and an off-center electrode. The cylinder and 

back plate are positioned with precision stepping-motor controlled slides. For a detailed description of 

the chamber see Refs. [8 and 9]. 

The air-kerma determination with the Attix chamber involves the collection of charge with various plate 

configurations. By changing the volume of the Attix chamber and knowing the corresponding change in 

length of the collecting electrode, the air kerma can be determined with a minimum of two different 

plate configurations. Although the minimum number of plate configurations needed to determine the air 

kerma is two, four measurements are conducted, with the fourth being a repeat of the first position. The 

electrode length is typically changed by 5 cm with each plate configuration. The average of the three 

resulting ratios of the change in charge to change in electrode length is calculated and used in the airkerma calculation as a component of the mass ionization current. For routine measurements, the 

defining point of the Attix chamber is positioned at one meter from the focal spot of the x-ray source. 

Figure 7 shows the Attix chamber configuration used for the measurement procedure. 

Figure 7 Attix primary standard measurement positions.

25 cm

Chamber Center

21.27 cm from Aperture

Aperture

1 m from source

20 cm

10.5 cm

8 cm

15 cm

Chamber Center

21.27 cm from Aperture

Aperture

1 m from source

10 cm

15.5 cm

13 cm

21.27 cm from aperture 

121.27 cm from source

21.27 cm from aperture 

121.27 cm from source
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Equipment

The required equipment is listed in Table 6.

All data are acquired through National Instrument's LabVIEW interfacing using the Hopewell Designs 

Inc. (HDI) controller, which is contained in the control console, seen in Figure 8 and shown enlarged in 

Figure 8a. The interaction with the HDI controller is explained in various procedures. The open and 

close buttons are used to control the shutters for non-automated shutter control. The x-ray tube keys are 

part of the interlock system and required for production of x-rays and the emergency off switch is 

connected to all motion associated with the LPS and the shutters and is explained in the safety feature 

section.

Figure 8 X-ray control console

Figure 8a HDI controller and x-ray tube key interlock
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HDI calibrators

The information on the HDI calibrators is from the HDI documentation. HDI designed the calibrators

specific for the geometry used at NIST. The HDI calibrator consists of a shielded enclosure of the tube 

head, head mount, cables, and high-speed shutter. The enclosure is made of steel, lead, and steel 

laminate with a nominal thickness of 15 mm between two layers of sheet metal. Exposure rates are 

limited to less than 5 mR/h at the surface of the enclosure when the tube is operating at any setting. 

When the side doors of the cabinet are opened, an interlock disables power to the tube head. Baffled 

penetrations allow the cables to exit without increasing radiation exposure rates. The axis of the 

radiation beam is horizontal and is 51 inches above floor height. Figure 9 shows the three calibrators, 

left to right in this order: mammography, 100 kV system and 300 kV system. The 300 kV tube is 

contained in the calibrator closest to the north wall. The right picture of Figure 9 shows the 300 kV tube 

with one of the side doors opened. The 100 kV tube is in the middle calibrator and the mammography

calibrator closest to the shielding door contains the Mo and Rh tubes. 

Figure 9 The three x-ray calibrators, showing the interior of the 300 kV calibrator with x-ray tube.
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The HDI mammography calibrator is of a different design than the other HDI calibrators, modified to 

incorporate the previous tube translator. The Mo and Rh x-ray tubes, shielded to 60 kV, are fully 

enclosed in 3⁄4 inch thick steel, which contains the previously used, lead-lined translating tube 

positioning mechanism. The unit consists of the translation tube support, shown in Figure 10, a 

collimated opening, the tungsten shutter and a pneumatic solenoid. Figure 10a shows the side view of 

the tube support with positioning interlock switches and Figure 10b is a closeup of the shutter 

collimator.

Figure 10 The mammography translation tube support.

Figure 10a Positioning switches

Figure 10b Tube collimator
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The HDI calibrator consists of a collimated opening, the tungsten shutter, a pivot mechanism, and the 

pneumatic solenoid. A lead collimator is installed in the beam port to establish a beam with an 

approximate 15-degree to 17-degree solid angle. The high-speed shutter consists of a tungsten disk that 

is 50 mm diameter x 25 mm thick (2 in. dia. x 1.125 in. thick) and completely covers the beam port to 

block radiation when closed. An electric solenoid operates an air cylinder to open and close the shutter 

in less than 150 milliseconds. Flow control valves allow the shutter speed to be controlled and shock 

absorbers slow movement at the end of travel. Electric switches indicate when the shutter is fully open 

or closed. When power is turned off to the solenoid, air pressure and a spring pulls the shutter closed in 

the same amount of time. A Panasonic FP0H PLC is used for the timing of each shutter which is 

triggered by a pair of solid-state switches mounted directly to the actuating air cylinder. The timer 

initiates once the shutter shielded switch is deactivated as the shutter uncovers the beamline. At the end 

of the preset exposure time, the shutter is signaled to close, which activates the exposed sensor and the 

duration time is achieved. The opening and closing transition should deliver approximately half the 

dose rate, so by counting during one transition, the average results in the elapsed time. The shutter 

motion open and close times are independently tracked and monitored. During the installation of the

shutters a quartz-crystal digital timer, accuracy of 0.001 %, was used to establish traceability for the 

PLC times. The shutter open and close transition times are initially adjusted by the switch placement and 

air pressure regulators, resulting in balanced shutter motion. Figure 11 shows the interior of the 100 kV 

x-ray tube and the closeup shutter details on the left and center and the 300 kV tube and shutter on the 

right. 

Figure 11 The 100 kV tube and closeup of the shutter assembly on left and center picture and the 300 

kV tube shown in the right picture.
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A pressure sensor for the compressed air shutter operation is contained in each calibrator and is 

connected to the pressure gauge in the interlock junction box in H127-2. Figure 12 shows the access to 

the pressure sensor at the bottom of the calibrator. The pictures on the right of Figure 12 show the 

interior of the calibrator bottom with the interlock system and the adjustable pressure sensor with a 

closeup of the pressor dial that displays the setpoint by the green pin, the lower limit by the green needle 

and the actual pressure by the black needle.

Figure 12 Calibrator pressure sensor and indicator controls.

Linear positioning system

The NIST x-ray calibration facility features a precision motion controlled 2 axis, X-travel: 3 m x Ytravel: 4 m, linear positioning system (LPS) that translates across the x-ray beam lines of three x-ray 
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calibrators allowing the direct comparison of the three primary x-ray standards and radiation detection 

instrumentation at source to detector distances from 60 cm to 370 cm, repositioning in both directions to 

within 0.05 mm. The platform of the LPS was designed to incorporate the unique positioning 

requirements of the three NIST primary x-ray standards and a HPGe detector and 3000 lbs to the same 

beam height of 51 inches, shown in Figure 2. The LPS features a linear encoder that has a resolution of 2 

microns. The HDI Operations and Maintenance Manual for the Sliding Stage Positioning System, 

shown in Figure 13 is located near the HDI console and explains the use of the LPS to the qualified 

trained operator. The manual includes the procedure for using the LPS with the HMI touchscreen 

located on the telescope stand, shown in Figure 14. The description for use of the LPS control PC 

software, interface shown in Figure 15, is also described in the manual. The configuration files for the 

LPS software are located on the HDI control computer: C:\ProgramData\HDI\NIST\X-Ray 

Lab\LPS\LabVIEW\X-Ray LPS\ParkerIPADrive Config XAxis.ini

C:\ProgramData\HDI\NIST\X-Ray Lab\LPS\LabVIEW\X-Ray LPS\ParkerIPADrive Config YAxis.ini

C:\ProgramData\HDI\NIST\X-Ray Lab\LPS\LabVIEW\X-Ray LPS\YAxis Presets.ini

The Y-Axis position presets are seen on the LPS vi. If a new preset is necessary, the change is made 

from the user interface and saved to the configuration file for future use as a pre-set.

Figure 15 LPS software interface

Alignment Lasers and instrument positioning controls

The use of the alignment lasers and instrument positioning controls is explained in the section, Set-up 

procedure using the alignment lasers. Two alignment lasers are essential for the calibration chamber 

Figure 13 HDI LPS manual. Figure 14 LPS touchscreen control 
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alignment process. One laser is referred to as the telescope alignment laser and verifies the plane of the 

alignment of all instruments. It is attached to the telescope stand, below the telescope. The other is 

mounted on the east wall, located between the EMO switch and the 100 kV tube laser. It is referred to as

the beam height alignment laser; both are shown in Figure 16. The four hand controls for the test 

chambers are also shown and labeled right to left at T1, T2, T3 and T4. These correspond to the test 

chamber positions.

Figure 16 Alignment lasers and test chamber positioning controls.
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Electrometers

The charge measurements are acquired through the use of Keithley 617, 617-HiQ, 6514 and 6512 

electrometers. The charge ranges of the electrometers are calibrated upon introduction into the system 

and any time the charge collection is suspect. Any necessary correction would be applied as part of the 

air-kerma calculation in the LabVIEW software, however no such calibration correction is required. The 

Keithley 6512 and 617 electrometers are interchangeable in the HDI data acquisition system. The HiQ is 

dedicated for use if a monitor chamber is required. The 20 nC capacitance range electrometer is used for 

the customer chamber. The 6514 electrometer is available as a backup or redundancy for the standard 

electrometer. LabVIEW control software is available for a seamless transition for the use of the 6514, 

and eventually for other electrometers as funds exist.

Temperature indicators

The temperature sensors are various models of platinum resistance probes and thermistors, and 

Hart/Fluke digital thermometers that are used to measure room, free-air chamber, and monitor-air 

temperatures. The thermometer calibration correction is applied internally to the digital meters. A 

Fluke/Hart meter 1504 (sn A95694) with probe model 5610-5 SNA932006 serves as the “in-house” 

temperature reference standard for x- and gamma-ray calibrations. The meter/probe will continue to be 

calibrated periodically as necessary to the NIST primary standard. Various probe and meter 

combinations are periodically directly calibrated by the NIST temperature experts. One liquid-in-glass 

thermometer that was a previous lab reference is maintained, but will no longer be calibrated, for 

resolving possible measurement discrepancies.

Pressure indicators

The atmospheric pressure for the x-ray range is measured by the Setra 370 digital barometer. No 

calibration correction is necessary, but if one was necessary, the correction would be documented and 

applied in the LabVIEW software. An aneroid barometer, Wallace and Tierman, Model FA 139, Serial 

Number XX11242 is the “in-house”, lab standard and has direct NIST traceability. Some of the Setra 

370 barometers have direct NIST traceability, serving as a backup or secondary standard. All barometers 

are compared to the in-house lab standard routinely. Calibrations of the lab barometers used at the 

various facilities are made by placing the “in- house” standard barometer alongside the instrument to be 

tested. If a correction factor is required, it would be clearly identified on the meter. This calibration 

procedure is conducted periodically.

Voltage dividers

Two customized voltage dividers were manufactured for the purpose of voltage calibration of the x-ray 

generators used in the calibration range. Both dividers were initially calibrated by the NIST Electricity 

group. These dividers are periodically used for routine calibrations and for measurement comparisons. 

A check of the voltage calibration is required after any of the x-ray generators are changed due to 

maintenance or replacement. The dividers must be directly placed in series with the generator cables 

into the x-ray tube and require a knowledgeable, experienced person for installation. A high precision 
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voltage meter must be used to read each divider. This equipment is listed in Table 6. Two commercially 

available GE voltage dividers are dedicated for use on the 300 kV system. Obtaining direct voltage 

calibrations for this specialized equipment is difficult and is not a service routinely offered at NIST. 

Support equipment calibration and validation

The temperature probes, pressure transducers and electrometers are considered essential support 

equipment to allow routine calibrations of ionization chambers. The details of maintenance and 

calibration of this support equipment follows. The quality of the ionization chamber calibration is not 

monitored by the calibration of the support equipment but rather by the NIST artifact quality procedure. 

The quality check of the calibration of all customer ionization chambers is determined by the 

reproducibility of the calibration coefficients of the NIST test chamber. Therefore, if a calibration 

coefficient 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, described below, if there is a question of 

reproducibility of the NIST transfer ionization chamber. Since the QA procedure for all x-ray 

calibrations requires the calibration of a NIST 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 and the instrument would be clearly identified on the Access equipment log 

in the data acquisition software, which serves as the instrument log for calibrations. All new equipment 

is calibrated against the “in house” reference standard and history is created and maintained. The new 

equipment is checked to verify it meets the specifications of the stated procurement, using the “in 

house” standards. The validation records are kept in L:\internal\846.02\x-ray\QMSvalidateH127.xlsx

located on the group server for calibration data, in the x-ray folder. The tracking of all equipment used 

for calibrations is performed by the LabVIEW data acquisition system and stored in Access log sheets. 

