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OCT 1 7 1975

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NBS SPECIAL PUBLICATION 331

Radioactivity

Calibration Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. Today,

in addition to serving as the Nation's central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

areas. These are: (1) basic measurements and standards, (2) materials measurements and

standards, (3) technological measurements and standards, and (4) transfer of technology. The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

Institute for Applied Technology, the Center for Radiation Research, the Center for Computer

Sciences and Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and

uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of an Office of Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering - —Time and Frequency - —Astrophysics - —Cryogenics.- THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; develops, produces, and distributes standard reference materials; relates the physical and chemical properties of materials to their behavior and their interaction with their environments; and provides

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the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations in the development of technological

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offices: Engineering Standards—Weights and Measures — Invention and Innovation — Vehicle Systems Research—Product Evaluation —Building Research—Instrument Shops—Measurement Engineering—Electronic Technology—Technical Analysis. THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and application of radiation to the solution of Bureau mission problems and the problems of other agencies and institutions. The Center consists of the following divisions: Reactor Radiation—Linac Radiation—Nuclear Radiation—Applied Radiation. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and

provides technical services designed to aid Government agencies in the selection, acquisition, and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data—Clearinghouse for Federal Scientific and Technical Information : ' —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg. Maryland, unless otherwise noted; mailing address Washington. D.C 20234.

2 Located at Boulder. Colorado 80302.

:1 Located at 5285 Port Royal Road, Springfield, Virginia 2215).
UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

Radioactivity Calibration Standards

Proceedings of a Special Session of the

International Conference of the American Nuclear Society

Meeting on the Constructive Uses of Atomic Energy

Held in Washington, D.C., November 10-15, 1968

Edited by

W. B. Mann and S. B. Garfinkel

Center for Radiation Research

National Bureau of Standards Washington, D.C. 20234

National Bureau of Standards Special Publication 331

Nat. Bur. Stand. (U.S.), Spec. Publ. 331, 120 pages (August 1970) CODEN: XNBSA

Issued August 1970

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C 13.10:331). Price $1.25
NATIONAL BUREAU OF STANDARDS MAR 1 5 1971

ABSTRACT

Experimental procedures and methods used in nine

international radioactivity standardization laboratories

are described by eight authors. The possibility of

attaining accuracies of the order of 0.01% are discussed.

The desirability of carrying out consistency checks of

gamma-ray standards with time, using a 4 Try ionization

chamber is emphasized. The validity of radioactivity

standards is examined. International comparisons organized by the Bureau International des Poids et Mesures

are discussed. An analogue method of liquid-scintillation

counting is described. Certain types of radioactivity

standards are criticized. The fields of interest of the

two Canadian laboratories are discussed. Details of the

work in progress at the Boris Kidric Institute and National

Bureau of Standards are presented.

Key words: Alpha-particle standards; calibration consistency; coincidence counting; decay-scheme effects; efficiency

tracing; gamma-ray standards; international comparisons;

liquid-scintillation counting; radionuclide half-lives;

radionuclide standards.

ii
CONTENTS

PAGE

Introduction

W. B. Mann and S. B. Garfinkel 1 Aspects of Radionuclide Standardization

in the IAEA

H. Houtermans 5 Some Aspects of Radioactivity Standardization

at the CBNM of Euratom

A. Spernol 13

International Comparisons of Calibrated

Radionuclide Sources

A. Rytz 25 Standardization of Radionuclides by EfficiencyTracing Methods using a Liquid-Scintillation

Counter

D. G. Jones and A. McNaLr 37 Validity of Radioactive Standards

Y. Le Gallic 53

Radioactivity Standardization in Canada

J. G. V. Taylor and A. P. Baerg 69 Metrology of Radioactive Isotopes at the Boris

Kidric Institute of Nuclear Sciences at VincaBeograd, Yugoslavia

Dj . N. Bek-Uzarov 79 Recent Activities of the NBS Radioactivity

Section, with Special Reference to the

Standardization of Thallium-208

S. B. Garfinkel and J. M. R. Hutchinson .... 105

iii
Acknowledgement

Very grateful acknowledgement is made to Mrs. S. C. Love and Mrs. P. A. Cannon who spent long days of continuous effort in typing the manuscript to meet deadlines, and

also to Mrs. R. Maddock for preparing and processing the

many tables and photographs and for final assembly of the

various parts. Deep appreciation is expressed to Mrs. M.

E. Guggenheimer who provided overall coordination of the

procedures

.

iv
INTRODUCTION

The National Bureau of Standards was invited by the

American Nuclear Society to organize a seminar on Radioactivity Calibration Standards at its November 1968 International Conference on the Constructive Uses of Atomic

Energy in Washington, D.C.

These Proceedings contain the papers given at that

seminar and the discussions that were generated by these

papers. The discussions were recorded on tape and are presented verbatim, with a minimum of editing, in order

to preserve their informality and vitality.

The delay in publication of just over a year, due to

a variety of reasons ranging from initial production to final reproduction, is unfortunate. The importance and

originality of this collection of papers on radioactivity

standardization from many of the world's leading laboratories is such, however, as to mitigate against any harm

due to delay, and most of the material and points of view

presented herein are still unavailable elsewhere.

We would like to thank Mr. Oscar M. Bizzell and

Dr. J. F. Black both for suggesting the seminar and for

their support in organizing it.

W. B. Mann National Bureau of Standards

S. B. Garfinkel Washington, D.C. 20234

1
2J. F . Black

I just wanted to take the opportunity to make a few

remarks here this afternoon to introduce our program on the

Radioactivity Calibration Standards to extend the appreciation of our division and society to the participants. This

is one of the two special sessions which the Isotopes and

Radiation Division is sponsoring here at the Washington

International Meeting and is distinguished by the fact that

first, it is one of the few sessions which consists entirely

of invited papers and secondly, I think this session exemplifies the international air of the meeting in a better manner

than most other sessions in view of the wide variety of the

countries represented. Now I think we all realize that the

organization of a meeting of this sort takes extensive time

and effort from a man who has an international reputation

which can draw the type of participation which has been

achieved here and I think that we've been very fortunate in

having such a man available as Chairman of the meeting in the

person of Dr. Wilfrid Mann. I would like to extend the

appreciation of not only the audience but the entire membership of the ANS Isotopes and Radiation Division to Dr. Mann

for his efforts in organizing this fine program and I would

also like to welcome the individual speakers whom he has

invited and to extend our appreciation for the opportunity

to hear about their work.

W. B. Mann

Thank you very much for your kind opening remarks and

I'd just like to say very briefly how happy we are to welcome

so many of our colleagues from abroad. The International

Radioactive Metrology fraternity is a most pleasant one, and

it has been one of my greatest joys to be able to work in

this field.
3

I have been asked to read a paper for Dr. Hans Houtermans , but before I do that I'd like to say that we are going to try to publish the proceedings of this session and we do have a microphone and tape recorder going here, in

order to get the discussion which can often be as interesting

as the papers. The papers will be published and will be

available I hope, and if anybody has a text and can give

this to Mr. Garfinkel or to me I would deeply appreciate

this, but the net result is that you will be able to skim

over your papers and just touch on the high spots knowing

that they will be published in their entirety, and if you

exceed 20 minutes there will not be much time for discussion

of your paper and as I say discussion can be very valuable.

I have glanced through Hans Houtermans ' paper and I hope

I can read it without detracting from it. In a covering

letter, he said it did not have any figures or any jokes,

and I am afraid I cannot supply any jokes. It's very short

and I feel I have to read it. I cannot give you a brief

resume. His paper is on "Aspects of Radionuclide

Standardization in the I.A.E.A.:

Aspects of Radionuclide Standardization in the IAEA

by

H. Houtermans

International Atomic Energy Agency, Laboratory

Vienna, Austria

One of the activities assigned to the laboratory of the

IAEA in 19 5 8 was the distribution of radionuclide standards.

The responsible bodies of the Agency felt at that time that

the supply of such standards was inadequate. Apparently,

the fabrication of standard sources for all the numerous important radionuclides was neither scientifically very

exciting nor interesting from a financial point of view.

Therefore, this job was left to national standardizing

institutes mainly, which were sometimes more inclined to improve calibration methods than to use their very limited

staff for the distribution of many calibrated radionuclides.

On the other hand, the performance of the necessary measurements just for a few samples of a not too frequently used

radionuclide would, by any method of cost accounting, raise

the price per sample to intolerable heights.

Since 1962, the IAEA laboratory has offered and

distributed standards for a number of radionuclides. Because

of its international clientele and by strict synchronization,

the number of samples per distribution was relatively large,

and thus the prices could be kept low.
6 Radionuclide Standardization in the IAEA

Now it has been pointed out that during the last years

the availability of radionuclide standards, perhaps due to a growing demand, has improved considerably, and standards of

most radionuclides can be obtained from several different

institutions. Therefore, it is appropriate to review the

whole situation.

The intercomparisons between specialized laboratories,

normally the "national standard" ones, organized by the BIPM,

have shown agreement within 0.5% or better between the values

of such institutions in some favorable cases. In principle,

the activities of such standard sources from different

laboratories may be expected to be consistent within these

limits. But, for the overwhelming majority of radionuclides

the situation is much less satisfying. Intercomparisons of

standard sources from different suppliers have only been made

in a few cases, probably, because even these relative measurements are by no means easy to perform to a precision

definitively better than the overall uncertainty stated on

the certificates.

Even in the most favorable case, gamma-ray emitters in

solution form, possible errors due to weighing, not exactly

reproducible conditions and non-linearity of the measuring

device make it difficult for a non-specialized laboratory to decide that there is a discrepancy. Moreover, the samples

are available nearly always at different times; uncertainties

in half-lives, possible effects of small radionuclidic

impurities and doubts about the long-term stability of the

instrument response increase the possible errors.

It is probably for these reasons that the IAEA

laboratory has, in all the past years, received practically

no complaints about the activities given for the standardized samples. Because of the inherent sources of error,
Houtermans 7 selling radioactivity standards, especially short-lived ones, seems to be a fairly safe business.

It is now being considered that the IAEA laboratory may

shift gradually its activity in the field of radionuclide

standards from distribution to intercomparison by relative

methods. There is no reason whatsoever that IAEA standards

are "better" than those fabricated elsewhere, but IAEA as an international organization may be in a favorable position

for registering and comparing calibration results obtained

elsewhere

.

At present, a scheme along the following lines, based

on a proposal by Dr. W. B. Mann, is being discussed. It is

assumed that the IAEA laboratory can preserve calibration

results by means of some relative device such as a 4 tt re- entrant ionization chamber for gamma-ray emitters. The

quantity to be determined and preserved may be, in the case

of a standardized solution of a special radionuclide, the

quotient Q of the response of the instrument, when measuring

a normalized sample of the solution, e.g. 2 grams, and the

activity per gram according to the calibrating laboratory.

For eliminating fluctuations of the instrument sensitivity,

the sample can also be compared with a long-lived standard

source. Then the essential quantity is Ag/ i.e. the activity of the nuclide, which gives the same instrument response

as the long-lived standard, when measured under standard

conditions. Of course care must be taken to keep all conditions such as ampoule dimensions, absorption in the liquid

etc. constant.

Institutions with practical experience in radionuclide

standardization may now be invited to convey standardized

samples in solution form of as many gamma-ray emitting

nuclides as possible. On the basis of the reported value
8 Radionuclide Standardization in the IAEA

for the activity per gram, IAEA will measure the equivalent

activity2 A . e If two samples have been received of the same radionuclide from different laboratories, a list of the A

e values for this radionuclide will be started and be sent to those laboratories. They will now be asked to grant permission to include their result in further lists; otherwise

their value will be kept confidential.

Laboratories sending samples later will receive their

A^ value together with the list as it stands at this date;

they will also be given the choice between inclusion in the

list or withdrawing their result.

Copies of the list containing new values will be sent

from time to time to all laboratories whose values are already included.

To increase the usefulness of the scheme, error estimation and other details to be included in the lists

should be supplied by the calibrating institutions. A

special form may be devised for obtaining uniformity.

One important point is the radionuclidic purity of the

samples. At the beginning, IAEA will determine A

g under

the assumption that impurities do not influence the results.

At a later stage of the program, the radionuclidic purity of

the samples may also be checked by IAEA in order to avoid

inclusion of results for "impure" solutions in the list.

If this scheme is successful, each laboratory can compare its results with those of many other specialized

institutions, irrespectively of the time the calibrations

were performed. The IAEA laboratory will, of course, parti-
Houtermans 9 cipate itself, if possible even furnishing the first A

g values on the lists.

No interpretation of the results by IAEA is planned,

but of course, each laboratory will draw its own conclusions.

The immediate effect of the scheme will certainly be an increase in consistency of radionuclide standards, but an improvement of the accuracies can also be expected.

If the scheme shows its usefulness for gamma-ray

emitters, it can be extended to other radionuclides also.

For example, special proportional counters, windowless or with a thin window, can be used for solutions containing

high- and medium-energy beta-ray emitters, liquid-scintillation counting for low-energy beta-ray emitters and electroncapture nuclides.

Many people think of a radioactive standard source, which has been calibrated by a relative method "only",

automatically as one of a minor quality. This opinion is

not justified and many unnecessary "absolute calibrations"

are performed, not taking into account earlier results and

those of the many other laboratories. If connected with

radioactivity measurements, "absolute" just means "not depending on other standardizations", or more simply "start- ing from zero". If done properly such calibrations are clearly elaborate and time-consuming; they should only be

performed occasionally for checking the relative method, or

if one feels one can now do better than last time. For a standard, the only figure which matters is the overall

uncertainty - "absolute" or "relative", indirect or more

or less direct is of no importance to the user.
10 Radionuclide Standardization in the IAEA

W . B . Mann

Is there any discussion? I don't propose to answer any

questions, because I cannot, on behalf of Dr. Houtermans of

the Agency. Maybe we should have some discussion of the

subject. I think Dr. McNair was making rather the same point this morning about the preservation of measurements.

Q. M. Bizzell

How does this compare with the early schemes the Bureau

used? For example, I recall the matter of standardization of

iodine-131 beginning about 19 47. This seems to be somewhat

similar to the approach that was taken in the standardization

of that medically important radioisotope. Would you care to comment on that, Dr. Mann?

W. B. Mann

19 47 was even before my time or that of Miss Cavallo who

is here, but who might be able to comment. I suspect that

much of the early work was done by 4tt6 counting.

L. M. Cavallo

Yes, it was, but earlier measurements were made by

determining the aluminum absorption curves for iodine-131

and for radium D + E and then comparing the responses at the

extrapolated intercept for zero absorber.

O. M. Bizzell

I have reference, specifically, to the cross-comparisons

of iodine-131 that George Manov did in late 1948 and in 1949.

Dr. Manov worked in cooperation with many hospitals around
Houtermans 11 the country that were using iodine-131 for diagnosis and

treatment. He found variations by factors of 3 to 4 in

activity measurements. As a result of these intercomparisons, iodine-131 activities administered to patients were brought into much better agreement with NBS calibrations.

The patient benefited tremendously and the medical effects

of iodine-131 became much more predictable.

W. B. Mann

That may have been by the extrapolation method that

Miss Cavallo has just mentioned. Actually I remember the

intercomparison as I was at Chalk River at the time and we agreed also to participate. Only later on did we construct

our "4tty" re-entrant ionization chamber at NBS and that is one of the most valuable devices that we ever had because

you have a constant check on your previous measurements and

all your past wealth of data is preserved.

