Search

This work focuses on the experimental search and characterisation of the super-heavy 7H, using a one-proton transfer reaction 12C(8He,7H)13N. A 8He primary beam at 15.4 A MeV was delivered by the GANIL-SPIRAL facility and impinged on a gaseous carbon target, the active target MAYA, where 7H was produced and detected. 7H was observed as a resonance 0.57+0.42-0.21MeV above the 3H+4n subsystem mass, with an associated width of 0.09+0.94-0.06MeV. The differential cross-section of 7H production for the 12C(8He,7H)13N reaction yields 40.1+58.0-30.6μb/sr. This experiment demonstrates the advantage of using the active target technique particularly well suited for the detection of low energy residues.

27Ne has been investigated through the one neutron transfer reaction 26Ne(d,p)27Ne in inverse kinematics at 9.7 MeV/nucleon. The results support the existence of a low lying negative parity state in 27Ne which is a signature of a reduced sd-fp shell gap in the N = 16 neutron rich region, at variance with stable nuclei.

Abstract.: This work focuses on the experimental search and characterisation of the super-heavy 7H, using a one-proton transfer reaction 12C(8He,7H)13N. A 8He primary beam at 15.4 A MeV was delivered by the GANIL-SPIRAL facility and impinged on a gaseous carbon target, the active target MAYA, where 7H was produced and detected. 7H was observed as a resonance 0.57+0.42-0.21 MeV above the 3H+4n subsystem mass, with an associated width of 0.09+0.94-0.06 MeV. The differential cross-section of 7H production for the 12C(8He,7H)13N reaction yields 40.1+58.0-30.6 μb/sr. This experiment demonstrates the advantage of using the active target technique particularly well suited for the detection of low energy residues.

This work focuses on the experimental search and characterisation of the super-heavy 7H, using a one-proton transfer reaction 12C(8He,7H)13N. A 8He primary beam at 15.4 A?MeV was delivered by the GANIL-SPIRAL facility and impinged on a gaseous carbon target, the active target MAYA, where 7H was produced and detected. 7H was observed as a resonance 0.57+0.42-0.21?MeV above the 3H+4n subsystem mass, with an associated width of 0.09+0.94-0.06?MeV. The differential cross-section of 7H production for the 12C(8He,7H)13N reaction yields 40.1+58.0-30.6??b/sr. This experiment demonstrates the advantage of using the active target technique particularly well suited for the detection of low energy residues.

A new direct technique using the CIME cyclotron as a high-resolution mass spectrometer is being developed in order to measure the masses of exotic nuclei. Tests have been performed to check the feasibility of the method with a mixed beam of stable ions extracted from the SPIRAL ion source and injected into the CIME cyclotron. Preliminary results obtained with this new technique are presented and discussed.

In order to study direct reactions of exotic beams on light particles target, we have developed a new detector: MAYA. This detector is an active target, using the gas as target and as a detection material. With such a device, thick target can be used without resolution loss. Furthermore, excitation functions can be studied from the incident beam energy down to the detection threshold with the same setting. This detector uses a multiplexed read-out electronic for the pads of the segmented detection plan. Several experiments have already been performed, providing results about detector characteristics.

The newly commissioned TIARA array has been coupled for the first time to the VAMOS spectrometer and the EXOGAM germanium array to study nucleon transfer reactions in inverse kinematics and using beams of low intensity, which are typical of radioactive beam experiments. This set-up offers a high geometrical efficiency for the charged particle detection, a high efficiency for γ-ray detection and a final energy resolution only limited by the Doppler broadening. This report demonstrates the potential of such an apparatus to study single-nucleon transfer reactions with radioactive nuclear beams. A beam of 14N at 10.6 MeV/nucleon with an intensity of 105 s−1 was directed onto a CD2 target of 1 mg cm−2. Proton angular distributions have been measured for the population of 15N in the ground state and in excited states at 7.15 and 7.56 MeV. Transferred l-values deduced from comparison to DWBA calculations are in very good agreement with the well-known shell structure of 15N. We have therefore demonstrated the effectiveness of this set-up in studying nucleon transfer reactions in inverse kinematics and with low-intensity beams.

The first physics results measured using the TIARA array are reported. The reaction 24Ne(d,p)25Ne has been studied in inverse kinematics with a radioactive beam of 24Ne provided by SPIRAL at GANIL. TIARA is very compact with a high geometrical coverage for charged particles, and is designed specifically for the study of transfer reactions in inverse kinematics, with radioactive beams. From the (d,p) differential cross sections, the ground state of 25Ne is assigned to have Jπ = 1/2+ and the lowest states with Jπ = 5/2+ and 3/2+ are tentatively identified at excitation energies of 1.70 and 2.05 (±0.05) MeV, respectively. Coincident gamma-ray data proved essential to resolve these two excited states. Strongly populated levels observed at 3.30 and 4.05 (±0.05) MeV are candidates for negative parity states. Further analysis of the differential cross sections is in progress, with the aim of improving the identification of the higher lying states and determining absolute spectroscopic factors.

Following mass measurements in the region N= 20 and N= 28, we have studied inelastic nuclear scattering for the nuclei 34Si, 33Al and 32Mg. No evidence for a low-lying shape isomeric 0+state was found in 34Si, and an upper limit for the population cross-section could be established, rendering its existence very unlikely. A new transition was found in 33Al, that is a good candidate for a 2p-2h state and therefore a determination of the 2p-2h gap at N= 20. Inelastic nuclear scattering strongly excites 3-states, as seen in 34Si. A strong transition was found in 32Mg that should correspond to the first 3-in this nucleus, lying very low as compared to theory and systematics in this region.

