Search Results (36 total)

The conductivity spectra of polycrystalline Na₂SO₄ have been investigated in the frequency range 42 Hz–1 MHz at different temperatures below and above the V-I phase transition temperature. The conductivity data have been analyzed using Almond-West formalism. The dc conductivity, the hopping frequency of the charge carriers, and their respective activation energies have been obtained from the analysis of the ac conductivity data, and the concentration of charge carriers was calculated at different temperatures. The power-law exponent n of the conductivity spectra has average values of 0.43 and 0.61 in phases V and I, respectively, which indicates different conduction properties in the two phases. Moreover, scaling of the conductivity spectra at the low- and high-temperature phases was performed in accord with Ghosh’s scaling approach. It is found that the scaling properties depend on the structure of the investigated material.

The thermal neutron cross section and resonance integral of the reaction ¹⁵⁹Tb(n,γ)¹⁶⁰Tb in the thermal and 1∕E regions, respectively, of a thermal reactor neutron spectrum have been experimentally determined. Literature data on the resonance integral of the reaction show a large scatter ranging from 313 to 780  b. On the basis of such data it becomes extremely difficult to produce a best value for application. In this work we performed a careful experiment in an effort to produce a precise and accurate measured value, which will be a valuable addition to the literature. The ¹⁵⁹Tb(n,γ)¹⁶⁰Tb reaction is studied by irradiating spectrographically pure Tb₄O₇ powder samples with thermal and epithermal neutrons in the Pakistan Research Reactor–1 (PARR-1) at PINSTECH. Thermal and epithermal neutron fluence rates were determined with Au and Co activation detectors. The α parameter, which accounts for the deviation of the neutron spectrum shape from the 1∕E law in the epithermal neutron region, was obtained with good accuracy by using Au, Co, Mn, and Zn activation detectors and adopting the regression and iterative analysis procedures. The induced activities in the target samples and the activation detectors were measured with a high purity germanium detector system. The cadmium ratio method was used to yield the thermal neutron cross section and resonance integral. The measured thermal neutron cross section at 0.0253  eV and resonance integral are 23.6±0.4  b and 443±21  b, respectively. The results are discussed and compared with the literature.

We propose a model for the measurement of an arbitrary multimode entangled state of the cavity field using two-photon correlated emission laser. We consider two cases: (a) The modes have different frequencies and are detected separately and (b) the modes consist of two orthogonal polarization states and are detected using a single balanced homodyne detector. The basic idea is to amplify the initial multimode state such that there is no-noise in the quadrature of interest and all the noise is fed into the conjugate quadrature component. The amplified noise-free quadrature is prepared in different phases and then corresponding quadrature distribution is measured. The Wigner function of the initial multimode entangled state is then reconstructed by using inverse Radon transformation. This scheme is insensitive to the noise associated with the nonunit efficiency of the detector in the homodyne detection measurement scheme.

Two recent theoretical studies [C. Liu , Phys. Rev. A 64, 010501 (2001); M. Žitnik , ibid. 65, 032520 (2002)] predict that the fluorescence lifetimes of helium doubly excited states converging to He⁺ N=2 should be longer than that of the He⁺ 2p ion state. This effect is caused by the electric field of the outer electron which, through Stark mixing, gives the inner fluorescing electron some series specific, stabilizing 2s character. We have obtained the first experimental evidence that confirms this effect by measuring the lifetime of the 2p3d(¹P⁰) doubly excited state. This was determined to be 190±30  ps compared to 100 ps for the He⁺ 2p ion state. The measurements were performed using short pulses of synchrotron radiation to form doubly excited states and recording the arrival time of photons from fluorescence.

Phase-sensitive amplification in a three-level atomic system exhibits interesting features. For example, in the zero-detuning limit and for sufficiently strong driving field, this system becomes an ideal parametric amplifier, whereas, for a weak driving field it is a phase-insensitive amplifier [Ansari et al., Phys. Rev. A 41, 5179 (1990)]. In this paper, we show that this system could be used to measure the quantum state of the radiation field inside a cavity. To reconstruct the quantum state, we amplify it through a three-level atomic system and in the parametric limit, when noise in both the quadratures approaches to zero measure the amplified field quadrature. The complete quadrature distribution is obtained by measuring the quadratures for different values of the driving field phases. The inverse Radon transformation is then employed to reconstruct the original quantum state. Our scheme is insensitive to the problems associated with nonunit detector efficiency in homodyne detection measurement.

