- Email: [email protected]
The work of the BIPM consultative committee for measurement standards of ionizing radiation section II
NUCLEAR
INSTRUMENTS
AND
METHODS
II2
(i973) 4r-45;
©
NORTH-HOLLAND
PUBLISHING
CO.
T H E WORK OF THE BIPM C O N S U L T A T I V E C O M M I T T E E FOR M E A S U R E M E N T S T A N D A R D S OF I O N I Z I N G R A D I A T I O N S S E C T I O N I I Radionuclide measurements* P. J. C A M P I O N
Section II, CCEMRI, Pavilion de Breteuil, 92310 Sdvres, France A brief account of the activities, both past and present, of this Committee is presented. The aim is the improvement of the accuracy and uniformity of radioactivity measurements. To date the Committee has organized 11 international comparisons and the results of some of these are examined. While these have revealed some systematic errors in measurement techniques, the results of the more recent comparisons have been disappointing in that the knowledge gained has been small in relation to the effort involved. In view o f this a different approach has been adopted. The Committee has set up a n u m b e r of small working groups
to study various aspects of radionuclide metrology where discrepancies may perhaps occur. It is hoped that, at least for a n u m b e r o f such studies, a m o n o g r a p h will be published highlighting the measurement difficulties. Until such m o n o g r a p h s are available there is, in the Committee's view, little point in carrying out further international comparisons. However, in order to fill a need created by this temporary moratorium, the Bureau International des Poids et Mesures has offered to both provide calibrated solid sources and train staff in radionuclide metrology.
The Bureau International des Poids et Mesures (BIPM) was created in 1875 under the Convention du M~tre for the purpose of making and processing standards of mass and length. The Bureau is located at S6vres near Paris and, since 1875 has gradually extended its scope to include thermometry, electricity, photometry and, even more recently, ionizing radiations. The work of the Bureau and its relationship with that of the national standardizing laboratories is surveyed by the Comit6 International des Poids et Mesures. Since this committee is relatively small it appoints a number of Consultative Committees to advise it on individual subjects. In the case of the Comit6 Consultatif pour les Etalons de Mesure des Rayonnements lonisants (CCEMRI), which was created in 1958, the subject is so broad that it has been found convenient to divide the Committee into four Sections. Section II deals with radionuclide measurements and it is the work of this Section which forms the subject of this paper. The aim of this Section is to improve the accuracy and uniformity of measurements of activity. This includes not only improvements in the techniques of measurement, but also the determination of those constants which are necessary both for disintegration rate measurements and their useful application. It is of course impossible to determine accuracy in any absolute sense, but the spread of results of an inter-
comparison of measurements carried out by competent laboratories is the traditional yardstick by which accuracy can be gauged. However, it must be emphasized that the results of such intercomparisons, even when the spread of results is consistent with the individual uncertainties, only provide an estimate of the absolute accuracy of that particular measurement. Nevertheless it is by making comparisons in this way that it is possible to uncover the presence of systematic errors which might otherwise go unsuspected and this is the prime reason for undertaking international comparisons at the highest level. In fulfilling its mission to improve radionuclide measurements the Consultative Committee arranges such comparisons when it is felt that they would produce useful results. Table 1 lists chronologically the intercomparisons that have been made during the last decade or so, the results for each, being fully documented by the BIPM, are available on request. In a short survey of this nature it is inappropriate to present a full statistical analysis of all these results but, in order to demonstrate the type of information that can be obtained, it is instructive to pick out from amongst these II intercomparisons the three that involved 6°Co. Only those results obtained by coincidence methods have been included in this limited survey and these are presented in fig. l as histograms of the deviations from the arithmetic mean. The number of results included in these comparisons varied from 16 to 24, but the areas under the histograms have been normalized in order to make visual comparisons easier. The figure at the top right of each histogram is the standard deviation
* Persons interested in more detailed information on the activity of the B I P M in this field may obtain a comprehensive list of publications and internal reports directly f r o m the Bureau International des Poids et Mesures, Pavilion de Breteuil, 92310 S6vres, France.
41 I. G E N E R A L
REVIEW
PAPERS
42
P. J. CAMPION
I
1962
|
7'1%,
,,.t 1963 Pycnometer
1963
1967
m t
/,.2 %.
,,
1967~ L ~ 31 . */,
3.~ */,o
Extrapo{ation
Strong solution
2"6*/**
1967
~~L
Solid sources
28 %.
ratio C ° n c e n t r a t i ° A h ~
I0 8 6 4 2 O 2 4 6 8 10 ~--
10 8 6 4 2 0 2 4 6 8 Deviation,'/,,-
10
Fig. 1. Results o f intercomparisons o f 6°Co obtained by coincidence m e t h o d s expressed as deviations in parts per thousand from the arithmetic mean. The figure at the top right o f each histogram is the standard deviation for that c o m p a r i s o n which is identified b y year (top left) and any further characterization as necessary. The cross-hatched areas in the 1963 solution c o m p a r i s o n represent results obtained by the extrapolation technique while the solid areas are those obtained by the pycnometer technique. N o distinction is m a d e in the results for the other comparisons. The histogram for the solid source c o m p a r i s o n includes only those sources in the range 2400 to 4600 s - l ; the c o m p a r i s o n was made by normalizing to reference-laboratory results.
