B(E1) Strengths from Coulomb Excitation of 11Be

The $B$(E1;$1/2^+\to1/2^-$) strength for $^{11}$Be has been extracted from intermediate energy Coulomb excitation measurements, over a range of beam energies using a new reaction model, the extended continuum discretized coupled channels (XCDCC) method. In addition, a measurement of the excitation cross section for $^{11}$Be+$^{208}$Pb at 38.6 MeV/nucleon is reported. The $B$(E1) strength of 0.105(12) e$^2$fm$^2$ derived from this measurement is consistent with those made previously at 60 and 64 MeV/nucleon, i n contrast to an anomalously low result obtained at 43 MeV/nucleon. By coupling a multi-configuration description of the projectile structure with realistic reaction theory, the XCDCC model provides for the first time a fully quantum mechanical description of Coulomb excitation. The XCDCC calculations reveal that the excitation process involves significant contributions from nuclear, continuum, and higher-order effects. An analysis of the present and two earlier intermediate energy measurements yields a combined B(E1) strength of 0.105(7) e$^2$fm$^2$. This value is in good agreement with the value deduced independently from the lifetime of the $1/2^-$ state in $^{11}$Be, and has a comparable p recision.Comment: 5 pages, 2 figures, accepted for publication in Phys. Lett.

Helium Clustering in Neutron-Rich Be Isotopes

Measurements of the helium-cluster breakup and neutron removal cross sections for neutron-rich Be isotopes A=10-12,14 are presented. These have been studied in the 30 to 42 MeV/u energy range where reaction measurements are proposed to be sensitive to the cluster content of the ground-state wave-function. These measurements provide a comprehensive survey of the decay processes of the Be isotopes by which the valence neutrons are removed revealing the underlying alpha-alpha core-cluster structure. The measurements indicate that clustering in the Be isotopes remains important up to the drip-line nucleus 14^Be and that the dominant helium-cluster structure in the neutron-rich Be isotopes corresponds to alpha-Xn-alpha.Comment: 5 pages, 2 tables and 3 figure

The GUINEVERE Project for Accelerator Driven System Physics

paper 9414International audienceThe GUINEVERE project is part of the EUROTRANS Integrated Project of the 6th EURATOM Framework Programme. It is mainly devoted to ADS on-line reactivity monitoring validation, sub-criticality determination and operational procedures (loading, start-up, shut-down, ...) as a follow-up of the MUSE experiments. The project consists in coupling a fast lead core, set-up in the VENUS reactor at SCK*CEN Mol (B), with a GENEPI neutron source under construction by CNRS. To accommodate the accelerator in a vertical coupling configuration, the VENUS building is being heightened. The fast core will be loaded with enriched Uranium and will be moderated and reflected with solid lead (zero power experiment). For the purpose of the experimental programme, the neutron source has to be operated not only in pulsed mode but also in continuous mode to investigate the current-to-flux reactivity indicator in representative conditions of a powerful ADS. In this latter mode it is also required to make short beam interruptions to have access to the neutron population decrease as a function of time: from this spectrum it will be possible to apply different analysis techniques such as "prompt decay" fitting techniques and "source jerk" techniques. Beam interruptions will be repeated at a programmable frequency to improve time spectra statistics. Different sub-criticality levels (keff=0.99, 0.97, 0.95, ...) will be investigated in order to obtain a full set of data points for the final overall validation of the methodology. This paper describes the status of the experimental facility assembling, and the foreseen experimental programme to be started

The GUINEVERE project at the VENUS facility

Proc. on CD Rom log315International audienceThe GUINEVERE project is an international project in the framework of IP-EUROTRANS, the FP6 program which aims at addressing the main issues for ADS development in the framework of partitioning and transmutation for nuclear waste volume and radiotoxicity reduction. The GUINEVERE project is carried out in the context of domain 2 of IP-EUROTRANS, ECATS, devoted to specific experiments for the coupling of an accelerator, a target and a subcritical core. These experiments should provide an answer to the questions of online reactivity monitoring, sub-criticality determination and operational procedures (loading, start-up, shutdown, …) in an ADS by 2009-2010. The project has the objective to couple a fast lead core, within the VENUS building operated by the SCK•CEN, with a neutron generator able to work in three different modes: pulsed, continuous and continuous with beam interruptions at the millisecond scale. In order to achieve this goal, the VENUS facility has to be adapted and a modified GENEPI-3C accelerator has to be designed and constructed. The paper describes the main modifications to the reactor core and facility and to the accelerator, which will be executed during the years 2008 and 2009, and the experimental programme which will start in 2009

