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1. Purpose and Scope

This document describes procedures used at NIST to calibrate protection-level betaparticle sources and laboratory–standard transfer instruments. Between 1982 and 1985, 

the National Bureau of Standards (NBS) as NIST was then known, with partial support 

from the U.S. Nuclear Regulatory Commission, developed a facility suited to establish a 

service for calibrating certain user-owned encapsulated beta-particle sources and certain 

laboratory–standard transfer instruments for application in radiation protection (Pruitt et 

al. 1988). This work was undertaken in response to the need of the radiation-protection 

community for better calibrations of beta-particle survey instruments and dosimeters. 

This task was ably performed by J.S. Pruitt who was also responsible for the first wave of 

automation of the extrapolation chamber and data acquisition. In 1988 Dr. C. G. Soares 

took over this service upon the retirement of Dr. Pruitt. He had been aided in further 

efforts to automate the system by Dr. M.G. Mitch, who was responsible for the first 

version of LabView code used for data acquisition and control. A new version of 

LabView code and upgraded instrumentation were implemented by Dr. C. Szabo-Foster 

in 2022. The methods and corrections as developed by Pruitt were applied until 2000, 

when the methods and corrections of ISO 6980-2 [ISO 6980-2, 2004] were implemented. 

1.1. Quantity for Reporting Calibration Results 

The physical quantity of interest in radiation protection is the absorbed dose (rate)1to 

tissue, from which the effective dose equivalent (rate) and the pertinent practical doseequivalent (rate) quantity (or quantities) may be derived. For the application in radiation 

protection, absorbed dose (rate) to tissue may be set equal to absorbed dose (rate) in 

water. Therefore, source-calibration results are reported by NIST in terms of absorbeddose rate to water at a fixed source-to-detector distance, and instrument-calibration 

results are reported as calibration coefficients, the quotients of absorbed-dose (rate) to 

water and the corresponding instrument reading ("scale indication"). Throughout this 

document, water is considered to be equivalent to tissue. 

1.2. Sources and Instruments Accepted for Calibration 

The following beta-particle sources and instruments are accepted: 

1) Encapsulated sources of 90Sr+90Y and 85Kr with activities leading to absorbeddose rates to water ranging from a few tenths of one Gy/s to a few mGy/s, for 

calibration in terms of absorbed-dose rate to water at fixed distances in air 

(corresponding to conditions specified in ISO 6980) [ISO 6980-1, 2006] at a 

reference depth of 0.07 mm. 

2) Thin-window parallel-plate ionization chambers suited for use as transfer 

instruments for the calibration of beta-particle sources in terms of absorbed-dose 

rate to water. (see also Section 2.5) 

1.3 Procedures for Calibration and Internal Quality Control 

1

 Throughout this document, when a statement is made that can refer either to a particular radiation quantity 

or to its rate, the name of the quantity is followed by the word "rate" in parentheses. 
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Calibrations are performed at fixed source-to-detector distances. If a beam-flattening 

filter is to be used with the submitted source (see Section 2.3), it is to be supplied with the 

source. The calibration uncertainty (for a coverage factor of k=2) for 90Sr+90Y is reported 

as 2.2 % and for 85Kr sources as 2.4 %.For thin-window parallel plate ionization 

chambers the calibration uncertainty is reported as 2.2 % when calibrated with 90Sr+90Y

and 2.4 % when calibrated with 85Kr (see Section 3.7). Consistency of calibration results 

is ensured through internal quality-control procedures comprising (a) the calibration in 

terms of absorbed-dose rate to water of a NIST beta-particle source of known absorbeddose rate along with the submitted unknown source incorporating the same radionuclide, 

or (b) the calibration of a NIST extrapolation chamber along with the chamber submitted 

in at least one of the calibration beams. Calibration data on the submitted source or 

chamber are considered acceptable if the data obtained (when corrected for decay) for the 

NIST source or instrument agree to within the statistical uncertainty (type A uncertainty) 

stated in Tables 5 and 6 of Section 4.7. 

1.4 International Comparisons of the Determination of Absorbed-Dose Rate to Water

Absorbed-dose rate to water were determined for the four Buchler beta-particle 

secondary standard sources (BSS1, described in Section 2.3) at the reference distances 

from measurements of ionization currents in the extrapolation ionization chamber. The 

methods used for this initial comparison were those applicable to calibrations performed 

before 2000 (Pruitt et al. 1988). The results are shown in Table 1. Since the sources had 

been initially calibrated at the Physikalisch-Technische Bundesanstalt (PTB), it was 

possible to compare the NIST calibrations with those obtained at the PTB in a similar 

geometry at the same reference distances. Table 1 shows a comparison for absorbed-dose 

rates to water at the surface. The results agree to 1 % or better for the 90Sr+90Y sources 

and to 2 % for 204Tl. A 4 % disagreement for 147Pm is not surprising, considering the size 

of the overall uncertainty in the measurement of the ionization current (see Table 5, 

section 4.7) and the relatively strong dependence of the measurement results on the 

calibration geometry, as well as the difference in calculation methods of some of the 

corrections applied to the measured currents.

One of the aims of the ISO Working Group 2, Sub Group 0 on reference radiations for 

beta particles was to harmonize the methods used to calculate the correction factors. The 

fruits of this effort are evident in the latest comparison between PTB and NIST, 

performed when the BSS2 set of sources was obtained. The results of this comparison are 

shown in Table 2, where now it is seen that these are in agreement to better than 2 % for 

all sources and distances. In this comparison, both parties used the methods specified in 

ISO 6980-2 for the calculation of correction factors for the measured currents.

2. Equipment

2.1 Rationale for Facility Choice and for Preliminary Studies

One of the reasons why protection measurements for beta particles are considerably more 

difficult than for photons is the beta particles' vastly greater interaction in the media 

intervening between the source and the point of measurement ("reference point"). This 
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results in change in the beta-particle spectrum with the change of measurement 

configuration – and in most instances also in detector-response characteristics. As a 

consequence, calibrations of radiation-protection instruments in beta-particle beams can 

be considered strictly valid only for the particular configuration in which they are 

performed and are applicable to field measurements in similar configurations only. 

Conversely, knowledge of the degree to which changes in measurement configuration 

may produce changes in beam characteristics and therefore in calibration results is of 

considerable importance. Therefore, prior to establishing the calibration service, the 

influence on spectral characteristics of beam-flattening filters and other absorbing 

materials in the beta-particle beams, and the influence of changes in source-to-detector 

distance, were investigated (Pruitt et al. 1988). Changes in absorbed dose rates with 

elevation above sea level were given special attention (Pruitt 1985).

2.2 Facility Design

The NIST facility for carrying out beta-particle measurements for radiation-protection 

purposes is located in Building 245, Room H015 and consists of:

1) a set of well-characterized beta-particle sources covering a wide energy and doserate range;

2) an extrapolation ionization chamber suited for the determination of absorbed-dose 

rates to water at distances from the beta-particle sources appropriate for the study 

of radiation-protection instruments;

3) an electrical interlock system causing the source shutter to close automatically 

when the safety perimeter (established by an infrared beam) is penetrated; and

4) measurement and control equipment capable of changing the air gap, proper 

biasing and measuring current produced in the extrapolation chamber, and for the 

determination of necessary corrections. The rest of Section 2 deals with a more 

detailed description of these systems and with their use.

