Fossil track studies of the Omolon pallasite crystals

The olivine and stanfildite crystals from Omolon pallasite meteorite were selected from some residue and from locations of an about 150 g fragment of that meteorite in the Meteoritic Committee, Moscow, Russia. The radiation age of Amnion pallasite was determined to be about 78 MY The Olivine crystals were mounted in epoxy resin, polished and etched in Krishnaswami solution, modified by adding 30 g per liter oxalic acid. The etching of the olivine crystals was performed in hermetically closed vessels during 36–72 hours at T = 100 ° C. The measured Fe- group track density varies from 3 × 10⁴ up to 2.6 × 10⁵ track. cm⁻². The measure in 12 Fe-group track rich olivine 43 track due to VVH nuclei were found. It means that the depth of ablation of Omolon meteorite does not exceed (8.2 ± 2.1) cm out of preatmospheric surface for the most favourable locations.     

Davydov splitting and two-photon excitation of cadmium tungstate (CdWO4) single crystal phosphorescence

Phosphorescence characteristics of CdWO₄ excited by one-photon (λ = 308 nm) and two-photon (λ = 570–590 nm) processes were measured. A Davydov splitting of 120 ± 20 cm⁻¹ was obtained in the phosphorescence spectra, suggesting a diffusion coefficient of about 1.2 × 10⁻² cm² s⁻¹, and a diffusion length of about 3.1 × 10⁻⁴ cm for the room temperature measured lifetime of 8μs. The phosphorescence quantum efficiency was less than 2% at low temperatures (only 0.25% at room temperature), indicating that the dominant decay mechanism was radiationless. The radiative lifetime was thus estimated as 1–2 ms. The two-photon phosphorescence excitation is characterized by an absorption cross-section of the order of 10⁻⁴⁹cm⁴s.     

Escape of charged particles at stopping annihilation of antiprotons in the heavy nuclei of nuclear photoemulsion

“Rare” annihilation channels for antiprotons stopping on heavy (Ag, Br) nuclei of photoemulsion, have been sought; 4872 stops of antiprotons on photoemulsion nuclei are analysed. Events of formation and decay of the hyperfragment _Λ⁴H, escape of ⁸He and ⁸Li nuclei, one-prong stars with the mean range 79.5±5.1 μm of secondary slow “b” particles are found among the annihilation stars at capture on nuclei (Ag, Br). The lower limits for the production probability of _Λ⁴H and ⁸He, ⁸Li nuclei per antiproton stopping in the nuclei (Ag, Br) are

W_Λ⁴H2×10⁻⁴ and W_⁸He,⁸Li=(1.3±0.6)×10⁻³.

The branching ratio for the production of one-prong stars with the secondary “b” particles is at least (1.3±0.6)×10⁻³. Possible mechanisms for a production of these events in annihilation processes are considered.     

Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Domain wall pinning in polycrystalline perovskite La0.7Sr0.3MnO3

Reversible and irreversible domain wall (DW) motions have been investigated in La_0.7Sr_0.3MnO₃ ceramic samples using frequency-response complex permeability with various amplitudes of AC field. We also examine the effects of temperature in the range from 293 to 368 K and transverse DC magnetic field with a maximum of 4.40×10⁵ A/m on the real part of permeability (μ′). Two relaxations corresponding to reversible wall motions and domain rotations occur in low and high frequency regions, respectively. The irreversible DW displacements can be activated as the amplitude larger than the pinning field of 3 A/m, leading to an increase in μ′. The μ′ obeys a Rayleigh law at the temperature below 343 K or under DC field of less than 4.22×10⁴ A/m. The Rayleigh constant η increases from 5.45×10⁻² to 1.54×10⁻¹ (A/m)⁻¹ as the temperature rises from 293 to 343 K, and η decreases from 5.58×10⁻² to 3.67×10⁻² (A/m)⁻¹ with increasing DC field from 1.99×10³ to 4.22×10⁴ A/m.     

