d′ production in heavy-ion collisions

The production of d′ dibaryons in heavy-ion collisions due to the elementary process NN → d′π is considered. The NN → d′π cross section is estimated using the vacuum d′ width Γ_d′ ≈ 0.5 MeV extracted from data on the double charge exchange reactions on nuclei. The d′ production rate per single collision of heavy ions is estimated at an incident beam energy of 1 A GeV within the framework of the Quantum Molecular Dynamics transport model. We suggest to analyse the invariant mass spectrum of the NNπ system in order to search for an abundance of events with the invariant mass of the d′ dibaryon. The d′ peak is found to exceed the statistical fluctuations of the background at a 6σ level for 2 × 10⁵ · A central collisions of heavy ions with the atomic number A.     

Polarization effects in inelastic deuteron scattering in the region of baryon-resonance excitation

Within the impulse approximation, polarization effects in inelastic deuteron scattering on nuclei are investigated in the region around the resonance of mass about 2190 MeV/c ². It is shown that the spin-dependent part of the NN → NN*(2190) amplitude plays a significant role at high momentum transfers. Predictions are obtained for some polarization observables and cross sections for various deuteron-spin-flip processes.     

Regge pole model for invariant functions. IV

TheNN→NN andγN→πN differential cross sections for polarized targets are discussed in terms of invariant functions. In former fits of high energy experimental data certain of these invariant functions came out to be small. Based on these fits thepn→np,p¯p→ n¯n, γp→ nπ ⁺ andγn→ pπ ^? differential cross sections are predicted to be independent of the target polarization.     

Collisional broadening of infra-red absorption lines

The Lorentz half-widths of collision-broadened lines in the rotation-vibration bands of diatomic molecules vary with line number |m| in the P and R branches. The observed variation of half-width for lines in the 0 → 1 and 0 → 2 bands of CO and HCl are interpreted in terms of a simple fitting procedure. One dominant source of line broadening is assumed to consist of diabatic hard collisions involving transitions from each rotational level to all higher rotational levels; the effectiveness of this process, which varies from line to line, is described in terms of an empirically adjusted collision cross section, a maximum collision parameter related to independently measured molecular properties, and upon the availability of the required energy and angular momentum in molecular collisions. The second source of line broadening, assumed to be the same for all lines, includes all other types of collisions and is represented by a single empirically adjusted cross section. The simple fitting procedure is applied successfully to self-broadening of CO lines and to foreign-gas broadening of CO and HCl; for HCl self-broadening an additional cross section for resonant-dipole processes must be included. Possible applications of the fitting procedure to HBr, HF, and CO₂ are discussed. The simple procedure presented represents an approximation that may prove useful pending the development of readily applicable complete theories based on first principles.     

Evolution of pion phase-space density and Bose-enhancement effects in high-energy heavy-ion collisions

The evolution of the pion phase-space density in the ultrarelativistic heavy-ion collision is investigated on the basis of an analytical solution of the kinetic equation. The rate of pion production is treated as a superposition of binary collisions of the projectile and target nucleons. It is calculated in accordance with the experimentalNN→πX cross section, neglecting the deceleration of nuclei during their interpenetration. Very large occupation numbers of the lowp _T -pions in the central rapidity region are predicted for typical heavy-ion collisions at Brookhaven and CERN energies. The role of the Boseenhancement of pion production is discussed. It is shown that this effect may be essentially suppressed due to the finite formation time of pions.     

Giant resonance in the total photoabsorption cross section of Z ≈ 90 nuclei

Total photoabsorption cross sections of ²³²Th, ²³⁵U, ²³⁸U and ²³⁹Pu have been measured in the giant resonance region by the absorption method. Measured cross sections were approximated by two Lorentz lines. Lorentz line parameters, integrated cross sections, deformation parameters and quadrupole moments are given. The analysis of the nuclear optical anisotropy evolution with the increase of Z shows that Z ≈ 90 nuclei seem to be transitional, similar to N ≈ 90 nuclei. A comparison of experimental cross sections with dynamic collective model calculations has been performed. The evidence that quadrupole photoabsorption occurs in the 20–25 MeV energy region is also presented.     

Photoabsorption cross section measurements of CO2 between 106.1 and 118.7 nm at 295 and 195 K

High-resolution vacuum ultraviolet carbon dioxide photoabsorption cross sections are required for modeling airglow emissions from the Martian and Venusian atmospheres and for photochemical models of those atmospheres. We report cross section measurements on CO₂ at 295 and 195 K between 106.1 and 118.7 nm at a spectral resolution of 0.005 nm. Significant deviations from the results of previous, lower-resolution measurements are apparent, particularly in regions of sharp spectral structure. Temperature dependence in the photoabsorption cross sections is evident throughout the region, being especially pronounced at wavelengths greater than 113 nm.     

