Criteria that must be satisfied
for the
Discovery of a New Chemical Element to be Recognized
I. ORGANIZATIONAL AND GENERAL INTRODUCTION
I.1. The idea that a few elementary substances could, combined in various
ways, build up the whole of matter goes back to the Greek philosophers
of the sixth century B.C. The earliest form of the atomic "hypothesis",
more or less as we know it today, goes back almost as far, to Leucippus
in the fifth century B.C. later elaborated by Democritus. The dead hand
of Aristotle relaxed its grip only in the 17th century: atomic science
began to emerge from the atomic philosophy of Democritus with Robert
Boyle's (1661) recognition of chemical elements as "certain primitive
and simple bodies not being made of any other bodies or of one another".
In the late 18th century Antoine-Laurent Lavoisier, defining a chemical
element as something that could be decomposed by chemical means into
simpler substances, recognized about thirty of what we now know to be
elements in the modern sense but also included a few particularly stable
chemical compounds that resisted attempts to break down. John Dalton,
in 1808, introduced the idea of atomic weight as a useful characterization
of a chemical element, and chemists began to seek relationships between
elements on the basis of such weights. The great step was taken by Dmitri
Mendeleev in 1869 who recognized that the chemical properties of the
elements (of which he knew about 65) were periodic in their atomic weights;
this permitted him to construct his periodic table and to make the dramatic
prediction of the existence of then-undiscovered elements, notably eka-boron
(scandium), eka-aluminium (gallium), and eka-silicon (germanium). With
the discovery of isotopes by J.J. Thomson in 1912 following their inference
by Frederik Soddy in 1910, and with the discovery of characteristic
X-rays by H.G.J. Moseley in 1913, atomic charge or atomic number placed
atomic weight as the chief determining character of an element.
The centuries-old history of the definition and discovery of chemical
elements has a deep scientific and general fascination. This is because
the problem is of an essentially finite scope: there can only be a limited
number of species of atomic nuclei containing different numbers of protons
that can be imagined to have an existence, though perhaps only fleeting,
in the chemical sense. But although the problem is of finite scope,
we do not know what the scope is: we do not as yet know how many elements
await discovery before the disruptive Coulomb force finally overcomes
the nuclear attraction. In this sense, the problem is open although
of finite scope, unlike the number of continents upon the surface of
the earth where we know with certainty that none still awaits discovery.
These considerations give to the discovery of new elements an importance,
an allure and a romance that does not attach to the discovery of, say,
a new comet or a new beetle where many more such discoveries are
to be anticipated in the future. This, together with, of course, the
insight that they give into the details of the construction of Nature's
most complex nuclear edifices and the laws that govern their construction,
explains the great investment of material and, most particularly, human
resources into the discovery of new elements. Lives are committed over
decades to this enterprise, and this is not surprising. Nor is it then
surprising that, although from the point of view of science itself (except
that of the "science of history") and the associated advance of human
understanding it does not matter who makes the discovery, immense importance
is attached, personally, institutionally and nationally, by those engaged
in the enterprise, to the public recognition of their discoveries. Not
less surprising is that differences of opinion have arisen, in certain
cases, as to the relative importance of the various contributions, by
various research groups, at various times, that might have been made
to the recognition of such new elements.
Specifically, such differences of opinion have arisen concerning priority
in the discovery of elements beyond Z=101. In part, these difficulties
have been caused by a change in the experimental methods needed to produce
these elements. The reported discoveries have been made using complete-fusion/neutron-evaporation
reactions. In them, overwhelming competition with prompt fission causes
low yields which, combined with the often quite short half-lives, made
experimental investigation difficult. Also, backgrounds with properties
similar to those expected for transfermium elements but in reality due
to lower-Z products of other type (transfer-) reactions, or to
reactions on impurities (especially in actinide targets), have sometimes
caused confusion.
