Interactions of neutrons with gallium and its isotopes

Coherent neutron scattering lengths and total cross sections were measured on elemental Ga, Ga-compounds and on isotopically enriched samples at neutron energies from 0.5 meV up to 143 keV using different techniques. From the experimental data the following quantities could be obtained:

the coherent scattering length of Ga and the spin state scattering lengths of the isotopes.

the potential scattering radii (R′)

the absorption cross sections of Ga,⁶⁹Ga,⁷¹Ga and⁷⁵As.

As derived quantities are given the coherent and incoherent cross sections and the resonance spin scattering lengths for theJ=1 andJ=2 states of⁷⁰Ga and⁷²Ga. The physical meaning of these data and ofR′ and the comparison with the resonance parameters are considered.     

Neutron scattering lengths and cross sections of the barium isotopes

Coherent neutron scattering lengths and total cross sections have been measured for barium compounds and on isotopically enriched samples for neutron energies from 0.5 meV up to 132 eV using different techniques. From the experimental data the following quantities were obtained:

the coherent scattering lengths (in fm) of Ba (5.07±0.03) and of the isotopes with mass numbers 130 (?3.6±0.6); 132 (7.8±0.3); 134 (5.7±0.1); 135 (4.66±0.10); 136 (4.90 ±0.08); 137 (6.82±0.10); and 138 (4.83±0.08).

the absorption cross sections σ_γ (at 0.025 eV in barn): of Ba (1.1±0.1) and of the isotopes: 130 (30±5); 136 (0.68±0.17); 137 (3.6±0.2); and 138 (0.27±0.14).

zero-energy scattering cross sections for Ba and the isotopes 136, 137 and 138. On the basis of these data, the isotopic- and spin-incoherent cross sections and the (s)-resonance contributions to the coherent scattering lengths have been determined and discussed.

     

Quasi-particles at finite temperatures

We study the consequences of the KMS-condition on the properties of quasi-particles, assuming their existence. We establish

1. If the correlation functions decay sufficiently, we can create them by quasi-free field operators.
2. The outgoing and incoming quasi-free fields coincide, there is no scattering.
3. There are may age-operatorsT conjugate toH. For special forms of the dispersion law ε(k) of the quasi-particles there is aT commuting with the number of quasi-particles and its time-monotonicity describes how the quasi-particles travel to infinity.

     

Interaction of slow neutrons with the isotopes of molybdenum

Coherent neutron scattering lengths and total cross sections were measured on samples of ordinary Mo and isotopically enriched samples. From the experiments with neutrons of 0.57 meV and 1.26 eV the following values have been obtained:

1. the coherent scattering lengths (in fm) of the bound atoms of Mo (6.715±0.020) and for the isotopes with the mass numbers 92 (6.93±0.08), 94 (6.82±0.07), 95 (6.93±0.06), 96 (6.22±0.06), 97 (7.26±0.08), 98 (6.60±0.07) and 100 (6.75±0.07);
2. the incoherent scattering cross section at zero-energy for ordinary Mo: σ_i=0.02 ± 0.02 barn;
3. the absorption cross sections (in barn) for Mo (2.48 ± 0.04) and for the isotopes with the mass numbers 95(13.4 ± 0.3), 97(2.5 ± 0.2) and 100(0.4 ± 0.2). The relation of the present results to the resonance parameters of the neutron — Mo interaction has been discussed.

     

Anisotropy of the electron-phonon scattering rate in metals

A method of measuring the temperature dependent part of the electron scattering ratev(T) for separate groups of electrons has been developed on the basis of the radio-frequency size effect. The temperature dependence of the amplitude of a radio-frequency size effect line yields information on the probability of electron scattering averaged over the vicinity of the corresponding extremal cross section of the Fermi surface (FS). This method was applied to two problems:

1. the determination of the electron-phonon scattering rate on the FS point-by-point in noble metals; and
2. the extraction of the contribution of electron-phonon scattering collisions with electron transfering from one sheet of the FS to another to the total electron-phonon scattering rate.

