Showing papers by "A. Bracco published in 2007"

Investigation of the properties of a 1″×1″ LaBr3:Ce scintillator

Abstract: The properties of a cylindrical 1 00 � 1 00 LaBr3:Ce scintillator (Brillance 380) were investigated. The energy resolution at g-ray energy of 662 keV was measured to be smaller than 3% and the intrinsic time resolution was found to be E230 ps. Two different aspects were investigated. The first is the detailed study of the crystal self-activity, emitting a and b particles and g-rays, by measuring coincidences with g-rays in HPGe, BGO and BaF2 detectors. In particular, the coincidence with an HPGe detector allowed to isolate clearly the different contributions from 227 Ac (chemical homologue of lanthanum) and its daughter nuclei down to the stable 207 Pb. The second aspect is the determination of the efficiency for g-ray detection, measured at 1 MeV and simulated using GEANT4 up to 15 MeV as a function of the detector size.

PROGRESS IN THE STUDY OF THE γ-DECAY OF THE GIANT DIPOLE RESONANCE IN HOT NUCLEI

Abstract: The problem of the damping of the Giant Dipole Resonance (GDR) at finite temperature at T>2 MeV is discussed here. The experimental results are based on fusion evaporation reactions. The most recent results on the mass region A = 132 (Ce isotopes) obtained in exclusive measurements are compared with the existing results on the A = 110–120 region (Sn isotopes). The comparison with the theoretical predictions based on thermal shape fluctuations is also discussed. The GDR width is found to increase also in the region T>2 MeV and this is accounted by the combined effect of the increase of the compound nucleus width (smaller lifetime) and to the increase of the average deformation of the nucleus.

Band structures extending to very high spin inXe126

Abstract: High-spin states in $^{126}\mathrm{Xe}$ have been populated in the $^{82}\mathrm{Se}$($^{48}\mathrm{Ca}$,4$n$)$^{126}\mathrm{Xe}$ reaction in two experiments, one at the VIVITRON accelerator in Strasbourg using the Euroball detector array, and a subsequent one with ATLAS at Argonne using the Gammasphere Ge-detector array. Levels and assignments made previously for $^{126}\mathrm{Xe}$ up to $I=20$ have been confirmed and extended. Four regular bands extending to a spin of almost $I=60$, which are interpreted as two pairs of signature partners with opposite parity, are identified for the first time. The \ensuremath{\alpha} = 0 partner of each pair is connected to the lower-lying levels, whereas the two \ensuremath{\alpha} = 1 partners remain floating. A fractional Doppler shift analysis of transitions in the strongest populated $(\ensuremath{\pi},\ensuremath{\alpha})=(\ensuremath{-},0)$ band provides a value of ${5.2}_{0.5}^{0.4}$ b for the transition quadrupole moment, which can be related to a minimum in the potential-energy surface calculated by the ULTIMATE CRANKER cranked shell-model code at $\ensuremath{\epsilon}\ensuremath{\approx}0.35$ and $\ensuremath{\gamma}\ensuremath{\approx}{5}^{\ifmmode^\circ\else\textdegree\fi{}}$. The four lowest bands calculated for this minimum compare well with the two signature pairs experimentally observed over a wide spin range. A sharp upbend at $\ensuremath{\hbar}\ensuremath{\omega}~1170$ keV is interpreted as a crossing with a band involving the ${j}_{15/2}$ neutron orbital, for which pairing correlations are expected to be totally quenched. The four long bands extend to within \ensuremath{\sim}5 spin units of a crossing with an yrast line defined by calculated hyperdeformed transitions and will serve as important stepping stones into the spin region beyond 60\ensuremath{\Elzxh} for future experiments.

Isospin symmetry breaking at high spin in the mirror nuclei Ar 35 and Cl 35

Abstract: High-spin states in $^{35}\mathrm{Ar}$ and $^{35}\mathrm{Cl}$ have been populated in the $^{24}\mathrm{Mg}$($^{16}\mathrm{O}$, $\ensuremath{\alpha}n$) and $^{24}\mathrm{Mg}$($^{16}\mathrm{O}$, $\ensuremath{\alpha}p$) reactions, respectively, at a beam energy of 70 MeV The comparison between the level schemes of these mirror nuclei shows a striking asymmetry in the population yield of high-spin analog states of positive parity, which indicates different intensities of $E1$ transitions connecting positive- and negative-parity structures in both nuclei Large energy differences are observed between analog states of negative parity with configurations of dominant pure single-particle character This results from the comparison with large-scale shell-model calculations in the ${s}_{1/2}{d}_{3/2}{f}_{7/2}{p}_{3/2}$ valence space It is shown that important contributions to the energy differences arise from the multipole Coulomb and the relativistic electromagnetic spin-orbit interactions

Progress in the study of the -decay of the giant dipole resonance in hot nuclei

Abstract: The problem of the damping of the Giant Dipole Resonance (GDR) at nite temperature at T > 2 MeV is discussed here. The experimental results are based on fusion evaporation reactions. The most recent results on the mass region A = 132 (Ce isotopes) obtained in exclusive measurements are compared with the existing results on the A = 110{120 region (Sn isotopes). The comparison with the theoretical predictions based on thermal shape uctuations is also discussed. The GDR width is found to increase also in the region T > 2 MeV and this is accounted by the combined eect of the increase of the compound nucleus width (smaller lifetime) and to the increase of the average deformation of the nucleus.

