arXiv subjects

Walter Greiner

Publications and source records attributed to Walter Greiner.

At least 37 records · Page 2Linked to original sources

arXiv2010

On possibility of synthesizing superheavy elements in nuclear explosions

The possibility to produce superheavy elements in the course of low-yield nuclear explosions is analyzed within a simple kinetic model which includes neutron capture, gamma-emission, fission and particle evaporation from excited nuclei. We have calculated average numbers of absorbed neutrons as well as mass distributions of U and Cm nuclei exposed to an impulsive neutron flux, as functions of its duration. It is demonstrated that detectable amounts of heavy nuclei absorbing from 20 to 60 neutrons may be produced in this process. According to an optimistic scenario, after multiple beta-decay such nuclei may reach the long-living elements of the predicted ``island of stability''.

nucl-th↗

arXiv2010

True ternary fission of superheavy nuclei

We found that a true ternary fission with formation of a heavy third fragment (a new type of radioactivity) is quite possible for superheavy nuclei due to the strong shell effects leading to a three-body clusterization with the two doubly magic tin-like cores. The simplest way to discover this phenomenon in the decay of excited superheavy nuclei is a detection of two tin-like clusters with appropriate kinematics in low-energy collisions of medium mass nuclei with actinide targets. The three-body quasi-fission process could be even more pronounced for giant nuclear systems formed in collisions of heavy actinide nuclei. In this case a three-body clusterization might be proved experimentally by detection of two coincident lead-like fragments in low-energy U+U collisions.

nucl-th↗

arXiv2010

Strong electric fields induced on a sharp stellar boundary

Due to a first order phase transition, a compact star may have a discontinuous distribution of baryon as well as electric charge densities, as e.g. at the surface of a strange quark star. The induced separation of positive and negative charges may lead to generation of supercritical electric fields in the vicinity of such a discontinuity. We study this effect within a relativistic Thomas-Fermi approximation and demonstrate that the strength of the electric field depends strongly on the degree of sharpness of the surface. The influence of strong electric fields on the stability of compact stars is discussed. It is demonstrated that stable configurations appear only when the counter-pressure of degenerate fermions is taken into consideration.

astro-ph.SR↗

arXiv2010

Deconfinement Phase Transition in Compact Stars : Maxwell vs. Gibbs Construction of the Mixed Phase

We study different realisations of the first order deconfinement phase transition inside a compact star by comparing the Gibbs and Maxwell construction for the mixed phase. The hadronic sector is described within the relativistic mean field model including hyperons. The quark sector is described by the MIT Bag model. We find that these two realisations lead to very different star properties, in particular, the composition of the stellar matter. We also find that for the Maxwell construction there is a sharp discontinuity in the baryon density and the electron chemical potential. We argue that a sharp jump in the elctron chemical potential should lead to the redistribution of electrons and formation of strong electric fields around the discontinuity surface.

nucl-th↗

arXiv2010

The Robertson-Walker Metric in a Pseudo-Complex General Relativity

We investigate the consequences of the pseudo-complex General Relativity within a pseudo-complexified Roberston-Walker metric. A contribution to the energy-momentum tensor arises, which corresponds to a dark energy and may change with the radius of the universe, i.e., time. Only when the Hubble function $H$ does not change in time, the solution is consistent with a constant $Λ$.

gr-qc↗

arXiv2009

Nuclear fragmentation reactions in extended media studied with Geant4 toolkit

It is well known from numerous experiments that nuclear multifragmentation is a dominating mechanism for production of intermediate-mass fragments in nucleus-nucleus collisions at energies above 100 A MeV. In this paper we investigate the validity and performance of the Fermi break-up model and the statistical multifragmentation model implemented as parts of the Geant4 toolkit. We study the impact of violent nuclear disintegration reactions on the depth-dose profiles and yields of secondary fragments for beams of light and medium-weight nuclei propagating in extended media. Implications for ion-beam cancer therapy and shielding from cosmic radiation are discussed.

physics.med-ph↗

arXiv2009

Crossover to Cluster Plasma in the Gas of Quark-Gluon Bags

We study a smooth crossover transition in the gas of quark-gluon bags. The equation of state at high temperature is that of the quark-gluon plasma. However, the system consists of the bags with finite volumes which are defined by the model parameters of the mass-volume bag spectrum. Possible structures in this {\it cluster} quark-gluon plasma are classified.