A. McNair

I'd just like to comment that at the Radiochemical

Centre we have been doing just this sort of thing for the

past seven or eight years. All of our absolute measurements

of gamma-emitting standards are checked against previous

measurements using a re-entrant type of ionization chamber.

We also do this for other people's standards, whenever these

are available to us, particularly those obtained from the

National Physical Laboratory in the United Kingdom. We have

measurements on Agency standards as well. We have this

wealth of preserved information going right back into the

past, and as Dr. Houtermans' points out, you can reduce a number of errors in absolute standardization just by averaging over a long period of time. The sort of thing that I am thinking of are aliquotting and dilution errors in an abso-
12 Radionuclide Standardization in the IAEA

lute standard, errors due to random background fluctuations,

etc., which for any one absolute standard are systematic

errors. In 20 or 30 absolute standards spread over a period

of time these become random errors in the overall calibration of the ionization chamber. There are also other errors,

like errors in assessing impurities in the material which may

or may not become effective random errors over a long period

of time. In principle I think it is quite feasible that one

can make relative measurements with an accuracy approaching

that of so-called absolute measurements in a number of cases.

W . B . Mann

Thank you Dr. McNair.

I remember too at the last meeting of the BIPM at Sevres

that Dr. Campion had graphs from international comparisons

showing the consistency of results from different laboratories over a period of years, 5 or 10 years. It's quite interesting. He was simply taking from reports of the BIPM and

measurements the NPL has made on solutions that they had been

distributing. Is there any more discussion on this paper?

I think probably we had better go on because as I say I am in no position really to answer for Dr. Houtermans, and so I would like to call on M. Spernol of Euratom to give his paper

on "Some Aspects of Radioactivity Standardization at CBNM of

Euratom.
Some Aspects of Radioactivity Standardization

at the CBNM of Euratom

by

A. Spernol

Central Bureau of Nuclear Measurements of Euratom

Geel, Belgium

Thank you, Professor Mann, for the kind invitation and

the opportunity to talk here about the work of our Euratom

laboratory at Geel in Belgium. The part of our work I would

mainly like to speak about pertains to recent refined improve

ments in the accuracy of radioactivity determinations. This

part is especially interesting, because, on the one hand, it

is intimately connected to current problems in nuclear physics, and, on the other hand, throws some light at the present

limits of accuracy in activity standardization. But first

some words about our laboratory and its activities in general

Our radionuclides laboratory is part of the CBNM,

Central Bureau of Nuclear Measurements, which is responsible

for standards and measurements in the nuclear energy field

in the European Community. The main interest of CBNM (comprised of about 200 people) lies therefore with neutron

standards and data and consequently an important part of the

activities of its radionuclides laboratory is tied to such

neutron objectives. The staff of our laboratory is kept
14 Radioactivity Standardization at the CBNM

below the maximum number, which according to one of Parkinson's laws still allows efficient working and is high enough

to allow extensive team work. Of the present 18 members,

four are guests from different countries. The instrumental

equipment of the group consists of more than 50, generally

self-built, counting devices, about 20 of which are in

current use. This includes two electromagnetic 3-ray spectrometers, 2 Ge (Li ) -detectors and 8 Si (Li)- and Si (Surfacebarrier)- detectors. Our annual budget is about $40,000,

and the production rate of publications nearly 10 per year.

The work of our group can be subdivided into three

parts, which absorb approximately equal amounts of our time:

1) direct neutron work, 2) standardization, 3) work for the

improvement of the accuracy of radioactivity measurements

.

Typical examples of the first kind of work are an international intercomparison of threshold detectors irradiated in

about 15 different reactors, which we are just finishing, or

the accurate determination of the thermal-neutron activation

cross section of Co-59. In the field of standardization we

took part in all the useful BIPM intercomparisons , supply

about 100 special standards per year to other laboratories,

and give assistance to other groups in and outside the CBNM.

Our efforts towards the improvement of the accuracy of

radioactivity measurements can again be subdivided into three

parts: 1) development and improvement of methods, 2) accurate determination of nuclear and atomic constants involved

in radioactive decay, mainly decay-scheme parameters, and 3) investigation of the physical effects, which limit the accuracy of radioactivity determinations, mainly effects of

higher order. About 20 papers pertaining to the first point

have been published in the last four years, ranging from 3- counting using Cerenkov radiation to the application of the

coincidence-counting method for the accurate determination of
Spernol 15 internal conversion coefficients. Recent work concerns, among

other minor investigations, the reduction of photon induced

afterpulses in proportional counters and the investigation of

the y~sensitivity of 3-counters in the 4tt3~y coincidence

technique. An interesting result is the confirmation of the

very recently established dependence of the time interval

between pulses and afterpulses in a proportional counter on the travel time of electrons between cathode and anode.

Consequently we reduced the diameters of our cylindrical 4tt 3- counters to about 2.5 cm and can now apply dead times of 0.4

ysec with less than 0.05% afterpulse contribution to the

counting rate. Another result of some interest is that the

widely applied tungsten and molybdenum wires are less suitable anodes for proportional counters, because they rapidly

deteriorate while stainless steel wires behave much more satisfactorily. With afterpulses eliminated, short dead

times, non-deteriorating anodes, suitable choice of the cutoff energies and proper correction of the y-sensitivi ty of

the B-counter we hope that we shall soon be able to reach an accuracy near 0.01% in 4tt3~Y counting. Some effort has also

recently been directed to the investigation of currentmeasuring methods of radioactivity determination, which might

replace pulse-counting methods at the accuracy level of

better than 0.01%.

Since the accuracy of radioactivity measurements is very

often limited by the inaccuracy of several percent, with

which most decay schemes are only known, much work has been

devoted in the last years to the accurate determination of

decay schemes and atomic constants. The schemes which we have re-investigated until now are shown in Figure 1. Only

our results are indicated. Always as many as possible

different methods have been used, for example, four for the

determination of the 3 + -branch in the Co- 58 decay. The re- sults on the Cs-137 decay andd on the low intensity 3-transi-
16 Radioactivity Standardization at the CBNM

Co-60,5.27y Cs-137,~30y

Co-58,71d

Figure 1.
Spernol ] 7 tions in the Co-60 decay are perhaps interesting. The re- sults on Zn-65 are not yet published, because we had a deviation of about 1% for the y -branch intensity, when we

used argon-methane instead of methane as counting gas in the

4tt3-y method. The argon-methane result is certainly wrong,

because all other methods gave excellent agreement on the

branching ratios. But we could not explain the difference

until now and are still working on the problem. The results

on Co-58 and Tm-170 are only preliminary, the final series of

measurements are not yet finished. The measurements on the

EC-branch in the Tm-170 decay are especially simple, since

the branching is calculated only from spectra obtained with

high resolution Si (Li ) -detectors , which permit resolution of

the K-X-rays of Er and Yb following electron capture or con- version after 3-decay, respectively. Of the decay schemes

which are at present under investigation that of Be- 7 seems

to be the most interesting one, also for neutron work.

Atomic constants involved in radioactivity, which have been

redetermined with an accuracy of better than 1%, are K-shell

fluorescence yields of Cr after Mn-54 decay, Cu after Zn-65

decay and CI after A-37 decay; those of several other nu- clides are under investigation. Perhaps the most interesting

of our results on nuclear constants are the conversion coefficients measured with our e-x-coincidence method to an accuracy of better than 0.5%. This accuracy could not be

reached before and is therefore of some importance for the

investigation of nuclear penetration effects in internal

conversion. Finally it could be mentioned, that we redetermined about 20 half-lives with a rather high accuracy, including a microcalorimetric measurement on Tl-20 4, which was just

finished. Many half-life measurements are still under way,

the most important being those on the long living uranium

isotopes

.
13 Radioactivity Standardization at the CBNM

The last point in our work for the improvement of

accuracy in radioactivity determinations is the most interesting one. It is the investigation of effects of, in

general, higher order intervening in radioactive decay. An

uncomplete list of these effects, which often are subjects

of recent studies in nuclear physics, is shown for a first

orientation in Table 1, together with an indication of their

order of magnitude. Some of them are under investigation in our laboratory. First results, for example, of an investigation of the ionization and simultaneous X-ray emission

during 3-decay of Pm-147 seem to confirm a recent theory,

which predicts a rather great difference between total 3- spectra and such taken in coincidence with X-rays. Also the

theoretically interesting dependence of the ionization rate

on the energy of the emitted 3-particles is under investigation. With 3 +-decays we have the complications connected

with the annihilation. Problems are here the uncommon modes

of annihilation, as one- or three-photon annihilation, directly or via positronium formation, and annihilation in

flight. For the latter, figures can be found in the literature ranging from zero to several percent (relative to normal

annihilation), a quite unsatisfactory situation. We estimated roughly the order of magnitude of this effect and try now

to confirm our estimate by experiment. It is not impossible,

that many EC/3+ -ratios measured so far have to be corrected

for annihilation in flight. Another rather interesting problem is the intensity and energy distribution of high-energy

electrons emitted during electron capture. This question is

particularly important for Be-7, which as a "pure" y-emitter

is widely used for the determination of y-eff iciencies of 3- counters. The total number of electrons with a continuous

energy distribution accompanying electron capture amounts in

this case probably to more than one percent, but the energy

distribution of these electrons shows a rapid decrease with

higher energy above about 50eV. Therefore it is not impossi-
Spernol 19 Table 1 Some higher order and environmental effects in

nuclear decay and their order of magnitude

With first order process: Order of magnitude

e decay

ionization and excitation of daughter atom % 10 ! " " " in the surrounding

material

internal bremss trahlung

external " recoils

double effects 10

a.

'X, -5

0.1' 1%

1%

e +decay

same as in e decay

annihilation other than at rest

EC decay

ionization and excitation of daughter atom

internal bremss trahlung

recoils

double effects

10

10

-3

- 1% M),01%

" 5 - 1% r transition

internal conversion

internal pair formation

monoenergetic positron emission

two y-quanta emission

two electron emission (2xIC or IC+Compton)

electron plus y-ray emission

recoils

ionization and excitation in inactive source material

^10_^%

vLO % * 0.1%

^ 0.1%

^ 0.1%

environmental effects on half life, capture ratios

conversion ratios etc. might reach several %.

ble, that all measurements with Be-7 of the y-efficiency of

3-counters might be wrong at low 3-cut-off energies. We are intensively investigating this problem, having obtained preliminary promising results in spite of difficulties with

impurities, and hope soon to have final reliable figures.

Disturbances in radioactivity measurements can also be caused

by the environmental effects on nuclear decay. Best known is
20 Radioactivity Standardization at the CBNM

the influence of the electron shell structure of the halflife, for example, of Be-7, which should be stable if it

would be fourfold ionized. We are investigating the dependence of the half-life of Be-7 on temperature and chemical

structure. First measurements at 300 °C did not confirm an old theory that predicts a considerable dependence of the

half-life on the temperature. Measurements at 2000 °C as well

as measurements with different Be-7 compounds are under way.

These investigations on the half-life are closely connected

to our L/K capture-ratio determinations on the same nuclide.

Here, besides the investigation of environmental effects, the

recently much discussed exchange and over- lap effects could

be decisively checked. Our proportional-counter measurements,

performed about three years ago using beryllium acety lacetonate at 300°C, failed, but new measurements using electrostatic

analyzers are under way.

Finally it should be mentioned, that all our work of

which I could only discuss examples, would not be possible

without the fine spirit within the group, an extensive

cooperation between its members and the excellent skill of our technicians.

Thank you!

W. B. Mann

Thank you, M. Spernol. Would anybody like to ask

M. Spernol questions on his paper?

A. McNair

Two questions, M. Spernol. When you speak of accuracies

of the 0.01% in 4tt3-y measurements, is this precision in

measuring a particular source, or is this the overall accu-
Spernol 21 racy of measuring the activity of a solution, including deposition on glassware of the radioactive material, the weighing

uncertainties and all this sort of thing?

A. Spernol

I speak of the overall accuracy, including all effects,

but I didn't mention in my talk all the other effects which

should also be investigated such as impurity, and adsorption,

and these effects include a lot of investigations that were

not mentioned here.

A. McNair

So 0.01% is the accuracy of the final result for the

solution?

A. Spernol

Yes, but one will not reach that in the first attempt;

at the moment 0.01% is the order of magnitude which we hope

to reach.

A. McNair

Do you take any special precautions about plating of

radioactive materials on to vessels when dispensing or storing materials?

A. Spernol

We have alot of different precautions, for example,

source preparation is made by an electro-balance which, as far as we know, has an accuracy better than one microgram but

is difficult to check for droplets because there is no better
22 Radioactivity Standardization at the CBNM

method. It was checked by standard weights and was better

than one microgram. This is necessary for 0.01% overall

accuracy

.

A. McNair

Yes, well, I ask this question because we've noticed

recently that for quite a large number of materials you can

get absorption on vessel surfaces, ampoules and this sort of

thing, of significant fractions of a microgram of material,

so that if you're going to try to approach the 0.01% level of

accuracy you're really going to have to have a lot of carrier

or something else.

A. Spernol

Yes, there is only a small range of activity and carrier

where you can avoid this effect, but this must be checked in

any case, this is quite clear.

A. McNair

That was one question. The second question: Have you

investigated environmental effects on fluorescence yields?

A. Spernol

Not sufficiently yet. What we did until now was to measure only K-shell fluorescence yields and environmental

54 effects on K-shell fluorescence in Mn decay. The latter

effect is certainly very low, because the nuclear charge

is rather high. This environmental effect on fluorescence

yields can only be expected to be considerable for the outer

shells so we have had no results on it until now. But there

are alot of papers where the effect is described as x-rayenergy shifts.
Spernol 2 3 A. McNair

Yes, I was thinking in particular of low-Z materials in

the iron region, where if you look at the literature it seems that the published values of the fluorescence yield do depend

on the state of the material when the measurement was made.

Of course the accuracies of the measurements are often not

very good, but it is very difficult to sort this sort of

thing out.

A. Spernol

The effect of location on the fluorescence yields and

their magnitudes is certainly below 0.1 or 0.2% and this

cannot be measured at present.

W . B . Mann

I have asked this question before, when you reach 0.01%

this seems to be our present limit of precision as you have

4 to count, say, a microcurie, which is 3.7 x 10 dps, for an hour, or 3.6 x 10"^ seconds. That is a total of approximately

8 4 10 counts, giving a precision of 10 , but you are already

counting for a long time and may run into problems of self

absorption. You say you had already achieved this

precision?

A. Spernol

Aiming for this. This is just one of the points which I mentioned. If you are to measure this number of counts you

need low dead times. With normal proportional counters you

cannot use dead times below, let us say, 5 or 10 microseconds

due to the photon-induced after pulses but if you reduce this

distance between the after pulses, by decreasing the diameter
24 Radioactivity Standardization at the CBNM

of the counter, then you get this dead time. Our normal

measurements last 12 to 24 hours and yield precisions much

below 0.01%.

J. Silverman

Am I to understand that the state of chemical combination

has no significant effect of fluorescence yields? There is,

I believe, a known measurable effect on the half-life of

beryllium-7 as a function of the state of chemical combination.

I think Segre reported it some time ago.

