X-rays from antiprotonic3He and4He

Antiprotonic X-rays from the helium isotopes have been observed at pressures of 36, 72, 375 and 600 mbar. The antiproton beam from LEAR with momenta of 309 and 202 MeV/c has been stopped at these pressures using the cyclotron trap. The X-rays were detected with Si (Li) and intrinsic Ge semiconductor detectors. Absolute X-ray yields were determined and the strong-interaction 2p shifts and the 2p and 3d broadenings measured to be _2p=(–17±4) eV, _2p=(25±9) eV and _3d=(2.14 ±0.18) meV for ¯p³He and _2p=(-18±2) eV, _2p =(45±5) eV and _3d=(2.36±0.10) meV for ¯p⁴He.The efforts of the LEAR staff and the help of P. Gauss from the CERN Cryogenic Group as well as the technical assistance of M. Dröge and M. Stoll are gratefully acknowledged. This work is part of the Ph.D. of one of us (M.S.), University of Karlsruhe (1987), KfK report no. 4222.     

Non-collective oblate states in121Te

Two new neutron-deficient isotopes,²¹³Pa and²¹⁴Pa were produced in complete fusion reactions of⁵¹V-ions with¹⁷⁰Er targets at (5.2–5.6) AMeV. The assignment was based on delayed evaporation residue - - time and position coincidences. The- decay energies of^213,214Pa were measured to be E=(8236±20) keV and E=(8116±20) keV, respectively. The half-lives of^213,214Pa were determined to be T_1/2=(5.3 _–1.6 ^+4.0 ms and T_1/2=(17 ±3) ms, respectively.This article was processed by the author using the LAT_EX style filecljour2 from Springer-Verlag.     

Parity violation in the 17/2−–17/2+ doublet in93Tc

The parity-violating mixing of the 17/2^– and 17/2⁺ levels in⁹³Tc nuclei, polarized by the tilted multifoil interaction, was measured by the observation of the forward-backward-ray asymmetry. The nuclear polarization, induced by the tilted multifoils, was measured directly for the neighboring^88,90Zr isomers. The forward to backward asymmetry was determined to be A=(2.5±2.1) 10^–3 which implies a parity violating matrix element ¦H _PV )¦=(4.0±3.7)meV.     

Diffusion of Al implanted into α-Hf studied by means of the nuclear resonance technique

The diffusion of Al in -Hf was studied in the 973–1298 K temperature range by using the nuclear resonance technique. The measurements show that the diffusion coefficient follows an Arrhenius behavior D(T)=D₀exp(-Q/RT), where D₀=(3±2)×10^-4 m²s^-l and Q=337±10 kJ/mol. The activation energy is typical of a substitutional mechanism of diffusion, as can be deduced from self-diffusion measurements. On the other hand, differences between the present and previous published results can be attributed to the different purity of the samples used in each experiment. PACS 66.30.-h; 66.30.Jt     

Internal conversion coefficient for the 165.8 keV transition in139La

The internal conversion process of the 165.8 keV-ray transition in the decay of¹³⁹Ce has been reinvestigated. The emission rate of the K-shell internal conversion electrons was determined by an (electron)-(X-ray)-(X-ray) triple coincidence experiment using a magnetic-spectrometer and two Si(Li) detectors. From electron spectra recorded as a function of momentum conversion electron ratios were also determined. The disintegration rate of the sources has been taken from 4- coincidence experiments. The following results were obtained: the probability for internal conversion in theK-shellk _K =0.1719±0.0020 and the conversion ratiosK/(L + M+...=5.84±0.17,K/L=7.45±0.20, andL/(M+...)=3.63±0.29. The internal conversion coefficients deduced area=(0.2520+0.0050), _K =(0.2152±0.0033), and _L =(0.0289±0.0012). The results are in good agreement with previous experimental values and with theoretical data calculated for a M1 transition of 165.8 keV in¹³⁹La.     