TABLE 1
International intercomparisons o f radionuclides. Nuclide
Date
Ref.
32p
1961 1961 1962 1962 1962 1962 1963 1963 1964 1965 1967
1 2 3 4 5 6 7 8 9, l 0 11, 12 13
131I
198Au 6OCo 2°4T1 z5S 6°Co 241Am 9°Sr-9°Y
54Mn COCo
expressed as parts per thousand. For the 1962 c o m parison, results lying outside _+ 1.5% of the mean have been excluded from the histogram and the calculation o f the standard deviation, whilst for all other compari-
sons, results outside + 1.0% have been excluded. Such limits are necessary since although an extreme result m a y have relatively little effect on the mean it has a considerable influence on the standard deviation. One or two results per comparison were rejected as a result of these limits. The 1962 comparison was a straightforward c o m parison of 60 Co in solution4); that is, laboratories were required to determine the activity per unit mass o f solution. The results are fairly typical of that era. The 1963 comparison was a rather special one7). The C o m mittee intuitively felt that all was not well with the diluting and dispensing techniques and hence arrangements were m a d e to distribute a radioactive solution together with a series of solid sources mounted on thin supports suitable for 4n/3- 7 coincidence counting. These solid sources were counted by reference laboratories before dispatch to participating laboratories and m o s t sources were recounted by one of the reference laboratories on their return in order to check that active material had not been lost from the mount in
THE
WORK
OF
transit. The Committee also went to some considerable effort to prepare a detailed form for reporting the results. This proved to be most worthwhile since the form was instrumental in bringing to light a difference between the results of those laboratories which dispensed sources by the pycnometer technique and those that used the extrapolation method. For this particular comparison (1963) the results of the laboratories using the former techniques are distinguished in fig. 1 by solid areas in the histogram, while those from laboratories using the extrapolation technique are shown as cross-hatched areas. The means of the two groups are shown by the vertical arrows, and even a visual inspection suggests that the difference may be significant. This result led to investigations in several laboratories concerning the inherent errors in both dispensing techniques and it was established that, under certain circumstances, there could be a considerable error involved in the extrapolation technique~4-1v); this error was of the right sign and order of magnitude to account for the difference shown in fig. I. This is an excellent example of an unsuspected systematic error being revealed by a carefully documented intercomparison. The second point that emerged from the 1963 comparison was that, in general, laboratories could get considerably better agreement on solid sources than on solutions at least for medium counting rates. Not all of this difference can be ascribed to the pycnometerextrapolation variation since even if only the results obtained using the pycnometer technique are considered the dispersion of the solution results is greater than that of the solid sources. It was therefore reasonable to conclude that there was some other factor associated with diluting and/or dispensing which contributed to the dispersion of the results. A third comparison was organized in 1967. This differed from the previous ones in that two solutions of 6°Co were distributed, a weak solution and a solution some twenty times stronger in radioactive concentration13). The ratio of the concentrations of these two solutions was known by gravimetric measurements to an accuracy considerably greater than that possible by activity measurements. Laboratories were asked to measure the radioactive concentration of the two solutions and, from these measurements, to derive the ratio of the concentrations. The reasoning behind this exercise was that, for any one laboratory, systematic errors in dispensing and counting sources should tend to cancel out in the ratio and hence result in a better agreement for the latter than for the individual concentration measurements. The results can be seen
THE
CCEMRI
43
II
in fig. 1. The dispersions of the individual concentration measurements for the weak and strong solutions were each found to be considerably less than that for the 1962 comparison but were no better than that obtained in 1963. On the other hand, the dispersion of the ratio results was significantly smaller. It was concluded therefore that, despite the increased interest in dispensing sources which took place over the four year interval between comparisons, the techniques had not been improved materially. One can attempt to identify the cause of this by comparing the observed standard deviation of 2.8%o with that estimated from the uncertainties quoted by each participating laboratory. Since for a given laboratory systematic uncertainties should, in principle at least, cancel out in the ratio results one need only consider the statistical uncertainties; estimates of these, derived from the scatter of individual source results, have been listed for the participating laboratories(~3). Using these data, one obtains an estimate of the standard deviation of 0.23 %0. An F test indicates that this value is significantly different from the observed value of 2.8%o and clearly some factor other than those due to counting and weighing statistics must be influencing the observed result. Of course it is possible that some of the systematic uncertainties do not entirely cancel out in the ratio. For example, an uncertainty in the dead time will give rise to slightly different errors in the numerator and denominator of the ratio if the counting rates are different; however, it is felt that such effects would be unlikely to account for such a large discrepancy. On the other hand, the fact that the strong solution required dilution whereas no such manipulation was necessary for the weak solution suggests that there may be a significant uncertainty due to this operation. Since the results of the more recent comparisons show no improvement over those of the earlier ones, together with the fact that the organization of such comparisons requires a very great deal of work, the Committee felt that the time had come to adopt a fresh approach to the general problem of improving accuracy on a worldwide basis. Thus the Committee has recently decided to set up a number of small working parties, the task of each of which is to examine in depth a particular problem associated with radionuclide metrology. By way of illustration the following is a list of some, but by no means all, of the topics at present under review: (1) Principles of the coincidence method. (2) Problems in microweighing. I.