A. Spernol

Yes, this is a very unsatisfying situation. These

theoretical investigations and measurements on beryllium-7

were done in the years 1948 to 1955, and the theory always

gave influences on the chemical compound of the order of some percent. The first measurements showed this, but it was proved afterwards that this was wrong and the final measurements then showed that the effect is below 0.1%, two orders

of magnitude below the theoretical prediction. But my belief

is that not all of these experiments were very well done

(especially the chemistry of the sources), so that the

problem is not yet finally solved.

W. B. Mann

Thank you. The next paper is by Dr. Rytz of the

International Bureau of Weights and Measures on "International

Comparisons and Calibrations of Radionuclide Sources".
International Comparisons of Calibrated Radionuclide

Sources

by

A. Rytz

Bureau International des Poids et Measures

Sevres, France

Radioactivity designates both, the property of undergoing

spontaneous nuclear transformation by emission of radiation

(particles or photons) and more restrictedly , the rate of decay of these so-called radionuclides. Activity is measured in

terms of disintegrations per unit time. The dimension being

an inverse time, the same as that of a frequency, it is important to stress the basic difference between activity and frequency of recurrent events which stems from the fact that

there is rigorously no connection between single decay events.

This gives rise to a random distribution of rare events known

under the name of the Poisson distribution. According to Poisson's law, the measurement of an activity A during seconds

has a mean error of /A/t. Since this error is of the same nature as A, both magnitudes should have the dimension s 1

, rather than dps. Besides s"

1

, the Ci is also in legitimate

use ( = 3.7 x 10 10

s -1

) and is very nearly the activity of

226

1 g of Ra. Finally, activity may be expressed as the

number of active atoms considered, multiplied by the decay

constant A, which is the decay probability per unit time. In all activity measurements we suppose that A is a characteristic constant of the radionuclide. Since A is a magnitude

25
26 International Comparisons of Radionuclides

which cannot be calculated exactly from other data, e.g. decay

energy, it has to be determined experimentally with the aid of

activity measurements. Thus there is an inevitable interdependence between half-life measurements and comparisons of

activity measurements. In other terms: the radioactive

impurities always set a limit of accuracy beyond which

comparison of radioactivity is meaningless.

Activity measurements can directly be carried out in

terms of s . But in many cases, considerable self-absorption takes place and calls for correction terms which are so great that the measurement loses its absolute character. A

super-purist might even argue that there is no really absolute activity measurement at all. I prefer a slightly less

extreme attitude and would call absolute those measurements

in which one counts single events, in an absolutely known

solid angle, in terms of counts per second and where corrections are based on a sound theoretical basis. Uncertainties

of corrections should not be much greater than the overall

imprecision. Thus, ionization measurements should not be

called absolute, but, under favorable conditions, proportional

4tt counting may give absolute results. The 4tt (PC) -y method,

on the other hand, is considered to be the most reliable

absolute method, provided it is applied to suitable

radionuclides

.

Scientific, medical, and industrial applications of

radioactivity considerably increased the need of reliable

absolute measurements of activity and availability of standard sources during the last decade. The smallness of the

energy involved and the fact that part of it may be carried

away by undetectable particles are the roots of the difficulties encountered in standardization of radionuclide

sources

.
Rytz 27 Speaking about sources, we mostly think of solid sources,

i.e. dry deposits on thin backing material. More generally,

I should like to extend the notion of source to solutions of

radionuclides from which solid sources may be prepared in a quantitative way. Liquid sources lend themselves better to the use of varying parameters and to shipping to distant laboratories. The quantity to be measured is, of course, activity per unit mass of solution.

If one carefully studies a certain method of

standardization one can eliminate a great many systematic

errors. If several laboratories compare their results

obtained by measuring the same sources by the same method,

some of the remaining systematic errors may further be discovered, i.e. eliminated. Yet there may still be systematic

errors of some more hidden nature which escape detection

unless different and independent methods of comparable precision are used. If this order of ideas and procedure is

changed one takes the risk of increasing the effect of

systematic errors.

The idea of international comparisons of sources and

methods is not new. Discussion started in 1958 and three

years later the decision was taken that the BIPM should

extend its activity to the measurement of ionizing radiation

and organize international comparisons of radionuclides. It was recognized that preparation and distribution was not the

task of the BIPM, but that the aim was: getting coherent

measurements between national standardizing laboratories.

This obviously included the creation of a laboratory at the

BIPM which can participate and take over the organization. A

very ambitious program of comparisons was fixed and the aim

was stated as follows: "An accuracy of + 1% is desirable,

going as far as 0.1% in special cases".
28 International Comparisons of Radionuclides

The practical work started in 1961 and during the first

18 months as many as six comparisons took place, including

some rather difficult radionuclides. Table 1 gives a very

rough account of the mutual agreement obtained. Further it

illustrates what I just said regarding systematic errors. Nevertheless, I should like to mention that the last two

comparisons enabled us to collect a great quantity of valuable information on many technical aspects of source prepara

tion and counting.

TABLE I Review of the first 7 international comparisons of radionuclides carried out under the auspices of the B.I. P.M.

Radio- Reference Number of Total Spread of Number of

nuclide date participants spread selected selected

(%) labs. labs.

32 p Jan. 61 16 6.0 2.0 13 131, Apr. 61 16 4.0 1.5 12 198AU Jan. 62 25 3.1 2.5

1.0

15

10 60 Co Jan. 6 2 21 9.0 3.0 18 May 6 2 19 14.2 5.9 14

35

s June 6 2 15 18.4 6.5 4

After a year's interruption a thoroughly prepared

comparison of the 4^3 (PC) -y method, using 60 Co, took place

and turned out to be a real landmark in the history of inter

national comparisons of radionuclides. Each of the 20 parti

cipating laboratories had to measure four solid sources of

different activities which had already been measured by two

experienced laboratories. In addition, a more usual

comparison of a

60 Co master solution was carried out.
Rytz 2 9 The results can be summarized briefly as follows: For

the first time, this comparison made it possible to discuss

differences between results of the order of fractions of a percent instead of several percent. Between the results

obtained by any two laboratories, the mean difference

amounted to 0.6%, whereas the two pilot laboratories had a mean difference of 0.3% with a probability of 95%. Each

participant had also to measure a thick source leading to a reduced 3-efficiency and hence a considerable decay-schemedependent correction. It was found that this correction can be determined with sufficient accuracy. The results of the

specific activity of the solution distributed gave a total

spread of 1.4% which contrasts advantageously with that of

preceding comparisons. Finally, there was some evidence of

a systematic error due to the so-called extrapolation

technique used in source weighing.

The solid sources had been most successfully prepared

by electroplating gold-covered VYNS films by A. Spernol and

his co-workers at Euratom (Belgium) . These sources proved to be extremely stable and strong mechanically. At the BIPM we have sorted out four of them which we measure regularly by

4tt3-y coincidence counting. These repeated absolute measurements of the same sources may be considered as the creation

of a "maintenance standard" of radioactivity. They raise a fundamental question: what should we trust more, the "con- ceptual standard", i.e. the 4tt3-y equipment or the "mainte- nance standard"? Before trying to answer, let us look at Figure 1. It represents a semi- logarithmic plot of 43 mea- surements of the sum of the 4 decay rates distributed over a little more than a half-life.

The first 17 results were obtained with the old equipment

in the old building; most of the following results with the new equipment in the new laboratory. Different selections can
30 International Comparisons of Radionuclides

therefore be made. I have calculated the half-life with four

different sets of results, using a least squares fit to a straight line, assuming equal weights, which is not far from

reality. All the half-lives thus obtained are slightly higher

than the value found in the literature. Figure 2 shows the

2 deviations from the best fit straight line. A x test confirms the visual impression that the 4 3 points do not seem to belong to a single Gaussian distribution. Even so, the mean deviation is only about 0.1%. We get even 0.05% with a more

2 satisfactory x test if we take only the 20 last measurements

obtained with the new equipment.
Rytz 31

0.01-

-0.01

43 DEVIATIONS FROM BEST FIT STRAIGHT LINE

0.05%

oo o o

'1 •• 2

• o

• old building

o new building

Figure 2. Deviations of 43 half-life measurements of 60 Co

It is not clear how we should interpret the deviations

from the generally adopted half-life. Contamination can most

certainly be ruled out. My personal feeling is that there is

a very weak count-rate-dependent systematic error, e.g. in

the dead-time correction. We are probably at the limit of

the accuracy attainable with this method. The maintenance

standard seems to be slightly more reliable than the conceptual one, but, of course, only during a limited time.
32 International Comparisons of Radionuclides

I shall not say much about the three subsequent

international comparisons since they have also been reviewed

at a panel meeting held in Vienna, two years ago. They con-

.r 241,, ... 90 o ^ 90,7 cerned measurements of Am, an a-y emitter, Sr + Y, two

54

pure 3-emitters in equilibrium, and Mn, an electron capturer. The latter, unexpectedly, contained a large

contamination of which led to some difficulties.

6 0 In 196 7, Co was again distributed in a large

comparison which mainly aimed at the dilution and source- preparation techniques. Two solutions of different concentrations in an accurately known ratio were distributed, a concept which had already been used, though with less suc90 90 cess, in the Sr + Y, comparison. The participants were

given very precise instructions as to how they should proceed

in order to get comparable results. They had to prepare 28 sources from four dilutions of the strong solution and 14 sources of the undiluted weak one, and to measure them all by

the 4tt3(PC)-y method following an exactly prescribed scheme.

About one half of the 25 participating laboratories have

succeeded to determine the specific activity to within 0.1%,

but there were also some results deviating by a percent or

more. Dilution errors may have been the reason for this, in

several cases. The concentration ratio has been obtained correctly

within 0.1% by one half of the participants. Four of them

got it right within even 0.01%.

Systematic errors have often been greatly underestimated.

In some cases the measurements can be shown to deviate significantly from a Poisson distribution or to be affected by a trend with the order of measurements or source preparation.

The latter statistical analysis is rather complicated and will

be explained in the report.
Rytz 33 A preliminary comparison between 3 laboratories has been

carried out before writing down the instructions and forms.

This has been found to be extremely useful. Finally, the

BIPM has worked out and distributed a detailed note on the

frequently neglected buoyancy correction in weighing

radioactive standard sources. All the 10 preceding comparisons have been thoroughly

discussed and described in great detail. The reports issued

by BIPM may be obtained on request and will soon be completed

by the report on the last Co comparison.

Summarizing, we see that a great amount of work has been

invested in 11 very different comparisons. Many illusions

have been destroyed, for hundreds of sources have been prepared and measured without any positive result. On the other

hand, a wealth of useful information has been collected. A definite improvement of standardizing techniques has been

reached. Many laboratories, especially the less advanced

ones, have come to know their capabilities and their position

with respect to others. Source preparation, dilution, and

4tt$-Y coincidence counting have been unified considerably.

On the whole the work has been quite successful and worth the

big effort made.

The success of comparisons of the kind described will

always depend on the interest with which they are undertaken

by the participants who may belong to one of two different

groups. The first group includes laboratories with great

experience in this field and for whom comparisons represent

only a small part of their activity. They must squeeze them

into an already heavily loaded standardization program. The

second group is formed by laboratories with medium or little

experience, wishing to improve their technique and test their

equipment. For these, comparisons are welcome opportunities
34 International Comparisons of Radionuclides

to estimate the value of their work. Laboratories of the

first type consider comparisons more or less as a sacrifice

which they take upon themselves for the community's sake.

For the second group comparisons may be of great help provided

advanced laboratories take part as well. However, comparisons are neither the only nor even the best way of helping

less advanced laboratories to improve their techniques. In many cases, a few calibrated sources or a standardized

solution would be more efficient, more rapid and much simpler.

The investment of time and money in a comparison is

always considerable on the organizer's side as well as on that of the participants.

Carrying out a comparison without a well defined aim or without sufficient interest on the part of the participants

would, therefore, be a waste of man-hours and money. We know

from experience that it is often very difficult, if not impossible, to compare results obtained by different methods, especially when they are of different levels of accuracy. On the

other hand, if we confine ourselves to a single method, systematic errors may be carried along, without the possibility of

detecting them.

A compromise between both concepts is necessary to get an undistorted view with regard to systematic errors. At this

moment, the BIPM does not have definite plans for future com- parisions. Better knowledge of special needs and interests of

the advanced standardization laboratories will have to be con- sidered and more discussion is needed before a decision can be

taken. I very much hope that this discussion will start

during this meeting.
Rytz

Note added in proof

Figure 1 contains results which might be misleading as far as the half-life of Co is concerned. After a careful

review of all the data and addition of five more recent mea- surements, we preferred to reject the 17 early measurements

and some of the following ones because of lacking coherence

in the measuring conditions. Thus only 21 measurements distributed over 3.3 years are retained. A least-squares fit

using weights inversely proportional to the square of the

experimental standard errors gives a value of (1927 + 2) days or (5.2 76 + 0.006) years, where the error stated is

purely random (one standard deviation) . We estimate that

the systematic error should not exceed twice this amount.

W. B. Mann

Thank you, Dr. Rytz. Are there any questions?

Perhaps I might just make a comment rather than ask a question. I'm not quite sure whether our laboratory is among

the advanced laboratories or not so advanced laboratories,

but Miss Cavallo recently said to me that there has not been

a single international intercomparison in which we have not

learned something, so we are very grateful for the stimulus

of these intercomparisons . Are there any questions?

R. S. Caswell

I'm a little concerned by this statement that there are no plans for the future for intercomparisons. Is there a mechanism that will review this? Is there some point in time

when a decision will be made concerning possible future

intercomparisions?
36 International Comparisons of Radionuclides

A. Rytz

I think we will decide this very soon. Even before the

end of the year.

W. B. Mann

Are there any other questions for Dr. Rytz? If not,

thank you very much, Dr. Rytz. It was a very interesting

talk.

The next paper is to be given by Dr. McNair of the

Radiochemical Centre in England. It is entitled "Standardization of Radioactive Solutions by Efficiency-Tracing

Methods Using a Liquid-Scintillation Counter".
Standardization of Radionuclides by Efficiency-Tracing

Methods using a Liquid-Scintillation Counter

by

D. G. Jones and A. McNair

Radiochemical Centre

Amersham, England

1 . Introduction

Radionuclides which emit only low-energy 3 radiation are notoriously difficult to standardize because coincidence methods cannot be applied directly to such nuclides. Gas-counting

methods can be used for certain members of the group such as tritium, carbon-14 and sulphur-35, but these methods, though

accurate, are rather difficult and tedious for routine standardization work. The simple 4tt3 method of counting solid sources prepared quantitatively from a solution is too inaccurate to be of real value, because of wide variations in the

self-absorption of the sources and the impossibility of making a direct experimental determination of the efficiency

with which electrons from the sources are counted.

Some of the advantages of coincidence methods can be

extended to the standardization of low-energy 3 emitters by a technique first proposed by Merritt et al. in 1960 (1,2) . In this "efficiency-tracing" technique a suitable 3~Y emitting

radionuclide is added to the "pure" 3 emitter and the relationship between the number of 3 particles of the pure 3 emit37
38 Efficiency-Tracing Methods

ter detected in the counter and the efficiency with which the

3 particles of the "tracer" radionuclide are detected (measured by coincidence methods) is determined experimentally.

