Density dependent resonance behaviour of a perturbed gamma-gamma directional correlation in a magnetic field

Using pure gaseous H₂ ⁷⁵Se sources with densities between 1.9×10¹⁵/cm³ and 3.8·10¹⁷/cm³, the directional correlation of the 121.1–279.5 keV gamma-gamma cascade of⁷⁵As was measured as a function of the strength of an external magnetic field. The anisotropy shows a resonance behaviour around zero magnetic field with a broadening of the curves for increasing density. A comparison with the theory of Leisi was possible for the source with the lowest density. Using the stochastic model of Bosch and Spehl, the theory of Leisi was extended to describe the broadening of the curves. The measured half widths of the curves were fitted to the theory yielding a cross section of 1.6 ×10^?13 cm² for charge transfer collisions in the eV energy region between As ions and H₂Se molecules.     

The slowing down of atoms with kinetic energies ofW≲5 eV in Noble Gases: Evidence for inelastic interactions

Profiles of the 264.6 keVγ-line of⁷⁵As emitted after the electron capture decay of⁷⁵Se have been measured using nuclear resonance fluorescence in combination withγγ-coincidences as a high resolution, variable-energyγ spectrometer. The sources consisted of gaseous⁷⁵SeH₂ with different amounts of Kr or Xe. From the line profiles information has been obtained about the velocity distribution of the radioactive⁷⁵As ions after electron capture decay and about the slowing down of the⁷⁵As ions in the moderator gas. The⁷⁵As ions obtain kinetic energies of up toW～5 eV by neutrino recoil and Coulomb fragmentation of the⁷⁵SeH₂ molecules, corresponding to velocities of the order of 10⁵ cm s^?1. The slowing down of the⁷⁵As ions has been interpreted in terms of the model of colliding elastic spheres, taking into account the thermalization in an exact way. No satisfactory agreement has been obtained between measured and calculated line profiles. This result indicates that inelastic processes are of great importance in the interaction of⁷⁵As ions or atoms with the noble gas moderator atoms.     

Feasibility of arsenic and antimony NMR spectroscopy in solids: An investigation of some group 15 compounds

The feasibility of obtaining ⁷⁵As and ^121/123Sb NMR spectra for solids at high and moderate magnetic field strengths is explored. Arsenic-75 nuclear quadrupolar coupling constants and chemical shifts have been measured for arsenobetaine bromide and tetraphenylarsonium bromide. Similarly, ^121/123Sb NMR parameters have been measured for tetraphenylstibonium bromide and potassium hexahydroxoantimonate. The predicted pseudo-tetrahedral symmetry at arsenic and the known trigonal bipyramidal symmetry at antimony in their respective tetraphenyl-bromide “salts” are reflected in the measured ⁷⁵As and ¹²¹Sb nuclear quadrupole coupling constants, C_Q(⁷⁵As)=7.8 MHz and C_Q(¹²¹Sb)=159 MHz, respectively. Results of density functional theory quantum chemistry calculations for isolated molecules using ADF and first-principles calculations using CASTEP, a gauge-including projector augmented wave method to deal with the periodic nature of solids, are compared with experiment. Although the experiments can be time consuming, measurements of ⁷⁵As and ¹²¹Sb NMR spectra (at 154 and 215 MHz, respectively, i.e., at B₀=21.14 T) with linewidths in excess of 1 MHz are feasible using uniform broadband excitation shaped pulse techniques (e.g., WURST and WURST-QCPMG).     

Nuclear magnetic resonance of 55Mn and 75As in MnAs

Four sets of NMR signals, two each, from ⁵⁵Mn and ⁷⁵As nuclei have been observed. The temperature dependences of ⁵⁵Mn resonances have been studied from 77 to 311 K and that of ⁷⁵As, from 77 K to about 250 K. The results show that there is a phase transition at T₁ ≈ 220 K. This transition may be due to introduction of a local spontaneous distortion in the region of the domain walls in the lattice, resulting in lowering of symmetry at low temperatures. Another possibility is the canting of spins which would lower the magnetic group symmetry. The observed resonances have been assigned to arise from the nuclei at the edge and the centre of the domain walls at temperatures T >T₁ and from two types of wall edges with inequivalent orientation of atomic spins at T < T₁. The isotropic hyperfine field at 0 K obtained by extrapolating the resonance frequencies are 227 and 285.1 kOe at ⁵⁵As nuclei, respectively. The anisotropy in the hyperfine field is nearly zero at ⁵⁵Mn nuclei and about 5.8 kOe at ⁷⁵As nuclei at 0 K.     