Subthreshold and near-threshold K +-meson production on light nuclei by protons

Inclusive K ⁺-meson production in proton-nucleus collisions in the near-threshold and subthreshold energy regimes is analyzed with respect to the one-step (pN → K ⁺ YN, Y=Λ, Σ) and two-step (pN → NNπ, NN2π; πN → K ⁺ Y) incoherent production processes. An appropriate folding model is used that properly takes into account the struck-target-nucleon-removal energy and the momentum distribution (nucleon spectral function), novel elementary cross sections for proton-nucleon reaction channel close to threshold, as well as nuclear-mean-field-potential effects on the one-step and two-step kaon-creation processes. A detailed comparison of the model calculations of the K ⁺ total and differential cross sections for the p+⁹Be and p+¹²C interactions with the existing experimental data is given, which displays both the relative role of the primary and secondary production channels at considered incident energies and those features of the cross sections that are sensitive to the high-momentum and high-removal-energy parts of the nucleon spectral function. It is found that, contrary to previous studies known in the literature, the pion-nucleon production channels do not necessarily dominate in pA collisions at subthreshold energies and that the relative strength of the proton-and pion-induced reaction channels for light target nuclei in the subthreshold energy regime is governed by the kinematics of the experiment under consideration.     

K + production in proton-nucleus collision

We present a dynamical study ofK ⁺ production in proton-nucleus collisions from 1.2 to 2.5 GeV bombarding energy. The evolution of the proton-nucleus collision is described by a transport equation of the Boltzmann-Uehling-Uhlenbeck type. We incorporate all known sources forK ⁺ production and study their momentum and angular distributions, and the excitation function. We show that at lower energies (E _b<1.5 GeV) theNΔ andNN* channels dominate the kaon yield for light systems. At higher bombarding energies the directNN channel accounts for almost the whole cross section.     

Measurement of the Baryon-exchange reactions K−p→Λη, K−p→Λπ0 at 6 GeV/c and the value of the ηNN coupling constant

In order to determine the ηNN coupling constant we have measured the two reactions K^?p→Λη and K^?p→Λπ⁰ with a magnetic wire chamber spectrometer which contained a gamma counter for the γγ decays of π⁰ and η. The Λ polarization and the differential cross sections are given. The latter have quite different u dependences. Their ratio is interpreted, in terms of a nucleon-Regge exchange model, as the effect of a small ηNN coupling constant for which we obtain G_ηNN² = G_πNN² · (0.26 ± 0.10) as allowed by SU(3). The large value given by Heisenberg's non-linear field theory, G_ηNN² = G_πNN² · 0.9, is excluded by this measurement if the characteristic u dependence of the Λπ⁰ channel is attributed to N_α Regge exchange.     

The fate of Li and Be in stars and in the laboratory

We connect the observed under-abundances of Li and Be in dwarfs, with recent results on nuclear cross sections at low energies: for collisions of protons with atomic or molecular targets, the measured cross sections seem too high with respect to extrapolations for bare nuclei. Phenomenologically, these anomalous nuclear interactions can be described in terms of an effective screening potentialU _lab in the range of few hundred eV: in the presence of the electron cloud, nuclei become more transparent to each other as if the effective collision energy is aumented byU _lab. This implies that fusion cross sections are enlarged and at the same time elastic cross sections are lowered. If something similar occurs in stellar plasma, the nuclear burning temperatures are lowered, whereas diffusion processes are enhanced. We find that the observed Li and Be abundances in the Hyades and in the Sun can be reproduced for effective screening potentials of the plasma in the range of 600–700 eV, close to that found by experiments in the laboratory.     

ω Meson photoproduction from deuterium at 3.9 GeV photon energy

Measurements are presented of the cross sections for ω meson photoproduction from deuterium at an average photon energy of 3.9 GeV. The cross section for the coherent process γd → dω is found to be 1.4 ± 0.5 μb and that for the incoherent process γd → npω is 3.3 ± 0.7 μb. The forward differential cross section for γd → dω is estimated to be (dσ/dt)_t=0 = 54 ± 20 μb/GeV². This result indicates that the value of the A₂ exchange amplitude in ω photoproduction from hydrogen is less than that predicted from the measurements of the difference between the total hadron photoproduction cross sections from neutrons and protons. We note that the world data tend to show that the A₂ exchange amplitude is more consistent with zero than with the value needed to explain these differences. Measurements are also presented of the ratio of the coupling constants of the photon to the ? and ω mesons which tend to favour the vector dominance model proposed by Das, Mathur and Okubo with exact SU(3) symmetry at infinite energies.     

Isospin symmetry and giant resonance in nuclei of mass number <Emphasis Type="Italic">A</Emphasis> = 14

An analysis of the cross sections for photon absorption by isobar nuclei ¹⁴C and ¹⁴N in the giant-dipole-resonance region demonstrates a high degree of isospin symmetry for this type of collective excitations in the above nuclei. Giant resonances in A = 14 isobar nuclei are related by a simple rescaling procedure that is based on the fact that, for these nuclei, the isospin remains a good quantum number in the process of dipole excitations of energy up to about 40 MeV. The features of the isospin splitting of the giant resonance in the ¹⁴C nucleus are refined. The shape of the cross section for photoabsorption in the ¹⁴C nucleus—in particular, a large width of the giant resonance in this nucleus—is exhaustively explained.     