2.3 The Beta-Particle Sources

NIST purchased its first complete Buchler beta-particle secondary standard source setup 

in 1982, referred to in the following as BSS1, and a second, modernized setup was 

obtained from AEA Technologies in 1998, referred to as BSS2. Both sets of sources were 

initially standardized by the PTB (see Section 1.4). BSS1 consists of four encapsulated 

beta-particle sources, a source holder on a stand equipped with a shutter, separate beamflattening filters for each type of source and associated electrical circuitry for shutter 

operation and timing. The beam flattening filters are those specified in ISO 6980: plastic 

discs, thicker in the center than on the periphery for 90Sr+90Y and 204Tl, and with a central 

circular opening for 147Pm. When installed perpendicular to the beams and concentric 

with the beam axis at a pre-set distance from the sources, they filter the beams selectively 

so as to achieve beam cross sections of relatively constant absorbed-dose rate over an 

area sufficient for instrument calibration (see also Sections 2.4.1 and 4.11), and consistent 

with the requirements of ISO 6980 for Series 1 reference radiation fields. Figure 1 shows 

a picture of BSS1 and the extrapolation chamber. BSS2 is of similar design to BSS1 

except that there are only three sources, and 204Tl is replaced by the longer lived and 
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higher activity 85Kr. This system is also safer, since each source now resides in its own 

holder which contains a shutter. The system is also more automated and contains sensors 

to verify that the correct distance and filter have been set. Irradiations at nonperpendicular angles are also facilitated, as well as operation of the system in a vertical 

beam geometry. Figure 2 shows pictures of BSS2 as well as the source capsules. 

Information on the properties of the beta-emitting nuclides employed and on source 

structure and nominal activity is given in Table 3. Figure 3 shows the approximate 

theoretical beta-particle spectra of the four radionuclides, covering a range of average 

energies from 0.06 MeV to 0.8 MeV.

2.4 The Extrapolation Ionization Chamber

2.4.1 Description

The extrapolation ionization chamber is a device designed to achieve a continuously 

variable air volume. This volume is defined by two electrodes and an insulating ring 

which separates the collecting region from the guard region. A potential is placed 

between the two electrodes which causes ionization products to be swept out (collected) 

of the defined volume. The radiation enters the chamber through a thin (0.006 mm thick) 

carbonized polyethylene terephthalate (PET) window which also serves as the "high 

voltage" electrode by having a voltage placed upon it. The radiation passes through the 

air in the gap between the electrodes, striking the other (collecting) electrode which is 

held at ground potential. The sign of the voltage placed on the high voltage electrode 

determines the sign of the ionization products collected by the collecting electrode. Since 

the beta particles are also stopped in the collecting electrode material, there is always a 

negative current being measured and recourse must be made to measurements at both 

polarities in order to remove this unwanted "parasitic" current from the desired ionization 

current. Figure 1 is a picture of the arrangement for producing the beta-particle fields

with BSS1 and for determining absorbed-dose rates to water in these fields by means of 

measurements with an extrapolation chamber. The beta-particle sources fit into the 

shuttered source holder. Different beam flattening filters are employed with the different 

radionuclide sources. The extrapolation chamber (shown before the stepping motor was 

added to the end to control the air gap remotely) on the left was designed by the PTB and 

built for NIST by Pychlau Technical Works (PTW). The body of the chamber is made of 

polymethyl methacrylate (PMMA). Its diameter is 140 mm – which makes it large 

enough to simulate a phantom of infinite size. The plate separation is variable and can be 

accurately determined by means of a micrometer measurement (see screw on the end of 

the chamber at left). The thin entrance window for the beta particles is constructed of 

carbonized polyethylene terephthalate (PET). The source shutters on both BSS1 and 

BSS2 are electrically interlocked with an infrared beam and photocell defining a safety 

perimeter as long as the shutter is open. Figure 2 shows a similar view of BSS2, with the 

extrapolation chamber absent. 

The chamber is mounted in a stand that can be moved from side to side across the table, 

for investigating beam uniformity in the calibration plane (which is the plane 

perpendicular to the beam axis at the calibration distance). The reference distances have 
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been chosen in accordance with ISO 6980 to be 20 cm for 147Pm, and 30 cm for 90Sr+90Y, 

204Tl and 85Kr. Variations in dose rate from BSS1 with reference distance can be studied 

by moving the chamber cart along the tracks.

2.4.2 Chamber Construction

Figure 4 shows a schematic cross-section through the front end of the extrapolation 

chamber, which was originally designed by [Böhm 1984]. The high-voltage electrode 

through which the incident beta particles enter the extrapolation chamber is a 0.67 

mg/cm2graphite-coated PET foil. The collecting electrode and the guard electrode are 

both graphite coatings on the acrylic piston. The plate separation, d, is changed by 

moving the piston using a stepping motor attached to a micrometer screw (not shown).

The ionization-chamber volume is taken to be the product of the area of the collecting 

area of the electrode, A = (/4)•(effective diameter)2, and the plate separation (air gap), d, 

of the electrodes. The effective diameter and consequently the area A can be measured 

accurately with a traveling microscope. Accurate determination of d is usually more 

difficult but can be readily accomplished by extrapolation measurements. 

In the current version of the NIST extrapolation chamber, all motions and the data 

collection are computer controlled. A single stepping motor is used to drive a rotating 

stage to move the collecting electrode towards the entrance window. Data is collected 

from the electrometer, and, since ionization chamber measurements must be corrected to 

reference conditions of temperature and pressure, from a thermometer and a barometer. 

The polarity and magnitude of the voltage applied to the entrance window is also under 

computer control.

2.4.3 Principle of Use

The following is a guide to the use of the extrapolation chamber for determining 

absorbed-dose rate to water at the points of interest in the beta-particle beams. Covered 

are steps leading from the initial ionization measurements to the fully corrected absorbeddose rates, including a description of the required corrections.

2.4.3.1 Measurement of Ionization Current as a Function of Chamber Depth

The ionization current is measured directly with an electrometer. Figure 5 shows the 

results of current measurements as a function of air gap with three types of sources, after 

application of certain corrections. These corrections are introduced in Eq. 4 below and 

are separately discussed in detail. The corrected currents vary linearly with chamber 

depth to a good approximation. Linearity is important since the slopes of the curves are 

used in the computation of absorbed-dose rate, as seen in Eq. 9, Section 4.5. The average 

of the absolute values of the deviations from the lines drawn, as determined from leastsquares computations, are too small to be shown in the figure.

The net measured current, I, is given by

I = (I+ + |I-|)/2, (1)
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where

I+,- = (Q2-Q1)/t, (2)

and Q1 and Q2 are the initial and final measured charge and t is the integration time. An 

associated quantity, known as the parasitic current, IB, is given by

IB = (I+ + I-)/2. (3)

This quantity represents the current in the collecting electrode due to incident beta particles 

stopping on the collecting electrode. It is only slightly dependent on chamber depth, 

increasing with decreasing chamber depth. It serves as a useful monitor of the measurement 

stability and should be recorded with the net current.

The measured currents are small to moderate, on the order of 100s of fA for most of the 

sources used and thus an extremely sensitive electrometer is required. The high voltage 

supply must be bipolar since measurements are made at both positive and negative 

polarities. The voltage gradient used is moderate, 10 V/mm, however the recommended air 

gaps are very small (≤ 2.5 mm), so only modest voltages are required.

2.4.3.2 Correcting the Net Measured Ionization Current

The corrected net ionization current, Ic, is given by

Ic = I iki jk'j, (4)

where I is the measured current, ki are corrections which may vary during the 

measurement of the extrapolation curve and k'j are corrections that are constant during the 

measurement. The following is a discussion of these correction factors. Note that 

correction factors in ISO 6980-2 which have the value of unity for all sources are not 

discussed.

a) k'ba, the backscatter correction, takes into account the difference in backscatter from 

water and backscatter from PMMA, the collecting electrode material. Values for k'ba

(taken from ISO 6980-2) are shown in Table 4. 

b) k'br, the bremsstrahlung correction, takes into account the contributions to the 

ionization current due to photons from material intervening between the source and the 

chamber air volume (chamber window plus added filtration, if any). Values for k'br (taken 

from ISO 6980-2) are shown in Table 4. 

