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Code ALICE B and B: a precompound/evaporation fission code emphasizing neutron yields

Description: This report describes a version of the code ALICE/LIVERMORE 82 which has been modified to output neutron yield information of the type one would measure in an experiment gated by measuring a fission fragment. The modified code does the same precompound/compound decay calculations as the code ALICE/LIVERMORE 82, but has additional buffers and algorithms for the neutron yield calculation. We calculate the pre-fission neutron spectra and multiplicities, and post-fission neutron spectra and multiplicities, predicted for measurements which are gated on fission events. In this report we describe the additional output, referring the reader to the more complete report on the original code for further details.
Date: November 1, 1983
Creator: Blann, M. & Howe, R.
Partner: UNT Libraries Government Documents Department

Covariances for neutron cross sections calculated using a regional model based on local-model fits to experimental data

Description: We suggest a procedure for estimating uncertainties in neutron cross sections calculated with a nuclear model descriptive of a specific mass region. It applies standard error propagation techniques, using a model-parameter covariance matrix. Generally, available codes do not generate covariance information in conjunction with their fitting algorithms. Therefore, we resort to estimating a relative covariance matrix a posteriori from a statistical examination of the scatter of elemental parameter values about the regional representation. We numerically demonstrate our method by considering an optical-statistical model analysis of a body of total and elastic scattering data for the light fission-fragment mass region. In this example, strong uncertainty correlations emerge and they conspire to reduce estimated errors to some 50% of those obtained from a naive uncorrelated summation in quadrature. 37 references.
Date: November 1, 1983
Creator: Smith, D.L. & Guenther, P.T.
Partner: UNT Libraries Government Documents Department

Delta-nucleus dynamics: proceedings of symposium

Description: The appreciation of the role in nuclear physics of the first excited state of the nucleon, the delta ..delta..(1232), has grown rapidly in the past decade. The delta resonance dominates nuclear reactions induced by intermediate energy pions, nucleons, and electromagnetic probes. It is also the most important non-nucleonic degree of freedom needed to resolve many fundamental problems encountered in the study of low-energy nuclear phenomena. Clearly, a new phase of nuclear physics has emerged and conventional thinking must be extended to account for this new dimension of nuclear dynamics. The most challenging problem we are facing is how a unified theory can be developed to describe ..delta..-nucleus dynamics at all energies. In exploring this new direction, it is important to have direct discussions among researchers with different viewpoints. Separate entries were prepared for the 49 papers presented. (WHK)
Date: October 1, 1983
Creator: Lee, T.S.H.; Geesaman, D.F. & Schiffer, J.P. (eds.)
Partner: UNT Libraries Government Documents Department

4. pi. data of relativistic nuclear collisions. [Plastic ball]

Description: During the past two years, complete events of relativistic nuclear collisions are being studied with the Plastic Ball, the first electronic nonmagnetic particle-identifying 4..pi.. spectrometer. It is well suited to handle the large multiplicities in these reactions and allows collection of data at a rate sufficient to make further software selections to look at rare events. The analysis of the data follows various lines covering topics like thermalization, stopping or transparency, cluster-production mechanism (--can it tell entropy), search for collective flow through various global analyzing methods that allow determination of the scattering plane, projectile fragmentation (--is there a bounce-off), pion distribution, two-particle correlations: Hanbury-Brown Twiss, and excited nuclear states (--nucleosynthesis at the freezeout point or from chemical equilibrium). We will cover in this contribution only two subjects: stopping and thermalization and cluster production.
Date: May 1, 1983
Creator: Gutbrod, H.H.; Gustafsson, H.A. & Kolb, B.
Partner: UNT Libraries Government Documents Department

Pion production at 180/sup 0/ in nucleus-nucleus collisions

Description: A survey experiment of pion production at 180/sup 0/ in nucleus-nucleus collisions is presented. Beams of 1.05 GeV/A and 2.1 GeV/A protons, alphas, and carbon were used, as well as proton beams of 0.80 GeV, 3.5 GeV, and 4.89 GeV, and argon beams of 1.05 GeV/A and 1.83 GeV/A. This is the first such experiment to use the heavier beams. Targets used ranged from carbon to lead. An in-depth review of the literature, both experimental and theoretical, is also presented. The systematics of the data are discussed, and comparisons are made both with prior experiments and with the predictions of the models reviewed. The cross sections appear consistent with a simple single nucleon-nucleon collision picture, without the need for collective or other exotic effects. Suggestions for future work are made.
Date: May 1, 1983
Creator: Chessin, S.A.
Partner: UNT Libraries Government Documents Department

