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Progress in collective flow studies from the onset to Bevalac/SIS

Description: Collective flow in heavy ion collisions was first observed experimentally more than a decade ago at the Bevalac by the Plastic Ball collaboration. Although early calculations had suggested that measurement of the flow would place tight constraints on the nuclear equation of state, uncertainties in other input parameters of microscopic models, which also affect the flow, led to large ambiguities in the equation of state. This talk will discuss recent flow studies that attempt to overcome these difficulties. The EOS and FOPI experiments at the Bevalac and SIS accelerators have measured flow in the 200--2000 A-MeV bombarding energy range with better acceptance, particle identification, and systematics than was previously available. Meanwhile, programs at MSU and GANIL are studying the disappearance of flow around 50 A-MeV. Systematic comparison of these data with predictions of microscopic models is beginning to reduce the ambiguities in the extraction of physics quantities. Also, new directions in flow studies, such as the flow of produced particles and radial flow, offer the possibility of further information from flow studies. Recent accomplishments and new directions in flow studies are discussed, and areas where further study is needed are pointed out.
Date: March 1, 1995
Creator: Lisa, M.A.
Partner: UNT Libraries Government Documents Department

Anharmonic vibrator description of collective nuclei

Description: It is shown that a simple anharmonic vibrator model can account for the properties of yrast states in collective nuclei, as well as the quasi-band states built on the {gamma}-vibration or on the O{sub 2}{sup +} state. This description extends from nearly harmonic vibrator nuclei to pure rotor nuclei and encompasses both energies and B(E2) values. It survives both ``horizontal`` and ``vertical`` perspectives on the data of nuclear physics and offers a challenge to microscopic theories of nuclear structure.
Date: July 1, 1995
Creator: Casten, R.F. & Zamfir, N.V.
Partner: UNT Libraries Government Documents Department


Description: Antiproton-production and nucleon-interaction cross sections were investigated for antiprotons in the energy range 0.5 to 1.0 Bev. The antiprotons were distinguished from other particles produced at the Bevatron by a system of scintillation- and velocity-selecting Cherenkov counters. The excitation function and momentum distribution were recorded for antiproton production in carbon and compared with statistical model expectations. The antiprotons were directed by a system of bending and focusing magnets to a liquid hydrogen target. An array of plastic scintillation counters, which almost completely surrounded the hydrogen target, was used to determine the p-p total, elastic, inelastic, and charge-exchange cross sections. Near 500 Mev the total d-p cross section was about 120 mb, and it slowly decreased to 100 mb near 1 Bev. The inelastic cross section, which is principally due to the annihilation process, represented nearly 2/3 of the total cross section. The elastic scattering distribution was highly peaked in the forward direction and could be fitted by an optical model. The total and partial cross sections were also determined for the collisions of antiprotons with deuterons. The p-d total and inelastic cross sections were found to be approximately 1.8 times the p-p cross sections. Corrections were made for the shielding of nucleons within the deuteron in order to ascertaln the p-n interaction. The results indicate that the p-p and p-n cross sections are very nearly equal in this energy region, and that they satisfy the inequalities required by charge independence. (auth)
Date: December 12, 1961
Creator: Elioff, T.; Agnew, L.; Chamberlain, O.; Steiner, H.M.; Wiegand, C. & Ypsilantis, T.
Partner: UNT Libraries Government Documents Department

Radioactive nuclear beams and the evolution of collectivity in nuclei

Description: The advent of Radioactive Nuclear Beams (RNBs) has opened up new nuclear territories to exploration. Recent theoretical work suggests that nuclear structure in these new regions may be unlike anything observed to date and that its study may substantially alter our concepts of shell structure, collectivity and the evolution of nuclear behavior. To face the challenge of inevitably low RNB intensities, new, more efficient signatures of structure and new experimental techniques are being developed. We discuss these topics and also summarize the current status of RNB facilities and facility planning in North America.
Date: November 1995
Creator: Casten, R. F. & Zamfir, N. V
Partner: UNT Libraries Government Documents Department

Proton Radioactivity Measurements at HRIBF: Ho, Lu, and Tm Isotopes

Description: Two new isotopes, {sup 145}Tm and {sup 140}Ho and three isomers in previously known isotopes, {sup 141m}Ho, {sup 150m}Lu and {sup 151m}Lu have been discovered and studied via their decay by proton emission. These proton emitters were produced at the Holifield Radioactive Ion Beam Facility (HRIBF) by heavy-ion fusion-evaporation reactions, separated in A/Q with a recoil mass spectrometer (RMS), and detected in a double-sided silicon strip detector (DSSD). The decay energy and half-life was measured for each new emitter. An analysis in terms of a spherical shell model is applied to the Tm and Lu nuclei, but Ho is considerably deformed and requires a collective model interpretation.
Date: November 13, 1998
Creator: Akovali, Y.; Batchelder, J.C.; Bingham, C.R.; Davinson, T.; Ginter, T.N.; Gross, C.J. et al.
Partner: UNT Libraries Government Documents Department

Numerical simulation of the electromagnetic decay of the nuclei {sup 152-154-156}Dy with selfconsistent strength functions

Description: The electromagnetic decay of the nuclei {sup 152-154-156}Dy is analyzed using microscopic Hartree-Fock calculations at finite temperature. The theoretical collective transition probabilities are implemented in numerical simulations to produce theoretical espectra. Thermal shape fluctuations are also taken into account. The inclusion of these correlation is crucial in order to understand the main features of the collective E2 spectra of these isotopes at different energies. The theoretical calculations suggest a shape change as responsible for the unusual features of the spectrum of the nucleus {sup 154}Dy at high energy.
Date: November 11, 1993
Creator: Martin, V.; Egido, J.L.; Khoo, T.L. & Lauritsen, T.
Partner: UNT Libraries Government Documents Department