NMR self-diffusion study of organic glasses: COANP,MBANP, PNP,NPP

A pulsed field gradient proton spin-echo NMR self-diffusion study of organic glasses COANP, MBANP, PNP and NPP in their liquid and weakly supercooled states was performed. The NMR phase diagrams, based on the proton NMR transverse relaxation time (T ₂) temperature hysteresis data of these materials, clearly give evidence of the onset of a glass phase on cooling the isotropic liquids below their respective melting temperatures. The self-diffusion data exhibit in the supercooled glassy state a non-Arrhenius behaviour and can be described in terms of the Vogel-Fulcher modification of the Arrhenius law,D=D exp{–E _a /[k _B (T–T _VF )]}. The activation energiesE _a and Vogel-Fulcher temperaturesT _VF are 83.2 meV and 239 K for COANP, 66 meV and 249 K for MBANP, and 85 meV and 245 K for PNP, respectively. The flow viscosity data obtained for COANP in the same temperature region as well conform to the Vogel-Fulcher behaviour, exp{E _a ⁽⁾ /[k _B (T–T _VF )]}, withE _a ⁽⁾ =80.4 meV andT _VF =239 K. In case of COANPD was found to increase with decreasing diffusion time in the supercooled (glassy) melt just belowT _M whereas no such behaviour was found aboveT _M .     

Nonlinear Optical Characteristics of BSO and BGO Photorefractive Crystals in Visible and Infrared Ranges

Nonlinear refraction, nonlinear absorption and optical limiting in photorefractive crystals Bi₁₂SiO₂₀(BSO) and Bi₁₂GeO₂₀(BGO) at the wavelengths of 1064 and 532 nm were investigated. It was shown that both BSO and BGO crystals possess by positive nonlinear refraction in two investigated spectral ranges (n ₂ ^BSO=(2.5 ± 0.5)× 10^–12 esu, n ₂ ^BGO=(6.3 ± 1.3)× 10^–12 esu at equals 1064 nm; n ₂ ^BSO=(4.4 plusmn; 0.9)× 10^–12 esu, n ₂ ^BGO=(7.4 ± 1.5)× 10^–12 esu at = 532 nm). The nonlinear absorption was due to three-photon absorption at the wavelength of 1064 nm ( ⁽³⁾ _BSO=(2.5 ± 0.8)× 10^–20cm³W^–2, ⁽³⁾ _BSO=(4.4 ± 1.3)× 10^–20cm³W^–2) and two-photon absorption at the wavelength of 532 nm ( ⁽²⁾ _BSO=(2 ± 0.4)× 10^–9cm W^–1, ⁽²⁾ _BGO=(3.7 ± 0.7)× 10^–9cm W^–1).     

Comparative kinetic stability of classical and non-classical fullerenes C46

Molecular dynamics is used to study the kinetic stability of a classical fullerene C₄₆ and its non-classical analog with a square. The lifetimes of both clusters till the moments of their isomerization are directly calculated as functions of temperature. The activation energies _Ea$E_a$ of isomerization processes are determined from the fits of the results obtained to the Arrhenius law. For the non-classical fullerene, the value of _Ea=3.1±0.2 eV$E_a=3.1\pm 0.2\text{eV}$ is found to be considerably below that for the classical one, _Ea=4.5±0.3 eV$E_a=4.5\pm 0.3\text{eV}$. In view of rather low kinetic stability of non-classical C₄₆, its experimental registration in a gas phase seems to be problematic.     

Dipolar magnetic field and fermi contact field at the μ+ site in α-Fe at very low temperatures

In a longitudinal ⁺SR experiment on a high-purity-Fe single crystal sphere magnetically saturated in a 111 direction damped oscillations (wiggles) were observed in a temperature range 30 mK to 600 mK and in a certain regime of applied magnetic fieldsB _appl. Meassurements of the wiggle frequency as a function ofB _appl give us directly the Fermi fieldB _Fermi=(–1.13±0.02)T and the dipolar magnetic field ¦B _dip ¦=(0.66±0.03)T.B _dip was used to determine the prefactor in the Arrhenius law obeyed by the ⁺ hopping rate between 100 K and 1000 K. A comparision with the corresponding values for protons and deuterons suggests diffusion via the adiabatic mechanism.     

