Fragmentation channels of neutral and charged sodium clusters

The Density Functional Formalism (with the local density approximation for exchange and correlation) is applied to jellium-like spherical particles to obtain heats of fragmentation of Na_n; Na⁺_n and Na²⁺_n (n ⩽ 90) following different decay paths. Masses of the fragments are analyzed resulting that the energetically more favorable channels are controlled by the tendency to fragments with a magic number of electrons, mainly 2 or 8. We have also obtained that, provided we consider parent clusters with the same number of electrons and decay modes involving a neutral fragment, the fragmentations of neutral and ionized clusters are very similar.     

Effect of synchrotron radiation on nuclear beta decay

The stimulation of the endothermic beta decay of stable nuclei by the field of synchrotron radiation has been analyzed theoretically in the framework of the photobeta decay mechanism. In contrast to works devoted to the effect of laser fields on beta decay, the action of the field directly on a nucleus rather than on a beta electron is considered (a sufficiently intense flux of hard photons whose energies exceed 60 keV allows this action). The rates of such a beta decay are calculated for a number of “parent nucleus-daughter nucleus” pairs for the relativistic case including Coulomb effects. For the most intense available sources of synchrotron radiation, the rate of stimulated beta decay for most nuclei under investigation appears to be characteristic of third-forbidden β^? transitions. The effect of synchrotron radiation on highly forbidden natural nuclear β^? decays is also analyzed. In particular, irradiation increases the rate of the β^? decay of the ₃₇ ⁸⁷ Rb and ₄₉ ¹¹⁵ In nuclei by 2% and by almost two orders of magnitude, respectively.     

Dielectronic recombination of positive ions—II. Rate coeffecients for Mo38+

Dielectronic recombination rate coefficients for Mo³⁸⁺ recombining to Mo³⁷⁺ are given for electron temperatures of 1.4, 2.8, and 5.6 KeV. Non-relativistic results for various transitions were obtained in the resonance approximation by direct evaluation of Auger and radiative decay probabilities A_a and A_r, respectively, as discussed in the preceding paper. It is found that a large number of transitions contribute to the total rate. Tables of selected A_a, A_r, fluorescence yields, and rates are given. Excitation of n = 2 electrons to higher shells is found to be the dominant contribution at the temperature considered.     

Quantum decay rate of a nonlinear dissipative system with fission-like potential nearT c

We use a thermodynamic scheme (imaginary free energy method) in terms of the path integral technique to study the quantum decay rates of a metastable state system coupled to a heat bath in the crossover temperature (T _c) region. In this region the transition between thermally activated decay and tunneling occurs. A nonlinear coupling form factor is used to overcome the divergent integral in the partition function nearT _c. The decay rate formula based on the steepest descent approximation has been improved. A method is developed to calculate the real and imaginary parts of the partition function which combines a random walk method with fast-Fourier transform Monte-Carlo evaluation. For a nonlinear dissipative system with a damping correlation kernel of exponential form, the accurate numerical calculations are presented. The effects of nonlinear and frequency-dependent damping on the rate are shown.     

Electronic properties of metals at low temperatures

A many body theory of an electron gas is developed to find the internal and correlation energies at low but finite temperatures. The contribution from the first order exchange, second order (regular and anomalous) exchange, and ring diagrams are treated. The Fermi momentum and the correlation energy are determined as functions of the density by two different methods, one being based on iteration and the other a direct solution of the number density relation. It was found that the iterative solutions which are correct to ordere ² ore ⁴ become negative forr _s of order 5 while the direct solutions do not, indicating the invalidity of the former. Hence, the correlation energy evaluated to the same orders by iteration will not be satisfactory in the same range. The highest order iterative solution which includes terms of ordere ⁶ does not show such a breakdown. These terms which give the contribution of orderr _s to the correlation energy are therefore important and tend to reduce the magnitude of the correlation energy. The corresponding curve is indeed close to that determined by the direct method for smallr _s but a significant deviation takes place at largerr _s . The Coulomb interaction seems less effective at higher temperatures. The internal energy is also determined as a function of density and temperature.     

