| Themes > Science > Chemistry > Nuclear Chemistry > Nuclear Reactions > Nuclear Stability Experiments |
The stability of heavy nuclei is governed by nuclear shell structure whose influence is dramatically amplified near closed proton and neutron shells. Since the mid-1960s, with growing confidence, nuclear theory predicts the spherical shells to be located at Z=114 and N=178-184. More recently, however, it was realized that this region of spherical superheavy nuclides might be connected by a "peninsula" of stability to the edge of the known heaviest elements. This far-reaching conclusion was based on the predicted existence of the deformed proton and neutron shell closures near Z=108 and N=162. We carried out a series of experiments designed to provide a direct test of the theoretical predictions regarding the existence of the new shell closures in the vicinity of Z=108 and N=162. Prior to our experiments, no evidence was available to make a definite conclusion about these predictions. Collaborative Dubna-Livermore experiments performed in 1993-1995 by employing the Dubna gas-filled recoil separator, with the beams of heavy-ion projectiles delivered by the JINR U400 cyclotron, have resulted in the discovery of the new nuclides 262Rf, 265Sg, 266Sg, 267Hs and 273110 - the heaviest isotopes of elements 104, 106, 108 and 110. In the experiment performed in April 1993 by using the 248Cm+22Ne reaction we discovered three new heavy nuclides, 266Sg, 265Sg and 262Rf. The ground-state decay properties that we established for 266Sg and 262Rf revealed a large enhancement in their stability as compared to that of nuclides with lower Z or N values. The transition from 262No to 266Sg at N=160 or from 258Fm to 262Rf at N=158, an addition of four protons, increases the stability against spontaneous fission (SF) by a factor of >3x103. It was observed for the first time that an increase in Z at a given N causes an elevation in the SF half-lives of even-even nuclides. The only explanation for this fact can be the approach to a nearby proton shell closure. Similarly, in going from 260Sg to 266Sg, the stability increases by a factor of >3x103 for SF decay and ~3x103 for alpha decay. Thus, the ground-state decay properties of 262Rf and 266Sg provide a strong indication of the existence of deformed shell closures near N=162 and Z=108. In March-April 1994 we carried out experiments designed to explore further the nuclear stability near N=162 and Z=108 by producing new heavy isotopes of element 108 in the complete fusion reaction 238U+34S. In a 36-day bombardment we identified the alpha-decaying N=159 isotope 267Hs. A critical test of the theory could be the observation of a decrease in stability for nuclides with Z, N beyond the predicted magic numbers. This would allow the exact Z, N localization of the new shell closures. Thus, the determination of whether the neutron closure is at N=162 or at a higher N value can be made by measuring alpha-decay properties of a nuclide with N=163 or 164. During the period from September 10 to December 30, 1994, we performed experiments to produce neutron-rich Z=110 nuclides by the 244Pu+34S reaction. After 43 days of actual 244Pu+34S bombardment, we observed the new nuclide 273110. The alpha-particle energy E=11.35 MeV measured for the Z=110 nuclide with N=163 provides direct and convincing evidence that a neutron shell closure indeed exists and is located at N=162 and not at a higher value of N. Decay properties determined for the new nuclides establish the existence of the shell closures at N=162 and Z=108 predicted by modern macroscopic-microscopic nuclear theory. The discovery of significantly increased nuclear stability near N=162 and Z=108 creates essentially new opportunities for extending the chart of the nuclides at its upper edge. Providing a decisive test of and a new credit for the current nuclear theory, this result offers predicted spherical shells at Z=114 and N=178-184 to be a major challenge for future experimental explorations. A new series of experiments with the Dubna gas-filled separator is aimed at the production of spherical superheavy nuclides with Z=114 and N=174-176 by the complete fusion reaction 244Pu+48Ca. The sensitivity of these experiments is expected to be 100-1000 times higher as compared to the most sensitive previous attempts to produce spherical superheavy nuclides using 48Ca-induced fusion-evaporation reactions on actinide targets. |
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