Ultra-relativistic heavy ions and cosmic rays

Description: The collisions of ultra-relativistic heavy ions, E/sub /N/ greater than or equal to 1 TeV/nucleon are most interesting, since, at these energies, matter is produced at sufficiently high energy density that a quark-gluon plasma has a good chance to form. Very heavy ions are also most interesting since the matter forms in a larger volume than for light ions, and the matter is at a somewhat higher energy density. At very high energies with very heavy ions there is great flexibility in the experimental signals which might be studied, as well as the nature of the matter which is produced. The fragmentation region and central region provide different environments where a plasma might form. The former is baryon rich while the central region is high temperature with low baryon number density and is not accessible except at very high energies.
Date: May 1, 1983
Creator: McLerran, L.
Partner: UNT Libraries Government Documents Department

Boson-mediated interactions between static sources

Description: The techniques are now available for doing accurate computations of static potentials arising from the exchange of virtual mesons. Such computations must take account of the fact that different approximation methods must be used in the regions where R is large and where R is small. In the asymptotic region, the distorted-field approximation provides an appropriate starting-point, but it must be improved before trustworthy results are obtained for all but the largest values of R. In the region of small R, accurate strong-coupling methods are based on the use of states with coherent meson pairs. For small R, it is also important to take account of the possibility of meson emission or near-emission. Current work is aimed at applying the techniques described to the case of static sources interacting via pion field. In particular, it will be interesting to see how sensitive the potential is to the value of the cutoff ..lambda... Other areas of application are the study of the effects of nonlinearity and models of quark-quark and quark-antiquark potentials. 17 references.
Date: January 1, 1983
Creator: Bolsterli, M.
Partner: UNT Libraries Government Documents Department

delta. /sub 33/ resonance in pion nucleus elastic, single, and double charge exchange scattering

Description: The ..delta../sub 33/ resonance is strongly excited in pion-nucleon scattering, but there is clearly only a limited amount of information that can be learned in scattering the pion from an isolated nucleon. One learns that there is a resonance of mass 1232 MeV, width 115 MeV, and, if one is willing to introduce a dynamical model, something about the off-shell extension of the amplitude. One stands to learn much more from pion-nucleus scattering because in this case the ..delta../sub 33/ resonance has an opportunity to scatter from nucleons, and how this occurs is not well understood. What do we know about the ..delta..-N interaction for pion-nucleus scattering. The isobar-hole model was invented to deal directly with the ..delta../sub 33/-nucleus dynamics, and a phenomenological determination of the isobar shell-model potential was attempted. The unknown dynamics deltaU/sub ..delta../ is contained in a central isoscalar spreading potential of strength W/sub 0/ and a spin orbit potential deltaU/sub 0/ = W/sub 0/rho + spin-orbit. The real part of W/sub 0/rho is measured relative to the nucleon-nucleus potential. From a more theoretical point of view, one would like to be able to calculate deltaU/sub ..delta../, including its isospin dependence, from an underlying dynamical model which is formulated in terms of the basic effective meson-baryon couplings. Some salient properties of these couplings can be determined from models of quark-bag structure, which raises the exciting possibility of learning about these fundamental issues from pion scattering. Attempts at Los Alamos to build a theoretical framework to deal with these and other issues are described. (WHK)
Date: January 1, 1983
Creator: Johnson, M.B.
Partner: UNT Libraries Government Documents Department

Dynamical calculation of thresholds for compound-nucleus formation

Description: A necessary condition for forming a compound nucleus in a heavy-ion reaction is that the dynamical trajectory of the fusing system pass inside the fission saddle point in a multidimensional deformation space. For nuclear systems lighter than a critical size and for relatively low angular momentum, the fission saddle point lies outside the point of hard contact, and this requirement is automatically satisfied once a one-dimensional interaction barrier is overcome. However, for heavier nuclear systems and/or for high angular momentum, the fission saddle point lies inside the contact point, and the center-of-mass bombarding energy must then exceed the maximum in the one-dimensional interaction barrier by an amount ..delta..E in order to form a compound nucleus. Dynamical trajectories for fusing systems have been calculated by use of several approaches. It is our major purpose here to calculate within a single unified model the dependence of the additional energy ..delta..E upon Z/sup 2//A for various types of dissipation, in an effort to ultimately determine the magnitude and mechanism of nuclear dissipation from comparisons with experimental data. Since both our predictions and those of other groups depend somewhat upon the details of the model, we also study the effect of the shape parametrization on the dynamical thresholds. (WHK)
Date: January 1, 1983
Creator: Davies, K.T.R.; Sierk, A.J. & Nix, J.R.P
Partner: UNT Libraries Government Documents Department

