J. Pochodzalla

Universality of spectator fragmentation at relativistic bombarding energies

Abstract Multi-fragment decays of 129Xe, 197Au and 238U projectiles in collisions with Be, C, Al, Cu, In, Au and U targets at energies between E A = 400 and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 Siue5f8CsI(Tl) telescopes the solid-angle coverage of the setup was extended to θlab = 16°. This permitted the complete detection of fragments from the projectile-spectator source. The dominant feature of the systematic set of data is the Zbound universality that is obeyed by the fragment multiplicities and correlations. These observables are invariant with respect to the entrance channel if plotted as a function of Zbound, where Zbound is the sum of the atomic numbers Zi of all projectile fragments with Zi ⩾ 2. No significant dependence on the bombarding energy nor on the target mass is observed. The dependence of the fragment multiplicity on the projectile mass follows a linear scaling law. The reasons for and the limits of the observed universality of spectator fragmentation are explored within the realm of the available data and with model studies. It is found that the universal properties should persist up to much higher bombarding energies than explored in this work and that they are consistent with universal features exhibited by the intranuclear cascade and statistical multifragmentation models.

Multifragmentation of spectators in relativistic heavy ion reactions

Abstract Using the ALADIN forward-spectrometer at SIS we have studied multi-fragment decays of 197 Au projectiles after collisions with C, Al, Cu and Pb targets at a bombarding energy of E A = 600 MeV . The new data presented in this work comprise the measured cross sections of multifragment processes, the N Z ratios of the produced fragments, and the differential distributions of fragment multiplicities and of observables built on the charge correlations within the event. The 197 Au+Cu data are analyzed with the statistical multifragmentation model. It is shown that all observables can be simultaneously reproduced on an absolute scale, apart from an overall normalization constant which relates the number of model events to the measured cross section. A continuous distribution of excited residual nuclei, used as input for the calculations, was derived from the criterion of a best fit to the data. It exhibits a correlation between decreasing mass number A and increasing excitation energy E x A and a saturation of the excitation energy at E x A ≈ 8 MeV .

Search at the Mainz Microtron for light massive gauge bosons relevant for the muon g-2 anomaly.

H. Merkel,1, ∗ P. Achenbach,1 C. Ayerbe Gayoso,1 T. Beranek,1 J. Beričič,2 J. C. Bernauer,1, † R. Böhm,1 D. Bosnar,3 L. Correa,1 L. Debenjak,2 A. Denig,1 M. O. Distler,1 A. Esser,1 H. Fonvieille,4 I. Friščić,3 M. Gómez Rodrı́guez de la Paz,1 M. Hoek,1 S. Kegel,1 Y. Kohl,1 D. G. Middleton,1 M. Mihovilovič,1 U. Müller,1 L. Nungesser,1 J. Pochodzalla,1 M. Rohrbeck,1 G. Ron,5 S. Sánchez Majos,1 B. S. Schlimme,1 M. Schoth,1 F. Schulz,1 C. Sfienti,1 S. Širca,2, 6 M. Thiel,1 A. Tyukin,1 A. Weber,1 and M. Weinriefer1 (A1 Collaboration) Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, D-55099 Mainz, Germany‡ Jožef Stefan Institute, SI-1000 Ljubljana, Slovenia Department of Physics, University of Zagreb, HR-10002 Zagreb, Croatia Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, BP 10448, F-63000 Clermont-Ferrand, France Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem 91904, Israel Department of Physics, University of Ljubljana, SI-1000 Ljubljana, Slovenia (Dated: April 22, 2014)

Charge correlations as a probe of nuclear disassembly

Abstract We have studied multi-fragment decays of Au projectiles after collisions with C, Al, Cu and Pb targets at a bombarding energy of 600 MeV/nucleon. We examine the correlations between the charges emitted in these reactions. These correlations are given as a function of the total charge in bound fragments, Z bound , at forward angles, which is a measure of the violence of the collision and can be related to the impact parameter. The charge distributions have been fitted by a power law and the extracted τ parameter exhibits a minimum as a function of Z bound . We observe a strong reduction in the maximum charge, Z max , of the event with decreasing Z bound . For those events where Z max is less than half Z bound , the relative sizes of the two largest charges within the event cover the full spectrum of possibilities. The charge-Dalitz plots indicate that the multi-fragmentation events are not an extension of symmetric fission reactions. The event-by-event charge moments are examined to measure the size of the charge fluctuations. All of the charge correlations are independent of the target when plotted as a function of Z bound . The results are compared to both nuclear statistical and percolation calculations. The model predictions differ from each other, establishing that the observables are sensitive to how the available phase space is populated. The sequential nuclear model predicts too asymmetric a decay, while the simultaneous model predicts too symmetric a break-up. The percolation model, which was adjusted to reproduce the size of Z max , correctly predicts the mean behaviour and the fluctuations of the lighter fragments.

