After a brief introduction of the role of shell effects in determining the limiting nuclear masses, the experimental investigation of the decay properties of the heaviest nuclei is presented. For the production of superheavy nuclides fusion, reactions of heavy actinide nuclei with 48Ca-projectiles have been used. The properties of the new nuclei, the isotopes of elements 112–118, as well as of their decay products, together with the known data for the light isotopes with Z ≤ 113, give evidence of the significant increase of the stability with the neutron number of the heavy nucleus. The obtained results are discussed in the context of the theoretical predictions about the 'island of stability' of the hypothetical superheavy elements.

https://iopscience.iop.org/article/10.1088/0954-3899/34/4/R01/pdf

Heaviest nuclei from 48Ca-induced reactions

The authors combine modern theoretical calculations with evaluated selected experimental data to produce a comprehensive data resource of K- and L-x-ray transition and absorption edge energies for all of the elements from neon to fermium. The theoretical and experimental components of this work are the result of programs of parallel development extending over more than 20 years. At each of several progressively more refined comparisons, it was possible to identify theoretical components whose systematic improvement then led to the next level of refinement in comparisons with an increasingly robust experimental reference data set. We have now reached a certain practical limit in what can be undertaken with reasonable levels of theoretical effort. This limit is not very different from the practical level of accuracy that can be meaningfully associated with the experimental data. For the more prominent diagram lines, experiment and theory are concordant with a zero-centered distribution of residuals whose statistical metrics allow the uncertainties to be estimated. For the light elements $(Zl20)$ and the very heavy elements $(Zg90)$ there are significant difficulties, as is also the case for a few isolated elements and transitions for $20lZl90.$ Overall, the results reported here represent improvements over previously available data compilations not only because of their scope but also because of their attempt to offer internal metrics of the database accuracy. The identified regions of difficulty are areas where further experimental work may be directed to see if there may remain theoretical issues that are still unresolved.

X-ray transition energies: new approach to a comprehensive evaluation

In bombardment of ${}^{244}\mathrm{Pu}$ with ${}^{48}\mathrm{Ca}$, we observed a decay sequence consisting of an implanted heavy atom, three subsequent $\ensuremath{\alpha}$ decays, and a spontaneous fission (SF), all correlated in time and position. The measured $\ensuremath{\alpha}$ energies and corresponding time intervals were ${E}_{\ensuremath{\alpha}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9.71$ MeV ( $\ensuremath{\Delta}t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}30.4$ s), 8.67 MeV ( $\ensuremath{\Delta}t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}15.4$ min), and 8.83 MeV ( $\ensuremath{\Delta}t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.6$ min); for the SF ( $\ensuremath{\Delta}t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}16.5$ min), the total deposited energy was approximately 190 MeV. The $\ensuremath{\alpha}$-particle energies together with the decay times and SF terminating the chain offer evidence of the decay of nuclei with high atomic numbers. This decay chain is a good candidate for originating from the $\ensuremath{\alpha}$ decay of the parent nucleus ${}^{289}114$, produced with a cross section of about 1 pb.

Synthesis of Superheavy Nuclei in the 48 C a + 244 Pu Reaction

The search for new elements is part of the broader field of investigations of nuclei at the limits of stability. In two series of experiments at SHIP, six new elements were synthesized via fusion reactions using 1n-deexcitation channels and lead or bismuth targets. The isotopes were unambiguously identified by means of correlations. Alpha decay, not fission, is the dominant decay mode. The collected decay data establish a means of comparison with theoretical data. This aids in the selection of appropriate models that describe the properties of known nuclei. Predictions based on these models are useful in the preparation of the next generation of experiments. Cross sections decrease by two orders of magnitude from bohrium (Z = 107) to element 112, for which a cross section of 1 pb was measured. The development of intense beam currents and sensitive detection methods is essential for the production and identification of still heavier elements and new isotopes of already known elements, as well as the measurement of small -, - and fission-branching ratios. An equally sensitive set-up is needed for the measurement of excitation functions at low cross sections. Based on our results, it is likely that the production of isotopes of element 114 close to the island of spherical superheavy elements (SHEs) could be achieved by fusion reactions using targets. Systematic studies of the reaction cross sections indicate that the transfer of nucleons is an important process for the initiation of fusion. The data allow for the fixing of a narrow energy window for the production of SHEs using 1n-emission channels. The likelihood of broadening the energy window by investigation of radiative capture reactions, use of neutron deficient projectile isotopes and use of actinide targets is discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/61/6/002/pdf

New elements - approaching

Superheavy nuclei from 48 Ca-induced reactions

The daughter x-ray identification technique has been applied to the identification of element 104. The characteristic $K$-series x rays from the $\ensuremath{\alpha}$-decay daughter isotope, nobelium ($Z=102$), have been observed in coincidence with $\ensuremath{\alpha}$ particles from the decay of 4.5-sec $^{257}104$, thus providing an unequivocal determination of the parent atomic number, $Z=104$.

X-Ray Identification of Element 104

The nuclide $^{260}105$ has been produced in the $^{249}\mathrm{Cf}(^{15}\mathrm{N},4n)$ reaction and its decay properties investigated. The $\ensuremath{\alpha}$-decay branch of 1.52 \ifmmode\pm\else\textpm\fi{} 0.13-sec $^{260}105$ has been measured to be (90.4\ifmmode\pm\else\textpm\fi{}0.6)% with spontaneous-fission decay the remainder [(9.6\ifmmode\pm\else\textpm\fi{}0.6)%]. Measured $\ensuremath{\alpha}$-particle energies and intensities (% per decay) are 9.041\ifmmode\pm\else\textpm\fi{}0.014 MeV (48\ifmmode\pm\else\textpm\fi{}5), 9.074\ifmmode\pm\else\textpm\fi{}0.014 MeV (25\ifmmode\pm\else\textpm\fi{}3), and 9.120\ifmmode\pm\else\textpm\fi{}0.017 MeV (17\ifmmode\pm\else\textpm\fi{}3). Characteristic $L$ series x rays of element 103 were observed in coincidence with these $\ensuremath{\alpha}$-particle groups, thus providing elemental identification. The $^{256}\mathrm{Lr}$ $\ensuremath{\alpha}$-decay daughter was observed in a direct time-correlated genetic linkage, thus providing the mass number assignment. The absence of characteristic $L$- or $K$-series x rays of element 104 in time coincidence with, but preceding, spontaneous-fission-decay events, has allowed an upper limit of 2.5% to be placed on the electron-capture decay branch of $^{260}105$ to the elusive nuclide 0.1-sec $^{260}104$. If $^{260}104$ is a spontaneous-fission emitter with a half-life \ensuremath{\le} 100 \ensuremath{\mu}sec, as some predictions would indicate, then the electron-capture decay branch of $^{260}105$ is \ensuremath{\mu}0.2%.

R.J. Silva

Papers

X-Ray Identification of Element 104

Production, L x-ray identification, and decay of the nuclide /sup 260/105

Identification of the atomic number of nobelium by an x-ray technique.

The new nuclide nobelium-259

Determination of the NO(II)NO(III) potential from tracer experiments