All NIST issued calibration reports for the critical support equipment are located in H127-1. In-house 

calibration records for the barometers and thermometers are maintained in electronic files in the folder 

named supportequipqa located on the group server for calibration data, in the x-ray folder.
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Procedures

Administrative procedures

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

establish contact with the Dosimetry Group by email, requesting information regarding techniques 

offered, charges, turnaround time, and shipping information. At this stage, there is generally an 

opportunity to discuss with the prospective customer the type of service being requested and methods of 

shipment to reduce the risk of damage. The customer is informed of the administrative procedures for 

acceptance of the requested work as outlined in the NIST QMI and in the RPD QMII, in accordance 

with the NIST calibration policy. In addition to completing the NIST payment authorization 

requirements, the customer must include a detailed description of the calibration request, instrument 

model and serial numbers, name and telephone number of a technical contact, the official address which 

will appear on the calibration report, the return shipping instructions including the address, any special 

handling requirements, the specified mail carrier with account number for payment, the value of the 

equipment, and instructions for the insurance amount, if any. The customer is directed to the complete 

instructions and policies for calibrations on the NIST calibration ordering webpage. The Calibration 

Information request form, see Table 8, can be completed by the NIST technical contact, as a method to 

organize the dates and information about the calibration request. The NIST order number is generated by 

the NIST online ordering system upon completion of the NIST policy for payment documentation. At 

the completion of the calibration, a copy of the purchase order if available, the final copy of the 

calibration report, summary sheets, and any significant documents of correspondence concerning the 

calibration are maintained in the customer’s calibration report folder filed by the unique dosimetry group 

(DG) number.

 

When instruments arrive for calibration, they are unpacked and inspected for damage. The customer 

chamber is stored in the H127-1 storage room on the customer shelf until the scheduled calibration. 

Special attention is given to the condition and type of connector on the customer chamber. If an adaptor 

is sent with the chamber, it should be noted on the inventory list along with the description of the 

chamber. Shipping damage is reported to the NIST shipping department. If an instrument arrives in a 

state of disrepair that is obvious by visual inspection, the customer would be notified, and the instrument 

returned without calibration. The instrument would be rejected if the window is obviously dented or 

punctured or if the chamber cable is damaged. If the shipping box is visually damaged, this should be 

noted, and the customer consulted.

After the instrument is calibrated, the calibration report is generated. Templates generated in Microsoft 

Word and Excel are available to simplify this procedure and to ensure consistency in the reporting 

format. The electronic reports are finalized with an official NIST logo. A sample report is found in 

Appendix B. A sample NIST report is provided in this procedure with the required elements identified 

by the corresponding letter in red font. Upon completion of the requested calibration, the final 

calibration report is reviewed and initialed by the preparer, the technical reviewer, the Group Leader and 

the Division Chief, electronic signatures are acceptable. A copy of the signed report is maintained in the 

customer’s DG folder located in room H127 of building 245 and in the customer’s electronic file. The

original or an electronic equivalent is sent to the customer within the container of the returned 

equipment, by email or by mail, if requested by the customer. An electronic version of the final report is 
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uploaded to the E-commerce ordering system. The instrument(s) is(are) then repackaged in the original 

container, or a more suitable one, for return shipping. The shipping page of the NIST online ordering 

system is printed and attached to the box. The NIST shipping department collects the box from outside 

of H127 or the designated collection point. 

General overview of the X-ray calibration process

X-ray calibrations are performed using the substitution method: the standard is used to measure the airkerma rate at a given point; the instrument to be calibrated is placed at that point and exposed to the 

same calibrated x-ray source under the same conditions. The probe's response is normalized to 101.325 

kPa (760 mmHg) pressure and 295.15 K (22 C) when the instrument is open to the atmosphere.

After mounting and aligning the chamber in the holder and applying the appropriate chamber voltage, 

which is often requested by the customer, the applied chamber bias is checked at the chamber voltage 

connection box located near the chamber mounts, shown in Figure 17. This verifies the voltage 

connection has been made. Certain triax-to-BNC adapters can accidentally ground, and a short circuit 

occurs if the triaxial portion of the adapter, which is held at high voltage, is not insulated; see Figure 18.

To prevent electrical shock, the adapter is insulated with heat shrink insulator and secured in place so 

that movement of the instrument does not remove the insulator; see orange and white insulators in 

Figure 17. Effort is made to keep all signal connections out of the radiation beam when possible.

Figure 17 Test chamber voltage connection boxes. 

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

assurance. Generally, the selected NIST QA chamber is similar in design or collection volume to the 

customer chamber. A NIST chamber is selected which has a previous calibration history to the 

reference radiation qualities in use for the customer calibration. 

After a pre-irradiation of around 1000 seconds, background current measurements are taken prior to 

calibration. If the background current is elevated compared to the exposure reading, either the connector 

of the chamber is inspected and resecured, or the chamber is not calibrated. This is rarely the case, since 

customers are asked to verify the function of the chamber prior to shipment to NIST. A typical signal to 

noise ratio is 0.01 %. In addition, background current measurements are made at the time of calibration. 

Figure 18 Chamber triax to BNC adapters
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Environmental parameters

During all calibrations, the laboratory temperature must be maintained between 22 C +/- 2 C and 

stable to +/- 0.1 C for typical measurement sets of 10 minutes. If the temperature is not stable during a 

typical measurement set, calibrations should be postponed. The laboratory humidity should be 

maintained between 20 % and 50 % relative humidity. Since the pressure is monitored and the charge 

data is normalized to pressure, it is good laboratory practice to postpone data collection and calibration 

work during sudden changes in pressure due to storms or weather fronts. If the pressure reading is 

stable during the calibration collection time, the measurements can continue. The pressure should be 

monitored and the influence on the normalization factor should be considered.

Set-up procedure using the alignment lasers

Prior to calibration, the position of the free-air ionization chamber (FAC) must be established and used 

to position the NIST QA chamber and the customer test chamber to the x-ray beam in use. The beam 

height alignment laser on the east wall is the position on the LPS that each chamber must be placed to 

verify the correct height. The laser on the LPS telescope stand is used to set the distance from the 

source. The defining point on the FAC aperture should be verified by the laser spot of the telescope 

stand laser. The center dot of the laser should shine on the alignment point of the FAC. Figure 19 shows 

all the alignment points on the standards with the laser dots. The telescope is used as a redundancy for 

positioning, see Figure 16. The cross-hair reticle of the telemicroscope is set to the defining plane of the 

appropriate free-air chamber prior to alignment of the test chambers. For the Wyckoff-Attix (50 kV to 

300 kV) chamber, the defining plane is the white line on the aperture holder. For the Ritz (20 kV to 100 

kV) chamber, the defining plane is exactly 15.00 mm beyond the white line on the aperture holder in the 

downstream direction from the x-ray source. For the Lamperti (10 kV to 20 kV) chamber, which has 

been dedicated to electronic brachytherapy calibrations, located in H116 but could still be used in the xray facility if needed, the defining plane is exactly 20.00 mm downstream from the source from the 

scribed line on the plastic insert that fits into the removable shield and touches the front face of the 

aperture. For the Attix (10 kV to 50 kV) chamber, the defining plane is the scribed line in the brass 

aperture holder. 

Figure 19 Alignment points of standards, showing laser crosshair.
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Once it is verified that the telescope alignment laser hits the defining point on the aperture and the crosshair of the telescope is established, if required, the NIST QA chamber position T1 or T3 is translated to 

the beam height alignment laser. The NIST QA chamber is placed in a holder and the height laser beam 

is used to determine vertical and horizontal alignment. Adjustment of the NIST QA chamber position to 

the defining plane, distance from the source, is accomplished by use of the telescope alignment laser and 

also sighting through the telemicroscope that was previously aligned on the defining plane of the free-air 

chamber. Motorized slides are then used to adjust the NIST QA chamber reference point to the standard 

defining plane. Control of the motorized slides is accomplished using the corresponding hand operated 

control box located on the telemicroscope stand, see Figure 16. In general, all chambers should be 

aligned to the center of the test-chamber volume, but this should be verified with the customer. Most 

chambers are mounted perpendicular to the beam axis. If the chamber has a window, then it should be 

mounted such that the window faces the x-ray source. If a chamber has a reference line, then it should 

be used for alignment purposes. An identification mark or serial number should be used to reference the 

rotation with respect to the x-ray source. The alignment conditions, orientation and rotation of the 

chamber should be recorded in the calibration report. This process is then repeated for the customer 

chamber which will be placed in position T2 or T4 by translation of the LPS.

For large diameter chambers, larger than the field of view of the telescope, the use of the telescope to 

verify the position is more difficult. After centering the chamber on the alignment laser dots, the scale 

reading of the electronic telemicroscope and the diameter of the chamber can be used to verify the 

position. A further consideration in the setup for x-ray calibrations is the choice of an appropriate beam 

size. For this purpose, a parameter called the "beam size" is compared to the largest dimension of the 

active volume of the chamber. The general 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. For x rays, as opposed to high-energy gamma rays, this is of secondary importance because the 

chamber stem attenuates the radiation considerably. The beam size of the x-ray beams, for all beamdefining apertures for the vertical and horizontal beam positions, is determined using an Exradin A1 ion 

chamber, using the method described in the ISO4037. The ratio of the intensity as measured with the ion 

chamber of the center to the outer point of the useful beam should also change by less than 0.5 percent. 

Table 9 lists the possible beam sizes for each calibration range. The aperture defining beam size is set in 

the HDI software with the selection of the aperture in use. The operator must select a beam size that 

fully irradiates the chamber. Once selected, the same beam defining aperture is used for the FAC, the 

NIST QA chamber and the customer chamber.
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Procedures for the calibration of chambers and collection of data

The following procedures are listed in numerical order. The actual order in which the steps are 

completed is required only where noted. 

100 kV or 300 kV x-ray calibration range

1. Turn on the HDI controller using the key on the front panel. This energizes the communication 

junction box that controls all systems. 

2. Turn on water supply and return for the 300 kV system. Failure to turn on water will result in an 

interlock error. The heat exchanger for the 100 kV system is a closed loop and energizes automatically 

during the Kimtron warmup procedure. 

3. Follow the Procedure for the Use of the Kimtron; Appendix A.

4. Data entry in Access

a. start Access and use C:\100-300 XRay Sys\100300 NIST Track Revcurrent#

b. select the form tab

c. select the switchboard option

d. enter appropriate data under the customer, instrument and work order options

5. Start the HDI LabVIEW software using the provided shortcut for Revcurrent#.vi

a. login by entering operator’s initials (CMO) and the configuration code (456)

b. select the range to use. The default is the 300 kV range; for the 100 kV range, click on the 

pink “300 x-ray system” selector key in the upper right of the front panel HDI software to toggle 

to the blue 100 kV system

6. Run the LPS vi, homing the x and y axes and secure LPS in the desired position and distance.

7. Mount chambers as described previously using the lasers and telescope for redundancy. Connect 

signal and high-voltage cables. 

8. Verify that the correct filter wheel is mounted; if not, then repeat the homing procedure of the filter 

wheel and verify that the filter corresponds to the selection. If an M-series beam is being used on the 300 

kV range, verify that the additional M-filtration was put in place.

9. Verify that all support equipment is energized.

10. Verify that the signal lead of the FAC in use is connected to the FAC electrometer.

11. Once the chambers are positioned and secured, apply high voltage to the test chamber and the NIST 

QA chamber. Enter the desired high voltage on the front panel display of the HDI software and flip the 

Bertan HV switch on. The voltage is maintained on the primary standards, but make a visual check that 

voltage is set on the Bertans and also that the 300 kV and the 100 kV FAC DC microampere meters 
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show applied voltage. See Figure 20 for the location and expected reading for the FACs. The 

mammography FAC does not have an independent meter and the application of voltage is assessed by 

performance. As the chamber bias of the FACs is well above the safe range for a hand-held voltmeter, 

never test the FAC bias at the required operating voltage. Select a lower voltage of nominally 100 volts 

to 300 volts.