Observation of H Λ 4 Hyperhydrogen by Decay-Pion Spectroscopy in Electron Scattering

At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a (9)Be target in order to study the Λ binding energy of light hypernuclei. Positively charged kaons were detected by a short-orbit spectrometer with a broad momentum acceptance at 0° forward angles with respect to the beam, efficiently tagging the production of strangeness in the target nucleus. Coincidentally, negatively charged decay pions were detected by two independent high-resolution spectrometers. About 10(3) pionic weak decays of hyperfragments and hyperons were observed. The pion momentum distribution shows a monochromatic peak at pπ≈133u2009u2009MeV/c, corresponding to the unique signature for the two-body decay of hyperhydrogen Λ(4)H→(4)He+π(-), stopped inside the target. Its Λ binding energy was determined to be BΛ=2.12±0.01u2009u2009(stat)±0.09u2009u2009(syst)MeV with respect to the (3)H+Λ mass.

Fragmentation of gold projectiles: From evaporation to total disassembly

We have studied the fragmentation of Au projectiles interacting with targets of C, Al and Cu at an incident energy ofE/A=600 MeV. The employed inverse kinematics allowed a nearly complete detection of projectile fragments with chargeZ≧2. The recorded fragmentation events were sorted according to three observables, the multiplicityMlp of light charged particles, the largest atomic numberZmax within an event, and a new observable,Zbound, representing the sum of the atomic numbersZ of all fragments withZ≧2. Using these observables, the impact parameter dependence of the fragmentation process was investigated. For all three targets, a maximum mean multiplicity of 3 to 4 intermediate mass fragments (IMFs) is observed. The corresponding impact parameters range from central collisions for theC target to increasingly peripheral collisions for the heavier targets. It is found that the correlation between the IMF multiplicity andZbound, extending from evaporation type processes (largeZbound) to the total disassembly of the projectile (smallZbound), is independent of the target nucleus. This universal behaviour may suggest an — at least partial — equilibration of the projectile fragment prior to its decay.

Thermal and chemical freeze-out in spectator fragmentation

Isotope temperatures from double ratios of hydrogen, helium, lithium, beryllium, and carbon isotopic yields, and excited-state temperatures from yield ratios of particle-unstable resonances in {sup 4}He, {sup 5}Li, and {sup 8}Be, were determined for spectator fragmentation, following collisions of {sup 197}Au with targets ranging from C to Au at incident energies of 600 and 1000 MeV per nucleon. A deviation of the isotopic from the excited-state temperatures is observed which coincides with the transition from residue formation to multi-fragment production, suggesting a chemical freeze-out prior to thermal freeze-out in bulk disintegrations.

Xi- production by Sigma-, pi- and neutrons in the hyperon beam experiment at CERN

Inclusive cross sections for Ξ- hyperon production in high-energy Σ-, π- and neutron induced interactions were measured by the experiment WA89 at CERN. Secondary Σ- and π- beams with average momenta of 345 GeV/c and a neutron beam of 260 GeV/c were produced by primary protons of 450 GeV/c from the CERN SPS. The influence of the target mass on the Ξ- cross section is explored by comparing reactions on copper and carbon nuclei. Both single and double differential cross sections are presented as a function of the transverse momentum and the Feynman variable xF. A strong leading effect for Σ- produced by Σ- is observed.

Electromagnetic fission of238U at 600 and 1000 MeV per nucleon

Electromagnetic fission of238U projectiles at E/A =600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsäcker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium.

Do medium heavy fragments give evidence for a liquid gas phase transition

Abstract An inclusive measurement of light and medium heavy fragments for eight projectile target combinations and incoming energies is presented. The energy spectra of all fragments are fitted consistently with a moving source parametrization. The results are compared to several model predictions. No evidence for the liquid gas phase transition of nuclear matter is found and considerable doubts in thermal interpretations arise. An alternative model of a cold shattering of the target nucleus is able to explain the energy spectra well but fails to reproduce the charge spectra.