Two Auger electrons, one very slow, one fast, have been detected in coincidence following near threshold 4d photoionization of the Xe atom. The distribution in the energy the two electrons share has been measured for the first time revealing the presence of post-collision interaction effects that provide unique information on the decay dynamics of the 4d hole. Analysis of the distorted line shapes indicates that the dominant process is decay of Xe⁺(4d^-1) to Xe³⁺ through cascade emission of a zero kinetic energy Auger electron followed by a fast Auger electron. The widths of the intermediate Xe^2+* states are estimated to be about 60 meV.

We propose a model for the quantum-state measurement via phase-sensitive amplification. The basic idea is to amplify the quantum state through a two-photon correlated-emission laser, such that there is no noise in the quadrature of interest and all the noise is fed into the conjugate quadrature. The noise-free quadrature is prepared in different phases and then corresponding quadrature distribution is measured. The Wigner function of the initial quantum state is then reconstructed by carrying out inverse Radon transformation familiar in tomographic imaging. This scheme allows us to avoid the deterioration of homodyne detection measurement due to the problem of detector efficiency.

Exclusive measurements of the analyzing power and two spin-transfer observables for nucleon knockout from an ¹⁶O target are presented, at kinematic conditions chosen to emphasize interactions in the nuclear interior. The analyzing power data are substantially reduced in comparison with values calculated in the distorted wave impulse approximation (DWIA) using the free nucleon-nucleon interaction, particularly for knockout of the deeply bound 1s_1/2 nucleons. Inclusion of density dependence for the interaction in the calculations improves the agreement with the data, but does not provide a satisfactory description for nucleon knockout from ¹⁶O. Spin-orbit distortions are shown to strongly affect the DWIA predictions of the 1s_1/2 analyzing powers over most of the experimental kinematic range, but notably not near the points of negligible recoil momentum. Hence these data offer constraints on the optical potentials and independently on the two-body effective interaction.

We measured the analyzing power A_y and the triple differential cross section d³σ/dΩ_pdΩ_ndEp for the ²H(p→,pn)¹H reaction at 200 MeV. Coplanar coincidence data were taken for all combinations of neutron angles θ_n=35°, 45°, or 55° with proton angles θ_p=35°, 45°, or 52°. Protons were detected with a ΔE-E telescope with a 1000-μm silicon surface barrier ΔE detector and a plastic scintillator E detector. Neutrons were detected with large-volume plastic scintillators at flight paths of 17.5 or 18 m. The overall neutron separation-energy resolution was about 3 MeV. Data are compared with plane-wave impulse-approximation calculations with a Hulthén deuteron wave function and p-n cross sections and analyzing powers obtained from N-N phase shifts. The agreement between these calculations and the data is generally good for the cross sections. The agreement for the analyzing powers is good near the point of zero recoil momentum. Our results suggest that the deuteron is a good ‘‘neutron target’’ for recoil momenta <100 MeV/c.

Evidence of the existence and distribution of an induced spin polarization of the conduction electrons in the Cu layers of Fe/Cu multilayers has been obtained by nuclear magnetic-resonance (NMR). Fine structure associated with the spin-echo signal of ⁶³Cu and ⁶⁵Cu nuclei shows that the spin polarization and an associated exchange field oscillate in sign, similar to the characteristics of the RKKY interaction. However, an accurate determination of the period of the oscillations cannot be determined solely from the NMR data at the present time.

Differential cross sections were measured and compared to distorted-wave impulse-approximation calculations for the J^π=4^-(νd_5/2,πp_3/2^-1) state excited in the ¹²C(n,p)¹²B (4.52 MeV) reaction, the J^π=6^-(ν,f_7/2πd_5/2^-1) state excited in the ²⁸Si(n,p)²⁸Al (5.175 MeV) reaction, and the J^π=10^-(νh_11/2,πg_9/2^-1) state excited in the ¹²⁰Sn(n,p)¹²⁰In (≃0.0 MeV) reaction at 300 MeV. We obtained a normalization factor of 0.68 for the J^π=10^-(νh_11/2,πg_9/2^-1) state excited in ¹²⁰Sn(n,p)¹²⁰In (≃0.0 MeV) reaction; this is the first ‘‘1ħω’’ stretched state known in this mass region.