Rather sophisticated experimental work by Williams et al (3) has shown that this relationship is not, in general, linear

but by working to high precision a meaningful curve can be

fitted in the observable region and extrapolated to 100% tracer efficiency to give a figure which is presumed to be the

true disintegration rate of the pure 3 emitter. Work, using

different tracers and the same pure 3 emitter, has shown

small, but apparently reproducible, systematic differences

between results obtained with different tracers (4). Baerg

(5) has considered the efficiency tracing method theoretically and has shown that the validity of the extrapolation

depends on an implicit assumption that the counting efficiency

depends only on the parameter being altered to change the

efficiency. To get a closer approximation to a one-parameter

system he prefers to change the efficiency of his sources by

adding external absorbers instead of employing the more usual

procedure of measuring a large number of sources containing

different amounts of inactive carrier to give a range of

counting efficiencies. Many of the difficulties that arise

with efficiency-tracing measurements using solid sources are probably caused by the measurement techniques not satisfying

Baerg 's "one variable parameter" criterion. The most usual

cause of the difficulties is generally ascribed to non-uniform

and variable distribution of the "traced" and "tracer" radionuclides in the sources causing a variation in the relative

efficiencies with which the 3 particles from the two radionuclides are detected. For this reason at the Radiochemical

Centre we decided to look at the possibility of using a liquid

scintillation counter for absolute standardization work.

In principle there are a number of attractive features

in using a liquid-scintillation counter with the radionuclide
Jones, McNair 39 dissolved in the scintillant as the electron detector in standardization work. The source preparation should be simple and

reproducible. There is no solid source to absorb the radiations emitted from the decaying nuclei, and a uniform mixture

of radionuclides should be obtainable for efficiency-tracing

measurements. A liquid-scintillation counter has the added

advantage that it is a reasonable spectrometer for electrons

of moderate energies and the counting efficiency can be varied

in an uncomplicated manner merely by altering the amplification or external discriminator levels, to give low-energy

discrimination

.

The main disadvantage of a scintillation counter is the

relatively low efficiency with which the energy of the primary

ionizing particle is converted into a useful electrical signal. As a rough rule of thumb, counting efficiencies obtained

for 3 emitters in a liquid-scintillation counter are not

greatly different from those obtained in counting solid sources

of the same radionuclides in a proportional counter. The difference is that the main loss of efficiency in a proportional

counter is the result of electron absorption in the source, whereas in a liquid-scintillation counter the loss of signals occurs in the statistical variations in the processes of

photon and, particularly, photoelectron production. These

processes may be treated mathematically or, as we have done at the Radiochemical Centre, by designing an analogue of the

liquid-scintillation counter that is close enough to reality

to permit us to make a measurement of the efficiency of the

counter which is accurate enough for standardization work.

The principles and design of the system were described in 19 66,

in a paper presented at the IAEA Symposium on Standardization

of Radionuclides (6). What follows is a brief description of

the system before going on to discuss the results.
40 Efficiency-Tracing Methods

2 . The Analogue System

The simplest and most accurate analogue system would use

as much of the electronic equipment of the real scintillation

counter as possible, and all that was done was to replace the

bottle of scintillant by a pulsed light source which simulated

in intensity and intensity distribution the scintillations

produced by the interactions of 3 particles in the scintillant.

A cathode-ray tube with an aluminized "killed-phospor

screen with a decay time of less than 0.1 ysec. proved to be

a satisfactory light source. Flashes of constant intensity

are produced by pulsing the tube with 200 nanosecond pulses

at the grid and these flashes illuminate the phototube cathode

after passing through an aperture, a series of diffusers and

filters and, finally, reflecting from a diffuse 45° reflector

replacing the sample bottle. The intensity of the flashes is

modulated by moving a shutter across the aperture. The position of the shutter is controlled by a cam and lever arrangement (Figure 1) . Any desired modulation of the intensity of

the flashes can be achieved by using a suitably shaped rotating cam to move the shutter across the aperture.

The instant at which each flash is produced is known

so that the output pulse, if any, can be identified and the

efficiency of the system measured under any desired conditions.

In order to simulate the scintillations produced in a bottle of scintillant by the 3 particles from a dissolved

radionuclide, the shape of the cam must incorporate:

A. The shape of the energy spectrum of the 3 particles. This is derived from Fermi theory.
Jones, McNair

Figure 1.

B. The relationship between the efficiency of

the scintillant and the energy of the exciting

electron. The relationship published by Horrocks

is used (7,8)

.

C. The relationship between light transmission

and radius of the cam. This was obtained by

using the cathode-ray tube as a continuous light source and measuring the current from the phototube as the lever was moved.

D. The energy range of the system. This is

the energy of a particle which will produce a scintillation of the same mean intensity (at

the photocathode ) as the flashes reaching the

photocathode when the shutter is fully open.
42 Efficiency-Tracing Methods

3 , Preliminary Tests of the Analogue System

The first tests of the system were made to see how

closely the pulse-height spectrum from simulated niobium-95

compared with the pulse-height spectrum obtained from a bottle of scintillant containing the radionuclide in Solution.

The intensity of the light flashes was adjusted so that the

efficiency measured using the analogue system was the same as the real efficiency (measured by 3~Y coincidence methods)

under particular conditions of gain and discrimination. Then

the conditions were altered in the same way for both real and

simulated spectra. Small systematic divergences between the

two spectra were observed and similar results were obtained

with real and simulated cobalt-60. However, there are a num- ber of factors which, either singly or in combination, can be

expected to create differences between the pulse-height spectra produced by the scintillation counter and its analogue.

Most of these factors are likely to distort the real and simulated spectra of different radionuclides in the same way. The

use of an efficiency-tracing type of measurement is therefore

likely to minimize the distorting effects, because in this

technique the spectrum of one 3 emitter is, in effect, compared

with the spectrum from another 3 emitter, and the simulated

spectra are also compared.

The most direct way of testing this is to measure the

relationship between the efficiencies with which the 3 particles from a mixture of two radionuclides in the scintillant

are detected, and compare this relationship with that measured

using the simulated spectra in the analogue system. In order

to make an unequivocal determination of the efficiency of

detecting real 3 particles, suitable 3~Y emitting radionuclides were used, and the efficiencies were measured by coincidence techniques. Unfortunately, there are relatively few

3-Y emitters with decay schemes which permit the selection of
Jones, McNair 43 a 3 spectrum with the correct shape and in the desired energy

range. The most suitable from the physical point of view are niobium-95, mercury-203 and cobalt-60. Measurements have been

made on mixtures of niobium-95 and cobalt-60 in scintillant,

and the results compared with the predictions of the analogue

system are shown in Figure 2. The predictions of the analogue

system are seen to be in good agreement with the measurements

on the real radionuclides. Small divergencies between the

real and simulated spectra observed at very low counting efficiencies may be caused by spectrum distortion due to wall

effects in the bottle of scintillant.

Figure 2.
44 Efficiency-Tracing Methods

4 . Measurements on Pure 3 Emitters

Measurements of the activity of low-energy pure 3 emitters cannot of themselves provide unambiguous proof that

the system gives correct results. This can only be implied,

by comparing the results with measurements of the activity

of the same materials by independent methods

.

For the purpose of this illustration I shall concentrate

on measurements that have been made on materials that have

been independently standardized, usually at other laboratories.

In particular, I shall discuss measurements on two solutions

of sulphur-35 (end-point energy 167 keV) standardized at the

National Physical Laboratory, a sample of n-hexadecane labelled with carbon-14 (end-point energy 159 keV) standardized by

the National Bureau of Standards, Washington, and a sample of

carbon-14 n-hexadecane measured at the Radiochemical Centre

but by a direct comparison against NBS-standardized sodium

carbonate-C14

.

The procedure adopted was to obtain the relationship

between e^ r the efficiency of detecting the 3 emitter, and

e

R

, the efficiency of detecting the 3 particles from the

Y tracer nuclide, using the analogue system. Then, using a mixture of the radionuclides in scintillant and varying the

conditions of measurement, values of N^, the number of 3 particles detected from the pure 3 emitter, and the corresponding

efficiency with which the 3 particles from the tracer were detected (measured by coincidence methods) were obtained experimentally. The values of N c divided by the corresponding values

p of Eg give what should be the disintegration rate of the pure

NR

3 emitter in the scintillant. Clearly _E must be constant

£

B irrespective of the value of e

r at which the measurements are

'y
Jones , McNair

made. This is a necessary though not a sufficient condition

for the system to be valid for measuring a particular pair of

radionuclides. Some examples of the results that have been

obtained are shown in Figures 3-5 and a summary of the results

of the measurements on the sulphur- 35 and carbon- 14 materials

is given in Table 1.

(JO

pCi gm.

^

0.5

[» -85.19 M Ci/gm

SULPHUR -35 SOLUTION TRACER: COBALT -60 EMULSION SCINTILLANT

0.6 0.7 0.8 0.9 EFFICIENCY FOR DETECTING P PARTICLES OF TRACER Figure 3

It is clear from the table that very good agreement is

obtained between measurements made using the analogue system

and the results obtained by other laboratories. (It is worth

noting that different methods of measurement were used at the

other standardizing laboratories. For the sulphur-35 measurements, NPL used solid source-efficiency tracing methods, fitting

a second-order polynomial to the results and extrapolating on the fitted curve. For the first of the two results quoted,

three different tracers were used but the third result was quoted to lower statistical accuracy than the first two. The
46 Efficiency-Tracing Methods

A

-85.19 M Ci/gm

SULPHUR -35 SOLUTION TRACER: NIOBIUM -95 EMULSION SCINTILLANT

0.5 0.6 0.7 0.8 0.9 EFFICIENCY OF DETECTING fi PARTICLES OF TRACER Figure 4.

1.07-

1 06- (i)

05-^

1.063 MCi/gm

n-HEXADECANE- 1-C14

TRACER: NIOBIUM -95 EMULSION SCINTILLANT

0.6 0.7 0.8 0.9 EFFICIENCY FOR DETECTING 0 PARTICLES OF TRACER Figure 5.
Jones, McNair

Table 1 Results of Comparison Measurements

4 7

Radionuclide Tracer Result from Comparison Comparison

Measured Used analogue system* Result Laboratory

(yCi/gm) (yCi/gm)

Sulphur-35 Cobalt-60 404.5+0.4

Sulphur-35 Cobalt-60 85.22+0.21 )) Niobium-95 85.35+0.12 ) Carbon-14 Niobium-95 0.783+0.002

406.91 ) 401.98 ) 394.46 ) 85.19+3.1

0 .781+0 .016

NPL

NPL

RCC, by

direct

comparison

with NBS

sodium car- bonate C-l

Carbon-14 Niobium 1.062+0.001 1.063+0.010 NBS

*The errors quoted for measurements on the analogue system

are the la statistical counting errors only. The errors

on the comparison results are the overall errors quoted

by the standardizing laboratory.

NBS measurement of hexadecane was achieved by oxidizing the

hexadecane to carbon dioxide, preparing sodium carbonate from

the carbon dioxide and comparing the activity of the sodium

carbonate directly with sodium carbonate standards.) This

good agreement, together with the results already discussed

for 3~Y emitters goes a long way to convincing us that the

system provides an analogue of the real scintillations which

is probably quite as accurate as is required for efficiencytracing measurements when the 3 spectra of the tracer and

traced radionuclides are not too dissimilar in end-point energy.

Calcium-45 (end-point energy 254 keV) has also been measured

by the analogue technique with apparently satisfactory results.
48 Efficiency-Tracing Methods

5 . Estimation of Residual Systematic Errors in the System

Having designed a system to measure the activity of

low-energy pure 3 emitters it is important to be able to make

a realistic assessment of the accuracy of the results obtained

using the system. Estimating the random errors in the ana- logue-system measurements is fairly straight-forward, but as usual, the main problem is to estimate the effects on the

results of systematic imperfections. So far, the only parameters that have been investigated have been the effects of

inaccuracies in setting up the intensity of the light flasher

for the analogue system, errors in measuring the light transmission law of the shutter, and the effects of uncertainties

in the assumed end-point energies of a few of the 3 spectra.

The correct flasher intensity is set up by measuring the

efficiency for detecting the 3 particles from the tracer under

specific counting conditions and then varying the intensity of

the flasher until the analogue system reproduces this efficiency for the simulated radionuclide. Then the relationship

between the efficiencies of detecting tracer and traced 3 particles is measured using the analogue system. This was then

repeated using a flash intensity only 50% of the "correct"

value and the change in the analogue-measured efficiency for

detecting sulphur-35 3 particles (when traced with cobalt-60)

was 0.3% of e 0 at e Q = 0.81, 0.4% of e D at e 0 = 0.64% and 1.3%

p p p p of e 0 at e 0 = 0.53. Tests show that the flasher intensity can

p p certainly be set to within 10% of the correct value.

The effects of inaccuracies in the end-point energies of

the 3 spectra act on the system in a uni-directional sense

and are not compensated by the use of an efficiency-tracing

technique. In order to investigate this (and other) parameters of the system a stepped cam was designed. With this,

the probability of detecting a light flash as a function of
Jones , McNair 4 9 its intensity can be measured. The results of a measurement

of this kind can then be combined with the other relevant data

by computer to give the appropriate relative counting efficiency curves. Measurements have shown that a 1% change in the

assumed end-point energy of calcium-45 causes about 1% change

N

8 u N

ft in -±- when e is 0.54 but only 0.2% change in when e 0 = 0.87

For sulphur-35 the corresponding figures are a 1.5% N

3 change m — when e Q is 0.34 and 0.3% change when e D is 0.80.

8

p p The stepped cam has also been used to investigate the

effect of changes in the law of transmission of light through

the shutter. Two versions of the relationship which differed

by amounts comparable with the measurement errors were invesN R tigated but the effect on _EL for sulphur-35 was very small at

£

8 e 0 = 0.34. A more extreme variation in which the mid-point

p transmission of light was assumed to be only 18.6% compared

N

R with the measured 30.2% was also tried. This altered —2. for

calcium-45 by about 0.2% at = 0.54

These results again show the value of using an efficiency-tracing measurement to reduce the significance

of variations in many parameters of the system. Clearly,

though, the investigations of systematic errors point out

the value of making measurements in the analogue scintillation system at counting efficiencies that are as high as possible, consistent with not counting spurious events, in

order to reduce the effects of systematic imperfections.
50 Efficiency-Tracing Methods

6 . Conclusions

Investigations of the behaviour of the liquidscintillation-counter system have shown that it is feasible

to design an analogue of the system which can provide accurate enough results for standardizing a number of low-energy

pure 3 emitters by efficiency-tracing measurements

.

It is particularly noteworthy that, unlike other forms

of the efficiency-tracing technique, the results are obtained

without having to make an unknown extrapolation outside the

observable region of efficiencies.
Jones , McNair

References

51

1. Merritt, J.S., Taylor, J.G.V., Merritt, W.F. and Campion,

P.J., Analyt. Chem. 3_2, 310 (1960).

2. Campion, P.J., Taylor, J.G.V. and Merritt, J.S., Intern. J. Appl. Radiat. Isotopes 8_, 8 (1960) .

3. Williams, A. and Goodier, I.W., Standardization of Radionuclides p. 153 (1967, IAEA Vienna)

.

4. Williams, A., Private Communication (1968).

5. Baerg, A. P., Metrologia, 3, 105 (1967).

6. Bryant, J., Jones D.G. and McNair, A., Standardization of

Radionuclides, p. 47 (1967, IAEA Vienna)

.

7. Horrocks, D.L., Nucl . Instrum. Meth. 27, 253 (1964).

8. Horrocks, D.L., Nucl. Instrum. Meth. 30_, 157 (1964).
52 Efficiency-Tracing Methods

K. D. George

In some of your previous slides, I recall one on hexadecane, you had some points that were triangles and some that were squares and consistent differences between these

families. What were the differences between these triangles

versus the squares?

