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Pure Appl. Chem., Vol. 63, No. 6, pp. 879-886, 1991.

Criteria that must be satisfied for the
Discovery of a New Chemical Element to be Recognized







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.


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