Direct observation of intracluster reactions induced in (CF<Subscript>3</Subscript>I)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> clusters by femtosecond ultraviolet radiation

Intracluster reactions that are induced in (CF₃I) _n clusters by femtosecond ultraviolet radiation, including the reaction of the formation of the I₂⁺ molecular ion, have been directly observed. It has been shown that there are two channels of the formation of I₂⁺ ions with the characteristic times τ₁ ≈ 1 ps and τ₂ ≈ 7 ps. A model of these reactions has been proposed that is in good agreement with the experimental data.     

Observation of Δ+→ pπ0 decay in heavy-ion collisions

Proton-π⁰ coincidences have been measured at the beam energy of 180A MeV in the reaction Ar+Ca studied by TAPS at SIS/GSI. In the proton-π⁰ invariant mass spectrum we observe a significant excess of strength above the background obtained by event mixing. We attribute this signal to the strength distribution N _Δ of the Δ baryonic resonance. No correlation is observed in the case of deuteron-π⁰ coincidences. Assuming isotropic emission of π⁰ and Δ⁺ from a midrapidity thermal source and isospin symmetry, we determined the global N _Δ/N _πratio of 0.79 ±0.30(stat) ±0.2(syst). This value indicates that most pions produced at subthreshold energy in heavy-ion reaction are mediated by the Δ-resonance. Received: 7 April 2000 / Accepted: 29 August 2000     

Radiative energy loss of high-energy quarks in finite-size nuclear matter and quark-gluon plasma

The induced gluon radiation of a high-energy quark in a finite-size QCD medium is studied. For a sufficiently energetic quark produced inside a medium we find the radiative energy loss ΔE _q∝L ², where L is the distance traveled by quark in the medium. It has a weak dependence on the initial quark energy E _q. The L ² dependence turns to L ¹ as the quark energy decreases. Numerical calculations are performed for a cold nuclear matter and a hot quark-gluon plasma. For a quark incident on a nucleus we predict ΔE _q ≈0.1E _q (L/10fm) ^β , with β close to unity. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 585–589 (25 April 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Normal-metal hot-electron bolometer with Andreev reflection from superconductor boundaries

This paper describes the design and experimental testing of a high-sensitivity hot-electron bolometer based a film of normal metal, exploiting the Andreev reflection from superconductor boundaries, and cooled with the help of a superconductor-insulator-normal metal junction. At the measured thermal conductivity, G≈6×10⁻¹² W/K, and a time constant of τ=0.2 μs, and a temperature of 300 mK, the estimated noise-equivalent power NEP=5×10⁻¹⁸ W/Hz^1/2, assuming that temperature fluctuations are the major source of noise. At a temperature of 100 mK, the thermal conductivity drops to G≈7×10⁻¹⁴ W/K, which yields NEP=2×10⁻¹⁹ W/Hz^1/2 at a time constant of τ=5 μs. The microbolometer has been designed to serve as a detector of millimeter and FIR waves in space-based radio telescopes. Zh. éksp. Teor. Fiz. 115, 1085–1092 (March 1999)     

Determination of very slow μ+ hop rates in Cu by LLF-μSRhop rates in Cu by LLF-μSR

Muon spin relaxation in low (weak) longitudinal magnetic field (LLF-μSR) provides a means of independently determining the static dipolar width Δ characterizing the μ⁺ lattice site and the correlation time τ_c for μ⁺ hopping, in a manner that is nearly model-independent for τ_c and especially accurate in the near-static limit (τ_c>τ_μ). The advantages of this method are illustrated by its application to muon hopping in Cu near the τ_c maximum around 50 K.     

Submillimetric cyclotron resonance linewidths and electron relaxation time in copper

For the first time submillimetric microwaves (λ<1 mm) are used to observe Azbel' Kaner cyclotron resonance in metals. The very high frequency used (typicallyF≅400 GHz) gives a large value ofωτ (typically 500) and therefore very sharp peaks. The fundamental resonance fieldH _c=m ^* cω/e is rather high (typically 200 KG), so subharmonicsH _c/n can be observed at many values ofH in the field region 0–27 KG. If relatively few electrons participate in the resonance and ifω _cτ≧50 (ω _c=eH/m ^* c,τ relaxation time) thenChambers has shown that the line shapes are independent of relaxation time while the fractional linewidthΔH/H varies as l/ωτ. For the belly orbit in pure copper the conditions of Chambers' theory are satisfied forH≧20 KG parallel to [111] axis.m ^* is a minimum andτ=1.8×10⁻¹⁰ s.     

