Radiation Physics Division 43010C, 43020C RPD-P-01

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Purpose

The purpose of this procedure is to describe the measurement and reporting procedures for 

gamma-ray-emitting radionuclides using the NIST secondary standard ionization chambers 

(SSICs). 

Scope

The NIST SSICs are “4”− re-entrant pressurized ionization chambers, which are well-type 

instruments used for measuring the radioactivity (disintegrations per second) of gamma-rayemitting radionuclides. Scheduled calibration services for submitted samples of gamma-ray 

emitters are offered as a complement to the Standard Reference Material (SRM) and 

Measurement Assurance programs. These calibration services are 43010C (half-lives longer than 

15 days) and 43020C (half-lives shorter than 15 days). The nominal range of activity for the 

gamma-ray-emitting radionuclides is 0.4 MBq to 60 MBq and the nominal uncertainty range is 

0.5 % to 2.3 % (expanded uncertainties, k = 2), both vary by radionuclide.

The SSICs use instrument-specific calibration factors (K-values) for each radionuclide 

previously established by direct methods of activity measurements (Hoppes 1984; NCRP 1985).

Records of the direct measurements of activity used to determine K-values are kept in the Kvalue Record Book. For further information on specific methods refer to NIST SP 250-10. SSIC 

calibrations are checked with those of other national standardizing laboratories through 

international measurement comparisons, particularly those conducted by the Bureau International 

des Poids et Mesures (BIPM) with the Système de Référence (SIR), SIR Transfer Instrument 

(SIRTI), or Key Comparisons (https://www.bipm.org/en/radionuclide-metrology). Results of 

these comparisons are obtainable from the Key Comparisons Database (KCDB, 

https://www.bipm.org/kcdb/ comparison/quick-search) maintained by the BIPM. 

The gamma-ray-emitting radionuclides which may be submitted to NIST for IC calibrations are

those listed in the K-value Record Book as having established, primary standardizations

Definitions

K-value: The relative calibration factor, KR, is given by RR/R, the ratio of the measured responses 

of the 226Ra reference source (RRS) and a known one becquerel (1 Bq) of a stated radionuclide.

Equipment

Equipment manuals are maintained either as paper copies in Building 245, Room H223 or as 

electronic copies on the attached computers. Current instrument models and serial numbers are 

listed in the instrument-specific Lab Notebooks (ICA and AutoIC) located in H223. Current 

software and database versions are also listed in these notebooks and referenced by name and file 

date.
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Hardware

• The NIST "4π"-γ (re-entrant) cylindrical ionization chamber “A” (ICA; described in NIST SP 

250-10)

• The NIST Automated Ionization Chamber (AutoIC; also referred to as ionization chamber “B”, 

ICB) [Described in Fitzgerald (2010)]

• Multi-range electrometer (for each SSIC)

• Sample holder(s)

• Radium-226 reference sources

• A network connected desktop computer

• A micro-analytical balance

Equipment Quality Control

Multi-range electrometer: Individual measurements of submitted sources are evaluated as ratios 

to the appropriate reference source, therefore normally fluctuating environmental conditions do 

not impact the measurement result. Multi-range electrometers may be subject to non-linearities 

between ranges. Upon installation correction factors are measured based on a short-lived 

decaying source or a variable current source and, if appropriate, incorporated in the data 

collection program. The linearity of the electrometers is checked biennially, as scheduling 

allows, not to exceed 2.5 years, using short-lived decaying sources. The result of a check for 

consistency, including any updates to correction factors, is recorded in the IC lab notebook.

Radium-226-reference sources: K-values are sometimes determined for only one RRS. K-values 

for other RRSs are calculated using RRS ratios. RRS ratios are checked at least every five years. 

The result of the measurements and comparison with historical data is recorded in the IC Lab 

Notebook. Failure of an individual RRS should be investigated if there is a change in ratio 

greater than three times the standard uncertainty on the ratio measurement. RRSs are checked for 

external contamination, indicating possible failure, prior to each measurement campaign.