Etude des systèmes non liés 16B et 13Be

A description of a study of the neutron-rich unbound systems 16B and 13Be is presented. The structure of these nuclei provides strong constraints on current models and vital information for a three-body description of the Borromean two-neutron halo nuclei 17B and 14Be. The experimental work was undertaken at GANIL. Single-proton removal reactions were employed to populate the nuclei of interest, starting with a secondary beam of 17C at 35 MeV/nucleon (for 16B) and 14B at 41 MeV/nucleon (for 13Be). The charged fragments were detected using a position-sensitive Si-Si-CsI telescope of the CHARISSA collaboration, and the neutron using the DéMoN modular array. The decay energy spectra for 16B and 13Be were thus reconstructed from the measured 15B-n and 12Be-n coincidences. A theoretical description, based on the sudden approximation, of the population of unbound final states was developed. The predicted decay energy spectra were compared with the measurements after folding in the response of the experimental setup. This involved the use of simulations based on GEANT and a code, BELZEBUTH, developed specifically for the present work. In the case of 15B+n the data were best reproduced assuming the population of a narrow, low-lying (about 85 keV) d-wave resonance(s). The 12Be+n data were consistent with the population of a very broad (1-2 MeV) s-wave state at around 600-800 keV and a d-wave resonance at 2.5 MeV.Ce travail porte sur l'étude de deux systèmes non liés, le 16B et le 13Be, dont la structure très exotique fournit de fortes contraintes sur les modèles actuels et des informations primordiales pour modéliser les systèmes à trois corps que sont les noyaux à halo borroméens 17B et 14Be. Les deux expériences réalisées au GANIL consistaient à peupler les noyaux d'intérêt à partir de réactions de perte d'un proton d'un faisceau secondaire de 17C à 35 MeV/nucléon pour le 16B et de 14B à 41 MeV/nucléon pour le 13Be. L'utilisation d'un télescope sensible en position Si-Si-CsI de la collaboration CHARISSA pour détecter les fragments chargés, et du multidétecteur DéMoN pour les neutrons, permettait de reconstruire les spectres en énergie de décroissance du 16B et du 13Be à partir des coïncidences 15B-n et 12Be-n. Une description théorique, fondée sur l'approximation soudaine, du peuplement des états finals non liés a été développée. Les spectres en énergie de décroissance prédits ont été comparés aux données après convolution avec la réponse du dispositif expérimental, obtenue en utilisant deux codes de simulation, le premier basé sur GEANT et le second, BELZEBUTH, développé spécialement pour cette étude. Le meilleur accord avec les données 15B+n suppose le peuplement d'une (de) résonance(s) d très étroites à basse énergie (environ 85 keV). Les données 12Be+n sont en accord avec le peuplement d'un état s très large (1-2 MeV) autour de 600-800 keV et d'une résonance d à 2,5 MeV

Kinetic parameters of the GUINEVERE reference configuration in VENUS-F reactor obtained from a pile noise experiment using Rossi and Feynman methods

International audienceA pile noise measurement campaign has been conducted by the CEA in the VENUS-F reactor (SCK-CEN, Mol Belgium) in April 2011 in the reference critical configuration of the GUINEVERE experimental program. The experimental setup made it possible to estimate the core kinetic parameters: the prompt neutron decay constant, the delayed neutron fraction and the generation time. A precise assessment of these constants is of prime importance. In particular, the effective delayed neutron fraction is used to normalize and compare calculated reactivities of different subcritical configurations, obtained by modifying either the core layout or the control rods position, with experimental ones deduced from the analysis of measurements. This paper presents results obtained with a CEA-developed time stamping acquisition system. Data were analyzed using Rossi-$\alpha$ and Feynman-$\alpha$ methods. Results were normalized to reactor power using a calibrated fission chamber with a deposit of Np-237. Calculated factors were necessary to the analysis: the Diven factor was computed by the ENEA (Italy) and the power calibration factor by the CNRS/IN2P3/LPC Caen. Results deduced with both methods are consistent with respect to calculated quantities. Recommended values are given by the Rossi-$\alpha$ estimator, that was found to be the most robust. The neutron generation time was found equal to 0.448 $\pm$ 0.012 $\mu$s and the effective delayed neutron fraction is 770.3 $\pm$ 19 pcm. Discrepancies with the calculated value (722 pcm, calculation from ENEA) are satisfactory: -6.3% for the Rossi-$\alpha$ estimate and -2.7% for the Feynman-$\alpha$ estimate

Neutron flux measurements in the TRADE experiment : Critical configuration

The TRiga Accelerator Driven Experiment (TRADE) to be performed in the existing TRIGA reactor of the ENEA Casaccia centre had the objective of coupling a spallation target, a proton accelerator and a sub-critical reactor core. On the way of the transmutation of minor actinides in dedicated facilities, the TRADE experiment represents the intermediate step between the validation of each component of an Accelerator Driven System (ADS), i.e. the spallation target, the high intensity accelerator and the sub-critical core, and the final facility aiming to incinerate radioactive nuclear waste. Measurements have been performed in the TRIGA - RC1 reactor of the ENEA Casaccia centre in the second semester of 2004 to characterize the neutron flux in a critical configuration. Gold and Indium samples were introduced in the core with the help of a "fast rabbit" pneumatic device. Bare samples and samples covered with Cadmium foils were successively irradiated to estimate the thermal and epithermal fluxes. Measurements have been carried out along a traverse of the core. Samples were put inside Aluminium shuttles travelling in stainless steel tubes. Trains composed of five shuttles allowed for simultaneous measurements in five different axial positions. γ spectrometry of the radioactive nuclides has been performed with two HPGe detectors

A telescope for monitoring fast neutron sources

In the framework of nuclear waste management, highly radiotoxic long-lived fission products and minor actinides are planned to be transmuted in a sub-critical reactor coupled with an intense external neutron source. The latter source would be created by a high-energy proton beam hitting a high atomic number target. Such a new system, termed an accelerator-driven system (ADS), requires on-line and robust reactivity monitoring. The ratio between the beam current delivered by the accelerator and the reactor power level, or core neutron flux, is the basis of one method which could give access to a core reactivity change. In order to test reactivity measurement technique, some experimental programs use 14-MeV neutrons originating from the interaction of a deuteron beam with a tritium target as an external neutron source. In this case, the target tritium consumption over time precludes use of the beam current for reactivity monitoring and the external neutron source intensity must be monitored directly. A range telescope has been developed for this purpose, consisting of the assembly of a hydrogenous neutron converter and three silicon stages where the recoiling protons are detected. In this article, the pergormances of such a telescope are presented and compared to Monte-Carlo simulations