Figure 20 The Ritz (left) and Wyckoff-Attix (right) chamber meters showing voltage application.

12. Close the shielding door and set the LMO switch to allow the use of the x-ray beam.

13. Verify that the HDI software is communicating with the Kimtron generator

14. Verify the collection time so as to not overload the electrometers. This depends on the air-kerma

rate for the beam quality. Put each chamber in the beam using the conditions for the calibration and 

establish the collection time. Press the “exposure enable” at the bottom of the screen so it is not red. 

Secure the interlocks for entrance to the rooms. Press the red “open” button on the HDI control panel to 

open the shutter. The time of the exposure will accumulate on the front panel of the HDI software.

 

15. Press “build sequence” to prepare an automated test sequence.
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16. Enter the work order number of the previously entered data in Access. When the “get work order” 

button is selected, the background information for the customer chamber automatically is entered. The 

toggle switch for the customer chamber position should correspond to the position used.

17. Create a unique cal run ID.

18. Enter the pressure, temperature and humidity from the front-panel displays.

19. Enter the beam aperture selected, the diameter of the beam, and the distance of the calibration.

20. Enter the identification information of the NIST reference-class chamber to be calibrated.

21. Enter the desired collection times and the number of measurements to be collected.

22. Finally, select the beam codes, enter the currents and select the ISO Al filter if appropriate.

23. Upon completion, press “build sequence”.

24. At the front panel press “start auto sequence” to begin the calibration.

25. Upon completion of the calibration, review the data from the printed reports. If some data needs to 

be eliminated go to Access.

a. open the “exphist” table and find the data points to delete. Place a check in the appropriate 

column to delete the data point. 

b. on the front panel of the HDI software select “Redo Report”. Enter the work order number 

and the unique cal run ID. Press “get records”. Select the appropriate beam code. Repeat until all 

data is recalculated.

26. A summary of the final data for both chambers is stored to an Access file named InstCalReport. 

Select the records desired and copy to an Excel spreadsheet to complete the summary calculations. The 

spreadsheet should be named by the DG number and stored in the locations as described below.

27. Repeat until a sufficient number of calibrations have been completed for each beam quality, 

typically three to five. 

28. The expected standard deviation on the calibration results depends on the chamber type but,

generally, a standard deviation near 0.1 % is acceptable. 

29. The averages of multiple calibrations are maintained in the Excel spreadsheets. These Excel files are 

named by the DG number and saved as described in the electronic-data section of this manual.

30. The data is also recorded on index card forms that have been maintained for many years for all 

previously calibrated chambers. The current calibration results are compared with previous results. This 

verifies the quality of the calibration. The previous data is also in electronic form for comparison.
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31. The summary data is then pasted into the Word or Excel templates for the calibration report. A 

sample report follows in Appendix B.

Mammo FAC steps

1. Turn on the HDI controller using the key on the front panel. This energizes the communication 

junction box that controls all systems. 

2. The heat exchanger for the mammography system is a closed loop and energizes automatically 

during the Kimtron warmup procedure. 

3. Follow the Procedure for the Use of the Kimtron; Appendix A., selecting the Mo or Rh tube.

4. Run the LPS vi, homing the x and y axes and secure LPS in the desired position and distance.

5. Mount chambers as described previously using the lasers and telescope for redundancy. Connect 

signal and high-voltage cables. 

6. Verify that the Attix chamber’s moveable alignment slide is in the position furthest from the source.

7. Verify that the Attix chamber is locked in the air-kerma position.

8. Verify power is on all instruments visible from the control area. These include four Keithley 

electrometers, four Bertan high-voltage supplies, three Hart thermometers and a Setra barometer. 

9. Start the HDI LabVIEW software using the provided shortcut for Revcurrent#.vi

10. Once the chambers are positioned and secured, apply high voltage. Enter the desired high voltage 

on the front panel display of the HDI software and flip the Bertan HV switch on. The voltage is 

maintained on the primary standards, but make a visual check that voltage is set.

11. Close door to range so that the safety interlocks will allow the opening of the x-ray shutter.

12. Data entry in Access

a. start Access and use C:\100-300 XRay Sys\100300 NIST Track Revcurrent#

b. select forms

c. select switchboard

d. enter appropriate data under the customer, instrument and work order forms

13. Verify the collection time, so as to not overload the electrometers. This depends on the air-kerma 

rate for the beam quality. Put each chamber in the beam using the conditions for the calibration and 

establish the collection time. Press the “exposure enable” at the bottom of the screen so it is not red. 

Secure the interlocks for entrance to the rooms. Press the red “open” button on the HDI control panel to 

open the shutter. The time of the exposure will accumulate on the front panel of the HDI software. 

14. Press “build sequence” to prepare an automated test sequence.
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15. Enter the work order number of the previously entered data in Access. When the “get work order” 

button is selected, the background information for the customer chamber automatically is entered. The 

toggle switch for the customer chamber position (T3 or T4) must correspond to the correct position.

16. Create a unique cal run ID.

17. Enter the pressure, temperature and humidity from the front-panel displays.

18. Enter the beam aperture selected, the diameter of the beam, and the distance of the calibration.

19. Enter the identification information of the NIST reference-class chamber to be calibrated.

20. Enter the desired collection times and the number of measurements to be collected.

21. Finally, select the beam codes.

22. Upon completion, press “build sequence”.

23. At the front panel press “start auto sequence” to begin the calibration.

24. Upon completion of the calibration, review the data from the printed reports. If some data needs to 

be eliminated, go to Access.

a. open the “exphist” table and find the data points to delete. Place a check in the appropriate 

column to delete the data point. 

b. on the front panel of the HDI software select “Redo Report”. Enter the work order number 

and the unique cal run ID. Press “get records”. Select the appropriate beam code. Repeat until all 

data is recalculated.

25. A summary of the final data for both chambers is stored to an Access file named InstCalReport. 

Select the records desired and copy to an Excel spreadsheet to complete summary calculations. The 

spreadsheet should be named by the DG number and stored as described in the electronic-data section of 

this manual.

26. Repeat until a sufficient number of calibrations have been completed for each beam quality, 

typically three to five. 

27. The expected standard deviation on the calibration results depends on the chamber type but 

generally a standard deviation less than 0.5 % is acceptable. 

28. The averages of multiple calibrations are maintained in the Excel spreadsheets. These Excel files are 

named by the DG number and saved as described in the electronic-data section of this manual.
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29. The data is also recorded on index card forms that have been maintained for many years for all 

previously calibrated chambers. The current calibration results are compared with previous results. This 

verifies the quality of the calibration.

30. The summary data is then pasted into the Word or Excel templates for the calibration report. A 

sample report follows in Appendix B.

Electronic data

All data collected with the NIST reference-class chambers is saved in the appropriate files located in the 

group server in the x-ray\chamberhistory folder. The folder to be used is either NISTREFSTD or 

NISTREFSTDMAMMO, depending on which type of chamber is used. The files are organized and 

named by model and serial number. The group server is backed up routinely.

Similarly, the data collected using the customer chamber and the associated files and the final calibration 

report is saved in the group server: L:\internal\846.02\x-ray\calibrationreports\allcustomerdata. Paper 

copies of the reports are kept in H127, as described in the administrative procedure section. All 

collected raw data is stored in the Access database mentioned in the above procedure, C:\100-300 XRay 

Sys\100300 NIST Track Revcurrent# on the Labview data acquisition system. 

In-house calibration checks and traceability

The long-term reliability of NIST dosimetry calibrations depends on the stability of the NIST air-kerma 

standards. For x rays, the NIST air-kerma standard is the response of the appropriate free-air chamber. 

The four free-air chambers are also periodically compared. The in-house calibration checks are intended 

to check both the stability of the NIST standards and the reliability of the calibration procedures. 

Comparison results are listed in Ref. 6 and in published reports. References 10-20 are the most recent 

published comparisons which establish and demonstrate traceability of air-kerma. 

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 as 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 0.5 % but dependent on the chamber type, an investigation is 

warranted. If 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. Any 

discrepancy found for a NIST check chamber of the order of magnitude mentioned above 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.

Uncertainty Analysis

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 
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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 (i.e., 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 a coverage factor of two (k=2) to give the expanded 

uncertainty. This expanded uncertainty is considered to have the approximate significance of a 95 % 

confidence limit. Table 10 lists the details of the assessment of uncertainty for the air-kerma rates 

determined for the tungsten x-ray beams by the free-air ionization chambers. Table 11 lists the details of 

the assessment of uncertainty in the calibration of a typical ionization chamber. Table 12 lists the details 

of the assessment of uncertainty in air-kerma rates determined for the mammography x-ray beams by the 

Attix chamber. Table 13 lists the uncertainty components for the calibration of instruments used for 

mammography. As the estimate of uncertainty varies slightly with beam qualities, methods of 

measurement, and rate, in each case the largest value is used for the estimate. In an official calibration, 

measurements could be repeated to maintain optimal conditions.

Safety

As described below, every effort is made to avoid any possibility of radiation exposure to lab occupants, 

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. There is no danger of high voltage related to the x-ray generators because the 

equipment has no exposed high voltage in a normal operating mode. The potential risk from the motion 

control associated with the LPS is addressed through various indicators, fully explained in the safety 

protocol, and shown below in the visual explanation of safety features. Risks due to trip hazards are 

reduced by secure floor cable ramp protectors and the reduction of occupancy to properly trained 

personnel.

High-voltage safety 

The only danger that exists from high voltage comes from the free-air ionization chambers, the customer 

chamber, and the x-ray-calibration-range monitor 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. The risk of high voltage from the Attix chamber is minimized by 

surrounding the chamber with its protective cover. 

Radiation safety

The radiation area is marked with the proper signs and a dosimeter must be worn by all personnel. 

Radiation safety training and assessment services are provided by the NIST Gaithersburg Radiation 

Safety Division. The x-ray range, H127-3 was designed to eliminate any possible exposure to x 

radiation. Details are listed in the safety protocols for each x-ray system. The entrance, shielding door 

features a pair of ISO13849-1 "PL-e" rated safety switches, with independent feedback to an equally 
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rated multifunction safety relay. This is the highest degree of machine safety rating, intended for 

systems which could cause instant death or dismemberment in the event of a failure, thus exceeding the 

facility safety requirements. Additionally, a LMO "Last Man Out " wall switch is present for the search 

and evict sequence to be performed prior to closing the door relay. Scram buttons are on the control 

panel and in the experiment room, any of which, in an unsafe state, will break the electrical circuit 

which allows the shutter air solenoid to energize forming a hardware interlock system independent of 

software control. Additionally, the high voltage from the generator will be deenergized. Hardware 

interlocks include an enable key for the shutter system and for high voltage production from each 

generator. The shared interlock monitoring PLC for the room also monitors these key signals to ensure 

that shutters close. For a further understanding of the safety features, a visual explanation is provided for 

the radiation safety lights and the motion control in the x-ray calibration range.
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Visual Explanation of Various Safety Lights in H127-3

Location of lights next to the shielding door for H127-3

O

Shutter indicator light Power on x-ray 

system indicator light

The H127-3 door “x-ray on” light 

illuminates when power is on the xray tube. The H127-3 door “Danger 

High Radiation Area” light 

illuminates when the shutter is open.
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Calibrator status lights glow 

red when shutter is open. This 

picture was taken during 

installation so the condition of 

being inside H127-3 with the 

red status lights is not 

achievable in normal 

operation.

EMO red 

status light on 

NORTH wall 

glows red 

when shutter 

is open.

When HDI 

controllers are 

energized, the 

calibrator status 

lights are green 

when the 

shutters are 

closed.

Calibrator status lights, showing green when shutters are closed Calibrator status lights visible 

through the lead glass window

Calibrator status lights showing red

Calibrator “x-ray on” light illuminates 

when the generator is energized
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Visual Explanation of Motion Control Safety Features in H127-3

Hand control LPS motion controller with 

EMO button on telescope stand

Telescope stand with 

motion control and EMO

North wall EMO 

and shutter open 

indicator light

EMO ready position

East wall EMO

West wall EMO

South wall EMO
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Filing and Retention

The administrative procedures section describes the handling of the calibration reports. The electronic 

data section describes the organization of the electronic data files. 

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

Copies of the current revision of this Procedure shall be placed in controlled Quality Manuals. 