Ion molecule collisions investigated via nuclear resonance fluorescence

The method of nuclear resonance fluorescence has been applied to analyse the profileN(E) of the 285 keV gamma line of⁷⁵As after electron capture decay of⁷⁵Se in gaseous⁷⁵SeH₂ sources. The profileN(E) depends on the velocity distribution of the radioactive As ions at the moment of γ-decay and, hence, is strongly dependent on the slowing down mechanism. Investigating sources with pressures of 1.5, 15, and 177 Torr different states of the slowing down process could be studied. A simple model is developed that describes the basic properties of the experimental line profiles.     

Magnetic resonance study of arsenic bonding sites in ternary chalcogenide glasses

⁷⁵As NQR and high-field NMR experiments have been performed on Ge_xAs_ySe_1−x−y glasses. Evolution of As bonding structure from arsenic sites with axially symmetric distribution of the electric field gradient (EFG) to highly asymmetric As surroundings has been revealed. Arsenic atoms form pyramidal structural units in Ge₂As₂Se₇ with no evidence of significant concentration of homopolar bonds. In Ge₂As₂Se₅ most of arsenic atoms form structural units with two As-As bonds per atom and asymmetric EFG distribution. Arsenic bondings become more complicated in Ge_0.33As_0.12Se_0.55 where all arsenic sites are highly distorted. The combination of NQR and NMR data provide valuable information on arsenic bonding dynamics in these glasses.     

Effect of gamma irradiation on the dielectric response of a Sr0.75Ba0.25Nb2O6 relaxor single crystal

The effect of gamma irradiation from a ⁶⁰Co source at a dose D = 4.75 × 10⁴ R on the dielectric properties of a Sr_0.75Ba_0.25Nb₂O₆ (SBN-75) single crystal with a smeared phase transition is investigated in the low-and infralow-frequency ranges. The behavior of the dielectric response is compared with the behavior of the depolarization currents induced in unirradiated and irradiated samples of the Sr_0.75Ba_0.25Nb₂O₆ compound. The observed changes in the electrical properties of the Sr_0.75Ba_0.25Nb₂O₆ single crystal in the temperature range of the smeared phase transition are associated with the stabilization of the ferroelectric phase of the material in an internal bias field generated by radiation-induced defects.     

A study of low-lying states of 76As using the 75As(d,p) reaction

The ⁷⁵As(d, p)⁷⁶As reaction was investigated with 11 MeV deuterons. Thirty levels of the residual nucleus were identified up to about 1.8 MeV excitation energy, and angular distributions were measured. Spectroscopic factors and l_n values were obtained for low-lying transitions by means of a distorted-wave analysis of the differential cross sections. The l_n = 4 transition to the ground state of ⁷⁶As and the presence of l_n = 2 transitions to low-lying levels provide a strong support to the assumption that nuclei in this mass region have a large deformation parameter. The results are compared with the ⁷⁵As(n, γ)⁷⁶As data.     

The perturbation of a γγ directional correlation of75As in gaseous H2 75Se sources

Using gaseous H₂ ⁷⁵Se sources the γγ directional correlation of the 121.0–279.5 keV γγ cascade of⁷⁵As was measured as a function of pressure. The attenuation coefficientG ₂ varied between 0.67 and 0.96 for pressures between 0.1 and 200 Torr. Since the perturbation of the directional correlation is caused by hyperfine interaction of the highly charged ion formed after electron capture, the pressure dependence may be understood as being mainly due to changes of the charge states during collisions with neutral molecules. A simple model is proposed yielding a reasonable fit to the experimental data.     