The rest of the corrections discussed vary with the chamber depth.
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c) kad, the correction for air density changes inside the chamber, arises because of 

temperature and pressure variations of the ambient air, since the chamber is of a type 

which is open to the atmosphere. It corrects to the reference temperature and pressure 

using the relationship, 

ρa = ρa0 [

p

100.726

−

r

114.2

(

T

T0

)

17.97]

T0

T

 (5)

where

T is the absolute temperature in K of the air in the collecting volume;

p is the air pressure in kPa; 

r is the relative humidity of the air, expressed as a fraction; 

a0 is the air density for the reference conditions which are defined for the following 

parameters: 

T0 = 293.15 K,

p0 = 101.325 kPa, 

r0 = 0.65. For these conditions, 

a0 = 1.19740 kg m-3

Eq. 5 is valid only for standard test conditions. The correction factor for the variations of 

the air density within the collection volume, kad, is then given by kad = a0/a.

d) kabs, the correction for variations in the attenuation and scattering of beta particles 

between the source and the collecting volume due to variations from reference 

conditions, is calculated using the functions and coefficients given in ISO 6980-2.

e) kac, the correction for the attenuation of beta particles within the chamber air gap, 

is calculated using the functions and coefficients given in ISO 6980-2.

f) kdi, the beam divergence correction, is necessary because of the relatively short sourcedetector distance and the point-like nature of the source. Assuming inverse-square law 

dependence over the short distances involved in the change in air gaps in the 

extrapolation chamber measurement, it is calculated from

kdi = 1+d/y0, (6)

where y0 is the source to chamber surface distance.
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g) kpe, the sidescatter correction, takes into account the perturbation of the beta particle 

flux by the side walls of the extrapolation chamber. It is calculated using the functions 

and coefficients given in ISO 6980-2.

h) ksat, the correction for ion recombination and diffusion in the air inside the chamber, is 

usually quite small for protection level measurements. It only becomes appreciable for 

large ion currents and/or small voltages. It is calculated by means of published 

relationships [Böhm 1976] as

ksat = ([1- 5.05 × 10

13 Id 3

/ aU 2][1- 4.4 d / U][1- 17.24 × 10-5 T / U])-1 (7)

where d is the chamber depth in m, I is the measured current in A, U is the biasing 

voltage in V, a is collector area in m2and T is the ambient temperature in K. 

i) kde, the correction for source decay, can be taken into account by the correction factor

kde = exp[(tm – t0)ln(2)/t1/2] (8)

where t1/2 is the half life of the radionuclide, tm is the time at which the measurement is 

made, and t0 is the time to which the measurement is being corrected to, which is the 

reference time for the measurement. For the measurement of an extrapolation curve, the 

reference time is usually taken as the beginning of the measurement. Table 3a shows 

values of half lives for the beta-particle emitting nuclides used in this service.

3. Health & Safety

While the absorbed dose rates involved with protection level measurements are generally 

low, the principle of As Low As Reasonably Achievable (ALARA) shall be followed. To 

this end, unshielded sources are only handled using the tool described in Section 4.12. 

The interlock system described in Section 2.2 shall be employed to prevent accidental 

exposure to the calibration beams. The entrance to the back part of Room H015 where the 

calibration fields are located is very narrow and shall be roped off when radiation fields 

are expected to be present. In addition, the area shall be posted as a Radiation Area.

4. Procedures

4.1 Source Acceptance

Once a customer has contacted NIST and indicated that a source is being shipped for 

calibration, a NIST 364 form must be completed and electronically submitted to the

Radiation Safety Division (RSD). The source is then assigned an RS# which will be used 

in future source inventories. The source will be removed from inventory when it is 

returned to the customer. (See Section 4.8).
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4.2 Chamber acceptance

Chambers need only be inspected for damage. Note that PTW extrapolation chambers 

should be shipped with the micrometer screw locked in place. This is accomplished by 

moving the micrometer to a setting of about 10 mm, which exposes a 5 mm Allen screw 

which is screwed out to lock the piston and screwed in to free the piston. This is to 

protect the window from the piston during shipment. The window should also be 

protected from the outside by a covering of thick plastic which is screwed into place on 

the face of the chamber. Check for damage to the window before proceeding with the 

calibration.

4.3 Opening a test folder

Once a customer completes the required online purchase using the E-Commerce system, 

an order number is generated for the calibration request.

4.4 Source leak testing

Prior to being released by RSD, the source will be checked for removable radioactive 

contamination. This test usually takes the form of swiping the inside and outside of the 

Type A container as well as the outside of the shielded source container.

4.5 Initial information to be logged in the Data Book

In the data book prior the start of the calibration the following information shall be 

recorded:

1) customer name, address and phone number

2) source RS#, assigned by RSD

3) source make, model and serial number (located usually on a flat on the handle)

3) customer purchase order number (PO#), from customer

5) NIST order number (O#), assigned by the E-Commerce system

6) NIST Report of Calibration number (DG#), assigned by Group or Division secretary

4.6 Generation of Report

Once the measurements have been completed and analyzed (see Sections 4.11 through 

4.16), a Word template is used to generate the final calibration report. Be careful in 

generating this report that the appropriate fields are changed, such as the DG number, 

RS# and dates in the footer, the customer and source information on the front page, the 

source ID and measurement data tables. A sample report for an instrument calibration is 

shown in Appendix A.

In addition, a copy of all pertinent raw and analyzed data results listed in the final 

calibration report is to be included in the review process. These values should be 

highlighted for the reviewer to facilitate comparison with the final reported values. This 

will enhance the accuracy check of reports and ensure that no mistakes were made when 

entering values into the final report template.
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4.7 Uncertainty of Calibration

The method of uncertainty assessment used here follows the recommendation of NIST 

1260 [Taylor and Kuyatt 1994]. Evaluations of the uncertainty estimates are of two kinds. 

Conventional statistical estimates of random uncertainties are given as standard 

deviations of the mean, designated “Type A”, which can be considered to be objective 

estimates. All other uncertainty estimates, designated “Type B”, are subjective estimates, 

based on the extensive experience of the calibration staff. The Type B uncertainties are 

estimated so as to have roughly the character of standard deviations. The estimates of 

each type are combined in quadrature, and then the two results are combined in 

quadrature to give a combined standard uncertainty. The combined standard uncertainty 

in turn is multiplied by two to give an expanded uncertainty with a confidence level of 95 

%.

Table 5 gives a breakdown of the uncertainties in the determination of absorbed-dose rate 

to water at the 7 mg/cm2depth of interest from the ionization current measured with the 

extrapolation chamber. The data in this table are based on the detailed presentation of 

corrections in Section 2.4.3 and measurement procedures in Sections 4.11 through 4.16. 

Many of these components are common to ionization chamber measurements and should 

be consistent with those of other Radiation Physics Division calibration service 

uncertainty budgets. A few, however, are unique to this service. The uncertainty in the 

determination of absorbed dose rate to water and known influence of measurement 

geometry on source calibration are then used in Table 6 to arrive at combined standard 

uncertainties in the calibration of beta particle sources and transfer ionization chambers 

submitted to NIST. The expanded uncertainty is quoted as two times the combined 

standard uncertainty. 

4.8 Source Shipment

When the calibration has been completed satisfactorily, the source is repacked in the 

Type A package in which it arrived along with the final signed calibration report. In cases 

where there is a delay in obtaining the required signatures on the report and the customer 

has an urgent need for the source to be returned, the source may be shipped with only a 

draft, not fully signed report to be used by the customer in the interim until the final 

report is ready. In this case it shall be clearly indicated in a note attached to the report that 

the report is a draft and that the final report will be sent separately. Shipping Radioactive 

Material (RAM) may be performed only by trained personnel. Contact RSD for 

assistance in preparing the package for shipment.