GENERAL
REVIEW
PAPERS
44
P. J. C A M P I O N
(3) A study o f reference techniques for the assay o f radionuclides. (4) The detection a n d e s t i m a t i o n o f spurious pulses. (5) F e a s i b i l i t y o f liquid scintillation counting for precision metrology. (6) D i s i n t e g r a t i o n rate m e a s u r e m e n t s involving nuclides with c o m p l e x decay schemes. In some cases e x p e r i m e n t a l investigations involving the j o i n t c o o p e r a t i o n o f two or three l a b o r a t o r i e s are in progress in o r d e r to d e t e r m i n e the limiting accuracy o f various techniques a n d m a n i p u l a t i o n s . In others, it is a m a t t e r o f r e c o r d i n g in a convenient f r o m the " k n o w - h o w " gained b y experience in m a n y different l a b o r a t o r i e s t h r o u g h o u t the world. The objective o f m o s t o f these projects is to p r o d u c e a series o f m v n o g r a p h s which will serve as a guide to g o o d l a b o r a t o r y practice in various aspects o f r a d i o n u c l i d e metrology. U n t i l these are available a n d have been assimilated by l a b o r a t o r i e s it is clear t h a t there is little p o i n t in c a r r y i n g o u t further i n t e r c o m p a r i s o n s . The C o m m i t t e e is fully aware that this t e m p o r a r y m o r a t o r i u m on c o m p a r i s o n s might affect some l a b o r a t o r i e s which find such m e a s u r e m e n t s a convenient way to i m p r o v e their technique a n d test their equipment. H o w e v e r , in o r d e r to fill this gap, the B I P M has offered to send calibrated r a d i o n u c l i d e s as solid sources to a n y n a t i o n a l l a b o r a t o r y for comp a r i s o n p u r p o s e s a n d indeed this is likely to be a c o n t i n u i n g activity o f the Bureau. In a d d i t i o n the B I P M has offered to a r r a n g e for staff from such l a b o r a t o r i e s to stay at S6vres or elsewhere for training a n d experience in r a d i o n u c l i d e metrology. The C o m mittee m e m b e r s are p a r t i c u l a r l y grateful to the B I P M for these services a n d wish to express their t h a n k s to the staff o f the Bureau. In c o n c l u d i n g therefore this review o f the last decade o f r a d i o n u c l i d e m e t r o l o g y the following points emerge. First, there has been a slow b u t steady i m p r o v e m e n t in the a g r e e m e n t o f i n t e r c o m p a r i s o n results, at least until the m i d 1960's. Second, it is possible to learn m u c h m o r e f r o m an i n t e r c o m p a r i s o n when there is a specific scientific question to be answered. Thus the 1962 6°Co c o m p a r i s o n yielded little i n f o r m a t i o n other than the s p r e a d o f results. In the 1963 c o m p a r i s o n the question was posed: C a n l a b o r a t o r i e s m e a s u r e solid sources m o r e accurately t h a n s o l u t i o n s ? In 1967 the question was: Is there any residual e r r o r in diluting sources ? So far as the latter question is concerned the results w o u l d seem to suggest that there m a y well be, b u t this has yet to be confirmed. The third p o i n t is that the value o f a c o m p a r i s o n is e n h a n c e d m a n y times if a certain a m o u n t o f p r e p a r a t o r y w o r k is carried o u t first. Such w o r k m a y
include a trial c o m p a r i s o n a m o n g s t two or three l a b o r a t o r i e s to elucidate a n y p r o c e d u r a l difficulties. It certainly includes the p r e p a r a t i o n o f a suitable form on which results are to be reported. A n y o n e who has a t t e m p t e d to analyse the results o f an i n t e r c o m p a r i s o n will confirm that it is very difficult to collect i n f o r m a t i o n that is missing, or n o t asked for, in the forms. The present a t t e m p t to p r o d u c e a series o f m o n o g r a p h s on r a d i o n u c l i d e m e t r o l o g y m a y be seen as a logical extension o f this p r e p a r a t o r y work. A n d the final p o i n t to emerge is the value o f g o o d d o c u m e n t a t i o n ; that is the collection a n d p u b l i c a t i o n o f the results after a c o m p a r i s o n and again the staff o f the Bureau m u s t be c o m m e n d e d for this.
