A Marin

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.

Bremsstrahlung photons as a probe of hot nuclei

Abstract Aside from the dominant production of hard photons in first-chance p-n collisions, a significant hard-photon production in a later stage of heavy-ion reactions is predicted by the BUU theory. These thermal hard photons are emitted from a nearly thermalized source and still originate from bremsstrahlung produced in individual p-n collisions. The calculations predict that the production of the thermal hard photons is strongly correlated with the incompressibility of nuclear matter. Tentative experimental evidence for their production is found in the hard-photon energy spectra measured in the systems 86 Kr + nat Ni at 60.0 A MeV, 181 Ta+ 197 Au at 39.5 MeV and 208 Pb+ 197 Au at 29.5 A MeV.

Density oscillations in systems of colliding heavy ions observed via hard-photon interferometry measurements☆

Abstract We have analyzed hard-photon intensity correlations measured in the systems 86 Kr+ nat Ni at 60.0 A MeV and 181 Ta+ 197 Au at 39.5 A MeV assuming that hard photons are emitted from two distinct sources in space-time. We confirm the existence of the Bose-Einstein correlation between independent hard photons and attribute the origin of the two sources to the density oscillations of nuclear matter generated in intermediate-energy heavy-ion collisions via the incomplete-fusion reaction mechanism.

Impact parameter dependence of hard photon production in intermediate energy heavy-ion collisions

Abstract Hard photon production was investigated in the system 68 Kr+ nat Ni at 60 A MeV for a wide range of impact parameters. The parameters characteristic of hard photon emission such as source velocities, angular distributions and energy spectra were studied from peripheral to central collisions. The energy spectrum is seen to be strongly dependent on the centrality of the collision reflecting both a static and dynamic momentum distribution of the nucleons inside the collision zone.

Pion reabsorption in heavy-ion collisions interpreted in terms of the Delta capture process

We have measured energy-differential cross sections for pi(0) production in Ar-36 + (197)AU collisions at 95 MeV/u. From an analysis of spectral features due to pion final-state interactions we have estimated the cross section of the capture process Delta + N --> N + N in the center-of-mass energy range root s similar or equal to 2.05-2.25 GeV. Within the frame of BUU calculations, our results support the extension of the detailed-balance principle to broad-width resonances.

Photon Production in Heavy-ion Collisions Close to the Pion Threshold

Abstract We report on a measurement of hard photons ( E γ >30 MeV) in the reaction Ar+Ca at 180 A MeV at an energy in which photons from the decay of π ° mesons are dominating. Simultaneous measurement with the TAPS spectrometer of the photon spectrum and photon-photon coincidences used for the identification of π ° enabled the subtraction of π ° contribution. The resulting photon spectrum exhibits an exponential shape with an inverse slope of E 0 =(53±2 (stat) −5 +8 (syst) ) MeV. The photon multiplicity equal to (1.21±0.03 (stat) +0.3 −0.2 (syst) )·10 −2 is roughly one order of magnitude larger than the value extrapolated from existing systematics. This enhancement of the hard photon production is attributed to a strong increase in the contribution of secondary np collisions to the total photon yield. We conclude that, on average, the number of np collisions which contribute to the hard photon production is 7 times larger than the number of first chance np collisions in the reaction Ar+Ca at 180A MeV.

Investigation of polar and azimuthal distributions of subthreshold pions at intermediate energies

Abstract We present data on π 0 emission coincident with projectile-like fragments from 36 Ar + 197 Au and 36 Ar + 12 C reactions at 95 MeV/u. The pion polar and azimuthal distributions, obtained for different impact parameters, are interpreted in terms of strong final-state interactions. We propose to exploit pion shadowing as a probe of the competition between attractive and repulsive forces in nucleus-nucleus collisions.

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.

Hard photons as a probe to study dissipation mechanisms

Abstract The probability for Bremsstrahlung has been measured as function of the mass of the projectile-like fragment in peripheral reactions of 36 Ar + 159 Tb at 44 MeV/nucleon. The Bremsstrahlung probability is found to depend on the amount of mass transferred and the direction of the transfer.

Heavy ion coulomb excitation and gamma decay studies of the one and two phonon giant dipole resonances in 208Pb and 209Bi

Abstract Projectile — photon coincidences were measured for the scattering of an 80 MeV/nucleon 64 Zn beam from 208 Pb and 209 Bi targets at the GANIL heavy ion accelerator facility. Projectile-like particles between 0.5° and 4.5° relative to the incident beam direction were detected in the SPEG energy loss spectrometer where their momentum, charge, and mass were determined. Photons were detected in the BaF 2 scintillation detector array TAPS. Light charged particles produced in the reaction were detected in the KVI Forward Wall. The analysis of the data acquired in this experiment is focused on three different phenomena: (1) the two phonon giant dipole resonance, (2) time dependence of the decay of the one phonon giant dipole resonance, and (3) giant resonance strength in projectile nuclei.