Hot fragmentation of nuclei

Today, we have a variety of reactions at hand that can be used to multi-fragment nuclei. In many of these reactions even several sources of fragments can be discerned and characterized. There is overwhelming evidence that these sources of fragments are hot. It is already less clear whether heat by itself is sufficient to initiate the fragment decay. What causes fragmentation, and when and how are the fragments (pre)formed? These questions have remained as much a challenge as the complementary class of questions to which they are related: What observations derive their significance from the liquid-gas phase behavior of extended nuclear matter? And, can we observe a phase transition in finite nuclei? Recent developments, largely coming from complex analyses of data sets measured in 4-#pi#-type experiments as well as from calculations based on advanced theoretical concepts, will be discussed. (orig.)68 refs.Available from TIB Hannover: RO 801(2000-44) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Multifragmentation in relativistic heavy ion reactions

Multifragmentation is the dominant decay mode of heavy nuclear systems with excitation energies in the vicinity of their binding energies. It explores the partition space associated with the number of nucleonic constituents and it is characterized by a multiple production of nuclear fragments with intermediate mass. Reactions at relativistic bombarding energies, exceeding several hundreds of MeV per nucleon, have been found very efficient in creating such highly excited systems. Peripheral collisions of heavy symmetric systems or more central collisions of mass asymmetric systems produce spectator nuclei with properties indicating a high degree of equilibration. The observed decay patterns are well described by statistical multifragmentation models. The present experimental and theoretical studies are particularly motivated by the fact that multifragmentation is being considered a possible manifestation of the liquid-gas phase transition in finite nuclear systems. From the simultaneous measurement of the temperature and of the energy content of excited spectator systems a caloric curve of nuclei has been obtained. The characteristic S-shaped behavior resembles that of ordinary liquids. Signatures of critical phenomena in finite nuclear systems are searched for in multifragmentation data. These studies, supported by the success of percolation in reproducing the experimental mass or charge correlations, concentrate on the fluctuations observed in these observables. Attempts have been made to deduce critical-point exponents associated with multifragmentation. (orig.)90 refs.Available from TIB Hannover: RO 801(96-52) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Chemical equilibrium and isotope temperatures

The measurement and interpretation of isotopic yield ratios in heavy ion reactions at intermediate and high energies are discussed and the usefulness of these observables for establishing equilibrium properties and for determining thermodynamic parameters is illustrated. The examples are mainly taken from work performed with lighter projectiles at intermediate and high energies and from studies of spectator reactions at relativistic energies. As an application, the caloric curve of nuclei, as derived for Au on Au collisions, is introduced and discussed. (orig.)94 refs.Available from TIB Hannover: RO 801(2000-27) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Thermodynamic variables from spectator decay

In this contribution, the focus is on the decay of excited spectator nuclei. New results are reported that were obtained in two recent experiments with the ALADIN spectrometer at SIS in which reactions of "1"9"7Au on "1"9"7Au in the regime of relativistic energies up to 1 GeV per nucleon were studied. In the first experiment, the ALADIN spectrometer was used to detect and identify the products of the projectile-spectator decay. The large-area neutron detector (LAND) was used do measure coincident free neutrons emitted by the projectile source. In the second experiment, three multi-detector hodoscopes, consisting of a total of 216 Si-CsI(Tl) telescopes, and three high-resolution telescopes were positioned at backward angles to measure the yields and correlations of isotopically resolved light fragments of the target-spectator decay. From these data excitation energies and masses, temperatures, and densities were deduced. Before proceeding to the discussion of these new data some of the indications for equilibration during the spectator decay will be briefly recalled. (orig.)Available from TIB Hannover: RO 801(98-20) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Isoscaling in light-ion induced reactions and its statistical interpretation

Isotopic effects observed in fragmentation reactions induced by protons, deuterons, and #alpha# particles of incident energies between 660 MeV and 15.3 GeV on "1"1"2Sn and "1"2"4Sn targets are discussed. The exponential scaling of the yield ratios with the third component of the fragment isospin t_3 = (N -Z)/2 is observed in all reactions, with scaling parameters that depend on the incident energy. Breakup temperatures for these reactions are deduced from double ratios of isotopic yields and tested for their relation with the isoscaling parameters. The quantum statistical (QSM) and the statistical multifragmentation (SMM) models are used for interpreting the results. The observed isoscaling can be understood as a consequence of a statistical origin of the emitted fragments in these reactions. The SMM analysis shows that the exponent describing the isoscaling behavior is proportional to the strength of the symmetry term of the fragment binding energy. Using this result, a symmetry-term coefficient #gamma# #approx# 22.5 MeV for fragments at breakup is deduced from the experimental data. This is close to the standard value and supports SMM assumptions for the breakup configuration. An alternative method of determining the symmetry-energy coefficient, by using isotope distribution widths, is also discussed. (orig.)Available from TIB Hannover: RO 801(02-07) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Experiments at GSI with ALADIN and INDRA

