[en] We carried out the Coulomb excitation, on a lead target, of an exotic beam of neutron-rich nucleus Ne²⁶ at 58 MeV/n, in order to study the possible existence of a pygmy dipole resonance above the neutron emission threshold. The experiment was performed at the Riken Research Facility, in Tokyo (Japan) and included a gamma-ray detector, a charged fragment hodoscope and a neutron detector. Using the invariant mass method in the Ne²⁵ + n decay channel, and by comparing the reaction cross section on the lead target and a light target of aluminum, we observe a sizable amount of E1 strength between the one neutron and the two neutron emission thresholds. The corresponding Ne²⁶ angular distribution confirms its nature and we deduce its reduced dipole transition probability value of B(E1) = 0.54 ± 0.18 e²fm². Our method also enables us to extract for the first time the decay pattern of a pygmy resonance. By detecting the decay photons from the excited states below the neutron emission threshold and by analyzing the angular distribution of the inelastically scattered Ne²⁶ we deduce the reduced transition probability of the first 2⁺ state, from the ground state. The value obtained of B(E2) = 87 ± 13 e²fm⁴ being in disagreement with a previous result. (author)

[en] Neutron capture cross sections on unstable nuclei are important for many applications in nuclear structure and astrophysics. Measuring these cross sections directly is a major challenge and often impossible. An indirect approach for measuring these cross sections is the surrogate reaction method, which makes it possible to relate the desired cross section to a cross section of an alternate reaction that proceeds through the same compound nucleus. To benchmark the validity of using the (d,pγ) reaction as a surrogate for (n,γ), the ^171,173Yb(d,pγ) reactions were measured with the goal to reproduce the known (1) neutron capture cross section ratios of these nuclei

[en] Coincident γ rays from a ²⁵²Cf source were measured using an array of six segmented high-purity germanium (HPGe) Clover detectors each enclosed by 16 bismuth-germanate (BGO) detectors. The detectors were arranged in a cubic pattern around a 1 (micro)Ci ²⁵²Cf source to cover a large solid angle for γ-ray measurement with a reasonable reconstruction of the multiplicity. Neutron multiplicity was determined in certain cases by identifying the prompt γ rays from individual fission fragment pairs. Multiplicity distributions from previous experiments and theoretical models were convolved with the response function of the array and compared to the present results. These results suggest a γ-ray multiplicity spectrum broader than previous measurements and models, and provide no evidence of correlation with neutron multiplicity.

[en] We are studying the gamma ray and neutron multiplicity of various fission processes, beginning with the spontaneous fission of ²⁵²Cf, for a variety of basic and applied science purposes. The Livermore-Berkeley Array for Collaborative Experiments (LiBerACE) consists of six high-purity germanium Clover detectors (HPGe) each enclosed by an array of 16 bismuth-germanate (BGO) detectors. These detectors were arranged in a cubic pattern around a 1 (micro)Ci ²⁵²Cf source to attempt to cover as much solid angle of gamma ray emission as possible with a high level of segmentation. The single-gamma detector response function is determined at several energies by tagging in a HPGe detector on the photopeak of one of two gamma rays in two-gamma ray calibration sources and observing the multiplicity of the remainder of the array. Summing these single-gamma responses in groups yields the response function of the array to higher multiplicity events, which are convolved with multiplicity distributions from theoretical models and compared to the measured results to test the models validity

[en] We report the measurement of the isoscalar monopole strength in the unstable nucleus "6"8Ni using inelastic alpha scattering at 50A MeV in inverse kinematics. This experiment has been performed at GANIL with LISE spectrometer using a dedicated detector: the active target MAYA. A part of the isoscalar giant monopole resonance (ISGMR) has been measured at 21.1 ± 1.9 MeV and indications for a soft monopole mode are provided for the first time at 12.9 ± 1.0 MeV. Distorted-wave born approximation (DWBA) with random-phase approximation (RPA) transition densities have been used to study angular distribution and indicate that the L=0 multipolarity dominates the cross-section for the ISGMR, and significantly contributes to the soft mode. (author)

