B. Laurent

Energy measurement of prompt fission neutrons in 239Pu(n,f) for incident neutron energies from 1 to 200 MeV

An experimental campaign was started in 2002 in the framework of a collaboration belween CEA-DAM and the Los Alamos National Laboratory to measure the prompt fission neutron spectra (PFNS) for incident neutron energies from 1 to 200 MeV with consistent error uncertainties over the whole energy range. The prompt neutron spectra in {sup 235,238}U(n,f) and {sup 237}Np(n,f) have been already studied successfully. A first attempt to characterize the prompt neutrons emitted during the fission of the {sup 239}Pu was done in 2007. This contribution will focus on the results obtained during the final experiment to measure the PFNS in {sup 239}Pu(n,f) performed in 2008. Prompt fission neutron spectra in the neutron-induced fission of {sup 239}Pu have been measured for incident neutron energies from 1 to 200 MeV at the Los Alamos Neutron Science Center. Mean energies obtained from the spectra are discussed and compared to theoretical model calculation.

How sharp is the transition into the N=20 island of inversion for the Mg isotopes ?

The N=20 island of inversion is an excellent playground for testing shell model calculations. The Mg chain is a region of shell evolution still far from being well understood. In this paper we present preliminary results of a single-neutron knockout experiment from 31Mg performed at GANIL to study the structure of 31Mg and of the core 30Mg. The level scheme and longitudinal momentum distributions were mesured and spectroscopic factors were deduced. Negative parity states arise at low energy and the spectroscopic factor for the isomeric in 30Mg was determined to be smaller than foreseen in the standard picture. The preliminary experimental results are compared to state-of the art shell model calculations revealing opposed interpretations.

Fission dynamics at high excitation energies investigated in complete kinematics measurements

Light-charged particles emitted in proton-induced fission reactions on 208 Pb have been measured at different kinetic energies: 370A, 500A, and 650A MeV. The experiment was performed by the SOFIA collaboration at the GSI facilities in Darmstadt (Germany). The inverse kinematics technique was combined with a setup especially designed to measure light-charged particles in coincidence with fission fragments. The data were compared with different model calculations to assess the ground-to-saddle dynamics. The results confirm that transient and dissipative effects are required for an accurate description of the fission observables.

New experimental approaches to investigate the fission dynamics

The first ever achieved full identification of both fission fragments, in atomic and mass number, made it possible to define new observables sensitive to the fission dynamics along the fission path up to the scission point. Moreover, proton-induced fission of 208Pb at high energies offers optimal conditions for the investigation of dissipative, and transient effects, because of the high-excitation energy of the fissioning nuclei, its low angular momentum, and limited shape distortion by the reaction. In this work we show that the charge distribution of the final fission fragments can constrain the ground-to-saddle dynamics while the mass distribution is sensitive to the dynamics until the scission point.

Multiplicity of light-charged particles investigated with proton-induced fission of

Spallation reactions produce large quantities of lightcharged particles (hydrogenand helium isotopes) which are a concern in spallation target design. For instance, the pro duction of tritium is a concern for radioprotection, especially in the case of liquid targets from which it can escape easily. Therefore, a reliable prediction of the light-char ged particle yields by high-energy transport codes [1] becomes important for the design of spallation targets. Furthermor e, light-charged particle emission has been well established as a sensitive tool in probing the dynamics of heavy-ioninduced nuclear reactions and it could help to investigate some fundamental questions about fission such as the dissipative and transient effects in the last stages of the fissi on process [2]. In the present work, we report on the first results of a new generation of accurate measurements on multiplicities of light-charged particles in spallation reaction of Pb at different relativistic energies: 370 A, 500A and 650A MeV. The experiment takes advantage of the inverse kinematics technique, in which fission fragments and light particles are emitted in forward direction. Time coincidence with the fission fragments discriminates the particles produced during fission from other reaction channels. The experiment [3] was performed at the ALADINLAND cave at GSI. Measurements were performed with a hydrogen target isolated by two windows consisting of aluminized mylar foils of 35μm. Fission events were selected in a double multi-sampling ionization chamber (Twin MUSIC) [4]. Between the target and the Twin MUSIC a pipe filled with helium gas was placed to transmit the fission fragments. A Time-of-Flight Wall detector (ToF Wall), based on plastic-scintillator paddles and two photomultipliers (PM) per paddle, was placed in front of the Twin MUSIC to detect the light-charged particles. It consists of two orthogonally-oriented planes with six paddles each (60×6×1 cm), which leave a square hole ( 12.5 × 12.5 cm) in the middle to transmit the fission fragments. Because the reaction kinematics, while most of the fission fragments go through the Twin MUSIC, a large fraction of the light-charged particles escape the target to reach th e ToF Wall. The time and charge of the scintillator signals are registered by using a TDC and a QDC, respectively. The particle multiplicity is provided by the number of fired paddles in each plane and the particle identification is obtaine d by using the time and the charge signals as shown in the inset of Fig. 1. Detection efficiency was determined from GEANT4 simulations [5] using INCL4.6-ABLA07 [1, 6] for the particle kinematics and the ToF Wall dimensions. Figure 1 shows the multiplicity for hydrogen-like particles measured at different energies. The multiplicity is strongly correlated with the impact parameter, so that central collisions lead to high multiplicities whereas low mul tiplicities are related to peripherical reactions that are th major contribution [7]. The average value of the distribution increases with the bombarding energy, as expected because the reactions are more violent.