The gluonic condensate from the hyperfine splitting M cog(XcJ) — M(h c) in charmonium

The precision measurement of the hyperfine splitting, Δ_HF(1P, c-c) = M _cog(xcJ) — M(h _c), in the Fermilab-E835 and CLEO experiments allows one to determine the gluonic condensate G ₂ with high accuracy if the gluonic correlation length T _g is fixed. In our calculations, the negative value of Δ_HF = −0.5 ± 0.4 MeV, as in the E835 experiment, is obtained only if the relatively small T _g = 0.16 im and G ₂ = 0.060(3) GeV⁴ are taken. For T _g ≥ 0.2 fm, the hyperfine splitting is positive and grows for increasing T _g. In particular, for T _g = 0.2 fm and G ₂ = 0.045(2) GeV⁴, the splitting Δ_HF = 1.0(5) MeV is just in accordance with the recent CLEO result. The values of G ₂ taken correspond to the “physical” string tension σ ≈ 0.18 GeV². The text was submitted by the authors in English.     

Excitation of the Roper resonance in single- and double-pion production in nucleon-nucleon collisions⋆

In most investigations the Roper resonance is sensed only very indirectly via complex partial-wave analyses. We find indications for its excitation in the invariant nπ⁺ mass spectrum of the pp → npπ⁺ reaction at M ≈ 1360 MeV with a width of ≈ 150 MeV . The values fit very favorably to the most recent phase shift results as well as to the observations at BES. In the near-threshold two-pion production pp → ppπ⁰π⁰ , where the Roper excitation and its subsequent decays via the routes N ^* → Δπ → Nππ and N ^* → Nσ are the only dominant processes, we find its direct decay into the Nσ channel to be the by far dominating decay process --in favor of a monopole excitation of the Roper resonance.     

Spectroscopy of quasiparticle excitations of superconducting bismuth cuprate at high pressures

We study the energy spectrum of Bi2223 (Bi_1.6Pb_0.4Sr_1.8Ca_2.2Cu₃O_x) at high hydrostatic pressures by Andreev-and tunneling-spectroscopy methods. We determine the gap anisotropy in the basal ab plane and find the following values for the parameters Δ(ϕ): Δ_max=42 mV, and Δ_min=19.5 mV (T _c =110 K and dT _c /dP=0.16 K/kbar). We detect an increase in the ratio R=2Δ_max/kT _c with pressure P; for Bi2223 cuprate, dR/dP≈0.017 kbar⁻¹. In the phonon-frequency region we detect a “softening,” due to pressure, of the high-frequency part of the phonon spectrum corresponding to “breathing” modes of oxygen, as well as other optical modes of Cu-O. The characteristic frequencies of the spectrum for ℏΩ>60 mV are found to decrease, with increasing pressure, at a rate d ln(ℏΩ)/dP≈−6.5±0.5×10⁻³ kbar⁻¹. This result explains the observed increase in the ratio 2Δ/kT _c (P) in the model of strong electron-phonon interaction. Zh. éksp. Teor. Fiz. 113, 1397–1410 (April 1998)     

Leading-order 2πγ exchange NN interaction: Central potentials proportional to g<Subscript>A</Subscript><Superscript>0</Superscript> and g<Subscript>A</Subscript><Superscript>2</Superscript>

We calculate at two-loop order in chiral perturbation theory the electromagnetic corrections to the leading-order 2π exchange NN interaction proportional to g _A ⁰ and g _A ². The resulting 2πγ exchange potential contains isospin-breaking components which reach up to about -2% of the corresponding isovector 2π exchange potential. With a value of only -17keV at r = m _π ^-1 = 1.4fm the charge-independence breaking central potential obtained here is negligibly small in comparison to the one generated by the isoscalar c₃ contact vertex. Our calculation confirms that the largest long-range isospin-violating NN potentials arise from the 2πγ exchange diagrams involving the large low-energy constants c ₄ ≃ - c ₃ ≃ 3.3GeV^-1 representing the important Δ(1232) dynamics.     

