2.5.3 Internal Gas Counters. Radionuclides in the gaseous form can be
introduced directly into the counting gas of proportional counters. With
Precautions for quantitative mixing and handling, and compensation for
boundary effects, low uncertainties can be obtained even for rather low-energy
beta spectra.

2.5.4 Defined-Geometry Counters. Counters with a 2n geometry are widely
used in routine counting, but backscattering from a thick backing often makes
them unsuitable for a direct measurement of beta particles, except under
special circumstances. However, in the case of alpha particles, quantitative
investigations of source and backing scattering have provided sufficient
information so that an activity can be deduced from a 2x alpha-counting rate
with moderate accuracy. The predominant emission of backscattered alpha
particles almost parallel to the backing suggests, however, that more
restricted geometries will give results of lower uncertainty.

Defined-geometry counters with restricting apertures are not usually
considered suitable for electron counting because of the ill-defined
scattering from edges. They are suitable for emission-rate measurements of
low-energy photons, but the large uncertainties in probabilities per decay
usually prohibit conversion into useful activity values.

2.6 Direct Methods Using Cascade Radiations

The methods discussed so far are most often used for radionuclides
without appreciable branching to excited nuclear levels. However, more
complex decays can give enough information to yield efficiency values for the
detectors as part of the activity measurement. The essence of most methods is
that not only are two radiations detected, each with some unknown efficiency,
but the simultaneous detection of both (which depends on the product of
efficiencies) is also recorded or inferred. With three measured quantities
expressed as functions of the efficiencies of the two detectors and the
activity of the sample, enough information exists for the three unknowns to be
calculated. As with the previously discussed counting methods, the effects of
backgrounds, dead-times, and pulse pileup must be considered, but with cascade
radiations there may also be timing and resolving-time considerations.

2.6.1 Coincidence Counting. For a simple alpha-gamma or beta-gamma
cascade, with each of two detectors sensitive to one radiation and not the
other, corrections and analysis are straightforward. Many systems using

proportional or liquid-scintillation counters to detect the particles, and
NaI(T1) or germanium detectors for photons, have been described.

Usually analysis is simplified if the detector for the particles has a
high efficiency, but sometimes the circumstances require other choices. The
reduction in gamma-ray rate due to branching from a level, or to internal
conversion, is incorporated into an apparent detection efficiency, but
explicit corrections must be made for other branchings from the level origina-
ting the cascade. Extra counts in the particle detector due to the detection
of gamma rays or internal-conversion electrons must be taken into account.

The effect of dead times for moderate counting rates was treated approximately
for many years with small error, but recent formulations allow accurate
correction even at high rates.

11


60 ionization-chamber measurements on this sample

solution composition and density
five coincidence measurements
efficiency extrapolation
gravimetric measurements
resolving time

background

half life

original ionization-chamber measurements
photon-emitting impurities in original calibration
radium 5000 to radium 1000 reference sources ratio

32

percent
0.01
0.05
0.11
0.10
0.03
0.05
0.01
0.20
0.01
0.04
0.25

overall uncertainty = 1.10
(3 x vz u2)


NBS-443
(Rev. 2-85)

U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS:
GAITHERSBURG, MARYLAND 20899

REPORT OF CALIBRATION

for

COMPANY NAHE

Radionuclides
Source identification

Source description

Solution composition

Activity

Reference time

Overall uncertainty
Photon-emitting impurities
(Activity ratios at
reference time)

Half life

Measuring instrument

Dale D. Hoppes, Group Leader
Radioactivity Group
Center for Radiation Research

Gaithersburg, MD 20899
February 26, 1985
NBS Test No.

28

ADDRESS

Indium-111
X5001

Liquid in NBS borosilicate
glass ampoule a

Stated to be carrier—free
in 0.1 M HCl

4.157 x 106 Bq
1200 EST February 19, 1985
0.73 percent (2)

114myn/1113n; (2,020.6) x 1074 (3)

2.8048 + 0.0005 days (4)

NBS pressurized "4n"y ionization
chamber calibrated by 4x(A,x)-7
coincidence efficiency-extrapolation

Randall S, Caswell, Chief
Nuclear Radiation Division
Center for Radiation Research

“Notes on next page


1.3.2 Packaging. Packaging for all sources must be in compliance with
the Department of Transportation (DOT) and Nuclear Regulatory Commission (NRC)
regulations. Copies of these regulations may be obtained from Operations
Division, Office of Hazardous Materials, Department of Transportation,
Washington, DC 20590. Postal regulations prohibit mailing radioactive
materials which require a caution label under DOT regulations.

1.3.3 NBS Shipping Address. All samples submitted for calibration are
checked by the NBS Health Physics Group for radiation level and source
integrity before transferring to the NBS Radioactivity Group for calibration.
The following shipping address must be used:

National Bureau of Standards

Atten: Name of NBS contact

Health Physics (Radioactivity Group)
Quince Orchard and Clopper Road
Gaithersburg, Maryland 20899

2. Design Philosophy and Theory

2.1 Physical Principles and Restrictions

The function and status of the "4n"y ionization-chamber calibration
service can be better appreciated if the difficulties in maintaining national
standards for the activity of radionuclides is considered. Each of the 80-100
radionuclides for which activity calibrations are sometimes required in the
United States presents a separate challenge for a "direct" calibration,
independent of existing standards. Such calibrations, discussed in a section
to follow, are usually time-consuming and not applicable to routine submitted
samples. Because radionuclides by definition decay, a comparative measurement
against an existing, laboriously directly- calibrated, sample may not be
possible after even a few days.

What is needed is a stable, easy-to-use instrument to serve as a
repository once the direct calibration has been performed for a particular
radionuclide. With due precautions, an ionization chamber serves this purpose
if gamma rays of sufficient energy and probability are emitted as part of the
decay process. Gamma-ray interactions in a gas, pressurized to maximize their
probability, produce jon-electron pairs in proportion to the gamma-ray energy.
A voltage between two electrodes causes these charges to be collected; the
resulting current is also proportional to the number of decays per second,
called the “activity” and assigned the name bequerel (Bq). One bequerel of a
given radionuclide always produces the same current if all conditions are the
same. Hence a calibration, or "K" value can be assigned for that radio-
nuclide, on a particular ionization chamber.

As a precaution against changes in the chamber (loss of gas; movement of
electrodes) or the sensitive electronics required to measure the small
currents produced by the usual-activity samples, the current for the sample is
measured relative to that from a long-lived-radionuclide sample which was also
measured when the original "K" value was determined. To account for possible
changes in leakage currents or differences in external radiation, "background"
measurements with no sample are also interleaved with sample and reference
source runs during a measurement series.


4x proportional
counter

Pressurized 47
proportional
counter

Radon pulse-
ionization
chambers

Microcalorimeter

Internal-gas
counters

Defined-geometry
system for
low-energy photons

Pin-well Nal(T1)

4nB

PPCX

Radon
PIC

Cal

Gas

Photon
Def.
Geom.