nucl-th↗

arXiv2008

Critical temperature of antikaon condensation in nuclear matter

We investigate the critical temperature of Bose-Einstein condensation of $K^-$ mesons in neutron star matter. This is studied within the framework of relativistic field theoretical models at finite temperature where nucleon-nucleon and (anti)kaon-nucleon interactions are mediated by the exchange of mesons. The melting of the antikaon condensate is studied for different values of antikaon optical potential depths. We find that the critical temperature of antikaon condensation increases with baryon number density. Further it is noted that the critical temperature is lowered as antikaon optical potential becomes less attractive. We also construct the phase diagram of neutron star matter with $K^-$ condensate.

nucl-th↗

arXiv2008

Pseudo-complex General Relativity

An extension of the theory of General Relativity is proposed, based on pseudo-complex space-time coordinates. The new theory corresponds to the introduction of two, in general different, metrics which are connected through specific conditions. A pseudo-complex Schwarzschild solution is constructed, which does not suffer any more by a singularity. The solution indicates a minimal radius for a heavy mass object. Consequences for the redshift and possible signatures for its observation are discussed.

gr-qc↗

arXiv2008

Stable propagation of a modulated particle beam in a crystal channel

The propagation of a modulated beam of charged particles in a planar crystal channel is investigated. It is demonstrated that the beam preserves its modulation at sufficiently large penetration depths to ensure the feasibility of using a crystalline undulator as a coherent source of hard X rays. This finding is a crucial milestone in developing a new type of lasers radiating in the hard X ray and gamma ray range.

physics.acc-ph↗

arXiv2008

One-dimensional Model of a Gamma Klystron

A new scheme for amplification of coherent gamma rays is proposed. The key elements are crystalline undulators - single crystals with periodically bent crystallographic planes exposed to a high energy beam of charged particles undergoing channeling inside the crystals. The scheme consists of two such crystals separated by a vacuum gap. The beam passes the crystals successively. The particles perform undulator motion inside the crystals following the periodic shape of the crystallographic planes. Gamma rays passing the crystals parallel to the beam get amplified due to interaction with the particles inside the crystals. The term `gamma klystron' is proposed for the scheme because its operational principles are similar to those of the optical klystron. A more simple one-crystal scheme is considered as well for the sake of comparison. It is shown that the gamma ray amplification in the klystron scheme can be reached at considerably lower particle densities than in the one-crystal scheme, provided that the gap between the crystals is sufficiently large.

physics.acc-ph↗

arXiv2008

Impurity effects on the melting of Ni clusters

We demonstrate that the addition of a single carbon impurity leads to significant changes in the thermodynamic properties of Ni clusters consisting of more than a hundred atoms. The magnitude of the change induced is dependent upon the parameters of the Ni-C interaction. Hence, thermodynamic properties of Ni clusters can be effectively tuned by the addition of an impurity of a particular type. We also show that the presence of a carbon impurity considerably changes the mobility and diffusion of atoms in the Ni cluster at temperatures close to its melting point. The calculated diffusion coefficients of the carbon impurity in the Ni cluster can be used for a reliable estimate of the growth rate of carbon nanotubes.

physics.atm-clus↗

arXiv2008

Phase transitions in fullerenes: Fragmentation and reassembly of the carbon cage

Phase transition in fullerenes C60 and C240 are investigated by means of constant-temperature molecular dynamics simulations. In the phase transition region, the assembly (and fragmentation) of the C60 cage from (and to) the gaseous state is demonstrated via the dynamical coexistence of two phases. In this critical region, the fullerene system is seen to continuously oscillate between the carbon cage (the solid phase) and the state of carbon dimers and short chains (the gas phase). These oscillations correspond to consecutive disintegration and formation of the fullerene. Furthermore, the temperature-dependent heat capacity of the fullerene features a prominent peak, signifying the finite system analogue of a first-order phase transition. The simulations were conducted for 500 ns using a topologically-constrained pairwise forcefield which was developed for this work. Results of the simulations were supplemented by a statistical mechanics analysis to account for entropy and pressure corrections, corresponding to experimental conditions. These corrections lead to a phase transition temperature of 3800-4200 K for pressure 10-100 kPa, in good agreement with available experimental values.

physics.atm-clus↗

arXiv2008

Ion-beam therapy: from electron production in tissue like media to DNA damage estimations

