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.     

On search and identification of relatively short-lived superheavy nuclei (<Emphasis Type="Italic">Z</Emphasis> ≥ 110) by fossil track studies of meteoritic and lunar olivine crystals

The main goal of the present work is the search for and identification of relatively stable nuclei of superheavy elements (SHE) (Z>110) in galactic matter by fossil track study of nonconducting crystals from the surface of meteorites and rocks from the lunar regolith. Nuclei of SHE are thought to be the products of nucleosynthesis in explosive processes in our Galaxy (supernova r-process nucleosynthesis and, especially, neutron-star formation, etc.). When accelerated to relativistic energies in the Galaxy, they can produce extended trails of damage in nonconducting extraterrestrial crystals. The lifetime of such SHE in galactic cosmic rays will range from 10³ to 10⁷ yr to be registered in extraterrestrial crystals. To search for and to identify the superheavy nuclei in the galactic cosmic rays, it was proposed to use the ability of nonconducting extraterrestrial crystals such as olivines, pyroxenes, and feldspars to detect and to store for many millions of years the trails of damage produced by fast Z≥23 nuclei coming to rest in the crystalline lattice. The track lengths of fast Z≥23 nuclei are directly proportional to Z² of these nuclei. The nuclei of SHE produce, when coming to rest in a crystal volume, tracks that are a factor of 1.6–1.8 longer than the tracks due to cosmic-ray Th and U nuclei. To identify the tracks due to superheavy nuclei, calibrations of the same crystals were performed with accelerated Au, Pb, and U nuclei. For visualization of these tracks inside the crystal volume, proper controlled annealing and chemical etching procedures were developed. Since 1980, fossil tracks due to Th and U nuclei have observed and unambiguously identified (1988) by subsequent calibrations of the olivine crystals with accelerated U, Au, and Pb ions. The number of tracks of Th and U nuclei measured in olivine crystals totaled more than 1600, as compared with the prior 30 events. The other approach to identifying SHE in nature is to search for tracks in phosphate crystals from spontaneous fission of Z ≥ 110 nuclei; these produce two-prong and three-prong fission fragment tracks and differ significantly from the tracks from spontaneous fission of ²³⁸U and ²⁴⁴Pu nuclei. Extraterrestrial phosphate crystals of lunar and meteoritic origin will be investigated. Such SHE nuclei can survive in crystals of extraterrestrial rocks and produce spontaneous fission tracks, if the lifetime is more than 5×10⁷ yr.     

On search and identification of tracks due to short-lived SHE nuclei in extraterrestrial crystals

The unique approach for search and unambiguous identification of short-lived (T_1/2=10³–10⁷ years) superheavy nuclei in cosmic-ray products of the recent nucleosynthesis in our Galaxy are discussed.It is based on: (a) the ability of non-conducting crystals to register and to store for many million years the tracks due to fast nuclei with atomic number Z20 (“fossil” tracks);(b) calibrations of the said crystals with accelerated heavy ions (20Z92) and on revealing the volume etchable track length (VETL) of the fast nuclei coming to rest inside crystals—both of fossil and “fresh” tracks—to determine the charge distribution of cosmic-ray nuclei tracks and(c) the so-called “four-zone” model of tracks in crystals (and also glasses) which provides not only the VETL track length dependence for 20Z92 nuclei but also demonstrates the regular annealing behavior of VETL of 20Z92 nuclei in a broad temperature interval.This approach was first applied in the early 1980s to investigate the “fossil” tracks due to 22Z92 cosmic-ray nuclei in olivine crystals from meteorites-pallasite Marjalahti and Eagle Station.The discovery of Th–U cosmic-ray nuclei tracks in 1980 was unambiguously confirmed by calibrations of the same crystals with ²³⁸U, ¹⁹⁷Au and ²⁰⁸Pb accelerated ions in the late 1980s. More than 1600 tracks due to cosmic-ray actinide nuclei were measured during the last two decades of the 20th century.Also, 11 anomalously long tracks (track length exceeds by a factor (1.6±0.1) the track length due to Th–U nuclei were measured. The detailed analysis shows that at least 5 of these tracks could not be attributed to the Th–U nuclei. It means that now we have a preliminary proof on the existence Z110 nuclei in cosmic-rays. The abundance is Z110/Th–U=(1–3)×10⁻³ in Z110 freshly formed cosmic-rays (time interval 10³–10⁷ years).The method proposed can provide the necessary and sufficient conditions for the discovery of Z110 nuclei in nature.     