Pin Well

Table 3 cont.
A metallized film carrying a thin source
forms the boundry between two hemispherical
proportional counters, each with a loop
electrode. It is used primarily for
high-energy pure-beta-ray-emitting
radionuclides,

Similar to the above 4a proportional
counter, but modified for pressures of a
few atmospheres. It is used for measuring
x-ray-emission rates of pure electron
capture radionuclides of low atomic number,

Radon gas is quantitatively flushed from
radium solutions or introduced from a
collection flask. Water and oxygen are
removed, and the gas transterred to one of
four 3.6-1 cylindrical pulse-ionization
counters,

A twin-cup radiation balance is used to
compare compact similar sources.
Radionuclides with radiations that can be
totally absorbed are compared with a
Peltier current.

Radioactive gases are quantitatively

mixed with counting gas and introduced

into a common chamber containing 3
copper-walled proportional counters of
different length and a comparable set

of 3 counters with walls of stainless steel,
Spectral extrapolation of the difference

in counting rates for counters of different
length is used.

A 0,015-cm-thick Pt diaphragm with a
hole 0.9505 + 0.0020 cm in diameter
quantitatively establishes the area
subtended by a 1.6-mm-thick NalI(T1)
crystal with a 0.13-mm-thick Be window,
Carefully measured source positions are
arranged in a vacuum housing containing
the apparatus. The emission rate of
photons between 5 and 40 keV can be
measured with an uncertainty of less than
one percent.

Two-inch-diameter NaI(T1) detectors with
thin-walled wells or central passages less
than 1 cm in diameter are used for measuring
photon-emission rates in the energy region
of 50 to 150 keV.

15


6.

8.

References

NBS Special Publication 260, National Bureau of Standards, Standard
Reference Materials Catalog 1986-1987 ed. [Available from the
Superintendent of Documents, U.S. Government Printing Office, Washington,
D.C. 20402].

NBS Special Publication 250, National Bureau of Standards Calibration
Services Users Guide 1986-1988 ed. [Available from the Superintendent of
Documents, U.S. Government Printing Office, Washington, D.C. 20402].

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).

Rytz, A., Activity Measurements of a Solution of 134Cs: Report of an
International Conparison, Nucl. Instrum, Methods 192, 427-431 (1982).

Keithly Digital Electrometer Instruction Manual, Model 6616, Keithly
Instruments Cleveland, Ohio, April 1976. (Mention of a commercial
product does not imply recommendation or endorsement by the National
Bureau of Standards, nor does it imply that the product identified is
necessarily the best available for the purpose.)

Radioactivity Calibrations Catalog, 1986 (unpublished).

Muller, J. W., J. Nucl. Instrum, Methods 163, 241-253 (1979).

33


PREFACE

The calibration and related measurement services of the National Bureau
of Standards are intended to assist the makers and users of precision
measuring instruments in achieving the highest possible levels of
accuracy, quality, and productivity. NBS offers over 300 different
calibration, special test, and measurement. assurance services. These
services allow customers to directly link their measurement systems to
measurement systems and standards maintained by NBS. These services
are offered to the public and private organizations alike. They are
described in NBS Special Publication (SP) 250, NBS Calibration Services
Users Guide.

The Users Guide is being supplemented by a number of special
publications (designated as the "SP 250 Series") that provide a
detailed description of the important features of specific NBS
calibration services. These documents provide a description of the:
(1) specifications for the service; (2) design philosophy and theory;
(3) NBS measurement system; (4) NBS operational procedures; (5)
assessment of measurement uncertainty including random and systematic
errors and an error budget; and (6) internal quality control procedures
used by NBS. These documents will present more detail than can be
given in an NBS calibration report, or than is generally allowed in
articles in scientific journals. In the past NBS has published such
information in a variety of ways. This series will help make this type
of information more readily available to the user.

This document (SP 250-10), NBS Measurement Services: Radioactivity
Calibrations with the "4n" Gamma Ionization Chamber and Other
Radioactivity Calibration Capabilities, by J. M. Calhoun, is the tenth
to be published in this new series of special publications. It
describes the use of the NBS "41" gamma ionization chamber to measure
the activity of gamma-ray-emitting radionuclides against national
standards (see test numbers 43010C and 43020D in the SP 250 Users
Guide). It also reviews NBS capabilities for making direct
radioactivity calibrations. Inquiries concerning the technical content
of this document or the specifications for these services should be
directed to the author or one of the technical contacts cited in SP
250.

The Center for Radiation Research (CRR) is in the process of publishing
21 documents in this SP 250 series, covering all of the calibration
services offered by CRR. A complete listing of these documents can be
found inside the back cover.

NBS would welcome suggestions on how publications such as these might
be made more useful. Suggestions are also welcome concerning the need
for new calibration services, special tests, and measurement assurance
programs.

George A. Uriano Chris E. Kuyatt
Director Director
Measurement Services Center for Radiation Research

iii


4.3.5 Measuring the Sample. When placing the solution sample in the
chamber, observe the ampoule to make certain that no liquid is in the neck of
the ampoule. The sample is then counted the same number of times in each set
of measurements as the radium reference source. At least two sets of

Measurements should be made on the sample, with an equivalent number of
measurements made on the radium reference source,

After all measurements have been completed on the submitted sample and

the radium reference source, it is recommended that background measurements be
repeated.

4.4 Data Analysis

4.4.1 Data Collection. The data collection is under control of a
FORTRAN program running on a multi-user super-microcomputer. The start of the
calendar scalar and other default parameters are entered as prompted, by the
operator. The operator enters the sample number, number of data points to be
collected, and capacitor ratio. The data is collected and stored to a disk
for off-line analysis. The programs are available from the Radioactivity
Group, upon request.

4.4.2 Processing the Data. The data processing is done using a FORTRAN
program. 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 entered K-value for that radium
reference source producing a current approximating that of the sample. The
results, with intermediate averages if desired, are printed out.

4.4.3 Correction for Impurities. Figure 2a and 2b show the relative

response curves for the NBS "4n"y pressure ionization chamber for monoenergtic
gamma rays. To construct the response for an impurity radionuclide for which
there is no measured calibration factor, one uses the relative response for
energy E; and a specific 226Ra reference source to calculate:

«Ione
a} Ee,

>
"
7 P

where A is the impurity activity, Rp is the reading for the reference source,
R is the reading for the impurity radionuclide emitting gamma rays of energies
E; with a probability of emission per decay of (Prde,» and (Xe, is the
response for a gamma ray of energy Ej.

The sharp drop-off of response at approximately 100 keV is due to the
absorption of low-energy gamma rays in the 1/8" brass liner in the reentrant
tube in which the source is placed (See fig. 1). The advantage of this liner
is the much lower response to low-energy x-rays, resulting in a more linear
response curve for most nuclides having considerable x-ray emissions.
However, a disadvantage is the low response to nuclides such as 125],

26


2.6.2 Anticoincidence Counting. If the efficiency of one detector is
very high, then it is reasonable to count those pulses from the other detector
which do not have a coincident pulse. As applied, this method has the
technical advantage that the timing is not critical and the effect of
long-lived states, which is difficult to measure by the coincidence method,
can be measured directly.