^{208}

A program to measure fission neutron spectra from neutron‐induced fission of actinides is underway at the Los Alamos Neutron Science Center (LANSCE) in a collaboration among the CEA laboratory at Bruyeres‐le‐Châtel, Lawrence Livermore National Laboratory and Los Alamos National Laboratory. The spallation source of fast neutrons at LANSCE is used to provide incident neutron energies from less than 1 MeV to 100 MeV or higher. The fission events take place in a gas‐ionization fission chamber, and the time of flight from the neutron source to that chamber gives the energy of the incident neutron. Outgoing neutrons are detected by an array of organic liquid scintillator neutron detectors, and their energies are deduced from the time of flight from the fission chamber to the neutron detector. Measurements have been made of the fission neutrons from fission of 235U, 238U, 237Np and 239Pu. The range of outgoing energies measured so far is from 0.7 MeV to approximately 8 MeV. These partial spectra and average fiss...

Pb at different kinetic energies

Combining STM, LEED, and density functional theory, we determine the atomic surface structure of rutile TiO{sub 2} (110)-(1x2): nonstoichiometric Ti{sub 2}O{sub 3} stripes along the [001] direction. LEED patterns are sharp and free of streaks, while STM images show monatomic steps, wide terraces, and no cross-links. At room temperature, atoms in the Ti{sub 2}O{sub 3} group have large amplitudes of vibration. The long quasi-1D chains display metallic character, show no interaction between them, and cannot couple to bulk or surface states in the gap region, forming good atomic wires.The 10.15 MeV resonance in {sup 10}Be has been probed via resonant {sup 6}He+{sup 4}He elastic scattering. It is demonstrated that it is the J{sup {pi}}=4{sup +} member of a rotational band built on the 6.18 MeV 0{sup +} state. A {gamma}{sub {alpha}} of 0.10-0.13 MeV and {gamma}{sub {alpha}}/{gamma}=0.35-0.46 were deduced. The corresponding reduced {alpha} width, {gamma}{sub {alpha}}{sup 2}, indicates one of the largest {alpha}-cluster spectroscopic factors known. The deformation of the band, including the 7.54 MeV, 2{sup +} member, is large (({Dirac_h}/2{pi}){sup 2}/2I=200 keV). Such a deformation and the significant degree of clusterization signals a well-developed {alpha} ratio 2n ratio {alpha} molecular structure.The mixed occurrence of s-wave and p-wave contributions in a first forbidden unique Gamow-Teller {beta} decay has been investigated for the first time by measuring the beta environmental fine structure (BEFS) in a {sup 187}Re crystalline compound. The experiment has been carried out with an array of eight AgReO{sub 4} thermal detectors operating at a temperature of {approx}100 mK. A fit of the observed BEFS spectrum indicates the p-wave electron emission as the dominant channel. The complete understanding of the BEFS distortion of the {sup 187}Re {beta} decay spectrum is crucial for future experiments aiming at the precise calorimetric measurement of the antineutrino mass.