Search for superheavy hydrogen isotopes in pion absorption reactions

Review of the experimental results on the search for and the spectroscopy of the superheavy hydrogen isotopes ^4–7H obtained in stopped π^?-meson absorption by ⁹Be and ¹¹Be nuclei is presented. Study of light neutron-rich nuclei is a principal line in developing modern views on the properties of nuclear forces and determination of nuclear properties near the drip line. The present method of investigation relies on precision measurements of energy of charged particles emitted after pion absorption by nuclei. Important advantages of this method are the practically accurate initial state energy and momentum, as well as the possibility to study a wide range of excitation energy. In the frames of this method new results were obtained on level structures of the isotopes ^4–6H and indications on ⁷H production. Comparison with other experimental and theoretical results is performed.     

Search for superheavy elements in galactic cosmic rays

The charge distribution of approximately 6000 nuclei with charge numbers above 55 in galactic cosmic rays has been obtained in the OLIMPIYA project. Three superheavy nuclei with the charge numbers in the range 105 < Z < 130 have been detected. The regression analysis has provided a more accurate estimate of the charge number of one of these nuclei (119 _?6 ⁺¹⁰ with a probability of 95%). Such nuclei should form stability islands. Their detection in nature confirms theoretical predictions and justifies efforts for their synthesis under terrestrial conditions. The model calculations performed in this work possibly can explain the results of some experiments on the investigation of the charge composition of cosmic rays in which particles with charge numbers in the range 94 < Z < 100 were detected (they cannot enter into the composition of primary cosmic radiation because their lifetime is very short). The calculations indicate that events with Z > 92 are due to the fragmentation of heavier nuclei from the stability island, rather than to methodical inaccuracies or fault of instruments. Several such events have been revealed. Thus, the track method makes it possible to obtain the results very important for understanding of the physical picture of the world. The results obtained within the OLIMPIYA project show that the study of tracks of galactic cosmic rays in olivine crystals from meteorites opens new capabilities for the investigation of fluxes and spectra in cosmic rays in the region of heavy and superheavy nuclei.     

Search for decay modes of heavy and superheavy nuclei

Spontaneous fission(SF) with a new formula based on a liquid drop model is proposed and used in the calculation of the SF half-lives of heavy and superheavy nuclei(Z = 90–120). The predicted half-lives are in agreement with the experimental SF half-lives. The half-lives of decay(AD) for the same nuclei are obtained by using the Wentzel-Kramers-Brillouin(WKB) method together with Bohr-Sommerfeld(BS) quantization condition considering the isospin-dependent effects for the cosh potential. The decay modes and branching ratios of superheavy nuclei(Z =104-118) with experimental decay modes are obtained, and the modes are compared with the experimental ones and with the predictions found in the literature. Although some nuclei have predicted decay modes that are different from their experimental decay modes, decay modes same as the experimental ones are predicted for many nuclei. The SF and AD half-lives, branching ratios, and decay modes are obtained for superheavy nuclei(Z = 119–120) with unknown decay modes and compared with the predictions obtained in a previous study. The present results provide useful information for future experimental studies performed on both the AD and SF of superheavy nuclei.     

Search for superheavy elements in monazite from beach sands of South India

Monazite minerals obtained from beach sands of South India were examined for the presence of superheavy elements with photon-induced x-ray fluorescence method. The accumulated data of a number of runs each of several days duration do not show any convincing peaks above the background at the expected locations for superheavy elements which are above the present sensitivity of detection of about 10 ppm by weight for element 126. However, some intriguing features pertaining to structures in the x-ray spectra around 27 keV were observed, which are of interest for further investigations.     

Search for superfluidity in solid hydrogen

A torsional oscillator study of solid para-hydrogen has been carried out down to 20 mK in a search for evidence of superfluidity. We found evidence of a possible phase transition, marked by an abrupt increase in the resonant period of oscillation and onset of extremely long relaxation times as the temperature was raised above 60 mK. In contrast to solid 4He, the change in the period for para-hydrogen is not a consequence of irrotational superflow. The long relaxation times observed suggest the effect is related to the motion of residual ortho-hydrogen molecules in the solid.     