2.6.3 Anticoincidence and Coincidence Counting with Efficiency
Extrapolation. One must normally correct for other radiations that can
produce counts in the beta detector when a beta particle is not detected. As
the efficiency for detection of the beta particle (or positron or Auger
electrons and x rays) approaches unity, however, these corrections go to zero.
Thus one can introduce solids into the source or add absorbing films to vary
the efficiency (as measured by the ratio of the coincidence rate to the gamma-
ray rate) to obtain an experimental function of beta rate versus efficiency.
This function can then be extrapolated to 100-percent efficiency, and
corrections for the extraneous counts no longer have to be made explicitly.

If the pulse height from the particle detector is monotonically related
to the energy of the particle detected, as it can be for liquid scintillators
and high-pressure gas proportional counters, pulse-height discrimination can
be used to vary the efficiency.

For a simple decay, without alternate beta branches or other complica-
tions, the extrapolation function of beta-particle count rate versus
efficiency should be a straight line. In general the extrapolation function is
taken to be a simple polynomial, with the extrapolated value considered more
reliable if a straight line gives a satisfactory fit. For complex decays,
explicit corrections may yet have to be made for differences in branch decay
properties in the extrapolation region. Radionuclides which have no coinci-
dent radiations can be “efficiency traced" with quantitative amounts of
another radionuclide which has a similar particle spectrum, but with coinci-
dent gamma radiations.

Efficiency extrapolation has become the method of choice for radio-
nuclides to which it can be applied. In an international intercomparison of
134Cs activity measurements organized by the Bureau International des Poids et
Mesures (BIPM) in 1978, the total range of values from 22 laboratories was
only 0.7 percent [5].

2.6.4 Sum-Peak Counting. This method, applicable to simple two- or
three-photon cascades, also does not use coincidence or anticoincidence
circuitry. The radiations are detected in the same spectrometry system, with
the probability of the simultaneous detection of one of each energy equal to
the product of the efficiencies for detecting each radiation singly. The
result is a spectral peak at the sum of their energies. The areas of the
single-photon and sum peaks are used to deduce directly the activity.

2.6.5 High-Efficiency NaIl(Tl) Counting. Although individual probabili-
ties per decay and detection efficiencies for gamma rays and x rays in cascade
decays may not be known with sufficient accuracy, the probability of not
detecting any radiation from a decay in large well-type NaI(Tl) detectors may
be small enough that a calibration of suitable uncertainty is possible.

12


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18


TABLE 3 INSTRUMENTS FOR NBS BASIC CALIBRATIONS NOT USING CASCADE RADIATIONS

Counting System

O.lma scintillation
counter

0.81 Robinson a
scintillation
counter

2na proportional
counter

Liquid scintillation
counters

Label

0.1aa

0.820

27a

LSa

LSb

LSc

LSd

Description and References

A carefully measured external baffle about
11 cm in diameter establishes the solid
angle subtended by a CaF(Eu) scintillator
in an evacuated tube. As with the
following alpha-particle-counting systems,
extrapolation to zero energy is used to
account for the small fraction of the
alpha spectrum masked by noise or the
detection of recoil nuclei or other
radiations.

The solid angle subtended by a CsI(T1)
scintillator greater than 15cm in diameter
is limited by a precise hole in a diaphragm
and by a central stepped cone. The latter
minimizes corrections for source size (up
to 1.6-cm diameter) and displacement.
Intervening air is replaced by hydrogen to
reduce energy losses,

A 12-cm-diameter hemispherical counter
with a wire-loop electrode is used for
measuring the alpha-particle-emission rate
of thin sources up to 9 cm in diameter.
Measured and calculated effects of source
thickness permit calculation of activity
from alpha-particle-emission rate.

A commercial sample-changing system
with two refrigerated phototubes
viewing 10-ml vials. Outputs of the
two tubes are summed, with a coincidence
requirement. Pulses are sent through
an external amplifier and then analyzed.

A commercial sample-changing system

with refrigerated phototubes,

logarithmic amplifier, and microprocessor
control and analysis.

A refrigerated NBS-built system with
the sample in a quartz spectrophotometer
cell optically coupled to a phototube
on each end.

The "LS Cascade" system but without
gamma-ray-detection requirements.

14


4.5 Format of Calibration Report

The following two pages show an example of a Report of Calibration. As
stated in section 1.2, at the end of a calibration exercise, the technical
user will be given a Report of Calibration stating primarily [3]:

a) the principal radionuclide

b) reference time and date

c) method of calibration

d) certified values for activity

e) decay-scheme assumptions

f) assessment of radionuclidic purity

g) overall uncertainty determinations for calibration

5. Assessment of Uncertainty

5.1 Precautions and Uncertainties in Service Calibrations

Measurements performed with the "4n"y ionization chamber (IC) 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
standard 5-mL type, the volume.in ampoules must be 5.0 + 0.2 mL, and the 226Ra
reference source must be corrected for decay.

Insufficient volume in the NBS standard 5-mL ampoule can be adjusted by
adding an inactive carrier solution to adjust the height of the solution.
Observed variations with height are used to estimate the uncertainty due to
inexact fillings. Uncertainty in activity measurements may be increased if
the contribution of the background is variable, large, or both. The uncer-
tainty 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.

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 times may be used to check consistency. Germanium
detector systems are used to identify and quantify the significant radio-
nuclidic impurities that may be present. The uncertainty in these
measurements is propagated through the calculated effect on the IC results.

The uncertainty in the ratio of radium reference sources used when the

initial sample and test sample differ greatly in activity were determined by
measuring different radium sources against the same and various radionuclides.

27


2.3 Comments on Direct Activity Measurements

All direct measurements depend to some degree on existing nuclear-decay-
scheme data. For example, calorimetric measurements require a knowledge of the
average energy per decay. The associated uncertainty for this method is
trivial for some radionuclides emitting only alpha particles of a single
energy which has been measured to five significant figures, but can be quite
significant for beta decays for which the spectral distribution and maximum
energy have not been measured well. Many of the methods developed for radio-
nuclides with cascade radiations do not depend sensitively on details of the
decay. In this article it is accepted that a requirement for nuclear data
will not rule out a method being called direct if the sensitivity to those
data, and the certainty of the values, will permit an acceptable overall
uncertainty. Usually at least one nuclear datum, the half life, is required if
the measured value is to have significance at any earlier or later time.

It should also be noted that all of the direct methods assume that no
impurity radionuclide is present. This can be tested by repeated measurements
over an appreciable fraction of a half life, but in practice a careful search
for impurities by other means is a prudent first step in any calibration.
Another very important topic not discussed here is the quantitative prepara-
tion of suitable and stable sources and solutions.