Fission-track analysis of meteorites: Dating of the Marjalahti pallasite

The results of the Marjalahti pallasite fission-track age determination are presented. Thorough examination of fossil tracks in the phosphate (whitlockite) crystals coupled with U-content determination in whitlockites can make it possible to estimate the contributions of all possible track sources to the total track density and to calculate a model fission-track age.

It is found that whitlockite crystals of the Marjalahti pallasite contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); fission of U and Th induced by cosmic rays; spontaneous fission of ²³⁸U; and spontaneous fission of extinct, short-lived ²⁴⁴Pu present in significant quantities in the early solar system.

A great track density attributed to the extinct ²⁴⁴Pu testifies to the high fission-track age. The model fission-track ages of (4.31±0.02)×10⁹ yr for the Marjalahti pallasite are calculated. Petrographic studies allow us to interpret the fission-track age as the time of the last shock/thermal event in the cosmic history of the pallasite.     

Chemically deposited copper oxide thin films: structural, optical and electrical characteristics

Thin films of copper oxide with thickness ranging from 0.05–0.45 μm were deposited on microscope glass slides by successively dipping them for 20 s each in a solution of 1 M NaOH and then in a solution of copper complex. Temperature of the NaOH solution was varied from 50–90°C, while that of the copper solution was maintained at room temperature. X-ray diffraction patterns showed that the films, as prepared, are of cuprite structure with composition Cu₂O. Annealing the films in air at 350°C converts these films to CuO. This conversion is accompanied by a shift in the optical band gap from 2.1 eV (direct) to 1.75 eV (direct). The films show p-type conductivity, 5×10⁻⁴ Ω⁻¹ cm⁻¹ for a film of thickness 0.15 μm. Electrical conductivity of this film increases by a factor of 3 when illuminated with 1 kW m⁻² tungsten halogen radiation. Annealing in a nitrogen atmosphere at temperatures up to 400°C does not change the composition of the films. However, the conductivity in the dark as well as the photoconductivity of the film increases by an order of magnitude. The electrical conductivity of the CuO thin films produced by air annealing at 400°C, is high, 7×10⁻³ Ω⁻¹ cm⁻¹. These films are also photoconductive.     

Cosmic history of some pallasites based on fossil track studies

Investigation of fossil charged-particle tracks in various mineral phases of extraterrestrial samples is a powerful method for research the early stages of the solar system. Examination of fossil charged-particle tracks in the phosphate and olivine crystals of 5 pallasites coupled with U content determination allowed the estimation of the contributions of all possible track sources to the total track density and calculation of a value of model fission-track age.

Phosphate crystals from all studied pallasites were established to contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); induced fission of U and Th by cosmic rays; spontaneous fission of ²³⁸U; spontaneous fission of extinct short-lived ²⁴⁴Pu nuclei presented in significant quantities in the early solar system. The discovery of a high track density attributed to the extinct ²⁴⁴Pu pointed obviously to the high value of the fission-track age. Model fission-track ages of (4.37±0.02) Gyr for the Marjalahti pallasite; (4.37±0.01) Gyr for the Omolon pallasite; (4.19±0.02) Gyr for the Bragin pallasite; (4.18±0.03) Gyr for the Krasnoyarsk pallasite; (4.21±0.02) Gyr for the Brenham pallasite were calculated. The comparison of represented data with petrographic analyses allowed us to interpret a value of fission-track age as the time of the last intensive shock/thermal event in the cosmic history of a pallasite.     

Ionic conductivity of crystalline and glassy Mg4.5Na7(P2O7)4

In our work single crystals of Mg_4.5Na₇(P₂O₇)₄ were prepared, pulverized, pressed into pellets and sintered in order to measure the electrical conductivity of polycrystalline specimens. The conductivity was also measured on glassy specimens obtained by the melting of previously prepared crystals. The electrical conductivities at 25°C with values of the order of 10⁻¹⁶ Ω⁻¹ cm⁻¹ for polycrystalline samples and a value of the order of 10⁻¹⁴ Ω⁻¹ cm⁻¹ for glass, show that the glassy phase of Mg_4.5Na₇(P₂ because of its greater molar volume and loosely packed structure, is a better matrix for ionic motion.     