Solar-neutrino variations: A manifestation of nonzero neutrino mass and magnetic moment, and mixing

Time variations of solar neutrino flux are investigated on the basis of available Homestake experimental data for more than two solar cycles (1970–1994). At first, we determine (with the weight-time function by taking into account ³⁷Ar decay), for each solar neutrino run n, the effective Earth’s heliolatitude L _eff(n), the effective Zurich sunspots number Z _eff(n), the effective latitude of sunspots distribution Λ_eff(n), and the effective surfaces of sunspots in different heliolatitude belts. Then, we consider the correlation of solar-electron-neutrino fluxes with these parameters for different periods of solar activity. It is found that correlation coefficients change sign in different periods of solar activity, so that for total period 1970–1994 the correlation coefficient is very small. The obtained information indicates that a neutrino should have nonzero mass and nonzero magnetic moment.     

T-odd correlation in the K_{13_\gamma } decay

The dependence of the K ⁺ → π ⁰ l ⁺ ν _lγ decay rate on the T-odd kinematical variable ξ = q · [p _l × p _π]/m _K ³ is investigated in the tree and one-loop approximations of the Standard Model (SM). It is shown that the partial width of the decay on the tree level is an even function of the variable ξ, whereas the odd component is generated by the electromagnetic final-state interaction and determined by the imaginary parts of one-loop diagrams. The ξ-odd components of the partial widths of the K ⁺ → π ⁰e⁺ ν _e γ and K ⁺ → μ ⁰μ⁺ ν _μ γ decays calculated in the one-loop approximation are smaller by four orders of magnitude than the even components evaluated from the tree-level SM diagrams.     

The decay of isoscalar giant resonances in 24Mg and 40Ca

In this paper we present data on the charged-particle decay of the isoscalar 2⁺ strength between 10 and 20 MeV excitation energy (E_x) in ²⁴Mg and ⁴⁰Ca. The isoscalar strength was excited by inelastic scattering of 120MeV α-particles at 14° and 12.5° for ²⁴Mg and ⁴⁰Ca, respectively. The charged particles originating from the decay were detected in coincidence with the α′ particles at several angles in the scattering plane. J^π assignments of the decaying states were made on the basis of the angular correlation pattern of the α₀ decay to the ground state of ²⁰Ne and ³⁶Ar, respectively, using a DWBA calculation for the m-state population of the decaying state.For ⁴⁰Ca, about 40% of the E2 EWSR is found to be located in the interval E_x = 13.5 ± 1.5 MeV, which is similar to what has been found from previous inelastic scattering experiments at E_x = 18 ± 2 MeV, but much more than such experiments located in the region E_x = 12–15 MeV. The difference for the region E_x < 16 MeV is due to the fact that from our α₀ angular correlation pattern we conclude that virtually no continuum is excited in the (α, α′) process up to E_x = 16 MeV while all previous inelastic hadron scattering experiments assumed such a continuum to be present. The E2 strength distribution for ⁴⁰Ca thus obtained is very different from what previous theoretical calculations predict. For ²⁴Mg about 30% of the E2 EWSR is present in the interval 12.5 ? E_x ? experiments. 16.5 MeV which again is about twice as much as deduced from previous inelastic scattering The observed branching ratios are compared with calculated ones assuming statistical decay. Reasonable agreement was obtained for ⁴⁰Ca, but for ²⁴Mg especially the α₀-decay branch and to a lesser extent also the p₁ one are much stronger than the statistical calculations predict, indicating that especially the α₀ decay occurs mainly in a non-statistical way.A similar conclusion can be drawn from the behaviour of the forward-backward asymmetry in the angular correlations of the decay particles as a function of the excitation energy FBA(E_x). For ⁴⁰Ca, FBA(E_x) for all decay channels increases smoothly on the average once E_x is above a well-defined threshold, which is due to the onset of knock-out processes. For ²⁴Mg, however, the FBA(E_x) for the α₀ shows a large fluctuation as a function of E_x, indicating an interference process between semi-direct decay and knock-out processes.     