Electronic copies of this Procedure are uncontrolled versions.
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Table 1. NIST Calibration Conditions for X-Ray Measuring Instruments

NIST Calibration Conditions for X-Ray Measuring Instruments

Beam 

code

Additional filtration a Half-value layerb (HVL) Homogeneity 

coefficient (HC)

Effective

energy

Al

(mm)

Cu

(mm)

Sn

(mm)

Pb

(mm)

Al

(mm)

Cu

(mm)

Al Cu (keV)

X-Ray Beam Qualities

L10 0.037 86

L15 0.059 70

L20 0.070 72

L30 0.30 0.23 60

L40 0.53 0.52 61

L50 0.71 0.79 63

L80 1.45 1.81 56

L100 1.98 2.80 58

M20 0.27 0.15 72

M30 0.5 0.36 65

M40 0.89 0.74 67

M50 1.07 1.04 68

M60 1.81 1.68 0.052 63 60

M80 2.86 3.08 0.1 67 61

M100 5.25 5.10 0.2 74 55

M120 7.12 6.77 0.31 76 53

M150 5.25 0.25 10.30 0.66 86 63

M200 4.35 1.12 14.73 1.64 94 68

M250 5.25 3.2 18.49 3.2 98 85

M300 4.25 6.5 21.77 5.3 99 97

H10 0.105 0.051 77

H15 0.5 0.16 87

H20 1.01 0.36 89

H30 4.50 1.2 86

H40 4.53 0.26 2.93 94

H50 4.0 0.1 4.16 0.14 92 93 38

H60 4.0 0.61 6.06 0.25 92 94 46

H100 4.0 5.2 13.51 1.15 98 92 80

H150 4.0 4.0 1.51 16.93 2.43 99 96 120

H200 4.0 0.6 4.16 0.77 19.72 4.1 99 99 166

H250 4.0 0.6 1.04 2.72 21.59 5.19 99 98 211

H300 4.1 3.0 5.0 23.55 6.19 98 98 252

S60 4.35 2.81 0.09 74 66

S75 1.50 1.81 58

aThe additional filtration value does not include the inherent filtration. The inherent filtration is approximately 1.0 

mm Be for beam codes L10-L100, M20-M50, H10-H40 and S75; and 3.0 mm Be for beam codes M60-M300, 

H50-H300 and S60.bThe x-ray tubes were installed in 2008 and 2015.
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Table 2. Mammography X-Ray Beam Qualitya Parameters 

Beam code Tube 

voltage

Additional

filtrationb

Half-value

layer

(kV) (mm) (mm Al)

Mo Anodec

Mo/Mo23 23 0.032 Mo 0.288

Mo/Mo25 25 0.032 Mo 0.313

Mo/Mo28 28 0.032 Mo 0.346

Mo/Mo30 30 0.032 Mo 0.370

Mo/Mo35 35 0.032 Mo 0.404

Mo/Rh28 28 0.029 Rh 0.420

Mo/Rh32 32 0.029 Rh 0.453

Mo/Mo25x 25 0.030 Mo + 2.0 Al 0.551

Mo/Mo28x 28 0.030 Mo + 2.0 Al 0.589

Mo/Mo30x 30 0.030 Mo + 2.0 Al 0.633

Mo/Mo35x 35 0.030 Mo + 2.0 Al 0.715

Rh Anode

Rh/Rh25 25 0.029 Rh 0.351

Rh/Rh30 30 0.029 Rh 0.438

Rh/Rh35 35 0.029 Rh 0.512

Rh/Rh40 40 0.029 Rh 0.559

Rh/Rh30x 30 0.029 Rh + 2.0 Al 0.814

Rh/Rh35x 35 0.029 Rh + 2.0 Al 0.898

a Kessler, C., Burns, D.T. and O’Brien, M., “Key comparison BIPM.RI(I)-K7 of 

the air-kerma standards of the NIST, USA and the BIPM in mammography xrays,” Metrologia 48 06014 (2011).

bThe additional filtration value does not include the inherent filtration, which is 

comprised of 1.0 mm Be from the x-ray tube window and 0.075 mm polyamide

from the transmission monitor.

cThe HVL values were measured directly using the Mo anode installed in Dec 

2008.
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Table 3a. ISO X-Ray Beam Quality Parameters Offered at NIST

Beam Additional filtration (mm)a First HVLb Second HVL

code Al Cu Sn Pb mm Al mm Cu mm Al mm Cu

HK10 0.042 0.045

HK20 0.15 0.128 0.170

HK30 0.52 0.408 0.596

HK60 3.19 0.079 0.113

HK100 3.90 0.15 0.298 0.463

HK200 1.15 1.669 2.447

HK250 1.60 2.463 3.37

HK280 3.06 3.493 4.089

HK300 2.51 3.474 4.205

WS60 0.3 0.179 0.206

WS80 0.529 0.337 0.44

WS110 2.029

5

0.97 1.13

WS150 1.03 1.88 2.13

WS200 2.01 3.09 3.35

WS250 4.01 4.30 4.50

WS300 6.54 5.23 5.38

NS10 0.095 0.049 0.061

NS15 0.49 0.153 0.167

NS20 0.90 0.324 0.351

NS25 2.04 0.691 0.762

NS30 4.02 1.154 1.374

NS40 0.21 0.082 0.094

NS60 0.6 0.241 0.271

NS80 2.0 0.59 0.62

NS100 5.0 1.14 1.19

NS120 4.99 1.04 1.76 1.84

NS150 2.50 2.41 2.57

NS200 2.04 2.98 1.003 4.09 4.20

NS250 2.01 2.97 5.34 5.40

NS300 2.99 4.99 6.17 6.30

LK10 0.30 0.061

LK20 2.04 0.441

LK30 3.98 0.18 1.492

LK35 0.25 2.21

LK55 1.19 0.260

LK70 2.64 0.509

LK100 0.52 2.0 1.27

LK125 1.0 4.0 2.107 2.094

LK170 1.0 3.0 1.5 3.565 3.592

LK210 0.5 2.0 3.5 4.726 4.733

LK240 0.5 2.0 5.5 5.515 5.542

a The additional filtration does not include the inherent filtration. The inherent filtration is a 

combination of the filtration due to the monitor chamber plus 1 mm Be for beam codes 

LK10-LK30, NS10-NS30, HK10-HK30; for all other techniques the inherent filtration is 

adjusted to 4 mm Al. Details of these reference radiation qualities can be found in the 

following: ISO/IS 4037-1:1996(E) X and Gamma Reference Radiations for Calibrating 

Dosimeters and Dose Rate Meters and for Determining Their Response as a Function of 

Photon Energy - Part 1; Radiation Characteristics and Production Methods. bThe x-ray tubes 

were installed in 2015 and 2008.
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Table 3b. CCRIa X-Ray Beam Quality Parameters Offered at NIST

Beam code Tube 

voltage

Added

filtrationb

Half-value

layerc

(kV) (mm Al) (mm Cu) (mm Al or mm Cu)

BIPM25 25 0.373 0.24 mm Al

BIPM30 30 0.208 0.167 mm Al

BIPM40 40 3.989 0.212 2.649 mm Al

BIPM50a 50 3.989 2.291 mm Al

BIPM50b 50 1.007 1.038 mm Al

BIPM100 100 3.248 0.149 mm Cu

BIPM135 135 1.060 0.265 0.496 mm Cu

BIPM180 180 3.842 0.482 1.003 mm Cu

BIPM250 250 3.842 1.618 2.502 mm Cu

aBIPM, Qualités de rayonnements, Consultative Committee for Ionizing 

Radiation (CCEMRI) (Section I), 1972, 2, R15. Details of these reference 

radiation qualities can be found in: Burns, D.T. and O'Brien, M., "Comparison of 

the NIST and BIPM Standards for Air Kerma in Medium-Energy X-Rays," J.

Res. Natl. Inst. Stand. Technol. 111, 385-391(2006) and Burns, D.T., Kessler, C. 

and O’Brien, M., “Key comparison BIPM.RI(I)-K2 of the air-kerma standards of 

the NIST, USA and the BIPM in low-energy x-rays,” Metrologia 49 06006 

(2012).

bThe additional filtration does not include the inherent filtration of the x-ray tubes

which is approximately 3 mm Be and 1 mm Be.

cThe HVL values were determined for the tubes installed in 2015 and 2008.
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Table 4. Important dimensions and parameters for the use of the NIST standard free-air ionization 

chambers.

Chamber

(alternative

name)

X-ray 

tube

potential

(kV)

Alignment 

offseta

Operating 

potential

(v)

Diaphragm

diameter/ID

(mm)

Air 

absorption

length

(mm)

Lampertib 10 to 60 20 mm 1500 4.994/5s 39.18

Attix

(Mammo)

10 to 50 None 2500 10.00/10u 212.7

c

Ritz

(LE or 100

kV)

20 to 100 15 mm 5000 10.00/10A 127.39

WyckoffAttix

(HE or 300 

kV)

50 to 300 None 5000 10.00/10B 308

a The distance down-stream from the chamber aperture to move the alignment telescope to properly 

align the chamber.

b The Lamperti Chamber is dedicated for use in Electronic Brachytherapy calibrations but has been 

established for use in low energy x-rays. 

cThis is variable; the value shown is used for routine use.
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Table 5. Features of x-ray systems.

Features Unipolara Bipolarb Mammographyc Mammographyd

Generator manufacturer Kimtron Kimtron Kimtron Kimtron

Calibrator HDI Model 

X80-100A-SP

HDI Model

X80-320kVA-SP

HDI Model

X82-100-Mo-RhA-Sp 

HDI Model

X82-100-Mo-RhA-Sp

Output voltage (kV) 7.5-180 kV 20-360 kV 7.5-180 kV 7.5-180 kV

Output current (mA) 0 to 45 mA 0 to 30 mA 0 to 45 mA 0 to 45 mA

Output power (kW) 4,5 4,5 4,5 4,5

kV adjustment (kV) 0.1 0.1 0.1 0.1

mA adjustment (mA) 0.1 0.1 0.1 0.1

Tube manufacturer Thales Comet Lohmann RTW

Fixed anode material W W Rh Mo

Tube window (mm Be) 1 3 1 1

Focal spot size (mm) 5.5 x 5.5 5.5 x 5.5 5 x 5 3 x 3

a Tube installed in November 2008.

b Tube installed in May 2015.

c Rh tube installed in 1994.

d Mo tube in 2008
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Table 6. Essential equipment to conduct calibrations in the x-ray range located in H127.

Description Model Serial number Use

300 kV Free-air chamber Wyckoff- Attix none FAC

100 kV Free-air chamber Ritz none FAC

Mammography Free-air chamber Attix none FAC

Transmission monitor chamber PTW 34014 0024 300kV

Transmission monitor chamber PTW 34014 0023 100kV

Transmission monitor chamber PTW 34014 072 Mammo

Oenoke voltage divider PN OE-908-160-V 100 kV

Oenoke voltage divider PN OE-908-160-V Mo kV

HP high-precision digital voltmeter 3456A 2201A11673 meter test

HP high-precision digital voltmeter HP34401A US36043465 divider

HP high-precision digital voltmeter HP34401A US36043467 divider

HP high-precision digital voltmeter HP34401A US36037412 divider

HP high-precision digital voltmeter HP34401A US36037414 divider

High-voltage power supply EDC 09417 FC51033 divider

Agilent 34411A 48000105 backup

Setra barometer 370 498554 DAS

Setra barometer 370 3318085 backup

Wallace-Tiernan barometer FA139 MM14869 QA check

Wallace-Tiernan barometer FA139 XX11242 Lab 

reference

Bertan high-voltage power supply Series 23005R 9188 WyckoffAttix 

Bertan high-voltage power supply Series 22501R 7192 Test 2

Bertan high-voltage power supply Series 22505R 70982 Test 1

Bertan high-voltage power supply Series 22505R 4183 Ritz

Bertan high-voltage power supply Series22501R 7191 PTW

Bertan high-voltage power supply Series 22503R 6252 Test 4

Bertan high-voltage power supply Series 22503R 04024 Test 3

Bertan high-voltage power supply Series 22505R 4172 Attix

Keithley electrometer 617-HIQ 06463833 PTW 

Keithley electrometer onitor 6512 0664956 FAC

Keithley electrometer 6512 0762177 test 1
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Keithley electrometer 6512 0664959 test 2

