Showing papers in "Journal of Physics G in 1994"
TL;DR: In this article, the authors review several important experimental and theoretical developments that during the past decade have revived interest in the stability properties of the heavy elements and discuss the importance of a "few-parameter approach" and of solving a Schrodinger equation for accurately obtaining microscopic effects.
Abstract: We review several important experimental and theoretical developments that during the past decade have revived interest in the stability properties of the heaviest elements. On the experimental side two accomplishments stand out. One is the extension of the known elements to 107Ns, 108Hs and 109Mt. The other is the collection of an extensive body of data on the transition between asymmetric and symmetric fission in the region close to proton number Z=2*50 and neutron number N=2*82. On the theoretical side it has become dear that some models that appropriately account for the most important nuclear-structure aspects are sufficiently reliable for meaningful applications to new regions of nuclei and to studies of new phenomena. We discuss here in particular the importance of a 'few-parameter approach' and of solving a Schrodinger equation for accurately obtaining microscopic effects. We show how such models now more reliably than earlier describe the stability properties of the heaviest elements when the following important points are treated carefully. In fission studies very precise shape specifications are necessary in the saddle-point region and beyond. Coulomb redistribution effects and higher-multipole effects are important for determining the ground-state masses. We review how consideration of these effects has influenced theoretical work in the last decade and present some current results on alpha -decay, beta -decay and fission properties in the heaviest region.
225 citations
TL;DR: Some aspects of heavy-ion reactions close to the Coulomb barrier are reviewed in experiment and theory in this article, both microscopic and macroscopic approaches to the theory are discussed.
Abstract: Some aspects of heavy-ion reactions close to the Coulomb barrier are reviewed in experiment and theory Fusion, complex reactions and quasi-elastic reactions are covered, both microscopic and macroscopic approaches to the theory are discussed A brief outlook is given
183 citations
TL;DR: In this paper, the authors study the temperature and angular momentum dependence of the shell effects in the fissioning nucleus by means of a theoretical analysis of the total fission probability.
Abstract: We study the temperature and angular momentum dependence of the shell effects in the fissioning nucleus by means of a theoretical analysis of the total fission probability. We find clear evidence of the disappearance of the shell correction to the fission barrier with both increasing spin and temperature and propose a general method to determine the level density at the saddle-point configuration. The results of the present study might explain the differences between the fission barriers extracted from the heavy-ion reactions and those obtained using light-particle induced fission.
88 citations
TL;DR: In this paper, the gamma-ray spectra resulting from p-p collisions for an energy range from 0.01 GeV to 107 GeV were calculated and shown to be observable with existing and future instruments such as EGRET on board the GRO satellite, Cerenkov telescopes and air shower arrays.
Abstract: Since Galactic cosmic rays are believed to be created in supernova remnants (SNRs), SNRs are expected to be a source of high energy gamma -rays through the decay of neutral pions produced by p-p collisions. The characteristics of cosmic rays such as the maximum accelerated energy, spectral index and total energy in SNRs can be probed by their gamma -ray emission. Since the resultant gamma -ray spectrum is flatter than that of the Galactic background reflecting the difference of cosmic ray spectra in SNRs and in the Galaxy, SNRs are likely to be observed by gamma -rays at higher energies. For various spectra of cosmic rays in SNRs, we have calculated the gamma -ray spectra resulting from p-p collisions for an energy range from 0.01 GeV to 107 GeV. We show that for a reasonable choice of numerical values SNRs are observable against the gamma -ray background and cosmic ray background with existing and future instruments such as EGRET on board the GRO satellite, Cerenkov telescopes and air shower arrays over various energy bands.
84 citations
TL;DR: In this paper, the radiative effects in deep inelastic scattering of polarized leptons by polarized targets (H,D,3He) are investigated both on the Born level and taking into account radiative corrections (RC).
Abstract: Principal contributions to QED radiative effects in deep inelastic scattering (DIS) of polarized leptons by polarized targets (H,D,3He) are investigated both on the Born level and taking into account radiative corrections (RC). Scattering in the case of transverse polarized targets is also considered. All quantities are presented in terms of covariant variables. Exact results are obtained for low-order corrections. Detailed numerical analysis under the conditions of forthcoming polarization experiments is carried out. Two points of view on calculation of RC to experimental data are discussed in detail. The results of a computer run in the regime of data-processing iteration procedure are presented.