The multi-fragment spectator decays following collisions of "1"9"7Au on "1"9"7Au and other heavy nuclei at relativistic bombarding energies have been studied by the ALADIN collaboration in recent years. The partitioning modes of the excited systems and their thermodynamic properties at breakup were in the center of interest in these experiments. Temperatures and excitation energies, but also breakup densities and emission times have been determined. More recently, the multifragmentation of highly excited heavy nuclei was studied with the 4#pi# multidetector INDRA in experiments at the GSI in 1998 and 1999. The motivation for these new experiments was to extend the investigation of fragmentation processes and of their link to the liquid-gas phase transition to bombarding energies beyond those used in previous INDRA campaigns at GANIL. First results from the analysis of these data will be discussed with particular emphasis on topics related to previous ALADIN work. (orig.)Available from TIB Hannover: RO 801(02-08) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Electromagnetic and nuclear fission of "2"3"8U in the reaction of 100, 500, and 1000 A.MeV "2"0"8Pb with "2"3"8U

The folding- and azimuthal-angle and velocity distributions for the "2"3"8U fission fragments have been measured in reactions with 100, 500, and 1000 A.MeV "2"0"8Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16#+-#0.07, 0.48#+-#0.08, and 0.60#+-#0.04, respectively. The electromagnetic fission cross section as a function of the "2"0"8Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (#approx#1.5 b) is approximately constant between 100 and 1000 A.MeV. (orig.)Available from TIB Hannover: RO 801(94-39) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Breakup temperature of target spectators in "1"9"7Au + "1"9"7Au collisions at E/A=1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction "1"9"7Au+"1"9"7Au 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 observed universality of the spectator decay at 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 #theta#_l_a_b=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. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Sorption of subgroup IV, V and VI elements on ion exchangers from HCI/HF solutions model experiments for chemical studies of the elements 105 and 106 in aqueous solution

The distribution coefficients of Zr, Hf, Nb, Ta, Mo, W, and Pa on cation and anion exchange resins from mixed HCl HF solutions were determined in batch experiments by radiometric measurements. From solutions of 0.05 to 0.1 M HCl with 1.0x10"-"3 M HF a separation of subgroup V and VI from subgroup IV elements is possible on DOWEX 50x8. This system is proposed for the fast on-line separation of the elements 105 and 106. In column experiments with cation exchangers W can be separated from Mo. This opens the possibility to determine the chemical behaviour of element 106 in comparison to the lighter homologs. Differences in the sorption behaviour of Ta, Nb and Pa on cation exchange resins from 0.1 M HCl 2.0x10"-"4 M HF solution should allow a statement as to which of these elements element 105 has the highest similarity. (orig.)SIGLEAvailable from TIB Hannover: RA 3692(95-48) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

First aqueous chemistry with Seaborgium (element 106)

For the first time, chemical separations of element 106 (Seaborgium, Sg) were performed in aqueous solutions. The isotopes "2"6"5Sg and "2"6"6Sg were produced in the "2"4"8Cm+"2"2Ne reaction at a beam energy of 121 MeV. The reaction products were continuously transported by a He(KCl)-jet to the computer-controlled liquid chromatography system ARCA. In 0.1 M HNO_3/5 x 10"-"4 M HF, Sg was found to be eluted within 10 s from 1.6 x 8 mm cation-exchange columns (Aminex A6, 17.5#+-#2 #mu#m) together with the hexavalent Mo- and W-ions, while hexavalent U-ions and tetravalent Zr-, Hf-, and element 104 ions were strongly retained on the column. Element 106 was detected by measuring correlated #alpha#-decays of the daughter isotopes 78-s "2"6"1104 and 26-s"2"5"7102. For the isotope "2"6"6Sg, we have evidence for a spontaneous fission branch. It yields a partial spontaneous-fission half-life which is in agreement with recent theoretical predictions. The chemical results show that the most stable oxidation state of Sg in aqueous solution is +6, and that like its homologs Mo and W, Sg forms neutral or anionic oxo- or oxohalide-compounds under the present condition. In these first experiments, Sg exhibits properties very characteristic of group 6 elements, and does not show U-like properties. (orig.)Available from TIB Hannover: RO 801(96-56) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman