arXiv subjects

A. Gobbi

Publications and source records attributed to A. Gobbi.

7 recordsLinked to original sources

arXiv2004

Energetics of midvelocity emissions in peripheral heavy ion collisions at Fermi energies

Peripheral and semi-peripheral collisions have been studied in the system 93Nb+93Nb at 38 AMeV. The evaporative and midvelocity components of the light charged particle and intermediate mass fragment emissions have been carefully disentangled. In this way it was possible to obtain the average amount not only of charge and mass, but also of energy, pertaining to the midvelocity emission, as a function of an impact parameter estimator. This emission has a very important role in the overall balance of the reaction, as it accounts for a large fraction of the emitted mass and for more than half of the dissipated energy. As such, it may give precious clues on the microscopic mechanism of energy transport from the interaction zone toward the target and projectile remnants.

nucl-ex↗

arXiv2002

Sub-threshold phi-meson yield in central 58Ni+58Ni collisions

The $ϕ$-meson production cross section is measured for the first time at a sub-threshold energy of 1.93 AGeV in \nuc{58}{Ni}+\nuc{58}{Ni} central collisions. The $ϕ$ data were obtained within the acceptance of the CDC/Barrel subsystem of FOPI. For a sample of $4.7\cdot 10^6$ central events, after background subtraction, 23 candidates were observed. Extensive GEANT simulations of the detector performance are shown in a thorough comparison to the real response, aiming at a good understanding of the apparatus and at a trustable determination of the efficiencies, production probability and possible systematic errors. A filter procedure is elaborated, which is meant to facilitate the comparison of any theoretical calculation or new data with the current ones. How to extrapolate the present value to a $ϕ$-meson cross section in $4π$ is also discussed. This result on pseudo-vector mesons can now be compared to existing experimental knowledge for the same reaction at the same incident energy for various outgoing channels, $K^+$ and $K^-$ included. A significant fraction (at least 20%) of the $K^-$-mesons is originating in the decay of the $ϕ$, supporting the statement that the two channels are strongly correlated.

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arXiv2001

A Large Area Timing RPC

A large area Resistive Plate Chamber (RPC) with a total active surface of 160x10 cm2 was built and tested. The surface was segmented in two 5 cm wide strips readout on both ends with custom, very high frequency, front end electronics. A timing resolution between 50 and 75 ps sigma with an efficiency for Minimum Ionizing Particles (MIPs) larger than 95% was attained over the whole active area, in addition with a position resolution along the strips of 1.2 cm. Despite the large active area per electronic channel, the observed timing resolution is remarkably close to the one previously obtained (50 ps sigma) with much smaller chambers of about 10 cm2 area. These results open perspectives of extending the application of timing RPCs to large area arrays exposed to moderate particle multiplicities, where the low cost, good time resolution, insensitivity to the magnetic field and compact mechanics may be attractive when compared with the standard scintillator-based Time-of-Flight (TOF) technology.

physics.ins-det↗

arXiv1999

Isospin-tracing: A probe of non-equilibrium in central heavy-ion collisions

Four different combinations of $^{96}_{44}$Ru and $^{96}_{40}$Zr nuclei, both as projectile and target, were investigated at the same bombarding energy of 400$A$ MeV using a $4 π$ detector. The degree of isospin mixing between projectile and target nucleons is mapped across a large portion of the phase space using two different isospin-tracer observables, the number of measured protons and the ${\rm t}/^{3}{\rm He}$ yield ratio. The experimental results show that the global equilibrium is not reached even in the most central collisions. Quantitative measures of stopping and mixing are extracted from the data. They are found to exhibit a quite strong sensitivity to the in-medium (n,n) cross section used in microscopic transport calculations.

nucl-ex↗

arXiv1997

Onset of Nuclear Matter Expansion in Au+Au Collisions

Using the FOPI detector at GSI Darmstadt, excitation functions of collective flow components were measured for the Au+Au system, in the reaction plane and out of this plane, at seven incident energies ranging from 100AMeV to 800AMeV. The threshold energies, corresponding to the onset of sideward-flow (balance energy) and squeeze-out effect (transition energy), are extracted from extrapolations of these excitation functions toward lower beam energies for charged products with Z>2. The transition energy is found to be larger than the balance energy. The impact parameter dependence of both balance and transition energies, when extrapolated to central collisions, suggests comparable although slightly higher values than the threshold energy for the radial flow. The relevant parameter seems to be the energy deposited into the system in order to overcome the attractive nuclear forces.

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arXiv1997

Abundance of Delta Resonances in 58Ni+58Ni Collisions between 1 and 2 AGeV

Charged pion spectra measured in 58Ni-58Ni collisions at 1.06, 1.45 and 1.93 AGeV are interpreted in terms of a thermal model including the decay of Delta resonances. The transverse momentum spectra of pions are well reproduced by adding the pions originating from the Delta-resonance decay to the component of thermal pions, deduced from the high transverse momentum part of the pion spectra. About 10 and 18% of the nucleons are excited to Delta states at freeze-out for beam energies of 1 and 2 AGeV, respectively.

nucl-ex↗

arXiv1996

Direct experimental evidence of non-equilibrium energy sharing in dissipative collisions

Primary and secondary masses of heavy reaction products have been deduced from kinematics and E-ToF measurements, respectively, for the direct and reverse collisions of 100Mo with 120Sn at 14.1 A MeV. Direct experimental evidence of the correlation of energy-sharing with net mass transfer and model-independent results on the evolution of the average excitation from equal-energy to equal-temperature partition are presented.

nucl-ex↗