Quasielastic transfer reactions in 238U + 197Au and 197Au + 197Au collisions near the Coulomb barrier

Differential cross sections as a function of cm angle were measured for 1n- and 2n-transfer reactions in ²³⁸U + ¹⁹⁷ Au and¹⁹⁷ Au + ¹⁹⁷ Au collisions in the energy range from 0.881 V_c to 1.093 V_c and 0.825 V_c to 0.964 V_c, respectively. For ¹⁹⁸Au and ¹⁹⁹Au from the ²³⁸U + ¹⁹⁷Au collisions, for reduced distances of closest approach d _o 1.55 fm, the angular distributions at all bombarding energies are well described by the semiclassical theory. Equivalently, the transfer probabilities show the expected exponential decrease with increasing d _o over many orders of magnitude. For all other transfer products from ²³⁸U + ¹⁹⁷Au collisions, and for all transfer products from ¹⁹⁷Au + ¹⁹⁷Au collisions, markedly reduced cross sections relative to the semiclassical theory are observed for central collisions at all bombarding energies, even for values of d _o that are well outside the region where absorption is known to set in. Only for the more peripheral collisions, one observes agreement of the angular distributions (transfer probabilities) with the semiclassical expectations. The deviations for central collisions are absent for reactions with positive Q _gg values and scale roughly with increasingly negative values of Q _gg, i.e. with increasing Q-value mismatch. Channel coupling is proposed as the relevant mechanism.     

Understanding of the dissipation mechanism in ternary fission for the system 197Au+197Au

The mass number distributions of three fragments from the ternary fission of the system 197Au+197Au are reproduced rather well by using the improved quantum molecular dynamics (ImQMD) model without any adjusting parameter. It is found that the probability of ternary fission evidently depends on the incident energy and the impact parameter, and the two-body dissipation is the main mechanism responsible for the formation of the third fragment with comparable mass.     

Directed and elliptic flow in 197Au+197Au at intermediate energies

Directed and elliptic flow for the ¹⁹⁷Au+¹⁹⁷Au system at incident energies between 40 and 150 MeV per nucleon has been measured using the INDRA 4π multi-detector. For semi-central collisions, the excitation function of elliptic flow shows a transition from in-plane to out-of-plane emission at around 100 MeV per nucleon. The directed flow changes sign at a bombarding energy between 50 and 60 MeV per nucleon and remains negative at lower energies. Molecular dynamics calculations (CHIMERA) indicate sensitivity of the global squeeze-out transition on the σ _NN and demonstrate the importance of angular momentum conservation in transport codes at low energies.     

Pion photoproduction on197Au to ground and isomeric states in197Hg

The lifetime of the 331.3 keV 0 ₂ ⁺ state in¹⁰⁰Zr has been measured at the gas-filled recoil separator for fission products JOSEF. By observing the delayed coincidences between theβ-particles populating the level and theE0 conversion electrons from its decay into the ground state, a half-life of 3.37±0.30 ns has been obtained. From the measured lifetime and the relative intensities of the 0 ₂ ⁺ →0 ₁ ⁺ and 0 ₂ ⁺ →2 ₁ ⁺ transitions, values of 0.493±0.015 for theE0 strength parameterρ, and of 16 single particle units forB(E2,2 ₁ ⁺ → 0 ₂ ⁺ ) have been deduced. The enhanced nature of theE0 transitions suggests mixing of the 0 ₁ ⁺ and 0 ₂ ⁺ states which may be estimated by comparing the experimentalB(E2) values for the 2 ₁ ⁺ →0 ₁ ⁺ and 2 ₁ ⁺ →0 ₂ ⁺ transitions with the predictions of the asymmetric VMI model.     

Energy dependence of collective flow of neutrons and protons in197Au+197Au collisions

We investigate the beam energy dependence of neutron and proton squeeze-out in collisions of¹⁹⁷Au+¹⁹⁷Au atE/A=400—800 MeV. The azimuthal anisotropy that describes the enhanced emission of mid-rapidity neutrons perpendicular to the reaction plane rises strongly with the transverse momentum of the neutrons. This dependence of the azimuthal anisotropy follows a universal curve — independent of beam energy — if the neutron momenta are measured in fractions of the projectile momentum per mass unit. Analogously, the kinetic energy spectra of mid-rapidity neutrons exhibit a universal behaviour as a function of the kinetic energy of the projectile. The members of the FOPI-collaboration: J.P. Alard, Z. Basrak, N. Bastid, I.M. Belayev, M. Bini, R. Bock, A. Buta, R. aplar, C. Cerruti, N. Cindro, J.P. Coffin, M. Crouau, P. Dupieux, J. Erö, Z.G. Fan, P. Fintz, Z. Fodor, R. Freifelder, L. Fraysse, S. Frolov, A. Gobbi, Y. Grigorian, G. Guillaume, N. Herrmann, K.D. Hildenbrand, S. Hölbling, O. Houari, S.C. Jeong, M. Jorio, F. Jundt, J. Kecskemeti, P. Koncz, Y. Korchagin, R. Kotte, M. Krämer, C. Kuhn, I. Legrand, A. Lebedev, C. Maguire, V. Manko, T. Matulewicz, G. Mgebrishvili, J. Mösner, D. Moisa, G. Montarou, P. Morel, W. Neubert, A. Olmi, G. Pasquali, D. Pelte, M. Petrovici, G. Poggi, F. Rami, W. Reisdorf, A. Sadchikov, D. Schüll, Z. Seres, B. Sikora, V. Simion, S. Smolyankin, U. Sodan, N. Taccetti, K. Teh, R. Tezkratt, M. Trzaska, M.A. Vasiliev, P. Wagner, J.P. Wessels, T. Wienold, Z. Wilhelmi, D. Wohlfarth, A.V. Zhilin.     