La NMR and As NQR study of the As-based filled skutterudite LaOs4As12

We report experimental results of nuclear magnetic resonance (NMR) at the La site and nuclear quadrupole resonance (NQR) at the As site in the normal state of the superconducting compound LaOs₄As₁₂. Measurements have been performed on powder sample obtained from high quality single crystals. The temperature dependences of the nuclear spin-lattice relaxation rates, 1/T₁, of ⁷⁵As and ¹³⁹La nuclei were measured. No scaling between them was found indicating a local character of relaxation processes. The relaxation of ⁷⁵As nuclei can consistently be understood in terms of antiferromagnetic spin fluctuations, as deduced from the T-dependence of (1/T₁T)=C/(T−θ)^1/2.     

Some properties of the two-point functions in a dilatationally covariant field theory

The object of our concern are some properties of the two-point functions in a model of dilatationally covariant field theory. We examine the one- and two-dimensional irreducible representations of the dilatation group. For the one-dimensional case we obtain either a massless free field theory or a theory of an interacting field which does not contribute on the mass shell μ=p²=0 and is characterized by a spectral function μ^r₊?1. In the two-dimensional case both fields differ from the free field, their spectral functions ρ_ij(μ) do not vanish identically and are products of two factors, a polynomial of order up to two in ln μ and μ^r₊?-1. The differences between the case of internal symmetry and the case of dilatations are emphasized. The formula for the form factor in the Araki-Haag limit is given.     

On a2(1) reflection matrices and affine Toda theories

We construct new non-diagonal solutions to the boundary Yang-Baxter equation corresponding to a two-dimensional field theory with U_q(a₂⁽¹⁾) quantum affine symmetry on a half-line. The requirements of boundary unitarity and boundary crossing symmetry are then used to find overall scalar factors which lead to consistent reflection matrices. Using the boundary bootstrap equations we also compute the reflection factors for scalar bound states (breathers). These breathers are expected to be identified with the fundamental quantum particles in a₂⁽¹⁾ affine Toda field theory and we therefore obtain a conjecture for the affine Toda reflection factors. We compare these factors with known classical results and discuss their duality properties and their connections with particular boundary conditions.     

Nontrivial critical behaviour in a lattice model of random surfaces

A model of “planar random surfaces without spikes” on hypercubical lattices was introduced some years ago as a discretization of quantum string theory. We review some general properties of this model and present results from a Monte Carlo study of its critical behaviour in d = 4, 8 and 10 dimensions. In d = 4 dimensions we find a Hausdorff dimension d_H ≈ 4 and an anomalous dimensions η ≈ 1. These critical exponents imply a deviation from mean field theory in contrast to other lattice random surface models. Furthermore, we find evidence for mean field behaviour in 8 and 10 dimensions, indicating an upper critical dimension d_c^u ⩽ 8.     

Energy levels in As from the As(p,d) As reaction

The ⁷⁵As(p, d) ⁷⁴As reaction has been used to identify 23 excited states in ⁷⁴As. The l_n values, or mixture of l_n values, to 19 levels have been determined from DWBA fits. The spectroscopic sum rules show a neutron shell filling which is consistent with other N = 42 isotones.     

Study of mercury surface diffusion on tantalum with and without an electric field by means of a field emission microscope

The surface diffusion of mercury atoms on tantalum substrate with and without high electric field was studied by means of a field emission microscope (Müller's). The activation energy during surface migration Q_m of mercury atoms with and without an electric field F on tantalum substrate depending on the thickness of the adsorbate was measured. It is shown that the electron density distribution at coverage θ < 0.65 with adsorbate is due to a dipole momentum P. At θ > 0.65 the slope of the curves of Q^F_m = ?(θ) is explained with the appearance of the effect of polarization. The energy of desorption Q_d as a function of the thickness of the adsorbed layer in the temperature range 100–300 K was measured also.     