I.2. It has long been felt that the scientific community should be
able to resolve differences of opinion such as those which reference
has just been made by appropriate objective study of the evidence. In
1974 IUPAC in collaboration with IUPAP appointed a group of experts
not themselves directly involved in the controversies, 3 from the USA,
3 from the USSR and 3 (including the chairman) from other countries,
specifically ".. to consider the claims of priority of discovery of
elements 104 and 105 and to urge the laboratories at Berkeley (USA)
and Dubna (USSR) to exchange representatives and in their presence to
repeat the experiments regarding these elements." This committee never
completed its work, nor issued a report nor, indeed, met as a group,
though there was some correspondence between members.
At its meeting in September 1985 the Executive Council of IUPAP, faced
with renewed calls for the solution of the continuing problem and in
view of the evident lapsing of the earlier initiative, decided to suggest
to IUPAC the establishment of a new joint group, the Transfermium Working
Group (TWG), to examine the issues ab initio. It was decided that the
members should not be drawn from the countries of the major laboratories
concerned with research into the heaviest elements which in addition
to the USA and the USSR now included West Germany through GSI in Darmstadt.
I.3. The Terms of Reference of the TWG as suggested by its Chairman
were approved by the Executive Council of IUPAP in September 1986 and
by the IUPAC Bureau in October 1986; they include that the work be divided
into two phases:
Phase (i) |
The establishment of criteria that must be satisfied
for the discovery of a new chemical element to be recognized; |
Phase (ii) |
The application of these criteria in practice. |
It was also agreed by IUPAP and by IUPAC that: "(TWG in Phase (i))
should to some degree work interactively with the major experimental
laboratories concerned so as to move to criteria that will command general
assent."
I.4. IUPAP membership of the TWG was determined by IUPAP in September
1987 and the IUPAC membership by IUPAC at that same time:
IUPAP |
D.H. Wilkinson (UK) Chairman
R.C Barber (Canada)
A. Hrynkiewicz (Poland)
M. Lefort (France)
M. Sakai (Japan)
I. Ulehla (Czechoslovakia)
A.H. Wapstra (Netherlands)
|
IUPAC |
N.N. Greenwood (UK)
Y.P. Jeannin (France)
|
The TWG appointed Messrs Wapstra and Ulehla as its joint secretaries.
I.5. The TWG has held the following meetings, of which the first
and last were "private", with the remainder in the laboratories of chief
concern:
3-5 February 1988 |
Nonant (France) |
12-17 December 1988 |
Darmstadt (FRG) |
19-23 June 1989 |
Berkeley (USA) |
12-16 February 1990 |
Dubna (USSR) |
16-20 April 1990 |
Prague (Czechoslovakia) |
At the meetings in the laboratories, the TWG divided its time roughly
equally between "private" meetings and sessions with the scientists
of the laboratories. The TWG is most grateful for the warm and open
discussions that it has enjoyed in all three laboratories.
I.6. Early in its work, the TWG realized that it was
not practicable to effect a clean separation between the establishment
of criteria and their application in the sense that the validity of
criteria, and the pitfalls that might be encountered in their application,
can be assessed only through discussion of their potential impact in
real examples. The TWG has, as far as possible, restricted its examination
in Phase (i) of such examples to cases that are not contentious in respect
of priority claims but only by such "shadow" application of the evolving
criteria has it been possible to establish a set of criteria that is
indeed applicable in practice. These shadow exercises, not infrequently
in respect of cases in which reported assignments to certain nuclides
were later found to be incorrect, have proved to be most informative.
A benefit of this practical evolutionary approach to the definition
of criteria is that the TWG is now fully ready to proceed to Phase (ii)
of operations, the adjudicatory phase, should the present report on
Phase (i) be accepted by IUPAP and IUPAC, who have agreed that the TWG
as presently constituted should remain unchanged for Phase (ii).