The spin of the 1643 keV state and multipole mixing ratios of γ-transitions in Ba

Directional correlations of the ¹³⁴Ba γ-rays following the β-decay of ¹³⁴Cs have been measured using two high-resolution 30 cm³ coaxial Ge(Li) detectors. Particular attention was paid to the various corrections that must be applied in measuring γ-γ directional correlations in complex decays. It was clearly established that the spin-parity of the 1634 keV excited state of ¹³⁴Ba is 3⁺. The multipole mixing ratios (in the Krane-Steffen definition) of the ¹³⁴Ba γ-rays are (energies in keV): |δ(475)| = > 18, δ(563) = −7.4 ± 0.9, δ(569) = +0.29 ± 0.02, δ(1038) = 1.85 ± 0.15.     

Dynamical intermediate-mass fragment production in197Au+197Au collisions at energies ≤15 MeV/u

We present radiochemical data on the production of intermediate-mass fragments (IMF, 11≤Z<≈25) in collisions of¹⁹⁷Au+¹⁹⁷Au at 9, 11, 13 and 15 MeV/u. We demonstrate that the IMFs are produced in two-step reactions: Highly excited heavy nuclei are formed in a binary deep-inelastic reaction, and the IMFs originate from fast mass-asymmetric sequential fission of these primary reaction products.     

Breakup temperature of target spectators in 197Au + 197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction ¹⁹⁷Au + ¹⁹⁷Au at 1000 MeV per nucleon. Pairs of ^3,4He and ^6,7Li isotopes and pairs of ^3,4He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the universality established for the spectator decayat relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at τ_lab = 150°, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage.     

Effective attenuation length dependence on photoelectron kinetic energy for Au from 1 keV to 15 keV

Hard X-ray PhotoElectron Spectroscopy (HAXPES) is an extremely powerful tool for the electronic, compositional, and chemical characterization of bulk materials and buried interfaces. Its success is based on the dramatic increase of the electron effective attenuation length (EAL) with increasing photoelectron kinetic energy. EALs are well established for electrons with kinetic energies up to several keV (below 3 keV). However, few data are available for kinetic energies up to 15 keV. In the present study we have determined the EAL dependency on kinetic energy for gold from 1 keV up to 15 keV. Two different approaches have been used. The first approach consists of following the signal rate from a core level for a fixed kinetic energy as a function of overlayer thickness (overlayer method). The second approach consists of following the signal rate from a core level as a function of the incident photon energy, i.e., electron kinetic energy, for a fixed overlayer thickness (depth profile method). An EAL dependency of EAL (nm) = 0.022 × E_kin (eV)^0.627 has been obtained from both methods. Hence, the EAL, for gold, is 4.7, 7.3 and 9.4 nm for 5 keV, 10 keV and 15 keV electron kinetic energies, respectively. A comparison between the experimental data and the EALs predicted by practical expressions available in the literature is also performed.     

On the momentum correlations in the fragmentation of <Superscript>197</Superscript>Au + <Superscript>197</Superscript>Au reactions

We aim to present the formation of fragments using static as well as momentum-dependent interactions and larger nucleonic cross-sections. We shall also examine, in detail, the role of momentum correlations in these situations which are close to the fire-ball as well as to the spectator matter decay. The role of momentum correlations will be studied by imposing an additional cut in the momentum space. A clear need of a momentum cut is demonstrated in both fire-ball as well as spectator matter physics. The momentum-dependent interactions and larger cross-sections play a drastic role in peripheral reactions, whereas the role of momentum cut remains the same in these situations.     

Experimental determination of penetration parameter forM1 conversion,and theE2 admixtures for 77 and 191 keV transitions in197Au

TheL-subshell conversion for 77 keV transition andK,L ₁,L ₂-shell conversion for 191 keV transition in¹⁹⁷Au, as well asK-shell conversion transition of 158 keV in¹⁹⁹Hg were measured by means of Π√2-iron free electron spectrometer. Relative gamma-ray intensities have been determined by Ge(Li) spectrometer. From these measurements the α _K conversion coefficient value has been deduced for 191 keV transition as α_K(191 keV)=0.86±0.03. This value shows that the spin of the level at 268 keV in¹⁹⁷Au is 3/2⁺. For the penetration parameter (λ) and intensity ratio \(\left( {\delta ^2 = \frac{{\left\langle {E2} \right\rangle ^2 }}{{\left\langle {MI} \right\rangle ^2 }}} \right)\) the following values are obtained: $$\begin{gathered} \lambda = 3.4 \pm _{1.5}^{1.9} \delta ^2 = 0.11 \pm 0.03for 77 keV transition \hfill \\ \lambda = 3.2 \pm _{0.6}^{0.8} \delta ^2 = 0.17 \pm 0.04for 191 keV transition. \hfill \\ \end{gathered} $$ The agreement of these results with the predictions of De Shalit model is discussed.     