4.9 Instrument Shipment

Instruments are packed securely inside the container in which they were received. Note 

the precautions to be taken to prepare PTW extrapolation chambers for shipment given in 

Section 4.2.
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4.10 Mounting Extrapolation Chambers in BSS2

The extrapolation chamber is attached at its base to a support rod which is in turn 

attached to the aluminum stand shown in Figure 1. A 30 cm distance rod is supplied, 

which goes into the source stand exactly as sources do (see Section 4.12). Remove any 

flattening filters before installation of the distance rod and be very careful not to poke 

out the window of the chamber with a distance rod! Changing windows on a PTW 

extrapolation chamber should not be a routine occurrence (see Section 5.2); they are 

expensive and not easy to change. It is prudent to set chambers up with some sort of 

covering over the window, even if it's only the 7 mg/cm2cowl. A better covering is the 

plastic protective piece which goes over the chamber for shipping. It is also convenient if 

the chamber center is marked on the covering used, although this is not crucial as the 

beams are fairly uniform and rigorous centering is not essential for good results. Small 

adjustments in the height of the chamber are possible by loosening the nut at the chamber 

base and screwing the support rod in or out. The face of the chamber is made parallel to 

the calibration plane by measuring the distance with a meter stick between the arms of 

the source support stand ("cross") on all four sides of the chamber face. Adjustments can 

then be made to the position of the table, turning it slightly and/or shimming the front or 

back legs. The last adjustment is to bring the chamber forward carefully until the distance 

rod just touches the window surface. This must be done with the cowl removed for the 

most accurate positioning. Unlike the centering, placing the chamber surface at the proper 

distance is crucial to good measurement results because the distances are relatively small 

and consequently the dose rate gradients appreciable. Once a chamber has been set up 

properly on the range, remove the distance rod and carefully place the proper cowl over 

the front face of the extrapolation chamber. The cowl is designed to add material at the 

chamber front to bring the measurement depth to the equivalent of 7 mg/cm2(0.07 mm) 

in water or unit density tissue. For the currently installed window of the NIST chamber, 

which is 0.67 mg/cm2, the cowl plastic added thickness is 6.87 mg/cm2 making a total 

thickness of 7.54 mg/cm2 which for PET is equivalent to 7.03 mg/cm2of water or unit 

density tissue (ISO 6980-2). The remaining small difference from the reference depth of 

7 mg/cm2is accounted for in the correction kabs (seen Section 2.4.3.2). To perform 

calibrations at distances other than 30 cm, unlock the positioning lock on the optical 

bench and slide the source support stand to the required position (11 cm, 20 cm, or 50 

cm) and relock the stand.

4.11 Mounting Sources in BSS2 

To install a source, take it from its storage pig and place it shutter first into the source 

stand. When it is all the way in, lock it in place using the lever at the back of the stand. 

Be careful not to open the shutter of the source capsule during manipulation; it's difficult 

but not impossible to so. Each source is keyed differently and the system can recognize 

which source has been installed. Finally, install the appropriate flattening filter. Each 

filter is also keyed differently and the system can recognize which filter has been 

installed. The filter supports are sized such that the filter will only go on the supports one 

way.
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4.12 Current Measurements

The basic measurement performed by all the data acquisition programs controlling the 

extrapolation chamber is the net current. Measurements at both positive and negative 

polarities are made. The net (uncorrected) current is obtained from the average of the 

magnitudes of the currents at the two polarities. 

4.13 Extrapolation Measurements using the LabView Extrapolation Chamber Control 

Programs

Data acquisition and control are accomplished using a suite of LabView programs. To 

initiate the measurement process, execute the program Main Extrapolation 

ControlMGM_CSF.vi, which can be found in the LV Beta folder of the host computer or 

starting the Beta control CSF.lvproj labview project. This will bring up the initial 

window allowing selection of the control mode. Choose Constant Time Extrapolation 

(the default) and then run the program by clicking on the arrow at the upper right. This 

brings up the PreExtReading page on which initial information about the measurement 

can be entered in as much or as little detail as desired. The main use of this information is 

for descriptive headers and production of input files for the PTB program BETA, which 

can be used for data analysis as an alternative to the Excel spreadsheet described in 

Section 4.15. When this page has been filled out to the user's satisfaction, click the 

Boolean switch. This starts the Extrapolation Chamber with setsMGM_CSF.vi LabView 

program, which is the main control page for the measurement. This is the most important 

page for the actual measurement as all the critical parameters controlling the 

measurement are entered here. First one chooses the number of readings per set. Choose 

six here to get five current measurements. Then choose the number of sets, a set being a 

series of measurements at both positive and negative polarities. Next enter the current 

micrometer reading on the barrel of the extrapolation chamber itself. This is the actual 

reading, which is about 0.06 mm less than the actual estimated air gap. Note that when 

"reading" is used it refers to the actual micrometer indication and when "air gap" is used 

it refers to the estimated actual air gap, which is the reading + 0.06 mm. Indicate which 

direction the air gap changes should go, either decreasing or increasing. If small currents 

(<20 fA), it is a good idea to try to have these measured out-of-hours; therefore, if 

starting in the afternoon, choose decreasing so that smaller air gaps will run in the middle 

of the night. If starting first thing in the morning, choose increasing air gaps to get the 

small air gaps done before most people arrive and start putting more load on the 

building’s electrical systems. Next choose the initial gap, 0.25 mm for increasing gap 

measurements and 2.5 mm for decreasing gap measurements. Next choose the gap 

increment, which should be 0.125 mm. Finally choose the number of air gaps, which will 

be 19 for the conditions specified. Check that everything on this page is correct and then

click the Boolean switch; this will bring up a smaller window on which the initial gap and 

the current reading are displayed. Make sure that these values are correct and then hit the 

Boolean switch. The extrapolation chamber should be moved to the initial gap. When the 

chamber has stopped moving, check that the micrometer reading is 0.06 mm less than the 

numerical value of the initial gap. Now the source and filter (if used) can be mounted if 

not already done so, and the area roped off. Reset the interlocks and open the shutter of 
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the BSS2 system using the BSS2 computer running the PTB software Beta3234 (shortcut 

available on the desktop of BSS2 computer) To open the shutter choose the Test menu in 

the PTB software (BetaSecondary Standard 2), click the Sensoren menu item to test the

functioning of the controls, then click Open Shutter in the Test menu. When the 

measurement is complete close the shutter by clicking Close Shutter in the Test menu of 

the PTB software on the BSS2 computer. At the end of the measurements the main 

control computer will provide a prompt to save the measured data. The first file is saved 

with the name PB#### (no extension) where #### is the measurement sequence number. 

This file contains all the data necessary for the absorbed dose measurement with the 

Excel file as described in Section 4.15. The other two files that can be saved are used by 

the BETA program for absorbed dose rate measurement. 

4.14 Calculation of absorbed dose rate

Absorbed dose rate to water, Ḋw, is given by the Bragg-Gray relationship

Ḋw =

(

W̅̅̅

e

)st,a

ρ0a

(

ΔIc

Δd

)

d→0

, (9)

where W̅

e

is the average energy in joules needed to produce one coulomb of charge of either 

sign in air at 65 % relative humidity (33. 83  0.06 JC-1), st,a is the ratio of the mean masscollision stopping power of water to that of air, 0 is the density of air at the reference 

temperature and pressure (T0, p0), a is the effective area of the collecting electrode, and 

(

ΔIc

Δd

)

d→0

is the rate of change of the corrected current with extrapolation chamber air-gap 

width as the width approaches zero. The values used for st,a for each source are given in 

Table 4. The value for the effective collecting area, a, is determined to be the collector 

diameter given by the manufacturer. For the NIST PTW chamber in Table 8, the value of 

the effective collecting area, a, is 7.0827 x 10-4 m2.

Excel templates are used to perform final calculations of the absorbed dose rate for the 

calibration report. The raw data from the extrapolation chamber control program are pasted 

into the template at the proper location. Current versus air gap curves are produced, as well 

as a plot of current per unit air gap as a function of corrected air gap. This latter curve is 

useful to assess any curvature in the current versus air gap function as well as the 

smoothness of the data since neither of these features are apparent in the current versus air 

gap plot. This plot is inserted in the data book as well as the final dose rate calculation, 

which is done in the Excel template, which shows all corrections applied to the data. The 

absorbed dose rate is quoted at a depth of 7 mg/cm2and represents an average over the 

central 30 mm diameter of the radiation field.

4.15 Acceptance of Results

Reference values for the sources used are given in Table 7. Quality assurance 

measurements are considered valid if the results obtained match those in Table 7 (after 

suitable decay correction) to within the uncertainties given in Table 5: (1.2 %). The same 
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criteria apply when a NIST reference chamber is calibrated for measurement assurance of 

a customer reference chamber.