A. McNair

These were just different samples of tracer plus

hexadecane dissolved up in liquid scintillant.

K. D. George

Different Samples?

A. McNair

Different samples, yes. All the same material but

different aliquots

.

W. B. Mann

Any more questions? If not, thank you very much

Dr. McNair, for your very interesting paper.

The next paper we had a little difficulty in translating

the title. It seems to have come out as a misprint in this

program too. I think it's "Validite des etalons radioactifs "

,

but when we looked up "validite" in the dictionary we found

it meant both "validity" and "availability" and either word

would have done all right! So we await Mr. Le Gallic to tell

us which it was

.
Validity of Radioactive Standards

by

Y. Le Gallic

Centre d' Etude Nucleaires

Saclay, France

Standard radioactive products are essential to users to enable them to quantify their research work on the various

applications of radionuclides. Further this is the logical

conclusion to the radioactive metrological studies carried

out by the national laboratories specializing in this particular field. Therefore , not only must radioactivity standards be developed and delivered but the measure of their

validity must also be determined.

The standards may be divided into two classes: primary

and secondary.

The primary standard is determined by a direct method of

activity measurements, without any intermediate handling, e.g.,

a solid source measured by the 4tt 3 coincidence method, or a

Y solution determined by calorimetry.

The secondary standard which results either from a direct

measurement of activity on an aliquot, or from an indirect

activity measurement, e.g., standard solutions calibrated, as from sources measured in 4tt3, or y sources measured in a previously calibrated ionization chamber, gas samples calibrated

by the compensated-counter method.
54 Validity of Radioactive Standards

The secondary standards are the most used by far and

this is why we shall study their validity first, but not with

out first exploding a myth,

1 . The False Standards

I shall deal very quickly with this aspect of the

question which, from the scientific angle, is of no great

interest but which nevertheless is worth mentioning because

this practice is growing.

The manufacturers of radiological protection measuring

instruments have to ascertain their reliability and to do so they use reference sources. This is quite in order of course but to keep abreast with rival firms it is essential for them

to have competitive efficiency, and hence they blithely trans

form their reference source into an activity standard. Here

are two examples of this:

A. Standard source for radiological protection

used to determine surface contamination

These sources are prepared by depositing a known amount of a standard solution on to a solid surface, stainless steel for instance.

The efficiency of the detector is then determined, and owing to backscatter of the 3 rays

this is obviously increased from 10% to 30%.

B. Standard source for a liquid-scintillation

counter

The industry making such equipment is a most

flourishing one with a very promising future

owing to the importance of these instruments
Le Gallic

in medical and biological research. But efficiency is a fundamental criterion of choice

for the user, so this is how high efficiency

is obtained:

1. By taking a solution that has really been

calibrated by a national laboratory, for

instance some tritiated toluene,

2. By placing the optimum quantity of the

tritiated- toluene- scintillator mixture

in the customary counting vial, taking

care to use only toluene triple-distilled

over sodium,

3. By pumping out, then removing all traces

of oxygen in the scintillating solution

by purging with an inert gas , and then

4. By sealing in vacuo, taking care also to cover the stopper of the flask with, say

a TiG^-base reflecting paint.

Providing all this is done for "^H, an efficiency of 30% to 40% greater will be obtained than that of the user who is

content to utilize classical scintillators, in air and

without a reflector.

Such processes involve the legal liability of the

manufacturers but they also involve the moral liability of

the national laboratories who unwittingly serve as cover.

Thus the radiological protection instruments which I mentioned a few moments ago are delivered with a secondary

standard from the Saclay laboratory of radioactivity metrology and some liquid-scintillation counters with a secondary

standard of the National Bureau of Standards.
56 Validity of Radioactive Standards

2 . Validity of Secondary Standards

A. Expression of the result ;

Let us consider for example the calibration

32 certificate of a solution of P. Let us assume it indicates:

3.7 x 10 4 dps/g + 1% on 1.1.1968 at 1200 hrs

.

How accurate is this figure?

B . Radioactive purity :

On the face of it a standard must be pure,

which means to say that the impurity content

must be of the second degree at least in

relation to the error indicated. In our

case, the radioactive impurity content

should be less than 0.1%. This is parti32 cularly not so for P which contains from

0.2 to 3% 33P.

If this reasoning is applied to the everyday production

of radioisotopes, it will be seen that only 50, out of every

90 commercially produced radionuclides delivered, meet this

specification. What should the metrologist do in such a case? Should he deliver standards only when the radioisotopes are really pure and thereby agree considerably to limit

the scope of application, or should he, on the contrary,

agree to deliver standards containing impurities and allow

for them when expressing results? We have opted for this

latter alternative, although it is spiritually not very satisfactory and even meets sometimes with insuperable difficulties

such as:
Le Gallic r,7

Non-identification of the impurity or impurities,

Difficult determination of the impurity content,

33 32 for example P in P.

C . Decay Scheme : Among the various measuring techniques,

the coincidence methods are presently

considered to be the most accurate. But

in order to use these methods it is necessary to know all about the decay scheme.

Now this is far from always being the case, since quite a number of currently used

radioisotopes have very indefinite schemes.

m, . . c . 133-, 140^ This is so for instance with Ba, Ba,

110m- , 170 m Ag, and Tm. Furthermore, the supposedly well known schemes still

produce surprises. Thus Mr. Spernol of the BCNM recently

discovered a new scheme for ^Co.

For the validity of a standard to be acceptable, it is

therefore necessary to study, or study again, each decay

scheme and also to deliver with the standard the decay scheme

utilized when the calibration was effected, in order that the user can make the necessary adjustments should there be any

modifications

.

D. Radioactive Half-life : This is a very important parameter because

it conditions the validity of the standard

in the course of time. Although measuring

techniques have improved over the last few
58 Validity of Radioactive Standards

years, large differences are unfortunately

still noticed today where half-life values

are concerned. Table 1 gives a number of

half-life values already published in Vienna

in 1966, but which highlights two aspects

of the problem:

24

1. Na case - a short-lived element,

relatively easy to determine, where the

values extend from 14.9 6 + 0.01 hrs

(Campion, 19 57) to 15.0 5 + 0.0 2 hrs

(Wyatt, Lyon, etc., 1961).

2.

"*" 33Ba case - a long-lived and fairly

difficult element to measure, where

deviations are significant since the

recent values observed varied from

10.2 + 0.8 years (Wyatt, Reynolds 1963) to 7.8 + 0.1 years (LMR, 1967).

TABLE I. RADIOACTIVE HALF LIVES

Radio- Determinations since 19 56 This work

nuclides

Minimum value Maximum value

56Mn 2 .56+0 .03 h 2 .59 1+0 .008 h 2. 58 2+0 .009 h 60 Co 5 .20 +0 .03 Y 5 .33 +0 .04 Y 5 . 27 +0 .02 Y 131, 8 .054+0 .010 d 8 .06 7+0 .007 d 8. 0 73+0 .008 d

24Na 14 .959+0 .011 h 15 .05 +0 .02 h 15. 0 0 +0 .02 h 51Cr 27 .701+0 .006 d 28 .04 + 0 .16 d 27. 7 +0 .2 d Mn 278 +5 d 313 .5 +0 .7 d 312. 2 + 0 .9 d 95 Nb 35 .0 +0 . 1 d 35 .8 +0 .5 d 35. 0 +0 .2 d 133 Ba 7 .2 +0 . 3 Y 10 .7 + 0 .2 y 7. 8 + 0 .1 y

203Hg 45 .4 +0 ,5 d 47 .2 + 0 . 7 d 47. 0 + 0 .1 d

204 T1 3 .56 +0 .05 Y 4 .11 +0 .13 Y 3. 85 +0 .04 y
Le Gallic 59 Table 2 shows the most recent results obtained in our laboratory and it also evidences the not inconsiderable

deviations, particularly for 1 ^°Tm.

Radionuclides

32^

35

S 87 Srm

99 Tcm HO* m

Ag

113 T m

In

TABLE II. RADIOACTIVE HALF LIVES

Determinations since 19 56

170

199

Tm

Au

Minimum value

14.233+0.015 d

86.35 +0.17 d

2.83+0.02 h

6.0 0+0.10 h

250.8 +0.3 d

1.70+0.0 3 h

125 +2 d

Maximum value

14.58+0.03 d

8 8.8+1 d

90+0.08 h

13+0.05 h

+ 7 d

75+0.25 h

2 +0.8 d

2

6 295

1 134

This work

3.14+0.02 d

14.268+0.042 d

87.48 +0.40 d

2.793+0.009 h

6.0 31+0.0 36 h

250.38 +0.90 d

1.658+0.004 h

127.10 +0.92 d

3.129+0.011 d

If we also assume that the acceptable error over the

half-life should be of the second degree in relation to the

final error on the standard, we can establish a valid time

for the standard which for example would be: 15 days for

133 32 170 24 Ba, one day for P, and Tm, and three hours for Na.

E . Expressing the error :

This is a major philosophical issue about

which much has already been written. Should

a distinction be made between systematic

errors and statistical errors? Should a probability of 65% - 90% or 99.7% corres- ponding to 1, 2 or 3 a be chosen? How are errors combined? By dealing with them

quadratically without distinction or by
Validity of Radioactive Standards

dealing with the random errors alone? All

these are difficult questions to answer. Nevertheless there is an evident conclusion to be drawn: the error ranges do not

tally when the various known values for

the same half-lives are examined (Figures

1, 2) or when the results of an international intercomparison are summarized

(Figure 3 )

.

>.EC MN

CENS

IKO

IMM

ISS JEN

PTB

AECL

IPM AM

ETL

A IEA

ISN NBS

NPL

NPRL

NRC

UVVVR

(1 January 1963)

0.1% 60

Intercomparison of V^O

i i i i I i

Arithmetic mean

7mi

1510 1520 1530 1540

Figure 1.
Le Gallic 61

15.10

15.05

15.00

14.95 24

Half life of Na

14.90 0.1%

1950 1954 1958 1962 1966

Figure 2

.
62 Validity of Radioactive Standards

2.30

2.20

2.10

2.00

1.90

134

Half life of CS

1957 1959 1961 1963 1965

Figure 3.

One has therefore no alternative but to agree that the

authors are optimistic, to say the least, and that if the same ideas were applied to radioactivity standards one would

be entitled to doubt their worth. This is why a number of

laboratories, including our own, has decided to deal quadratically with random errors to express the error, by adopting
Le Gallic G3

a probability of 0.99 7 which corresponds to 3a, and to find

the arithmetic sum of the errors known as systematic, S. These two kinds of errors are then combined arithmetically,

3a + S.

This philosophy of errors is not quite strict; in

particular it is not correct to combine systematic errors by

finding their arithmetic sum. We are, of course, aware of

this but prefer to be cautious rather than optimistic where

metrology is concerned.

This point of view has another drawback, needless to

say, it lowers the value of the standards of the laboratory

expressing the error in this way as compared to those who

use for example, an "effective standard deviation" such that

E S D = i [ (3a + S) = (3a + S') ].

o From this it will be seen that the accuracy expressed

with an E S D will be three times better than that achieved

by using the formula we advocate, but then these are commercial considerations which have nothing to do with metrology.

F . Validity of secondary standards :

In point of fact, from what has been stated,

the validity of a secondary radioactive standard frequently depends much more on the foregoing factors than on the standard of measure- ment as such. This is obviously no excuse for

not improving the measurements, but it does go

to show that radioactive metrology research is

not uniform.
64 Validity of Radioactive Standards

3 . The Primary Standards Problem

The question of the validity of the determinations

effected in the national laboratories remains to be posed.

In theory this is the job of the International Bureau of

Weights and Measures (IBWM) , but the task to be assumed is

overwhelming and it is hardly possible to undertake more

than one intercomparison every other year. Furthermore it

is often difficult to put the results to use. Figure 3 which

summarizes an intercomparison of Co made in 1965, illustrates the following points:

A. A dispersion of 1.5% is observed and the

arithmetical average shown on this figure

is only for the record.

B. It is impossible to quote the activity of

this ^°Co solution to 0.1% accuracy, but

the most that may be said is that the most

likely activity is determined within a bracket of + 0.5% and this is far too

imprecise

.

C. It may also be assumed that one group of

laboratories using a given weighing method

may work with, say greater accuracy than

another group using another method. But

the inter-comparison does not show this up for many errors can offset each other, and

as a consequence this requires extra studies

that each laboratory must undertake for

itself.

This gives rise to the problem of measurement reliability

and, thus, of the national primary standards, since if each
Le Gallic 65 national laboratory taking part in this 60 Co inter-comparison

had calibrated - in parallel with the solution delivered by

the IBWM - a solution or sources which would subsequently form

its primary standards, we would now find ourselves face to face with the following situation:

1. In different countries there would be

primary standards of activity which

would sometimes differ by several

percent

.

2. With accurate transfer instruments it would

be possible to obtain excellent secondary

standards relative to these primary ones, which could in turn be internationally

inter-compared, thereby enabling the

divergencies to be elucidated.

3. It would then be possible to refer back,

define the true value of the standards

as inter-compared several years before,

adjust the value of any erroneous national

primary standard and, above all, advise

the users of the secondary standards delivered by this same national laboratory as to adjustments to be made on them.

In this spirit our laboratory has undertaken the

development of the primary standards of activity, having

selected the following as reference radionuclides:

—, 1

• j u 60^ 55^ 241^ Solid phase : Co Fe Am

Liquid phase : 3 H

14

C 60 Co 90 Sr 133 Ba 241Am

3 14 Gas phase : H C
66 Validity of Radioactive Standards

To conclude, we consider that the validity of

radioactivity standards depends on the following:

1. The development of primary standards

in each metrological laboratory.

2 . Joint international action to undertake

the study of fundamental constants (decay

scheme, half-life etc.).

3. International agreement to standardize

the statement of results and errors.

It should be finally observed that activity units must

not be the sole preoccupation of nuclear metrology laboratories. This is why it seems to us also necessary to under

take the development of exposure rate and energy fluence

primary standards.

W . B . Mann

Are there any questions for M. LeGallic? Dr. McNair:

A. McNair

I think it's not so much a question as a comment. I think that M. LeGallic 's point about impurity is quite an important one and I think that standardizing laboratories

are in danger of operating in rather a vacuum by not really

being too concerned about what the users of the standard

want. In some ways I think it is better to prepare a user

a standard which has the same nuclidic composition as the

type of material that he is wanting to standardize. A
Le Gallic 67 particular case which comes to mind here, is some discussion

that we had with a user of strontium-89 standards which he

got from us. We produce our strontium-89 for standards

work by irradiation of enriched strontium-88 because this

gives a lower impurity figure and a more easily measured

one because the main impurity is s trontium-85 . But this

user, without telling us, wanted to use this standard to calibrate fission-product strontium-89 where the impurity,

of course, is s trontium-90 . It was quite useless for this

purpose, although in fact it was a much purer material than

he would have had if we had used strontium-89 from fissionproduct material. I think that we are always running into

this danger if we try to improve purity to the ultimate

extreme and we may find that the resultant standards are

of little use to the user.

Y. LeGallic

I agree with your point.

