W. U. Schröder

Reaction parameters for heavy-ion collisions

Abstract These tables present reaction parameters for all combinations of 27 projectile and 16 target nuclei in a laboratory bombarding energy range of 1–50 MeV/u. The reaction parameters are derived from the Fresnel model of heavy-ion scattering, the droplet model, and the rotating liquid-drop model, or from systematics of experimental data.

Mechanisms of very heavy-ion collisions: The 209Bi + 136Xe reaction at ELab = 1130 MeV

Abstract The mechanisms of kinetic-energy dissipation and nucleon exchange operating in damped heavy-ion collisions, and the corresponding time scales are investigated. As an example of a very heavy system, the reaction 209 Bi + 136 Xe at E Lab = 1130 MeV is choosen for a comparison with various reaction models. The experimental data include atomic number, energy and angular distributions of the projectile-like fragments and the correlations between these experimental observables. The integrated fragment Z distribution is found to be broad, but centered at the charge of the projectile. The kinetic energy distribution extends from quasi-elastic down to Coulomb energies of highly deformed fragments. The angular distribution of the reaction products is focussed into a narrow angular range a few degrees forward of the quarter-point angle. Examining the correlations of experimental observables with fragment charge and energy loss in detail, it is concluded that the kinetic-energy loss is a fundamental parameter indicating the stage of evolution of the reaction, i.e., the interaction time. The energy loss is inferred to decrease linearly with angular momentum, and this relation is used to construct an average experimental deflection function. The latter is compared to the predictions of classical dynamical models employing various potentials and frictional forces. These calculations do not provide a consistent description of the experimental results. Employing a simple phenomenological reaction model, l -dependent interaction times are deduced which are used to evaluate proton-number diffusion coefficients. A rather model-independent comparison of the time dependence of the mechanisms of nucleon exchange and kinetic energy dissipation is achieved by using the microscopic time scale provided by the nucleon-exchange mechanism. Energy dissipation is found to be of the one-body type, nucleon exchange contributing about 30% to the total energy loss. A consistent description of the observed reaction phenomena has been achieved in terms of a statistical mechanism of damped reactions evolving in time as nucleons are exchanged and energy is dissipated. It connects in a continuous fashion the domains of quasi-elastic scattering and strongly damped collisions.

Study of X-rays and nuclear γ-rays in muonic thallium

Abstract Energies and intensities of muonic X-rays, nuclear γ-rays and μ-capture γ-rays were measured in natural muonic thallium with Ge(Li) detectors. The absolute intensities of higher μ X-rays were reproduced by a cascade calculation starting with a statistical population at n = 20 including K-, L- and M- conversion. The electron screening effect was deduced from energies of higher μ X-rays. Eight prompt nuclear μ-rays were found, due to an excitation of (4.1 ±1.0) % and (14.6 ± 3.0) % of the first 3 2 + level in μ 203 T1 and μ, 205 T1, respectively, and of (8.8 ± 0.9) % of the 5 2 − level at 2.63 MeV in 205 T1. This excitation explains the anomalous intensity ratios of the 2p-ls and 3d-2p fine structure components. From the nuclear y-rays of the first excited states were deduced: the magnetic h.f. splittings e 203 ( 1 2 ) = 2.57 ± 0.15 keV , e 205 ( 1 2 ) = 2.30 ± 0.02 keV, muonic isomer shifts ΔE is (203) = 0.26 ± 0.15 keV, ΔE is (205) = −(0.31 ± 0.11) keV; E2/M1 mixing ratios δ μ 2 (203) = 0.89 ± 0.27, δ μ 2 (205) = 1.00 ± 0.05, and the half-life in the presence of the muon T μ ( 3 2 ) = 1.4 ± 0.1 nsec in 205 T1. The values of e( 1 2 ) and ΔE ls were compared with shell-model calculations. From δ μ 2 and T gm ( 3 2 ) evidence for a magnetic nuclear polarization was found. From the 2p-ls, 3d-2p, 3p-2s and 2s-2p transitions the size and shape of the nuclear charge distribution and a nuclear polarization due to the 1s 1 2 muon of ΔE np = 8.3 ± 1.2 keV and 9.5 ± 1.2 keV for 203 Tl and 205 T1 respectively were determined in agreement with theoretical estimates. An isotope shift of ΔE = 10.35 ± 0.25 keV was measured for the 1s 1 2 state which is compared with data from optical spectroscopy. From an analysis of the time distribution of delayed γ-rays from μ-capture, an isotopic effect of (5.4 ± 2.5) % was found for the capture rates, in fair agreement with predictions from Primakoffs μ-capture formula.