79 citations
TL;DR: The Coulomb field of heavy nuclei provides an intense source of quasi-real photons acting on passing nuclear particles as discussed by the authors, which has many applications in studies of problems of nuclear structure and particle physics.
Abstract: The Coulomb field of heavy nuclei provides an intense source of quasi-real photons acting on passing nuclear particles. This fact has many applications in studies of problems of nuclear structure and particle physics. In 1984 Coulomb dissociation of fast projectiles was proposed as a novel method to investigate radiative capture processes (time-reversed to photodisintegration). This was a new access to a specific class of radiative capture reactions at low energies, relevant for nuclear astrophysics, and it can overcome various limitations of direct measurements. Since then, various dedicated experiments in this field have been performed and many interesting results have been obtained. The status of this approach is critically reviewed and future directions of experimental and theoretical investigations suggested. After sketching the theoretical basis and the basic experimental requirements, some examples are scrutinized. Emphasis is put on a general discussion of the favourable experimental conditions, of related theoretical problems of the analysis and possible pitfalls of the approach. Various astrophysically relevant cases of current and future applications are discussed.
74 citations
TL;DR: In this paper, a prototype of a large hadron calorimeter was used to measure the vertical cosmic ray flux in the range from 5 GeV to 10 TeV, and the vertical flux was observed to be 1.59+or-0.24.
Abstract: Using a prototype of a large hadron calorimeter, vertical cosmic ray hadrons were recorded and the all-hadron flux was measured in the range from 5 GeV to 10 TeV. Hadron reconstruction and identification are described. We observe a vertical flux of dI/dEh=(1.59+or-0.24)*10-5(Eh/100 GeV)-2.72+or-0.10 (m2 s sr GeV)-1. The flux compares well with values obtained in other experiments. Total inelastic cross sections for protons scattered by nuclei in air are deduced from the unaccompanied hadron flux and compared with the values reported by other authors.
68 citations
TL;DR: Partial restoration in nuclear matter of the chiral symmetry of QCD is discussed together with some of its possible signals in this article, where estimates of corrections to the leading, linear dependence of the quark condensate are found to be small, implying a significant reduction of that condenate in matter.
Abstract: Partial restoration in nuclear matter of the chiral symmetry of QCD is discussed together with some of its possible signals. Estimates of corrections to the leading, linear dependence of the quark condensate are found to be small, implying a significant reduction of that condensate in matter. The importance of the pion cloud for the scalar quark density of a single nucleon indicates a close connection between chiral symmetry restoration and the attractive two-pion exchange interaction between nucleons. This force is sufficiently long-ranged that nucleons in nuclear matter will feel a significant degree of symmetry restoration despite the strong correlations between them. Expected consequences of this include reductions in hadron masses and decay constants. Various signals of these effects are discussed, in particular the enhancement of the axial charge of a nucleon in matter.
67 citations
TL;DR: In this article, double fine structures (i.e., decays to the excited states of both fragments) are observed experimentally in contrast to fine structure in only the heavy partner populated by the light partner in earlier cluster radioactivity work.
Abstract: By using the triple gamma coincidence technique with 20 Ge detectors at the Holifield Laboratory in the spontaneous fission of 252Cf, neutronless fragmentations, like 104Mo-148Ba, 106Mo-146Ba, 108Mo-144Ba and 104Zr-148Ce, are experimentally observed directly for the first time. When zero neutron emission spontaneous fission occurs, essentially all the available energy goes into the total kinetic energy of the fragments (cold fission). This process is seen theoretically to be an extension of cluster radioactivity, which involves the emission of one light fragment like 14C, 20O, (24)Ne or 28Mg to nearly equal fragments. In the neutronless spontaneous fission reported here, double fine structures (i.e. decays to the excited states of both fragments) are observed experimentally in contrast to fine structure in only the heavy partner populated by the light partner in earlier cluster radioactivity work. Neutronless spontaneous fission is discussed in the framework of cluster radioactivity.