Über die selbständige Polarisierung des Spins der Elektronen durch Ausstrahlung bei der Bewegung auf einer Spiralbahn im Magnetfeld

The nuclear half-life of the 287 keV second excited state of⁷⁵Se, populated by the reaction⁷⁵As(p, n)⁷⁵Se^* has been measured by the delayed coincidence technique. NaJ(Tl)- and Ge(Li)-detectors were used for measuringγ-energies and time-spectra. By slope analysis of the time-spectra we obtained a half-life of T_1/2(⁷⁵Se, 287 keV)=(30.0±0.4) ns. Possible spin assignments for the 287 keV state in⁷⁵Se from logft-values derived fromβ ⁺-decay data of⁷⁵Br to this level byBaskova et al are 3/2^? and 1/2^?. Application of an external magnetic field of 24 kG to the target nuclei yielded no measurable perturbation of the time-spectra, thus a spin value of 1/2^? for this level seems to be more probable.     

A decoupling experiment on the α-γ correlation in Am243

The directional correlation betweenα particles from the decay of Am²⁴³ to the 75 keV level in Np²³⁹ and the deexciting 75 keVγ-rays has been studied. The recoiling atoms were allowed to escape into vacuum. This brings about a strong hfs interaction between the heavily ionized daughter atom and the nuclear magnetic moment, which has been shown earlier to result in an almost isotropic correlation pattern. By the application of a magnetic field along the direction of the emission of theα-particlesAsaro andSiegbahn showed that the correlation pattern could be restored to, or almost to, the theoretical value, depending on the theory chosen. This is interpreted as a decoupling of the Ms-interaction, where the degree of decoupling is dependent on the strength of the applied field. The highest field used in the earlier experiment was 12 kG. For this experiment a decoupling magnet giving 24 kG was constructed. Applying this magnetic field on three different sources gave a result (A ₂=_?0.404±0.010) which confirms the so called quadrupole correction ofChasman andRasmussen to the theory ofBohr et al. of alpha decay of deformed nuclei. The decoupling magnet used in the experiment is described.     

The potts glass in uniform and random fields: a monte carlo investigation

As a simple model of an anisotropic orientational glass with short range forces, the 3-state Potts model on the simple cubic lattice with nearest neighbor interactions drawn from a Gaussian distribution is considered. With Monte Carlo methods we study the response of the system to a uniform “field” which favors one of the states. This is motivated by experiments which apply stress that favors one molecular orientation of the quadrupolar glass. The responsem to that fieldh=H/k _BT is analyzed in terms of an expansionm= χ₁ h+χ₁ h ²+χ₁ h ³+..., where χ₁ is the linear susceptibility, and χ₂,χ₁₃ are nonlinear susceptibilities. Unlike the case of spin glasses, where the spin inversion symmetry of the system in the absence of fields implies χ₂≡0,χ₂ is nonzero here and diverges to ?∞ at the zero temperature transition of the model, while χ₃ diverges to +∞ as in spin glasses. At inifinite temperature, however, χ₁=1/3, χ₂=1/18 and χ₃=-1/54, i.e. the nonlinear susceptibilities have a different sign as at low temperature. In contrast, a random field does not induce a uniform order parameterm but only a glass order parameterq. The temperature dependence of this glass order parameterq(T) shows for intermediate field strength order parameterq(T) shows for intermediate field strength a maximum of the slopedq(T)/dT very similar to corresponding experiments.     

Heavy quarks in electroproduction

The idea of an effective light-particle field theory is used to analyze the effects of heavy quarks in electroproduction. The b-quark distribution is predicted at large Q² and a systematic treatment of the changing number of quark flavors in a QCD analysis of F₂ is presented.     

Relativistic Hartree-Bogoliubov description of the lithium isotopes

Here we report the development of the relativistic Hartree-Bogoliubov theory in coordinate space. Pairing correlations are taken into account by both density dependent force of zero range and finite range Gogny force. As a primary application the relativistic HB theory is used to describe the chain of Lithium isotopes reaching from ⁶Li to ¹¹Li. In contrast to earlier investigations within a relativistic mean field theory and a density dependent Hartree Fock theory, where the halo in ¹¹Li could only be reproduced by an artificial shift of the 1p _1/2 level close to the continuum limit, the halo is now reproduced in a self-consistent way without further modifications using the scattering of Cooper pairs to the 2s _1/2 level in the continuum. Excellent agreement with recent experimental data is observed.