W. B. Mann

Thank you. In the early days when we were standardizing phosphorus-32 , there was always phosphorus-33 present,

but we didn't worry about this because the medical people

and the hospitals got the same samples from Oak Ridge which,

of course also had phosphorus-33 present.

I would like to make just two comments. Apropos the

certification of standards, there is a very valuable and

interesting report out, ICRU #12 by Mr. Garfinkel and Mr. Newbery of the Radiochemical Centre, which might be of

interest to people.
68 Validity of Radioactive Standards

And another comment that I have. I recall that the

Russians were in the habit so I was told, I've never seen

a certificate, of stamping their certificates that the

validity of this certificate expires after such and such a date, and this is something perhaps that we'd want to consider, that one can cancel out a certificate after its run

so many days, so many weeks, so many years.

W. B. Mann

The next paper is by Mr. J. G. V. Taylor from Chalk

River, Atomic Energy of Canada Limited, and it is entitled:

"A Review of Some Applications of Radioactivity Standardization Methods to the Determination of Nuclear Data".

Mr. Taylor:

J. G. V. Taylor

Mr. Chairman and Colleagues,- the title which Dr. Mann

gave you is indeed what I intended to speak on, but some

time after that, Dr. Mann asked if Dr. Baerg and I would

say something about radioactivity standardization in Canada

in general.
Radioactivity Standardization in Canada

J.G.V. Taylor

Atomic Energy of Canada Limited,

Chalk River Nuclear Laboratories

Chalk River, Ontario, Canada

and

A. P. Baerg

National Research Council, Ottawa, Canada

In Canada, two government organizations are engaged in

radioactivity standardization.

The National Research Council (NRC) in Ottawa has

responsibility for fundamental physical standards, and in

this respect its position is somewhat analogous to that of

the National Bureau of Standards, the National Physical

Laboratory, and other national standardizing laboratories.

The NRC group engaged in radioactivity measurements (under

A. P. Baerg) is in the X-Rays and Nuclear Radiations Section

of the Applied Physics Division and is associated with other

groups involved with standards of ionizing radiations (X-rays

under C. Garrett and W. H. Henry, and neutrons under K. W. Geiger) . L. E. Howlett, the Division's recently retired

Director, was from 1964 to 1968 President of the International Committee of Weights and Measures which sponsors

international collaboration in radioactivity standardization

through the International Bureau of Weights and Measures

69
70 Radioactivity Standardization in Canada

(BIPM) . C. Garrett, K. W. Geiger and A. P. Baerg are members of the BIPM working group on X-ray, neutron, and radioactivity standardization, respectively and A. P. Baerg is a member of the NAS-NRC (USA) Subcommittee on the Use of

Radioactivity Standards.

The other Canadian government group active in radionuclide standardization is at the Chalk River Nuclear Laboratories (CRNL) of Atomic Energy of Canada Limited (AECL)

.

AECL is a Crown Corporation, that is, an establishment

wholly owned by the government but set up as a company to allow more administrative and commercial flexibility than

a department of the Civil Service. The position of CRNL

is analogous to that of other national atomic energy laboratories such as those at Oak Ridge or Harwell. J. G. V. Taylor of the Radioisotope Standardization Section in the

General Physics Branch at CRNL is also presently a member

of the NAS-NRC (USA) Subcommittee on the Use of Radioactivity Standards

.

Historically, the Chalk River laboratories are an offshoot of the National Research Council in Ottawa. Radium

standardization began at NRC in 19 30 as part of its work

in radiology. Then the wartime atomic energy program resulted

in the Chalk River Project which became a division of NRC.

At Chalk River, absolute counting techniques were developed

to meet the needs of nuclear physics and chemistry, reactor

technology, and radioisotope production programs. During

the years following the formation of AECL in 1952, NRC had

to make up for the loss of the Chalk River group by expanding

its Ottawa standardization facilities to meet its statutory

responsibilities in this field. This is why, during the

1950 's, it was AECL rather than NRC which collaborated with

the NBS and NPL in international comparisons of radioactivity

standards 1/.
Taylor, Baerg 71 Since, except for a few long-lived nuclides, it is not

possible to maintain physical standards of radioactivity

analogous to a standard kilogram, laboratories responsible

for radioactivity standardization must concentrate on maintaining and improving apparatus and methods for making

absolute measurements. In common with many other laboratories, NRC and CRNL depend on 4tt3-Y coincidence counting

as the primary absolute method.

At NRC, measurements are made in response to requests

and intercomparisons are carried out with other national

and international laboratories. The main effort, however,

is directed toward basic research on absolute methods. In recent work, the emphasis has been on theoretical and

experiemental studies of the optimum experimental conditions

and methods of data analysis for the determination of efficiency functions 2,3,4/ . These empirical functions allow

for the influence of complex decay schemes and other effects

and are determined experimentally for each measurement. At

NRC, this is done by varying the electronic discrimination

level in the 4tt 3 channel. This method requires a 4tt proportional counter operated at high pressure. The counter

is designed for flow operation with automatic gas-density

regulation 3/.

Figure 1 is a photograph of one of the NRC 4tt3-y

coincidence systems incorporating a pressurized 4tt counter.

The PDP-8S on-line computer controls the operation of the

system to collect efficiency function data. When the efficiency functions are known in advance, the disintegration

rates are given immediately by the computer. More elaborate

data-fitting computations are handled by an IBM-360 computer

via a teletype link located in the laboratory.
72 Radioactivity Standardization in Canada

Figure 1. NRC 4tt3~Y coincidence apparatus. The detectors

can be seen through the open door of the castle

in the center. The computer-based data acquisition and control equipment is on the right;

the rest of the electronics and the gas density

regulating system are on the left.

Current research at NRC includes studies of resolution

and gain stability in 4tt proportional counters, coincidence

measurements of electron-capture decay, and decay-scheme

investigations using absolute activity measurements and high

resolution electron and photon spectroscopy. Recently a wal

less multiwire, 4tt proportional counter for electron-capture

studies has been set up by R. J. Adams.
Taylor, Baerg 73 At CRNL, the general picture is similar. Standardizations

are made as requested and the group participates in international and other intercomparisons of standards. The research

emphasis is on improving the techniques and apparatus for radioactivity standardization and on using absolute counting methods

to determine decay parameters and other nuclear data. 5,6,7/

P. J. Campion's original Chalk River 4tt3-Y coincidence apparatus 8/ recently has been modified to include an automatic

sample changer. The very fragile source mounts did not permit

the use of any commercially available changer. In Figure 2,

Figure 2. CRNL 47r$-y coincidence system partly dismantled

to show the automatic sample-changer details.

The digital stepping motor under the table to

the left positions the 36-sample vertical magazine stack to select the sample. A sample in

the sample carrier (center) can be seen half

way between the stack and the 4tt3 counter. The

upper Nal(Tl) y-ray detector has been raised and

laid across the only piece of the upper lead

shielding not removed for this picture.
74 Radioactivity Standardization in Canada

much of the shielding has been dismantled and the upper y-ray

detector moved to show the 47r3-counter , the sample insertion

and withdrawal mechanism with a sample halfway between the

counter and the magazine stack, and the sample-selecting

mechanism which drives the 36-sample stack vertically. Figure 3 is a general view of the associated electronics. The

output data are stored on punched paper type which is sent to the computation center. The system was intended for use with

a PDP-8 computer but the interface for this is not yet available. Other equipment used at CRNL includes a precision highpressure 4tt3 ion-chamber system for y-ray measurements, a large pressurized 4tt x-ray proportional counter for electroncapture measurements detectors for x-ray - x-ray coincidence

Figure 3. The electronics for the CRNL 4tt3"Y apparatus.

The temporary sample-changer control and data

readout system is on the right. The assembled

castle is partly visible on the left.
Taylor, Baerg 75 counting, and sets of compensated gas counters for gas-phase

j= j. • -u • 14 _ 85 T, counting of tritium, C, Kr, etc.

Radioactivity standards are not issued on a regular

schedule by either government laboratory because this would

not be justified by the present Canadian demand. Both organizations attempt to fill any requests they receive, especially

if the requirements cannot be met by standards available from

the IAEA , NBS, RCC or other commercial suppliers.

Turning to universities, L. Yaffe, well known for his

studies of 4tt$ counting at McGill in the mid-1950's, still

maintains absolute-counting facilities but is no longer

active in standardization research. Nor is his collaborator

in that work, B. D. Pate, who is now at Simon Fraser University. J. C. Roy at 1 Universite Laval recently has been

studying source-preparation techniques for 4tt 3 counting,

especially electrodeposition methods 9/.

In closing we would like to draw attention to the major

contribution to radioactivity standardization represented by

the alumni of the Chalk River Project dating from both the

NRC and AECL eras. P. J. Campion, who heads the Radiation

Science Division at the National Physical Laboratory in

England, spent six years at Chalk River where he became well

known for his work establishing 4tt3-Y coincidence counting

as the primary method in radionuclide standardization. A. P. Baerg, presently at NRC, spent a considerable part of his seven years at Chalk River doing absolute counting in connection with fission-yield and cross-section measurements. While

at Chalk River, L. Yaffe was a prominent figure in the pre-47Tcounter era of absolute measurements; his detailed studies of

4tt3 counting at McGill followed up the pioneer investigations

of this method at Chalk River by R. C. Hawkings and W. F.
76 Radioactivity Standardization in Canada

Merritt. J. C. Roy, now of Laval, also spent seven years at Chalk River before going to the BIPM in Paris for two years.

Finally, the Chief of the NBS Radioactivity Section and chairman of this session, W. B. Mann, was at Chalk River for three

years in the late 1940 's when, among other achievements, he

developed his first set of compensated internal gas counters

14 for absolute determinations of C to measure its half life.

W. B. Mann

Are there any questions for Mr. Taylor?

Anon

Such a good talk, there are no questions.

W. B. Mann

Thank you, Mr. Taylor. I'm honored to be an alumnus

of so distinguished a laboratory.

One of the very exciting laboratories I've visited in

recent years is the Boris Kidric Institute in Yugoslavia.

Dr. Bek-Uzarov has very kindly come at short notice to talk

about the work in the Radioactivity Group at the Boris Kidric

Institute. Dr. Bek-Uzarov.
Taylor, Baerg 77 References

[1] Astin, A. V., Bullard, E. C, and Lewis, W. B., Nature 177

12 (1956) and Science 123, 895 (1956).

[2] Baerg, A. P., Bowes, G. C, and Adams, R. J., Standard- ization of Radionuclides , p. 91, STI/PUB/139, IAEA,

Vienna (1967)

.

[3] Baerg, A. P., Metrologia 2, 23 (1966).

[4] Adams, R. J. and Baerg, APP . , Standardization of Radionuclides , p. 123, STI/PUB/139, IAEA, Vienna (1967).

[5] Taylor, J. G. V., Can. J. Phys . 40, 383 (1962).

[6] Merritt, Janet S., and Taylor, J. G. V., Anal. Chem. 3_7, 351 (1965).

[7] Merritt, Janet S., Taylor, J. G. V., and Boyd, A. W.

,

Nucl. Sci. Eng. 28, 286 (1967) and 3_4 , 195 (1968).

[8] Campion, P. J., Intern. J. Appl. Rad. Isotopes 4_, 232 (1959).

[9] Ujihira, Y. and Roy, J. C, Can. J. Chem. 4_6 , 1221 (1968).

Metrology of Radioactive Isotopes at the Boris Kidric

Institute of Nuclear Sciences

by

Dj.N. Bek-Uzarov

Vinca-Beograd, Yuogslavia

A survey of the activities of the department of

Radioactive Isotope Metrology of the Boris Kidric Institute

of Nuclear Sciences at Vinca is given. Some new problems

with which this Department is dealing in developing counters

used for absolute measurement of the number of decays are presented. Work on the estimation of air-equivalent materials for the production of air-equivalent cavity ionization

chambers is also surveyed.

The first problems of radioactive-isotope measurement

were encountered with the needs for neutron-flux measurements when the reactor of the Boris Kidric Institute and,

later, the Hot Laboratory were put into operation. In 1956,

a group was formed whose task was to develop methods for

absolute measurement of radioactive isotopes. Later, when

the Hot Laboratory began normal work it was necessary to extend the spectrum of radioisotopes to a large number of

commonly used ones which are daily produced and measured.

79
80 Metrology of Radionuclides in Yugoslavia

1 . Basic Activities of the Department of Isotope Metrology

A. The Department deals with concepts and

development of new methods of absolute and

relative measurement of radioactive isotopes.

B. It takes part in investigations of problems

of other Departments concerned with isotope

measurement

.

C. It produces radioactive standards for the

entire country.

D. It takes part in international intercomparisons

of radioactive isotopes organized by the Bureau

International des Poids et Mesures, thus

representing Yugoslavia in this field.

E. It produces and designs instruments for precise

measurement and analyses of radioactive isotopes.

F. It cooperates with research centres and

industrial laboratories in the country.

The Department is located in a separate building to protect the instruments from contamination by radioactive

isotopes and to provide satisfactory electric power supply,

gas (compressed air, noble gases) and water supply. The

gas installations, compressed air, methane and fuel gas, are independent of the installations of the whole Institute.

This supplies maximum safety in case the central installation

system of the Institute fails.
Bek-Uzarov HI The Department avails itself of the following basic

instruments for absolute measurement of radioactive isotopes;

beta-gamma coincidence equipment (with 4tt proportional counter and solid-angle method), two 4tt counters, apparatus for

internal gas counting and apparatus for measurement of exposure gamma radiation dose rate. The relative measurements

are made with: 2tt and 4tt proportional counters, a 2tt alpha

ionization chamber and a 4ir integral ionization chamber for

the measurement of the beta-gamma radioactive isotopes or bremsstrahlung radiation of pure beta emitters (Figures 1-7)

.

Figure 1. 4tt3-y coincidence apparatus. Two scintillation

counters with Nal (Tl) crystals, 100 mm in diameter

and 60 mm in height, are visible. To the right

side there is a door behind which a 4tt proportional counter is positioned.
82 Metrology of Radionuclides in Yugoslavia

Figure 3. Solid-angle-coincidence apparatus

.
Figure 5

.

4tt counter for sources of small diameter.
84 Metrology of Radionuclides in Yugoslavia

Figure 7. Arrangement for 4tt integral ionization chamber

for relative measurements.
Bek-Uzarov

1) Development methods of measurement

- Efficiency of the liquid-scintillation detector

The efficiency as a function of energy and

geometry, of the 4tt liquid-scintillation

counter, has been investigated.

The mean beta-particle energy absorbed in

the scintillation solution, required for the

production of one photoelectron on the photocathode is defined as the energy threshold

for detection. The energy threshold was determined from the difference between the

positions of the maximum tritium energy in a system of two electron-multiplier phototubes coupled in coincidence with a single

phototube

.

The dependence of the geometry factors was investigated with spherical glass flasks

filled with a liquid scintillator. These

flasks have different diameters and the

solution is of the same "specific activity

relative to the scintillator", i.e. of the

same number of disintegrations per gram of

scintillation solution 1/.

- Coincidence method of measurement.

In developing the coincidence method of

measurement it was concluded that two types

of counters were required: a real proportional and a nonproportional counter (which

is called semiproportional or H-counter)

(Figures 8 and 9) . Semiproportional counters
86 Metrology of Radionuclides in Yugoslavia

Figure 9. 4^ flow counter mounted in a coincidence

apparatus

.
Bek-Uzarov

are flow counters which are non-proportional

because a semiconductor diode is put in the

anode circuits instead of the operating

resistance. By employing the diode and a nonoverloading amplifier, one obtains a counter which is very much like a GM counter

except for its counting rate , which is many

times faster.

