R. Turrisi

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.

Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions

Photon energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy nuclei to investigate the influence of the multiple-scattering process on the photon production. Relative to the predictions of models based on a quasifree production mechanism, a strong suppression of bremsstrahlung is observed in the low-energy region of the photon spectrum. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.

Thermal bremsstrahlung probing the thermodynamical state of multifragmenting systems

Inclusive and exclusive hard-photon (E-gamma > 30 MeV) production in five different heavy-ion reactions (Ar-36+Au-197, Ag-107, Ni-58, C-12 at 60A MeV and Xe-129+Sn-120 at 50A MeV) has been studied coupling the TAPS photon spectrometer with several charged-particle multidetectors covering more than 80% of 4 pi. The measured spectra, slope parameters and source velocities as well as their target-dependence, confirm the existence of thermal bremsstrahlung emission from secondary nucleon-nucleon collisions that accounts for roughly 20% of the total hard-photon yield. The thermal slopes are a direct measure of the temperature of the excited nuclear systems produced during the reaction.

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.

Emission patterns of neutral pions in 40A MeV Ta plus Au reactions

Differential cross sections of neutral pions emitted in {sup 181}Ta+{sup 197}Au collisions at a beam energy of 39.5A MeV have been measured with the two-arm photon spectrometer (TAPS). The kinetic energy and transverse momentum spectra of neutral pions cannot be properly described in the framework of the thermal model, nor when the reabsorption of pions is accounted for in a phenomenological model. However, high energy and high momentum tails of the pion spectra can be well fitted through thermal distributions with unexpectedly soft temperature parameters below 10 MeV.

Transverse momentum distributions of neutral pions produced at deep subthreshold energies

We present in this paper transverse momentum distributions of π0-mesons measured in 36Ar+197Au reactions at 25A and 35A MeV beam energies with the spectrometer TAPS at the KVI AGOR accelerator. Our data are consistent with thermal-like emission of π0-mesons with temperature parameters much lower than in earlier measurements.

Hard photon and neutral pion production in cold nuclear matter

Abstract The production of hard photons and neutral pions in 190 MeV proton induced reactions on C, Ca, Ni, and W targets has been for the first time concurrently studied. Angular distributions and energy spectra up to the kinematical limit are discussed and the production cross-sections are presented. From the target mass dependence of the cross-sections the propagation of pions through nuclear matter is analyzed and the production mechanisms of hard photons and primordial pions are derived. It is found that the production of subthreshold particles proceeds mainly through first chance nucleon–nucleon collisions. For the most energetic particles the mass scaling evidences the effect of multiple collisions.

Results from proton-proton bremsstrahlung measurements at 190 MeV

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 experiment to date on the proton-proton system. The accuracy is high enough to investigate higher order effects such as the virtual Δ-isobar and meson exchange currents. Surprisingly, none of the microscopic calculations is able to describe the data both in magnitude and in shape.