Techniques discussed here are assigned, somewhat arbitrarily, to one of
two general classes: (1) those where the detector efficiency can be calcu-
lated from geometrical and physical information, without the use of
information obtained from cascade radiations; and (2) those where the cascade
radiations form the basis of the method.

2.4 Nuclear Radiations and Direct Activity Measurements

In order to relate the counting rate or current from a detector to the
activity of a sample of a radionuclide being measured without use of a
reference standard, several points usually have to be addressed: (1) What is
the radiation to be detected, what is its energy, what is its probability per
decay, and what is the effect of other radiations?; (2) What is the effect of
the source matrix and support?; and (3) What fraction of the emitted
radiations interact in the detector and what is the response?

Some of the methods discussed later can bypass these questions, but to
see the difficulty in devising methods to answer the questions with suffic-
jently small uncertainty, it is useful to consider the properties of different
radiations. Alpha particles are absorbed readily in thick sources and are
accompanied by nuclear recoils that can cause confusion, but they have large
and unique eneryies, backscatter from supports only at near-grazing angles,
and can be restricted by a defining aperture with little contribution from
edge effects. Beta particles are emitted with a spectrum of energies from
zero to some characteristic maximum energy, with energy-dependent absorption
or scattering in sources or supports. Positrons share these characteristics
and produce annihilation photons. Positron decays also have electron-capture
decays competing. Electron capture, which happens even when there is insuffi-
cient energy for positron emission, can occur in different atomic shells; and
the subsequent atomic deexcitation produces both characteristic x rays and
Auger electrons.


1. Introduction
1.1. General Information

The NBS "4n"y ionization chamber (IC) is a well-type pressurized
instrument which is used for measuring the activity (decays per second) of
gamma- ray-emitting radionuclides. Scheduled calibration services for
submitted samples of gamma-ray-emitting radionuclides are offered as a
complement to the Standard Reference Material (SRM) program as described in
NBS Special Publication 260 [1]. These calibration services are advertised in
the NBS Special Publication 250 (SP250) [2] as the “old" 8.2C (for radio-
nuclides with half lives greater than 15 days)and 8.2D (for radionuclides with
half lives less than 15 days) which are now the “new" 43010C and 43020C,
respectively. The range of activity for the gamma-ray-emitting radionuclides
is 0.4 - 60 MBq, and the nominal uncertainty range is 0.7 - 3.4 percent
(corresponding to 3 standard deviations). Special tests for beta-particle-
emitting solution sources are also available. These special tests are
advertised in the SP250 as the old 8.2Q, R, and presently 43060S and 43070S.
The specifics of these special tests are not discussed in this paper, but
general information is given about the direct calibration of the beta-particle
emitting radionuclides in section 2, Design Philosophy and Theory.

The "4n"y chamber uses calibration factors for each radionuclide
previously established by direct methods of activity measurements (1,2). A
description of the instrumentation used to make direct measurements is given
in the section on Design Philosophy and Theory. Many IC calibrations have
been checked with those of other national standardizing laboratories through
the Bureau International des Poids et Mesures (BIPM).

The gamma-ray-emitting radionuclides which may be submitted to NBS for "4n"y
ionization chamber calibrations are:

Na-22 Fe-59 Mo-99 1-123 Ba-140 Au-195
Na-24 Co-60 Te-99m 1-131 Ce-141 Hg-197
Sc-46 Ga-67 Cd-109 Ba-133 Eu-152 Au-198
Cr-51 Se-75 Ag-110n Cs-134 Eu-154 71-201
Mn-54 Sr-85 In-111 Cs-137 Eu-155 Hg-203
Co-57 Y-88 Sn-113 Ce-139 Yb-169 Pb-203

1.2 Description of Service

1.2.1 Purpose and Results of Service. The calibration service for the
gamma-ray-emitting radionuclides is available for the technical user who must
make measurements consistent with national standards or who requires higher-
accuracy calibrations than are available with commercial standards. When the
calibration is completed, the technical user is provided with a report
stating:



Questions are as follows:
New Bkg-0; Skip 1
# BKGS=-1
Time: Day#
Input Start of Calender Scalar HR,MIN,SEC
Reset clock on clock scalar
Computer Time: HR:MIN:SEC
Statement of agreement between computer and calender clock
Answer Y or y if OK, Norn to reenter cal, start time
Enter Sample #: 0-BKG, 999-DONE, -999 change CAPR, PTS.
4.3.2 Background Measurements. Background measurements are made
initially and should be examined before continuing. If the background is

inconsistent, or abnormally different, constant repetitive measurements may be
necessary.

4.3.3 Setting the Electrometer. Set the coulomb dial on the
electrometer to 10- ‘or background or 10-9 for a sample.

Set the sensitivity dial to the proper setting. The range of sensitivity
is 0.1 to 1000. The sensitivity setting for background is 0.1. Refer to
Table 6 for determining the 1-1000 range settings.

24


3.2 Characteristics and Functions

Several important characteristics of the NBS "4n"y ionization chamber
include: sensitivity, long-term stability, simplicity of operation, and
simple source preparation [2]. The instrument is not only used to maintain
results of direct standardization, but also provides checks of consistency
between results of direct measurements made at different times. It is also
useful for testing dilutions and sampling methods. It also serves to check or
establish half-life data for most gamma-ray-emitting radionuclides for which
basic calibrations have been performed, All samples measured are carefully
checked for impurities with germanium gamma-ray spectrometers and appropriate
corrections are applied.

3.3 Current Measurements

The electrometer is used as a coulombmeter, where the voltage developed
across a capacitor is measured [5]. As shown in figure 3 this output voltage
is fed into the comparator, which triggers a timer scalar to start counting
the elapsed time between a low and high voltage reading on the capacitor. The
triggering of the timer scalar is typically one volt to start the scalar and
10 volts to stop it. The critical point is that the comparator circuit is
designed to always measure a fixed difference in voltage, i.e., nine volts,
whether from 1 to 10 or from 2 to 11 volts, in order to minimize temperature
fluctuations in the comparator circuits. After measuring this elapsed time,
the comparator triggers a print control circuit which transmits the timer
scalar and calendar scalar data to the computer for storage and further
processing.

“an
IONIZATION — . Multirange

ONIZATION Electrometer [>] COMPARATOR

START STOP
SCALER

|

CALENDAR
Lt. SCALER

{

PRINT- OUT
CONTROLLER

COMPUTER

COMPUTER
TERMINAL

Fig. 3 Schematic Diagram of Electronic System for the "4n"y Ionization Chamber.

21


TABLE OF CONTENTS

Page
-iv
wi

Abstract..
List of Tables..