We studied the energy dependence of the excitation of the ‘‘1ħω,’’ J^π=6^-, state in the ²⁸Si(p,n)²⁸P(4.95 MeV) reaction and the ‘‘0ħω,’’ J^π=9⁺, state in the ⁸⁸Sr(p,n)⁸⁸Y(1.48 MeV) reaction at E_p=100, 200, 300, and 400 MeV. Differential cross sections for these reactions were measured and compared to distorted-wave impulse-approximation (DWIA) calculations. The ratio of the ⁸⁸Sr to the ²⁸Si DWIA normalizations is independent of beam energy, indicating that the systematic difference in DWIA normalizations reported earlier between 0ħω and 1ħω stretched states is a nuclear-structure effect. Additionally the DWIA normalization for each reaction is independent of the beam energy indicating that the necessary ingredients for DWIA calculations to describe these transitions consistently are well understood.

Recent experimental and theoretical studies indicate that atomic-layer by atomic-layer oscillatory compositional modulation can occur in the near surface layers of alloys in surface segregation. We present here evidence of such an oscillatory feature down to a depth of several atomic layers in surface cosegregation of Pt-Rh(S) alloys.

In order to improve existing I=0 phase shift solutions, the spin correlation parameter A_NN and the analyzing powers A_0N and A_N0 have been measured in n-p elastic scattering over an angular range of 50°–150° (c.m.) at three neutron energies (220, 325, and 425 MeV) to an absolute accuracy of ±0.03. The data have a profound effect on various phase parameters, particularly the ¹P₁, ³D₂, and ε₁ phase parameters which in some cases change by almost a degree. With the exception of the highest energy, the data support the predictions of the latest version of the Bonn potential. Also, the analyzing power data (A_0N and A_N0) measured at 477 MeV in a different experiment over a limited angular range [60°–80° (c.m.)] are reported here.

Recent measurements of charge symmetry breaking in the np system at 477 MeV, and of A_00nn for np elastic scattering at 220, 325, and 425 MeV also yield accurate analyzing power data. These data allow the energy dependence of the analyzing power zero-crossing angle and the slope of the analyzing power at the zero-crossing angle to be determined. The incident neutron energies span a region where the zero-crossing angle is strongly energy dependent (E_n<250 MeV) to where it is almost independent of energy (E_n>350 MeV). The results are compared to current phase-shift analysis predictions, recently published data, and the predictions of the Bonn and Paris potentials.

Absolute composition depth profiles of true single-atomic-layer depth resolution have been obtained for the near surface layers of the (001) and (111) planes of Pt-Rh and Pt-Ru alloys in surface segregation and cosegregation with sulfur of these alloys. For both alloys, the segregation species is found to be Pt, and its concentration approaches the bulk value nonmonotonically; the composition of the second layer is reversed. We did not detect a significant dependence of surface segregation on crystal planes and also find the plane-edge effect to be very small for Pt-Rh alloy. Also no reversed segregation at the two dilute ends of the alloy composition of Pt-Rh is found. For Pt-Rh, if the sample contains a very small amount of sulfur, then sulfur and Rh cosegregate to the surface, with sulfur forming an adsorption overlayer. This reversal of segregation species by impurity sulfur is not observed for Pt-Ru alloys. We believe that atom-probe composition depth profiles can also be used as references for calibrating other surface analytical techniques.

A model is presented to investigate the problem of interaction between an N-level atom and N-1 modes of the field. The model includes detuning. Constants of motion are obtained. The evolution operator is calculated, and the probability distribution function for the photon numbers is computed for different initial atomic states. The characteristic functions are computed. Different statistical quantities concerning the photons or the atomic system are given. The case of a three-level atom and two modes is considered for its different configurations. The phenomenon of collapses and revivals is discussed for squeezed light, and the effect of squeezing is shown in this phenomenon.

Neutron spectra from the ²⁷Al(d,n)²⁸Si reaction have been measured with an energy resolution of better than 10 keV. Bombarding energies ranged from 2.5 to 8 MeV, permitting various regions of excitation in ²⁸Si to be populated with 1 to 3 MeV outgoing neutrons. Levels corresponding to contaminants could be identified by the energy shift with angle. We have identified four new levels below 13 MeV and 22 new levels above 13.4 MeV. The (d,n) reaction at low bombarding energies appears to be nonselective; we observe every previously known level as well as the new ones, with the only exception being those which are unresolved (E₁-E₂≲8 keV) from neighboring levels. We are not able to deduce the spins and parities of these levels from the present data.