Figure 10 shows results of the comparison

of pulse-height-spectral distribution. In the upper part of the graph there are integral spectra obtained after pulse amplification so that their maxima do not exceed

80 V. It is apparent that the slope of the

integral spectrum for the H-counter is almost

a straight line. The magnitude of the slope

is compared using the conventional formula:

(N 9 - N ) 100

V I + Nl t% • V" 1

] (1)

2 <W

where denotes the counting rate at the

beginning of the integral curve (figure 10)

where the magnitude of the pulse amplitude is

Vj.; N

2 denotes the counting rate at the end

of the rectilinear part of the curve where

the pulse amplitude is .

Because of the different counting rates all curves were normalized. In the lower part of

the graph, besides the differential spectra,

forms of analyzed pulses with time parameters
88 Metrology of Radionuclides in Yugoslavia

Figure 10 . Differential and integral pulse-height spectra

taken with halogen, GM and H-counters

.

are shown. T . denotes the minimum rise time mm

which could be estimated and beside the differential spectrum, estimation of the pulse time

is given. For the first two counters the pulse

form at the output of the amplifier is shown.

The use of the limiter in the nonoverloading

amplifier prevented cutoff in any circuit.

To estimate the relative width of differential

spectra R

g

, the following expression has been

adopted

R

s = f r 100 [ % ]

,

p p

. (2)
Bek-Uzarov 8 9 where denotes the pulse amplitude in the

peak of the spectrum, while AV denotes the

peak width at half the height (in voltage) ; N

P denotes the normalized counting rate in the

peak and the counting rate at the beginning

of the integral curve

.

Figure 11 shows the characteristics of plateaus

(I, II, III) when resistors and diode are employed on the counter. By using a higher

amplification factor of the amplifier we obtain

a plateau at lower voltages which is shown by

curve IV. Curve V shows a plateau with an inversely polarized diode (in which case the

diode is used as the current limiter) . The

plateaus for pure alpha-particle counting are about 1500 V below beta (or alpha + beta)

Jm

s

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 H.V. [kV]

Figure 11. Different plateaus obtained by using resistors

and semiconductor diodes in anode circuit of

methane flow counter.
90 Metrology of Radionuclides in Yugoslavia

counting plateaus. The beta (or beta + alpha)

plateaus shown in figure 11 are obtained using

cobalt-60 sources.

All elements of the input circuit of the

counter are immersed in oil or epoxyde . The

diode is protected from too high an inverse

voltage by using small gaseous 60-V discharge

tubes denoted by T.

To measure electron-capturing nuclides, a proportional counter filled with a mixture

of argon (90%) and methane (10%) with constant

pressures, up to 10 atmospheres, is used

(figure 12). The sides of the counter facing

Figure 12. Proportional counter for measuring the number

of decays of electron-capture nuclei.
Bek-Uzarov 91 towards Nal(Tl) crystals can be replaced by

thinner plates if absorption of radiation is to be reduced when making coincidence measurements .

- Air equivalent ionization chamber for

measuring the exposure gamma radiation

dose rate. Data are available 2 , 3/ which assist in the

choice of materials for the construction of

air-equivalent cavity chambers. In our

case, too, a cavity-type ionization chamber

has been made of air-equivalent material

used for measuring gamma radiation in roentgen units. The air-equivalent material has

also been estimated on the assumption that

mass-absorption coefficients for the photoelectric, Compton effect and pair production

are equal to the product of two functions

the function depending on the energy of

gamma radiation and the function depending

on the atomic number of the element 4/.

If u f , y c and denotes the mass-absorption

coefficients for photoelectric, Compton and

pair-productio effects respectively, then

the following three relations can be written:

n

. . . (3) i=l
Metrology of Radionuclides in Yugoslavia

±

(y c )i = (y c )air, . . . (4)

. (y ) . = (y ) air

,

1

M p 1M p

' ... (5)

where the air-equivalent material contains

the i-th element whose percentage content

in the whole material is p^. This system

also involves an equation that defines the

percentage composition of each component of

the material of which the air-equivalent

mixture is made, i.e., the following

equation can be written:

Hence, the following approximation can be

made, i.e. relations (3), (4) and (5) are assumed to be:

= 1. ... (6)

= (Z 4.1/A) f (E ) ,

= (Z/A)C(E),

= (Z 2 /A)P(E)',
Bek-Uzarov 93 where Z and A denote the atomic and mass numbers of the element concerned, while

f (E) ,C (E) ,P (E) denote the functions of

energy of the above phenomena; for estimating the material, it is not necessary to know the form of functions

.

Using the above approximations and knowing

the percentage composition, by weight, of

air (76.5% nitrogen, 23.2% oxygen and 1.3%

argon) , the system of equations (3) , (4)

,

(5) and (6) and be reduced to the following

system:

a jiP i = b j' ( 3 = 1,2,3,4) ... (7)

where the coefficients have the following

values

Z 2

Z N

4 -1 z 4-l

z 4.1

1 ; b n = 0 . 755 -4 + 0.232 -?— + 0.013 -4— = 233.7 A. ' 1 A^^

(8)

z. z z z b

2 = 0 .755 j£ + 0.232 ^— + 0.013 ^— = 0.499

i N o A

(9)

z

2

z

2

z

2

z 2 b^ = 0.755 ^ + 0.232 ^— + 0.013 ^— = 3.65

l

J o AA

. (10)
Metrology of Radionuclides in Yugoslavia

1 ; b

4 = 1

The system of equations (8), (9), (10) and

(11) is not equivalent to the system of

equations (3), (4), (5) and (6). However,

the former represents the approximation to the latter, which depends on the use of the

approximation formulae for mass coefficients

of energy absorption.

Since the estimation has been made with

certain approximations 5/ it is necessary

to check the equivalence of the calculated

mixtures. By calculating the relation A which is defined as:

X - <"aW ( VA

(where AEM denotes the air-equivalent

material and A denotes air) for different

gamma-radiation energies, and by calculating the stopping powers of air and airequivalent material

sA

S = - for different electron energies,

AEM

we make a definite check of the mixture of

elements

.

Figures 13 and 14 give the dependence of A on gamma-radiation energies for the two

estimated mixtures. The figures also show

the dependence of A and S as a function of
Bek-Uzarov 9 5 the gamma-radiation energy and beta-particle

energy respectively. The estimation was made using the tables cited by Yoyet et al

2/ and Nelius 6/.

ELECTRON ENERGY [k*V]

) 200 400 600 800 1000 1200 1400 1600 1800

s - SA 1 Saem n

a S o I /

* o COMPOUND AEM I GRAPHITE -83.20V.

Al 203 -14.65

Al ~ 2-15

ai io GAMMA RADIATION ENERGY [m*v]

Figure 13. A and S-function for AEM I mixture.

2 ) Research work in cooperation with other

Departments

In cooperation with the Medical Department

and the Health Physics Department, our Department has constructed an apparatus for wholebody radioactivity measurement (whole-body

monitor) . The equipment was made of instruments and materials produced in our country

except for the crystal and steel used for

the cabin. The equipment is so constructed

that it can be used both for total-body
96 Metrology of Radionuclides in Yugoslavia

Figure 14. A and S-function for AEM II mixture.

measurement and, using the scanning system,

for detection of internal contamination.

The construction of the cabin and the equipment for position indication has interesting

original technical solutions and applications.

The production of the radioactivity standard

of the human body is one of the difficult

problems of radioisotope metrology.

The problem of measurement and indication

of atmospheric radioactivity in nuclear

power stations has also been solved in our Department by the construction of ionization chambers which measure the air

ejected from the reactor.
Bek-Uzarov

3 ) Production of radioactive reference

97

sources

As our Department has the best standard in our country it naturally undertakes the

production of radioactive standards for

all customers.

Solid standards ready for measurement are put in water-proof cases in which a drying

agent is put to keep the series of sources

always dry and ready for measurement with

necessary reproducibility.

Figure 15. Case for preserving dry solid sources.
Metrology of Radionuclides in Yugoslavia

Comparisons of the radioactive sources of consumers in our country have been made

twice. Our Department distributes the

radioactive solutions among the interested

laboratories and then collects their results

of measurements. In our country we repeated

what the Bureau International in Sevres had

done on an international scale.

4 ) Participation in intercomparisons of

radioactive solutions

Our Department has participated in

intercomparisons ever since 1958/19 59 when

first organized by the International Atomic

Energy Agency and later by the Bureau International des Poids et Measures. It need not

be pointed out what the usefulness of this

work is. However, the burden of the work

predicted in the field of intercomparison is ever growing. On the one hand this is positive, but on the participation of small laboratories in the measurements in which many

people and instruments and much involvement

and time are required, is more and more difficult.

5) Applied instruments for precise

measurements

The results of almost every intercomparison

are many new technical improvements , new constructions, some new solutions, which are used in the construction of small systems or individual instruments for different consumers

in the country (Figure 16).
Bek-Uzarov 9 9

Figure 16. Some proportional and scintillation counters

of home design and construction.

It very often happens that the instruments

constructed and used in our Department are given to the consumers (hospitals or industrial laboratories) and then our Department

designs for its own use, new, better and more perfect equipment. This is satisfactory

both for our Department and the consumers

the consumers get the most up-to-date instruments and our Department gets the best ones. Our Department designs scintillation detectors,

proportional counters and ionization chambers

(for alpha-and gamma-ray measurement) . Naturally, the number of these instruments is not
100 Metrology of Radionuclides in Yugoslavia

large , only a few of them are made per year.

This activity is considered fairly useful

because the technical staff is always in a constant working condition.

6 ) Cooperation with research centres

As the problems of metrology are often

investigated in different laboratories,

our Department most often appears as the

place where both foreign and home students

get preliminary training. A number of

experts presently working in our Institute,

or in some other institutions, have passed

through this preliminary phase of training in our laboratories for six months up

to one year.

There are also some regular exercises

included in the programme of the University

of Beograd which are carried out in our Institute

.

Cooperation with Faculties is of mutual

benefit. Our laboratories get young personnel while the Faculties are provided

with practical work of highest quality in our country.

Physicists and engineers trained to work

in the Health Physics or Medical Department,

as a rule, first pass through our Laboratory.
Bek-Uzarov 101

There have been several projects concerned

with the application of radioisotopes in

industry in which our Department took part.

Unfortunately we must point out that in this

field the possibilities have not been exhausted and, so far, we have not achieved all

the expected results.

References

[1] Vienna (1966). Standardization of Radionuclides. IAEA

Proceedings, Vienna, 10-14 October, 1966.

[2] Yoyet, G. , Hugentoblerant , E. , Mysyriwics, A., Helv.

Phys. Acta, 34, 414 (1961).

[3] Grodstein, Gladys White, NBS , Circ. 583 (1959).

[4] Vacetovic, P., Diploma work. Internal documentation

of Boris Kidric Institute, Vinca, 1961 (in Serbocroatian)

.

[5] Novkovic, D. , Master of Sciences thesis. Internal

documentation of Boris Kidric Institute, Vinca, 1961

(in Serbocroatian)

.

[6] Nelms, A. T. , NBS, Circ. 577 (1956).
102 Metrology of Radionuclides in Yugoslavia

W. B. Mann

Are there any questions for Dr. Bek-Uzarov?

S. B. Garfinkel

You mentioned that you were able to reduce the dead time

of a proportional counter by a factor of 20?

Dj . N. Bek-Uzarov

Yes, up to 20.

S. B. Garfinkel

How were you able to do this?

Dj . N. Bek-Uzarov

But not just the dead time. We have replaced the

proportional-counter load resistor with a semi-conductor

diode. As this has a non-linear voltage-current characteristic, the larger counter pulses are reduced in amplitude,

so that there are many fewer pulses large enough to saturate

the amplifier.

W. B. Mann

Any more questions? M. LeGallic:

Y. LeGallic

When you use your automatic sample changer what's the

contribution of the sources in storage to the background?
Bek-Uzarov 103

Dj . N. Bek-Uzarov

In the case of cobalt-60 we measure only 2 sources but

in other cases the contribution may be 5 to 10%.

W . B . Mann

Thank you very much, Dr. Bek-Uzarov. Finally in this

program it is a great pleasure to introduce my friend and

colleague Mr. Garfinkel to talk about our work at the

National Bureau of Standards.

Recent Activities of the NBS Radioactivity Section with

Special Reference to the Standardization of Thallium-20

8

by

S. B. Garfinkel and J. M. R. Hutchinson

National Bureau of Standards

Washington, D.C. , U.S.A.

The Radioactivity Section of NBS has the mission of

developing, preparing and maintaining radioactivity standards

for the United States. We also distribute a majority of

these standards and provide a calibration service covering

almost all the radionuclides with which we have worked, particularly those we no longer distribute. This calibration

service is available on a cost basis to individuals, private

companies, government agencies, and research laboratories, in

the United States. Some of the standards have been dropped

from distribution either due to lack of demand or because

they are available from commercial distributors who have

demonstrated their willingness and competence to maintain

their calibration facilities in good enough order to meet the

requirements and demands of the public. Table 1 is a list of

the radionuclides which we are distributing. Some of the

radionuclides which are marked by an asterisk are not always

available, either due to the short half-life or lack of customer demand. A great many techniques are used in arriving at standardized sources of these various nuclides, such as 4tt 3 / 2i\a, and lira counting; 4tt$-Y/ 2tt|3-Y/ 4-rrx-ray-Y/ Y~Yf an^ x-ray

105
106 Recent Activities at NBS

Table I

Radioactivity Standards GAS STANDARD: RADIUM SOLUTIONS: Polonium-210

Americium-2lil

, y, EC SOLUTION SOURCES: Hydrogen-3

(water, 2 intensities;

toluene

)

Carbon-llj (water, benzoic acid ir toluene, hexadecane) Sodium-22 Chlorine-36

Iron-55 #Niobium-95 Tin-113-Indium-113m —

*Iodine-125

-x-Cerium-llO. Cerium-Praseodymium-lijlj Promethium-llj7 *Mercury-197

*Mercury-203

Krypton-85 Y-RAY POINT SOURCES (Energy in MeV) : Sodium-22 (0.511, 1.27k) Manganese-5k (0.835) Cobalt-60 (1.173, 1.332) Zinc-65 (l.llit) *Yttrium-88 (I.836)

Niobium-9lj (0.702,0.871) *Niobium-95 (0.765) Cadmium-109 (0.088) Cesium-137 (0.662) Cerium-139 (0.166) *Mercury-203 (0.279) Thorium-228-Thallium-208 (2.615)

Contemporary Standard

for Carbon-lii Dating

Laboratories: Oxalic Acid

For Radon Analysis: 10" gram

10~9 gram

10 gram Blank

For Y-ray Measurements:

10 intensities from 0.1 to 100 u-g UNDER DEVELOPMENT : Nickel-63 p-ray solution standard

Krypton-85 Y~ray gas standard

Plutonium-238 a-source DISCONTINUED STANDARDS

Scandium-^6

Strontium-85

Strontium- 89

*Not always available, due to combination of half-life and customer demand.

- x-ray coincidence counting; compensated internal gas counting; micro-calorimetry ; and total-absorption x-ray counting.