I.7. The TWG has also come to the conclusion
that it is not feasible to specify criteria, or combinations of criteria,
that, in the words of its Terms of Reference "must be satisfied ..."
in order to achieve recognition of the existence of a new chemical element
and that would cover all cases. Very few properties indeed, of which
perhaps the only uncontentious example is the characteristic X-ray spectrum,
unambiguously determined, are suficient themselves to establish the
existence of a new element. For the rest, identification must rely upon
combinations of properties that will vary from case to case and that
cannot usefully be exhaustively codified as a set of criteria *.
To adopt any such codification would be to force research into
a strait jacket inimical to the spirit of free enquiry. The TWG has
therefore discussed, and here presents, those criteria (section
II) and properties (sections III and IV)
that have been used in the past and that are seen as being of relevance
for the future, and gives some indication of the store that it sets
by them, but recognizes that their relative importance will vary from
case to case depending upon the circumstances in which they are displayed
and the manner in which they are combined
To this degree, therefore, the TWG departed from the letter of its
Terms of Reference and would draw attention to the resultant disjunction
between the title of this Report, taken from those Terms, and its contents;
it holds strongly, however, that the position it presents in this Report
is the correct one.
I.8. The TWG has had to adopt an historical perspective in establishing
its criteria and testing them through the shadow exercises but of course,
the application of the criteria lies largely in the future. However,
it is evident that, historically, new elements were proposed, and accepted,
on the basis of evidence that would not meet the criteria of today,
even prior to the codification upon which the TWG has been engaged.
In terms of published scientific evidence and also in terms of the public
presentation of that evidence, there often appear what we can only describe
as significant inadequacies. The standards of the times have markedly
tightened, not least because of the development of new experimental
technologies and, particularly, of the computer. However, this presents
us with a problem which must be recognized at this stage although its
impact will be felt only in Phase (ii), namely that some of the contentious
cases, still to be resolved, now lie in the fairly remote past at a
time when standards were different from those of today. We must be constructively
sensitive to this when we move to Phase (ii). It would not be fair,
or indeed possible, to apply to yesterday the criteria of today without
regard for the circumstances of the times.
I.9. The TWG has not been charged with, and will not express opinions
about, any matter to do with the naming of the new elements, either
in Phase (i) or in Phase (ii). The following information may be useful:
The body primarily concerned with recommending names for the new elements
is the IUPAC Commission II.2 on the Nomenclature of Inorganic Chemistry.
This Commission does not "decide" on the adoption of names, but only
publishes recommendations for international use. The 1990 version of
its "Red
Book"
("Nomenclature of Inorganic Chemistry". Blackwell Scientific
Publications, Oxford) states in chapter I.3:
"The names approved by IUPAC are based on considerations of practicality
and prevailing usage. It is emphasized that the IUPAC selection carries
no implication regarding priority of discovery."
Systematic names have been proposed by IUPAC for all elements with
Z=100-999. These names were only intended for use in the period
when no "official" names were yet available. However, they have not
met with favour among nuclear chemists nor among physicists. They will
not be discussed in either phase of the present work.
Although the TWG is not concerned with names it feels that, in due
course, following its Phase (ii) report, it would be felicitous if the
laboratories concerned in the work leading to the establishment of a
new element were to present to IUPAC an agreed joint suggestion as to
the name.
I.10. Another matter involving IUPAC custom and practice is of concern
here: the TWG will follow the IUPAC Red Book definitions for
elements and isotopes and their atomic number Z, atomic mass
number A and atomic mass M (in atomic mass units). In
cases where the difference is important or instructive, the term "isotope"
(which strictly speaking refers to two or more atomic species having
the same atomic number Z but different atomic masses) will be
replaced by "nuclide" (for which no such restriction applies.)
We also refer to the Red Book for the notation for nuclear reactions
and for the definition of their cross sections.