Effective charges, the valence p-n interaction, and the IBM

Description: There are three recent themes in nuclear structure that come together in an interesting and useful way via the concept of effective charges and the framework of the IBM. These three concepts are the importance of dynamical symmetries in describing nuclear structure and the benefits that accrue from their exploitation, secondly, the critical role of the p-n interaction in the onset and development of collectivity in nuclei, and, thirdly, the importance of the valence nucleons in determining structure and its evolution. We will illustrate this by showing that the interpretation of measured B(E2) values in the context of the dynamical symmetries of the IBM leads to new insights into the meaning of effective charges and offers new avenues to understand the role of the proton-neutron (p-n) interaction in modulating the nature of the valence space and the growth of collectivity. In particular, we will show that effective charges in valence models, such as the IBM, can be interpreted in terms of derivatives of the collectivity of the low lying levels, that is, as measures of the rate of change of collectivity as the proton and neutron numbers vary. This paper is based on recent work by the authors.
Date: October 1, 1992
Creator: Casten, R. F. & Wolf, A.
Partner: UNT Libraries Government Documents Department

Evolution of pre-collective nuclei: Structural signatures near the drip lines

Description: Recent studies have shown that the phenomenology of single-magic and near-magic nuclei has universal characteristics analogous to those of collective nuclei and that, moreover, this phenomenology attaches smoothly to that describing collective nuclei. This has led to a number of new signatures of structure as well as to a new, tripartite, classification of nuclear structure that embraces the gamut of structures from magic, through pre-collective, to fully collective and rotational nuclei. Aside from the natural appeal of simple global correlations of collective observables, these results have particular significance for soon-to-be accessible exotic nuclei near the drip lines since they rely on only the simplest-to-obtain data, in particular, the energies of just the first two excited states, E(4{sub 1}{sup +}) and E(2{sub 1}{sup +}), of even-even nuclei, and the B(E2:2{sub 1}{sup +}{yields}0{sub 1}{sup +}) value. Indeed, without the need for more extensive level schemes, these basic data alone can reveal information about the goodness of seniority, about the validity of pair-addition mode relationships of adjacent even-even nuclei, about underlying shell structure (validity of magic numbers) and even about the shell model potential itself (e.g., the strengths of the l{center_dot} and l{sup 2} terms).
Date: October 1, 1994
Creator: Casten, R. F. & Zamfir, N. V.
Partner: UNT Libraries Government Documents Department

N/sub p/. N/sub n/ systematics and their implications

Description: A substantial simplification of the systematics in nuclear phase transition regions is obtained if the data are plotted against the product, N/sub p/.N/sub n/, of the number of valence protons and neutrons instead of against N, Z, or A as is usually done. Such a scheme leads to a unified view of nuclear transition regions and to a simplified scheme for collective model calculations. 7 refs., 5 figs.
Date: January 1, 1985
Creator: Casten, R.F.
Partner: UNT Libraries Government Documents Department

Theoretical studies in nuclear structure. Progress report, June 1, 1991--November 30, 1991

Description: In this period, the work has centered on two topics. The first is the study of a novel type of collective rotation in which an atomic nucleus with an inversion-symmetric shape rotates uniformly about an axis that is not a principal axis of the quadrupole tensor of the density distribution. This mode is referred to as tilted rotation. By using the cranking model together with higher-order corrections, it was shown that tilted rotation is indeed possible, not only within a microscopic framework, but also within the framework of collective models such as the IBM. The maximum tilt angle of {pi}/4 is realized for a certain class of states in the U(5) limit. The second topic, which actually was suggested during the course of the first investigation, is concerned with a new way of representing collective harmonic-oscillator algebras using boson-mapping techniques. In this approach, the many-phonon eigenvectors of a 2{lambda}+1-dimensional oscillator having good angular momentum are represented by simple products of boson operators acting on a vacuum. This representation may simplify the calculation of reduced matrix elements of arbitrary operators in collective models, but more work needs to be done.
Date: November 1, 1991
Creator: Marshalek, E. R.
Partner: UNT Libraries Government Documents Department


Description: Values of the distance of closest approach, the centerof-mass scattering angle, and the time integral are tabulated as functions of the impact parameter and the relative kinetic energy for each of three repulsive potentials. The classical theory of scattering was used as the basis for the calculations. The potentials used are the Born-Mayer (exponential), the Bohr (exponentially screened Coulomb), and the Thomas-Fermi. (auth)
Date: September 23, 1963
Creator: Robinson, M.T.
Partner: UNT Libraries Government Documents Department

A Statistical Model of Nuclear Level Spacings

Description: A generalization of Wigner's simple model for the distribution of nuclear level spacings is studied. The generalization is based on a stochastic process which reproduces the correct joint probabllity distribution of N energy levels for small spacings. The case N = 3, which includes the effect of the correlation between adjacent spacings, is discussed in detail. The resulting distribution and the correlation coefflcient are compared with experimental data. No definite conclusion can be drawn except that the effect of the correlations on the spacing distribution is very small. (auth)
Date: December 1, 1960
Creator: Dresner, L. & Inonu, E.
Partner: UNT Libraries Government Documents Department