Quantum Monte Carlo Investigation of the magnetic properties of weakly interacting antiferromagnetic chains with an alternating exchange interaction with spin S = 1/2

An approximation dependence of the spontaneous magnetic moment at a site, σ/σ(0) ? 1 = and the antiferromagnet-singlet state phase boundary, J ₂/J ₁ = 0.52(3)δ, are determined by the quantum Monte Carlo method in the self-consistent sublattice molecular field approximation for weakly inter-acting (J ₂) antiferromagnetic chains with spin S = 1/2 and alternating exchange interaction (J ₁ ± δ). The Néél temperature and a number of critical temperatures which could be related with the filling energy of two singlets (ΔS ^z = 0) and one triplet (ΔS ^z = 1) spin bands, each of which is split by the sublattice field (h ^{x, y} ≠ h ^z into two subbands, are determined on the basis of the computed correlation radii of the two-and four-spin correlation function, the squared total spin 〈 (S ^z)²〉 with respect to the longitudinal components, the dimerization parameter, and the correlation functions between the nearest neighbors with respect to longitudinal and transverse spin components. On the basis of the Monte Carlo calculations, the critical temperatures and possible energy gaps at the band center are determined for the antiferromagnets CuWO₄ and Bi₂CuO₄ and for the singlet compounds (VO)₂P₂O₇ and CuGeO₃, agreeing satisfactorily with existing results, and new effects are also predicted.     

A detailed study of the 12C + 16O fusion reaction at Ecm = 19 to 24 MeV

Excitation functions of evaporation residues following the ¹²C + ¹⁶O reaction are investigated at E_cm = 19 to 24 MeV. At the resonances which have been previously identified in the ¹²C(¹⁶O, ⁸Be)²⁰Ne two-body channel, an order of magnitude larger resonance cross section is observed in the fusion channel than that in the two-body one and is shared among a few evaporation residues (²⁰Ne, ²³Na and ²⁴Mg). Here the resonance cross section is defined as the resonance part of the cross section on a resonance in an excitation function. It is found that for each resonance the relative intensity of the resonance cross sections of these residues is fairly well explained by assuming that they are coming from the decay of a compound nuclear resonance. It indiates that these intermediate resonances have a large probability to proceed to the formation of a more complicated configuration prior to its decay into exit channels. However, the resonant enhancement in the fusion channel is found to be weak at three energies (E_cm = 19.7, 22.0 and 22.6 MeV), where prominent resonant structures have been observed in inelastic channels. These distinct situations may be attributed to the magnitude of level density of the compound nucleus.     

An experimental check of the quark-model relations for inclusive reactions

The quark-model relations for inclusive reactions obtained in the previous paper by two of the authors are compared with existing experimental data. The x and p_T² distributions are considered for hadrons produced as a result of fragmentation of an incident meson or nucleon, with the kinematics of the resonance decay being taken into account. The predictions of the quark model are in a reasonable agreement with experiment, both in the shape of spectra and in the absolute magnitude of cross sections. A number of predictions is obtained for cross sections of some inclusive reactions which are not yet measured.     

The method of unitary transformations in the theory of nuclear matter

The theory of nuclear matter is investigated by means of the method of unitary transformations in the special case of point transformations. The general formula for the energy per particle as a function of the density is given in Hartree-Fock approximation being neglected the induced three and more body forces for reasonable correlation functions. This function always shows saturation due to a term proportional tok _F ⁵ in the direct part of the approximation. The physical connexion of this term with the scattering amplitudes of the potential is shown. We point out the equivalence to orderk _F ³ of the energy per particle function of the unitary method with Jastrow and separation methods in their simplest form. The saturation properties are calculated for certain classes of correlations using the realistic potential of Gammel, Christian and Thaler.     

Instantons: Dynamical mass generation,chiral Ward identities and the topological charge correlation function

When dynamical mass generation resulting from the breakdown of chiral symmetry is taken into account, instanton dynamics treated within the dilute gas approximation may satisfy the constraints on the quark condensates and the topological charge correlation function derived by Crewther from an analysis of the chiral Ward identities assuming the absence of a physical axial U(1) Goldstone boson. From a consideration of the contribution of the η′ to the topological charge correlation function, a relationship is derived in which m_η′²f_η′² is proportional to the vacuum energy density.     

Induced decay of positronium and grasers

The differential cross section and the total cross section for the stimulated decay of positronium by an incident photon of frequencyΩ is calculated as a function of the dimensionless variableξ=hΩ/mc ². Forξ?1 the total cross section is found to decrease asξ ^?2. We also look at the particular case of positronium in a black-body radiation field. Expressions for the number of induced annihilations per second as functions of the dimensionless ratiomc ²/kT and the number of positronium atoms are obtained. It is found that this rate is proportional to (kT/mc ²)² forkT?mc ² and to (kT/mc ²)ln(kT/mc ²) forkT?mc ². The possibility of utilizing induced two-photon decay of positronium as a γ-ray laser at the wavelength λ _C /2 is examined, where λ _C is the Compton wavelength.     