Micro-analytical balance: If a customer source is supplied in a container other than the NIST 

type-1 flame sealed ampoule a micro-analytical balance is used to transfer the solution to a NIST 

type-1 ampoule. It is the intent of the Radiation Physics Division (RPD)/Radioactivity Group 

(RG) that there are enough balances available so that no one piece of equipment is essential to 

the completion of this calibration service. For example, the preparation of 5 g radioactive point 

sources may require a microbalance with a capacity of >7 g, a readability of <9.5 μg, and a 

standard uncertainty of <50 μg (relative to the SI). The RPD/RG currently has at least 6 

balances that meet these specifications, all of which are serviced and calibrated once per year 

with NIST traceable masses. This step is normally carried out by, or in cooperation with, the 

Coordinator of radioactivity SRM production and with the concurrence of the RG leader. A 

report of the calibration is maintained by the SRM Coordinator. The due date for the next 

calibration is noted on the balance.
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Software

Data acquisition software

ICi_[x].vi: ICi is a LabVIEW program running on the ICA desktop computer. The current 

version of the software is linked by a desktop shortcut. It is used to collect data from the ICA

with input from the user. Data are stored offline for further processing. Data are backed up in 

accordance with the Data Management Plan for ICs.

Robot.exe: Robot is a Visual Basic program running on the AutoIC desktop computer. It calls 

the current version of a LabVIEW program that collects data from the AutoIC with input from 

the user. Data are stored offline for further processing.

Data analysis software

Ionol4.exe: Ionol is a FORTRAN program running on a desktop computer. It is used to process 

the data file created by the acquisition programs using further input from the user and 

information from ICDAT4.TXT.

ICDAT4.TXT: ICDAT4.TXT is a data file containing the most recent K-values listed in the Kvalue Record Book for both the ICA and AutoIC. Note, the ICA K-values listed in this file have 

been corrected for the height effect (Fitzgerald 2012), as documented in the K-value Record 

book. For each radionuclide, the database also includes the half-life to be used for any decay 

corrections by the analysis software.

Validation

The validation of equipment is performed upon installation and subsequent to any change in 

equipment, and subsequent to any significant environmental event. 

Validation of processing software is performed by: manual calculation of an experimental result 

and comparison of this value to that obtained from the program or processing of a test data set 

yielding a known activity result. This is performed upon the initial version and subsequent to any 

changes in the program.

Results of validations are recorded in the IC Lab Notebooks.
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Safety

Radiation safety

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

Division (RSD). The room used for ionization chamber calibrations is designated as a Radiation 

Area. A countertop lead cave is used to house samples during measurement and to store the 226Ra 

reference sources. In the case of high-activity sources, a lead-lined “L” shield might be used 

while changing samples.

Ampoules to be measured must be checked and be found to be free of any external radioactive 

contamination or defects in the flame-seal before being loaded into the ionization chamber 

sample holder.

All source containers must be labelled in accordance with RSD requirements for labelling of 

radioactive sources. Ampoules may be labelled in pen but must not have affixed paper labels. 

Paper labels could cause the ampoule to become stuck in the ionization chamber source holder. 

Chemical safety

Typical solutions are dilute acids with microgram quantities of carrier ions per gram of solution. 

Any proposed solution must fall withing the requirements defined in the associated hazard 

review.

Procedures

Source Container and Contents

The solution to be assayed in the SSICs should have as nearly as possible the same composition 

as the standards used to calibrate the chamber (described in NIST SP 250-10). The gamma-rayemitting radionuclide solution should be (5.0 ± 0.2) mL in a standard, NIST-type, 5 mL ampoule. 

The specifications of these ampoules are as follows: 

body diameter 16.5 ± 0.5 mm 

wall thickness 0.60 ± 0.4 mm 

barium content less than 2.5 percent 

lead oxide content less than 0.02 percent 

other heavy elements trace quantities 

If necessary, the transfer of the radionuclides in solution is done using a pycnometer and mass 

difference to dispense the solution. The received container (i.e., ampoule, v-vial, dose vial, 

bottle, etc.) is opened and the solution is drawn with a pycnometer and transferred into a NIST-1 

ampoule for measurement. The balance used is noted on the mass sheet for cross reference to the 

yearly calibration. Only balances marked as being within their calibration period may be used. 

The NIST-1 ampoule is then sealed and checked for external contamination using approved 
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procedures. Since the radionuclides have certified values of activity per unit mass, the total mass 

of the solution in the ampoule must be measured and included in the report.