Measurements have been carried out for 14.6 MeV neutrons incident on natural iron using the associated particle time-of-flight technique. The elastic differential and neutron emission cross sections have been measured in the energy range 4.5–13.5 MeV and laboratory angular range 30°–130°. Monte Carlo simulation has been used for multiple scattering and flux attenuation corrections. A value of 477±48 mb has been obtained for the neutron emission cross section in the energy range 4.5–13.5 MeV. The present measurements have been compared with previous measurements and model calculations.

Structure in the spectra of neutrons emitted from iron upon bombardment with 14.6 MeV neutrons has been investigated and explained in terms of excitation of levels in ⁵⁶Fe. The energies of scattered neutrons have been measured by the time-of-flight technique based on the associated particle method. The observed excitations have been correlated with the reported levels in a satisfactory manner. Evidence for new excitations at 8.8±0.2, 9.8±0.1, 10.2±0.1, 12.44±0.03, and 12.52±0.03 MeV has been obtained. The excitation of possible components of the M1 giant resonance in ⁵⁶Fe is discussed.

The infrared reflectivity, the temperature-dependent conductivity, and thermopower of the one-dimensional conductor Li_0.82[Pt(S₂C₂(CN)₂)₂] · 2H₂O, LiPt(mnt), is presented. It undergoes a simple Peierls transition at T_c=215 K, which is not influenced by correlations or by cation ordering. The metallic LiPt(mnt) is characterized by a transfer integral |t_∥|≈0.1 eV. When 155 K<T<T_c it behaves as an intrinsic Peierls semiconductor with Δ≈36 meV. At lower temperatures the thermopower indicates extrinsic behavior due to the presence of donor impurities. Our results agree with those obtained from magnetic-susceptibility studies.

The reactions ⁴⁰Ca(p, pn)³⁹Ca and ⁴⁸Ca(p, pn)⁴⁷Ca were studied at 149.5 MeV in coplanar geometries. An overall separation energy resolution of about 1 MeV was achieved. Probable neutron-hole strength is seen for 1d_{5 / 2} and 1p shell knockout from both targets. The data are compared with distorted-wave-impulse-approximation calculations and spectroscopic factors are extracted for neutron-hole states.

NUCLEAR REACTIONS ^40,48Ca(p, pn)^39,47Ca, E=149.5 MeV; measured p-n coincidences; extracted neutron separation energy spectra and cross sections d³σ / dΩ_ndΩ_pdE_p for neutron-hole states. Compared cross sections with distorted-wave-impulse-approximation calculations; extracted spectroscopic factors C²S for neutron-hole states.

We present the ratio of experimental 0° (p, n) cross sections with the measured B(M1) values for transitions in several nuclei. Taking the strong spin-flip transition in ¹²C for normalization, we find overall good agreement with shell-model predictions for ²⁴Mg, ²⁸Si, and ⁴⁸Ca. The values of the ratios vary considerably and are very sensitive to the relative amounts of spin and current contributions to each transition. Experimental ratios are also presented for ¹⁶O and ²⁶Mg.

NUCLEAR REACTIONS Charge exchange (p, n) at 0°, E_p=62-160 MeV, cross sections; targets ¹²C, ¹⁶O, ²⁴Mg, ²⁶Mg, ²⁸Si, ⁴⁸Ca. Compared σ_p,n(0°) with B(M1) values from inelastic electron scattering.

The spectrum of states of superfluid ⁴He is described by the representation theory of groups. Starting from the symmetry group, describing the fluctuations of Bose-Einstein condensed states of zero momentum, we have described the excited states by noncompact groups. This contains complete information about the systems including degeneracy and energy levels, etc. This leads to the possibility of all realistic interactions. As this formalism is equivalent to the Hamiltonian formalism, this description restricts the ambiguous framework of the quantum-mechanical description where interactions or parameters can be viewed arbitrarily. Since we have all possible groups and representations, we can in principle describe all the excited states. Here we are mainly concerned with the phonon spectrum, and the rotonlike and vortex excitations are also taken into consideration.