Self-consistent approximations for correlation functions in the theory of triplet pairing superfluidity in3He

A self-consistent and “spin conserving” approximation scheme is developed for the two-particle correlation functions in superfluid³He. It is shown that the irreducible vertex part occuring in the Bethe-Salpeter equation for the generalized correlation function (having 4⁴ components) must be equal to the functional derivative of the approximate self-energy part with respect to the Green's function. The theory is specialized to the Hartree-Fock approximation yielding the RPA susceptibility belowT _c . An important feature of the resulting diagrammatic expansion is that the spin-fluctuation effects in the superfluid phase arise from infinite iteration of pairs of anomalous propagators inboth the particle-hole and in the particle-particle channels.     

Polarization correlations in the two�Cphoton decay of hydrogen�Clike ions

Polarization properties of the photons emitted in the two?Cphoton decay of hydrogen?Clike ions are studied within the framework of the density matrix and second-order perturbation theory. In particular, we derive the polarization correlation function that gives the probability of the (two?Cphoton) coincidence measurement performed by polarization?Csensitive detectors. Detailed calculations of this function are performed for the 2s _1/2 ??1s _1/2 transition in neutral hydrogen as well as Xe^53?+? and U^91?+? ions. The obtained results allow us to understand the influence of relativistic and non?Cdipole effects on the polarization correlations in the bound?Cbound two?Cphoton transitions in heavy ions.     

QCD corrections to b→sγγ and exclusive Bs→γγ decay

The short distance QCD corrections to b→sγγ are calculated in the leading logarithmic approximation. The equivalence of operator basis reduction for S-matrix elements by using the equations of motion or by proving a low energy theorem is discussed. We apply the above results to the exclusive B_s→γγ decay. The branching ratio of this decay is found to be 5×10⁻⁷ in the Standard Model. We also found that QCD corrections modify considerably the ratio between CP-even and CP-odd two-photon amplitudes.     

Investigation of the nuclear quadrupole interaction in ZnO by the method of perturbed angular correlations of nuclear radiations

It has been demonstrated that the sign and magnitude of the nuclear quadrupole interaction in ZnO can be determined using perturbed angular correlations of nuclear radiations. Information on the magnitude of the nuclear quadrupole interaction for the 184-keV ⁶⁷Zn level can be obtained from the observation of the gamma-gamma perturbed correlation of the directions for the ⁶⁷Ga → ⁶⁷Zn decay, and information on the magnitude and sign of the nuclear quadrupole interaction can be derived from the experiment on the induced beta-gamma correlation of the directions for the ⁶⁷Cu $ \xrightarrow{{\beta ^ - }} $ \xrightarrow{{\beta ^ - }} ⁶⁷Zn decay. The results of measurements have been interpreted using the calculation of the electric field gradient in the ZnO crystal by the full-potential linearized augmented plane wave method with the generalized gradient approximation of the exchange-correlation potential.     

Energies, Auger and radiative probabilities of the doubly-excited 1s23l3l′ states for Be-like neon

Energy levels, Auger decay and radiative transition rates of the doubly-excited 1s ²3l3l′ states for Be-like neon are calculated using relativistic Multiconfiguration Dirac-Fock (MCDF) method. Electron correlation and relaxation effects are taken into account. The magnitude of Auger rates for the 35 doubly-excited states and their decay channels are discussed. Present calculated values are in agreement with the latest experimental electron spectra as well as other theoretical results.     

Electron neutrino tagging through tertiary lepton detection

We discuss an experimental technique aimed at tagging electron neutrinos in multi-GeV artificial sources on an event-by-event basis. It exploits in a novel manner calorimetric and tracking technologies developed in the framework of the LHC experiments and of rare kaon decay searches. The setup is suited for slow-extraction, moderate power beams and it is based on an instrumented decay tunnel equipped with tagging units that intercept secondary and tertiary leptons from the bulk of undecayed π ⁺ and protons. We show that the taggers are able to reduce the ν _e contamination originating from K _e3 decays by about one order of magnitude. Only a limited suppression (～60%) is achieved for ν _e produced by the decay-in-flight of muons; for low beam powers, similar performance as for K _e3 can be reached supplementing the tagging system with an instrumented beam dump.