Description of the Actual Measurement Process 

The stability of these chambers is monitored by measuring response to a reference source of 

226Ra in equilibrium with its daughters whenever calibrations are made. The activity of the 

sample is taken to be proportional to the ratio of the chamber response (current) for the sample to 

that for the reference source. The relative calibration factor, KR, is given by RR/R, the ratio of the 

measured responses of the 226Ra reference source and a known one becquerel of a stated 

radionuclide.

The activity, A, of the radionuclide in a sample to be measured, S, is given by KRRS, where RS is 

the response (current) for the sample relative to that for the radium source. Background currents,

measured just before and after counting the sample and radium, must be subtracted from the 

response for each. The KR diminishes from the value at the time of the original calibration with 

the 1600-year half-life of the 226Ra, and a decay correction for this must be made.

The activity per unit mass may only be reported if the mass of solution is determined by NIST.

Preliminary

• Customer contact: give specifications for chemical composition and activity limits, source must 

either be provided as 5 mL solution in a NIST-type 5 mL flame-sealed ampoule or transferred to 

one by NIST.

• NIST paperwork and acceptance procedure: submit completed NIST 364 (or NIST 365), 

Radioactive Material Request, for approval before arrival of materials. Specifically notify the 

Chief of RSD or designate of the arrival and departure of all isotopes of plutonium and uranium. 

Complete any other necessary forms (NIST 64, Test Record; NIST 77, Calibration and Test Fee 

Computing Form; NIST 796A, Shipper’s Declaration for Radioactive Materials, etc.).

• E-Commerce: Note dates of material received (and returned) in the E-commerce system.

• Schedule time to use the IC’s by blocking and initialing the required amount of time on the 

calendar in 245/H223. Possible conflicts should be resolved in person with other users. Generally 

shorter half-life sources take precedence. 

• Deliver source, or a related sample, to the appropriate person to be measured on a calibrated 

germanium spectrometry system according to instructions for an impurity measurement. The 

results and source are delivered to the analyst at the completion of this measurement.
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Receiving Instructions for Radioactive Solution Samples 

All radioactive sources arriving at NIST are delivered to RSD where they are examined to 

determine the radiation level and possible contamination of containers and packaging. 

After RSD releases the source to the analyst, the analyst in turn should verify that the correct 

radionuclide has been shipped in the activity range which has been previously discussed with the 

customer. Quality control procedures that should be implemented by the analyst are as follows: 

1. Re-examine the source for pin holes of any type and check the ampoule seal. 

2. Inspect the label attached to the primary container. This helps to ensure that it is the expected

radionuclide and the activity amount is correct.

3. Read technical data sheets and package inserts accompanying the radionuclide package 

carefully for special instructions on stability, quality inspection, and chemistry of the product. 

This information should conform to the calibration guidelines specifying the physical and 

chemical properties of the submitted sample solution. 

Providing that no defects are observed, and clean swipes have been documented by RSD or by 

the analyst, the sample may be transported for measurement in the SSICs. In the case of above 

criteria not being met, contact customer.

Step-by-Step Operating Instructions for the SSICs

Complete an IC Log Book entry.

Starting the SSIC system

The SSIC system to be used is started from its connected computer using the following 

procedure: 

1. Check schedule and observe system to verify it is not currently in use. 

2. Verify computer system time is set to the correct Eastern Standard Time (EST). If necessary, 

correct the time using NIST network time (check nist.time.gov and be sure to convert to EST if 

checked during daylight saving time). 

3. For ICA, click on desktop icon ICi_[x].vi. For AutoIC, click on desktop icon Robot.exe. and 

click “OK” when prompted. 

Background Measurements

Background measurements are made initially and should be examined before continuing. If the 

background is inconsistent, or abnormally different, investigate the cause. High background may 
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be due to an unshielded source in the vicinity. It is possible to make measurements under high 

background conditions by measuring the background more often. Typical backgrounds can be 

viewed using the program Control Chart Viewer.vi on the respective attached computer.

Loading Sources

Complete procedures for safely handling radioactive sources and placing them into the source 

holders, including personal protective equipment (PPE) and tools, are provided in the appropriate 

Standard Operating Procedures.

When placing the solution sample in the chamber, observe the ampoule to make certain that no 

liquid is in the neck of the ampoule. 