Keithley electrometer 617 0566314 test 3

Keithley electrometer 617 388587 test 4

Keithley electrometer 6514 1466987 All FAC 

backup

Hart thermometer Chub-E4 and 4 

probes

1529/ PT1000 A1B165/

#1N31P300606

#2N31P300603

#3N31P300604

#4N31P300605

#1WyckoffAttix FAC 

#2Ritz FAC 

#3 test1/2 

#4 test3/4

Hart thermometer and probe 1502/ PT1000 62495/

N31P300608

PTW

Hart thermometer and thermistor 1504/5611 C11444/

RC-201031284

backup

Fluke thermometer and probe 1502A/5613

PT1000

A63236/763272

N31P300609

Attix

backup

Fluke thermometer and probe 1504/5610-6 A95694/A932006 Lab 

reference

300 kV generator Polaris III 360kV C360-100080 Cathode -

300 kV generator Polaris III 360kV A360-100080 Anode +

100 kV generator Polaris III 180kV C180-100079 100 kV

Rh generator Polaris III 180kV C180-100085 Rh tube

Mo generator Polaris III 180kV C180-100086 Mo tube

Thales x-ray tube THX160/1055 581082 100 kV

Comet x-ray tube MXR-320/26 415574 300 kV

Rhodium x-ray tube Lohmann 84005 047 Rh kV

Molybdenum x-ray tube RTW MCD-100H3MO6464 Mo kV

Kimtron Polaris water cooler 3500-WA 11798-01-20WA Mo/Rh

Kimtron Polaris water cooler 3500-WA 11796-12-19WA 100 kV

GE water cooler OW4002 115001 300 kV

Hopewell Designs, Inc. controller NIST#585204 & 

NIST#583270
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Electronic rack NIST#585799

HDI Calibrator 320 kV HDI X80-320-A-SP 0521-5406 300 kV

HDI Calibrator 100 kV HDI X80-100-A-SP 0521-5405 100 kV

HDI Calibrator Mammo HDI X80-100-A-SP 0521-5407 Mo/Rh

Linear Positioning System (LPS) HDI T20-2-X3-Y4 0521-5408
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Table 7. Example of the optional checklist for calibration order details

Calibration Information Request Form

Required Dates Optional Dates

PO received Estimated job start

Estimated completion Equipment arrival 

Report mailed Inspection complete

Equipment returned 

Contact Information

NIST Technical Contact: Michelle O’Brien x2014

Company: 

Technical Contact: 

DG Number: 

Instrument Description

Manufacturer Model Serial Number

Calibration Request and Cost

SP 250 Cal 

ID/SKU

46011C

Item Description 

X-Ray Calibration

Qty Cost for this Cal ID

$

TOTAL
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Table 9. Nominal calculated beam sizes 

Distance from source

(cm)

Beam defining 

aperture

(cm)

Diameter at

measurement point 

(cm)

300 kV X-Ray Calibrator

100

1.27 4

1.90 7

2.54 10

5.08 22

200

1.27 7

1.90 14

2.54 20

5.08 44

100 kV X-Ray Calibrator

100

1.27 4

1.90 7

2.54 10

Mammography X-Ray Calibrator

100

1.27 4

2.54 10
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Table 10. Typical uncertainty analysis for tungsten x-ray air-kerma rates, relative uncertainties shown in 

%.

Uncertainty components Type A Type B

air density 0.01 0.07

charge 0.1 0.06

humidity 0.03

volume 0.04 0.01

g 0.02

W/e 0.35

air attenuation, ka 0.05 0.02

electric field distortion, kd 0.2

initial ion·mean energy per ion 

pair, kiikw

0.05

electron loss, ke 0.1

penetration of aperture, kl 0.04

penetration of chamber face, kp 0.01

polarity difference 0.05

recombination loss, ks 0.1

fluorescence, kfl 0.03

scattered photons, ksc 0.07

quadratic sum 0.163 0. 439

combined standard uncertainty 0.469

expanded uncertainty 0.937
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Table 11. Uncertainty analysis for x-ray calibrations, shown in %

Uncertainty components Type A Type B

air-kerma rate 0.163 0.439

air density 0.01 0.07

charge 0.1 0.06

distance 0.01

humidity 0.03

radiation background

quadratic sum 0.192 0.450

combined standard uncertainty 0. 489

expanded uncertainty 0.978
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Table 12. Uncertainty analysis for air-kerma rates with the Attix chamber, shown in %

Uncertainty components Type A Type B

air density 0.010 0.070

charge 0.120 0.060

humidity 0.030

g 0.020

W/e 0.35

air attenuation, ka 0.050 0.010

ion pair mean energy kiikw 0.11

aperture area 0.010 0.010

plate separation 0.010 0.070

electron loss 0.05

recombination loss, ks 0.060

scattered photons, kp 0.07

polarity 0.1

quadratic sum 0.162 0.396

combined standard uncertainty 0.428

expanded uncertainty 0.856

Table 13. Uncertainty analysis for mammography calibrations, shown in %

Uncertainty Components Type A Type B

air density 0.01 0.07

air kerma 0.162 0.396

charge 0.1 0.06

humidity 0.03

distance 0.02

quadratic sum 0.191 0.408

combined standard uncertainty 0.451

expanded uncertainty 0.902
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REFERENCES

1. Marshal, J.L.,ed. NIST Calibration Services Users Gide, NIST (U.S.) Spec. Publ. 250; 1998 January. 

2. Boutillon, M. and Perroche-Roux, A.M., Re-evaluation of the W value for electrons in dry air, Phys. 

Med. Biol., 32, 213-219 (1987).

3. Wyckoff, H. O. and Attix, F. H., Design of free-air ionization chambers, National Bureau of 

Standards Handbook 64, 1957.

4. Ritz, V. H., Standard free-air chamber for the measurement of low energy x rays (20-100 

kilovolts-constant-potential), J. Res. Nat. Bur. Stand. (U.S.), 64C, 49-53 (1960).

5. Lamperti, P. J. and Wyckoff, H. O., NBS free-air chamber for measurement of 10 to 60 kV x rays, J. 

Res. Nat. Bur. Stand. (U.S.), 69C, 39-47 (1965).

6. Lamperti, P. and O’Brien, M., Calibration of X-Ray and Gamma-Ray Measuring Instruments, NIST 

Special Publication 250-58, (2001).

7. Attix, F. H.,"Electronic equilibrium in free-air chambers and a proposed new chamber design," Naval

Research Laboratory Report No. 5646, 1961.

8. Coletti, J.G., Pearson, D. W. and DeWerd, L.A., "Mammography exposure standards: Design and 

characterization of free-air ionization chamber," Rev. Sci Instrum. 66, 2574-2577 (1995).

9. Coletti, J. G., Pearson, D. W., DeWerd, L.A., O'Brien, C. M. and Lamperti, P.J. "Comparison of 

exposure standards in the mammography x-ray region," Med. Phys. 24(8), 1263-1267 (1997).

10. Burns, D.T., Lamperti, P. and O’Brien, M., “Comparison of the NIST and BIPM Air-Kerma

Standards for Measurements in the Low-Energy X-Ray Range,” J.Res. Natl. Inst. Stand.

Technol. 104, 135 (1999). 

11. Burns, D.T.,and O’Brien, M,.et.al “Comparison of the NIST and BIPM Medium-Energy X-Ray AirKerma Measurements,” J.Res. Natl. Inst. Stand. Technol. 108, 383-389 (2003). 

12. Burns, D.T.,and O’Brien, M,. “Comparison of the NIST and BIPM Standards for Air Kerma in 

Medium-Energy X-Rays,” J.Res. Natl. Inst. Stand. Technol. 111, 385-391 (2006). 

13. Büermann, L., O'Brien, M., Butler, D., Csete, I., Gabris, F., Hakanen, A., Lee, J-H., Palmer, M., 

Saito, N. and de Vries, W. , “Comparison of National Air Kerma Standards for ISO 4037 Narrow 

Spectrum Series in the Range 30 kV to 300 kV,” Metrologia 45 06013 (2008).

14. O’Brien, M. and Büermann, L.,”Comparison of the NIST and PTB Air-Kerma

Standards for Low-Energy X-Rays,” J. Res. Natl. Inst. Stand. Technol. 114, 321-331 (2009).
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15. O’Brien, M., Minniti, R. and Masinza, M., “Comparison Between the NIST and the KEBS

for the Determination of Air Kerma Calibration Coefficients for Narrow X-Ray Spectra and 137Cs 

Gamma-Ray Beams,” J. Res. Natl. Inst. Stand. Technol. 115, 7-13 (2010).

16. Burns, D.T., Kessler, C. and O’Brien, M., “Key comparison BIPM.RI(I)-K2 of the air-kerma 

standards of the NIST, USA and the BIPM in low-energy x-rays,” Metrologia 49 06006 (2012).

17. Kessler, C., Burns, D.T. and O’Brien, M., “Key comparison BIPM.RI(I)-K7 of the air-kerma 

standards of the NIST, USA and the BIPM in mammography x-rays,” Metrologia 48 06014 (2011).

18. Seltzer, S.M., O’Brien, M. and Mitch, M.G., “New National Air-Kerma Standard for Low-Energy 

Electronic Brachytherapy Sources,” J. Res. Natl. Inst. Stand. Technol. Volume 119 (2014). 

19. M O'Brien, J McCaffrey, H Shen, M Saraví, A Stefanic, G Montaño Ortiz, M Carlos, C da Silva, J

Álvarez and V Tovar,“SIM.RI(I)-K3 comparison of calibration coefficients at radiotherapy level for 

orthovoltage x-ray beams,” Metrologia 52 06015, Tech. Suppl., (2015).

20. D. T. Burns et al, "Key comparison BIPM.RI(I)-K3 of the air-kerma standards of the NIST, USA 

and the BIPM in medium-energy x-rays," Metrologia 54 06006 (2017).
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Appendix A: Procedure for the Use of the Kimtron X-ray Generators Outside the HDI Automated 

Control Interface

System Initialization

1) Power on the HDI control computer, located in the instrument control console of H127. Use the 

screensaver password to unlock. This computer is not connected to the internet. 

2) Verify all four x-ray tube position keys are in position OFF. 

3) Turn on the POWER key in the bottom right corner of the HDI controller, as shown in picture. The 

HDI power switch enables the system and starts the controller. 

4) In the x-ray range room, H127-3, verify that the status lights on the HDI x-ray tube 

cabinets/calibrators are green to indicate system status as ready and safe. This indicates safe 

entrance and that the tube shutters are all closed.

Turn on

key.

Verify top four keys are off.

Power on.
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Energize the X-ray System

5) In the equipment room, H127-2, turn on the power breaker of the generator in use, these are 

labeled as follows: HE (300 kV), LE (100 kV), Mo and Rh.

Verify green

lights

HE

LE

Mo Rh
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6) In H127-2, verify that the Interlock Control junction box is energized. The interlock junction box is 

towards the center of the room.

7) Inside the Interlock box, verify that the compressed air indicator light is NOT RED. A red light 

indicates no air pressure. The air pressure should be in the safe green range of approximately 80 

psi.

Interlock control junction 

box

Compressed air indicator
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8) On the HDI key control, turn the key to STANDBY of the x-ray system in use. The generator and heat 

exchanger will energize automatically as the key is switched to STANDBY.

9) Once the tube position key is in standby mode, the corresponding green light on the Edge Router in 

the interlock junction box illuminates. In this picture the HE tube is selected.

HE
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10)In the x-ray range, H127-3, after verifying the room is empty of all persons, the Last Man Out (LMO) 

button must be pressed, prior to closing the door. Press the LMO, which will have an audible alarm 

and flashing yellow light. Then the door must be closed within 40 seconds.
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11)Open the Kimtron vi found on the desk top screen of the HDI computer. Once opened, the name is 

Modbus Master R3.vi Front Panel.

12)In the Slave Address entry, change the suffix place, shown here as ##, 192.168.100.##, to the 

corresponding system in use: HE 192.168.100.41, LE 192.168.100.42, Mo 192.168.100.43 and Rh

192.168.100.44. Initializing appears in comment box.

13) Select connect. Connected appears in text box.

14) Select update.

15)In the Set Control Bits Enum selector, chose long warmup.

16)Turn tube selector key to the on (HV ENABLE) state.

17)In the Operating State box confirm ready.

