E. Vardaci

Search for ternary fragmentation in the reaction 856 MeV 98Mo+51V: kinematic probing of intermediate-mass-fragment emissions

Abstract A new technique has been applied to coincidence measurements between fission fragments (FF) and intermediate mass fragments (IMF) emitted from the composite system 149 65 Tb at an excitation energy of 224 MeV. The method permits simultaneous observation of IMF emissions along and normal to the FF separation axes. For the integrated total of 0.10± 0.02 IMF emitted per fission, we find no significant correlation with FF direction, suggesting that IMFs associated with fission reactions are predominantly emitted from the system prior to fission.

Fission dynamics In 132Ce composite nuclei at Ex = 122 MeV

We are carrying out a research program with the 8πLP apparatus at the Laboratori Nazionali di Legnaro (Italy), aimed at studying fission dynamics in systems of intermediate fissility. These systems, compared to the heavier ones, are characterized by larger pre‐scission charged particle multiplicities as well as by comparable fission and evaporation residue cross sections. Therefore, the measurement of observables in both channels allows putting severe constraints on the models, in order to obtain more reliable information on the dynamics of the process. We present the results for the composite system 122Ce, produced at Ex = 122 MeV in 200 MeV 32S+100Mo reaction. The comparison of the data with the predictions of a dynamical model, based on 3‐D Langevin equations, indicates that full one‐body dissipation plays a dominant role in the fission process, with a viscosity parameter independent from the temperature and a fission delay of ∼25*10−21 s.

Statistics vs. dynamics in fission: light and shade from systems of intermediate fissility

Systems of intermediate fissility are characterized by an evaporation residues cross section comparable or larger than the fission cross section, and by a relatively higher probability for charged particle emission in the pre‐scission channel. In a theoretical framework in which time scale estimates of the fission process rely on statistical model calculations, the analysis of particle emission in the evaporation residues channel is the source of additional constraints on the statistical and dynamical models. This contribution will focus on our statistical and dynamical analysis of a more complete set of data from the system 32S+100Mo at ELab = 200 MeV. Statistical model fails in reproducing the whole set of data and no convincing estimate is possible of the fission time scale. In particular, while pre‐scission multiplicities can be reproduced without delay, the model strongly overestimates proton and alpha particle multiplicities in the evaporation residues channel irrespective of the statistical model i...

γ‐ray Spectroscopy of Proton Drip‐Line Nuclei in the A∼130 Region using SPIRAL beams

A fusion‐evaporation experiment has been performed with a SPIRAL 76Kr radioactive beam in order to study the deformation of rare‐earth nuclei near the proton drip‐line. The experimental setup consisted in the EXOGAM γ‐array, coupled to the light‐charged particles (LCP) DIAMANT detector and to the VAMOS heavy‐ion spectrometer. The difficulties inherent to such measurements are enlightened. The coupling between EXOGAM and DIAMANT has been used to decrease the huge background caused by the radioactivity of the beam. It further permits assigning new γ‐ray transitions to specific residual nuclei. A γ‐ray belonging to the 130Pm level scheme has thus been observed for the first time.

{gamma}-Spectroscopy and Radioactive Beams: How To Perform Channel Selection ?

An experiment has been performed using a SPIRAL 76Kr radioactive beam at GANIL to investigate rare‐earth nuclei near the proton drip‐line. The EXOGAM gamma array was coupled with the DIAMANT light charged‐particle detector and the VAMOS spectrometer. We report here on the powerful of this setup to extract fusion‐evaporation γ‐rays from a large beam contamination.