Determination of the isotope shift in theD 1 line between197Au and195Au

The isotope shift between¹⁹⁷Au (stable) and¹⁹⁵Au (T _1/2=183 d) was determined by resonance fluorescence and polarization spectroscopy in the 6s² S _1/2 -6p ² P _1/2, λ =267.6nm line. The result is δν^{195, 197}=-2.9(2) GHz corresponding to a change of the mean-square charge radius by δ〈r ²〉^{195, 197}=0.063(9) fm². The sample was confined to a resonance cell heated to temperatures of up to 1500°C.     

The spin of the 1029 keV level in200Hg

The spin of the 1,029 keV level in²⁰⁰Hg has been determined to be 0 through measurements of theγ-γ-angular correlation of the 661 keV and 368 keVγ-rays following slow neutron capture in¹⁹⁹Hg. The upper limit for theγ-intensity ratioIγ(1,029 keV)/Iγ(661 keV) obtained from the curved crystal spectrometer (n, γ)-data is much lower than the ratio found during the²⁰⁰Tl→²⁰⁰Hg decay studies. This implies that the 1,029 keVγ-line seen in the decay work does not lead from the 1,029 keV level to the ground state in²⁰⁰Hg, but that it depopulates a level which is only weakly fed through the (n, γ)-process.     

On the ground state energy scaling in quasi-rung-dimerized spin ladders

On the basis of periodic boundary conditions we study perturbatively a largeN asymptotics (N is the number of rungs) for theground state energy density and gas parameter of a spin ladder with slightly destroyedrung-dimerization. Exactly, rung-dimerized spin ladder is treated as the reference model.Explicit perturbative formulas are obtained for three special classes of spin ladders.     

Light charged particle emission in 485 MeV 56Fe + 197Au reactions: Correlations with heavy fragments and relationships to spin and lifetime

Emission of ⁴He and ¹H has been studied in reactions of 485 MeV ⁵⁶Fe + ¹⁹⁷Au, using low-threshold detector arrays for light charged particles, and two heavy-fragment trigger detectors placed at symmetrically opposite angles with respect to the beam direction. The light charged particles were measured both in singles and in coincidence with heavy products of deeply inelastic and fission reactions detected near the grazing angle. Statistical model analyses of the data show that most of the ⁴He/¹H intensity is due to evaporation from energy equilibrated emitters both in deeply inelastic and fission reactions. In deeply inelastic reactions, the observed ⁴He/¹H emission can be attributed to evaporation from the post-scission reactant-like products, while in fusion-like fission reactions evaporation from pre- and post-scission sources are observed in comparable amounts. Angle-integrated multiplicities for ₄He and ¹H are deduced for each source of emission, and are compared with results from similar systems. The experimental ⁴He/¹H multiplicity ratios indicate roughly equal probability for ⁴He and ¹H evaporation from a given excited source. The observation of pre-scission ⁴He/¹H in fusion-like fission reactions supports the notion that thermal equilibration and subsequent particle evaporation proceed more rapidly than the collective motions that drive the system toward fission. Conversely, the lack of appreciable ⁴He/¹H emission from pre-scission sources in deeply inelastic reactions implies that the interaction times are short compared to the time required for particle evaporation.     

Nuclearg-factors of the 328 keV state in194Pt,the 240 keV state in195Pt,and the 356 keV state in196Pt determined by recoil-implantation into iron and cobalt hosts

The iron-implantation-perturbed-angular-correlation technique (IMPACT) has been used to measure the Larmor precessions of the 328, 240, and 356 keV states of¹⁹⁴Pt,¹⁹⁵Pt, and¹⁹⁶Pt nuclei, respectively, in ferromagnetic iron and cobalt hosts. The observed angular precessions in iron, corrected for the transient field effect, wereω τ ₁₉₄(328)=0.103±0.006 rad,ω τ ₁₉₅ (240)=0.142±0.012, andω τ ₁₉₆(356)=0.084±0.010. For cobalt hosts, the observed Larmor precessions were ω τ₁₉₄(328)=0.068±0.006, ω τ₁₉₅(240)=0.096±0.012, andω τ ₁₉₆(356)=0.057±0.013. Agreement between theg-factors obtained from the measurements in iron hosts and the measurements in cobalt hosts is very good, and a weighted average of the two measurements givesg ₁₉₄(328)=0.349±0.031,g ₁₉₅ (240)=0.136±0.013, andg ₁₉₆(356)=0.329±0.039.