ICA

Gently place the 226Ra reference source or ampoule into the rigid, tight-fitting Lucite source 

holder (“holder 2”). With the bottom part of the holder seated into the top part, align the 

markings on the RRS with the alignment mark on the source holder. Gently lower the source 

holder into the chamber using the string; do not drop. Align the markings on the source holder 

with those on the instrument shielding. (Note – The source holder is an integral part of the IC 

system. Care must be taken to place the holder shaft in a position where it will not roll and fall. If 

a cart is used, it must have walls or a tray to restrict movement of the holder.)

AutoIC

Gently place the 226Ra reference source and/or ampoule(s) into rigid, tight-fitting source holders, 

screw the top and bottom pieces together taking care not to cross-thread or overtighten, and place 

them in appropriate positions on the AutoIC source rack. 

Measuring the RRS and Sample(s)

Alternate measurements of RRS and samples. Each measurement duration is typically set for at 

least 60 seconds, with longer times used for weaker sources. At least four sets of 10 

measurements should be made on the sample, with bracketing measurements made on the radium 

reference source in each cycle.

For ICA, each measurement requires completing the appropriate fields (highlighted in blue) in 

the software for each source before each measurement. For AutoIC, the parameters for all of the 

measurements to be performed are entered by setting up a sample database in the Robot.exe 

software.

To commence ICA measurements, click “Start Run”. To commence AutoIC measurements, click

“Run samples using database”.
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After all measurements have been completed, it is recommended that background measurements 

be repeated.

Data Collection

The data collection is under control of data acquisition software. All data are collected and stored 

to a disk for off-line analysis. Data are backed up in accordance with the Data Management Plan

Pibida gamma spec.

Measurement of impurities

The following gives a summary of the procedures documented in the Procedure for gamma-ray 

spectrometry measurements for activity calibration and impurity measurements using High 

Purity Germanium (HPGe) detectors.

Impurity-check measurements of radioactive sources to be calibrated by SSICs are performed 

using the Radiation Physics Division (RPD) Gamma-Ray Spectrometry Systems located in 

Building 245, Rooms C015 and H119. This system consists of six High Purity Germanium 

(HPGe) detectors and one Lithium-drifted Silicon detector. These detectors are calibrated using 

NIST Standard Reference Materials (SRMs), which are, in turn, validated through measurement 

comparisons with other national metrology institutions and the BIPM through the periodic 

submission of measured sources to the SIR for gamma-ray emitting sources. The calibration of 

each of the seven detectors is done through the development of efficiency curves for different 

source geometries and various source-to-detector distances. These efficiency curves are verified 

monthly or more frequently if needed (such as when there has been a change in the instrument 

configuration), using previously calibrated 57Co and 60Co point sources to determine the 

efficiency value, shape (FWHM) and energy calibration reproducibility. Efficiency curves for 

point sources (solids), 5 mL ampoules (liquids) and 33 cm3spheres (gas) for gamma-ray emitting 

sources for the energy range 14 keV < E < 3.6 MeV are available for impurity and activity 

calibration measurements.

Gamma-ray spectra are acquired using GammaVisionTM (version 8.00.03 as of this writing), a 

commercially available software developed and marketed by Ametek/EG&G ORTEC or using 

Genie 2000 from Canberra (Version 3.4.1) depending on the detector. Data analysis is performed 

using Genie 2000 from Canberra (Version 3.4.1). The choice of data analysis software was 

based on in-house comparisons of the results obtained by several (4) different software packages. 

In addition, the data analysis software used by customers was taken into account. (Such 

commercial products are identified in order to specify these procedures accurately; the RPD does 

not recommend or endorse any commercial product, nor intend to imply that these products are 

necessarily the best available for the purposes of these procedures.)

The combined standard uncertainty (uc, corresponding to k = 1) on the activity values for the 

impurities and/or source measurement is given by 
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uc = √ueff

2 + umeasure

2

where ueff is the uncertainty of the efficiency for the measured gamma-ray energies for the 

impurity and/or source, and umeasureis the uncertainty calculated based on the gamma-ray lines 

used to determine the activity of the measured impurity and/or source. As impurity levels are 

generally much lower than the nuclide being calibrated, the uncertainty on the impurity 

measurement is often on the order of 5 % to 20 %, for very small impurity levels the uncertainty 

maybe higher. 

