Steady state self-diffusion at low density

We prove that the motion of a test particle in a hard sphere fluid in thermal equilibrium converges, in the Boltzmann-Grad limit, to the stochastic process governed by the linear Boltzmann equation. The convergence is in the sense of weak convergence of the path measures. We use this result to study the steady state of a binary mixture of hard spheres of different colors (but equal masses and diameters) induced by color-changing boundary conditions. In the Boltzmann-Grad limit the steady state is determined by the stationary solution of the linear Boltzmann equation under appropriate boundary conditions.Supported in part by NSF Grant No. PHY 78-15920-02.Supported by a Heisenberg Fellowship of the Deutsche Forschungsgemeinschaft.     

Laboratory measurements on molecules of astrophysical interest

Summary The infra-red absorption bands of a representative polycylic aromatic hydrocarbon (PAH) molecule, coronene, have been examined in the laboratory at various temperatures up to 240 °C. This paper is devoted to a systematic analysis of the temperature variations of the integrated band intensity. A tentative interpretation of the observed behaviour is given in terms of anharmonicity effects. These new laboratory results may be helpful in the context of the attribution of the ?unidentified? infra-red band to PAH molecules. In addition, for the first time thermal-emission spectra of three different PAHs recorded at 240 °C are presented. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

A simplified equation of state near nuclear density

The equation of state near nuclear density influences shock formation in stellar collapse Supernovae. The drop in the adiabatic index below $\text{4}\text{3}$ in this region, due to the negative nuclear pressure, disturbs the homology of the inner core and decreases its size. The initial shock energy and formation dynamics are particularly sensitive to matter in this regime.Only matter at low entropies (S ? 1.5) in the unshocked inner core approaches nuclear densities. We derive a simple equation of state for this material and find that nuclear properties are close to those at S = 0. The entropy associated with the nuclear surface can be absorbed into an “effective mass” which decreases towards one with increasing density, giving an accurate accounting for the storage of entropy in the excitation of the large nuclei. Such thermal excitation drains energy with little contribution to the pressure and thus may have important effects on the launching of the shock.Two phase transitions are considered. The first, from the heavy nucleus to the “bubble” phase, occurs at half nuclear matter density and is accomplished by use of simple expressions for the energy and pressure that include effects of the transition implicitly. The second, that to uniform nuclear matter, is done by requiring continuity of the pressure and entropy at the transition. The density at which this transition takes place is calculated and is found to decrease with entropy in a simple manner.With the use of suitable approximations, the equation of state is presented in a non-iterative form easily adapted for use in full hydrodynamical calculations of the supernovae process. Comparison with more detailed equations of state is made and the simplified one is found to represent well all important features.     

Intermediate state perturbation in low temperature nuclear orientation

This paper deals with the perturbation of low temperature nuclear orientation in an intermediate state by an interaction not axially symmetric around the orientation axis. The directional distribution of the emitted radiation is factorized into initial orientation tensors and perturbation factors known from perturbed angular correlation theory. Low temperature nuclear orientation coefficients have been calculated numerically for¹¹¹Cd in a polycrystalline sample with axially symmetric electric field gradient. Nuclear orientation experiment on¹¹¹In in indium host confirms theoretical perturbation predictions and demonstrates the sensitivity of the method on different parameters. Moreover this orientation allows the determination, in magnitude and sign of¹¹¹In (ground state) nuclear magnetic moment: = +5.48(10)_n.     

Excited state positronium formation in low density gases

Excited state Ps atoms formed in low density Ne, Ar and H₂ gases have been observed for the first time. The maximum yield was estimated to be ≈ 5.7 × 10^?2 excited Ps atoms per stopped positron of energy ≈ 16 eV in H₂. This is about 14 times greater than previous maximum yields.     

Towards understanding astrophysical effects of nuclear symmetry energy

The European Physical Journal A - The properties of the quantum electrodynamic (QED) vacuum in general, and of the nuclear vacuum (ground) state in particular, are determined by virtual processes...     