54 citations
TL;DR: In this article, the lateral distribution of charged particles associated with giant air showers and the attenuation of the local particle density at 600 m from the core, S600, with atmospheric depth using data collected with the Akeno Giant Air Shower Array (AGASA).
Abstract: We have studied the lateral distribution of charged particles associated with giant air showers and the attenuation of the local particle density at 600 m from the core, S600, with atmospheric depth using data collected with the Akeno Giant Air Shower Array (AGASA). The lateral distribution at distances of more than 1 km from the core has been observed to be much steeper than that suggested by some of the earlier measurements. The shape of the lateral distribution function has been observed to depend on the zenith angle of showers but there is no significant evidence for the dependence on the primary energy, within the resolution of the array. The systematic errors in energy estimation due to the uncertainties in the lateral distribution and the attenuation length of S600 are smaller than statistical errors. These errors have been estimated to be approximately 5% and approximately 3% respectively for near vertical showers with sec theta =1.1, and approximately 10% and approximately 12% respectively for showers with sec theta =1.4.
53 citations
TL;DR: In this paper, an improved determination is made of the energy spectrum of the so-called diffuse component of extragalactic gamma rays over the range 30 MeV-10 GeV.
Abstract: An improved determination is made of the energy spectrum of the so-called diffuse component of extragalactic gamma rays over the range 30 MeV-10 GeV. The data used comprise gamma ray spectra from the EGRET instrument on the Compton Gamma Ray Observatory and a variety of information on the column densities of gas in various forms (atomic and molecular hydrogen and ionized material) together with model predictions of the contribution from inverse Compton interactions, all the latter being necessary in order to evaluate the corrections to be applied for gamma rays of Galactic origin. Deleting only one known extragalactic source (the gamma -bright quasar 3C 279) the differential intensity is given by I(Egamma )=9.6*10-7 E- gamma cm-2 s-1 sr-1 GeV-1 with gamma =2.11+or-0.05 above 50 MeV. At lower energies there is some evidence for a somewhat lower intensity. The corresponding energy density in extragalactic space is 1.9*10-6 eV cm-3 in the range 100 MeV-10 GeV. The significance of the measured flux is examined.
TL;DR: In this article, the authors extended the exotic decay half-life calculations of their previously reported model to include decays from odd-A nuclei and decays where the experimental determination of the mass of the emitted ion is ambiguous.
Abstract: We extend the exotic decay half-life calculations of our previously reported model to include decays from odd-A nuclei and decays where the experimental determination of the mass of the emitted ion is ambiguous. We also present calculations for the decay 114Ba to 102Sn+12C which is currently being studied, so that we have now made predictions for the half-lives of all exotic decays for which positive experimental evidence of detection is extant. We find that the decays from even-even nuclei are well described by our model as ground-state to ground-state transitions, but that the decays from odd-A nuclei appear to involve predominantly an excited final state.
TL;DR: In this article, a relativistic constituent quark model was used to give nucleon form factors in agreement with data for both low and high momentum transfer, and it was shown that the model and the specific form of the wave function are essential for the result.
Abstract: We demonstrate that a relativistic constituent quark model can give nucleon form factors in agreement with data for both low and high momentum transfers. The relativistic features of the model and the specific form of the wavefunction are essential for the result.
TL;DR: A comprehensive compilation of experimental results on two-photon initiated reactions with hadronic final states is presented in this paper, where the data divide into results with exclusive and inclusive anal states.
Abstract: A comprehensive compilation of experimental results on two-photon initiated reactions with hadronic final states is presented. Processes with both quasi-real and virtual photons are encoded. The data divide into results with exclusive and inclusive anal states. The exclusive channels listed include vector meson production. The inclusive results encompass total cross-sections, photon structure functions and jet measures. Where appropriate these are compared with theoretical models and expectations. The two photon widths of meson resonances are tabulated, being the principal outcome of many experiments. This compilation was completed in march 1994. All the data in this review can be found in and retrieved from the Durham-RAL HEP databases (HEPDATA on CERNVM and DURPDG VAX/VMS, and on the World-Wide-Web) together with data on a wide variety of other reactions.