18)In the Set Control Bits Enum selector, chose x-ray on.

19)X-ray door lights illuminate when power is on x-ray tube during warmup.

20)Turn the key to STANDBY mode after warmup.

21)The door x-ray on light goes off when key is in STANDBY.

22)When prepared for x-ray use, turn the key to HV ENABLE.

23)In the Set Control Bits Enum selector, chose x-ray off then x-ray on.

24)Enter desired x-ray kV and mA in the request entry boxes and select set.

25)At completion of use, select x-ray off and turn key to STANDBY.

26)Deselect update. Select disconnect, then wait at least 5 minutes or go touch the cooling lines of heat 

exchanger to see if cool for HE heat exchanger or see if the other heat exchanger’s temperature is 

near set point temperature.

27)Press the red STOP VI square button next to disconnect.
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Appendix B: Sample Calibration Report

REPORT OF AIR-KERMA CALIBRATION(a)

OF

Name and complete postal address (f)

Radiation Detection Chamber: chamber/model (h, m)

(p)

Calibrations performed by Michelle O’Brien

Report reviewed by Ronaldo Minniti

Report approved by Michael G. Mitch, Dosimetry Group Leader

James M. Adams

Chief of the Radiation Physics Division

Physical Measurement Laboratory

For the Director of the National Institute of Standards and Technology

Information on technical aspects of this report may be obtained from Michelle O’Brien, National 

Institute of Standards and Technology, 100 Bureau Drive Stop 8460, Gaithersburg, MD 20899, 

michelle.obrien@nist.gov, or (301) 975-2014. 

Report format revised 6/22

(b,c) Official NIST letterhead is used.

Dosimetry Group Number DG: #####-20

NIST ID 46011C Order Number: 682.02/O-0000000###-20 (d)

Report Date: 00/00/2020 (l)

Page 1 of 8 (e)
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REPORT OF AIR-KERMA CALIBRATION

OF

Name and complete postal address

Radiation Detection Chamber: chamber/model 

(h to k)

Chamber orientation: The cavity was positioned XXX

Chamber collection potential: A positive or negative XXX volts, with respect to the outer electrode was 

applied to the chamber and positive or negative charge was collected by the electrometer.

Chamber rotation: The window faced the source of radiation.

Environmental conditions: The chamber is assumed to be open to the atmosphere.

Average background current: The background current is 0.00 % of exposure current.

Temperature range: example 293.6 K (20.5 °C) to 294.1 K (21.0 °C)

Pressure range: example 100.7 kPa to 101.8 kPa

Calibration date: date range

Current ratio: The current ratio at the full to half collection potential is X.XXX for beamXXX. The 

results relate only to the instrument calibrated in this report. (m)

Results

Beam 

Code

Half-Value

Layer

(mm Al/mm 

Cu)

Calibration Coefficient 

(Gy/C)

295.15 K (22 oC) and

101.325 kpa (1 Atm)

Air- Kerma

Rate 

(Gy/s)

Beam

Diameter

(cm)

Calibration

Distance

(cm)

Expanded, Combined 

Uncertainty, in Percent

Beam Code

Air-Kerma Rate (Gy/s) NIST Calibration Coefficient 

(Gy/C)

Dosimetry Group Number DG: #####-20

NIST ID 46011C Order Number: 682.02/O-0000000###-20

Report Date: 00/00/2020

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NIST Calibration Conditions for X-Ray Measuring Instruments

Beam 

code

Additional filtration a Half-value layerb (HVL) Homogeneity 

coefficient (HC)

Effective

energy

Al

(mm)

Cu

(mm)

Sn

(mm)

Pb

(mm)

Al

(mm)

Cu

(mm)

Al Cu (keV)

X-Ray Beam Qualities

L10 0.037 86

L15 0.059 70

L20 0.070 72

L30 0.30 0.23 60

L40 0.53 0.52 61

L50 0.71 0.79 63

L80 1.45 1.81 56

L100 1.98 2.80 58

M20 0.27 0.15 72

M30 0.5 0.36 65

M40 0.89 0.74 67

M50 1.07 1.04 68

M60 1.81 1.68 0.052 63 60

M80 2.86 3.08 0.1 67 61

M100 5.25 5.10 0.2 74 55

M120 7.12 6.77 0.31 76 53

M150 5.25 0.25 10.30 0.66 86 63

M200 4.35 1.12 14.73 1.64 94 68

M250 5.25 3.2 18.49 3.2 98 85

M300 4.25 6.5 21.77 5.3 99 97

H10 0.105 0.051 77

H15 0.5 0.16 87

H20 1.01 0.36 89

H30 4.50 1.2 86

H40 4.53 0.26 2.93 94

H50 4.0 0.1 4.16 0.14 92 93 38

H60 4.0 0.61 6.06 0.25 92 94 46

H100 4.0 5.2 13.51 1.15 98 92 80

H150 4.0 4.0 1.51 16.93 2.43 99 96 120

H200 4.0 0.6 4.16 0.77 19.72 4.1 99 99 166

H250 4.0 0.6 1.04 2.72 21.59 5.19 99 98 211

H300 4.1 3.0 5.0 23.55 6.19 98 98 252

S60 4.35 2.81 0.09 74 66

S75 1.50 1.81 58

aThe additional filtration value does not include the inherent filtration. The inherent filtration is approximately 1.0 

mm Be for beam codes L10-L100, M20-M50, H10-H40 and S75; and 3.0 mm Be for beam codes M60-M300, 

H50-H300 and S60.bThe x-ray tubes were installed in 2008 and 2015.

Dosimetry Group Number DG: #####-20

NIST ID 46011C Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020Page 3 of 8
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ISO X-Ray Beam Quality Parameters Offered at NIST

Beam Additional filtrationa First HVLb Second HVL

Code Al

(mm)

Cu

(mm)

Sn

(mm)

Pb

(mm)

Al

(mm)

Cu

(mm)

Al

(mm)

Cu

(mm)

HK10 0.042 0.045

HK20 0.15 0.128 0.170

HK30 0.52 0.408 0.596

HK60 3.19 0.079 0.113

HK100 3.90 0.15 0.298 0.463

HK200 1.15 1.669 2.447

HK250 1.60 2.463 3.37

HK280 3.06 3.493 4.089

HK300 2.51 3.474 4.205

WS60 0.3 0.179 0.206

WS80 0.529 0.337 0.44

WS110 2.029

5

0.97 1.13

WS150 1.03 1.88 2.13

WS200 2.01 3.09 3.35

WS250 4.01 4.30 4.50

WS300 6.54 5.23 5.38

NS10 0.095 0.049 0.061

NS15 0.49 0.153 0.167

NS20 0.90 0.324 0.351

NS25 2.04 0.691 0.762

NS30 4.02 1.154 1.374

NS40 0.21 0.082 0.094

NS60 0.6 0.241 0.271

NS80 2.0 0.59 0.62

NS100 5.0 1.14 1.19

NS120 4.99 1.04 1.76 1.84

NS150 2.50 2.41 2.57

NS200 2.04 2.98 1.003

3

4.09 4.20

NS250 2.01 2.97 5.34 5.40

NS300 2.99 4.99 6.17 6.30

LK10 0.30 0.061

LK20 2.04 0.441

LK30 3.98 0.18 1.492

LK35 0.25 2.21

LK55 1.19 0.260

LK70 2.64 0.509

LK100 0.52 2.0 1.27

LK125 1.0 4.0 2.107 2.094

LK170 1.0 3.0 1.5 3.565 3.592

LK210 0.5 2.0 3.5 4.726 4.733

LK240 0.5 2.0 5.5 5.515 5.542

a The additional filtration does not include the inherent filtration. The inherent filtration is a combination of the 

filtration due to the monitor chamber plus 1 mm Be for beam codes LK10-LK30, NS10-NS30, HK10-HK30; for all 

other techniques the inherent filtration is adjusted to 4 mm Al. Details of these reference radiation qualities can be 

found in the following: ISO/IS 4037-1:1996(E) X and Gamma Reference Radiations for Calibrating Dosimeters and 

Dose Rate Meters and for Determining Their Response as a Function of Photon Energy - Part 1; Radiation 

Characteristics and Production Methods. bThe x-ray tubes were installed in 2008 and 2015.

Dosimetry Group Number DG: #####-20

NIST ID 46011C Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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Mammography X-Ray Beam Quality Parameters

Beam code Tube voltage Additional filtrationa Half-value layer

(kVp) (mm)

Al

(mm)

Mo Anodeb

Mo/Mo23 23 0.032 Mo 0.288

Mo/Mo25 25 0.032 Mo 0.313

Mo/Mo28 28 0.032 Mo 0.346

Mo/Mo30 30 0.032 Mo 0.370

Mo/Mo35 35 0.032 Mo 0.404

Mo/Rh28 28 0.029 Rh 0.420

Mo/Rh32 32 0.029 Rh 0.453

Mo/Mo25x 25 0.030 Mo + 2.0 Al 0.551

Mo/Mo28x 28 0.030 Mo + 2.0 Al 0.589

Mo/Mo30x 30 0.030 Mo + 2.0 Al 0.633

Mo/Mo35x 35 0.030 Mo + 2.0 Al 0.715

Rh Anode

Rh/Rh25 25 0.029 Rh 0.351

Rh/Rh30 30 0.029 Rh 0.438

Rh/Rh35 35 0.029 Rh 0.512

Rh/Rh40 40 0.029 Rh 0.559

Rh/Rh30x 30 0.029 Rh + 2.0 Al 0.814

Rh/Rh35x 35 0.029 Rh + 2.0 Al 0.898

aThe inherent filtration is approximately 1.0 mm Be from the x-ray tube window and 0.075 mm 

polyimide, graphite coated, from the transmission monitor. bThe HVL values were measured directly 

using the Mo anode installed in Dec 2008. The linking comparison for mammography beams: Kessler, 

C., Burns, D.T. and O’Brien, M., “Key comparison BIPM.RI(I)-K7 of the air-kerma standards of the 

NIST, USA and the BIPM in mammography x-rays,” Metrologia 48 06014 (2011).

Dosimetry Group Number DG: #####-20

NIST ID 46011C Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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CCRIa X-Ray Beam Quality Parameters Offered at NIST

Beam code Tube 

voltage

Added

filtrationb

Half-value

layerc

(kV) (mm Al) (mm Cu)

BIPM25 25 0.373 0.240 mm Al

BIPM30 30 0.208 0.167 mm Al

BIPM40 40 3.989 0.212 2.649 mm Al

BIPM50a 50 3.989 2.291 mm Al

BIPM50b 50 1.007 1.038 mm Al

BIPM100 100 3.248 0.149 mm Cu

BIPM135 135 1.060 0.265 0.496 mm Cu

BIPM180 180 3.842 0.482 1.003 mm Cu

BIPM250 250 3.842 1.618 2.502 mm Cu

aBIPM, Qualités de rayonnement, Consultative Committee for Ionizing Radiation 

(CCEMRI) (Section I), 1972, 2, R15. Details of these reference radiation qualities 

can be found in the following: D. T. Burns et al, "Key comparison BIPM.RI(I)-

K3 of the air-kerma standards of the NIST, USA and the BIPM in mediumenergy x-rays," Metrologia 54 06006 (2017) and Burns, D.T., Kessler, C. and 

O’Brien, M., “Key comparison BIPM.RI(I)-K2 of the air-kerma standards of the 

NIST, USA and the BIPM in low-energy x-rays,” Metrologia 49 06006 (2012).

bThe additional filtration does not include the inherent filtration of the x-ray tubes 

which is approximately 3 mm Be and 1 mm Be.

cThe HVL values were determined for the tubes installed in 2008 and 2015.

Dosimetry Group Number DG: #####-20

NIST ID 46011C Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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Explanation of Terms Used in the Calibration Procedures and Tables (o)

Air Kerma: (g) The air–kerma rate at the calibration position is realized by a free-air ionization chamber 

for x radiation and is expressed in units of gray per second (Gy/s). (m) This realization of the air kerma 

establishes the national standard for air kerma which can be transferred through a suitable measuring 

instrument, thus establishing traceability to the national standard. For a free-air ionization chamber with 

measuring volume V, the air-kerma rate is determined by the relation:

=

− 

i

i

k

e g

W

V

I

K

air

air

air 1

1





where

I / (airV) is the ionization current, measured by the standard, divided by the mass of air in the 

measuring volume

Wair is the mean energy expended by an electron of charge e to produce an ion pair in dry air; 

the value used at NIST is Wair/e = 33.97 J/C

gair is the fraction of the initial kinetic energy of secondary electrons dissipated in air 

through radiative processes; the value is 0.0 (negligible) for x rays with energies less than 300 

keV, and

 ki is the product of the correction factors to be applied to the standard.