Screened Coulomb potentials for astrophysical nuclear fusion reactions

The electron-screening acceleration of laboratory fusion reactions at astrophysical energies is an unsolved problem of great importance to astrophysics. That effect is modeled here by considering the fusion of hydrogen-like atoms whose electron probability density is used in Poisson's equation in order to derive the corresponding screened Coulomb potential energy. That way atomic excitations and deformations of the fusing atoms can be taken into account. Those potentials are then treated semiclassically in order to obtain the screening (accelerating) factor of the reaction. By means of the proposed model the effect of a superstrong magnetic field on laboratory hydrogen fusion reactions is investigated here for the first time showing that, despite the considerable increase in the cross-section of the dd reaction, the pp reaction is still too slow to justify experimentation. The proposed model is finally applied on the H² d, pH³ fusion reaction describing satisfactorily the experimental data although some ambiguity remains regarding the molecular nature of the deuteron target. Notably, the present method gives a sufficiently high screening energy for hydrogen fusion reactions so that the take-away energy of the spectator nucleus can also be taken into account. Received: 19 May 2000 / Accepted: 4 September 2000     

Laboratory simulations of astrophysical blast waves with high energy lasers

We review recent efforts to simulate aspects of supernova remnants in laboratory experiments by creating energetic explosions with high energy lasers. High energy pulsed lasers are uniquely suited for these kinds of studies. By focusing a laser with pulse energy of a few joules to many hundreds of joules onto a solid target or into a dense gas target, explosive shock waves of very high Mach number can be created. With a well chosen set of laser and target parameters it has been shown by a number of groups that radiative blast waves can be produced. Such blast waves have dynamics dominated by radiation transport and exhibit unusual characteristics, the most important of which include hydrodynamic instabilities which may play an important role in the structure of the interstellar medium. As a result there are now prospects for gaining new insights into astronomical observations of supernova remnants by studying laboratory laser driven plasma systems.     

Constraining nuclear equations of state using gravitational waves from hypermassive neutron stars

Latest general relativistic simulations for the merger of binary neutron stars with realistic equations of states (EOSs) show that a hypermassive neutron star of an ellipsoidal figure is formed after the merger if the total mass is smaller than a threshold value which depends on the EOSs. The effective amplitude of quasiperiodic gravitational waves from such hypermassive neutron stars is approximately 6-7 x 10(-21) at a distance of 50 Mpc, which may be large enough for detection by advanced laser interferometric gravitational wave detectors although the frequency is high, approximately 3 kHz. We point out that the detection of such signals may lead to constraining the EOSs for neutron stars.     

Influence of nuclear physics inputs and astrophysical conditions on r-process

The rapid neutron-capture process(r-process) is one of the main mechanisms to explain the origin of heavy elements in the universe. Although the past decades have seen great progress in understanding this process, the related nuclear physics inputs to r-process models include significant uncertainty. In this study, ten nuclear mass models, including macroscopic, macroscopicmicroscopic, and microscopic models, are used to calculate the β-decay rates and neutron-capture rates of the neutron-rich isotopes for the r-process simulations occurring in three classes of astrophysical conditions. The final r-process abundances include uncertainties introduced by the nuclear mass model mainly through the variation of neutron-capture rates, whereas the uncertainties of β-decay rates make a relatively small contribution. The uncertainties in different astrophysical scenarios are also investigated,and are found to be connected to the diverse groups of nuclei produced during nucleosynthesis.     

Influence of shell structure and pairing correlations on the nuclear state density

A microscopic calculation of nuclear state densities was performed starting from realistic single-particle levels. Within this concept, a correspondence between microscopic ground state energy corrections and microscopic effects on state densities was deduced. It is shown that the approximations introduced by a simple analytical expression for the nuclear state density are comparable to the uncertainties of microscopically calculated state densities for nuclei in their ground state configuration. Guidelines for the determination of the parameters of this analytical expression were deduced from the microscopic computations.     

Modern energy density functional for nuclei and the nuclear matter equation of state

We discuss a method of determining a modern energy density functional (EDF) in nuclei. We adopt a Skyrme type EDF and fit the Skyrme parameters to an extensive set of experimental data on the ground-state binding energies, radii, and the breathing mode energies of a wide range of nuclei. We further constrain the values of the Skyrme parameters by requiring positive values for the slope of the symmetry energy S, the enhancement factor κ, associated with the isovector giant dipole resonance, and the Landau parameter G ₀^′. This is done within the approaches of Hartree-Fock (HF) and HF with the inclusion of correlation effects, using a simulated-annealing based algorithm forminimizing χ ².We also present results of HF based random phase approximation for the excitation strength function of the breathing mode and discuss the current status of the nuclear matter incompressibility coefficient.     

A compressible bag model and equation of state for high density nuclear matter

We propose a compressible bag model, in which a nucleon bag responds microscopic thermal pressure of the other bags. The volume exclusion effect and the particle exchange type interaction ensure the saturation property of the nucleus at normal density and bring about the deconfinement transition in high density region. The critical values of chemical potential and baryon number density for nuclear/neutron matter are estimated. Our equation of state is applied to neutron stars, and shown to be consistent with the observed rotation periods of millisecond pulsars.     

