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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







In the following list, we give properties connected with the preparation of nuclides as just described. The first five concern reaction properties, the others sample preparation. The note in the last column describes whether they are "characterization properties" C or "assignment properties" A(Z) for Z or A(A) for A or A(A, Z) for both (see section II.3).

Ei Energy of bombarding particles C
Cs Cross section C
Ey Yield curve C, A(A, Z)
Cb Cross bombardments C, A(A, Z)
Ad Angular distribution A(Z)
As Angular selection A(Z)
Ms Mass separation A(Z)
Vf Velocity filter A(Z)
Tf Time of flight selection A(Z)
Ch Chemistry A(Z)

Notations and introduction. Let a new nuclide AZ be formed in the bombardment of a target AtZt (it may be a mixture of isotopes) with particle az energy Ei, with a probability expressed as a cross section s.

Two types of reactions * are used to produce transfermium elements:

(i). Hot fusion reactions using ions with 4<z<12 impinging on actinides produce compound nuclei with high excitation energies (typically about 40 MeV at projectile energies just above the fusion threshold)

(ii). Cold fusion reactions using with 17<z<29 impinging on Bi, Pb or Tl produce compound nuclei with much lower excitation energies.

Such reactions in which g-rays or neutrons are emitted accompanied by at most one proton or a-particle will be called evaporation reactions. So called transfer reactions, in which the final charge is significantly smaller than Zt+z , are known to have produced unwanted and confusing backgrounds in measurement of evaporation reactions.

As to the target, the isotopic composition of the primary material is always known with sufficient accuracy in the experiments considered here. The presence of impurities in the target is not expected to be a source of uncertainty in cold fusion reactions. Impurities of, especially, Pb in actinide targets have been known to have produced confusion in the past.



Ei,Cs (Energy of bombarding particles and reaction cross section). The minimum necessary and admissible information about nuclear reactions is the energy of the particles impinging on a possibly thick target (in which they lose energy before reacting) and some information on the yield.

Ey (Yield curve). The most complete information is a yield curve: production cross section as a function of energy of the particle impinging on a target nucleus.

Yield curves for (az, axn) reactions tend to have tails and so can be distinguished from the previous two, if measured with good statistics. (Their maxima occur at higher energies than those for (az,xn) reactions). For hot fusion, the maxima for (az,xn) reactions can be considerably larger than those for (az,xn) reactions, but for cold fusion they are found to be about two orders of magnitude smaller. Yield curves for transfer reactions are much broader. At least for hot fusion, the yield for transfer processes need not be small compared with those for (az,xn) processes.

The theoretical understanding of reaction cross sections, especially in the region where fission seriously competes with evaporation, is insufficient to allow extrapolation to unknown Z cases with the confidence necessary to establish absolute priorities. Empirical evidence for the ratios of different kinds of evaporation reactions as a function of Z and A, which is now available, is, however, a useful guide.

Cb (Cross bombardments). Comparison of the probability of production of AZ in different combinations of AtZt and az can sometimes give valuable assignment criteria.

Ad, As (Angular distribution, Angular selection). The dependence of the production of AZ in (az,xn) processes is strongly forward peaked, more than in (az,axn) or transfer reactions. Thus, determination of angular dependences, or comparison of yields behind two different collimators, may yield a good criterion for assigning Z. This property can also be used to suppress unwanted backgrounds.

Ms (Mass separation). A well calibrated mass spectrometer with a resolution significantly better than 1/2 mass unit can yield an excellent criterion for assigning the mass number of the reaction products. One should, of course, be certain that one does not accidentally observe molecular fragments with the same SA.

Although in ion sources for mass spectrometry some chemical differentiation occurs, no useful information concerning Z can be drawn from A, except of course that an exceptionally high A would point strongly to a new (high) value for Z. Also, the value from evidence from other data might be strengthened by combination with mass spectroscopic evidence (e.g. when a possible daughter was observed.)

Even with limited resolution, a mass ("isotope") separator can be used to suppress unwanted backgrounds.

Vf (Velocity filter). A velocity filter can give a quite good (though not complete) separation of evaporation products from the results of transfer reactions. If combined with the result of an energy determination (e.g. by measuring the signal in a semiconductor counter catching the reaction product), it can act as a low resolution mass spectrometer. Its main use is suppression of unwanted backgrounds.

A variation of the velocity filter is to make use of the differences in range in matter between evaporation products and those resulting from transfer reactions.

Tf (Time of flight selection). Measurement of time of flight of the reaction products can replace or complement the use of a velocity filter.

Ch (Chemistry). Chemical methods can yield excellent assignment criteria. Observations of analogies of chemical properties of compounds involving the elements of unknown Z with those of compounds of the same chemical type of known elements may suggest specific Z-assignments.

Chemistry can be done with few, or even single atoms of an element. In these cases, many repeated reactions take place with those few atoms. This occurs in methods ion exchange (Ci), gas chromatography (Cg), gas thermochromatography (Ct) or chemical vapour transport (Cv).

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We here disregard the reported production of a new element ascribed to secondary particles of unknown energy themselves produced by the bombardment of a target with an intense beam of high energy. (Back to text)


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