5. Maintenance

5.1 List of critical equipment

A list of the critical equipment used in this calibration service is given in Table 8.

Calibration certificates showing NIST traceability exist for each piece of critical 

equipment (electrometer, thermometer, and barometer). Calibration frequency is as 

needed, based on control charts. Measurements using the extrapolation chamber are 

performed in Building 245, Room H015. All equipment is located in Building 245. 

None of the devices receive routine calibration. The uncertainties of the measurements 

are such (see Table 5) that small corrections introduced by routine recalibrations would 

not influence the overall results. Furthermore, the consistency of the measurement system 

as a whole is routinely monitored via measurements with the reference sources.

5.2 Changing Entrance Foils/HV Electrodes

Although this is a rare occurrence, entrance foils should be replaced when they show any 

sign of wear, such as tears, stretch marks or clear areas where the carbonization has been 

removed. To replace the foil, the window mounting ring is removed from the chamber by 

removing the four plastic screws which attach it. Take the mounting ring to a desk or 

other smooth work surface and remove the four screws which attach the window 

mounting plate to the mounting ring. Set the mounting ring aside, and remove the eight 

small screws which hold the tension ring in place and remove the old entrance window. 

Take a fresh window (obtainable from PTW) from the stock supply, making sure it 

contains no creases or other flaws. Place the PET with the carbonized side downward, 

which will put it on the inside of the chamber. The side which is carbonized is somewhat 

less shiny than the uncarbonized side but if uncertain, check conductivity with a portable 

ohmmeter; the carbonized side is conducting, the other side not. Now use pieces of 

adhesive tape to stretch the window over the mounting place and hold it in place; taping 

in 8 places is sufficient to remove all wrinkles in the foil. While doing this, it is prudent

to use a finger to press in the foil over the hole for one of the eight screws holding the 

tension ring, making a circular mark on the foil to make it easier to properly orient the 

tension ring during mounting. Mount the tension ring, which stretches the foil tight; use a 

sharp object to punch holes in the foil where the tension ring mounting screws will go, 

and attach the screws, working in a 180 pattern and keeping pressure on the tension ring 

until it has been securely attached. Use a knife or scalpel to cut the foil around the outer 

edge of the tension ring. The finished product should be mirror-like with no wrinkles 

evident. Reattach the window mounting plate to the holding ring with the four metal 

screws, and then attach the holding ring to the extrapolation chamber with the plastic 

screws, being sure to remember to put the insulating ring on the holding ring before 

fastening down. Care should be taken in all the screw tightening not to slip and punch a 

hole in the window
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5.3 Troubleshooting the Protection-Level Beta-Particle Measurements

Measurements in protection level beta-particle fields are relatively free from problems 

because of the relatively large distances employed. Problems usually occur only at very 

low signal levels, on the order of 20 fA or less. One precaution is to be sure that all 

sources in the measurement are stored in their Pb containers prior to any low level 

measurements 

Indication of problems is most often evident in signal drifting or noisy signals. Problems 

also seem to be associated with conditions of low humidity. The acceptable range of 

ambient conditions under which calibrations may be performed are humidity between 20 

% and 70 % and temperatures between 20 C and 24 C. There are no restrictions on 

barometric pressure. In addition, temperature changes must be limited to less than 1

C/hr. One of the most common problems is the absence of polarizing voltage on the high 

voltage electrode. Always make sure that the high voltage supply is switched on.

There are also some known problems with long exposure times using the BSS2 range, 

specifically premature shutter closing. There is no known cure for this, the only expedient 

is the placement of a shim under the shutter plunger on the front of the unit to prevent the 

shutter from closing. This must be done carefully to prevent radiation exposure to the 

extremities. 

Finally, there are sometimes problems with the interlock system which will prevent the 

shutters from being opened. Check that the beam is aligned with the receptor by 

observing the LED on the receptor.

6 References

Behrens, R., et al “International comparison EUROMET.RI(I)-S2 of extrapolation 

chamber measurements of the absorbed dose rate in tissue for beta radiation” 

(EUROMET project No 739): Final report. Metrologia 44 (2007).

Böhm, J., "Saturation corrections for plane-parallel ionization chambers," Phys. Med. 

Biol. 21, 754-759 (1976).

Böhm, J., "Standardization and calibration in beta dosimetry," Proceedings of the 

International Beta Dosimetry Symposium; 1983 February; Washington, DC, US Nuclear 

Regulatory Commission NUREG/CP-0050, 73-97 (1984).

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).

ISO 6980-1: 2003, Reference beta particle radiations – Part 1: Methods of Production.

(draft).
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ISO 6980-2:2003, Reference beta particle radiations – Part 2: Calibration fundamentals 

related to basic quantities characterizing the radiation field. (draft).

Pruitt, J.S., "The effect of altitude on beta-ray source calibrations," Pad. Prot. Dosim. 11, 

151-157 (1985).

Pruitt, J. S., C. G. Soares and M. Ehrlich, Calibration of Beta-Particle Radiation 

Instrumentation and Sources, NBS Special Publication 250-21 (1988).

Taylor, B.N. and Kuyatt, C.E., Guidelines for Evaluating and Expressing Uncertainty of 

NIST Measurements, NIST Technical Note 1297 (1994).

7. Uncertainty Analysis

See Tables 5 & 6.

8. Records

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

notebook that is stored in 245/H015. Printouts of the Excel spreadsheets that contain the 

results of all extrapolation chamber measurements along with all plots used to determine 

dose-rates are also kept in a folder in 245/H015.

9. Definitions

Absorbed dose to water: defined as the energy from ionizing radiation absorbed by a 

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

Beta particle: electron (unit negative charge) or positron (unit positive charge) 

spontaneously emitted by certain unstable atomic nuclei in the radioactive disintegration

process of beta decay.

10. Filing and Retention

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 for an Instrument

National Institute of Standards and Technology

REPORT OF CALIBRATION

OF

PTW Extrapolation Ionization Chamber (Model: xxxxxx)

S/N -###

SP250 Service ID# 47036C

FOR 

####

####

Calibration performed by Csilla Szabo-Foster

on XXX XX, XXXX

Report reviewed by Ronaldo Minniti

Report approved by Michael G. Mitch, Dosimetry Group Leader

Alan K. Thompson

Acting 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 Csilla Szabo-Foster, 

NIST, 100 Bureau Drive Stop 8460, Gaithersburg, MD 20899, csilla.szabofoster@nist.gov or 301-975-4334.

Report format revised 02/23

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

NIST ID 47036C Order Number: 682.02/O- #########

Report Date: 

 Page 1 of 7
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Extrapolation ionization chambers are calibrated by means of measurements in a reference 

beta-particle field. The readings obtained are compared to readings of a standard 

extrapolation chamber used to establish the absorbed-dose rate to tissue of the reference 

beta-particle field. Absorbed-dose rate to tissue in Gy/s is given by 

where is the absorbed-dose rate to tissue, is the average energy required to produce 

an ion pair in air (33.83 J/C), Sairtissue is the ratio of the average mass stopping power of tissue 

to air, 0 is the density of dry air at the reference conditions of 22 C and 760 Torr (1.197 

kg/m3), A is the area of the collecting electrode and (Ic/d) is the fitted slope in A/mm. 

Values for the stopping power ratio are given in Table 1. For the chamber submitted, a 

calibration was performed in a reference 90Sr+90Y beta-particle field at a distance of xx cm. 