References 1) B.I.P.M., R6sultats de la comparaison du phosphore 32 (Jan. 1961); Comit6 Consultatif pour les Etalons de Mesure des Rayonnements lonisants C.C.E.M.R.I., 4th Session (1963) p. 29. 2) A. P. Baerg, Rapport sur la comparaison de l'iode 131 (April 1961); C.C.E.M.R.L, 4th Session (1963) p. 35.
a) p. j. Campion, Rapport sur la comparaison internationale de For 198 (Jan. 1962); C.C.E.M.R.I., 4th Session (1963) p. 47. 4) A. E. Kotchine, Rapport sur la comparaison internationale du cobalt 60 (Jan. 1962); C.C.E.M.R.I., 4th Session (1963) p. 53. ~) J. c. Roy and L. Cavallo, Rapport sur la comparaison internationale du thallium 204 (May 1962); C.C.E.M.R.I., 4th Session (1963) p. 59. ~) J. C. Roy and A. Rytz, Analyse des r6sultats de la comparaison internationale du soufre 35 (June 1962); C.C.E.M.R.I., 5th Session (1964) p. 28. 7) A. Rytz, Rapport sur la comparaison internationale de la m6thode 4nfl(CP)- 7 au moyen du cobalt 60 (March-April 1963); C.C.E.M.R.I., 5th Session (1964) p. 67. 8) A. Rytz, Rapport sur la comparaison internationale de l'am6ricium 241 (July 1963); C.C.E.M.R.L, 5th Session (1964) p. 49. 9) A. Rytz, Comparaison internationale d'une solution de 90Sr+9°Y (F6vrier 1964); Premiere partie: R6sultats des laboratoires participants (B.I.P.M., •964). to) A. Rytz, Comparaison internationale d'une solution de 90Sr + 90y par la m6thode 4n fl(CP); Deuxi~me partie: analyse des r6sultats, Recueil de travaux du B.I.P.M., vol .1 (B.I.P.M., 1966-1967). 11) A. Rytz, Compte rendu des 16sultats de la comparaison internationale d'une solution de ~4Mn (April 1965). 12) Compte rendu de la comparaison internationale d'une solution de ~4Mn (April 1965); Recueil de travaux du B.I.P.M., vol. 2 (ed. H. M. Weiss, B. 1. P. M., 1968-1970). la) j. W. Mfiller and A. Rytz, Report on the international comparison of dilution and source preparation methods by means of 6°Co; part 1: Results of the individual laboratories, Recueil de Travaux de B.I.P.M., vol. 1 (B.I.P.M., 1966-1967). 14) p. j. Campion, J. W. G. Dale and A. Williams, Nucl. Instr. and Meth. 31 (1964) 253. a~) W. Van der Eijk and H. Moret, Precise determination of
THE
WORK
OF THE
drop weights, Proc. Symp. Standardization of radionuclides (IAEA, Vienna, 1967) p. 529. 16) y . Le Gallic, B. Grinberg a n d M. Th6nard, La micropes6e dans les mesures d'activit6 de solutions, Proc. Syrup. Standardization of radionuclides (IAEA, Vienna, 1967) p. 499. 17) A. E. Oakley a n d G. C. Lowenthal, Accurate m e a s u r e m e n t of drop weights by the extrapolation m e t h o d , Proc. Symp. Standardization ofradionuclides (IAEA, Vienna, 1967) p. 519.
Discussion Rytz: D r C a m p i o n already m e n t i o n e d the reports on the various intercomparisons which were prepared by the B I P M . 1 should like to add that there is a considerable n u m b e r o f B I P M reports on other topics related with standardization o f radionuclides. W e have prepared also a list o f these reports a n d will gladly show it to a n y person interested. Please, call on D r Miiller or myself; we shall send y o u copies o f the reports y o u wish to receive, after the conference. M y second point is m u c h m o r e o f a question. Activity per unit m a s s o f a solution is an often used m a g n i t u d e . 1 heard Dr C a m p i o n speaking a b o u t
CCEMR1
II
45
"radioactive c o n c e n t r a t i o n " , whereas we generally use "specific activity". Is this term a better one, i.e., should it be preferred to "specific activity" ? Mrs. Merritt: T h e term "specific activity" refers to the quantity o f a radionuclide per unit q u a n t i t y o f its element and is therefore n o t suitable for quantitative s t a t e m e n t s o f the radioactivity c o n t e n t o f solutions; instead the term "radioactivity concent r a t i o n ' , is preferred for the quantitative s t a t e m e n t o f solutions. Taylor: Possibly "radioactivity concentration" w o u l d be preferable to "radioactive c o n c e n t r a t i o n " . Mann: These terms were defined in the I.C.R.U. 1962 Report as follows: Radioactive concentration is the quotient o f activity by m a s s or by v o l u m e o f the material in question. Specific activity is the quotient o f activity by m a s s of the element whose radioisotope is considered. Campion: I personally feel that, in English at least, the term "specific activity" is concerned with the activity per unit m a s s o f the element or o f a molecular c o m p o u n d containing that element. T h u s , in terms o f for example ~4Na, one could speak o f the specific activity o f a sample o f s o d i u m metal or o f salt, hut not o f brine.
