Improvement in the reconstrution method for VAMOS Spectrometer

International audienceThe VAMOS spectrometer operational at GANIL is a large acceptance variable mode spectrometer designed for nuclear reaction studies using radioactive and stable ion beams. The spectrometer coupled with ancillary detectors like EXOGAM has been successfully used in recent experiments on (in)elastic, few nucleon transfer reactions in inverse kinematics and search for nuclei far from stability using deep inelastic transfer reactions In large acceptance spectrometers, the experimental resolution is worsened by aberrations of third and higher orders. Hardware corrections are limited and cannot completely correct the non-linear effects. Thus software reconstruction of trajectories (ray tracing) is essential to obtain the required resolution and identification of the products. A numerical method has been developed for reconstruction of ion trajectories and correction of aberrations in VAMOS. We have devised a procedure to select an optimum subset of closest trajectories for each focal plane event (x, θ, y, ϕ) from the database (generated by an ion-optics calculation). A polynomial fit to the momentum vector of the reaction product in terms of (x, θ, y, ϕ) is made only to this subset. Such an approach is found to give improved resolutions compared to fitting a single polynomial over the entire phase space. Extraction of charge state and angular distributions are rather difficult due to the variation of acceptance over the large phase space. Mass identification of the reaction products and characteristics of the spectrometer acceptance with its variation for different rigidities have been obtained. Applications to 238U+48Ca and 238U+58Ni systems at 5.5 MeV/u will be presented

Performance of VAMOS for reactions near the Coulomb barrier

Détecteur VAMOSVAMOS (VAriable MOde Spectrometer) is a large solid angle ray-tracing spectrometer employing numerical methods for reconstructing the particle trajectory. Complete identification of reaction products has been achieved by trajectory reconstruction. Equipped with a versatile detection system, VAMOS is capable of identifying reaction products from diverse reactions using beams at GANIL. The technique for trajectory reconstruction and its application for identifying reaction products are presented. The angular acceptance of the spectrometer has been studied using Monte Carlo simulation by an ion optics code. The spectrometer was coupled to the high efficiency EXOGAM γ-array to obtain γ-recoil coincidences for studying nuclei far from stability. The main features of the spectrometer as well as some results applied to experiments in deep inelastic collisions are described

Performance of the improved larger acceptance spectrometer: VAMOS++

International audienceMeasurements and ion optic calculations showed that the large momentum acceptance of the VAMOS spectrometer at GANIL could be further increased from $\sim$ 11% to $\sim$ 30% by suitably enlarging the dimensions of the detectors used at the focal plane. Such a new detection system built for the focal plane of VAMOS is described. It consists of larger area detectors (1000 mm × 150 mm) namely, a Multi-Wire Parallel Plate Avalanche Counter (MWPPAC), two drift chambers, a segmented ionization chamber and an array of Si detectors. Compared to the earlier existing system (VAMOS), we show that the new system (VAMOS++) has a dispersion-independent momentum acceptance . Additionally a start detector (MWPPAC) has been introduced near the target to further improve the mass resolution to $\sim$ 1/220. The performance of the VAMOS++ spectrometer is demonstrated using measurements of residues formed in the collisions of 129Xe at 967 MeV on 197Au

Nuclear Fission: : A Review of Experimental Advances and Phenomenology

In the last two decades, through technological, experimental and theoretical advances, the situation in experimental fission studies has changed dramatically. With the use of advanced production and detection techniques both much more detailed and precise information can now be obtained for the traditional regions of fission research and, crucially, new regions of nuclei have become routinely accessible for fission studies.
 This work first of all reviews the recent developments in experimental fission techniques, in particular the resurgence of transfer-induced fission reactions with light and heavy ions, the emerging use of inverse-kinematic approaches, both at Coulomb and relativistic energies, and of fission studies with radioactive beams.
 The emphasis on the fission-fragment mass and charge distributions will be made in this work, though some of the other fission observables, such as prompt neutron and γ-ray emission will also be reviewed.
 A particular attention will be given to the low-energy fission in the so far scarcely explored nuclei in the very neutron-deficient lead region. They recently became the focus for several complementary experimental studies, such as β-delayed fission with radioactive beams at ISOLDE(CERN), Coulex-induced fission of relativistic secondary beams at FRS(GSI), and several prompt fusion-fission studies. The synergy of these approaches allows a unique insight in the new region of asymmetric fission around <sup>180</sup>Hg, recently discovered at ISOLDE. Recent extensive theoretical efforts in this region will also be outlined.
 The unprecedented high-quality data for fission fragments, completely identified in <i>Z</i> and <i>A</i>, by means of reactions in inverse kinematics at FRS(GSI) and VAMOS(GANIL) will be also reviewed. These experiments explored an extended range of mercury-to-californium elements, spanning from the neutron-deficient to neutron-rich nuclides, and covering both asymmetric, symmetric and transitional fission regions.
 Some aspects of heavy-ion induced fusion-fission and quasifission reactions will be also discussed, which reveal their dynamical features, such as the fission time scale. The crucial role of the multi-chance fission, probed by means of multinucleon-transfer induced fission reactions, will be highlighted.
 The review will conclude with the discussion of the new experimental fission facilities which are presently being brought into operation, along with promising 'next-generation' fission approaches, which might become available within the next decade

Nuclear symmetry energy in calcium-calcium collisions (INDRA-VAMOS)

The density dependence of the symmetry energy is of great interest to many fields of nuclear physics and nuclear astro-physics. The E503 INDRA-VAMOS experiment performed at GANIL in 2007 is intended to provide further sub-saturation constraints using calcium-calcium collisions around the Fermi energy (35AMeV). In these proceedings this experiment will be discussed in the context of the physics it is aiming to study and will give a brief summary of the current progress of the data analysis

Fusion-fission studies on

Fission process is strongly influenced by entrance channel dynamical variables. Among these, the nuclear charge product, mass asymmetry and deformation play important role in fission dynamics. Reaction characteristics are distinguished by investigating the properties of fission mass and angular distributions. Experiments using actinide targets are challenging due to many conflicting results making unambiguous identification of quasi-fission difficult. At IUAC accelerator facility many experiments have been performed to make a systematic study of fission mechanism and role of entrance channel parameters and deformation. Fragment mass distribution, angular distribution and neutron multiplicity measurements are performed to study reactions using spherical and deformed targets