Calibrations of radiation-protection instruments in beta particle fields can be considered valid 

only for the particular geometry and configuration in which they are performed. In this field, 

currents were measured with both the submitted extrapolation chamber and the reference 

extrapolation chamber at 19 air gaps ranging from 0.25 mm to 2.5 mm. The geometrical 

properties of each of the extrapolation chambers are given in Table #. For each chamber, a 

thickness of 6.87 mg/cm2of polyethylene terephthalate (PTP) was added to make the total 

window thickness equivalent to 7 mg/cm2of tissue; all measured dose rates are reported at 

this depth. Because of the high signal levels available through the use of the NIST 90Sr+90Y 

source, no corrections for non-source-induced currents, IB, were made. It was verified that 

these currents are below 1 fA, and could be safely ignored. The corrected current Ic is given 

by

where I is the measured current, corrected for ion recombination (ksat), beam divergence 

(kdi), attenuation in the chamber air gap (kac), scatter from the chamber side walls (kpe), the 

effect of the air path between source and detector (kabs), photon contribution (kbr), 

difference in backscatter between tissue and polymethyl methacrylate collecting electrode 

material (kba), temperature and pressure (kad) and decay between the measurement date and 

the reference date (kde). The first four corrections are functions of the air gap, and they are 

given in Table #. The other corrections, or coefficients used in determining them, are 

shown in Table 1. Explanation of all the corrections used is given in NIST Special 

Publication 250-21 which is available upon request. 

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

NIST ID 47036C Order Number: 682.02/O- #########

Report Date: 

Page 2 of 7

)

d

I

(

A

(W /e)S D =

c

0

tissue

air







 Eq. 1

Ic = Iksatkdikackpekabskbrkbakadkde Eq. 2

D



W / e
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The realization of the radiation quantity absorbed dose-rate done at NIST establishes the 

National Standard. This can in turn be transferred to other measurement facilities 

through a suitable measuring instrument, thus enabling traceability to the National 

Standard. Calibration results are reported as calibration coefficients, the quotients of 

absorbed-dose (rate) to water and the corresponding instrument reading. 

For the day on which the calibration of the customer chamber was performed (xxxx ##, 

####), ambient laboratory conditions were a temperature range of ###.## K to ###.## K, 

a pressure range of ##.## kPa to ##.## kPa, and a relative humidity range of ## % to ##

%.

The corrected currents measured at each polarity at each air gap are shown in Tables # and 

#. Each current value shown in this table is the average of ten measurements at each 

polarity. The uncertainty shown represents a single standard deviation about this average. 

The sets of nominal air gap versus corrected net current data for both the customer and the 

NIST chambers shown in Tables # and # were least-squares fit to a straight line. The results 

of these fits are shown in Table #. The value shown for the X intercept is the correction 

necessary for the micrometer reading used to set the air gap. The absorbed-dose rate, 

corrected to the reference date of xxxx ##, ####, at a depth of 7 mg/cm2in tissue is 

calculated from Equation 1 using the fitted slope and the constants shown in Tables # and #. 

The last column shows the ratio of absorbed-dose rate measured with the submitted 

extrapolation chamber and with the NIST reference extrapolation chamber along with the 

associated standard deviation. Type B uncertainties associated with the various correction 

factors shown in Tables # and # cancel since a ratio has been formed. However, observed 

variations in the NIST measurements of these radiation fields (0.7 % at 1 standard 

deviation) must be considered. Therefore, the standard deviations of the fitted slopes 

have been combined in quadrature with the NIST measurement uncertainty and the 

resulting ratio and uncertainty are shown in Tables # through #; the latter represents a 

single standard deviation. From the values of the ratio we concluded that no corrections 

to the quoted collecting electrode are shown in Table # were necessary. The results relate 

only to the instrument calibrated in this report.

Table 1. Source-Dependent Correction Factors or Coefficients 

Correction Factor or Coefficient 90Sr+90Y

Decay constant, λ, d-16.587 x 10-5

kba 1.010

kbr 1.000

Khu 1.000

Stopping power ratio, Sairtissue 1.110

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

NIST ID 47036C Order Number: 682.02/O- #########

Report Date: 

Page 3 of 7
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Table #. Extrapolation Chamber Characteristics

Chamber

Stated Window 

Thickness (mg/cm2)

Collecting Electrode

Diameter (mm) Area, A (mm2)

Customer #.## ##.## ###.##

NIST 0.67 30.03 708.27

Table #: Air-Gap Dependent Correction Factors for 90Sr+90Y at ## cm

Source Air Gap, d

(mm)

 kabs kac kad kdi kpe ksat

90Sr+90Y

at ## cm

with no 

flattening 

filter, at 

depth of

7.6 mg/cm2

0.250

0.375

0.500

0.625

0.750

0.875

1.000

1.125

1.250

1.375

1.500

1.625

1.750

1.875

2.000

2.125

2.250

2.375

2.500

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

NIST ID 47036C Order Number: 682.02/O-#########

Report Date: 

 Page 4 of 7
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Table #. Measured Currents ( 1 ) for the Customer Extrapolation Chamber at 7 mg/cm2

Depth Using the 90Sr+90Y Source at ## cm SSD with No Flattening Filter

Nominal

Air Gap, d

(mm)

Corrected Current 

Using Positive 

Polarity

(fA)

Corrected Current 

Using Negative 

Polarity

(fA)

Corrected

Net Current

(fA)

0.250

0.375

0.500

0.625

0.750

0.875

1.000

1.125

1.250

1.375

1.500

1.625

1.750

1.875

2.000

2.125

2.250

2.375

2.500

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

NIST ID 47036C Order Number: 682.02/O-#########

Report Date: 

 Page 5 of 7
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Table #. Measured Currents ( 1 ) for the NIST Extrapolation Chamber at 7 mg/cm2

Depth Using the 90Sr+90Y Source at ## cm SSD with No Flattening Filter

Nominal

Air Gap, d

(mm)

Corrected Current Using 

Positive Polarity

(fA)

Corrected Current Using 

Negative Polarity

(fA)

Corrected

Net Current

(fA)

0.250

0.375

0.500

0.625

0.750

0.875

1.000

1.125

1.250

1.375

1.500

1.625

1.750

1.875

2.000

2.125

2.250

2.375

2.500

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

NIST ID 47036C Order Number: 682.02/O- #########

Report Date: 

 Page 6 of 7
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Table #: Dose-Rate Calculations from Least-Squares Fit Results

 Source Chamber Y Intercept 

(fA)

X 

Intercept 

(mm)

Slope,

(ΔIc/Δd)

(fA/mm)

Dose Rate,

 

(μGy/s)

Ratio, 

customer/ 

NIST

90Sr+90Y

at xx cm

SSD

customer

NIST

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

 

NIST ID 47036C Order Number: 682.02/O- #########

Report Date: 

 Page 7 of 7
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Appendix B. Sample Calibration Report for a Source

National Institute of Standards and Technology

REPORT OF CALIBRATION

OF

AEA Technology Beta-particle Secondary Standard Sources

85Kr Model XXXXX, S/N XXXX

90Sr/Y Model XXXXX, S/N XXXX

SP250 Service ID# 47035C

FOR 

XXXXXXXXXXXXXXX

XXXXXX

XXXXX

Calibration performed by Csilla Szabo-Foster

Reference Date:

Report reviewed by Ronaldo Minniti

Report approved by Michael G. Mitch, Dosimetry Group Leader

Alan K. Thompson

Acting 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 Csilla Szabo-Foster, 

NIST, 100 Bureau Drive Stop 8460, Gaithersburg, MD 20899, csilla.szabofoster@nist.gov or 301-975-4334. 

