SPIRAL Facility Research Articles

Interaction of He8 with Pb208 at near-barrier energies: He4 and He6 production

Angular distributions for the inclusive He4 and He6 production cross sections in the He8+Pb208 system at incident energies of 16 and 22 MeV measured at the SPIRAL facility of the GANIL laboratory are presented. Using a combination of kinematical arguments and distorted wave Born approximation (DWBA) calculations, neutron transfer reactions were inferred to be the dominant contributors to both inclusive cross sections. Model-dependent values for the ratios of two- to one-neutron stripping, σ2n/σ1n, were derived and compared with previous results for He8 and He6 projectiles incident on other heavy targets. Three- and four-neutron stripping were inferred to be the main processes leading to He4 production, although the exact mechanism remains to be elucidated.

A new FEBIAD-type ion source for the upgrade of SPIRAL1 at GANIL

The SPIRAL facility at GANIL is being upgraded with a new FEBIAD ion source in order to extend its production capabilities to condensable elements. The coupling of such an ion source with the present GANIL target was recently tested online at SPIRAL at nominal power (1200W). The results are promising as the calculated yields are in the range of those extrapolated from previous measurements at lower power and sometimes higher. This experiment and its results are presented.

New excited states in the halo nucleus 6He

The low-lying spectroscopy of 6He was investigated via the 2-neutron transfer reaction p(He8,t) with the 8He beam delivered by the SPIRAL facility at 15.4 AMeV. The light charged particles produced by the direct reactions were measured using the MUST2 Si-strip telescope array. Above the known 2+ state, two new resonances were observed: at E⁎=2.6±0.3 MeV (width Γ=1.6±0.4 MeV) and at 5.3±0.3 MeV with Γ=2±1 MeV. Through the analysis of the angular distributions, they correspond to a 2+ state and to an L=1 state, respectively. These new states, challenging the nuclear theories, could be used as benchmarks for checking the microscopic inputs of the newly improved structure models, and should trigger development of models including the treatments of both core excitation and continuum coupling effects.

Spectroscopy of 18Na: Bridging the two-proton radioactivity of 19Mg

The unbound nucleus 18Na, the intermediate nucleus in the two-proton radioactivity of 19Mg, was studied by the measurement of the resonant elastic scattering reaction H(17Ne,p)17Ne performed at 4 A.MeV using a radioactive beam from the SPIRAL Facility. Spectroscopic properties of the low-lying states were obtained in an R-matrix analysis of the excitation function.

Nuclear structure and reaction studies at SPIRAL

The SPIRAL facility at GANIL, operational since 2001, is described briefly. The diverse physics program using the re-accelerated (1.2–25 MeV/u) beams ranging from He to Kr and the instrumentation specially developed for their exploitation are presented. Results of these studies, using both direct and compound processes, addressing various questions related to the existence of exotic states of nuclear matter, evolution of new ‘magic numbers’, tunnelling of exotic nuclei, neutron correlations, exotic pathways in astrophysical sites and characterization of the continuum, are discussed. The future prospects for the facility and the path towards SPIRAL2, a next generation ISOL facility, are also briefly presented.

Fusion of the most neutron-rich nucleus8He: Recent results from GANIL

Re-accelerated radioactive ions beams of 8He from the SPIRAL facility at GANIL have been used to perform the first experimental investigations at energies around the Coulomb barrier of fusion and direct reactions with the most neutron rich 8He. Two experiments (in the 8He+65Cu and 8He+197Au systems) have been performed, using the complementarity of in-beam and off-beam techniques, to measure integral and differential cross sections for elastic scattering, fusion and neutron(s) transfer. These results, compared with Coupled Reaction Channels calculations as well as phenomenological approaches allowed to probe and understand the different facets of the interplay between the intrinsic structure of 8He and reaction mechanisms.

RES with RIB

The Resonant Elastic Scattering (RES) method has been used at GANIL with Radioactive Ions Beams (RIB) in order to study important unbound states. Three examples of studies are given: the first one is related to the important astrophysical reaction 18 F(p,α)15 O, the second one to the two-proton radioactivity of 19 Mg, and the third one to the prediction of the existence of a narrow state in the unbound nucleus 15 F. The three experiments have benefited from high quality radioactive beams produced by the Spiral Facility.

NEUTRON CORRELATIONS IN 6He VIEWED THROUGH NUCLEAR BREAK-UP

Nuclear break-up as a tool to study neutron correlations is investigated both theoretically and experimentally. First a time dependent theory going beyond mean field has been developed and shows that correlations between nucleons can be probed through nuclear break-up. Secondly, nuclear break-up of 6 He on 208 Pb target was studied at 20 A.MeV using a secondary beam of 6 He produced by the SPIRAL facility at GANIL. From the correlation function, a strong contribution of the di-neutron configuration of 6 He is found.