TL;DR: In this article, a cascade mechanism of electromagnetic shower evolution in a strong magnetic field is considered, based on the kinetic equations for distribution functions of such particles as electrons, positrons and photons.
Abstract: The cascade mechanism of electromagnetic shower evolution in a strong magnetic field is considered. It is based on the kinetic equations for distribution functions of such particles as electrons, positrons and photons. The differential-difference equations for Mellin transforms of the distribution functions are derived and solved in both adiabatic and modified adiabatic approximations. The authors calculate the differential and integral spectra of particles and other characteristics of the shower, i.e. the 'track' length, the position of the centre of gravity, the longitudinal spread, and the positions of the maxima of the distribution functions.
TL;DR: In this article, a rotational model is developed from a new version of the two-parameter quantum algebra Uqp(u2), which is applied to the description of some recent experimental data for the rotating superdeformed nuclei 192-194-196-198Pb and 192 -194Hg.
Abstract: A rotational model is developed from a new version of the two-parameter quantum algebra Uqp(u2). This model is applied to the description of some recent experimental data for the rotating superdeformed nuclei 192-194-196-198Pb and 192-194Hg. A comparison between the Uqp(u2) model presented here and the Raychev-Roussev-Smirnov model with Uq(su2) symmetry shows the relevance of the introduction of a second parameter of a 'quantum algebra' type.
TL;DR: In this article, the role of medium effects on the production of antiprotons in nucleus-nucleus collisions at sub-threshold energies was investigated and the reaction dynamics was simulated within the framework of quantum molecular dynamics.
Abstract: We investigate the role of medium effects on the production of antiprotons in nucleus-nucleus collisions at subthreshold energies The reaction dynamics is simulated within the framework of quantum molecular dynamics The antiprotons, produced through elementary baryon-baryon processes, are propagated in its time-dependent self-consistent mean field Both absorption and rescattering by the baryonic environment are evaluated dynamically The density and temperature dependence of the associated scalar and vector self-energies are calculated on the basis of an effective Nambu-Jona-Lasinio model We found that the inclusion of the self-energies enhances the production of antiprotons The calculated spectra are in good agreement with the experimental data and consistent with the formation of a very hot phase from which antiprotons are produced
TL;DR: In this paper, the conditions of formation of hot nuclei in nucleus collisions, as a function of the entrance channel parameters (bombarding energy, mass asymmetry, impact parameter) are studied in detail.
Abstract: Several aspects of hot nuclei are examined in the present review article. After defining what is usually meant by hot nuclei, the new tools which have been developed in order to determine the temperature are described. Using several examples, it is demonstrated that the 4 pi neutron multiplicity filters constitute a powerful and reliable means for nuclear temperature measurements. So far, hot nuclei have been essentially investigated through nucleus-nucleus collisions. The conditions of formation of hot nuclei in such collisions, as a function of the entrance channel parameters (bombarding energy, mass asymmetry, impact parameter) are studied in detail. Two specific decay channels of hot nuclei: binary fission and emission of intermediate mass fragments, as well as the combined effect of temperature and spin are reviewed. It is shown that energetic light particles can also be successfully used to generate hot nuclei. Such nuclei left with little initial compression and spin are of special interest in order to investigate the effect of heat and only heat on the properties and on the fate of nuclei. This is not possible in nucleus-nucleus collisions. Prospects are finally given, with the annihilation of energetic antiprotons in nuclei appearing as the most promising method of reaching the limiting temperature that a nucleus can sustain.
TL;DR: In this paper, the M1 excitations in three isotope chains, 142,146,148,150Nd, 144,148-150,152,154Sm, and 156,158Gd, were studied within the quasi-particle random-phase approximation using a mean field given by a deformed Woods-Saxon potential and including quadrupole-quadrupole and spin-spin residual interactions.