     

Neutron star magnetism

It is shown that

1. an appreciable change of magnetic moment of a neutron star cannot occur via ohmic dissipation
2. pulsars provide evidence for large internal magnetic fields in main sequence stars. If pulsars are born from stars with masses exceeding 3 ?_⊙ the internal field must be of the order of 10³-10⁴ Gauss while if they derived from less massive urstars 10² Gauss are sufficient to give rise to a magnetic moment ofM～10³⁰ Gauss cm³.

     

Two-dimensional quantum field theories containing a massless scalar field

The following new findings are briefly reported:

1. A consistent quantum theory can be formulated for a free massless scalar field in two-dimensional spacetime.
2. Satisfactory operator solutions in terms of asymptotic fields can be constructed in the Thirring and Schwinger models.
3. Gauge invariance is spontaneously broken in the Thirring model as well as in the Schwinger model.

     

Detection of highly ionizing particles: Nonlinear near-surface phenomena in silicon radiation detectors

As far as the detection system is concerned, experimens on synthesis and study of the properties of superheavy nuclei is one of the most difficult tasks. In fact, these experiments can be considered extreme in many senses:

• —extremely low (fractions of a picobarn-picobarns) formation cross sections of the products under investigation
• —extremely high heavy ion beam intensities for example, ～1.1–1.5 pμA^{1 48}Ca
• —high radioactivity of actinide targets, which are used in the experiments aimed at the synthesis of super-heavy nuclei
• —very long duration of the experiment (as long as a year)
• —extremely low yield of the products under investigation (sometimes less than 1 per month)
• —very high sensitivity of the detection system
• —radical suppression of the background products (method of “active correlations”).

The two last points are the subject of the present paper, as well as the subject of two of my reviews published before. It is evident that without knowledge of the nature of the internal processes in semiconductor detectors it is virtually impossible to provide clear detection of ultra rare signals. In the present paper, the author reports on the investigation of near-surface phenomena in silicon radiation detectors, first of all bearing in mind the theoretical-methodological aspect of these phenomena. Non-equilibrium electron-hole recombination, pulse height defect formation, charge multiplication, and formation of “hot” electron system, are considered. With just these phenomena one can observe nonlinearity of energy-charge-amplitude conversion for heavy ion (recoil nucleus) registered by a silicon detector. Practical applications are also considered. One of them is a deeply modified method of “active correlations”. Projection of applying the method in the experiments with the modernized cyclotron (DC-280 FLNR project) is projected as well as possible applications in the heavy-ion-induced complete fusion nuclear reactions.     

Neutrinos in general relativity: Four(?) levels of approach

The three subsequent levels of approach to the problem of the neutrino in general relativity which have been exploited till now, are:

1. ‘classical particle’ approach, i.e. a study on the neutrino as a classical particle in a classical, given gravitational field;
2. ‘quantum particle’ approach, i.e. considering the Dirac equation for the neutrino in a given gravitational field;
3. ‘classical field’ approach comprising the study of combined neutrino-gravitational fields.

Many results obtained along these lines are presented, with emphasis upon the geometrical theory of two-component neutrino-gravitational fields. A synthesis of the particle and fields aspects of the neutrino could provide a possible fourth, till now non-existing, ‘quantum field’ level of approach. This should deal with a guantized neutrino field in a curved space-time (which might be also quantized, but perhaps this should belong already to a next, fifth level of approach). Studies on the neutrino physics in gravitational fields reveal not only a series of results which are of interest in se, and which may be used as the basis to a unified theory of neutrino and gravitational fields (the Rainich problem for the neutrino). They provide in addition the necessary material for astrophysical and cosmological applications; some of these are outlined in relation to the results presented.     

Neutron interactions with germanium isotopes and amorphous and crystalline GeO2

Coherent neutron scattering lengths and total cross sections have been measured on elemental and oxide samples of ordinary Ge and of isotopically enriched substances. From the experimental results the following values were obtained:

the coherent scattering lengths (in fm) of the bound atoms Ge(8.185±0.020);⁷⁰Ge(10.0±0.1);⁷²Ge(8.51±0.10);⁷³Ge(5.02±0.04);⁷⁴Ge(7.58±0.10) and⁷⁶Ge(8.2 ±1.5);

the absorption cross sections at 0.0253 eV (in barn) for Ge(2.20±0.04);⁷⁰Ge(2.9±0.2);⁷²Ge(0.8±0.2);⁷³Ge(14.4±0.4) and⁷⁴Ge(0.4±0.2);

the free cross sections for epithermal neutrons and the zero energy scattering cross sections.