Radiation damage induced by ion beams is traditionally treated at different levels of theoretical approaches, for the different scales and mechanisms involved.We present here details of a combined approach that, from a method at a nanoscopic scale, attempts to merge with higher scales existing results, by tuning the analytical method employed when extended to larger scale and so yielding a consistent picture of the entire process. Results will show the possibility to get a good agreement with macroscale methods and, on the other hand, to produce a reliable electron energy spectra to be used for DNA damage estimations.

physics.bio-ph↗

arXiv2007

The influence of the structure imperfectness of a crystalline undulator on the emission spectrum

We study the influence of an imperfect structure of a crystalline undulator on the spectrum of the undulator radiation. The main attention is paid to the undulators in which the periodic bending in the bulk appears as a result of a regular (periodic) surface deformations. We demonstrate that this method of preparation of a crystalline undulator inevitably leads to a variation of the bending amplitude over the crystal thickness and to the presence of the subharmonics with smaller bending period. Both of these features noticeably influence the monochromatic pattern of the undulator radiation.

physics.acc-ph↗

arXiv2007

PET monitoring of cancer therapy with He-3 and C-12 beams: a study with the GEANT4 toolkit

We study the spatial distributions of $β^+$-activity produced by therapeutic beams of $^3$He and $^{12}$C ions in various tissue-like materials. The calculations were performed within a Monte Carlo model for Heavy-Ion Therapy (MCHIT) based on the GEANT4 toolkit. The contributions from $^{10,11}$C, $^{13}$N, $^{14,15}$O, $^{17,18}$F and $^{30}$P positron-emitting nuclei were calculated and compared with experimental data obtained during and after irradiation. Positron emitting nuclei are created by $^{12}$C beam in fragmentation reactions of projectile and target nuclei. This leads to a $β^+$-activity profile characterised by a noticeable peak located close to the Bragg peak in the corresponding depth-dose distribution. On the contrary, as the most of positron-emitting nuclei are produced by $^3$He beam in target fragmentation reactions, the calculated total $β^+$-activity during or soon after the irradiation period is evenly distributed within the projectile range. However, we predict also the presence of $^{13}$N, $^{14}$O, $^{17,18}$F created in charge-transfer reactions by low-energy $^3$He ions close to the end of their range in several tissue-like media. The time evolution of $β^+$-activity profiles was investigated for both kinds of beams. Due to the production of $^{18}$F nuclide the $β^+$-activity profile measured 2 or 3 hours after irradiation with $^{3}$He ions will have a distinct peak correlated with the maximum of depth-dose distribution. We found certain advantages of low-energy $^{3}$He beams over low-energy proton beams for reliable PET monitoring during particle therapy of shallow located tumours. In this case the distal edge of $β^+$-activity distribution from $^{17}$F nuclei clearly marks the range of $^{3}$He in tissues.

physics.med-ph↗

arXiv2007

Nuclei embedded in an electron gas

The properties of nuclei embedded in an electron gas are studied within the relativistic mean-field approach. These studies are relevant for nuclear properties in astrophysical environments such as neutron-star crusts and supernova explosions. The electron gas is treated as a constant background in the Wigner-Seitz cell approximation. We investigate the stability of nuclei with respect to alpha and beta decay. Furthermore, the influence of the electronic background on spontaneous fission of heavy and superheavy nuclei is analyzed. We find that the presence of the electrons leads to stabilizing effects for both $α$ decay and spontaneous fission for high electron densities. Furthermore, the screening effect shifts the proton dripline to more proton-rich nuclei, and the stability line with respect to beta decay is shifted to more neutron-rich nuclei. Implications for the creation and survival of very heavy nuclear systems are discussed.

nucl-th↗

arXiv2007

Pseudo-Complex Field Theory

A new formulation of field theory is presented, based on a pseudo-complex description. An extended group structure is introduced, implying a minimal scalar length, rendering the theory regularized a la Pauli-Villars. Cross sections are calculated for the scattering of an electron at an external Coulomb field and the Compton scattering. Deviations due to a smallest scalar length are determined. The theory also permits a modification of the minimal coupling scheme, resulting in a generalized dispersion relation. A shift of the Greisen-Zatsepin-Kuzmin-limit (GZK) of the cosmic ray spectrum is the consequence.

hep-th↗