Abstract: The M1 excitations in three isotope chains, 142,146,148,150Nd, 144,148,150,152,154Sm, and 156,158Gd, are studied within the quasi-particle random-phase approximation using a mean field given by a deformed Woods-Saxon potential and including quadrupole-quadrupole and spin-spin residual interactions. A residual rotation-vibration coupling ensures the exclusion of the spurious state. The behaviour of the M1 strength distribution is studied systematically in the whole range of experimentally explored excitation energies and compared with the large variety of experimental information, which includes both low-lying orbital and higher-lying spin-flip 1+ excitations. We have found a quadratic dependence of the M1 strength summed up to 4 MeV on the nuclear deformation reproducing at the same time the main features of the low-lying M1 spectra. The double-peaked structure of the spin M1 strength distribution found experimentally in several nuclei is qualitatively reproduced and interpreted as isoscalar and isovector peaks.
TL;DR: In this paper, the existence of non-statistical fluctuations in the azimuthal plane of interaction in high energy relativistic interactions initiated by 16O at 2.1 GeV and 12C and 24Mg at 4.5 GeV/nucleon was revealed.
Abstract: Analysis of target fragmented 'black' particles in nuclear emulsion from high energy relativistic interactions initiated by 16O at 2.1 GeV/nucleon and 12C and 24Mg at 4.5 GeV/nucleon reveal the existence of nonstatistical fluctuations in the azimuthal plane of interaction. The asymmetry or the non-statistical fluctuations, while found to be independent of projectile mass or incident energy, are dependent on the excitation energy of the target nucleus.
TL;DR: In this article, the authors considered the Lindblad axiomatic approach for open quantum systems and derived the relations between the friction and diffusion coefficients in the case of a general potential.
Abstract: In the framework of the Lindblad axiomatic approach for open quantum systems various models for heavy-ion reactions are considered Assuming the existence of a stationary solution for the Lindblad equation in Gibbs form, the relations between the friction and diffusion coefficients are derived In this case the evolution operator depends on temperature Models and Lindblad's evolution operators, which are constructed with the generators of SO(3) and SO(4) algebras, are considered A master equation for the density operator is derived for the case of a general potential and its connection with similar approaches in nuclear physics is discussed
TL;DR: In this article, multishell interacting boson-fermion model (IBFM) calculations have been performed both for the positive and negative-parity states in the odd-mass Xe isotopes 119 to 131.
Abstract: Multishell interacting boson-fermion model (IBFM) calculations have been performed both for the positive- and negative-parity states in the odd-mass Xe isotopes 119 to 131. The theoretical predictions are compared with the existing experimental data and the possible structure of the low-lying states is discussed.
TL;DR: In this article, the authors investigate the capabilities of an artificial neural network for impact parameter determination in a heavy-ion collision and show that the neural network is capable of improving the accuracy of the impact parameters based on observables such as flow angle, the average directed inplane transverse momentum and the difference between transverse and longitudinal momenta.
Abstract: Accurate impact parameter determination in a heavy-ion collision is crucial for almost all further analysis. We investigate the capabilities of an artificial neural network in that respect. First results show that the neural network is capable of improving the accuracy of the impact parameter determination based on observables such as the flow angle, the average directed inplane transverse momentum and the difference between transverse and longitudinal momenta. However, further investigations are necessary to discover the full potential of the neural network approach.
TL;DR: In this paper, an extensive air shower (EAS) muon production and propagation through the atmosphere were simulated and the incidence angles and arrival time at the observation level were determined for each muon.
Abstract: Extensive air shower (EAS) muon production and propagation through the atmosphere were simulated. The incidence angles and arrival time at the observation level were determined for each muon. It is shown that for large distances from EAS cores and for GeV-muons, time and track measurements could be used complementarily to improve the accuracy of the muon production height determination. For TTC detectors of 5 GeV muons attached to EAS arrays at mountains and placed at a distance of about 200-250 m from its centre, the accuracy of the muon production depth estimate is about 35 g cm-2. The influence of the detector location, the muon energy threshold, the observation level and experimental errors is analysed.
TL;DR: In this article, a re-examination of COS B and SAS II satellite data on cosmic gamma rays, using contemporary information in related areas, has strengthened the earlier view (Bhat et al 1985) that supernova remnants (SNR) accelerate cosmic rays, at least for energies up to 1010 eV, for protons, and 109 eV for electrons.