Mechanism of light fragment production in the interaction of 8.8 GeV 4He with 208Pb

The events with at least three fragments with Z>2 from the interaction of 8.8 GeV ⁴He with ²⁰⁸Pb nuclei have been examined using a CR-39 plastic track detector. The analysis shows that the majority of these events is produced in central collisions. But, it is also observed that no negligible part of them originates from peripheral collisions and has one heavy and two light fragments (2<Z_L<20). This indicates that various mechanisms contribute to the production of light fragments in high energy nuclear interactions.     

Alpha decay chains from superheavy nuclei

Magic islands for extra-stable nuclei in the midst of the sea of fission-instability were predicted to be around Z=114, 124 or, 126 with N=184, and Z=120, with N=172. Whether these fission-survived superheavy nuclei with high Z and N would live long enough for detection or, undergo α-decay in a very short time, remains an open question. α-decay half lives of nuclei with 130≥Z≥100 have been calculated in a WKB framework using density-dependent M3Y interaction with Q-values from different mass formulae. The results are in excellent agreement with the experimental data. Fission survived Sg nuclei with Z=106, N=162 is predicted to have the highest α-decay half life (∼3.2 h) in the Z=106-108, N=160-164 region called small island/peninsula. Superheavy nuclei with Z>118 are found to have α-decay half lives of the order of microseconds or less.     

A search for superheavy nuclei tracks in extraterrestrial olivine crystals

A study is made for the search of superheavy nuclei in Marjalahti, Eagle Station and in other pallasite olivines. The olivine crystals are calibrated for heavy ion track lengths by using heavy ion beams from cyclotrons. The calibration for ultra heavy ions which are presently not available with sufficient energy to produce volume tracks in olivine crystals, is based on Katz and Kobetich model of track formation. The length spectrum of volume tracks, revealed by puncturing them with focussed Nd-glass laser beam, is measured and the abundances of different nuclei groups are calculated. Partial annealing has been used at 430°C for 32 hr which eliminates the interfering tracks due to nuclei of atomic numberS ≤ 50. During the scanning 4 cm³ olivine crystals, about 360 long tracks of uranium group as well as two very long tracks have been found. If these tracks belong to superheavy nuclei, the relative abundance of super heavies is found to be 6 × 10⁻¹¹ in galactic cosmic rays.     

Attenuation of cosmic ray heavy nuclei fluxes in the upper atmosphere by fragmentation

Two plastic stacks containing cellulose nitrate as detector material were flown several times with balloons at 40 g cm^?2 for a total time of 25 hours. From the tracks of 920 heavy nuclei the vertical intensities of the particles of four charge groups (6 ? Z ? 9, 10 ? Z ? 14, 15 ? Z ? 19, 20 ? Z ? 28) have been derived for atmospheric depths from 40 – 100 g cm^?2. Combining these intensities with other results obtained with emulsions and in counter measurements, the fragmentation probabilities in air have been determined by comparison with the primary spectra.     

Static electric quadrupole and hexadecapole moments of nuclei

Ground-state static electric quadrupole and hexadecapole moments of even nuclei are calculated microscopically. Rare-earth, actinide and neutron-deficient nuclei with 50 <Z, N < 82 and neutron-rich nuclei with 28 < Z < 50, 50 < N < 82 are studied. Fairly good agreement with experiment is found. In the actinide region, the electric moments of fission isomers are also examined.     

Argon L x-ray spectra from collisions of argon ions with various solid state targets

Spectral measurements have been made of Ar-L-shell X-rays produced by Ar ions incident on various solid state targets. Different spectra have been observed for targets with Z < 18 and Z > 18 respectively. The data are discussed within the framework of the electron promotion model of Barat and Lichten.     