List of Figures. vii
Introduction

1.1 General Informations.....6. 1
1.2 Description of Service....... 1

1.3 Specifications for Samples Submitted for Calibration in the
NBS "4n"y LTonization Chamber..scscccccccccccccccccscccvsccsvece 2

Design Philosophy and Theory

2.1 Physical Principles and Restrictions.....
2.2 Direct Calibrations...cceccccccccccece

2.3 Comments on Direct Activity Measurements.
2.4 Nuclear Radiations and Direct Activity Measurements
2.5 Direct Methods Using Calculated Efficiencies.
2.6
2.7

Direct Methods Using Cascade Radiations....
Radioactivity Group Instrumentation at the U.S. National
Bureau of StandardS..cecccseccecccccccvcvece
2.8 Present Basic Radionuclide Calibrations at NBS

Principles of Operation
3.1 Construction of the NBS "4n"y Ionization Chamber.
3.2 Characteristics and Functions... .scecccceceees
3.3 Current Measurements....sceee
3.4 Source Container and Contents

13
al
a
22

Operational Procedures
4.1 Description of the Actual Measurement Process........0-
4.2 Receiving Instructions for Radioactive Solution Samples.
4.3 Step-by-Step Operating Instructions for the NBS "4n"y Pressure
Tonization Chamber. seseccccsecvcccvccccccccscecssscccsscsesees 23
4.4 Data AnalysisSccceccscccccscvees
4.5 Format of Calibration Report.

Assessment of Uncertainty
5.1 Precautions and Uncertainties in Service Calibrations.
5.2 Significance of Uncertainty Statements........
5.3 Examples of Uncertainty Components....cessveee

References... 33



ABSTRACT

This paper describes the use of the NBS "4n"y ionization chamber — an
instrument which provides an indirect method of comparing the activity (decays
per second) of gamma-ray-emitting radionuclides with national standards for a
routine calibration service by the National Bureau of Standards Radioactivity
Group. A description of the chamber's construction and characteristics, the
operational procedure, and the associated equipment is included.

A description of NBS capabilities for direct radioactivity calibrations

is also presented. Many of these capabilities are used to establish
calibration factors for the “4n"y ionization chamber.

Key Words: activity; calibration; gamma ray; ionization chamber;
radionuclides; radium reference source; standard

iv


Subsequent to the primary decay, excited levels in the daughter nuclide
then decay by gamma-ray emission or by internal conversion. The latter pro-
cess also gives rise to x rays and Auger electrons. Long-lived excited states
can contribute extra counts for the fraction of decays occurring beyond the
resolving times of counting circuits. Radionuclides decaying only by isomeric
transitions, without alpha or beta radiations, must also be calibrated.

The many types of radiations, and the large range of energies and half
lives encountered, suggest that there is no universal method for direct
calibrations, but rather that each radionuclide must be considered as a
separate challenge. Also, most standardizing laboratories consider it prudent
to check a calibration with an alternate method wherever possible. The
reduction in the uncertainty of nuclear-decay data in recent years, together
with improvements in technique, make measurements with efficiency-calibrated
germanium-spectrometry systems a common indirect way of making a final check
for many radionuclides.

2.5 Direct Methods Using Calculated Efficiencies

Methods using calculated efficiencies can be divided into those for which
the efficiency is very high ("4x" methods) and those for which the efficiency
is smaller, usually because the solid angle is appreciably smaller, but where
the latter can be specified with sufficient accuracy ("defined-geometry" meth-
ods). Both methods carry a requirement that at least the sum of the
probabilities per decay for each of the radiations to be detected must be
known with an acceptable uncertainty. Both usually require some extrapolation
or correction for events too low in energy to be separated from system noise.

2.5.1 High-Efficiency Counters. Two instruments commonly used for "4x"
measurements are gaS-proportional counters and liquid-scintillation counters.
In the former, attenuation in the source and backing materials is the usual
limitation for solid sources. In the latter, difficulties may arise from

extrapolation complexity, resolution limitations, "quenching" (reduction of

light output with the addition of the radioactive material), and the
sensitivity to radiations other than the ones desired.

For radionuclides emitting beta particles of sufficiently high energy,
4n proportional counters can yield results with a satisfactory uncertainty.
Suitable precautions in preparing thin conducting support films and thin
sources must be observed.

For alpha particles, liquid-scintillation counting avoids the problem
of energy loss in a support, but any energetic electrons may interfere with a
measurement because of the much higher light output for a given energy absorp-
tion relative to alpha particles. If such interferences are taken into
account, simple liquid-scintillation counting can be quite effective.

2.5.2 Calorimetry. If the average energy per decay for a radionuclide
is known and almost all of this energy can be transformed into heat in a
calorimeter, measurements of low uncertainty are possible, even for
radionuclides emitting low-energy beta particles.

10


Fig.
Fig.
Fig.
Fig.

lL
2a.
ab.
3.

LIST OF FIGURES
Page
Sectional View of a One-Inch Diameter reentrant-well......e.0. 7
Relative Response of Chamber vs. Gamma-Ray Energy 0-500 keV.... 8
Relative Response of Chamber vs. Gamma-Ray Energy 500-3000 keV. 8
Schematic Diagram of Electronic System for the "4n"y
Toni zation Chamber.scsecsscccccccceccsccsccscccccevcsccssesees 2b

vii


TABLE 4 INSTRUMENTS FOR BASIC NBS CALIBRATIONS USING CASCADE RADIATIONS

Counting System Label
Thin-crystal Thin-Xtal
scintillation Cascade
system
Large sodium 8" Nal
iodide system
Liquid scintillation Ls
system Cascade
Early atmospheric PC-y
proportional counter —
gamma-ray coincidence
system
atmospheric PC Wheel
proportional
counter system
with source changer
Pressure-proportional PPC
counter system Cascade

Description and References

A NaI(T1) crystal 1.6mm thick is used
with a thin plastic scintillator for
measurement of gamma rays in coincidence
with low-energy beta rays, or in summation
with another thin NaI(T1) crystal for
sum-peak counting of 125],

Two 8" by 4" NaI(T1) crystals with
shallow wells are brought together to form
a@ system for sum—peak or high-efficiency
cascade counting.

A hemisphere containing a liquid-
scintillation cocktail is optically coupled
to the phototube on which it sits. A 4"
Nal(Tl) crystal with a 2" well is

inverted over the hemisphere. Coincidence
or anticoincidences are used to measure
the efficiency for selected beta or
electron-capture branches as a function of
energy.

A spherical counter has opposing wire
loops in top and bottom, with the source
at the center plane on a gold-coated
collodion film at ground potential. The
2-in x 2-in Nal(T1) crystal is external
to the counter,

Two "pill-box" proportional counters
operating at atmospheric pressure are
separated by a thin metallized film carried
ina slide, Shielded 3" x 3" Nal(T1)
gamma-ray detectors are above and below the
proportional counters. Coincidences are
recorded, An automatic sample changer
inserts in turn one of 30 samples. Cover
films are added for efficiency extrapolation.

