A systematic study of coupling effects in heavy ion reactions

Target of our interest is the study of heavy ion reactions, such as multinucleon transfer, quasi-fission and/or fusionfission, at energies close to the Coulomb barrier as well as their possible application for the synthesis of new isotopes, especially in the high-Z neutron-rich region. The conventional time-of-flight technique, combined with the dE-E method, allows us (in general) to identify the mass and the charge of the reaction products and to study the reaction kinematics. Nonetheless, studying heavy particles at low energies needs a special approach. There are some limitations: pulse hight defect, radiation hardness, etc. Moreover, heavy ions at low energy have very short ranges, which means that we have to find a very thin detector capable of measuring energy loss and time simultaneously. Due to their unique properties (a good energy resolution comparable to silicon detectors, e.g. ∆E=20keV for alphas; time resolution on the order of few ten ps due to very high electron and hole mobilities, high count rate up to 10 due to very short rise and fall times (∼20ps) and high radiation hardness[1]) diamonds are good candidates for this application. Very recently, Pomorski et al. succeeded in fabricating ultra-thin diamond membranes with only few micrometers thickness that are able to measure energy loss and produce simultaneously time signals with excellent resolution[2]. Based on these findings we initiated a program to investigate single crystal (sc) diamond detectors for identification of low-energy heavy ion reaction products. For this application we realized the very first dE-E-ToF telescope [3] which consists of 2 sc diamond detectors: ”dE/Start“ and ”E/Stop“. First measurements were carried out with a mixed nuclide α-source. The diamond ”dE/Start“-detector had a thickness of ∼4 μm and the diamond ”E/Stop“detector ∼50 μm. The distance between the detectors was 14 mm. In Fig. 1a-1c the obtained result is shown. Fig. 1a shows the two-dimensional dE versus residual energy Er spectrum measured with the mixed nuclide α-source. There are three well separated ridges which correspond to the three α-lines. Fig 1b shows the one-dimensional energy spectrum where line 1 is the energy loss in the membrane detector, line 2 is the residual energy measured in the stop detector and line 3 is the sum of dE and Er for each event. Fig. 1c shows the time of flight spectrum. Furthermore, we applied the diamond dE-E-ToF telescope to study reaction products from collisions of

We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons is described via the excitation of nucleon resonances. Medium effects in heavy ion reactions are discussed.

A review on experimental results for direct photon production in heavy ion reactions is given. A brief survey of early direct photon limits from SPS experiments is presented. The first measurement of direct photons in heavy ion reactions from the WA98 collaboration is discussed and compared to theoretical calculations. An outlook on the perspective of photon measurements at RHIC is given.

Role of the Coulomb interaction in the flow and the azimuthal distribution of kaons from heavy ion reactions

The stopping of baryons in heavy ion collisions at beam momenta of plab = 20–160A GeV is lacking a quantitative description within theoretical calculations. Heavy ion reactions at these energies are experimentally explored at the Super Proton Synchrotron (SPS) and the Relativistic Heavy Ion Collider (RHIC) and will be studied at future facilities such as FAIR and NICA. Since the net baryon density is determined by the amount of stopping, this is the pre-requisite for any investigation of other observables related to structures in the QCD phase diagram such as a first-order phase transition or a critical endpoint. In this work we employ a string model for treating hadron–hadron interactions within a hadronic transport approach (SMASH, Simulating Many Accelerated Strongly-interacting Hadrons). Free parameters of the string excitation and decay are tuned to match experimental measurements in elementary proton–proton collisions, where some mismatch in the xF distribution of protons is still present. Afterwards, the model is applied to heavy ion collisions, where the experimentally observed change of the shape of the proton rapidity spectrum from a single peak structure to a double peak structure with increasing beam energy is reproduced. Heavy ion collisions provide the opportunity to study the formation process of string fragments in terms of formation times and reduced interaction cross-sections for pre-formed hadrons. A good agreement with the measured rapidity spectra of protons and pions is achieved while insights on the fragmentation process are obtained. In the future, the presented approach can be used to create event-by-event initial conditions for hybrid calculations.

The elliptic-flow ratio of neutrons with respect to protons or light complex particles in reactions of heavy ions at pre-relativistic energies has been proposed as an observable sensitive to the strength of the symmetry term of the nuclear equation of state at supra-saturation densities. In the ASY-EOS experiment at the GSI laboratory, flows of neutrons and light charged particles were measured for 197Au+197Au collisions at 400 MeV/nucleon. Flow results obtained for the Au+Au system, in comparison with predictions of the UrQMD transport model, confirm the moderately soft to linear density dependence of the symmetry energy deduced from the earlier FOPI-LAND data.

In collisions of heavy ions at relativistic energies highly excited nuclear matter is produced. Hadronic and electromagnetic probes are investigated in order to study the composition and the thermodynamical properties of the short-liver interaction volume and its expansion characteristics. The ultimate goal is to identify signals for a phase transition of hadronic matter to the quark-gluon plasma state. Because of the high multiplicity of particles in the final state various experimental techniques and analysis methods have been development in order to study exclusive signals with electromagnetic and hadronic probes. A selection of detector concepts which have been applied in heavy ion reactions at the CERN SPS will be presented here.