I. G E N E R A L
REVIEW
PAPERS
INSTRUMENTS
AND
METHODS
II2
(i973) 4r-45;
©
NORTH-HOLLAND
PUBLISHING
CO.
T H E WORK OF THE BIPM C O N S U L T A T I V E C O M M I T T E E FOR M E A S U R E M E N T S T A N D A R D S OF I O N I Z I N G R A D I A T I O N S S E C T I O N I I Radionuclide measurements* P. J. C A M P I O N
Section II, CCEMRI, Pavilion de Breteuil, 92310 Sdvres, France A brief account of the activities, both past and present, of this Committee is presented. The aim is the improvement of the accuracy and uniformity of radioactivity measurements. To date the Committee has organized 11 international comparisons and the results of some of these are examined. While these have revealed some systematic errors in measurement techniques, the results of the more recent comparisons have been disappointing in that the knowledge gained has been small in relation to the effort involved. In view o f this a different approach has been adopted. The Committee has set up a n u m b e r of small working groups
to study various aspects of radionuclide metrology where discrepancies may perhaps occur. It is hoped that, at least for a n u m b e r o f such studies, a m o n o g r a p h will be published highlighting the measurement difficulties. Until such m o n o g r a p h s are available there is, in the Committee's view, little point in carrying out further international comparisons. However, in order to fill a need created by this temporary moratorium, the Bureau International des Poids et Mesures has offered to both provide calibrated solid sources and train staff in radionuclide metrology.
The Bureau International des Poids et Mesures (BIPM) was created in 1875 under the Convention du M~tre for the purpose of making and processing standards of mass and length. The Bureau is located at S6vres near Paris and, since 1875 has gradually extended its scope to include thermometry, electricity, photometry and, even more recently, ionizing radiations. The work of the Bureau and its relationship with that of the national standardizing laboratories is surveyed by the Comit6 International des Poids et Mesures. Since this committee is relatively small it appoints a number of Consultative Committees to advise it on individual subjects. In the case of the Comit6 Consultatif pour les Etalons de Mesure des Rayonnements lonisants (CCEMRI), which was created in 1958, the subject is so broad that it has been found convenient to divide the Committee into four Sections. Section II deals with radionuclide measurements and it is the work of this Section which forms the subject of this paper. The aim of this Section is to improve the accuracy and uniformity of measurements of activity. This includes not only improvements in the techniques of measurement, but also the determination of those constants which are necessary both for disintegration rate measurements and their useful application. It is of course impossible to determine accuracy in any absolute sense, but the spread of results of an inter-
comparison of measurements carried out by competent laboratories is the traditional yardstick by which accuracy can be gauged. However, it must be emphasized that the results of such intercomparisons, even when the spread of results is consistent with the individual uncertainties, only provide an estimate of the absolute accuracy of that particular measurement. Nevertheless it is by making comparisons in this way that it is possible to uncover the presence of systematic errors which might otherwise go unsuspected and this is the prime reason for undertaking international comparisons at the highest level. In fulfilling its mission to improve radionuclide measurements the Consultative Committee arranges such comparisons when it is felt that they would produce useful results. Table 1 lists chronologically the intercomparisons that have been made during the last decade or so, the results for each, being fully documented by the BIPM, are available on request. In a short survey of this nature it is inappropriate to present a full statistical analysis of all these results but, in order to demonstrate the type of information that can be obtained, it is instructive to pick out from amongst these II intercomparisons the three that involved 6°Co. Only those results obtained by coincidence methods have been included in this limited survey and these are presented in fig. l as histograms of the deviations from the arithmetic mean. The number of results included in these comparisons varied from 16 to 24, but the areas under the histograms have been normalized in order to make visual comparisons easier. The figure at the top right of each histogram is the standard deviation
* Persons interested in more detailed information on the activity of the B I P M in this field may obtain a comprehensive list of publications and internal reports directly f r o m the Bureau International des Poids et Mesures, Pavilion de Breteuil, 92310 S6vres, France.
41 I. G E N E R A L
REVIEW
PAPERS
42
P. J. CAMPION
I
1962
|
7'1%,
,,.t 1963 Pycnometer
1963
1967
m t
/,.2 %.
,,
1967~ L ~ 31 . */,
3.~ */,o
Extrapo{ation
Strong solution
2"6*/**
1967
~~L
Solid sources
28 %.
ratio C ° n c e n t r a t i ° A h ~
I0 8 6 4 2 O 2 4 6 8 10 ~--
10 8 6 4 2 0 2 4 6 8 Deviation,'/,,-
10
Fig. 1. Results o f intercomparisons o f 6°Co obtained by coincidence m e t h o d s expressed as deviations in parts per thousand from the arithmetic mean. The figure at the top right o f each histogram is the standard deviation for that c o m p a r i s o n which is identified b y year (top left) and any further characterization as necessary. The cross-hatched areas in the 1963 solution c o m p a r i s o n represent results obtained by the extrapolation technique while the solid areas are those obtained by the pycnometer technique. N o distinction is m a d e in the results for the other comparisons. The histogram for the solid source c o m p a r i s o n includes only those sources in the range 2400 to 4600 s - l ; the c o m p a r i s o n was made by normalizing to reference-laboratory results.