Pion photoproduction and T> versus < analog states in N>Z nuclei

The T_> and T_< components of giant resonance levels in N>Z nuclei cannot be unambiguously identified in photonuclear reactions. In charged pion photoproduction, their analogs may be so identified (at least for the spinflip levels), as illustrated here with a ¹³C target.     

Excitation of the T> components of the giant dipole states in N > Z nuclei by muon capture

Muon capture on N > Z nuclei is proposed as a means of studying the T_> isospin component of giant dipole states. Calculations for nickel isotopes (except ⁵⁶Ni) indicate that these states are strongly excited by muon capture. Possible experiments to detect these states are discussed.     

Intermediate mass fragments emitted in the reaction Ag+14N at 100, 160 and 250 MeV bombarding energy

The fragments produced in the reaction between a ¹⁴N beam of various energies and a natural Ag target have been studied. The atomic numbers of the fragments have been identified up to Z = 17 by means of a E-ΔE counter telescope. The cross sections, the kinetic energy distributions as well as the angular distributions have been measured for each atomic number. The kinetic energy distributions show two components: a high-energy component (quasi-elastic), prevailing at angles close to the grazing angles and for atomic numbers close to Z = 7, and a low-energy component (relaxed), at energies close to the Coulomb repulsion energy, present at all angles and for all the Z. A detailed study of the relaxed components of the kinetic energy seems to account for both them means and the widths of these distributions on a purely statistical basis. The cross sections of the relaxed components appear to be quite large at low Z and to decrease rapidly to a fairly constant value in the region of 10 ≦ Z ≦ 17. A marked even-odd alternation in the cross sections is observed. The angular distributions are strongly forward peaked for Z < 7. For Z > 7 the forward peaking decreases rapidly until, for Z > 13, the limiting form 1/sinθ is attained. Evidence for the existence of a diffusion process along the mass asymmetry coordinate is discussed.     

Search for the “stability island” of superheavy nuclei using natural track detectors

The method of using natural track detectors, i.e., meteorite olivine crystals, is developed and improved applied to the problem of searching for superheavy nuclei in nature, in galactic cosmic rays (GCR). The new technique implements the sequence of etching, grinding, and track identification operations using the automated PAVICOM facility. The data on the track length and etching rate in combination with the results of calibration on heavy nucleus accelerators allowed the development of a technique for determining the GCR nucleus charge with an accuracy of ±2. On this basis, a significant set of experimental data on superheavy nuclei of natural origin was obtained (21743GCRheavy nucleiwithZ >20, including three nucleiwith a charge of 119_?6⁺¹⁰). The minimum lifetime T_min of the last-mentioned is within 50 years< T_min < 100 years, which exceeds the lifetime of transfermium nuclei synthesized on accelerators by many orders of magnitude. The long-lived superheavy nuclei detected in the GCR spectrum can belong to the “stability island”.     

Simulation of passage of gallactic cosmic ray nuclei in meteorite-pallasite substance

The results are presented of model calculation of the interaction and yield of galactic-cosmic-ray superheavy-element nuclear fragmentation products from the 50 < Z < 92 range of their charges as they pass through a certain meteorite-pallasite substance thickness.     

A new interpretation of an event attributed to a magnetic monopole

An alternate and a new interpretation is given for the event attributed to a magnetic monopole by Price and coworkers found in an emulsion plastic sandwich stack flown from Sioux City, Iowa, USA on 18 September 1973. The electron pick-up and stripping cross-sections of nuclei ofZ∼70–80 andv∼0.6–0.7c in Lexan polycarbonate are calculated using the formulae given by Nikolaev. It is shown that the corresponding mean free paths are of the order of thickness (∼250μ) of Lexan plastic sheets used by them. In such a case asnapshot of these processes is believed to have been observed in plastic sheets. Monte-Carlo simulations of the event have been made for three values of charges at the top of the main Lexan stack, namelyZ=83, 78 and 70 respectively. The event is thus interpreted as a cosmic ray nucleus ofZ=70–83 andv=0.6–0.7c losing and capturing electrons (mainly the latter) as it passes through the stack. The probability of the occurrence of such an event is estimated by several methods.     