Abstract: A re-examination of COS B and SAS II satellite data on cosmic gamma rays, using contemporary information in related areas, has strengthened our earlier view (Bhat et al 1985) that supernova remnants (SNR) accelerate cosmic rays, at least for energies up to 1010 eV, for protons, and 109 eV for electrons. Specifically, there is an enhanced cosmic ray intensity inside the Loop I SNR with particularly high intensities in the surface region of the shock-both features being predicted by theory.
TL;DR: In this article, the structure of the collective bands of 76,78,80,82Sr were studied within our deformed configuration mixing shell model based on Hartree-Fock states.
Abstract: The structure of the collective bands of 76,78,80,82Sr are studied within our deformed configuration mixing shell model based on Hartree-Fock states. The self-conjugate nucleus 76Sr is found to be most deformed in the mass A=80 region in agreement with experiment. The deformation is found to decrease for heavier isotopes. The calculated positive and negative parity bands agree quite well with experiment. The calculated B(E2) values are compared with the available experimental results. The agreement is quite satisfactory.
TL;DR: In this paper, a three-dimensional Kadyshevsky reduction of the Bethe-Salpeter equation was used for particle-nucleon scattering and the resulting potential included the direct and crossed N, Delta terms, and the t-channel sigma and rho -exchange terms.
Abstract: Pion-nucleon scattering is investigated using the Kadyshevsky three-dimensional reduction of the Bethe-Salpeter equation. The resulting potential includes the direct and crossed N, Delta terms, and the t-channel sigma and rho -exchange terms. The nucleon pole condition is imposed to define the renormalization of the nucleon mass and the pi NN coupling constant. A mixture of the scalar and vector sigma pi pi couplings is introduced to simulate the effect of the broad width of the s-wave correlated two-pion exchange mechanism. Good descriptions of the pi N phase shifts up to 400 MeV have been obtained in all S- and P-waves. The off-shell behaviours for our model differ significantly from those obtained using different reduction schemes.
TL;DR: In this article, the quark distributions for the ground state of doubly-magic nuclei, i.e. 4He, 16O and 40Ca, were calculated using the non-relativistic quark model of the nucleons and considering quark exchange between at most two nucleons.
Abstract: Using the non-relativistic quark model of the nucleons and considering the quark exchange between at most two nucleons, we calculate the quark distributions for the ground state of the doubly-magic nuclei, i.e. 4He, 16O and 40Ca. We take into the account the effect of two-body nucleon correlations which depend on spin, isospin, total angular momentum, tensor operators and the relative and centre-of-mass coordinates of two nucleons. The harmonic-oscillator wavefunctions are used as the uncorrelated states and a local density approximation is performed to convert the infinite nuclear-matter correlation functions to finite ones. The result is compared with results due to Catara and Sambataro (1992).
TL;DR: In this article, an exact analytical fragmentation function which embeds most of the kinematical and dynamical properties of the process is derived and demonstrated in the case of c and b quarks.
Abstract: We review the present heavy-quark fragmentation models briefly and derive an exact analytical fragmentation function which embeds most of the kinematical and dynamical properties of the process. Using this function we demonstrate these properties in the case of c and b quarks and compare them with similar models and recent experimental data.
TL;DR: In this article, the relative intensities and reduced probabilities of the E1, E2, M1 and M2 transitions between excited states in 168Er are calculated within the quasiparticle-phonon nuclear model.
Abstract: The intensities and reduced probabilities of the E1, E2, M1 and M2 transitions between excited states in 168Er are calculated within the quasiparticle-phonon nuclear model. The intensities and reduced probabilities are in reasonably good overall agreement with experimental ones. It is shown that the contribution of the two-phonon configuration to the wavefunction of excited states with energy below 2 MeV is small. The wavefunction of the first Knu pi =41+ state contains the largest double-gamma vibrational term equal to 30%. The dominance of the decay from the first excited 01+ state to the gamma vibrational state over that to the ground state band is in agreement with the experimental data. The specific features of the calculated B(E1), B(E2) and B(M1) values and the relative intensities of the gamma-ray transitions are discussed.