Production of medical radioisotopes with high specific activity in?photonuclear reactions with γ-beams of high intensity and?large brilliance

We study the production of radioisotopes for nuclear medicine in (γ,xn+yp) photonuclear reactions or (γ,γ′) photoexcitation reactions with high-flux [(10¹³–10¹⁵)γ/s], small diameter ∼(100 μm)² and small bandwidth (ΔE/E≈10⁻³–10⁻⁴) γ beams produced by Compton back-scattering of laser light from relativistic brilliant electron beams. We compare them to (ion,xn+yp) reactions with (ion = p,d,α) from particle accelerators like cyclotrons and (n,γ) or (n,f) reactions from nuclear reactors. For photonuclear reactions with a narrow γ-beam the energy deposition in the target can be managed by using a stack of thin target foils or wires, hence avoiding direct stopping of the Compton and pair electrons (positrons). However, for ions with a strong atomic stopping only a fraction of less than 10⁻² leads to nuclear reactions resulting in a target heating, which is at least 10⁵ times larger per produced radioactive ion and often limits the achievable activity. In photonuclear reactions the well defined initial excitation energy of the compound nucleus leads to a small number of reaction channels and enables new combinations of target isotope and final radioisotope. The narrow bandwidth γ excitation may make use of the fine structure of the Pygmy Dipole Resonance (PDR) or fluctuations in γ-width leading to increased cross sections. Within a rather short period compared to the isotopic half-life, a target area of the order of (100 μm)² can be highly transmuted, resulting in a very high specific activity. (γ,γ′) isomer production via specially selected γ cascades allows to produce high specific activity in multiple excitations, where no back-pumping of the isomer to the ground state occurs. We discuss in detail many specific radioisotopes for diagnostics and therapy applications. Photonuclear reactions with γ-beams allow to produce certain radioisotopes, e.g. ⁴⁷Sc, ⁴⁴Ti, ⁶⁷Cu, ¹⁰³Pd, ^117m Sn, ¹⁶⁹Er, ^195m Pt or ²²⁵Ac, with higher specific activity and/or more economically than with classical methods. This will open the door for completely new clinical applications of radioisotopes. For example ^195m Pt could be used to verify the patient’s response to chemotherapy with platinum compounds before a complete treatment is performed. Also innovative isotopes like ⁴⁷Sc, ⁶⁷Cu and ²²⁵Ac could be produced for the first time in sufficient quantities for large-scale application in targeted radionuclide therapy.     

Production of delta isobars on tantalum nuclei in CTa collisions at a projectile momentum of 4.2 GeV/<Emphasis Type="Italic">c</Emphasis> per nucleon

A procedure that relies on analyzing the angle between the momenta of the proton and the pion originating from CTa interaction at a projectile momentum of 4.2 GeV/c per nucleon was used to investigate for the first time the production of Δ⁰ and Δ⁺⁺ resonances in such interactions and to determine their parameters. The criterion of stability in the parameter a characterizing the background contribution was presented for the first time and was used to reconstruct the mass distributions of Δ⁰ and Δ⁺⁺ resonances. The contributions of Δ⁰- and Δ⁺⁺-resonance decays to the production of charged pions were estimated. The results of this study are compared with similar data obtained by using the analogous procedure for Δ⁰ and Δ⁺⁺ resonances produced on carbon nuclei in CC and ⁴HeC collisions at a projectile momentum of 4.2 GeV/c per nucleon and in π ⁻¹²C interactions at a projectile momentum of 40 GeV/c.     

Dynamic-mechanical and nuclear magnetic resonance study of relaxation processes in ultra-high molecular weight polyethylene fibres

The relaxation processes (α, β, and γ) in UHMW PE fibres drawn to different draw ratios λ have been studied by dynamic-mechanical and nuclear magnetic resonance methods. The temperature dependences of tensile loss moduli E″ and spin-lattice relaxation times T ₁ have been analyzed assuming distribution of correlation times τ according to the Davidson—Cole function. The activation energies E _a and parameters ε characterizing widths of distribution, and asymptotic value τ ₀ for correlation times have been determined from experimental data for low-temperature γ-process and for high-temperature α-process. A weak β-process has been found by dynamic-mechanical method and quantitative analysis was made only for fibres with λ = 9. The temperature dependences of second moment M ₂ of the broad-line NMR spectra have been analyzed according to the Gutowsky—Pake formula, which includes only a single correlation time τ _c without distribution. In this case the activation energies E _a and values of τ ₀ have been determined.     