Two “pill-box" proportional counters
operating at pressures of about 1.5 MPa are
separated by a slide carrying a metalized
thin film on which the source is deposited.
Shielded 3" by 3" gamma-ray detectors are
above and below the proportional counters.
The system is operated in the coincidence
and anticoincidence modes, with discriminator
efficiency variation for extrapolation,

16


NBS Measurement Services:

Radioactivity Calibrations

with the “4x” Gamma lonization
Chamber and Other Radioactivity
Calibration Capabilities

NBS
Special
Publication
250-10

U.S. Department of Commerce
National Bureau of Standards

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a) the principal radionuclide

b) reference time and date

c) method of calibration

d) certified value of activity

e) decay-scheme assumptions

f) assessment of radionuclidic purity
g) overall uncertainty

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 NBS, for the activity would be too low for measurement after
the NBS calibration.

1.3 Specifications for Samples Submitted for Calibration in the NBS "4n"y
Tonization Chamber

1.3.1 Submitted Solution Specifications. Samples submitted for calibra-
tion must be chemically and physically stable. Tables 1 and 2 list the
chemical forms which are suggested for these solution sources. Tables 1 and 2
also list the activity ranges and their associated nominal uncertainties.

The solution sample should consist of a specific radionuclide in
5.0 + 0.2 milliliters of liquid flame-sealed in a NBS-type borosilicate-glass
ampoule. The ampoule will be provided by the Radioactivity Group.


The activity, A, of the radionuclide in a sample, S, to be measured is
given by KpRs, where Ro 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. Kp diminishes from the value at the time of the original calibration
with the 1600-year half life of the 226Ra, and a correction for this must be
made.

4.2 Receiving Instructions for Radioactive Solution Samples

All radioactive sources arriving at NBS are delivered to the Health
Physics Group where they are examined to determine the radiation level and
possible contamination of containers and packaging.

After Health Physics releases the source to the analyst, the analyst
in turn should follow a quality control procedure to 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:

a. The analyst should re-examine the source for pin holes of any type
and check the ampoule seal.

b. The analyst should inspect the label attached to the primary
container. This helps to insure that it is the required radionuclide
and the activity amount is correct.

c. Technical data sheets and package inserts accompanying the radio-
nuclide package should be read 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, or see Tables 1 and 2.

Providing that no defects are observed, the sample may be transported for
measurement in the NBS “4n"y ionization chamber.

4.3 Step-by-Step Operating Instructions for the NBS “4n"y Pressure Ionization
Chamber

4.3.1 Starting of the Ionization Chamber System. The system is started
from the control terminal using the following procedure:

Log on as user- “ondata" (lower case)

Password is (blank), carriage return, "“datax" will be displayed
on screen in Tower case letters

Type “iconline", carriage return

Answer Y or y for default para., Nor n to change and supply answers
(generally use default values)

Carriage return after each. answer

23


The electrometer has a sensitivity stated to be 10uV. The input
amplifier provides high input resistance (2 x 10!4 ohms), low offset current
(less than 5 x 10-15 A), and low noise. The double-shielded input section
permits floating operations up to 1000 V above chassis. The construction of
the electrometer keeps unshielded capacitance from input HI to chassis ground
below 0.1 pF.

3.4 Source Container and Contents

The solution to be assayed in the NBS "4n"y ionization chamber should
have as nearly as possible the same composition as the standards used to
calibrate the chamber [4]. The gamma-ray-emitting radionuclide solution
should be 5,0 + 0.2 mL in a standard 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

Where variation in sample volume cannot be avoided, corrections should be
made from data obtained by investigation of changes in the pertinent
parameters [4]. Corrections can be empirically determined for different
solution heights, the function being only slightly dependent on the energy of
the radiation.

4. Operational Procedures

The operational procedures for calibrations using the "4n"y ionization
chamber has been divided into several sections. Each section is designed to
provide instructions for a specific operation. These sections are:

4.1 Description of the actual measurement process

4.2 Receiving instructions for radioactive solution samples

4.3 Step-by-step operating instructions for the NBS "4n"y Pressure
Ionization Chamber

4.4 Data Analysis
4.5 Format of Calibration Report

4.1 Description of the Actual Measurement Process

The stability of this chamber, although excellent, is made moot by
measuring its response to a reference source of 226Ra in equilibrium with its
daughters whenever calibrations or measurements 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 cali-
bration factor, Kps is given by Rp/R, the ratio of the measured responses of
the 226Ra reference source and a Brown one bequerel of a stated radionuclide.

22


Table

Table

Table

Table

Table

Table

LIST OF TABLES

Specifications for Calibration of Gamma-Ray Emitting Solutions
Which are Submitted for Calibration (Radionuclides having half
lives greater than 15 dayS)..cscccceccccecccseccccscececcececcs

Specifications for Calibration of Gamma-Ray Solution Sources
(Radionuclides having half lives less than 15 dayS)..ecessssees

Instruments for NBS Basic Calibrations Not Using Cascade

Radi ationS..scccccccccccccccccccccceccevecceceeccessseeccececes

Instruments for Basic NBS Calibrations Using Cascade Radiations

Radionuclide Measurement Capabilities in the Radioactivity
Group at the U.S. National Bureau of Standards

Sensitivity Range Settings...ccccccccccceccccccccccccccccscsces

vi

Page

14

16

7

25


2.7. Radioactivity Group Instrumentation at the U.S. National Bureau of
Standards

Some 40 counting systems are in use by the NBS Radioactivity Group for
the direct calibration of radionuclides, checking for impurities, -retaining
calibrations, counting special samples, measuring half lives and the probabili-
ties per decay of radiations, and intercomparing samples of production SRMs.
Some of these systems have been described in the past or are similar to
instruments described by others. In many cases the detector or source-handling
portions were constructed at NBS, but in general commercial modular electronics
are used, with provision for direct or indirect transfer of data to a
minicomputer for analysis.

Table 3 lists and describes some of the equipment currently used for
establishing basic calibrations without using cascade radiations. Table 4 is
a similar list for instruments using cascade radiations.

2.8 Present Basic Radionuclide Calibrations at NBS

Finally, Table 5 lists most of the radionuclides for which the NBS Radio-
activity Group has developed calibrations in the past three decades, together
with the methods used, the estimated uncertainties as given on recent certifi-
cates, and the conformity with results from other national standardizing
laboratories in many cases. NBS uncertainties in Table 5 are given as the sum
of half of the 99-percent confidence interval of the mean, plus the linear sum
of estimates of the limits of suspected systematic uncertainties [4].