Dynamical fusion thresholds in macroscopic and microscopic theories

Description: Macroscopic and microscopic results demonstrating the existence of dynamical fusion thresholds are presented. For macroscopic theories, it is shown that the extra-push dynamics is sensitive to some details of the models used, e.g. the shape parametrization and the type of viscosity. The dependence of the effect upon the charge and angular momentum of the system is also studied. Calculated macroscopic results for mass-symmetric systems are compared to experimental mass-asymmetric results by use of a tentative scaling procedure, which takes into account both the entrance-channel and the saddle-point regions of configuration space. Two types of dynamical fusion thresholds occur in TDHF studies: (1) the microscopic analogue of the macroscopic extra push threshold, and (2) the relatively high energy at which the TDHF angular momentum window opens. Both of these microscopic thresholds are found to be very sensitive to the choice of the effective two-body interaction.
Date: January 1, 1983
Creator: Davies, K.T.R.; Sierk, A.J. & Nix, J.R.
Partner: UNT Libraries Government Documents Department

Dynamical thresholds for complete fusion

Description: It is our purpose here to study the effect of nuclear dissipation and shape parametrization on dynamical thresholds for compound-nucleus formation in symmetric heavy-ion reactions. This is done by solving numerically classical equations of motion for head-on collisions to determine whether the dynamical trajectory in a multidimensional deformation space passes inside the fission saddle point and forms a compound nucleus, or whether it passes outside the fission saddle point and reseparates in a fast-fission or deep-inelastic reaction. Specifying the nuclear shape in terms of smoothly joined portions of three quadratic surfaces of revolution, we take into account three symmetric deformation coordinates. However, in some cases we reduce the number of coordinates to two by requiring the ends of the fusing system to be spherical in shape. The nuclear potential energy of deformation is determined in terms of a Coulomb energy and a double volume energy of a Yukawa-plus-exponential folding function. The collective kinetic energy is calculated for incompressible, nearly irrotational flow by means of the Werner-Wheeler approximation. Four possibilities are studied for the transfer of collective kinetic energy into internal single-particle excitation energy: zero dissipation, ordinary two body viscosity, one-body wall-formula dissipation, and one-body wall-and-window dissipation. (WHK)
Date: January 1, 1983
Creator: Davies, K.T.R.; Sierk, A.J. & Nix, J.R.
Partner: UNT Libraries Government Documents Department

Effect of the. delta. (1236) resonance on magnetic dipole properties of nuclei

Description: The effect of the ..delta..(1236) resonance on magnetic moments, M1 transition rates and beta decay is discussed. The main effect of including this resonance is to renormalize the g-factor of the valence nucleons and explicit values for this renormalization are given. The effect on l-forbidden M1 transitions is investigated.
Date: January 1, 1983
Creator: Lawson, R.D.
Partner: UNT Libraries Government Documents Department

How do nuclei really vibrate or rotate

Description: By means of the adiabatic cranking model the properties of the current and velocity fields of nuclear quadrupole vibrations for even-even nuclei in the rare-earth region are investigated. BCS correlated wave functions based on the Nilsson single particle Hamiltonian have been used. The current fields are analyzed in terms of vector spherical harmonics. The realistic microscopic currents show a vortex structure not present in the classical irrotational flow. The microscopic origin of the vortex structure is investigated.
Date: January 1, 1983
Creator: Andresen, H.G.; Kunz, J.; Mosel, U.; Mueller, M.; Schuh, A. & Wust, U.
Partner: UNT Libraries Government Documents Department

Pairing correction for particle-hole state densities

Description: The pairing correction proposed by Ignatyuk and Sokolov for particle-hole state densities has been examined. It has been found that the accuracy of the correction is sufficient for practical applications only if the system is in its normal state (..delta.. = 0). In the superfluid state (..delta.. not equal to 0), a consistent pairing-Pauli correction is developed here for improved accuracy. Practical implementations of the pairing correction are given and further developments are outlined.
Date: January 1, 1983
Creator: Fu, C.Y.
Partner: UNT Libraries Government Documents Department