Search for long-lived superheavy elements in the reaction of136Xe with238U

A search with radiochemical methods for long-lived superheavy elements in 238U targets bombarded with intense beams of¹³⁶Xe ions produced negative results. A formation cross section of ≤1×10^?35 cm² is deduced at 95% confidence level for nuclides with half-lives between 1 and 200 d.     

Spectroscopy of superheavy hydrogen isotopes in stopped-pion absorption by nuclei

The structure of levels of superheavy hydrogen isotopes ^4–6H is analyzed on the basis of a record statistics of experimental data on the absorption of negatively charged pions by light nuclei. Qualitatively new experimental data are obtained for the spectroscopy of the superheavy hydrogen isotopes ⁵H and ⁶H. Peaks due to four resonance states of ⁵H are observed in the missing-mass spectra for the reaction channels ⁹Be(π ^?, pt)X and ⁹Be(π^?, dd)X. A structure that is associated with four resonance states of ⁶H is observed in the missing-mass spectra for the reaction channels ⁹Be(π^?, pd)X and ¹¹B(π^?, p ⁴He)X. On the basis of the results presented for ground-state parameters, it can be concluded that the binding energy of superheavy hydrogen isotopes decreases as the number of neutrons increases. Excited levels of the isotopes ⁵H and ⁶H are observed for the first time. On the energy scale, all of these states lie above the threshold for decay to free nucleons.     

Search for delayed fission products from superheavy nuclei in the reaction136Xe+208Pb

The production of superheavy elements in binary reactions of the type²⁰⁸Pb (¹³⁶Xe, X) Y was investigated atE _c.m.=470 MeV. The experiment was designed to search for delayed fission products from elements withZ between 108 and 116 and fission lifetimes ofΤ?10^?12 s. No fission events were observed the upper limit for the formation cross section being 1.2 Μbarn.     

Spectroscopy of 7He and superheavy hydrogen isotope 5H

The neutron-transfer reaction d(⁶He, p)⁷He is investigated. The data obtained show that, in the range between 1 and 7 MeV above its ground-state resonance, ⁷He does not have well-pronounced narrow excited states with a single-particle structure. A resonance state of ⁵H with an energy of 2 MeV above the n+n+³H decay threshold is obtained for the first time by making use of the reaction p(⁶He, ²He)⁵H.     

Searches for the superheavy hydrogen isotope 7H in the absorption of stopped π − mesons

Experimental searches for the superheavy hydrogen isotope ⁷H were performed in reactions involving the absorption of stopped π ^? mesons on ⁹Be and ¹¹B nuclei. In the reaction ⁹Be(π ^?, pp)X, the missing-mass spectrum shows evidence for the formation of ⁷H states, that of E _r = 16 ± 1 MeV and Γ ? 2 MeV and that of E _r = 21 ± 1 MeV and Γ ? 5 MeV (E _r is the resonance energy with respect to breakup into a trition and four neutrons, and Γ is the observed level width).     

Resonance states of superheavy hydrogen nuclei obtained in transfer reactions with exotic beams

A resonance state situated at 1.8±0.1 and, most likely, another state positioned at 2.7±0.1 MeV above the t+n+n decay threshold were observed in the missing mass energy spectrum of the ⁵H nucleus produced in the reaction ³H(t,p)⁵H. The peak located close to $E_{^5 H} = 1.8$ MeV also was seen in the ⁵H spectrum obtained from the energy distributions of ³H nuclei emitted in the reaction ²H(⁶He,⁵H)³He. The width (Γ_obs ≤ 0.5 MeV) obtained for the two ⁵H resonance states is surprisingly small. A state of ⁴H with E _res = 3.3 MeV and γ² = 2.3 MeV was obtained in the reaction ²H(t,p)⁴H from the proton spectrum.     