The spin-isospin decomposition of the nuclear symmetry energy from low to high density

The energy per particle BA in nuclear matter is calculated up to high baryon density in the whole isospin asymmetry range from symmetric matter to pure neutron matter.The results,obtained in the framework of the Brueckner-Hartree-Fock approximation with two-and three-body forces,confirm the well-known parabolic dependence on the asymmetry parameterβ=(N?Z)/A(β^2 law)that is valid in a wide density range.To investigate the extent to which this behavior can be traced back to the properties of the underlying interaction,aside from the mean field approximation,the spin-isospin decomposition of BA is performed.Theoretical indications suggest that theβ^2 law could be violated at higher densities as a consequence of the three-body forces.This raises the problem that the symmetry energy,calculated according to theβ^2 law as a difference between BA in pure neutron matter and symmetric nuclear matter,cannot be applied to neutron stars.One should return to the proper definition of the nuclear symmetry energy as a response of the nuclear system to small isospin imbalance from the Z=N nuclei and pure neutron matter.     

Isomeric state of 80Y and its role in the astrophysical rp-process

A careful investigation of the isomeric transition of the long-lived state at 228.5 keV excitation energy in ⁸⁰Y has been done. The HIGISOL facility at the Jyv?skyl? isochronous cyclotron has been used. We used the electron magnetic transporter to prepare an appropriate source and to measure the electron spectra in clean background conditions. The measured internal conversion coefficient α_K = 0.50±0.07 allows unambiguous 1^- identification for the 228.5 keV first excited isomeric state in ⁸⁰Y. With a “bare" half-life of 6.8±0.5 s found in this work, this state is strongly populated in the rp-process during X-ray bursts and has therefore to be taken into account in X-ray burst model calculations. However, because of the similarity of the β-decay half-lives of isomeric and ground states, we find a maximum reduction in the effective β-decay lifetime of ⁸⁰Y of only 17±2%. Our results pave the way for a future investigation of the impact of the isomeric state on the “effective" ⁸⁰Y proton capture rate. Received: 14 March 2001 / Accepted: 10 July 2001     

Condensed matter effects on nuclear fusion rates in laboratory and astrophysical environments

Previously overlooked condensed matter effects (CME) can significantly influence nuclear fusion rates in both laboratory and astrophysical environments. In dense plasmas, the ensemble of fusing particles has a significant exchange of kinetic and potential energies. Thus, there are diminished effective flux velocities resulting in a significant selective reduction of fusion rates. Our CME predictions are testable in laboratory experiments and have broad-ranging implications on the fusion rates for stellar media in general. By calculating reaction rates forp(p, e ⁺ v _e ) D and⁷Be(p, )⁸B in the sun, we show that CME help to solve the solar neutrino problem.     

The determination of the nuclear ground state and transition charge density from measured electron scattering data

The evaluation of elastic and inelastic electron scattering cross sections is discussed in terms of the Fourier-Bessel expansion of the nuclear ground state and transition charge density, respectively. The method allows one to deduce the charge distributions and the moments as well as the corresponding errors, which originate on the one hand from the uncertainties in the measured data and on the other hand from the lack of knowledge about the large-q behaviour of the form factors; these two contributions are determined separately. The method is described and proved with an evaluation of pseudodata and then applied to ²⁰⁸Pb cross sections. For this nucleus, detailed results concerning the possible structure of the charge density are presented.     

On the self-consistency requirement in the low density expansion of the optical potential in nuclear matter

We critically discuss the choice of the auxiliary potential U which is introduced in the low density expansion of the mass operator M(k, w). This choice is related to the analytical properties of M(k, w) in the complex w-plane and we take due account of momentum conservation in the intermediate states appearing in the diagrams associated with M(k, w). We also provide a computation of the one-hole line, rearrangement and renormalization contributions to the optical potential, of the hole state spectral function and of the momentum distribution in nuclear matter. We use a real auxiliary potential which is self-consistent up to the order considered here, i.e. which takes into account the rearrangement and the renormalization corrections. Rearrangement is treated rigorously. The dependence of the obtained results on the choice of the nucleon-nucleon interaction, namely the Hamman-Ho Kim one in our calculation, is discussed.     

Analysis of K-prime equations of state

We present an analysis of the $K$-prime equations of state (EOS) due to Keane and Stacey. It is found that the two EOS differ significantly from each other in some important respects. $K$-prime represents the pressure derivative of the bulk modulus. It is shown that the volume dependence of $K$-prime and higher derivative properties predicted from the Keane EOS are compatible with those predicted from Stacey’s reciprocal $K$-prime EOS only when the Murnaghan approximation is valid. It has been emphasized that the Stacey EOS is more appropriate for describing the relationship between pressure and the bulk modulus and its pressure derivative. The results based on the two EOS have been compared and discussed.