In addition we use the "4tty" ionization chamber and 3" x 3"-

sodium-iodide crystals for relative calibrations, these detectors having been calibrated in terms of other radionuclides

which had been standardized by direct counting techniques. We use the "4tty" chamber and sodium-iodide crystals to calibrate

such radionuclides as tin 113 - indium 113m or cesium-137.

These nuclides, in particular, are not amenable to coincidence

counting as a means of determining the gamma-ray emission rates

of a given source.
Garfinkel, Hutchinson 107

One of the standards, which is currently under

development, is nickel-6 3. We have measured two samples of

nickel-63 in our microcalorimeter . To give you an idea

of what is involved here: the two nickel-63 samples have an activity of ^ 1-1/3 curies each, which corresponds to a heat

output of 133 microwatts. We have not yet completed

calculations on the total activity.

In the case of krypton-85, we have in the past issued

only 3-ray gas standards. These consist of sealed ampoules

containing ^ 1 microcurie of krypton-85. In order to use these,

it is necessary to transfer the gas from the ampoule into the

measuring system and to determine the quantity of the gas and

its activity by means of 3-particle counting or ionizationcurrent measurements. These samples are calibrated in units

of activity per mole of krypton. Because of the low abundance

0.4%) of the y ray in krypton-85, these 3-particle gas

standards do not permit of accurate y-ray counting. We have

prepared ^ 100 ampoules of krypton-85 y-ray standards which

are about three orders of magnitude stronger in activity, 1 millicurie each) . These will be individually calibrated as to total activity, and will be able to be used without opening

the ampoule. This sort of sample is of interest to people who

work with very strong krypton sources such as in fuel-burn-up

studies

.

A recently acquired facility of the Radioactivity

Section and the National Bureau of Standards is the isotope

separator. Although not obtained only for use in radioactivity standards, we hope to exploit the capabilities of this

instrument. One case in point is the production of niobium-9 4 y-ray standards. This is a very useful radionuclide having a half-life of some 20,000 years, and a decay scheme very similar

to that of cobalt-60 except for the 3- and y-ray energies.

Niobium-94 is produced by irradiation of stable niobium-93 in
108 Recent Activities at NBS

a reactor and this, of course, requires a long irradiation, of

14 -1 the order of 2 years. With a neutron flux of about 10 s -2 cm , one ends up with activities of the order of a millicurie

per gram of niobium but, in addition to niobium-94, one obtains

niobium-93m and niobium-95. As the latter has a 35-day halflife, one can wait a year or two for it to decay, but the longerlived niobium-93m, having a decay energy of 30 kilovolts, would

still preclude the use of 4tt3-y coincidence counting of the

niobium-9 4. In the past we have counted the niobium-9 4 by y~Y

coincidence counting. This can be done quite easily and allows

one to discriminate against the niobium-9 3m. However, we hope

to obtain some more niobium-9 4 and niobium-9 3m material and

separate these in the isotope separator (provided that the yields

are sufficient) and thus get two useful materials out of this:

first of all, enough carrier-free niobium-94 for a laboratory

working standard which we could calibrate by an independent

method, (4tt3-y coincidence counting) , and also some niobium-9 3m

which, depending on the measurement, may possibly provide a rather nice, low-energy standard of ^ 10-year half-life. (This

half-life is not well known.)

A yray standard which has been developed recently and

which has been in demand for some time is that of 1.9-year

thorium-228. The y ray of interest, 2.614-MeV, arises in the

decay of the last radioactive member of the decay chain (3.1-

minute thallium-20 8 , Figure 1). Since an intermediate member

of the chain is a gas, radon-220, it is important for the standard sources to be made in such a manner that the radon is con- tained within the source. The standards, for distribution, were in the form of thorium-228, sandwiched between two layers

of 0 .00 19 -cm-thick gold foil, which were in turn sandwiched

between double layers of 0 . 0 36-cm- thick polyurethane film tape.

Tests showed that this configuration allowed no radon to escape.

Gold foil was used to protect the plastic tape from damage

which might arise as a result of a-particle irradiation. These
Garfinkel, Hutchinson 109

Radionuclide Half -life

a-particle

energies

MeV

7 -ray

energies

MeV

Thorium-228

(Radiothorium)

1

1-91 y -5-2 - 1%

5-338 — 28%

5-421 — 71 %

0-084 — -2%

others — very weak

Radium-224

(Thorium X)

3-6 d 5.445 _ 4.9%

5-681 — 95%

0-241 — 3-7%

others —very weak ^

Radon-220

(Thorpn) 54 s 6-28 100%

^_ —

Polonium-216

(Thorium A)

1 0.0h% 199- 96$

0-158 s 6-775 100% —

Astaitine-216 1 3 X 10-* s 7.79 — 0 04%

*

Lead- (Th

f- \

212 orium B)

i

10-6 h 0-12 — 0 y0 0-24 — —36%

0-30 — —3%

Bisnaiuth-2i: "horium

1 Q

60-5 m , 6-04 — 25%

6-08 — 10%

others — 1 %

0-04 — ~0%

others — very weak

« Zg J most

1^08 r abU

fnv

nt

1- 62 J ~10 ^

_^

6k$>

0

>nium-212

'horium C)

<

3 X 10 7 s 8-78 —~64 %

others of

higher energy—very weak

>

Thallii (The

C \

im-2( rium

r

)8<n3-1 m 0-28 — —3%

0-51 — ~8%

0-58 — 31%

0-86 — 4%

Lea0

d-208

fhoriurr Stable

Figure 1. Thorium - (4n) s eries starting at thorium-22 8.
110 Recent Activities at NBS

standards were calibrated by comparing their y-ray emission

rates, in a reproducible geometry, with that of an NBS working

standard of thorium-22 8 which was prepared by plating some thorium-228 onto a platinum disc, and then flaming it to red

heat. This provides an ideal source for absolute a-particle

counting, upon which the calibration of the working standard

was based.

Since each decay of thorium-228 leads to precisely 5 a

(2) particles and the 2.614 MeV y-ray abundance is known the

equilibrium y~ray emission rate can be determined.

In order to reduce radon escape during calibration of the

working standard, it was covered with thin mylar films which

resulted in ct-particle-energy absorption in the films and limited the amount of film that could be used. For example, when

60-um-thick mylar was used as a covering layer on a polonium210 source, the energy degradation and a-particle scattering

caused losses of the order of 1% in the counting rate, which

was unacceptable. Counting rates, with 2 covering layers of

4-um- thick mylar over a polonium-210 source, were checked in

the Ittoc chamber and there was no loss in count rate due to the

mylar within +0.2%. We used 2 or 3 layers of this 4-ym mylar

in order to minimize the probability of a direct escape path

for radon atoms from the source. The source was covered,

placed in a vacuum chamber, and allowed to come to equilibrium,

which was checked by monitoring the count rate of the 8.78-MeV

polonium-212 a particles, using a surface-barrier silicon

detector (Figures 2 and 3) . From observation of a background

with a 10-hour half-life immediately after a source had been

withdrawn from the chamber, we concluded that some radon atoms were still escaping from the source.
Garfinkel, Hutchinson 111

ZERO TIME 100 HOURS AFTER COVERING

5 6 7 8 9 5 6 ENERGY, MeV

Figure 2. a-particle spectra of a thorium-228 source immediately after covering the source with thin

plastic and 100 hours after (see text) . The

highest energy peak, 8.78 MeV, has increased

significantly over the initial value indicating

accumulation of progeny of radon-2 20 behind the

mylar covering layer.

The problem, then, was to determine how much radon was

escaping. We counted the source for a-particle activity, then

took it out of the counter, placed some very thick plastic tape over the source and within, say, five minutes, started counting

the source for yray count rate above 84 keV with a Nal(Tl)

detector. Since some radon had been escaping from the source,

the daughters following radon were not in equilibrium with the

thorium. We followed the y-ray count rate of the heavily-taped
112 Recent Activities at NBS

00 0 10 20 30 40 50 60 70 80 90 100 TIME, HOURS

Figure 3. 8.7 8-MeV a-particle counting rate for a thorium- 22

8

source, as a function of time, using excessivelyporous mylar covering layer (not included among

points shown in Figure 5)

.

source over a period of time, and found the initial rate increased and finally came to equilibrium after 3 or 4 days (Figure

4), this period having been determined by 10.6-hour lead-212.

We repeated this experiment several times. Each time, after

measuring the a-particle and y-ray count rates, we removed all

the heavy and thin tape, recovered the source with new thin

tape, and after a new "equilibrium" a-particle count rate was obtained, remeasured the source in the same geometry counter

as before, following which, the source was re-encapsulated with

the heavy tape and y-ray counted again.
Garfinkel, Hutchinson 113

TIME AT WHICH SOURCE WAS

COVERED WITH THICK TAPE

3

O 34|—

2 3 4

ELAPSED TIME

5 DAYS

Figure 4. y~ ray count rate for a thorium-228 source as function of time after thick plastic encapsulation.

Figure 5 (curve B) shows the result of 3 sets of counts.

The ordinate of figure 5 is the thorium-22 8 source count rate

with only the thin mylar-film coverings. As the porosity of

the thin mylar film is a random property, there were different

amounts of radon escaping during the three experiments. The

abscissa, labelled Ay, is the difference (in percent) between

the initial y

- ray count rate immediately after encapsulating

the source and the final equilibrium count rate. Extrapolating

to Ay=0 (the condition for zero radon loss) gives a value,

which, when normalized by the vacuum-chamber-surface barrierdetector geometry factor, gives the a-particle activity for
114 Recent Activities at NBS

0 2 4 6 8 10 12 Ay, PERCENT

Figure 5. a-particle counting rate of thorium-228 source under three different conditions of radon escape

(curve B) as a function of Ay (defined in text)

.

Escape is least for condition 1, greatest for 11. Inset shows curve A for Ay from 0 to 100%.

thorium-228 in equilibrium with its daughters. This number

then is divided by 5 to give the activity of bismuth-212, and

multiplied by 0.3600+0.000 3,

(2) the number of 2.614-MeV y rays per decay of bismuth-212.

A few explanatory words are in order with reference to Figure 5. Of the five a-particle decays per decay of thorium228 (cf Figure 1), two occur before radon 220, and, 95% of the

y rays above 2 40 keV occur in the progeny of radon-220. Note,
Garfinkel, Hutchinson 115

that if all the radon escapes the source, (condition at point

a) the source would still emit 2 of the 5 a particles, but

essentially none of the y rays. The curve actually obtained

(B) has a slope about half that of A, due to detection of a particles from some of the decaying radon atoms (and/or their

daughters) which had escaped the source. Calculations show

that this detection efficiency amounted to about 0.75 percent

of these "external" decays, and the fact that the experimental

values fall on a straight line show that this efficiency was constant for the three source conditions.

One other interesting standard developed this year which

is now available is that of cadmium-109. Cadmium-109 decays by

electron capture. It has an 88-keV y ray

^

of approximately

4% abundance. One might calibrate this radionuclide for total

activity by conversion-electron-x-ray coincidence counting, but

we were interested only in the y~ ray emission rate. In Table

1, our y-ray point sources are listed in the center column,

and as we are trying to cover as large a range of energies as possible, and inasmuch as the y-ray abundance of cadmium-10 9 is not known very accurately, an activity calibration would

not have been as valuable for our purposes, as the y~ray emission rate. The calibration was accomplished with the use of a 47Ty-sodium iodide well crystal. Corrections had to be determined for some eight effects, and this work will be described

in a forthcoming publication.

Recognizing the importance of impurities in standards

and the troubles that can arise from their presence, we have

been looking with improved capability at the impurity levels

since we acquired several lithium-drifted germanium detectors. Whereas in previous years we could easily have had

small amounts of y-^ay impurities which could not be detected

by conventional sodium-iodide y-ray spectrometry, their presence can now be very easily seen with a germanium detector
116 Recent Activities at NBS

and we go to considerable pains and effort to remove the

impurities

.

The point that was brought up a few minutes ago by

Dr. McNair is a very important one, and it emphasizes the

fact that the user of a radioactivity standard should be

well aware of not only the purpose for which he wants it

but what actually is in the standard. To this end, we not

only try to determine the level of impurity, but we include

this information in our certificates and also indicate the

method used to determine such levels.

Another interesting experiment which was not directly

related to a standardization but which illustrates an elegant

technique for nuclear-instrument calibration, was associated

with the measurement of the half-life of the 74-keV state of

(4

)

neptunium-239 arising in the decay of americium-24 3 . In order to measure very short half-lives (this one turned out to be 1.38+0.03 nanoseconds), a time-to-pulse-height converter is

customarily used in conjunction with a pulse-height analyzer,

the latter being calibrated in terms of time-per-channel . Conventionally, this calibration is accomplished by insertion, into

the electronics system, of different lengths of coaxial cable

of known time delay. This method is frequently subject to systematic errors arising from subtle effects due to types of

connectors and fittings used with the cables. In this experiment, the calibration was accomplished without disturbing any

of the cables in the instrumentation. A universal standard

available to everyone - at no cost -, the speed of light, was

used. By increasing the distance between a pulsed light source

and a phototube, and measuring the corresponding times of

flight, the system was readily and accurately calibrated.
Garfinkel, Hutchinson 117

References

[1] NBS Circular 594, June 11, 1958.

[2] Walker, J., and Salyer, T. , The branching • ratio of

212 Bi (ThC). Proc. Phys . Soc . , 86_, 423, (1965).

[3] Hutchinson, J. M. R. , Mann, W. B., Smith, C. N .

,

and Whittaker, J. K. , The half lives of two excited

239 states in Np. Int. Jour, of Appl. Rad. and Isotopes,

2Q_, 493, (1969) .

[4] Schima, F. J., and Hutchinson, J. M. R. , Energy of

10 9 the Isomeric transition in Ag, Nuclear Physics

A, 102, 667, (1967) .

A. Spernol

Would Mr. Garfinkel be so kind and inform us if any of

your niobium-9 3m is available, as we are very much interested

in pure niobium-9 3m.

S. B. Garfinkel

We hope to do it. We haven't done it yet. If you want

some niobium-9 3m now, we have some, but it's in the presence

of niobium-94!
118 Recent Activities at NBS

W. B. Mann

Thank you very much, Mr. Garfinkel.

May I make one more comment perhaps on Dr. Rytz's

previous statement, and Dr. Caswell's expressing suprise that

there will be no more intercomparisons . I forgot to mention

at that point that we were asked with NRC Ottawa, Dr. Baerg,

to look into the question of trying to do tracer intercomparisons of carbon-14 using possibly niobium-95 oxalate which has

an end-point energy of the beta spectrum about the same as carbon-14. We were asked to do this at the November meeting

in Sevres last year. I suppose you consider this to be still

on the program, Dr. Rytz?

A. Rytz

Yes . W. B. Mann

It may be difficult, Dr. Rodriguez-Pasques , Miss Cavallo,

Dr. Landgrebe, and Mr. Garfinkel are all looking into this,

and I am too, with Dr. Baerg. So this is still on the program apparently. But it is so difficult that I'm not quite

sure that we are going to find a solution in the near future.

I think most carbon-14 standards are unfortunately compared

with the National Bureau of Standards' standards and this is

rather an invidious position and we'd rather have an international comparison on this as soon as possible. I hope that

the participants this afternoon will not mind perhaps letting

us have the manuscripts so we can make some effort to publish

these proceedings. I think it would be very interesting. I must say it has been a great privilege to have so many distinguished people here today and talking. It has been a great

pleasure and I wish to thank you all.


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