Study of collisions of the radioactive 24Ne beam at 7.9 MeV/u on 208Pb

Cross-sections of the main reaction channels for the collision 24Ne + 208Pb at 7.9MeV/u were studied using the radioactive ion beam delivered by the SPIRAL facility and the VAMOS-EXOGAM experimental set-up. Angular distributions for the elastic and inelastic channels were extracted, together with distributions for the +1n and -1p channels. A comprehensive description of the present data is made within the GRAZING model approach.

STUDY OF THE 20O(d,t) REACTION WITH THE TIARA-MUST2-VAMOS-EXOGAM SETUP

The reaction 20 O ( d , t ) has been studied in inverse kinematics using a secondary radioactive beam produced with the SPIRAL facility at GANIL. Fragments, light charged particles and gamma rays were measured with the TIARA, MUST2, VAMOS and EXOGAM detectors and preliminary results are reported. The level scheme of 19 O is built and the spin and parity of one state is tentatively assigned using the one-neutron transfer angular distribution.

Shape ofAr44: Onset of deformation in neutron-rich nuclei nearCa48

The development of deformation and shape coexistence in the vicinity of doubly magic $^{48}\mathrm{Ca}$, related to the weakening of the $N=28$ shell closure, was addressed in a low-energy Coulomb excitation experiment using a radioactive $^{44}\mathrm{Ar}$ beam from the SPIRAL facility at GANIL. The ${2}_{1}^{+}$ and ${2}_{2}^{+}$ states in $^{44}\mathrm{Ar}$ were excited on $^{208}\mathrm{Pb}$ and $^{109}\mathrm{Ag}$ targets at two different beam energies. $B(E2)$ values between all observed states and the spectroscopic quadrupole moment of the ${2}_{1}^{+}$ state were extracted from the differential Coulomb excitation cross sections, indicating a prolate shape of the $^{44}\mathrm{Ar}$ nucleus and giving evidence of an onset of deformation already two protons and two neutrons away from doubly magic $^{48}\mathrm{Ca}$. New Hartree-Fock-Bogoliubov based configuration mixing calculations have been performed with the Gogny D1S interaction for $^{44}\mathrm{Ar}$ and neighboring nuclei using two different approaches: the angular momentum projected generator coordinate method considering axial quadrupole deformations and a five-dimensional approach including the triaxial degree of freedom. The experimental values and new calculations are furthermore compared to shell-model calculations and to relativistic mean-field calculations. The new results give insight into the weakening of the $N=28$ shell closure and the development of deformation in this neutron-rich region of the nuclear chart.

Neutron correlations in 6He viewed through nuclear break-up

The nuclear break-up of 6He on a 208Pb target was studied at 20A MeV using a secondary beam of 6He produced by the SPIRAL facility at GANIL. $ \alpha$ -particles were detected in coincidence with two neutrons with a large angular coverage and the reaction mechanism was identified. From the distribution of the relative angles between the two neutrons the correlation function was extracted. It shows a strong correlation at small relative angles attributed to the contribution of the di-neutron configuration of 6He .

Status of the LPCTrap facility at GANIL

The LPCTrap is a device whose central element is a transparent Paul trap for the efficient trapping of low energy radioactive ions produced by the SPIRAL facility at GANIL. The installation comprises a linear RFQ cooler and buncher for the beam preparation, and a decay chamber where the Paul trap is located. The system has been tested with ions having mass-to-charge ratios ranging from 4 to 40. The first run with radioactive 6He+ ions has demonstrated the overall performance of the system. In this contribution we describe the current status of the device, we present in particular the results of a numerical study carried out to determine the properties of the ion cloud inside the trap and provide updated values for the overall efficiency of the system.

Shape coexistence in 74Kr and 76Kr

The coexistence of prolate and oblate shapes in the light krypton isotopes was investigated by Coulomb excitation of radioactive 74Kr and 76Kr beams at sub-barrier energies from the SPIRAL facility at GANIL. States up to the 8+ in the ground-state band and several non-yrast states were populated in both cases via multi-step Coulomb excitation. A large set of both transitional and diagonal matrix elements was extracted from the measured differential Coulomb excitation cross sections. A complementary lifetime measurement using the recoil-distance Doppler shift method gave important constraints for the determination of the matrix elements. The signs of the quadrupole moments of the yrast and excited bands represent direct evidence for the coexistence of prolate and oblate shapes in both isotopes under study. The results are compared to recent configuration mixing calculations based on the generator coordinate method and Skyrme Hartree-Fock-Bogolyiubov mean-field states.

Shape coexistence in neutron-deficient krypton isotopes

Shape coexistence in the light krypton isotopes was studied in two low-energy Coulomb excitation experiments using radioactive {sup 74}Kr and {sup 76}Kr beams from the SPIRAL facility at GANIL. The ground-state bands in both isotopes were populated up to the 8{sup +} state via multi-step Coulomb excitation, and several non-yrast states were observed. Large sets of matrix elements were extracted for both nuclei from the observed {gamma}-ray yields. Diagonal matrix elements were determined by utilizing the reorientation effect. In both isotopes the spectroscopic quadrupole moments for the ground-state bands and the bands based on excited 0{sub 2}{sup +} states are found to have opposite signs. The experimental data are interpreted within a phenomenological two-band mixing model and model-independent quadrupole invariants are deduced for the relevant 0{sup +} states using the complete sets of matrix elements and the formalism of quadrupole sum rules. Configuration mixing calculations based on triaxial Hartree-Fock-Bogolyubov calculations with the Gogny D1S effective interaction have been performed and are compared both with the experimental results and with recent calculations using the Skyrme SLy6 effective interaction and the full generator-coordinate method restricted to axial shapes.