I.11. It has become very clear to the TWG, particularly in its review
of historical cases referred to in I.6 above, that
the situation in respect of the discovery a new element is by no means
always black-and-white in the sense that it may be unequivocally asserted
that a new element was discovered, with the required certainty, by a
certain group, using a certain method on a certain date. Sometimes this
is the case, and this is what is popularly thought of as "discovery".
Perhaps more often , however, the situation is one in which data accumulated
over a period of time, perhaps of years, perhaps in two or more laboratories,
gradually bring the scientific community to the conviction that indeed
the existence of a new element has been established. However, different
individuals or different groups may take different views as to the stage
in the accumulation of evidence at which conviction is reached and may
take different views as to the existence or otherwise of crucial step
leading to that conviction and as to which those crucial steps were.
Such differences can be perfectly legitimate scientifically, in that
they may depend upon, for example, differing views as to the reliability
of the inference that might be drawn from certain types of evidence,
while not disputing the reliability of the evidence itself. So,
although scientific community may reach consensus as to the existence
of a new element, the reaching of that consensus is not necessarily
a unique event and different views may, in all scientific honesty, be
taken as to the steps by which it was reached.
It will be the business of the TWG, in Phase (ii), to analyze these
cases in detail and to attempt, where the situation is indeed not black-and-white,
to trace, with the aid of criteria now to be discussed, a kind of
discovery profile and to delineate the steps by which certainty in "discovery"
was reached and, if possible, to indicate the relative importance of
those steps. In other words, it is the TWG's conviction that there may
be cases in which it would be unjust to assign an absolute priority
in the "discovery" of a new element but where the credit should be appropriately
apportioned.
The TWG is sensitive to the fact that it may be thought by some that
such an apportionment of credit would be shirking the issue. This, however,
is not the case and such an opinion could be held only by those disregarding
the high complexity of the researches involved and the frequent lack
of total specificity in their application. Demand for absolute priority
assignments in all cases related to the new elements would imply an
attachment to outmoded concepts of the nature of discovery.
I.12. The TWG realizes, consonant with its views expressed in I.11,
that certain things relating to discovery will have to be made matters
of individual definition in the they stand outside scientific criteria
as such. An example of this might be Paper I that presents evidence
relating to a possible new element but which is not adequate of itself
to establish the existence of the new element without the evidence published
later in Paper 11, perhaps from a different laboratory, that, together
with Paper I, carries certainty but which is also not sufficient of
itself. It is now certain the Paper I "saw" the new element but could
not prove it at the time. Where does the priority lie? Is it with Paper
I or is it shared between Paper I and Paper II? And if, between Paper
I and Paper II, there was published a Paper III that was complete in
itself and carried conviction, does the credit lie wholly with Paper
III even though, after the publication of Paper II, it is evident that
Paper I saw the new element first?
We draw attention to such problems not to propose universal and rigid
resolutions such as might lead to the assignment of absolute priorities
but rather to support our concept of the discovery profile which we
feel will lead to a more equitable appreciation of the range of contributions
that might have been made. An absolute priority would often equate to
an absolute injustice.
A similar situation in which the discovery profile would offer a fairer
assessment would be one in which an early paper could not, at the time,
carry conviction but which was later realized to have reported correctly
signals from the new element in question, the existence of which was
definitely established by subsequent work following up the lead of the
early paper. Although it would clearly be wrong to assign absolute priority
to that early paper, it would, in our view, be appropriate to recognize
its seminal importance.
The discovery profile will also accommodate cases in which two groups
correctly report a new element within a brief time interval of each
other and in evident independence. In the TWG's view it would be absurd
and unjust to accord an absolute priority to the group that, in such
circumstances, simply happened to submit first. The discovery profile
will enable people who wish to attach importance to such matters to
do so.
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Notes
We distinguish between properties and
criteria. Properties are the objective chemical or physical attributes
of atoms and of nuclei or of processes through which they and their
behaviour are categorized. Criteria are the conditions that must be
met for those properties to be admitted as diagnostic in respect of
the character (e.g. the Z-value) of the bodies concerned.
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