NMR study of KCuF3 at low temperatures

The NMR of F¹⁹ nuclei in KCuF₃ has been measured in the a-type single crystal at 1.7 K. Two types of magnetic domains exist; one occupies most part of the crystal and has easy axis along <110>, and the other occupies the rest of the crystal and has easy axis along <100>. In both domains the moment directions are distributed around the easy axes over a considerable angular range. The spin-flop begins with nearly zero applied field in the <110>;-domain. When the magnetic field is rotated in the c-plane, an angular dependence has been observed for those F¹⁹ nuclei which lie on the c-axis. This dependence arises from the alternate stacking of the ground state wave functions of Cu²⁺ ions.     

Growth of iron single crystals in the etched ion tracks of polymer foils

Pulse reverse electrolysis in an ultrasonic field is used to grow iron single crystals of micron size in templates formed by etching the tracks of swift ions in polymer foils. High-grade crystals are produced from high-temperature ferrous chloride baths. The crystals are oriented along their <110>, <100>, and <111> crystallographic axes. Their orientation turns out to depend on supersaturation during the growing process. At low overvoltages of deposition, <110> and <100> orientations are observed. The crystals of <111> orientation appear more frequently at higher cathode pulse current density. The crystals possess prominent resistance to corrosion. Received: 20 February 2001 / Accepted: 21 February 2001 / Published online: 3 May 2001     

Excitation of the giant dipole resonance in the scattering of isoscalar projectiles

The excitation of the giant dipole resonance in nuclei with N > Z by isoscalar projectiles α and d is discussed within a simple collective model for isoscalar dipole excitations. Calculations have been performed for ²⁰⁸Pb; they are compared to recent data on the excitation of the new giant resonance at E_x = 13.8 MeV. For α scattering the effect of dipole excitation is quite weak but significant contributions are obtained for d scattering.     

Collective plasma corrections to thermonuclear reaction rates in dense plasmas

General kinetic equations are derived for nuclear reactions in dense plasmas by taking into account first-order collective plasma effects. We show that, apart from the corrections proportional to the product of the charges Z _i and Z _j of two reacting nuclei i and j, new corrections comparable in magnitude and proportional to the squares of the nuclear charges Z _i ² and Z _j ² arise. The Salpeter corrections [1] to the nuclear reaction probabilities due to the plasma screening of the interaction potential are shown to be at least a factor of r/d (r is the nuclear size and d is the Debye screening length) smaller than those assumed previously. These are zero in the approximation where the terms of order r/d are disregarded. The correlation corrections proportional to Z _iZ_j have a different physical meaning than those in [1], can have a different sign, and arise for reactions with zero Salpeter corrections. For the correlation corrections that substitute for the previously used Salpeter corrections, strong correlations are difficult to describe analytically. The interpolation formulas between weak and strong Salpeter screenings previously used in many astrophysical applications are inapplicable, because the interpolation formulas between weak and strong correlations cannot yet be obtained. We found a new type of corrections that are proportional to the squares of the charges of reacting nuclei. These are attributable to a change in the collective electrostatic self-energy of the plasma system during nuclear reactions. Plasma corrections for the hydrogen-cycle nuclear reactions are numerically calculated for the temperature, density, and abundances in the solar interior.     

Laser spectroscopy measurements of neutron-rich tellurium isotopes by COMPLIS

Laser spectroscopy based on resonant ionization of laser-desorbed atoms has been used to study the neutron-rich tellurium isotopes with the COMPLIS facility at ISOLDE-CERN. Isotope shifts and hyperfine structures of several neutron-rich Te isotopes: ^120?136Te and ^{123 m?133 m}Te have been measured. From the hyperfine structure we have extracted magnetic and quadrupole moments. Changes in the mean square charge radii have been deduced and their comparison with the known data for the other elements near Z?=?50 is presented. The experimental δ?<?r ²> values are compared with those obtained from relativistic mean field calculations.