TABLE 1
International intercomparisons o f radionuclides. Nuclide
Date
Ref.
32p
1961 1961 1962 1962 1962 1962 1963 1963 1964 1965 1967
1 2 3 4 5 6 7 8 9, l 0 11, 12 13
131I
198Au 6OCo 2°4T1 z5S 6°Co 241Am 9°Sr-9°Y
54Mn COCo
expressed as parts per thousand. For the 1962 c o m parison, results lying outside _+ 1.5% of the mean have been excluded from the histogram and the calculation o f the standard deviation, whilst for all other compari-
sons, results outside + 1.0% have been excluded. Such limits are necessary since although an extreme result m a y have relatively little effect on the mean it has a considerable influence on the standard deviation. One or two results per comparison were rejected as a result of these limits. The 1962 comparison was a straightforward c o m parison of 60 Co in solution4); that is, laboratories were required to determine the activity per unit mass o f solution. The results are fairly typical of that era. The 1963 comparison was a rather special one7). The C o m mittee intuitively felt that all was not well with the diluting and dispensing techniques and hence arrangements were m a d e to distribute a radioactive solution together with a series of solid sources mounted on thin supports suitable for 4n/3- 7 coincidence counting. These solid sources were counted by reference laboratories before dispatch to participating laboratories and m o s t sources were recounted by one of the reference laboratories on their return in order to check that active material had not been lost from the mount in
THE
WORK
OF
transit. The Committee also went to some considerable effort to prepare a detailed form for reporting the results. This proved to be most worthwhile since the form was instrumental in bringing to light a difference between the results of those laboratories which dispensed sources by the pycnometer technique and those that used the extrapolation method. For this particular comparison (1963) the results of the laboratories using the former techniques are distinguished in fig. 1 by solid areas in the histogram, while those from laboratories using the extrapolation technique are shown as cross-hatched areas. The means of the two groups are shown by the vertical arrows, and even a visual inspection suggests that the difference may be significant. This result led to investigations in several laboratories concerning the inherent errors in both dispensing techniques and it was established that, under certain circumstances, there could be a considerable error involved in the extrapolation technique~4-1v); this error was of the right sign and order of magnitude to account for the difference shown in fig. I. This is an excellent example of an unsuspected systematic error being revealed by a carefully documented intercomparison. The second point that emerged from the 1963 comparison was that, in general, laboratories could get considerably better agreement on solid sources than on solutions at least for medium counting rates. Not all of this difference can be ascribed to the pycnometerextrapolation variation since even if only the results obtained using the pycnometer technique are considered the dispersion of the solution results is greater than that of the solid sources. It was therefore reasonable to conclude that there was some other factor associated with diluting and/or dispensing which contributed to the dispersion of the results. A third comparison was organized in 1967. This differed from the previous ones in that two solutions of 6°Co were distributed, a weak solution and a solution some twenty times stronger in radioactive concentration13). The ratio of the concentrations of these two solutions was known by gravimetric measurements to an accuracy considerably greater than that possible by activity measurements. Laboratories were asked to measure the radioactive concentration of the two solutions and, from these measurements, to derive the ratio of the concentrations. The reasoning behind this exercise was that, for any one laboratory, systematic errors in dispensing and counting sources should tend to cancel out in the ratio and hence result in a better agreement for the latter than for the individual concentration measurements. The results can be seen
THE
CCEMRI
43
II
in fig. 1. The dispersions of the individual concentration measurements for the weak and strong solutions were each found to be considerably less than that for the 1962 comparison but were no better than that obtained in 1963. On the other hand, the dispersion of the ratio results was significantly smaller. It was concluded therefore that, despite the increased interest in dispensing sources which took place over the four year interval between comparisons, the techniques had not been improved materially. One can attempt to identify the cause of this by comparing the observed standard deviation of 2.8%o with that estimated from the uncertainties quoted by each participating laboratory. Since for a given laboratory systematic uncertainties should, in principle at least, cancel out in the ratio results one need only consider the statistical uncertainties; estimates of these, derived from the scatter of individual source results, have been listed for the participating laboratories(~3). Using these data, one obtains an estimate of the standard deviation of 0.23 %0. An F test indicates that this value is significantly different from the observed value of 2.8%o and clearly some factor other than those due to counting and weighing statistics must be influencing the observed result. Of course it is possible that some of the systematic uncertainties do not entirely cancel out in the ratio. For example, an uncertainty in the dead time will give rise to slightly different errors in the numerator and denominator of the ratio if the counting rates are different; however, it is felt that such effects would be unlikely to account for such a large discrepancy. On the other hand, the fact that the strong solution required dilution whereas no such manipulation was necessary for the weak solution suggests that there may be a significant uncertainty due to this operation. Since the results of the more recent comparisons show no improvement over those of the earlier ones, together with the fact that the organization of such comparisons requires a very great deal of work, the Committee felt that the time had come to adopt a fresh approach to the general problem of improving accuracy on a worldwide basis. Thus the Committee has recently decided to set up a number of small working parties, the task of each of which is to examine in depth a particular problem associated with radionuclide metrology. By way of illustration the following is a list of some, but by no means all, of the topics at present under review: (1) Principles of the coincidence method. (2) Problems in microweighing. I.
