P. Lautridou

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.

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.

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.

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.

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.