Some radionuclides have been compared bilaterally with other nations, but
for gamma-ray-emitting radionuclides the ionization chambers maintained at the
Bureau International des Poids et Mesures have proven an efficient mechanism.
NBS has 27 radionuclides recorded on these chambers at the present time. The
uncertainties shown for the international intercomparisons in Table 5 are the
simple estimated standard deviations of the means. -

3. Principles of Operation

3.1 Construction of the NBS "4n"y Ionization Chamber

The detecting element is a "4n" (re-entrant) cylindrical ionization chamber
in which the current produced by gamma rays from the sample is compared with
current from long-lived reference sources (see fig. 1). The chamber used for
most of the calibration services was constructed at the Chalk River Nuclear
Laboratories of Atomic Energy of Canada, Limited, but a commercial version from
20th Century Electronics, Ltd. in England is used in a later automatic-sample-
changing system on which basic NBS calibrations are also being stored. The
automatic chambers may later be used in the calibration service in the same way
as the present manual chamber.

All chambers are filled with 20 atmospheres of dry argon. A 4-inch-thick
lead housing surrounds the chamber presently used for service calibrations to
reduce environmental background effects and prevent a change of efficiency from
movable scattering material in the vicinity. The voltage for this chamber is
600 volts. A rigid tight-fitting holder made of low-Z material is used for the
samples and reference sources to insure reproducibility of position.

13


The origin of the uncertainty estimates (as standard deviations of means,
or suitable approximations) in a direct comparison are shown in the following
excerpt from the reporting form for an international intercomparison of
activity measurements of 133Ba, Measurement was by anticoindences between
electron capture radiations (Auger electrons and x rays) and gamma rays
measured by NaI(Tl) detectors. The efficiency of the proportional counter was
varied and an extrapolation of its rate to an efficiency of 100% was made to
give the activity.

133Ba Uncertainty Components

Due to: Uncertainty Obtained by:

counting statistics ), standard deviation of the mean

weighing 0.1 calculated effect of limits

dead time 0.02 measured accuracy of live-time
determination

background 0.1 calculation using estimated

background uncertainty

timing 0.004 measured oscillator accuracy

fitting procedure 0.13 error on intercepts

absorption 0.05 limit of measured effects

radionuclidic impurities 0.001 limit of 134Cs fraction

half-life 0.01 calculated effect of half-life
uncertainty

Combined Uncertainty 0.21 (Square root of summed squares)

5.2 Significance of Uncertainty Statements

The uncertainties given in most certificates prepared in past years were
divided into "random" and "systematic" parts. The former, derived from the
well-defined estimated standard deviation of the mean of a stated number of
measurements and the assumption of a normal distribution, was given as one-
half the 99-percent confidence interval of the mean. The "systematic" part
was given as the linear addition of “limits” of other possible errors in the
measurement system. The "systematic" part was clearly dominant in most radio-
activity calibrations. If an overall uncertainty was required, the two parts
were added linearly; if the source uncertainty needed to be propagated as a
component of the uncertainty in a subsequent measurement, it was sometimes
divided by three and treated as a standard deviation of the mean.

30


Discussions among representatives of several national standardizing
laboratories, following the distribution of a questionnaire about uncertainty
statements by the Bureau International des Poids et Mesures, led to a
Suggestion that an approach akin to that used for those components susceptible
to ordinary statistical analysis be applied to those that are not. This
suggestion followed at least the spirit of the article by J. W. Maller
Ref. [8]. With the idea that even the subsidiary quantities have distribu-
tions (perhaps measured poorly or not at all), an approximation of a standard
deviation should be devised for them. A possible virtue is that deducing,
say, the two-thirds coverage of an ill-defined distribution may be less
subjective than setting a "99% limit." Moreover, if all uncertainties are
taken to be equivalent to the corresponding standard deviations (or standard
deviations of the mean, if appropriate), the propagation of uncertainties is
straightforward and an approximation to the “total standard deviation of the
mean," called the combined uncertainty, is easily formed. A more conservative
overall uncertainty formed by taking three times this quantity is roughly
equivalent to the former total uncertainty in many cases, and this is done in
present certificates. No matter which method is used, the uncertainty for a
radioactivity calibration cannot be considered an accurate quantity, but
rather a best estimate.

5.3 Examples of Uncertainty Components

The uncertainties which have been determined for the NBS "4n"y ionization
chamber originate predominantly from the direct calibration method for the
particular radionuclide. Uncertainty is also introduced in the operation of
the ionization chamber and in the comparison with radium reference sources to
preserve the initial activity calibration of a specific radionuclide. As
examples, consider two samples submitted to NBS for measurements on the IC.
The overall uncertainty was formed by taking three times the quadratic
combination of the standard deviations of the mean, or approximations thereto,
for the following:

111In
percent

a) 59 ionization-chamber measurements on this sample 0.02
b) standard deviation of the mean of six coincidence measurements 0.06
c) dead time 0.01
d) resolving time 0.02
e) backyround 0.17
f) gravimetric measurements 0.10
g) standard deviation of the inean of the original ionization

chamber measurements 0.05
h) photon-emitting impurities 0.03
i) efficiency extrapolation 0.10
j) half-life 0.05

overall uncertainty = 0.73
(3 x vz u2)

31


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19


Library of Congress Catalog Card Number: 87-619870

National Bureau of Standards Special Publication 250-10
Natl. Bur. Stand. (U.S.), Spec. Publ. 250-10, 41 pages (Oct. 1987)
CODEN: XNBSAV

Commercial products —materials and instruments—are identified in this document for the sole pur-
pose of adequately describing experimental or test procedures. In no event does such identification
imply recommendation or endorsement by the National Bureau of Standards of a particular product;

nor does it imply that a named material or instrument is necessarily the best available for the purpose
it serves.

U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON: 1987

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325


Samples which are too low in activity produce a current insufficiently
different from background; samples which are too strong may produce so many
jon pairs that recombination can occur before the electric field sweeps them
away. Any change in container, solution nature, or density can cause changes
in gamma-ray absorption. Sample location must always be the same; in order to
achieve this, and to maximize the current, the chamber is made re-entrant so
that the sample can be placed at the center (see fig. 1).

The chamber cannot distinguish between currents produced by different
radionuclides. Hence all samples must be examined with calibrated gamma-ray-
spectrometry systems, and the effect of any impurity radionuclides removed.
The amount (activity) of the impurity present is deduced from the spectrometry
measurements, but a calibration factor may not have been established for some
uncommon radionuclides. In this case, a response (current) can be synthesized
from the observed gamma-ray-emission rates at different energies and an
ionization chamber "response" curve constructed from the measured response of
radionuclides emitting one or two gamma rays of known probability per decay
(see fig. 2a and 2b).

Obviously, the ionization chamber is not suitable for radionuclides which
emit only alpha particles or electrons, or for which NBS direct calibrations
of sufficient accuracy have not yet been performed. However, the chamber
offers a useful means of propagating national standards for, at present, some
36 radionuclides, including many for which SRM's are not regularly available.

2.2 Direct Calibrations

The following excerpt is from an article [1] that summarizes most of the
direct calibrations which have been performed at NBS, including those used in
establishing the National Bureau of Standards IC calibration values. All
methods and radionuclides are given to indicate other possible NBS services
beyond those listed in SP250. The uncertainties given in the article,
obviously different for each radionuclide, usually contribute the major uncer-
tainty in a calibration certificate. A change in the method of specifying the
uncertainties retains their "30" significance, as given in this article on
pages 40-44. Detailed descriptions of the direct activity measurement
techniques are given in Ref. [2].