On the basis of this data, the isotopic- and spin-incoherent cross sections and thes-wave resonance contributions to the coherent scattering lengths have been determined and discussed. Transmission measurements at 0.57 meV on amorphous and crystalline GeO₂ yielded for the amorphous sample an inelastic cross section eight times larger than for the crystalline samples. This effect corresponds to a clearly higher density of low energy states in the amorphous than in the crystalline substances.     

The possibility of using a semiconductor thermal probe to investigate the power flow distribution in waveguides

Conclusions

1. The temperature profile of an absorbing film for a given microwave power flux distribution has been considered.
2. The conditions under which agreement is found between the temperature relief and the flux distribution has been clarified.
3. An experimental investigation has been carried out of the temperature profile in a matched film for the TE₁₀ mode.
4. The possibility of using a semiconductor thermal probe to investigate the microwave power flux distribution has been demonstrated.

     

Small-angle X-ray scattering on spinodal,nucleotic or coarsening processes in two-phase systems containing spherical,fibrillar or lamellar inhomogeneities

We study the effects of some of the most important and typical structural changes in two-phase systems on selected structural parameters obtained from small-angle x-ray scattering (SAXS) measurements. To limit the present study, it was assumed that the Phase, 1, embedded in the matrix

1. is monodispersed and homogeneous,
2. possesses one of the three most extreme shapes (spherical, fibrillar or lamellar) and
3. changes its behaviour

1. through type change (spinodal or nucleotic or coarsening), without changing the shape,
2. through a change of the shape only, or
3. through a) (type change) and b) (shape change) simultaneously.

To find the type of change for three basically different shapes of Phase 1 and to calculate its intensity (amount of the change) the following three SAXS parameters must be compared before and after the treatment of the system:

1. chord lengthl ₁ (and/or radius of gyrationR),
2. volume partw ₁ of the Phase 1, and
3. relative inner surfaceS _v of the system.

It is shown by this comparison that by the pure type change in the case of

1. spinodal change, all three SAXRS parameters are increasing or decreasing simultaneously and proportional to a power of the intensity of the change,
2. nucleotic change,l ₁ (and/orR) is unchanged, the other two (w ₁ andS _v ) are increasing or decreasing simultaneously and directly proportional to the intensity,
3. coarsening change,w ₁ is unchanged and anincreasing ofl ₁ is always accompanied by adecreasing ofS _v and vice versa.

In addition to this type change, the cases of mere changes of the shape (“shape change”) and finally of simultaneous type and/or shape change are studied. For the case of pure shape change the alteration of the dimensions (chord lengthl ₁ and/or radius of gyrationR) must be followed. The limitations of the analyses of the simultaneous type and/or shape change are discussed in detail.     

Polarization effects of picosecond CARS in liquids

Coherent Raman scattering of delayed probing pulses after ultrafast excitation is investigated under generalized polarization conditions. Three factors are shown to contribute to the scattering signal:

1. scattering off the isotropic part of the resonant material excitation via the isotropic component of the Raman polarizability
2. scattering via the anisotropic part of the Raman polarizability from a second, oriented component of the resonant material excitation
3. four-wave mixing via the non-resonant part χ_NR of the third-order susceptibility. We demonstrate theoretically and experimentally that different polarization conditions lead to drastic changes of the signal transients in liquids. For the ring breathing mode of C₆H₅Br the ratio of non-resonant to resonant contributions is measured to be χ_NR/χ_res=0.037±0.015.

     

On the existence of crossing symmetric and unitary scattering amplitudes with Regge poles

We give a complete proof of the existence of scattering amplitudesA(s,t,u) with the following properties

1. the amplitudes are total symmetric ins,t, andu.
2. they satisfy elastic unitarity for 4≦s≦16, and
3. they develop resonances forl≧2 on a bounded Regge trajectory which dominates the asymptotics for large energies.