Observation of muonic HF molecule formation with TF μ− SR

Negative muon polarization has been measured in pure gaseous Ne (24 atm) and in Ne+H₂ mixtures (24 atm Ne+1.8 atm H₂, 8 atm H₂ and 16 atm H₂). The experiment was performed at JINR (Dubna) on aSR-spectrometer [1] with 200 G transverse magnetic field at room temperature. In pure Ne no polarization was observed (a ₀=0.09±0.1%) while in Ne+H₂ mixtures clear precession signals were detected at the free-muon frequency with asymmetries a_1.8=0.33±0.13%,a ₈=0.33±0.14% anda ₁₆=0.59±0.09%. The fact that polarization appears in the muonic HF molecule shows that at the moment of the molecule formation (t10^–10 s) the muon is not completely depolarized. The estimate of the reaction constantk=(2.3±1.6)×10^–11cm³ s^–1 agrees with the experimental values obtained by other methods. The result achieved demonstrates that ^– SR-method can be applied for studying fast kinetics processes in the gas phase and in particular for measuring chemical reaction rates of halogen atoms and ions.     

Diffusion of selenium and tellurium in silicon

Diffusion of selenium and tellurium in silicon has been investigated in the temperature range 1000°C to 1310°C by sheet conductivity. For SiSeD ₀= 0.3±0.1 cm²/s andh=2.6±0.1 eV, and for SiTeD ₀=0.9±0.3 cm²/s and h=3.3±0.1eV have been obtained. The surface concentrations for both dopants were of the order of 5 × 10¹³ to 6×10¹⁶cm^–3. The Hall coefficient leads to an energy level of 300±15meV for selenium and 200±20meV for tellurium.     

Precision measurement of nuclear muon capture by3He

In this article we report the results of an experiment performed in 1993 at PSI. The goal was to determine the absolute rate of nuclear muon capture by³He. In the experiment we used a new technique recently developed at Gatchina. As a preliminary result from this experiment we obtained _c=(1496±3(stat)-3(syst)) s^–1.     

Magnetic properties and antiferromagnetic Cu ordering in Pr2CuO4

Bulk-magnetisation measurements and neutron-scattering experiments were performed both on a polycrystalline sample and on a single-crystal of Pr₂CuO₄ in the temperature range from 1.5 to 300 K. Pr₂CuO₄ crystallizes with theT (Nd₂CuO₄)-type structure. We observed antiferromagnetic ordering of the Cu moments belowT _N =(190±2) K in a single crystal and belowT _N =(250±10)K in powder material. The magnetic unit cell dimensions area _m =2a _o ,c _m =c _o ;the Cu moments are oriented in the basal plane with a magnetic saturation moment of _Cu=(0.45±0.12) _B . Pr³⁺ does not order magnetically above 1.5K due to its crystal-field induced singlet ground state as verified by inelastic neutron scattering.     

Annealing behaviour of C-implanted α-Hf

The annealing behaviour of C in the group IVa metal Hf has been studied in the previously uninvestigated temperature range 650–800°C using ion-beam techniques. Diffusion couples were created by ion implantation. The time evolution of the carbon profiles were monitored by the use of the Nuclear Resonance Broadening (NRB) technique. The linear Arrhenius plot extracted from the measurements indicates that the diffusivity of implanted C in polycrystalline h.c.p. -Hf can be described by the activation energyQ=3.9±0.2 eV and the pre-exponential factor D₀=(50± ₄₇ ¹²⁰ )×10⁵ cm²/s. The implanted distributions were used to determine the solubility of C in -Hf for the first time.     

Break junction tunnelling on Bi2Sr2CaCu2O8+δ-single crystals

Break junction tunnelling experiments on Bi₂Sr₂CaCu₂O₈₊-single crystals were performed at different temperatures. Carefully adjusted samples exhibited conductance curves showing only one large peak at positive and negative bias voltages. From these measurements a gap value =(21.2±2.8) meV, i.e. 2/k _B T _c =(5.5±0.8), was inferred. The behaviour of the conductance curves is similar to that of conventional superconductors but with a much larger broadening of the tunnelling characteristic. Therefore, several models which could explain this smearing are compared with the experimental data. However, it is found that none of them is capable of fitting the conductance curves convincingly. In some cases tunnel characteristics similar to those predicted by a charging effect model or a multigap model were obtained.This work was supported financially by the Deutsche Forschungsgemeinschaft through SFB 252.     