合成和发现超铀化学元素、探索超重核稳定岛

合成新元素拓展元素周期表、探索原子核存在极限是核物理前沿研究领域。相关研究工作涉及的重大物理问题有：存在多少种化学元素？超重元素具有什么样的化学性质？超重元素化学性质是否符合元素周期律的外推预期？最大的核幻数是什么？是否存在稳定的或长寿命的超重核素？在极强库仑场中原子核具有哪些奇特的结构和性质？等等。经过逾半个世纪的不懈努力,元素合成取得了巨大成就,已将周期表从92号扩展至118号元素,完成了周期表上第七周期元素填充。文章回顾了在自然界寻找超重元素的历程,评述了利用人工核反应合成超铀元素和探索理论预言的超重核稳定岛的进展、成果、现状以及目前面临的困难,展望了未来的研究工作,简介了基于国家重大科技基础设施——强流重离子加速器装置开展超重研究的计划。     

Search for the metastable 2s state in muonic hydrogen

Results of an experiment to search for the metastable 2s state in muonic hydrogen at 150 and 600 Torr are presented. About 6% of all ωp formed go to the 2s state. Most of these (ωp)_{2 s} decay in less than 100 nsec; assuming a calculated lifetime of 2 ωsec at 150 Torr (0.5 ωsec at 600 Torr) for the long-lived fraction f of all 2s states, we find f to be less than 4% (16%) with 90% confidence level.     

Search for low-lying resonances in electron scattering by atomic hydrogen

Measurements of the transmission of an energy-selected electron beam through atomic hydrogen fail to indicate any resonance below 9·5-eV electron energy. The negative result contradicts recent suggestions by Rudkjøbing and by Van Rensbergenthat some diffuse interstellar absorption bands might be attributable to preionizing transitions in the hydrogen negative ion. A bulk of reliable theoretical data, however, justifies our finding.     

Search for superheavy hydrogen isotopes <Superscript>6</Superscript>H and <Superscript>7</Superscript>H in stopped π-absorption reactions

Experimental search for superheavy hydrogen isotopes ⁶H and ⁷H was performed in the stopped pion absorption on ⁹Be and ¹¹B nuclei. The structures in the missing-mass spectra were observed in the reaction channels ⁹Be(π^-, pd )X and ¹¹B(π^-, p⁴He)X . Four states of ⁶H were proposed. Evidences for ⁷H formation were obtained in the reaction channels ⁹Be(π^-, pp)X and ¹¹B(π^-, p³He)X .     

Theory of fusion for superheavy elements

A new model is proposed for fusion mechanisms of massive nuclear systems, where so-called fusion hindrance exists. The model describes the whole process in two steps: two-body collision processes in an approaching phase and shape evolutions of an amalgamated system into the compound nucleus formation. It is applied to ⁴⁸Ca-induced reactions and is found to reproduce the experimental fusion cross sections extremely well, without any free parameter. A schematic case is solved in an analytic way, the results of which shed light on fusion mechanisms. Combined with statistical decay theory, residue cross sections for superheavy elements can be readily calculated. Examples are given.     

Search for superheavy hydrogen isotopes <Superscript>4,5</Superscript>H in reactions of pion absorption by <Superscript>10,11</Superscript>B nuclei

The results of searching for production of superheavy hydrogen isotopes ^4,5H in reactions of absorption of stopped π^? mesons by ^10,11B nuclei are reported. A peak near 3 MeV was observed in the missing mass spectra measured in the reactions ^10,11B(π^?, t⁴He)X, ¹⁰B(π^?, d⁴He)X, and ¹⁰B(π^?, t³He)X. A structure caused by two ⁵H states with the resonance energies E _R = 5.2 and 10.4 MeV was observed in the missing mass spectra measured in the reactions ¹¹B(π^?, d⁴He)X, ¹¹B(π^?, t³He)X, and ¹⁰B(π^?, d³He)X.     

Search for production of superheavy elements via “fusion after instantaneous fission” in the reaction238U+208Pb

Superheavy elements (SHE) might be formed via a reaction mechanism called “fusion after instantaneous fission”, which is supposed to occur during the collision of a deformed very heavy nucleus with a spherical one. We bombarded natural uranium targets with lead ions and searched for alpha-emitting and spontaneously fissioning reaction products. Different techniques were used: a rotating wheel, a gas jet system and a radiochemical procedure. No SHE have been found. The upper cross section limits are between 10^?33 cm² and 10^?34 cm² within the half-life range from 1 ms to 1 year. In addition, the production cross sections of some Cm and Cf isotopes and of Am fission isomers were determined.