Processing the IC Data

The data processing is done using Ionol4.exe. The program averages the data for each sample 

and corrects them for background and radioactive decay (using the established half-life). The 

sample activity is calculated using the K-value for that radium reference source most closely

producing a current approximating that of the sample. The results are printed out.

Correction for impurities is handled by the processing program. Results of impurities are entered 

as an activity ratio relative to the main radionuclide and a measurement time.

The K-value used in the determination of the activity is checked against the K-value documented 

in the K-Value Record Book for the radionuclide being measured. In case of a discrepancy, the 

K-Value Record Book holds the official K-value.

In specific cases, documented in the K-Value Record Book, the K-value may be dependent on 

solution composition, particularly the density of the solution. This is normally seen in lowenergy gamma-emitters. If the solution composition of the measured sample is different from the 

composition of the sample used to determine the K-value, a correction to the K-value must be 

made using the equation listed in the K-value Record Book.

The Decay Data Evaluation Project (DDEP) online database should be consulted to be certain the 

half-life used for any decay corrections is the most recent value.

Acceptance Criteria

The data are examined to see if the results show the expected activity, and the standard deviation 

of repeated measurement on the source is checked to see if it is in the range expected for the 

radionuclide (normally less than 0.1 percent at 1 sigma). If there is a discrepancy in activity, the 

source may be measured in the other SSIC. Alternatively, the source may be measured in a 

commercial re-entrant ionization chamber, or “dose calibrator,” as a confirmatory measurement. 

This measurement is performed by placing the sample in the dose calibrator source holder and 

entering the correct instrument setting for the radionuclide in question. Settings are found on a 

list attached to the instrument or in the instrument manual. The analyst must be familiar with the 

use of the dose calibrator and the accuracy of the settings. If the standard deviation is abnormally 
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high, the data is examined for possible trends. The customer is contacted if a discrepancy cannot 

be resolved.

Calibration Report

At the end of a calibration exercise, the customer will be given a Report of Calibration stating 

primarily:

a) the principal radionuclide

b) reference time and date

c) SSIC used and method of calibration (traceability to primary standard)

d) certified value for activity or activity per unit mass with associated mass

e) decay-scheme assumptions

f) assessment of radionuclidic purity

g) overall uncertainty determinations for calibration

If the notes section of the report includes a comparison to customer provided values for the 

activity of the source, that note must include the following text: “As guidance for the proper use 

of this Report, it should be emphasized that the National Institute of Standards and Technology is 

concerned only with fostering good measurements capability and consistency with the national 

measurements system. The assurance of the proper application of that capability to the ultimate 

consumer products is the responsibility of each manufacturer of these products and of the Federal 

regulatory agencies.

A continuing traceability program in radioactivity demonstrates, to the degree established by the 

periodic assays of calibrated radioactivity samples, a continuing competence to maintain the 

methods and standards necessary for accurate measurement. Such a program cannot, however, 

endorse each and every measurement nor the final product, any more than a spot check can 

vouch for every unchecked item. Care should be taken, therefore, not to imply such 

endorsement. The proper use of this Report is governed by section 200.114 of Title 15 of the 

Code of Federal Regulations. These regulations may be met if Reports are quoted only in their 

entirety. Excerpts out of context may be misleading.”

Reported values are checked against the data on the IC printout and this check is noted by 

initialing the printout. Drafts of reports are stamped as such and identified by draft number. 

Results of draft proofreading are noted and initialed on the draft.

Signatures are obtained at the appropriate level and, if paper, the original report is embossed with 

a Department of Commerce seal. A copy marked file copy and two marked copies are 

maintained with the calibration printouts, stored in folders identified by company, radionuclide, 
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date, and service number. If requested, the original paper report is sent to the customer.

Electronically signed reports are stored in the calibration record of the E-commerce system.

Customer Sources

When the calibration service is completed, the technical users are notified, and the calibrated 

sample returned, if desired by customer. Short-lived radionuclide samples are usually measured 

by the customer only before transmittal to NIST, for the activity would be too low for 

measurement after the NIST calibration.

Uncertainty Analysis

The bases for the determination of uncertainties are the ISO Guide to the Expression of 

Uncertainty in Measurement and the Guidelines for Evaluating and Expressing the Uncertainty 

of NIST Measurement Results.