Report format revised 02/23

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

NIST ID 47035C Order Number: 682.02/O-#########

 Report Date: 

 Page 1 of 3 
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Beta particle sources are calibrated by means of measurements with a standard extrapolation 

chamber, in terms of absorbed-dose rate to tissue. This is determined from current 

measurements at a range of air gaps. The absorbed-dose rate to tissue in Gy/s is given by: 

where D is the absorbed-dose rate to tissue, is the average energy required to produce 

an ion pair in air at reference conditions of temperature, pressure and humidity (33.83 J/C), 

Sairtissue is the ratio of the average mass stopping power of tissue to air, 0 is the density of air 

at the reference conditions of 293.15 K (20 C), 101.325 kPa (760 Torr) and 65 % relative 

humidity (1.1974 kg/m3), A is the area of the collecting electrode and (Ic/d) is the fitted 

slope in A/m. Calibrations of radiation-protection instruments in beta particle fields can be 

considered valid only for the particular geometry and configuration in which they are 

performed. Values for the measured absorbed dose rates are given in Table 1 for each of the 

geometries used. For each geometry, currents were measured with the reference extrapolation 

chamber at 19 air gaps ranging between 0.25 mm and 2.5 mm. The reference extrapolation 

chamber is equipped with an entrance window that is 0.67 mg/cm2thick, and a polymethyl 

methacrylate (PMMA) collecting electrode with an area, A, of 708.27 mm2. A thickness of 

6.9 mg/cm2of polyethylene terephthalate (PTP) was added to make the total window 

thickness equivalent to 7 mg/cm2of tissue; measured dose rates are reported at this depth. The 

corrected current Ic is given by

where I is the measured current, corrected for difference in backscatter between tissue and 

PMMA (kba), photon contribution (kbr), the effect of relative humidity (khu), the effect of the 

air path between source and detector (kabs), attenuation in the chamber air gap (kac), 

temperature and pressure (kad), decay between the measurement date and the reference date 

(kde), beam divergence (kdi), scatter from the chamber side walls (kpe), and ion recombination 

(ksat). Explanation of all the corrections used is given in ISO 6980-2, “Reference beta-particle 

radiation – Part 2: Calibration fundamentals related to basic quantities characterizing the 

radiation field,” (2004), which is available upon request. 

The realization of the radiation quantity absorbed dose-rate done at NIST establishes the 

National Standard. This can in turn be transferred to other measurement facilities through 

a suitable measuring instrument, thus enabling traceability to the National Standard. 

Source calibration results are reported in terms of absorbed dose-rate to water at a fixed 

source-to-detector distance.

Dosimetry Group Number DG: #####/##

NIST ID 47035C Order Number: 682.02/O-#########

 Report Date:

Page 2 of 3 

)

d

I

(

A

(W /e)S D =

c

0

tissue

air







 Eq. 1

Ic = Ikbakbrkhukabskackadkdekdikpeksat Eq. 2

W / e
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For the days on which the calibrations were performed, ambient laboratory conditions were 

at a temperature range of ####.## K to ###.## K, a pressure range of ##.## kPa to ##.## kPa

and a relative humidity range of ## % to ## %. The corrected current measured at each air 

gap was the average of ten measurements at each polarity. The nominal air gap versus 

corrected net current data were least-squares fit to the functions recommended in ISO 6980-

2. The terminal slopes determined from these fits were used in Eq. 1 to calculate the absorbeddose rate at the reference depth of 7 mg/cm2

, which is shown in Table 1. The expanded, 

combined uncertainty in these measurements is also given in Table 1. The Type A 

uncertainty components are calculated as standard deviations of the mean of replicate 

readings; other components are estimated using the values recommended in ISO 6980-2, 

and can be assumed to have the approximate character of standard deviations. The 

expanded, combined uncertainties are two times the square root of the quadratic sum of all 

the component uncertainties; they are considered to have the approximate significance of 

95 % confidence limits. The results relate only to the source calibrated in this report.

Table 1. Calibration results

Source

Distance, 

cm

Geometry

Tissue 

Depth, 

mg/cm2

Reference date Absorbed Dose Rate 

in tissue, Gy/s

85Kr ## xxx 7 xxx ##, #### ### +/- ###

90Sr/Y ## xxx 7 xxx ##, #### ### +/- ###

90Sr/Y ## xxx 7 xxx ##, #### ### +/- ###

Dosimetry Group Number DG: ######/##

NIST ID 47035C Order Number: 682.02/O-#########

 Report Date: 

Page 3 of 3 
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List of Tables

1. Comparison of absorbed dose rates of the BSS1 sources

2. Comparison of absorbed dose rates of the BSS2 sources

3. Characteristics of NIST beta-particle sources

4. Source dependent correction factors and constants

5. Uncertainties in the determination of absorbed-dose rate to water

6. Uncertainties in the calibration of beta-particle sources and instruments

7. Reference absorbed-dose rates for the sources available at NIST

8. List of critical equipment

List of Figures

1. BSS1 beta-calibration facility

2. BSS2 calibration facility with source holders

3. Theoretical beta-particle spectra for the radionuclides employed at NIST

4. Extrapolation ionization chamber schematic representation

5. Current measurements with the extrapolation chamber
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Table 1. Comparison of absorbed-dose rates for the original BSS1 sources, at the surface, 

obtained by NIST and PTB; and absorbed-dose rates to water at the depth of interest in 

radiation protection for three of the NIST sources

Source Sourcetodetector 

distance 

(cm)

Flattening 

Filter 

used?

Absorbed-Dose Ratea

Type

Nominal 

activity 

(mCi)

to water, at the surface to water at 

a depth of 7 

mg/cm2

(Gy/s)

NIST 

(Gy/s)

PTB 

(Gy/s)

Ratio 

NIST/PTB

147Pm 14 20 yes 0.267 0.257 1.04 0.06

204Tl 0.5 30 yes 0.341 0.333 1.02 0.33

90Sr+

90Y

2 30 yes 1.93 1.90 1.01 2.01

" 50 11 no 513 506 1.01 -

" " 30 no 70.6 69.9 1.01 -

" " 50 no 25.2 25.1 1.00 -

aReferred to January 1, 1983 and 20 C at 1 standard atmosphere (101.325 kPa).

Table 2. Comparison of absorbed-dose rates for the BSS2 sources, at a depth of 7 

mg/cm2, obtained by NIST and PTB

Source Sourcetodetector 

distance 

(cm)

Flattening 

Filter 

used?

Absorbed-Dose Ratea

Reference 

date

Type

Nominal 

activity 

(mCi)

to water at a depth of 7 

mg/cm2

NIST 

(Gy/s)

PTB 

(Gy/s)

Ratio 

NIST/PTB

147Pm 100 20 yes 2.97 3.00 0.990 5-Mar-98

85Kr 100 30 yes 50.3 50.2 1.002 6-Mar-98

90Sr+

90Y

25 30 yes 10.3 10.2 1.010 9-Mar-98

" " 11 no 118.2 116.4 1.015 20-Mar-98

" " 20 no 36.4 35.9 1.014 18-Mar-98

30 no 16.3 16.0 1.019 13-Mar-98

" " 50 no 5.70 5.66 1.007 4-Mar-98

aReferred to 20 C at 1 standard atmosphere (101.325 kPa).
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Table 3. Characteristics of NIST Beta-Particle Sources

a) Radionuclide data 

Radionuclide

Half life

(days)

Maximum energy 

emitted, Emax

(MeV) Photon radiations

147Pm 958.2 0.225 : 0.121 MeV (0.01%)

Sm X-rays: 5.6 to 7.2 

and 39.5 to 46.6 keV

85Kr 3915 0.687 : 0.514 MeV (0.4%)

204Tl 1381 0.763 Hg X-rays: 9.9 to 13.8 

and 68.9 to 82.5 keV

90Sr + 90Y 10523 2.274 None

b) Source Structure and activity

Radionuclide

Source 

Set Structure

Window mass 

per unit area

Activity (MBq) and 

reference date

147Pm BSS1a 147Pm carbonate 

pressed into Ag 

foil

5 mg/cm2 Ag 518 on 16-Dec-82

147Pm BSS2 147Pm carbonate 

pressed into Ag 

foil

2 mg/cm2 Ti 3700 on 5-Mar-98

204Tl BSS1a 204Tl chromate 

pressed into Ag 

foil

20 mg/cm2

Ag

18.5 on 16-Dec-82

204Tl BSS1b

not specified not specified 1850 on 14-Aug-95 

85Kr BSS2 gaseous 22 mg/cm2 Ti 3700 on 6-Mar-98

90Sr + 90Y BSS1 90Sr carbonate 

pressed into Ag 

foil 

50 mg/cm2

Ag + 0.1 mm 

(77 mg/cm2) 

steel

74 on 16-Dec-82

90Sr + 90Y BSS1 90Sr carbonate 

pressed into Ag 

foil 

50 mg/cm2

Ag + 0.1 mm 

(77 mg/cm2) 

steel

1850 on 24-May-82

90Sr + 90Y BSS2 90Sr carbonate 

pressed into Ag 

foil

80 mg/cm2

stainless steel

460 on 20-Mar-98

aSource has decayed too much to be now used and has not been replaced.