Prompt fission neutron spectra and average prompt neutron multiplicities

Description: We present a new method for calculating the prompt fission neutron spectrum N(E) and average prompt neutron multiplicity anti nu/sub p/ as functions of the fissioning nucleus and its excitation energy. The method is based on standard nuclear evaporation theory and takes into account (1) the motion of the fission fragments, (2) the distribution of fission-fragment residual nuclear temperature, (3) the energy dependence of the cross section sigma/sub c/ for the inverse process of compound-nucleus formation, and (4) the possibility of multiple-chance fission. We use a triangular distribution in residual nuclear temperature based on the Fermi-gas model. This leads to closed expressions for N(E) and anti nu/sub p/ when sigma/sub c/ is assumed constant and readily computed quadratures when the energy dependence of sigma/sub c/ is determined from an optical model. Neutron spectra and average multiplicities calculated with an energy-dependent cross section agree well with experimental data for the neutron-induced fission of /sup 235/U and the spontaneous fission of /sup 252/Cf. For the latter case, there are some significant inconsistencies between the experimental spectra that need to be resolved. 29 references.
Date: January 1, 1983
Creator: Madland, D.G. & Nix, J.R.
Partner: UNT Libraries Government Documents Department

Quark distributions in nuclei: results from lepton probes

Description: Deep inelastic lepton scattering from nuclei indicates that quark-parton distributions are significantly different from the distributions in isolated nucleons. The EMC and SLAC data on iron and deuterium targets show enhanced and suppresed scattering for kinematic regions where Bjorken x < 1. In addition, the SLAC data on /sup 3/He show strong enhancement over results with conventional nuclear theory for x > 1. These data and recent model interpretations are summarized. Detailed comparisons with the data are presented for the quark cluster model. 53 references.
Date: January 1, 1983
Creator: Vary, J.P.
Partner: UNT Libraries Government Documents Department

Rapid disappearance of shell effects in the fission of transfermium nuclei

Description: In the last fifteen years we have learned that nuclear shells have a very broad and pervasive impact on the fission process. In the first few decades after the discovery of nuclear fission, the nucleus was treated as a drop of liquid with smoothly varying attractive and repulsive forces. Although this model still forms the underlying basis for fission, we also observe large effects from the superimposition of shell corrections derived from coupling the quantum states of individual nucleons. The consequences of single-particle coupling on the fission process can be striking and may often overshadow that originating from the intrinsic liquid-drop component. Here, we point out several major features attributable to shell effects in the spontaneous fission (SF) of the lighter actinides, the sudden transition to symmetric fission in the fermium isotopes, and finally new experimental information indicating another transition in the SF of transfermium nuclides due to the disappearance of shell perturbations. In each transition, the abruptness is surprising, and for the moment, such rapid changes in fission behavior lack a theoretical rationale.
Date: January 1, 1983
Creator: Hulet, E.K.
Partner: UNT Libraries Government Documents Department

SO/sub 8/ model of collectivity in nuclei

Description: The interacting boson model (IBM) has been very successful in giving a unified and simple description of the spectroscopic properties of a wide range of nuclei, from vibrational through rotational nuclei. The three basic assumptions of the model are that (1) the valence nucleons move about a doubly closed core, (2) the collective low-lying states are composed primarily of coherent pairs of neutrons and pairs of protons coupled to angular momentum zero and two and (3) these coherent pairs are approximated as bosons. Thus the IBM is an approximation to the nuclear shell model in which the number of degrees of freedom are reduced drastically. Once these assumptions are made, the effective Hamiltonian can then be written in terms of neutron monopole (angular momentum zero) and quadrupole (angular momentum two) bosons and proton monopole and quadrupole bosons. Since each nucleus will have a fixed number of nucleon pairs this boson Hamiltonian will conserve the number of bosons. Phemonologically, the Hamiltonian is assumed to be finite and to be a sum of a boson self-energy Hamiltonian plus pairwise interactions between bosons. However, the parameters of the boson Hamiltonian depend smoothly on the mass of the nucleus. The mass dependence of these parameters presumably arise because of two effects resulting from the approximations made. In this review we shall show how it is possible to have fermion Hamiltonians which have a class of collective eigenstates composed entirely of monopole and quadrupole pairs of fermions. Hence these models satisfy the assumptions (1) and (2) above but no boson approximation need be made. Thus the Pauli principle is kept in tact. Furthermore the fermion shell model states excluded in the IBM can be classified by the number of fermion pairs which are not coherent monopole of quadrupole pairs. Hence the mixing of these states ...
Date: January 1, 1983
Creator: Ginocchio, J.N.
Partner: UNT Libraries Government Documents Department