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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







The following list mentions properties connected to the radioactive decay of the produced nuclides.

Ki Kind of decay (a, b, g, SF=spontaneous fission) C
Br Branching ratio C
T Half-life C
Ea Energy of a-particles C
Eb Maximum energy of b-particles C
Eg Energy of g-radiations C
X X-ray spectrum (K or L) C,A(Z)
Fc Fission characteristics C

Genetic relation between ancestor and nth generation descendant (there may be more than one)



Ki (Kind of decay). This property need not necessarily refer to the kind of decay of the new isotope itself, but to that of a descendant. In the latter case, though, proof is necessary that the hypothetical ancestor really occurred.

Above, b stands for all weak interaction processes (b-, b+, e), g for all electromagnetic ones (e.g. also for conversion electrons and emitted electron-positron pairs) and X also for Auger electrons. For SF see also under Fc.

Br (Branching ratio). A nuclide might show more than one decay mode. Their intensity ratios, if determined with reasonable accuracy, are rather characteristic properties.

T (Half-life). The information on T is sometimes very imprecise, e.g. in the case of poor statistics, or if it is only known that the descendant has been seen after a specified time which may then be much longer than the half-life of the hypothetical parent. Some information of this kind is inevitably available. Evidently, T is a more distinctive property the more precisely it is known.

With few exceptions, a half-life cannot be used as an assignment property. Theoretical understanding of half-lives is insufficient for this purpose. Combination of a very high a-particle energy with a relatively long half-life is a strong indication for Z>100. Cases are known, however, where high spin isomers with Z around 84 combine the same characteristics. Similarly, fast SF occurs not only for transfermium elements but also for SF isomers with Z around 94.

The hindrance factor in a-decay is known to be nearly equal to 1 in ground state transitions between nuclei with even A and even Z. In other cases, it may (but not necessarily must) be much larger. Thus, observation of a relatively long half-life (high hindrance) in a-decay excludes assignment to a transition between ground states of even-even nuclides. Such a hindrance can be considered to be an assignment property. Also in other cases, half-lives can sometimes be used to exclude specific assignments.

Ea (Energy of a-particles). The energy of an a-particle can often be determined very accurately and can then be a very distinctive characterization property. For nuclide, with a complex a-spectrum, good counting statistics may be necessary. Rare cases do exist where different nuclides have quite similar combinations of Ea's and T.

Eb (Minimum energy of a b-spectrum). The maximum energy of a continuous b-spectrum, if present, can be determined with moderate precision and is then a rather characteristic property.

Eg (Energy of a g-radiation). The energy of a g-radiation, if present, can be determined accurately and is then a good characterization property.

X (X-ray spectrum). The energy of X-rays can be determined in the same way as those of g-rays. They can be distinguished from g-rays if observed with reasonable statistics, since X-rays (both K and L) show very characteristic patterns. Similarly, Auger electrons might be distinguished from conversion electrons. The presence of X-radiations of the correct energy is an unambiguous assignment property yielding the atomic number of the atom emitting those X-rays.

Fc (Fission characteristics). SF allows use of a sensitive technique for the detection of the presence of several actinide and trans-actinide nuclides. But most fission characteristics (such as total kinetic energy (TKE) and fission fragment mass distribution), even if measured with reasonable statistics, are not good assignment properties. If the nuclear charges of coincident fission fragments could be measured, this would determine the Z-value for the fissioning nuclide.

Gn (Genetic relations). (with nth descendant.) This can yield an excellent assignment criterion, but only in the case that the descendant has a well assigned value of Z and, preferably, also of A. The reality of the proposed genetic relation, which must be well established, can be demonstrated (even in the case of poor statistics) by the observation of one or, preferably, all of the following properties:

Tc time correlations between the decays of a parent and a daughter,
Ic a correct ratio of parent and daughter decay intensities,
Pc a position correlation (e.g. observation of two a particles -assigned to an ancestor and a descendant-starting from the same place).


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In this Phase (i) Report we have enumerated various characterization properties and assignment properties that relevant to the discovery of new elements having atomic numbers greater than 100. We have not referred specifically to earlier publications in this field. In Phase (ii) we will apply these ideas so as to develop discovery profiles for each of the individual transfermium elements. In the Phase (ii) Report*, we will refer in detail to all relevant publications on those elements and also mention earlier reviews dealing with the discovery of the transfermium elements.

* Note:
Part ii. Introduction to Discovery Profiles, Part iii. Discovery Profiles of the Transfermium Elements, Responses from the concerned laboratories in Berkeley, Dubna and Damstadt, and Reply to responses by Transfermium Working Group, Pure Appl. Chem. 1993, 65, 1757-1814.


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