GENERAL
REVIEW
PAPERS
44
P. J. C A M P I O N
(3) A study o f reference techniques for the assay o f radionuclides. (4) The detection a n d e s t i m a t i o n o f spurious pulses. (5) F e a s i b i l i t y o f liquid scintillation counting for precision metrology. (6) D i s i n t e g r a t i o n rate m e a s u r e m e n t s involving nuclides with c o m p l e x decay schemes. In some cases e x p e r i m e n t a l investigations involving the j o i n t c o o p e r a t i o n o f two or three l a b o r a t o r i e s are in progress in o r d e r to d e t e r m i n e the limiting accuracy o f various techniques a n d m a n i p u l a t i o n s . In others, it is a m a t t e r o f r e c o r d i n g in a convenient f r o m the " k n o w - h o w " gained b y experience in m a n y different l a b o r a t o r i e s t h r o u g h o u t the world. The objective o f m o s t o f these projects is to p r o d u c e a series o f m v n o g r a p h s which will serve as a guide to g o o d l a b o r a t o r y practice in various aspects o f r a d i o n u c l i d e metrology. U n t i l these are available a n d have been assimilated by l a b o r a t o r i e s it is clear t h a t there is little p o i n t in c a r r y i n g o u t further i n t e r c o m p a r i s o n s . The C o m m i t t e e is fully aware that this t e m p o r a r y m o r a t o r i u m on c o m p a r i s o n s might affect some l a b o r a t o r i e s which find such m e a s u r e m e n t s a convenient way to i m p r o v e their technique a n d test their equipment. H o w e v e r , in o r d e r to fill this gap, the B I P M has offered to send calibrated r a d i o n u c l i d e s as solid sources to a n y n a t i o n a l l a b o r a t o r y for comp a r i s o n p u r p o s e s a n d indeed this is likely to be a c o n t i n u i n g activity o f the Bureau. In a d d i t i o n the B I P M has offered to a r r a n g e for staff from such l a b o r a t o r i e s to stay at S6vres or elsewhere for training a n d experience in r a d i o n u c l i d e metrology. The C o m mittee m e m b e r s are p a r t i c u l a r l y grateful to the B I P M for these services a n d wish to express their t h a n k s to the staff o f the Bureau. In c o n c l u d i n g therefore this review o f the last decade o f r a d i o n u c l i d e m e t r o l o g y the following points emerge. First, there has been a slow b u t steady i m p r o v e m e n t in the a g r e e m e n t o f i n t e r c o m p a r i s o n results, at least until the m i d 1960's. Second, it is possible to learn m u c h m o r e f r o m an i n t e r c o m p a r i s o n when there is a specific scientific question to be answered. Thus the 1962 6°Co c o m p a r i s o n yielded little i n f o r m a t i o n other than the s p r e a d o f results. In the 1963 c o m p a r i s o n the question was posed: C a n l a b o r a t o r i e s m e a s u r e solid sources m o r e accurately t h a n s o l u t i o n s ? In 1967 the question was: Is there any residual e r r o r in diluting sources ? So far as the latter question is concerned the results w o u l d seem to suggest that there m a y well be, b u t this has yet to be confirmed. The third p o i n t is that the value o f a c o m p a r i s o n is e n h a n c e d m a n y times if a certain a m o u n t o f p r e p a r a t o r y w o r k is carried o u t first. Such w o r k m a y
include a trial c o m p a r i s o n a m o n g s t two or three l a b o r a t o r i e s to elucidate a n y p r o c e d u r a l difficulties. It certainly includes the p r e p a r a t i o n o f a suitable form on which results are to be reported. A n y o n e who has a t t e m p t e d to analyse the results o f an i n t e r c o m p a r i s o n will confirm that it is very difficult to collect i n f o r m a t i o n that is missing, or n o t asked for, in the forms. The present a t t e m p t to p r o d u c e a series o f m o n o g r a p h s on r a d i o n u c l i d e m e t r o l o g y m a y be seen as a logical extension o f this p r e p a r a t o r y work. A n d the final p o i n t to emerge is the value o f g o o d d o c u m e n t a t i o n ; that is the collection a n d p u b l i c a t i o n o f the results after a c o m p a r i s o n and again the staff o f the Bureau m u s t be c o m m e n d e d for this.