We study the energy dependence of the fusion cross-section with the goal of understanding the evolution of the underlying reaction mechanism as a function of input channel parameters and the causes of its (non-)disappearance. The heart of this work is a systematic and as wide as possible overview of measured cross-sections. Normalized by the reaction cross-section and plotted as a function of the available energy per ucleon corrected for the Coulomb barrier, fusion crosssection show a rather universal behavior leading to a disappearance of the fusion around 10A MeV, except in the case of very asymmetric systems for which incomplete fusion persists and tends towards a constant value that can be explained in the frame of a simple geometrical approach. These trends are confirmed by the semiclassical Landau-Vlasov transport calculations.\n

New reaction mechanisms occurring in heavy ion collisions at low energy (10- 30 MeV/A) are investigated within the Stochastic Mean Field model. We concentrate on the analysis of ternary breakup events, of dynamical origin, occurring in semi-central reactions, where the formation of excited systems in various conditions of shape and angular momentum is observed. We show how this fragmentation mode, which can be considered as a precursor of the neck emission observed at higher beam energies, emerges from the combined action of surface (neck) instabilities and angular momentum effects. Interesting perspectives are opening towards the investigation of this mechanism in neutron-rich (or exotic) systems, with the possibility to access information on the low-density behavior of the nuclear symmetry energy.

The use of heavy ions to induce nuclear reactions was reported as early as 1950. Since then it has been one of the most active areas of nuclear research. Intense beams of ions as heavy as uranium with energies high enough to overcome the Coulomb barriers of even the heaviest elements are available. The large variety of possible reactions gives rise to a multitude of products, which have been studied by many chemical and physical techniques. Chemical techniques have been of special value for the separation and unequivocal identification of low-yield species from the plethora of other nuclides present. Heavy-ion reactions have been essential for the production of the transmendelevium elements and a host of new isotopes. The systematics of compound nucleus reactions, transfer reactions and deeply inelastic reactions have been elucidated using chemical techniques. The variety of chemical procedures and techniques which have been developed for the study of heavy-ion reactions and their products has been examined. The determination of the chemical properties of the transmendelevium elements, which are very short-lived and can only be produced an “atom at a time” via heavy-ion reactions, is discussed.

Calculation of the imaginary part of the heavy ion potential

Strangeness production has long been proposed as a diagnostic tool for understanding the dynamics of relativistic heavy ion collisions. In this presentation we review the traditional picture of strangeness enhancement as a signature for quark-gluon plasma formation. We then review, in order, some experimental data on strange particle production in $e^{+}e^{-}$, $pp$, $p\bar{p}$, proton-nucleus and nucleus-nucleus collisions. This is not a comprehensive review, but rather an emphasis of a few significant points. Any clear interpretation of strange particle yields measured in heavy ion reactions is impossible without a physical understanding of the production mechanisms in elementary particle collisions.

The effects of angular momentum and excitation energy on neutron emission were studied in the production of Tb/sup 149g/, Dy/sup 150/, and Dy/sup 151/, by heavy ions reactions. The heavy ions employed were B/sup 10/, B/sup 11/ , C/sup 12/, N/sup 14/, O/sup 16/, F/sup 19/, and Ne/sup 20/. The number of neutrons emitted in the reactions ran from 3 to 9. The angular and range distributions and excitation functions were determined. The target materials were Pr/sup 141/, Nd/sup 146/, Nd/sup 142/, Nd/sup 144/, Ba/sup 136/, and Ce/sup 140/ . (C.E.S.)

Actinide Nuclei 4. Study of the 234 U(d,t) 233 U Reaction.... 25 5. Study of the Levels in 236 Np and 238 Np by the 237 Np(d,t) and 237 Np(d,p) Reactions 28 239 6. Nuclear Levels of Np Excited by the 3 ( He,d) and (a,t) Reactions.-307. Study of Proton-Hole States in Actinide Nuclei with the (t,a) Reaction B. Heavy Ion Reactions 1. Study of Strongly Damped Collisions.... 2. A Classical Model for Strongly Damped Collisions in Heavy Ion Reactions 40 84 3. Elastic Scattering of Ar and Kr 209_., 238 rT co on Bi and U TABLE OF CONTENTS (Continued) PAGE 4. Heavy Ion Total Reaction Cross Section Systematics Determined from Elastic Scattering 69 C. Muon Lifetime Measurements in the Actinide Elements 81 D. Radioactive Decay Rate Studies: Effect of Compression on the Radioactive Decay Rate of 22 Na 88 E. Statistical Description of Interacting Fermions Including Angular Momentum .... out at the Berkeley Superhilac where our group is one of the active users groups and, hence, this program requires extensive travel funds.A number of interesting results have already been obtained from this program.The muon experiments at LAMPF were initiated during the past year.These experiments are performed also as a Users Group and coordinated by one of our postdoctoral research associates (W.K. Hensley) who is in residence at Los Alamos.Following the summer shutdown at LAMPF we anticipate considerable running time in the early fall.

A variation on the inward-outward method of solving coupled Schr\"odinger equations is presented, which facilitates calculations for reactions induced by massive heavy ions in which many channels are open. Application is made to the 340 MeV $^{40}\mathrm{Ar}$ + $^{238}\mathrm{U}$ reaction and a new physical phenomenon is predicted: nuclear deexcitation to the elastic channel.NUCLEAR REACTIONS Method of calculating with coupled channels model. Calculated $^{238}\mathrm{U}$ ($^{40}\mathrm{Ar}$, $^{40}\mathrm{Ar}$\ensuremath{'}) $^{238}\mathrm{U}$ (${0}^{+}$,...${12}^{+}$).