bSource has developed corrosion problem and is now no longer used.
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Table 4. Source Dependent Correction Factors and Constants for Extrapolation Chamber 

Measurements Which are Independent of Chamber Depth

Factor Description 90Sr+90Y

85Kr, 204Tl 147Pm

k'ba backscatter correction 1.01 1.01 1.00

k'br bremsstrahlung correction 1.000 1.000 0.990

st,a stopping-power ratio 1.110 1.121 1.124

Table 5. – Uncertainties in the Determination of Absorbed-Dose Rate to Water from 

Measurements with the Extrapolation Ionization Chamber

a) Corrected Ionization Current (Extrapolation Chamber)

Parameter Symbol

Percent Uncertainty

Type A Type B

147Pm 204Tl, 85Kr 90Sr+90Y147Pm 204Tl, 85Kr 90Sr+90Y

current I 0.4 0.3 0.1 0.1 0.1 0.1

backscatter k'ba 0.4 0.3 0.3

bremsstrahlungk'br 0.2 0.2 0.2

air density kad 0.6 0.6 0.6

absorption kabs 0.6 0.2 0.3

attenuation kac 0.1 0.05 0.05

divergence kdi 0.1 0.1 0.1

sidescatter kpe 0.1 0.1 0.1

recombinationksat 0.2 0.2 0.2

decay kde 0.02 0.01 0.01

Corrected 

ionization 

current 

Ic 0.4 0.3 0.1 1 0.8 0.8
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b) Absorbed –Dose Rate to Water at a Depth of 7 mg/cm2

Parameter Symbol

Percent Uncertainty

Type A Type B

147Pm 204Tl, 

85Kr

90Sr+90Y147Pm 204Tl, 85Kr 90Sr+90Y

slope (ΔIc⁄Δd)d→0 0.4 0.3 0.1 1 0.8 0.8

Air 

density

ao 0.04 0.04 0.04

area a 0.1 0.1 0.1

Energy per 

ion pair

W̅ ⁄e 0.2 0.2 0.2

Stopping 

power 

ratio

st,a 0.7 0.7 0.7

Absorbed 

dose rate 

to water at 

7 mg/cm2

Ḋw 0.4 0.3 0.1 1.2 1.1 1.1

Table 6. Uncertainty in the Calibration of Beta-Particle Sources and Transfer Ionization 

Chambers Submitted to NIST

a) Source Calibration

Parameter Percent Uncertainty

Type A Type B

147Pm 204Tl, 85Kr 90Sr+90Y147Pm 204Tl, 85Kr 90Sr+90Y

Positioning of 

extrapolation 

chamber and sources

1 0.2 0.2

Absorbed dose rate 

to water at 7 mg/cm2

0.4 0.3 0.1 1.2 1.1 1.1

Overall Uncertainty 147Pm 204Tl, 85Kr 90Sr+90Y

Combined 

uncertainty

1.6 1.2 1.1

Expanded 

uncertainty (k=2)

3.2 2.4 2.2
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b) Calibration of Transfer-Ionization Chamber

Parameter Percent Uncertainty

Type A Type B

147Pm 204Tl, 85Kr 90Sr+90Y147Pm 204Tl, 85Kr 90Sr+90Y

Positioning of 

transfer chamber and 

sources

1 0.2 0.2

Absorbed dose rate 

to water at 7 mg/cm2

0.4 0.3 0.1 1.2 1.1 1.1

Measured ionization 

current

0.1 0.1 0.1

Source decay 

correction

0.05 0.05 0.05

Overall Uncertainty 147Pm 204Tl, 85Kr 90Sr+90Y

Combined 

uncertainty

1.6 1.2 1.1

Expanded 

uncertainty (k=2)

3.2 2.4 2.2

Table 7. Reference dose rates for the sources available at NIST

Protection level dose rates (BSS2):

Isotope S/N Distance Geometry Halflife, d

Referenc Dose 

Rate at 7 mg/cm2, 

Gy/s 

Reference 

Date

85Kr 5828 BX 30 cm filter 3915 50.3 3/6/1998

90Sr/Y FU 991 30 cm filter 10523 10.3 3/9/1998

90Sr/Y FU 991 50 cm no filter 10523 5.7 3/4/1998

90Sr/Y FU 991 30 cm no filter 10523 16.3 3/13/1998

90Sr/Y FU 991 20 cm no filter 10523 35.9 3/18/1998

90Sr/Y FU 991 11 cm no filter 10523 116.4 3/20/1998
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Table 8. List of critical equipment

Device Manufacturer Model S/N NIST # Location

BSS2 AEA 

Technology

001 586972 H015

Extrapolation 

chamber*

PTW 24670 H015

Electrometer Keithley 6514 4517668 H015

Pressure, 

Humidity and 

Temperature 

Transmitter

VAISALA PTU303 T2530845 H015

* Includes motorized chamber air gap setting and cowls to add the window thickness to 

bring to 7 mg/cm2 water equivalent thickness
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Figure 1. BSS1 Beta-Calibration Facility. The source support stand is shown at the right, 

the extrapolation ionization chamber (in its pre-automation condition) at the left, and a 

beam flattening filter supported by wires in-between, The pictured flattening filter is for 

the 147Pm source, and is designed for use at the pictured source-to-chamber distance of 20 

cm. The source shutter is shown in the open position. The chamber is mounted on a stand 

that can be moved from side to side across the table, which in turn is mounted on a cart, 

with wheels running on tracks shown at the bottom.
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Figure 2. BSS2 with Source Holders. The source holder support stand is shown at the left, 

and examples of the source holder with its shutter at the right. A flattening filter is shown 

mounted in the system. The extrapolation chamber is mounted on a similar table as 

shown in Figure 1, and sits on a platform which is attached to the round goniometer stand 

shown at the bottom. The position of the chamber remains fixed while the source-tochamber distance is set by sliding the source holder support stand along the attached 

optical bench.
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0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2 2.5

Figure 3. Theoretical Beta-Particle Spectra for the Radionuclides Employed calculated 

under the LOG-FT approximation. Plotted is the number of beta particles per MeV for 

one transition from the original state, against beta-particle energy in MeV. 

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8

0

0.5

1

1.5

2

2.5

3

0 0.2 0.4 0.6 0.8

0

2

4

6

8

10

12

0 0.05 0.1 0.15 0.2 0.25

Number of Beta Particles per MeV

90Y 90Sr + 90Y

204Tl 85Kr

147Pm

Beta-Particle Energy, MeV Beta-Particle Energy, MeV

Beta-Particle Energy, MeV Beta-Particle Energy, MeV

Beta-Particle Energy, MeV
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Figure 4. Extrapolation Ionization Chamber: Cross Section through Front Portion

(Schematic Representation)

The collecting and guard electrodes (graphite coating on PMMA) are separated by a 0.2

mm wide circular groove (shown of exaggerated size). The distance between the 

midpoints of the two grooves (distance between the indicated dashed lines) defines the 

effective diameter of the collecting volume. In this particular instrument, the diameter is 

30 mm. The entrance window is a 0.67 mg/cm2carbonized PET foil. For the 

measurements, the plate separation, d, (also referred to as air gap or chamber depth) is 

usually varied between 0.25 mm and 2.5 mm. During any experiment, the voltage 

gradient across the chamber is kept constant at 10 V/mm.
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0

50

100

150

200

250

300

0 0.5 1 1.5 2 2.5

Corrected chamber current, fA

Chamber depth, mm

Figure 5. Current measurements with the extrapolation chamber. Lines represent linear 

least-squares fits to the data shown. 

147Pm

85Kr x 0.08 90Sr + 90Y x 0.5