Air kerma K, in gray (Gy), is related to exposure X, in roentgens (R), by the following equation:

e

W

g

E

K

X

air

1 air

2.58 4

−

−

=

To obtain exposure in roentgens, divide air kerma in grays by 8.76E-03 for x rays with energies less than 

300 keV.

Beam Code: The beam code identifies important beam parameters and describes the quality of the 

radiation field. NIST offers four types of reference beam qualities, as well as the ISO reference radiation 

qualities. NIST beam codes are referred to as L, M, H, and S beams, which stand for light, moderate, 

heavy, and special filtration, respectively. The number following the letter is the constant potential across 

the x-ray tube. The mammography beam codes are a combination of the chemical symbol of the anode

and the filter respectively, followed by the constant potential, in kilovolts across the x-ray tube. The exit 

beam qualities, which represent the transmission of the x-rays through the breast, are created by an 

additional filtration of 2.0 mm of aluminum. The letter “x” ends the beam codes which refer to exit beam 

qualities.

Calibration Distance: The calibration distance is that between the radiation source and the detector center 

or the reference line. For thin-window chambers with no reference line, the window surface is the plane 

of reference. The beam diameter at the stated calibration distance is appropriate for the chamber 

dimensions.

Dosimetry Group Number DG: #####-20

NIST ID 46011C Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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Calibration Coefficient: (g,m,o) The calibration coefficients given in this report are quotients of the air 

kerma and the charge generated by the radiation in the ionization chamber, in units of Gy/C. 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 (see below). 

(q)The calibration coefficients listed in this report can be used to calculate the air kerma through the 

quantification of charge collected from the test instrument in calibration conditions that approximate NIST 

beam codes. A detailed study of ionization recombination was not performed. No recombination 

correction was applied to the calibration coefficient(s). If the chamber is used to measure an air-kerma 

rate significantly different from that used for the calibration, it may be necessary to correct for 

recombination loss.

Effective Energy: The effective energy is shown for those beams where it is considered a meaningful 

characterization of the beam quality. The effective energy for gamma radiation is the mean photon energy 

emitted by the radionuclide, and for x radiation it is computed from good-geometry copper attenuation 

data. The initial slope of the attenuation curve is used to determine the attenuation coefficient, and the 

photon energy associated with this coefficient is given as the "effective energy." The energy vs 

attenuation-coefficient data used for this purpose were taken from J. H. Hubbell, Int. J. Appl. Radiat. Isot. 

33, 1269 (1982). For beam codes H50-H300, the effective energy is well represented by the equation: 

effective energy = 0.861V - 6.1 keV where V is the constant potential in kilovolts. 

Half-Value Layer: The half-value layers (HVL) in aluminum and in copper have been determined by 

measurements with a free-air chamber for x radiation.

Homogeneity Coefficient: The homogeneity coefficient is the quotient of the first HVL and the second 

HVL, generally expressed as a percent.

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.

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)

Uncertainty: (n) The expanded, combined uncertainty is formed by taking two times the square root of 

the sum of the squares of the standard deviations of the mean for component uncertainties obtained from 

replicate determinations, and assumed approximations of standard deviations for all other uncertainty 

components; it is considered to have the approximate significance of a 95 % confidence limit. Details of 

the uncertainty analysis are given in: Lamperti, P.J., O’Brien, M., "Calibration of X-Ray and GammaRay Measuring Instruments," NIST Special Publication 250-58 (2001).

Dosimetry Group Number DG: #####-20

NIST ID 46011C Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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Appendix C: Proficiency Test Requirements for ISO 17043:2010 Conformity assessment –

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 identified by the corresponding 

letter in red font.

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. Since the NIST QA procedure requires stability for the chamber measurements, any 

change in the reproducibility above 0.2 % may require an investigation into the chamber and/or the support 

equipment. If the chamber completes the procedure at NIST with acceptable stability and is found to 

become unstable at the participant’s facility, using the acceptable stability limits of the said 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 during all calibrations at NIST. 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 will not provide the 

normalized error or the Z-score. 

Assigned values

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

calibration service, the proficiency test protocol and the proficiency test report. The details are repeated 

here. The air–kerma rate at the calibration position is realized by a free-air ionization chamber for x 

radiation and is expressed in units of gray per second (Gy/s). This realization of the air kerma establishes 

the national standard for air kerma which can be transferred through a suitable measuring instrument, thus 

establishing traceability to the national standard. The calibration coefficient is the quotient of the air kerma 

and the charge generated by the radiation in the ionization chamber, in units of Gy/C. The calibration 

coefficients used for the proficiency test are revealed at the end of the test in the form of a report issued
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to the participant. Early disclosure of the results is not allowed.

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 at the distance listed 

in the protocol, typically 1 meter.

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 with the required elements identified by the corresponding letter in red font. 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);
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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;

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

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. The 

communication with the participant is documented in the data file associated with the proficiency test. If 

statements of participation or performance are issued by a NIST proficiency testing program, the 

statements will contain sufficient information as to not be misleading. 
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PROFICIENCY TEST PROTOCOL

FOR AIR KERMA MEASUREMENTS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

A NIST proficiency test is being conducted to test the ability of your calibration facility to transfer a 

measurement of air kerma and to provide NIST traceability. 

NIST Technical Coordinator (b): Michelle O’Brien 

Laboratory Technical Contact (e): Name of Contact

Start Date of Test: Month, Year (k)

Anticipated Completion Date of Test: Month, Year (k)

Information on technical aspects of this measurement comparison may be obtained from (b) Michelle 

O’Brien, National Institute of Standards and Technology, 100 Bureau Drive Stop 8460, Gaithersburg, 

MD 20899, michelle.obrien@nist.gov, (301) 975-2014. 

Report format 6/2022

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

Page 1 of 6
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PROFICIENCY TEST PROTOCOL

FOR AIR KERMA MEASUREMENTS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Scope, Method and Traceability (a, d, e, f, g and q)

This (e) single participant proficiency test has been requested to fulfill the testing requirement for 

accreditation body, if applicable. An ionization chamber (g) was calibrated using the NIST primary 

free-air ionization chamber in the (a) NIST x-ray calibration facility in building 245, room H127. The 

air kerma rate, in terms of Gy/s, at the calibration position is realized by a NIST free-air ionization 

chamber. (q) This realization of the air kerma establishes the National standard for air kerma, which can 

be transferred through a suitable measuring instrument, thus establishing traceability to the National 

standard.

A NIST reference class ionization chamber has been sent for the requested proficiency test with NIST. 

The protocol for proficiency testing involves the calibration of a NIST reference class, transfer 

ionization chamber, by the participating facility. After the participating facility calibrates the NIST 

ionization chamber, using their appropriate NIST equivalent beam quality(s), the chamber is returned to 

NIST. The participant provides NIST with the results of the calibration coefficient(s) for the NIST 

chamber found at said facility in units of Gy/C in terms of the HVL of the NIST beam quality(s) 

specified, including uncertainty. The chamber is re-calibrated upon arrival at NIST, to ensure that no 

damage occurred while in transit. The participant’s calibration coefficient(s) should be provided to 

NIST, normalized to one standard atmosphere (101.325 kPa) and 295.15 K (22 oC) for a direct 

comparison with the average of the NIST calibration coefficients. The comparative results will be 

included in a proficiency test report issued upon the completion of the test. The results will reveal the 

degree to which the participating calibration facility can demonstrate proficiency in transferring a NIST 

air-kerma calibration under the conditions of the said facility at the time of the proficiency test. 

Questions or concerns should be directed to the NIST contact.

The criteria (d) for the test is that the participant must have a linking traceable measurement of airkerma. Using the participant’s established measurement methods and procedures, the participant should 

provide NIST with the (f) calibration coefficient(s) for the chamber found at their facility for each beam 

in units of Gy/C. The average current used to compute the calibration coefficient is based on 

measurements with the chamber at the stated polarity and potential. At the conclusion of the test, each 

calibration coefficient will be compared to the NIST calibration coefficient and the results will be 

provided as a percent difference from the NIST measured value. The results reveal the degree to which 

the participating calibration facility can demonstrate proficiency in calibrating an ionization chamber 

under the conditions of the said facility at the time of the test. The comparative results and the NIST 

uncertainty will be included in a NIST report issued upon the completion of the test. 

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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PROFICIENCY TEST PROTOCOL

FOR AIR KERMA MEASUREMENTS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Conditions for the Chamber and Sources in the NIST Facility: (k and m)

The values of the calibration coefficients are determined using the procedures outlined in the NIST quality 

measurement system for service ID 46011C. The chamber is open to the atmosphere and all measurements 

were normalized to one standard atmosphere and 22 degrees Celsius. Use of the chamber at other pressures 

and temperatures requires normalization of the ion currents to these reference conditions. No correction is 

made for the effect of water vapor on the instrument being calibrated. 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. Leakage corrections are applied. A detailed study of ion recombination was not 

performed. No recombination correction was applied to the calibration coefficient(s). If the chamber is 

used to measure an air-kerma rate significantly different from that used for the calibration, it may be 

necessary to correct for recombination loss.

The safe handling procedures of the chamber manufacturer are used, and the chamber is kept at safe 

laboratory conditions between 20° C and 24° C and relative humidity < 50 %. The results relate only to 

the instrument calibrated in this report.

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

Page 3 of 6

Chamber Exradin A4 Sn XP######

Bias 500 V (negative charge collected on electrometer)

Rotation The white mark faced the source of radiation.

Orientation The chamber was positioned in the center of the beam with 

the base of the chamber perpendicular to the beam direction

Distance 100 cm to center of volume

Beam size 10 cm diameter

Air Kerma rates

and Beam Codes

1.9 E-5 Gy/s (H200) and 4.6E-4 Gy/s (M250)
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PROFICIENCY TEST PROTOCOL

FOR AIR KERMA MEASUREMENTS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Participant Shipping Address and Dates (b and u)

Following is the contact information required for the proper shipment of the chamber. After the request 

for the proficiency test by the participant was processed through the NIST payment system, an agreed 

upon date was established for the shipment of the chamber, at the owner’s expense. At the completion of 

the proficiency test measurements at the participant’s facility, the results should be provided to NIST 

electronically, prior to returning the chamber to NIST. Once NIST deems the test complete, the chamber 

should be returned to NIST in the reusable container. The tracking information will be provided by the 

participant using shipping insurance which would cover loss or damage. 

Ship to participant:

Ship Date: month/day/year

Shipped to:

complete address

Email: 

Telephone:

Return to NIST:

Ship Date: month/day/year

Shipped to:

NIST,100 Bureau Dr.,Bldg 245, Rm H127, Stop 8460 Attn: Michelle O’Brien

Gaithersburg, MD 20899-8460

Phone: 301-975-2014

Email: michelle.obrien@nist.gov

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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PROFICIENCY TEST PROTOCOL

FOR AIR KERMA MEASUREMENTS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Format of Participant Results (s) (o and r)

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, normalized to 295.15 K (22 oC) and 101.325 kPa (1 Atm)

limiting the value to significant digits. The equipment used for the measurement should be provided in a 

report with details to include the model and serial number of the chamber, most recent calibration 

history, the bias potential and the electrometer identification. The date of the calibration, the complete 

uncertainty budget and a description of the geometry should be included in the participant’s results. The 

method the participant used to calculate the calibration coefficient should be included in the report.

Format of Final NIST Report

The results presented to the participant, at the conclusion of the test, will be in a table similar to that 

below. The NIST value for each coefficient is an average. The uncertainty is determined by the standard 

deviation of the mean of the charge measurements.

Comparative Results for the NIST Transfer Standard 

NIST

Beam Code

NIST Calibration Coefficient

295.15 K (22 oC) and

101.325 kPa (1 Atm)

(Gy/C)

Participant Calibration Coefficient 

295.15 K (22 oC) and

101.325 kPa (1 Atm)

(Gy/C)

Difference 

in Percent

M250 1.###E+06 1.###E+06 0.31

H200 1.###E+06 1.###E+06 0.24

Confidentiality, Collusion and Falsification of Results (j and t)

The identification of the test results will stay confidential and secure on the NIST server, with limited 

access by the authorized quality management system QMS calibration staff. Summary reports may be 

published but the identification of the participant will be withheld. It is the decision and the 

responsibility of the participant to provide the proficiency test results to the accreditation organization. If 

collusion or falsification of the results is suspected then the test will be terminated, fees applied and 

another test, with an associated fee, will be required.

