V. Wagner

Production of pi(0) and eta mesons in carbon-induced relativistic heavy-ion collisions

The production of π0 and η mesons has been investigated in the system 12C+12C at 0.8A GeV, 1.0A GeV, and 2.0A GeV using the TAPS photon detector. The production cross sections and transverse-momentum distributions measured around midrapidity extend the existing systematics of neutral-meson production to light systems and the highest energy available at the GSI accelerator facility. At all beam energies scaling of the differential production cross section with the transverse mass of the emitted meson is observed. In addition π0 production has been studied in the mass-asymmetric systems 12C+197Au and 197Au+12C at 0.8A GeV.

Hard photons and neutral pions as probes of hot and dense nuclear matter

Abstract The dynamics of heavy-ion collisions is studied in an energy domain in the vicinity of the Fermi energy. The early history of the collision is analyzed from the theoretical and experimental point of view in which the message conveyed by bremsstrahlung photons and neutral pions is exploited. The Boltzmann-Uehling-Uhlenbeck model and the Dubna Cascade Model, both based on similar principles but each adopting different computation technics, are briefly described and their respective predictions are discussed. In particular the emission pattern of bremsstrahlung photons is discussed. The photon production has been measured in the systems 86 Kr+ 58 Ni at 60 A MeV, 181 Ta+ 197 Au at 40 A MeV and 208 Pb+ 197 Au at 30 A MeV and energy spectra, angular distributions and two-photon correlations have been analyzed. We find that bremsstrahlung photons are emitted from two distinct sources that can be correlated with nuclear-matter density oscillations. The properties of photon emission are discussed in terms of collective properties of nuclear matter. The high energy tail of the photon spectrum is interpreted by π 0 and Δ decay but predominantly by radiative capture of pions. The π 0 absorption in the nuclear medium is further analyzed by examining their emission pattern.

Radiative proton capture on (^6)He

Radiative capture of protons is investigated as a probe of clustering in nuclei far from stability. The first such measurement on a halo nucleus is reported here for the reaction 6He(p,gamma) at 40 MeV. Capture into 7Li is observed as the strongest channel. In addition, events have been recorded that may be described by quasifree capture on a halo neutron, the alpha core, and 5He. The possibility of describing such events by capture into the continuum of 7Li is also discussed.

Evidence for thermal equilibration in multifragmentation reactions probed with bremsstrahlung photons

The production of nuclear bremsstrahlung photons (E{sub {gamma}}>30 MeV) has been studied in inclusive and exclusive measurements in four heavy-ion reactions at 60A MeV . The measured photon spectra, angular distributions, and multiplicities indicate that a significant fraction of the hard photons is emitted in secondary nucleon-nucleon collisions from a thermally equilibrated system. The observation of the thermal component in multifragment {sup 36}Ar+{sup 197}Au reactions suggests that the breakup of the thermalized source produced in this system occurs on a rather long time scale.

DETECTION OF CHARGED PIONS AND PROTONS IN THE SEGMENTED ELECTROMAGNETIC CALORIMETER TAPS

We present the characteristics of the segmented BaF2 calorimeter TAPS for the measurement of charged pions and protons. The method of particle identification exploits the relation between the kinetic energy of a particle, its mass and the time-of-flight required to reach the detector. The detection efficiency is calculated using GEANT-GCALOR simulations. The analysis method is applied in the reaction Ar-40 + Ca-nat at 0.8A GeV. The simultaneous detection of charged pions and protons can be used to search for correlated pairs signalling the de-excitation of the Delta(1232) resonance

Exclusive πo- and η-meson production in 40Ar + natCa at 800A MeV☆

Abstract The impact-parameter dependence of π o - and η-meson production is reported for the system 40 Ar+ nat Ca at a beam energy of 800 A MeV. Scaling of the meson abundances with the transverse mass is observed. The experimental results are compared to calculations within the BUU model.

Measurements of the virtual bremsstrahlung yields in the p+p system

Abstract In this paper recent results obtained from the p + p → p + p + e + + e − experiment are presented. This experiment has been performed with a 190 MeV polarized proton beam obtained from the new cyclotron AGOR at KVI in Groningen. Differential cross sections have been obtained in exclusive measurements in which all four exit particles have been measured in a coincidence setup between SALAD and TAPS. The data are compared with LET calculations. A reasonable agreement is found for virtual-photon invariant masses up to 80 MeV/c 2 .

Exclusive pi^{0} and eta-meson Production in ^{40}Ar+^{nat}Ca at 800 A MeV

Abstract The impact-parameter dependence of π o - and η-meson production is reported for the system 40 Ar+ nat Ca at a beam energy of 800 A MeV. Scaling of the meson abundances with the transverse mass is observed. The experimental results are compared to calculations within the BUU model.

Coherent Bremsstrahlung in \alpha + p Reactions at 50 MeV\nucleon

Bremsstrahlung emitted in the α + p reaction at 50 MeV/nucleon was studied using a 200 MeV α -particle beam and a liquid hydrogen target. Double-differential photon cross sections were measured over a wide range of angles and energies. Coherent bremsstrahlung was found to be strong for all photon energies. The cross section is enhanced at the highest photon energies and is characteristic of direct capture to the unbound 5 Li ground and first excited state. With direct capture as a limiting case of coherent bremsstrahlung a consistent reaction picture can be obtained.

Probing Few-body Systems with Bremsstrahlung

A series of bremsstrahlung measurements have been performed at KVI with the 190 MeV polarized proton beam of the new superconducting cyclotron, AGOR. The aim of these measurements is to investigate the dynamics of interactions involving intermediate off-shell nucleons. Nucleon-nucleon bremsstrahlung is the most fundamental process used for such studies as it involves the strong interaction between two nucleons and the well-known electromagnetic interaction. The present experiment provides the most accurate data on the proton-proton system. In addition the first exclusive proton-proton virtual-bremsstrahlung data ever is presented. The accuracy of the real bremsstrahlung data is high enough to investigate higher-order effects such as the virtual Δ-isobar and meson-exchange currents. None of the microscopic calculations is able to describe the data both in magnitude and in shape. The virtual-bremsstrahlung data are integrated over the detector acceptance. This same integration is done for two different soft-photon calculations for comparison.