Optical absorption of isotopically enriched Li2B4O7 single crystals irradiated by thermal neutrons

Induced absorption spectra in the range 200–900 nm at 77 and 290 K for Li₂B₄O₇ single crystals, isotopically Li and B enriched are presented after irradiation of these crystals by thermal neutrons with fluence 1.8×10¹⁶ cm⁻². The dependence of induced absorption spectra on the isotope composition was revealed: for ⁶Li₂¹⁰B₄O₇ and ⁷Li₂¹⁰B₄O₇ crystals intensive band in the region of 280–294 nm was observed. Under substitution of ⁷Li isotope by ⁶Li in the lithium tetraborate lattice no changes in the absorption spectra were observed. The nuclear reaction ¹⁰B(n,)⁷Li is proposed to be the main mechanism of formation of the radiation defects.     

On the luminescence of LiBaF3:Eu

From the temperature dependence of the line—band luminescence intensity ratio of LiBaF₃:Eu²⁺ a 4f−5d activation energy (Δ) of 800 cm⁻¹ is derived, being much higher than the value reported in the literature (100 cm⁻¹). The temperature dependence of the luminescence decay can be well described with Δ = 800 cm⁻¹ and with 4f−4f and 5d−4f radiative probabilities of 4×10²s⁻¹ and 6×10⁵s⁻¹, respectively.     

Conductivity anisotropy in directionally solidified CaZrO3---CaSZ and MgO-MgSZ eutectics

The results of the impedance spectroscopy measurements on eutectic samples based on zirconium oxide are presented here. Samples of CaZrO₃---ZrO₂(cubic) and MgO---ZrO₂(cubic) have been grown by a directional solidification procedure such that the different phases appear nearly oriented along the growth direction (lamellae in the system of CaZrO₃-ZrO₂(cubic) and fibers of MgO in a ZrO₂ matrix in the other system). The DC electrical conductivity has been measured by impedance spectroscopy along and across the growth axis. For CaZrO₃---ZrO₂ the coductivity is clearly anisotropic. The following values for σT have been obtained: the conductivity at 600 °C equals 2.0 × 10⁻⁶ Ω⁻¹ cm⁻¹ perpendicular to the fiber axis and 1.4 × 10⁻⁵ Ω⁻¹ cm⁻¹ parallel to it and with an activation energy of 1.3 eV for σT. For MgO---ZrO₂(cubic) the isotropic value of the conductivity at 600 °C is 10⁻⁴ Ω⁻¹ cm⁻¹ with activation energy for σT of 1.5 eV. The anisotropic conductivity in the CaZrO₃---ZrO₂ (cubic) system has been explained by a model of an ordered stacking of oxygen conducting (cubic ZrO₂) and non-conducting (CaZrO₃ or MgO) phases.     

A new measurement of the weak magnetism term in the betaspectra of B and N

The betaspectra of ¹²B and ¹²N have been measured with a NaI crystal as spectrometer. Assuming a shape correction factor 1 + aW + bW² and b₋ = 1.106 × 10⁻⁴ MeV⁻², b₊ = −1.397 × 10⁻⁴ MeV⁻², the spectra yield a₋ = (+0.91 ± 0.11) × 10⁻² MeV⁻¹ and a₊ = (−0.07 ± 0.09) × 10⁻² MeV⁻. the a₋ − a₊ = (+0.98 ± 0.09) × 10⁻² MeV⁻¹ is in agreement with the weak magnetism prediction.     

