Criteria that must be satisfied
for the
Discovery of a New Chemical Element to be Recognized
II. CRITERIA
II.1. Discovery of a chemical element is the experimental demonstration,
beyond reasonable doubt, of the existence of a nuclide with an atomic
number Z not identified before, existing for at least 10-14
s.
Note 1. This lifetime is chosen as a reasonable estimate of the time
it takes a nucleus to acquire its outer electrons. It is not considered
self-evident that talking about an "element" makes sense if no outer
electrons, bearers of the chemical properties, are present.
Note 2. Discovery of an element can be based on chemical or physical
methods or on both.
Note 3. The exact value of Z need not be determined, only
that it is different from all Z-values observed before, beyond
reasonable doubt.
Note 4. Neither is it required that the exact value of the mass number
A be known. Even if a value for A is suggested that
is later proved incorrect (but if an isotope with a nearby value of
A, which could also have been produced, has the reported properties),
this does not necessarily invalidate discovery of the new element
(see also II.5.).
II.2. The TWG realizes that the term "reasonable doubt" is necessarily
somewhat vague. Cases occur where experts at the time did not feel reasonable
doubt about reports the later were found to be incorrect. Conversely,
a case is known where expressed doubts could initially not be called
unreasonable, but where they later appeared to be based on circumstances
proven to be accidental. For these reasons, it is often considered
desirable to wait with the assignment of priority until the reported
results have been confirmed by later work. Generally speaking, such
confirmation should not consist merely in a repetition of the same procedure
with the same material, since this would imply too high a probability
of repetition of the same unsuspected error (although such a repetition
is not without value.)
Confirmation demands reproducibility, which is also related to
setting up discovery profiles. All scientific data, other than those
relating to unique events such as a supernova, must be susceptible of
reproduction. In the case of the new elements the TWG attaches considerable
importance to reproducibility and would indeed like to be able to suggest
that no new element should be recognized officially until the data upon
which the claim is based have been reproduced, preferably in another
laboratory and preferably by a different technique. However, it cannot:
given the immense labour and the time necessary to detect perhaps even
a single atom of a new element, it would appear unreasonable to apply
such a demand of demonstrated reproducibility in all rigidity. We do
not believe that recognition of the discovery of a new element should
always be held up until the experiment or its equivalent have been repeated,
desirable in principle as this may be. However, we would waive this
requirement only in cases where the data are of such a nature that no
reasonable doubt is possible (for instance for data with a high degree
of internal redundancy and of the highest quality), and under circumstances
where a repetition of the experiment would imply an unreasonable burden.
II.3. An experiment designed to demonstrate the existence
of a new element must have two aspects The first establishes physical
and/or chemical properties of samples suspected of containing (at
least one atom of) the new element and that are sufficient to categorize
it; these we call characterization properties. The second extends
to properties that are used to demonstrate that the "characterization
properties" are indeed those of an unknown element; these we call assignment
properties. Some properties can be used for both purposes.
II.4. Most assignment properties do not alone allow sufficient certainty
for assigning a unique value to Z. A combination of them may,
but it is usually not easy, if possible at all, to quantify the degree
of certainty reached.
The only realistic way for deciding how far the criteria have been
adequately met is for a group of informed "neutral" physicists
and chemists to determine this, after study of the material presented
and after consulting experts in the field.
II.5. The assignment of A can in principle
influence the assignment of Z, which is one of the reasons why
criteria for the assignment of A are included in the evaluation
below. If this is the case, a later change in the A-assignment
can throw doubt on the Z-assignment. It must be insisted, however,
that the priority cannot be denied if a wrong A-assignment does not
influence the Z-assignment.
II.6. In the specific cases under consideration, nuclides of supposedly
new elements have been obtained by bombarding targets of known composition
with known particles, sometimes followed by chemical purification. Impurities
in the targets are known to have caused confusion in some cases. In
the region of the transfermium elements, however, their influence is
now known and can be rather easily recognized, since impurities only
matter if they produce SF (spontaneous fission) or high-energy a-particle
radioactivities.
The criteria developed below are also applicable to cases where a nuclide
is obtained by chemically purifying natural material or debris of (thermo-)nuclear
explosions. Chemical methods must then be used to purify the nuclides.
As in the above case, these methods themselves might be sufficient for
proving that a new element is present.
II.7. The highest Z elements that one can hope to produce in
interactions between available stable or long-lived nuclides, even being
very optimistic, have atomic numbers around 190. The production properties
mentioned below cannot necessarily be expected to apply to higher Z
cases.
II.8. Finally, we must have a word about publication. The TWG has
a strong preference for publication in regular journals of international
standing. However, it does not wish to take up a rigid position on this
matter and would not wish to exclude from admissibility any form of
bona fide publication of wide general accessibility.