The purpose of this section is to explain the derivation of the various components of uncertainty.

Measurements performed with a SSIC must be done under completely fixed conditions. This 

requires a stable background, no saturation losses due to incomplete charge collection because of 

recombination at high rates, and corrections for all impurities. Ampoules must be of the NIST 

standard 5 mL type (NIST-1), the volume in ampoules must be 5.0 ± 0.2 mL, and the 226Ra 

reference source must be corrected for decay.

Uncertainty in activity measurements may be increased if the contribution of the background is 

variable, large, or both. The uncertainty quoted is derived from measurement of the variations. 

The operator of the chamber should make frequent measurements of the background and look for 

the source of any unusual variations observed.

• K-value (Type A and Type B) – uncertainty in the determination of the K-value, including the 

measurement of activity by primary methods and measurements to transfer that calibration to the 

ionization chamber (specific uncertainties for individual K-values are listed in the K-value 

Record Book)

• RRS ratio (Type A) - uncertainty in the ratio of radium reference sources, when the radium 

source used in the measurement is different from the radium source used for the determination of 

the K-value

• Half-life (Type B) – uncertainty in the half-life value(s) used for decay corrections

• N measurements on this sample (Type A) – standard deviation determined from N

measurements of activity ratios

• Impurities in this sample (Type A and Type B) - Radionuclidic impurities may or may not have 

a significant effect, depending on the radiations of the principal and impurity radionuclide and 

the amount of impurity present. If their half-lives are relatively short, measurements at different 
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times may be used to check consistency. High purity germanium detector systems are used to 

identify and quantify the significant radionuclidic impurities that may be present. The 

uncertainty in these measurements is propagated through the calculated effect on the IC results. 

• Gravimetric (Type B) – typical uncertainties from gravimetric transfers of solution, if necessary

• Density (Type B) – uncertainty from correction for differences between density of submitted 

source solution and the solution used to determine the K-value, if necessary

Records

Calibration printouts and report copies are stored in folders identified by company, radionuclide, 

date, and service number.

If the source is returned, the date that the source is shipped back to the customer should be 

entered into the folder.

Folders of calibration records are maintained in 245/C300-3 for at least five years.

Documentation

K-Value Record Book – maintained in 245/C300-WSBF

Ionization Chamber Laboratory Notebooks – maintained in 245/H223

Ionization Chamber Log Book (tabs for ICA and AutoICs) – maintained in 245/H223

SSIC software manuals – maintained in 245/H223

Standard Operating Procedures for SSICs – 846.04-0035 and 846.04-0037 posted in the 

245/H223

Procedure for gamma-ray spectrometry measurements for activity calibration and impurity 

measurements using High Purity Germanium (HPGe) detectors – maintained in 245/C015 and 

245/H119.
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References

Hoppes, D. D., Basic Radionuclide Measurements at the United States National Bureau of 

Standards, Environmental Int. 10, 99-107 (1984).

NCRP Report 58, A Handbook of Radioactivity Measurements Procedures, 

National Council on Radiation Protection and Measurement, Bethesda, Maryland, 

(1985).

Taylor, B.N., and Kuyatt, C.E. “Guidelines for Evaluating and Expressing the 

Uncertainty of NIST Measurement Results,” NIST Technical Note 1297 (1994).

Calhoun, J.M., Radioactivity Calibrations with the NIST “4π” Gamma Ionization Chamber, 

NIST Special Publication 250-10, 1987

Fitzgerald, R. An automated ionization chamber for secondary radioactivity standards, App. 

Radiat. Isotop. 68, 1507 (2010).

Fitzgerald, R. NIST Ionization Chamber “A” Sample-Height Corrections, J Res Natl Inst Stand 

Technol Vol 117 (2012).

Filing and Retention

Calibration printouts are stored in folders identified by company, radionuclide, date, and service

number.

If the source is returned, the date that the source is shipped back to the customer should be 

entered into the folder. 

For customer calibration, prepare calibration report and obtain required signatures. Make a copy 

for customer file and send original. Reports created and signed digitally are downloaded to the 

calibration record in the E-commerce system.

Records are maintained in 245/C300-3 for at least five years.

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

Procedure.
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Appendix A. Sample Calibration Report
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