Nuclear excitation and isomer shifts in muonic atoms: (I). Experiment and evaluation

Abstract Energies and intensities of 53 nuclear γ-lines in muonic atoms of Sm, Gd, Eu, Tm, Yb, Ta, W, Re, Os, Ir, Pt, Hg, Tl, and Bi have been measured with Ge(Li) detectors. By comparison with γ-lines from radioactive sources the isomer shifts for transitions from nuclear rotational levels, quadrupole and octupole vibrational levels, and single-particle levels have been deduced. The effect limiting the accuracy of the extracted isomer shifts is the correction for the magnetic hyperfine splitting of the nuclear levels.

Correlated mass and charge transport induced by statistical nucleon exchange in damped nuclear reactions

Abstract Experimental relations between mass, charge and energy loss of fragments from damped reactions are analyzed in terms of an independent-particle transport model. A quantitative description of data is achieved including the energy-loss dependence of isobaric and isotopic fragment distributions.

Particle identification via pulse shape analysis for large-area silicon detectors of the CHIMERA array

Mass and atomic-number identification (ID) of reaction products is a fundamental requirement of any nuclear reaction study. An effective particle-ID method is demonstrated, based on pulse shape analysis/discrimination (PSD) applied to large-area, single-element silicon detectors. This technique uses commercial electronic modules and achieves atomic number resolution rivaling that typically obtained with multi-element (/spl Delta/E-E) detector telescopes. The method is applied to the CHIMERA detector system without compromising its time-of-flight (TOF) resolution. In-beam tests of the PSD method have been performed with large-area, 300-/spl mu/m thick CHIMERA silicon detectors, measuring particles from the /sup 19/F+/sup 12/C reaction at Tandem energies. Performance of a simple PSD set up is discussed, for front and rear particle injection.

TEST OF QUANTUM-ELECTRODYNAMICAL CORRECTIONS IN MUONIC ATOMS.

Abstract Energies of the 5g-4f muonic X-rays have been measured in nat Hg and 203 Tl with an accuracy of about 25 eV. They are found to be smaller than theoretical energies including recently revised calculations of higher-order vacuum polarization, by about two standard deviations.

Neutron emission in the reactionHo165+Fe56atElab=8.5MeV/u

Neutron emission and ..cap alpha..-particle emission in the /sup 165/Ho + /sup 56/Fe reaction have been measured for the damped-collision and fusion-fission components of this reaction at a bombarding energy of 8.5 MeV/u. The c.m. neutron energy spectra for damped collisions indicate equal temperatures for the light and heavy fragments, a result consistent with the equilibration of the excitation energy during the interaction time. Pre-equilibrium neutron emission is found to contribute less than 5%. An out-of-plane neutron anisotropy A/sub 2/ = 0.2 +- 0.1 is found. The measured ratios of the neutron multiplicities for the light and heavy fragments suggest a rapid approach of the fragment N/Z ratios during the collision to that corresponding to minimum potential energy of the composite system.

Shapes of α-particle spectra from energetic heavy-ion fusion reactions

Abstract The shapes of α-particle spectra from energetic heavy-ion fusion reactions are analyzed within the framework of a statistical model. Contrary to claims made in a number of earlier papers, good agreement is obtained between calculated and experimental spectra without the need to lower the evaporation barriers relative to the respective absorption barriers.

Dynamical production of intermediate-mass fragments in peripheral 209Bi+136Xe collisions at ElabA=28 MeV

Abstract Intermediate-mass fragments (IMF) from the reaction 209 Bi+ 136 Xe at E lab A =28 MeV have been measured in coincidence with other charged reaction products, as well as with neutrons. The IMF emission patterns are seen to exhibit signatures of two mechanisms — statistical emission from the fully accelerated massive reaction partners and a fast dynamical emission from a single effective source. The latter mechanism, possibly involving a multiple neck rupture, becomes dominating for least dissipative collisions characterized by low associated light particle multiplicities.