     

Lattice dynamics and transport equations for quantum crystals including singular interactions

The influence of frequency modulation of the rf field and modulation of the static magnetic field on NMR-response is investigated. The mathematical treatment using density operator formalism results in a different dynamical behaviour of the nuclei whether frequency—or field modulation is applied. Therefore a general equivalence of the two kinds of modulation does not exist. Explicite solutions for the nuclear magnetization with simultaneous frequency—and field modulation are evaluated for systems obeying Bloch equations. Resonance experiments have been performed using different detection methods:

1. rf spectrum analysis of the nuclear induction emf from a decoupled (zero leakage) crossed coil setup.
2. conventional lock-in detection of the signal information from an NMR-detector with single resonant circuit.

     

On a classical spin glass model

A simple, exactly soluble, model of a spin-glass with weakly correlated disorder is presented. It includes both randomness and frustration, but its solution can be obtained without replicas. As the temperatureT is lowered, the spin-glass phase is reached via an equilibrium phase transition atT=T _f . The spin-glass magnetization exhibits a distinctS-shape character, which is indicative of a field-induced transition to a state of higher magnetization above a certain threshold field. For suitable probability distributions of the exchange interactions.

1. A mixed phase is found where spin-glass and ferromagnetism coexist.
2. The zero-field susceptibility has a flat plateau for 0≦T≦T _f and a Curie-Weiss behaviour forT>T _f .
3. At low temperatures the magnetic specific heat is linearly dependent on the temperature.

The physical origin of the dependence upon the probability distributions is explained, and a careful analysis of the ground state structure is given.     

Versuche zum toroidalen Einschluß von Hoch-Beta-Plasmen mit Hilfe von helischen Multipolfeldern

Helical multipole fields with multiplicities of 1 to 3 and up to 16 field cycles around the circumference of the torus are superposed on a toroidal theta pinch. The vacuum fields of the configurations are calculated and regions of closed flux surfaces are found. The experimental results are:

1. Equilibria of High-Beta plasmas also exist, if the vacuum field has no absolute minimum, provided that the region of closed flux surfaces in the vacuum field has a greater diameter than the plasma, and that the helical multipole fields do not periodically increase and decrease the gradient of the toroidal main field. These equilibria are possibly unstable.
2. If the helical multipole fields periodically increase and decrease the gradient of the theta pinch field along the circumference of the torus there is no equilibrium, even if the helical curvature of the field lines is great.
3. The main cause of the losses is the escape of particles along the field lines.

     

Magnetic field and doppler velocity correlation in quiescent solar prominences

Analyzing statistically the magnetic and Doppler velocity data for 85 quiescent prominences observed in 1983–1987 by Nikolsky's magnetograph, we came to the following preliminary conclusions:

The average longitudinal magnetic field of the prominence determines the dynamic velocity of the latter: the stronger magnetic fields correspond to the higher Doppler velocities.

A longitudinal magnetic field less than 25 G allows the material to move with arbitrary velocity within the limits of several kilometers per second. A magnetic field higher than 25 G suppresses such movements.

The horizontal length of the flux tube exceeds its vertical part by 1.5 orders of magnitude (the upper limit).

There is an angle of 10°between the horizontal component of the velocity vector in quiescent prominences and the long axis of the filament.

The maximum velocity in quiescent prominences is about 7 km/s.

     

Dynamical enhanced electron emission and discharges at contaminated surfaces

Broad-area electrodes show electron emission already at electric field strengthsF≈10⁷ V/m. This enhanced field emission (EFE) occurs only for contaminated surfaces. EFE is accompanied by photon emission and gas desorption yielding finally discharges. EFE is caused by dust and contaminants initiating the following effects:

an electron is stochastically emitted in a trigger zone

the electron gains energyΔE?eΔxF ^*

which excites electronic states

which relax by the emission of electrons, photons, and atoms

where the positive charges left behind enhanceF ^*=βF (β?1) initiating so an electron avalanche, i.e., a high conductivity channel. Because of charge migration and neutralization, this avalanche has a life time. This pulsating EFE is accompanied by light emission and gas desorption yielding finally a gas cloud and a discharge.

The pulsating, self-sustained EFE has the same root as:

the enhanced secondary emission found first by Malter

the conductivity switching exhibited by thin (≈ 1 μm) layers of semiconductors or insulators

the normal cathode fall and

the firing-wave instability in neurodynamics.