Poly(ethylene oxide-co-epichlorohydrin)/NaI: a promising polymer electrolyte for photoelectrochemical cells

We have investigated the thermal and ionic conductivity properties of the elastomer poly(ethylene oxide-co-epichlorohydrin) filled with NaI and I₂. The reason for using this composition is its potential application as electrolyte in photoelectrochemical cells. This copolymer was characterized as a function of NaI concentration, temperature and relative humidity. According to the data obtained, the Na⁺ ion interacts with the ethylene oxide repeating units by means of Lewis type acid–base interactions. The empirical Vogel–Tamman–Fulcher equation was used to model the conductivity and temperature relationships, indicating that the conduction occurs in the amorphous phase of the copolymer. The sample with 9.0% (w/w) of NaI presents a conductivity of 1.5×10⁻⁵ S cm⁻¹ in a dry atmosphere (30°C, [H₂O]<1 ppm) and 2.0×10⁻⁴ S cm⁻¹ at 86% relative humidity (22°C).     

Conductivity of two-dimensional crystals over liquid helium for different holding fields

The results are presented of the experimental study of electron crystals over liquid helium with surface electron density of 3.2×10⁸ and 6.4×10⁸ cm⁻² (melting temperatures 0.4 and 0.58 K) at temperature 83 mK and for holding electric fields 300–1200 V/cm. The measurements are performed in the frequency range 1–14 MHz where the coupled phonon–ripplon resonances are observed in the experimental cell. The real and imaginary parts of the complex conductivity of the crystal have been obtained as a result of analysis of an electron layer response on exciting ac voltage with frequency corresponding to the mode (0,1) of the coupled phonon–ripplon oscillations. An analysis of the results allows us to suppose that structural defects of the electron crystal play an essential role in dissipation processes.     

Search for parity violation in the 1081 keV gamma transition of F

The weak neutral current results in a parity-violating one-pion-exchange potential in nuclei. A search for this force has been carried out with the 1081 keV γ-radiation of ¹⁸F, which is expected to be circularly polarized. Excited ¹⁸F nuclei were produced with the ¹⁶O(³He, p)¹⁸F reaction and the γ-ray polarization was measured with a multiple transmission polarimeter system. The experimental result P = (− 1.0 ± 1.8) × 10⁻³ is smaller than recent theoretical estimates.     

Observation of two-photon emission in nuclear pion capture

The emission of two photons above 25 MeV energy was observed in the capture of stopped pions by beryllium and carbon with a rate of (1.0 ± 0.1) × 10⁻⁵, respectively (1.4 ± 0.2) × 10⁻⁵ per capture. These rates are about two to four times greater than various free-nucleon estimates based on the π⁻+π⁺→γγ annihilation mechanism alone.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

Conversion of VUV to UV and visible in K5Li2LnF10 containing rare-earth from cerium group (Ln=La, Ce, Pr, Nd)

The potential of single crystals K₅Li₂LnF₁₀, for applications as VUV excited phosphors has been examined. The crystals were doped with lanthanide ions (Ce³⁺–Nd³⁺) with concentrations up to 100 at%. The self-quenching of luminescence is strongly reduced in this material. Luminescence spectra in the 50,000–10,000 cm⁻¹ range and excitation spectra in the 40,000–250,000 cm⁻¹ spectral range have been recorded at room and low temperature. Intense visible emission has been observed for Nd³⁺ and Pr³⁺. For the Pr³⁺ ions strong the ¹S₀–¹I₆ transition has been recorded only for concentrated crystals. Efficient conversion of VUV excitation to UV–VIS emission was observed in crystals doped with 3 and 100 at% of cerium. In both cerium-activated crystals the lifetimes of f–d transitions were equal to 32 ns.     

Search for GUT magnetic monopoles and nuclearites with the MACRO experiment

We present the final results obtained by the MACRO experiment in the search for GUT magnetic monopoles and nuclearites. Several searches were performed with different subdetectors, i.e. scintillation counters, limited streamer tubes and nuclear track detectors. No magnetic monopole or nuclearite candidates were found. The MACRO upper limit to the local flux of GUT magnetic monopoles is at the level of 1.4×10⁻¹⁶ cm⁻² s⁻¹ sr⁻¹.