References 1) B.I.P.M., R6sultats de la comparaison du phosphore 32 (Jan. 1961); Comit6 Consultatif pour les Etalons de Mesure des Rayonnements lonisants C.C.E.M.R.I., 4th Session (1963) p. 29. 2) A. P. Baerg, Rapport sur la comparaison de l'iode 131 (April 1961); C.C.E.M.R.L, 4th Session (1963) p. 35.
a) p. j. Campion, Rapport sur la comparaison internationale de For 198 (Jan. 1962); C.C.E.M.R.I., 4th Session (1963) p. 47. 4) A. E. Kotchine, Rapport sur la comparaison internationale du cobalt 60 (Jan. 1962); C.C.E.M.R.I., 4th Session (1963) p. 53. ~) J. c. Roy and L. Cavallo, Rapport sur la comparaison internationale du thallium 204 (May 1962); C.C.E.M.R.I., 4th Session (1963) p. 59. ~) J. C. Roy and A. Rytz, Analyse des r6sultats de la comparaison internationale du soufre 35 (June 1962); C.C.E.M.R.I., 5th Session (1964) p. 28. 7) A. Rytz, Rapport sur la comparaison internationale de la m6thode 4nfl(CP)- 7 au moyen du cobalt 60 (March-April 1963); C.C.E.M.R.I., 5th Session (1964) p. 67. 8) A. Rytz, Rapport sur la comparaison internationale de l'am6ricium 241 (July 1963); C.C.E.M.R.L, 5th Session (1964) p. 49. 9) A. Rytz, Comparaison internationale d'une solution de 90Sr+9°Y (F6vrier 1964); Premiere partie: R6sultats des laboratoires participants (B.I.P.M., •964). to) A. Rytz, Comparaison internationale d'une solution de 90Sr + 90y par la m6thode 4n fl(CP); Deuxi~me partie: analyse des r6sultats, Recueil de travaux du B.I.P.M., vol .1 (B.I.P.M., 1966-1967). 11) A. Rytz, Compte rendu des 16sultats de la comparaison internationale d'une solution de ~4Mn (April 1965). 12) Compte rendu de la comparaison internationale d'une solution de ~4Mn (April 1965); Recueil de travaux du B.I.P.M., vol. 2 (ed. H. M. Weiss, B. 1. P. M., 1968-1970). la) j. W. Mfiller and A. Rytz, Report on the international comparison of dilution and source preparation methods by means of 6°Co; part 1: Results of the individual laboratories, Recueil de Travaux de B.I.P.M., vol. 1 (B.I.P.M., 1966-1967). 14) p. j. Campion, J. W. G. Dale and A. Williams, Nucl. Instr. and Meth. 31 (1964) 253. a~) W. Van der Eijk and H. Moret, Precise determination of
THE
WORK
OF THE
drop weights, Proc. Symp. Standardization of radionuclides (IAEA, Vienna, 1967) p. 529. 16) y . Le Gallic, B. Grinberg a n d M. Th6nard, La micropes6e dans les mesures d'activit6 de solutions, Proc. Syrup. Standardization of radionuclides (IAEA, Vienna, 1967) p. 499. 17) A. E. Oakley a n d G. C. Lowenthal, Accurate m e a s u r e m e n t of drop weights by the extrapolation m e t h o d , Proc. Symp. Standardization ofradionuclides (IAEA, Vienna, 1967) p. 519.
Discussion Rytz: D r C a m p i o n already m e n t i o n e d the reports on the various intercomparisons which were prepared by the B I P M . 1 should like to add that there is a considerable n u m b e r o f B I P M reports on other topics related with standardization o f radionuclides. W e have prepared also a list o f these reports a n d will gladly show it to a n y person interested. Please, call on D r Miiller or myself; we shall send y o u copies o f the reports y o u wish to receive, after the conference. M y second point is m u c h m o r e o f a question. Activity per unit m a s s o f a solution is an often used m a g n i t u d e . 1 heard Dr C a m p i o n speaking a b o u t
CCEMR1
II
45
"radioactive c o n c e n t r a t i o n " , whereas we generally use "specific activity". Is this term a better one, i.e., should it be preferred to "specific activity" ? Mrs. Merritt: T h e term "specific activity" refers to the quantity o f a radionuclide per unit q u a n t i t y o f its element and is therefore n o t suitable for quantitative s t a t e m e n t s o f the radioactivity c o n t e n t o f solutions; instead the term "radioactivity concent r a t i o n ' , is preferred for the quantitative s t a t e m e n t o f solutions. Taylor: Possibly "radioactivity concentration" w o u l d be preferable to "radioactive c o n c e n t r a t i o n " . Mann: These terms were defined in the I.C.R.U. 1962 Report as follows: Radioactive concentration is the quotient o f activity by m a s s or by v o l u m e o f the material in question. Specific activity is the quotient o f activity by m a s s of the element whose radioisotope is considered. Campion: I personally feel that, in English at least, the term "specific activity" is concerned with the activity per unit m a s s o f the element or o f a molecular c o m p o u n d containing that element. T h u s , in terms o f for example ~4Na, one could speak o f the specific activity o f a sample o f s o d i u m metal or o f salt, hut not o f brine.
I. G E N E R A L
REVIEW
PAPERS