Study of transients in the propagation of nonlinear waves in some reaction diffusion systems

We study the transient dynamics of single species reaction diffusion systems whose reaction terms f(u) vary nonlinearly near u ≈ 0, specifically as f(u) ≈ u² and f(u) ≈ u³. We consider three cases, calculate their traveling wave fronts and speeds analytically and solve the equations numerically with different initial conditions to study the approach to the asymptotic front shape and speed. Observed time evolution is found to be quite sensitive to initial conditions and to display in some cases nonmonotonic behavior, ascribable to the disparity in time scales between the evolution of the front interior and the front tail.     

The time arrow in a Planck gas

In this paper the quantum heat transport equation (QHT) is applied to the study of thermal properties of Planck gas, i.e., a gas of massive particles with mass equal to the Planck massM _P = (łc/G)^1/2 and whose relaxation time equals the Planck timeτ _p = (łG/c ⁵)^1/2. The quantum of thermal energy for a Planck gas,E ^Planck = 10¹⁹GeV, and the quantum thermal diffusion coefficientD ^Planck = (ħG/c)^1/2 are calculated. Within the framework of QHT the thermal phenomena in a Planck gas can be divided into two classes: for a time period shorter thanτ _p , the time reversal symmetry holds and for a time period longer thanτ _p , time symmetry is broken, i.e., a time arrow is created.     

Electron Raman scattering in Tl2Ba2CuO6+x crystals

It is inferred from the position of the 2Δ peak in the electron light scattering spectra that in a sample with high T _c ≈90 K the ratio 2Δ/T _c is approximately equal to 6 and 7 for different polarizations, while in overdoped samples with T _c ≈40 and 35 K the 2Δ peak is observed only in x′y′ polarization with a substantially lower ratio 2Δ/T _c ≈3.2. With decreasing T _c in Tl₂Ba₂CuO_6+x crystals, a transition occurs from strong to weak coupling; the superconducting gap remains anisotropic at different doping levels. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 760–765 (10 December 1996)     

Strong reaction channels at barrier energies in the system 6Li + 208Pb

Large cross-section reaction channels were measured in the systems ⁶Li( ⁷Li) + ²⁰⁸Pb with high statistical accuracy at 5(3) energies around the Coulomb barrier from 29 to 39 MeV. These channels were assigned (mainly) to the breakup of ⁶Li, very loosely bound, into α + d and to the breakup of ⁵Li, produced by n-transfer to the target, into α + p and to similar processes with ⁷Li beam. The cross-sections with ⁶Li, S _α = 1.475 MeV, are systematically larger than the ⁷Li ones. This reflects, most likely, the higher binding energy of ⁷Li, S _α = 2.468 MeV. Theoretical predictions for the ⁶Li + ²⁰⁸Pb system which include for ⁶Li breakup to continuum states within a continuum discretized coupled-channels approach (CDCC) and resonant breakup plus n-transfer with DWBA reproduce the angular distribution shapes but still underestimate the cross-sections by a factor ∼ 3. Received: 15 January 2001 / Accepted: 3 March 2001     

Chemical freeze-out temperature in the hydrodynamical description of Au + Au collisions at $$ \sqrt {^S NN} = 200 GeV $$

We study the effect of separate chemical and kinetic freeze-outs to the ideal hydrodynamical flow in Au + Au collisions at RHIC ( energy). Unlike earlier studies we explore how these effects can be counteracted by changes in the initial state of the hydrodynamical evolution. We conclude that the reproduction of pion, proton and antiproton yields necessitates a chemical freeze-out temperature of T ≈ 150MeV instead of T = 160–170 MeV motivated by thermal models. Contrary to previous reports, this lower temperature makes it possible to reproduce the p T spectra of hadrons if one assumes very small initial time, τ ₀ = 0.2 fm/c. However, the p T differential elliptic flow, v ₂(p T) remains badly reproduced. This points to the need to include dissipative effects (viscosity) or some other refinement to the model.     

On the investigation of magnetic critical phenomena in ferromagnets by μSR and PAC

On the basis of current theoretical views on the critical phenomena in isotropic Heisenberg ferromagnets the power temperature behavior Λ=c(τ)λ₀τ^-w has been derived for the muon spin relaxation rate Λ as π-T _c ⁻¹ (T-T _c ) → 0⁺. It is shown that the crossover from an exchange critical regime to a dipolar one is accompanied not only with the change in the critical exponentw in the above law, but also with the reduction of the coefficientc(π). A comparison with the temperature behaviour of the inverse nuclear relaxation timet _R ⁻¹ measured in the PAC experiment is carried out.