0.10 -

0.05 +

RELATIVE RESPONSE

0 1 L 1 1
0 100 200 300 400 500

ENERGY, keV

Fig. 2a. Relative Response of Chamber vs. Gamma-Ray Energy 0-500 keV.

xto-8
0.6

O05

0.4

RELATIVE RESPONSE

02

n
500 1000 1500 2000 2500 3000
ENERGY, keV

Fig. 2b. Relative Response of Chamber vs. Gamma-Ray Energy 500-3000 keV.


NBS MEASUREMENT SERVICES:
RADIOACTIVITY CALIBRATIONS
WITH THE “4r” GAMMA
IONIZATION CHAMBER

AND OTHER RADIOACTIVITY
CALIBRATION CAPABILITIES

Jacqueline M. Calhoun

Center for Radiation Research
National Measurement Laboratory
National Bureau of Standards
Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Clarence J. Brown, Acting Secretary
NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1987


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(2)

(3)

(4)

NOTES

Approximately five milliliters of solution. Ampoule specifications:

body diameter 16.5 + 0.5 mm
wall thickness 0.60 + 0.04 mm
barium content less than 2.5 percent
lead oxide content less than 0.02 percent
other heavy elements trace quantities

The overall uncertainty was formed by taking three times the
quadratic combination of the standard deviations of the mean, or
approximations, thereto, for the following:

a) 59 ionization~chamber measurements

on this sample 0.02 percent
b) standard deviation of the mean of

six coincidence measurements 0.06 percent
c) dead time 0.01 percent
d) resolving time 0.02 percent
e) background 0.17 percent
£) gravimetric measurements 0.10 percent

g) standard deviation of the mean of
the original ionization chamber

measurement 0.05 percent
h) photon-emitting impurities 0.03 percent
i) efficiency extrapolation 0.10 percent
j) half-life 0.05 percent

Limits of detection as a percentage of the gamma-ray-emission rate of
the 245-keV gamma rays emitted in the decay of indium-111 are:

0.1 percent between 30 and 240 keV
0.01 percent between 250 and 1900 keV,

provided that the impurity photons are separated in energy by five keV
or more from photons emitted in the decay of indium-111l.

NBS-measured half-life value. NCRP Report No. 58, 2nd Edition,
February 1985, lists a half-life value of 2.805 + 0.001 days.

For further technical information please contact J.M. Calhoun at 301-921-2383.

29


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Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National
Bureau of Standards. The Center provides the Nation with standards and measurement services for fonizing
radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support
to the National Measurement Support System-for ionizing radiation; conducts research in radiation related
fields to develop improved radiation measurement methodology; and generates, compiles, and critically
evaluates data to meet major national needs. The Center consists of five Divisions and one Group.

Atomic and Plasma Radiation Division

Carries out basic theoretical and experimental research into the Atomic Spectroscopy
spectroscopic and radiative properties of atoms and highly ionized Atomic Radiation Data
species; develops well-defined atomic radiation sources as radiometric  @ Plasma Radiation

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

Provides the central national basis for the measurement of far ultra- © Far UV Physics
violet, soft x-ray, and electron radiation; develops and disseminates e@ Electron Physics
radiation standards, measurement services, and data for for these radia- @ Photon Physics
tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applica-

tions; determines fundamental mechanisms of electron and photon inter-

actions with matter; and develops advanced electron- and photon-based

measurement techiques. The Division consists of the following Groups:

Radiometric Physics Division

Provides national measurement standards and support services for altra- e Spectral Radiometry

violet, visible, and infrared radiation; provides standards dissemination © Spectrophotometry

and measurement quality assurance services; conducts research in optical @ Radiometric Measurement Services
radiation, pyrometry, photometry, and quantun radiometry; and develops

spectroradiometric and spectrophotometric standards and calibrat fon

Procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division
Develops, operates, and improves major NOS radvation Facilities
including the electron Linac and race track microtron; develops,
designs, and builds electronic and mechanical instrumentation for
radiation programs and facilities; provides national leadersnip
in the standardization of nuclear instrunentat ion; and develops
new competence in radiation sources and instrumentation. The
Division consists of the following Groups:

Accelerator Research
Linac Operations
Electronic Instrumentation
Mechanical Instrumentation

veoe

lonizing Radiation Division

Provides primary national standards, measurement services, and basic
data for applications of ionizing radiation; develops new methods of Radiation Theory

chemical and physical dosimetry; conducts theoretical and experimental Radiation Chemistry and Chemical
research on the fundamental physical and chemical interactions of Dosimetry

ionizing radiation with matter; provides essential standards and Neutron Measurements and Research
measurement support services to the National Measurement Support Neutron Dosimetry

Office of Radiation Measurement

.
.

System for Ionizing Radiation; and develops and operates radiation © Radioactivity

sources needed to provide primary radiation standards, fields, and @ X-Ray Physics

well-characterized beans of radiation for research on radiation © Dosimetry

interactions and for development of measurement methods. The Division
consists of the following Office and Groups:

Nuclear Physics Group

Engages tn forefront research in nuclear and elementary particle physics:
performs highly accurate measurements and theoretical analyses which
probe the structure of nuclear matter; and improves the quantitative
understanding of physical processes that underlie measurement science.



Table 6. Sensitivity Range Settings

Capacitor Ratio Radium Reference SourceTM
1.0 Backyround :
10.0 3, 10, 20
100.0 20, 50, 100, 200
1000.0 200, 500, 1000, 5000

*The number of the radium reference source used in approximately equivalent to
the number of micrograms of radium per source.

SENSITIVITY

10
19 © 100
e

Ole #1000

Note: The capacitor ratio and the 226Ra reference source to be used must be
determined based on the activity provided by the supplier when the
sources are submitted, with appropriate corrections for decay.

4.3.4 Measuring Reference Source. The 226Ra reference source is then
placed into a rigid, tight-fitting Lucite holder and lowered into the
chamber.

After the radium reference source is placed in the chamber, change the
timer controller switch from the manual to the automatic mode. This step is
repeated each time a radium reference source or solution sample is measured.

After the designated number of measurements have been made on the 226Ra
reference source, or a sample, type on the terminal "-1", then press line feed
key. This should be done at the end of every set of measurements. This will
display the average count time for the sample after measurement. The sample
number will be requested for the next sample which is to be counted.

Remove the holder from the chamber and replace the 226RA reference source
with the sample which is to be assayed.

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Fig. 1. Sectional view of a one-inch diameter reentrant-well IG = 11 pressure
ionization chamber. (From Centronic, 20th Century Electronics Ltd.,
King Henry's Drive, New Addington, Croydon CRQ OBG, England.)