Optical pumping method for studying nuclear-spin polarization in alkali dimers

Magnetic resonance of¹³³Cs nuclei was observed in free Cs₂ dimers employing the method of optical pumping in combination with atom-molecule nuclear orientation transfer. This novel method allows measurements ofn m r spectra in free diamagnetic molecules, atom-dimer exchange cross sections, dissociative collision cross sections of dimers against buffer gases and intramolecular relaxation. The dimer-atom exchange cross section for Cs-Cs₂ isσ _DE =(1.9±0.4)·10^?14cm², the dissociative dimer-buffergas collisions show the expected exponential behaviour in temperature.     

Fusion cross-section measurements for the 13C+13C reaction

Absolute γ-ray yields from characteristic low-lying levels in nuclei produced in the ¹³C+ ¹³C reaction have been measured from E_c.m. = 4.0 to 15.8 MeV using an intrinsic germanium detector. Statistical-model calculations of the decay modes of the compound nucleus have been used to deduce absolute cross sections for the production of the observed residual nuclei and to determine the fusion cross section. Consistency checks on the adopted procedure lead to an estimated absolute uncertainty of ± 15 % on the deduced cross sections. Over the energy range covered, no striking evidence has been found for either broad single-particle resonances or for narrow non-statistical resonances in the cross sections for individual channels. Comparisons are made with optical-model calculations of the reaction cross section and with different expressions for the fusion cross section.     

The deformation and orientation effect on reaction cross section with deformed targets

The reaction cross section (σ_R) for a deformed target nucleus and spherical projectile is calculated using the optical-limit approximation of the Glauber-Sitenko theory. A method is presented to include both the density-dependent NN interaction and the higher order deformations of the target nucleus in the collision process. We studied both the orientation and the deformation dependence of σ_R within the energy range 30–900 MeV/A. We found that the orientation of the heavy target nucleus (A ≥ 120) can produce a difference in the calculated σ_R up to 30%. The averaged σ_R over all directions of the symmetry axis of the deformed nucleus differs by less than 1% compared with σ_R calculated for a spherical target with the same rms matter radius as the deformed nucleus. For certain orientation, it was found that σ_R is highly dependent on the hexadecapole deformation. The orientation-averaged cross sections show almost no variation with either the sign or the value of the hexadecapole deformation. We compared the average cross section with the experimental data for several mass numbers; fair agreement is obtained.     

He原子和BH分子碰撞体系的转动激发能量转移

在碰撞体系He+BH的CCSD(T)二维势能面基础上,应用密耦方法,研究了 He+BH分子碰撞转动激发过程.计算了该体系的转动态-态激发的弹性和非弹性的微分和积分截面,分析了计算结果与势能面特征间的关系.结果表明: He原子以从H原子端共线形式碰撞BH分子对j=0→j'=2的激发最为有效;短程排斥对Δj=2的激发作用较大;态-态跃迁总截面出现振荡结构,长程部分分波只对j=0→j'=1的跃迁总截面有较大贡献,j'≥ 关键词： He+BH体系 转动激发 散射截面     

Schwingungsanregung von H2-Molekülen durch Zentralstoß mit Li+-Ionen

An apparatus is described for measuring the inelastic differential cross section for vibrational excitation in collisions of diatomic molecules with monoenergetic ions at laboratory energies between 10 and 50 eV. The method consists of measuring the time of flight of single ions with a time amplitude converter and displaying the results on a 100 channel pulse height analyzer. From the shift in the time of flight relative to that expected for elastic scattering the final state of the molecule excited in a single collision is identified. By studying only central collisions with almost zero impact parameter rotational excitation is strongly suppressed. Measured times of flight after collisions of monoenergetic Li⁺ ions with H₂ show that with increasing energy the most probable vibrational quantum jump increases from 0→1 to 0→2,0→3 etc. Contrary to the usual assumption of a small steric factor for vibrational excitation the results show that the inelastic cross section is larger than the elastic cross section. Using reported potential parameters the energy dependence of the most probable excited state is compared with the calculations of Secrest and Johnson for a one-dimensional collinear collision. The satisfactory agreement suggests that the steric factor is close to 1. From measurements at different scattering angles at 10 eV the integral inelastic cross section is found to be about 0.2 Ų corresponding to a differential cross section of 0.4 Ų/sr. Measured values of integral and differential total cross sections for Li⁺-He andLi⁺-H₂ are reported and compared with theory. Direct dissociation of D₂ by Li⁺ in the energy range from 25 to 55 eV was not observed, yielding an upper limit for the cross section of 4 · 10^?4 Ų/sr.