[en] Light neutron-rich nuclei provide an excellent opportunity to study the changes in nuclear shell structure that occur with increasing neutron number and are an important testing ground for shell model theories. Probably one of the most striking examples of shell modification is the occurrence of intruder ground states, which signal an inversion of the normal shell ordering. Intruder ground states are observed around ³²Mg (Z=10-12), ''the island of inversion'', and in ¹¹Be. An analogous situation appears in the Z=2 He isotopes, where the intrusion of sd excitations in p-shell configurations becomes important in the heavy helium isotopes. Finally, for Z=8, recent data on ²⁰O [1] show a reduction in the p-sd shell gap with increasing neutron number. It remains an open question whether the observed diminishing of the p-sd shell gap is restricted to O and F isotopes or extends also to neighboring nuclei. Here, we report preliminary results on ¹⁸N (Z=7), which is sufficiently far from stability to exhibit modified shell structure and yet still within the reach of stable beam facilities utilizing state-of-the art detector systems. ¹⁸N was produced in the ⁹Be(¹¹B,2p)¹⁸N reaction at the 88'' Cyclotron at LBNL and studied using the LIBERACE-STARS detector array--an array of large area segmented silicon detectors (E-ΔE) and six HPGe Clover detectors. This experiment was the first to use a fusion-evaporation reaction to populate ¹⁸N. Previous information on the excited states of 18N came from ¹⁸C beta-decay [2] and charge-exchange reactions [3]. These are highly selective reactions and the fusion-evaporation reaction used here can provide a more comprehensive picture of the excitation spectrum. The beam energy of 50 MeV was chosen to optimize the cross section for the evaporation of 2 protons while simultaneously suppressing the evaporation of additional neutrons in conjunction with the 2p channel. The two proton tag cleanly selects the weak (sub milli-barn) ¹⁸N products. A natural lead catcher foil was mounted between the target and Silicon detectors (3 cm distance) to detect gamma-rays emitted from long lived (t_1/2 < 1 (micro)s) states. The ¹⁸N γ-ray spectrum is shown in figure 1 and a preliminary level scheme in figure 2. New transitions were observed at 628 and 155 keV. The 628 and 114 keV transitions are shown to be in coincidence. The origin of the 298 keV line is currently being investigated. In ref. [2] a lifetime of > 600 ns was assigned to the first excited state at 114 keV. However, from our measurement we estimate a lifetime value of < 30 ns for this state; far shorter than the value of > 600 ns given from the beta decay experiment

[en] An inelastic α-scattering experiment on the unstable N=Z, doubly-magic ${}^{56}$Ni nucleus was performed in inverse kinematics at an incident energy of 50 A.MeV at GANIL. High multiplicity for α-particle emission was observed within the limited phase-space of the experimental setup. This observation cannot be explained by means of the statistical-decay model. The ideal classical gas model at kT = 0.4 MeV reproduces fairly well the experimental momentum distribution and the observed multiplicity of α particles corresponds to an excitation energy around 96 MeV. The method of distributed mα-decay ensembles is in agreement with the experimental results if we assume that the α-gas state in ${}^{56}$Ni exists at around 113${}_{\text{â<!-- ?? -->}\text{ˆ<!-- ?? -->}\text{’<!-- ??? -->}17}^{+15}$ MeV. These results suggest that there may exist an exotic state consisting of many α particles at the excitation energy of 113${}_{\text{â<!-- ?? -->}\text{ˆ<!-- ?? -->}\text{’<!-- ??? -->}17}^{+15}$ MeV.

[en] The surrogate reaction ²³⁸U(³He,tf) is used to determine the ²³⁷Np(n,f) cross section indirectly over an equivalent neutron energy range from 10 to 20 MeV. A self-supporting ∼761 (micro)g/cm² metallic ²³⁸U foil was bombarded with a 42 MeV ³He²⁺ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS), consists of two 140 (micro)m and one 1000 (micro)m Micron S2 type silicon detectors. The ²³⁷Np(n,f) cross sections, determined indirectly, were compared with the ²³⁷Np(n,f) cross section data from direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets. Use of the (³He,tf) reaction as a surrogate to extract (n,f) cross section in the 10 to 20 MeV equivalent neutron energy is found to be suitable.

No abstract available

[en] The ^44,46Ar(d,p) reactions were performed at GANIL using the ^44,46Ar beams delivered by the SPIRAL facility. The protons were detected at backward angles using the highly segmented Si detector MUST. The transfer-like products ^45,47Ar were selected by the SPEG spectrometer and identified through their position, energy loss and time of flight at the focal plane of the spectrometer. From proton energy and angular spectra, the energies, spin values and spectroscopic factors of bound and unbound states in ^45,47Ar have been determined for the first time. This spectroscopic information will be used to examine the evolution of the N = 28 shell closure below the doubly magic nucleus ⁴⁸Ca. Shell model calculations nicely agree with the observed p-state levels, which give confidence to the description of the shell erosion phenomenon observed in the S isotopes. This spectroscopic information will be used to determine the DC rates around the N = 28 shell closure, which are of great importance in understanding the large ⁴⁸Ca/⁴⁶Ca abundance ratio in certain inclusions of meteorites. (author)