SPIRAL at GANIL: Latest Results and Plans for the Future

The first accelerated exotic beam of the SPIRAL (Production System of Radioactive Ion and Acceleration On-Line) facility at GANIL at Caen has been delivered for experiments in September 2001. After working for almost 5 years, 32 experiments were performed in the facility using exotic isotopes of helium, oxygen, neon, argon and krypton. The intensities of the radioactive beams increased since the first beam was delivered. Nominal intensity values are achieved for most of noble gas beams. Developments of new beams as well as the increasing of present intensities for a number of isotopes are being undertaken. In particular, in this contribution it is presented the first results obtained for the production of light alkali beams. Other developments are also envisaged in the close future.

Probing the maximally deformed light rare-earth region around the drip-line nucleus 130Sm

The neutron deficient rare-earth nuclei of the A∼130 region are of particular interest since highly deformed prolate ground states are expected. Indeed these nuclei are predicted to show maximal ground-state deformations of β2 ∼ 0.40 (axis ratio of 3:2), comparable to the deformation deduced for superdeformed cerium isotopes at high spin. A fusion–evaporation experiment was performed with radioactive ion beams at GANIL in October 2004 which had the goal to reach very proton-rich exotic nuclei located near the proton drip-line. A radioactive 76Kr beam, delivered by the SPIRAL facility, was used to bombard a thin 58Ni target. Emitted γ-rays were detected by the EXOGAM γ-ray spectrometer which was, for the first time, coupled with both the DIAMANT charged-particle array and the VAMOS spectrometer.

A cryogenic target for direct reaction studies with exotic beams

A cryogenic system has been designed to make a thin target of solid hydrogen isotopes usable under vacuum, with a particle beam. In order to insure the homogeneity of the target, an equivalent pressure is maintained with helium on both sides of the target windows during the formation of the solid hydrogen. The system developed is described and the results obtained with 1 mm thick solid hydrogen and deuterium targets, in two different experiments using the radioactive beams from the SPIRAL facility, are presented. The flatness achieved was better than a few percent in both experiments.

Experimental evidence for subshell closure inHe8and indication of a resonant state inHe7below 1 MeV

The spectroscopy of the unstable $^{8}\mathrm{He}$ and unbound $^{7}\mathrm{He}$ nuclei is investigated via the $p(^{8}\mathrm{He}$, d) transfer reaction with a 15.7A MeV $^{8}\mathrm{He}$ beam from the SPIRAL facility. The emitted deuterons were detected by the telescope array MUST. The results are analyzed within the coupled-channels Born approximation framework, and a spectroscopic factor ${C}^{2}S=4.4\ifmmode\pm\else\textpm\fi{}1.3$ for neutron pickup to the ${}^{7}{\mathrm{He}}_{g.s.}$ is deduced. This value is consistent with a full p3/2 subshell for $^{8}\mathrm{He}$. Tentative evidence for the first excited state of $^{7}\mathrm{He}$ is found at E${}^{*}=0.9\ifmmode\pm\else\textpm\fi{}0.5$ MeV (width $\ensuremath{\Gamma}=1.0\ifmmode\pm\else\textpm\fi{}0.9$ MeV). The second one is observed at a position compatible with previous measurements, E${}^{*}=2.9\ifmmode\pm\else\textpm\fi{}0.1$ MeV. Both are in agreement with previous separate measurements. The reproduction of the first excited state below 1 MeV would be a challenge for the most sophisticated nuclear theories.

Study of the N = 28 shell closure in the Ar isotopic chain

The structure of the neutron-rich nucleus 47Ar has been investigated through the d(46Ar,47Ar)p transfer reaction. The radioactive beam of 46Ar at 10 A · MeV was provided by the SPIRAL facility at GANIL. The protons corresponding to a neutron pick-up on bound or unbound states mechanism in 47Ar nuclei were detected at backward angles by the position-sensitive Si array-detector MUST. The transfer-like ejectiles were detected in the SPEG spectrometer. Level scheme, spin assignments and spectroscopic factors have been deduced for 47Ar and compared to shell model predictions. They suggest a slight erosion of the N=28 shell gap from the weakening of the spin-orbit interaction arising from the f and p orbitals. These spectroscopic information are subsequently used to infer (n, γ) reaction rates in the Ar isotopic chain to understand the origin of the 48Ca/46Ca abnormal isotopic ratio observed in certain inclusions of meteorites.