Mass measurements of the N=Znuclei 80Zr, 76Sr, 68Se were performed for the first time and a new measurement was obtained for 80Y, using the second cyclotron CSS2 of GANIL as a high-resolution spectrometer. Ions around N=Zwere produced by fusion-evaporation in the inverse 58Ni (4.32MeVA) + 24Mg and 12C reactions. New masses were measured by a time-of-flight method, with a precision of 2⋅10−6, by using well-known masses as references. Study of the double binding energy difference δVnpis then performed leading to a strong N=ZWigner effect around N=Z=40. Knowledge of new masses in this region also plays a crucial role in the modelling of the astrophysical rp process.

The measurement of masses (or binding energy) of nuclei far from stability is of fundamental interest for our understanding of nuclear structure. Their knowledge over a broad range of the nuclear chart is an excellent and severe test of nuclear models. This is why considerable experimental and theoretical efforts have been and are invested in this domain. Here we want to give a description of direct mass measurements combining high-resolution time-of-flight determinations and accurate momentum measurements in a spectrometer. This very fast and direct method was developed at GANIL with the high-resolution spectrometer SPEG. The mass measurements realised and scheduled provide important first indications of new regions of deformation or shell closures very far from stability.

A mass measurement experiment by a time of flight method with the SPEG spectrometer at GANIL has been performed to investigate the N=20 and N=28 shell closures far from stability. The masses of 31 neutron-rich nuclei in the range A=29–47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S and P isotopes are seen to exhibit a change in shell structure around N=28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. The evidence of an isomeric state in the 43S and its interpretation by a shell model calculation confirm the analysis of the masses and constitutes the first evidence of the predicted shape coexistence around N=28.

The mass-measurement programme at GANIL aims to measure the masses of heavy nuclei close to the N=Zline which is the ideal region to study neutron-proton pairing. An original direct time-of-flight mass-measurement method was developed at GANIL which uses the CSS2 cyclotron as a high-resolution spectrometer. The masses of ions of A=68,76,80 and 100 have been measured with a precision of a few 10−6. Mass measurements will be performed with the new CIME cyclotron of SPIRAL using a similar method based on the measurement of the phase of the accelerated ions for different radio-frequencies. A recently approved experiment will help develop this new technique and aims to measure the mass of 31Ar radioactive nuclei with a precision of 10−6.

The quadrupole moments of the first two superdeformed (SD) bands in the nucleus ¹⁵¹Tb have been measured with the Doppler Shift Attenuation Method (DSAM) using the EUROGAM -ray spectrometer. The first excited band (B2) is identical to the yrast SD band of ¹⁵²Dy in terms of dynamical moments of inertia and -ray energies. The measured relative quadrupole moments of the yrast band (B1) with respect to the band B2 are nicely reproduced by self-consitent Hartree-Fock calculations. The variation of the Q₀ value at the bottom of the yrast band suggests a large admixture between wave functions in the normal and the SD wells. This could explain the rather high deexcitation spin for the yrast band.

Abstract.: 27Ne has been investigated through the one neutron transfer reaction 26Ne(d,p)27Ne in inverse kinematics at 9.7 MeV/nucleon. The results support the existence of a low lying negative parity state in 27Ne which is a signature of a reduced sd-fp shell gap in the N = 16 neutron rich region, at variance with stable nuclei.

27Ne has been investigated through the one neutron transfer reaction 26Ne(d,p)27Ne in inverse kinematics at 9.7?MeV/nucleon. The results support the existence of a low lying negative parity state in 27Ne which is a signature of a reduced sd-fp shell gap in the N = 16 neutron rich region, at variance with stable nuclei.

The inclusive (d, xd’) reaction on 9Be, 12C, 27Al, 58Ni, 93Nb, 181Ta, and 208Pb were investigated at an incident energy of 100 MeV for scattering angles between 10° and 120°. The measured spectra are rather smooth and featureless and reveal very similar qualitative features of the continuum compared to existing (p, xp’) and (α, xα’) data. The continuum angular distributions at various emission energies are described reasonably well with parameterized systematics which indicates a multistep mechanism as the underlying process of pre-equilibrium emission.

The58Ni(30Si,apn)82Y reaction at a beam energy of 134 MeV was used to populate the odd-odd nucleus82Y and coincident? rays depopulating high spin states in the evaporation residues were detected with the EUROGAM Phase 2 array. A superdeformed structure has been observed with an intensity of 1.5±0.5%. The cascade of 9 coincident? rays has consecutive energy spacing of ~160 keV. This corresponds to a dynamic moment of inertia which is almost constant at approx mately 25h2 MeV-1. The band is interpreted as a signature=0, negative parity SD band with ap51V52 intrinsic intruder configuration.

The EUROGAMarray has been used to investigate the decay out of the yrast superdeformed (SD) band in 194Pb. Eigth single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy (4877 ± 1.5 keV) and the spin (6+) of the lowest observed SD state in 194Pb are established.

Following mass-measurements in the region N = 20 and N = 28, we have studied inelastic nuclear scattering for the nuclei 34Si, 33Al and 32Mg. No evidence for a low lying shape isomeric 0+ state was found in 34Si, and an upper limit for the population cross section could be established, rendering it's existence very unlikely. A new transition was found in 33Al, that is a good candidate for a 2p-2h state and therefore a determination of the 2p-2h gap at N = 20. Inelastic nuclear scattering strongly excites 3- states, as seen in 34Si. A strong transition was found in 32Mg that should correspond to the first 3- in this nucleus, lying very low as compared to theory and systematics in this region.