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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PROFICIENCY TEST PROTOCOL

FOR AIR KERMA MEASUREMENTS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Potential Source of Errors (h)

Differences in geometry can contribute to errors. Failure to normalize data to 295.15 K (22 oC) and 

101.325 kPa (1 Atm) will result in an error. The same voltage bias used by NIST should be used by the 

participant.

Statistical Design (d, p and r)

Each chamber is measured at least three times by NIST in each beam and the average is used. 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 

will not provide the normalized error or the Z-score.

Initial Stability Check and Quality Control of Ionization Chambers (i and n)

Only stable and reproducible ionization chambers are accepted for use for proficiency testing. NIST 

prepares the ionization chamber through a series of redundant measurements in highly reproducible 

radiation fields prior to using the chamber for the proficiency test and repeats those checks at suitable 

intervals. Since the NIST QA procedure requires stability for chamber measurements, any change in the 

reproducibility above 0.2 % may require an investigation into the chamber or support equipment. If the 

chamber is found to become unstable at the participant’s facility, using the acceptable stability limits of 

the said facility, NIST should be notified, and the chamber should be excluded from the measurement 

comparison and a replacement chamber will be provided. 

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. 

Evaluation of proficiency testing scheme results

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.

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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REPORT OF PROFICIENCY TEST FOR AIR-KERMA CALIBRATIONS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Calibrations performed by Michelle O’Brien (c)

Report technical review by Ronaldo Minniti (c)

Report approved by Michael G. Mitch, Dosimetry Group Leader (c)

James M. Adams

Chief of the Radiation Physics Division

Physical Measurement Laboratory

For the Director of the National Institute of Standards and Technology

Information on technical aspects of this report may be obtained from Michelle O’Brien, National 

Institute of Standards and Technology, 100 Bureau Drive Stop 8460, Gaithersburg, MD 20899, 

michelle.obrien@nist.gov, or (301) 975-2014. (a, b)

Report format version 6/22.

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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REPORT OF PROFICIENCY TEST FOR AIR-KERMA CALIBRATIONS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Proficiency Testing Scheme (i, q)

The protocol for proficiency testing involves the calibration of a NIST reference class, transfer ionization 

chamber, by the participating facility. After the participating facility calibrates the NIST ionization 

chamber, using their appropriate NIST equivalent beam quality(s), the chamber is returned to NIST. The 

participant provides NIST with the calibration coefficient(s), for the NIST chamber found at their facility 

in units of Gy/C in terms of the half-value layer of the NIST beam quality(s). The chamber is re-calibrated 

upon arrival at NIST, to detect damage or any changes which may have occurred while in transit. The 

participant calibration coefficient is compared to the average of the NIST calibration coefficients. The 

comparative results are given in the following tables. The results reveal the degree to which the 

participating calibration facility can demonstrate proficiency in transferring a NIST air-kerma calibration 

under the conditions of the said facility at the time of the proficiency test. It is the responsibility of the 

participant to inform the accrediting body of the results of this proficiency test.

Stability Assessment

Only stable chambers are used for the NIST proficiency test. The NIST QA procedure requires stability 

for the chamber and primary chamber measurements. Any change in the reproducibility of the charge 

above 0.2 % will be investigated.

Statistical Design (l, n, r)

The chamber is measured at least three times for each beam by NIST and the average is used. The 

standard deviation on the charge measurement is used in the uncertainty analysis as the type A 

uncertainty component. 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 will not provide the normalized error or 

the Z-score. 

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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REPORT OF PROFICIENCY TEST FOR AIR-KERMA CALIBRATIONS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Calibration Conditions at NIST

Date Chamber shipped to participant: dd/mm/yr

Chamber return date to NIST: dd/mm/yr

Calibration dates: dd/mm/yr

Calibration distance: The center of the volume was positioned 100 cm from the source.

Chamber orientation: The cavity was positioned in the center of the beam with the stem of the chamber 

perpendicular to the beam direction.

Chamber collection potential: 500 volts to collect a negative charge on the electrometer

Chamber rotation: The white mark faced the source of radiation.

Temperature range: 293.2 K (20 °C) to 294.6 K (21 °C)

Pressure range: 99.2 kPa to 101.4 kPa

Background current: less than 0.02 % of signal

The results relate only to the instrument calibrated in this report. 

Calibration Results from the NIST Facility

Beam Code Half –Value 

Layer

(mm Al)

Calibration Coefficient 

295.15 K (22 oC) and

101.325 kPa (1 Atm)

(Gy/C)

Air- Kerma 

Rate

(Gy/s)

Beam 

Diameter

(cm)

Calibration

Distance

(cm)

M250 18.49 1.###E+06 #.##E-04 10 100

H200 19.72 1.###E+06 #.##E-05 10 100

Expanded, Combined Uncertainty, in Percent

NIST

Beam Code Air-Kerma Rate (Gy/s) NIST Calibration Coefficient (Gy/C)

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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REPORT OF PROFICIENCY TEST FOR AIR-KERMA CALIBRATIONS

Participants complete name and address

Radiation Detection Chamber: Exradin A4 Sn XP######

Participant’s Calibration Conditions as reported by Participant (d, j) 

Calibration distance: The center of the volume was positioned 100 cm from the source.

Chamber orientation: The cavity was positioned in the center of the beam with the stem of the chamber 

perpendicular to the beam direction.

Chamber collection potential: -500 volts to collect a negative charge on the electrometer

Chamber rotation: The white mark faced the source of radiation.

Beam size: Used the reference delivery system for the 30 cm x 30 cm field with optimized uniformity 

across the central 15 cm x 15 cm area. 

Nominal Air-Kerma rates: 5.0 E-04 Gy/s (M250) and 2.3 E-05 Gy/s (H200)

Comparative Results for the NIST Transfer Standard 

NIST

Beam Code

NIST Calibration Coefficient

295.15 K (22 oC) and

101.325 kPa (1 Atm)

(Gy/C)

Participant Calibration Coefficient 

295.15 K (22 oC) and

101.325 kPa (1 Atm)

(Gy/C)

Difference 

in Percent

M250 1.###E+06 1.###E+06 0.31

H200 1.###E+06 1.###E+06 0.24

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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Radiation Physics Division 46011C, 46050S RPD-P-03

CALIBRATION OF X-RAY RADIATION DETECTORS

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4.70 4/5/2023 CMO AKT 82 of 84 Procedure03v470

Comments and advice: (p, s, t)

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

analyze the results applying the established criteria. The expectations are on the PT customer and their 

accrediting body to use the purchased PT report as they find necessary within their own quality system. 

The NIST PT results do not mean, and should not be implied to mean, evaluation, endorsement, or 

certification of commercial firms' products and services. 

Confidentiality, Collusion and Falsification of Results (g)

The identification of the test results will stay confidential and secure on the NIST server, with limited 

access by the authorized quality management system (QMS) calibration staff. Summary reports may be 

published but the identification of the participant will be withheld. It is the decision and the 

responsibility of the participant to provide the proficiency test results to the accreditation organization. If 

collusion or falsification of the results is suspected then the test will be terminated, fees applied and 

another test, with an associated fee, will be required.

All beam code tables provided but not shown here for efficiency.

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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Radiation Physics Division 46011C, 46050S RPD-P-03

CALIBRATION OF X-RAY RADIATION DETECTORS

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4.70 4/5/2023 CMO AKT 83 of 84 Procedure03v470

Explanation of Terms, Establishment of Assigned Values and Metrological Traceability (m)

Air Kerma: The air–kerma rate at the calibration position is realized by a free-air ionization chamber for 

x radiation and is expressed in units of gray per second (Gy/s). This realization of the air kerma establishes 

the national standard for air kerma which can be transferred through a suitable measuring instrument, thus 

establishing traceability to the national standard. For a free-air ionization chamber with measuring volume 

V, the air-kerma rate is determined by the relation:

=

− 

i

i

k

e g

W

V

I

K

air

air

air 1

1





where

I / (airV) is the ionization current, measured by the standard, divided by the mass of air in the 

measuring volume

Wair is the mean energy expended by an electron of charge e to produce an ion pair in dry air; 

the value used at NIST is Wair/e = 33.97 J/C

gair is the fraction of the initial kinetic energy of secondary electrons dissipated in air 

through radiative processes; the value is 0.0 (negligible) for x rays with energies less than 300 

keV, and

 ki is the product of the correction factors to be applied to the standard.

Air kerma K, in gray (Gy), is related to exposure X, in roentgens (R), by the following equation:

e

W

g

E

K

X

air

1 air

2.58 4

−

−

=

To obtain exposure in roentgens, divide air kerma in grays by 8.76E-03 for x rays with energies less than 

300 keV.

Beam Code: The beam code identifies important beam parameters and describes the quality of the 

radiation field. NIST offers four types of reference beam qualities, as well as the ISO reference radiation 

qualities. NIST beam codes are referred to as L, M, H, and S beams, which stand for light, moderate, 

heavy, and special filtration, respectively. The number following the letter is the constant potential across 

the x-ray tube. The mammography beam codes are a combination of the chemical symbol of the anode

and the filter respectively, followed by the constant potential, in kilovolts across the x-ray tube. The exit 

beam qualities, which represent the transmission of the x-rays through the breast, are created by an 

additional filtration of 2.0 mm of aluminum. The letter “x” ends the beam codes which refer to exit beam 

qualities.

Calibration Distance: The calibration distance is that between the radiation source and the detector center 

or the reference line. For thin-window chambers with no reference line, the window surface is the plane 

of reference. The beam diameter at the stated calibration distance is appropriate for the chamber 

dimensions.

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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Radiation Physics Division 46011C, 46050S RPD-P-03

CALIBRATION OF X-RAY RADIATION DETECTORS

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4.70 4/5/2023 CMO AKT 84 of 84 Procedure03v470

Calibration Coefficient: The calibration coefficients given in this report are quotients of the air kerma 

and the charge generated by the radiation in the ionization chamber, in units of Gy/C. 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 (see below). 

The calibration coefficients listed in this report can be used to calculate the air kerma through the 

quantification of charge collected from the test instrument in calibration conditions that approximate NIST 

beam codes. A detailed study of ion recombination was not performed, and no correction was applied to 

the calibration coefficient(s). If the chamber is used to measure an air-kerma rate significantly different 

from that used for the calibration, it may be necessary to correct for recombination loss.

Effective Energy: The effective energy is shown for those beams where it is considered a meaningful 

characterization of the beam quality. The effective energy for gamma radiation is the mean photon energy 

emitted by the radionuclide, and for x radiation it is computed from good-geometry copper attenuation 

data. The initial slope of the attenuation curve is used to determine the attenuation coefficient, and the 

photon energy associated with this coefficient is given as the "effective energy." The energy vs 

attenuation-coefficient data used for this purpose were taken from J. H. Hubbell, Int. J. Appl. Radiat. Isot. 

33, 1269 (1982). For beam codes H50-H300, the effective energy is well represented by the equation: 

effective energy = 0.861V - 6.1 keV where V is the constant potential in kilovolts. 

Half-Value Layer: The half-value layers (HVL) in aluminum and in copper have been determined by 

measurements with a free-air chamber for x radiation.

Homogeneity Coefficient: The homogeneity coefficient is the quotient of the first HVL and the second 

HVL, generally expressed as a percent.

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.

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)

Uncertainty: (m) The expanded, combined uncertainty is formed by taking two times the square root of 

the sum of the squares of the standard deviations of the mean for component uncertainties obtained from 

replicate determinations and assumed approximations of standard deviations for all other uncertainty 

components; it is considered to have the approximate significance of a 95 % confidence limit. Details of 

the uncertainty analysis are given in: Lamperti, P.J., O’Brien, M., "Calibration of X-Ray and GammaRay Measuring Instruments," NIST Special Publication 250-58 (2001).

Dosimetry Group Number DG:#####-20

NIST ID 46050S Order Number: 682.02/O-0000000###-20 

Report Date: 00/00/2020

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