Rate of abstraction of hydrogen atoms from ethane by muonium

Thermal reaction rates for the gas-phase reaction Mu+C₂H₆MuH+C₂H₅ have been measured bySR over the temperature range 510–730 K. The usual Arrhenius expression,k=Aexp(–E _a /RT), fits the data well, giving parametersA=1.0×10^–9 cm³ molecule^–1 s^–1 andE _a =15.35 kcal/mol. The activation energyE _a is 5.5 kcal/mol higher than for the H atom variant of this reaction, indicating a marked difference in reaction dynamics. Preliminary analysis indicates a still greater difference between Mu and H for the corresponding CH₄ reaction.     

EPR and optical study of Mn-doped bis(l-Asparaginato)Zn(II)

EPR and optical studies of single crystals of Mn²⁺: bis(l-Asparaginato)Zn(II) are reported. The spin-Hamiltonian parameters are determined employing the positions of a large number of resonance lines for various orientations of the external magnetic field. The best-fit zero-field parameters to the observed EPR spectra are obtained as, D=(228±2)×10⁻⁴ cm⁻¹, E=(58±2)×10⁻⁴ cm⁻¹ and a=(−12±1)×10⁻⁴ cm⁻¹,whereas g=2.0002±0.0002, , and . From the optical absorption study, the lattice distortion is suggested. The electron repulsion parameters (B and C) and crystal field parameters (Dq and α) evaluated from the fitting of observed optical spectra are: B=858 cm⁻¹, C=2620 cm⁻¹, Dq=950 cm⁻¹, and α=76 cm⁻¹.     

Optical,photoelectric and electric properties of single-crystalline Sb2Se3

Measurements of absorption coefficient in the region of the absorption edge, of spectral distribution of photoconductivity and dependence of electrical conductivity upon temperature on Sb₂Se₃ single crystals are given. The absorption of light was proved to correspond to indirect forbidden transitions. The value of optical gapE _g ^opt =(1·11±0·02) eV forE a andE c was determined. From photoconductivity and conductivity measurements the values of the gaps areE _g ^opt =1·11 eV andE _g ^el =1·04 eV. The anisotropy of the electrical conductivity parallel and perpendicular to the cleavage plane is 2·2.     

Search for superconducting effect on the decay of99m Tc

An experimental investigation of the influence of superconductivity on the decay rate of^99m Tc, _T, has been performed by means of a differential method. The^99m Tc samples containing⁹⁹Tc as a carrier were prepared by electrodeposition on a copper plated tungsten wire. For production of metallic technetium the samples were reduced in pure hydrogen gas at 800~1000°C. X-ray analysis of the samples showed they were surely metallic with anhcp structure. The lattice constants observed area=2.741 Å andc=4.398 Å. The transition temperature was found to be 7.5±0.2K. Comparison of two sources, normal (room temperature) and superconducting (4.2 K), did not show an appreciable effect of superconductivity on the decay rate of^99m Tc exceeding the limit of uncertainty of our experiment:/ _T=(1.1±2.7) × 10^–4.     

Beta-decay asymmetries in polarized12B and12N and theG-parity non-conservation

The decay asymmetries (A) in polarized¹²B and¹²N have been measured as a function of -ray energies (E). The coefficients _± inA = ±P(p/E) (1+_± E) have been determined to be _–(¹²B) =+(0.31±0.06)%/MeV and ₊(¹²N) = –(0.21±0.07)%/MeV. The experimental value, _––₊=(0.52±0.09)%/MeV, is larger than the prediction according to conservation of vector current which includes no second-class current, _––_{+ CVC}0.27%/MeV, and indicates the existence of the second-class induced-tensor current.This work has been done in collaboration with I. Tanihata and J. Göring.