The EXODET Apparatus And Its First Experimental Results: 17F Scattering By 208Pb Below The Coulomb Barrier

A new detector apparatus has been designed and developed to be used in experiments performed with radioactive ion beams. It consists of 16 highly segmented silicon strip detectors arranged in two‐layer telescopes and subtending a large solid angle (about 70% of 4π sr). An innovative readout system for the position information that uses highly integrated electronics (ASIC chips) has been implemented. A first successful experiment has been performed at the Argonne National Laboratory (USA) to study the 17F scattering by a 208Pb target at 90.4 MeV of incident energy. The 17F angular distribution has been analyzed and the optical model potential best‐fit parameters determined. The same analysis performed on 17F data taken at higher incident energy, in completely different experimental conditions, gives consistent results. The comparison with experiments performed with stable beams (19F, 16O, 17O) indicates a behavior for the 17F more similar to that of the Oxygen isotopes than to the 19F one. Despite of the s...

EXODET: a new approach to detection systems for RIB nuclear physics based on ASIC chips developed for high-energy experiments

A new experimental apparatus for charged particle detection, named EXODET (EXOtic DETector) has been designed to be used in nuclear physics experiments involving RIBs (radioactive ion beams). Typical feature of RIBs presently available at the first-generation production facilities is the low beam intensity (105-106 pps) therefore a large solid angle coverage and a high granularity are required to allow the reconstruction of the complete event kinematics. In this work are presented the main features of the EXODET apparatus, fulfilling the previous requirements, and the first experimental results.

Studies of Light Charged Particle Emission From Fission and ER Reactions in the System 344 Mev 28Si+121Sb-149Tb (E=240 MeV)

Abstract Light charged particles (LCP) have been measured for the reaction 344 MeV 28 Si + 121 Sb in singles and in coincidence with evaporation residues (ER), fusion–fission fragments (FF), and other LCP. A major feature of this experiment was the use of a gas-filled magnetic spectrometer in the forward direction to separate ER from the much more abundant yield of elastically scattered projectiles and projectile-like fragments. The dominant sources of evaporative 1 H and 4 He emission are the ER (approximately 75%), with the remainder being largely associated with fission reactions. For these latter reactions, most of the 1 H and 4 He can be well accounted for by evaporation from the composite system prior to fission and by evaporation from the postfission fragments. LCP emission cross sections were determined for each identified source, and a comparison has been made to previous studies. From this comparison, indications were found for significant entrance channel effects, with the more asymmetric channels exhibiting much larger LCP cross sections. Statistical model predictions for ER emissions are in good agreement with observed LCP energy spectra, angular distributions, and integrated inclusive and exclusive cross sections, with all calculations using the same unique set of model parameters. This result contrasts strongly with recent reports for light mass systems, where model calculations were unable to simultaneously reproduce all observables.

Production of n-rich Nuclei along the Closed Shell N=126 in the collision 136Xe + 208Pb @E lab =870 MeV

Multi-nucleon transfer reactions are nowadays the only known mean to produce neutron-rich nuclei in the Terra Incognita. The closed-shell region N=126 is crucial for both studying shell-quenching in exotic nuclei and the r-process, being its last ”waiting-point”. The choice of suitable reactions is challenging and a favorable case is Xe+Pb, near the Coulomb barrier, because their neutron shell-closures play a stabilizing role, favoring the proton-transfer from lead to xenon. TOF-TOF data were analyzed to reconstruct the mass-energy distribution of the primary fragments. Preliminary results of an experiment held at Laboratori Nazionali di Legnaro with PRISMA, aimed at A and Z identification of the products, will be shown.

Gamma rays as probe of fission and quasi-fission dynamics in the reaction 32S + 197Au near the Coulomb barrier

Compound nucleus fission and quasi-fission are both binary decay channels whose common properties make the experimental separation between them difficult. A way to achieve this separation could be to probe the angular momentum of the binary fragments. This can be done detecting gamma rays in coincidence with the two fragments. As a case study, the reaction 32S + 197Au near the Coulomb barrier has been performed at the Tandem ALTO facility at IPN ORSAY. ORGAM and PARIS, two different gamma detectors arrays, are coupled with the CORSET detector, a two-arm time-of-flight spectrometer. TOF-TOF data were analyzed to reconstruct the mass-energy distribution of the primary fragments coupled with gamma multiplicity and spectroscopic analysis. Preliminary results of will be shown.