Recent Results on Neutron-Rich Nuclei Spectroscopy with the CLARA-PRISMA Setup

Abstract: The CLARA–PRISMA setup consisting of the array of Euroball Clover detectors CLARA, coupled to the LNL large acceptance magnetic spectrometer PRISMA, has been successfully working since spring 2004. Recently, new developments such as the ancillary device DANTE and the use of the differential plunger technique has improved the capabilities of the setup. Results of experiments studying the structure of neutron-rich nuclei together with the preliminary results on the commissioning of the ancillary detector array DANTE and of the plunger technique will be presented.

STRONG DEFORMATION EFFECTS IN HOT ROTATING 46 Ti

Abstract: Ti nucleus were investigated by analy-sing the high-energy γ-ray and the α-particle energy spectra. One of theexperiments was performed using the charged-particle multi-detector ar-ray ICARE together with a large volume (4in×4in) BGO detector. Thestudy focused on simultaneous measurement of light charged particles andγ-rays in coincidence with the evaporation residues. The experimental datashow a signature of very large deformations of the compound nucleus in theJacobi transition region at the highest spins. These results are compared todata from previous experiments performed with the HECTOR array cou-pled to the EUROBALL array, where it was found that the GDR strengthfunction is highly fragmented, strongly indicating a presence of nuclei withvery large deformation.PACS numbers: 24.30.Cz, 21.60.Ev, 25.70.Gh, 24.60.Dr

g FACTOR MEASUREMENTS ON RELATIVISTIC ISOMERIC BEAMS PRODUCED BY FRAGMENTATION AND U-FISSION: THE g-RISING PROJECT AT GSI

Abstract: Within the RISING (Rare ISotope INvestigations @ GSI) Collaboration at GSI, g factor measurements have been performed on isomeric states in neutron-rich isotopes approaching Sn-132 and in the neutron deficient Pb-region (the g-RISING campaign). We present the experimental technique and some typical aspects related to such studies on relativistic beams selected with the FRS fragment separator. First results are presented for the (19/2(+)) 4.5 mu s isomeric state in Sn-127, which has been produced by means of fission of a relativistic U-238 beam on the one hand, and by the fragmentation of a relativistic Xe-136 beam on the other hand. Spin-alignment has been observed in both reactions. It was the first time that spin-alignment has been established in a relativistic fission reaction. (Less)

Selective studies of the excited rotational bands in the superdeformed nucleus {sup 151}Tb

Abstract: The experimental study of the unresolved rotational bands, forming ridge structures in {gamma}-{gamma} spectra, has been performed on the superdeformed nucleus {sup 151}Tb. {gamma} transitions from the reaction {sup 27}Al, at 155 MeV, on {sup 130}Te were measured with EUROBALL IV in high fold coincidence. The analysis of the intensities and count fluctuations of the ridge structures shows the existence of {approx_equal}30 discrete rotational bands of superdeformed nature, half of which is in direct coincidence with the superdeformed yrast band. A comparison with band mixing model predictions and with a previous work on the superdeformed nucleus {sup 143}Eu is presented.

High-spin spectroscopy of Xe-124,Xe-125,Xe-126

Abstract: Author(s): Al-Khatib, A; Huebel, H; Bringel, P; Engelhardt, C; Neusser-Neffgen, A; Hagemann, GB; Hansen, CR; Herskind, B; Sletten, G; Bracco, A; Camera, F; Benzoni, G; Fallon, P; Clark, RM; Carpenter, MP; Janssens, RVF; Khoo, TL; Chowdhury, P; Amro, H

Nuclear structure far off stability — rising campaigns

Abstract: Nuclear structure studies at CST attracted recently increased interest for the results of present activities as well as for the future project FAIR. A broad range of physics phenomena can be addressed by high-resolution in-beam gamma-ray spectroscopy experiments with radioactive beams offered within the Rare Isotopes Spectroscopic INvestigation at GSI (RISING) project. It combined the EUROBALL Ge-Cluster detectors, the MINIBALL Ce detectors, the HECTOR-BaF detectors, and the fragment separator FRS. The secondary beams produced at relativistic energies were used for Coulomb excitation or secondary fragmentation experiments to study projectile like nuclei by measuring de-excitation photons. The first results of the "fast beam campaign" is discussed in comparison to various shell model calculations. The discussion focuses on the N = 32, 34 sub-shell closure based on neutron rich Cr isotopes. Alternatively, the relativistic radioactive beams, both in their ground and isomeric states, were implanted and their decay could be investigated. The "stopped beam campaign" has started in October 2005 with a series of g-factor measurements. It continued from February 2006 with the next, configuration and the main goal of identification of new isomers and angular momentum population in fragmentation reactions. (Less)

Light Charged Particles as Gateway to Hyperdeformation

Abstract: The Euroball-IV \gamma -detector array, equipped with the ancillary charged particle detector array DIAMANT was used to study the residues of the fusion reaction $^{64}$Ni + $^{64}$Ni \Rightarrow $^{128}$Ba at E_{beam} = 255 and 261 MeV, in an attempt to reach the highest angular momentum and verify the existence of predicted hyperdeformed rotational bands. No discrete hyperdeformed bands were identified, but nevertheless a breakthrough was obtained through a systematic search for rotational ridge structures with very large moments of inertia J(2) \ge 100 {\mathchar h}^2 MeV^(-1), in agreement with theoretical predictions for hyperdeformed shapes. Evidence for hyperdeformation was obtained by charged particle + \gamma -ray gating, selecting triple correlated ridge structures in the continuum of each of the nuclei, $^ {118}$Te, $^{124}$Xe and $^{124,125}$Cs. In 7 additional nuclei, rotational ridges were also identified with J(2) = 71--77 {\mathchar h}^2 MeV^(-1), which most probably correspond to superdeformed shape. The angular distributions of the emitted charged particles show an excess in forward direction over expectations from pure compound evaporation, which may indicate that in-complete fusion plays an important role in the population of very elongated shapes.