[en] This work is a contribution to the study of nucleus-nucleus collisions at the LHC with ALICE. The aim of this experiment is to search for a new phase of matter, the quark-gluon plasma (QGP). The dimuon forward spectrometer should measure one of the most promising probes of the QGP, the production of heavy quark vector mesons (J/ψ, γ, γ', γ'') through their muonic decays. The dimuon trigger selects the interesting events performing a cut on the transverse momentum of the tracks. The trigger decision is taken by a dedicated electronics using RPC (''Resistive Plate Chambers'') detector information. We have made our own R and D program on the RPC detector with various beam tests. We show the performances obtained during these tests of a low resistivity RPC operating in streamer mode. The ALICE requirements concerning the rate capability, the cluster size and the time resolution are fulfilled. We have optimised the trigger with simulations which include a complete description of the read-out planes and the trigger logic (algorithm). In particular, a technique of clustering is proposed and validated. A method called ''Ds reduction'' is introduced in order to limit the effects of combinatorial background on the trigger rates. The efficiencies and the trigger rates are calculated for Pb-Pb, Ca-Ca, p-p collisions at the LHC. Other more sophisticated cuts, on the invariant mass for example, using again the RPC information have been simulated but have not shown significant improvements of the trigger rates. (author)

[en] The neutron radiative capture is a nuclear reaction that occurs in the presence of neutrons on all isotopes and on a wide energy range. The neutron capture range on Lutetium isotopes, presented here, illustrates the variety of measurements leading to the determination of cross sections. These measurements provide valuable fundamental data needed for the stockpile stewardship program, as well as for nuclear astrophysics and nuclear structure. Measurements, made in France or in United-States, involving complex detectors associated with very rare targets have significantly improved the international databases and validated models of nuclear reactions. We present results concerning the measurement of neutron radiative capture on Lu¹⁷³, Lu¹⁷⁵, Lu¹⁷⁶ and Lu^177m, the measurement of the probability of gamma emission in the substitution reaction Yb¹⁷⁴(He³,pγ)Lu¹⁷⁶. The measurement of neutron cross sections on Lu^177m have permitted to highlight the process of super-elastic scattering

[en] We report on a measurement of the neutron detection efficiency in a LaBr₃ detector in the energy range 0.1–2 MeV. The experiment was carried out using a 4 MV Van-de-Graaff accelerator which provided monoenergetic neutrons from T(p,n)³He and ⁷Li(p,n)⁷Be reactions. The five most intense γ-rays from the LaBr₃(n, n′γ) reaction have been studied to deduce the neutron sensitivity of the detector. GEANT4 simulations with different nuclear data libraries have been done and a comparison with the data is provided

[en] We report on the dedicated implementation of the blocking technique for a LaBr₃(Ce) detector as well as associated electronics and data acquisition system for border protection applications. The detector is meant to perform delayed γ-ray spectroscopy of fission fragments produced via photofission induced by a high intensity pulsed photon beam. The gating technique avoids saturation of the detection chain during irradiation. The resulting setup allows us to successfully perform delayed gamma-ray spectroscopy starting only 30 ns after the gating operation. The measured energy resolution ranges from 5% to 6.5% at 662 keV depending on the γ-ray detection time after the gating operation. (authors)

[en] The cross-section of the ²³⁹Pu (n,2n)²³⁸Pu reaction is relatively well-known between 8 and 12 MeV for incident neutron energy but different models disagree between 6 and 8 MeV as well as beyond 12 MeV. A new measurement method has been developed, this method is based on the recovery of the quantity of Pu²³⁸ produced by the reaction during a few hundreds hours of irradiation and at an energy of 6.88 MeV for the incident neutron. The result confirms the disagreement between the JEFF3.3 evaluation and previous data. One of the issues of this method is to determine the right efficiency of the recovery of the quantity of Pu²³⁸. Another issue was to get a quantity of ²³⁹Pu pure enough as the experiment has shown that some Pu²³⁸ initially present in the Pu²³⁹ target was recovered as reaction product. (A.C.)

[en] The theoretical description of the cross-section of the neutron radiative capture is difficult to get because it can vary dramatically from one nucleus to a neighbouring one. As a consequence measurements are still essential. This measurement has been recently made for an instable nucleus: ¹⁷³Lu whose half-life is 1.37 y. The ¹⁷³Lu target has been obtained with a 14-day long irradiation of natural hafnium by a proton-beam of 92 MeV. Several chemical separations have been necessary to get a pure lutetium target. The 200 μg lutetium target itself was too radioactive (3*10⁹ becquerel) and a lead shield was placed before the detectors to reduce the radiation rate by 2 orders of magnitude. The neutron beam from the LANSCE accelerator has been used to irradiate the lutetium target. The response of the γ detectors of the 4π γ DANCE multi-detector has been influenced by the presence of the shield and accurate simulations have been necessary to mask this influence. (A.C.)

[en] The techniques used to produce a ^177mLu (J^π=23/2^-,T_1/2=160.4 days) target are described in this paper. Firstly, an isotopic separation of an enriched lutetium sample was used to reach a purity of ¹⁷⁶Lu close to 99.993%. Afterwards, the high neutron flux of the Grenoble Institut Laue-Langevin reactor was used to produce the ^177mLu isomer by the ¹⁷⁶Lu(n,γ) reaction. Finally, a chemical separation was performed to extract 10¹³ nuclei of ^177mLu. Thanks to this experiment, we have been able to estimate the destruction cross-section of the ^177mLu

[en] Nowadays, reliance on nuclear models to interpolate or extrapolate between experimental data points is very common, for nuclear data evaluation. It is also well known that the knowledge of nuclear reaction mechanisms is at best approximate, and that their modeling relies on many parameters which do not have a precise physical meaning outside of their specific implementations in nuclear model codes: they carry both specific physical information, and effective information that is related to the deficiencies of the model itself. Therefore, to improve the uncertainties associated with evaluated nuclear data, the models themselves must be refined so that their parameters can be rigorously derived from theory. Examples of such a process will be given for a wide sample of models like: detailed theory of compound nucleus decay through multiple nucleon or gamma emission, or refinements to the width fluctuation factor of the Hauser-Feshbach model. All these examples will illustrate the reduction in the effective components of nuclear model parameters, through the reduced dynamics of parameter adjustment needed to account for experimental data. The significant progress, recently achieved for the non-fission channels, also highlights the difficult path ahead to improve our quantitative understanding of fission in a similar way: by relying on microscopic theory. (authors)

[en] The inelastic neutron acceleration - INNA (also called super-elastic neutron scattering) occurs during the collision of a neutron with an excited nucleus. In this reaction, the nucleus transfers some of its excitation energy to the scattered neutron. The 160-day isomer in ^177mLu (E_X=970 keV) is a good candidate to observe the INNA reaction. In this paper we report on experimental results on the direct measurement of high energy neutrons in the reactions between the long-lived isomer ^177mLu and cold neutron at the Orphee reactor facility. The ^177mLu target was got through the irradiation of a natural Lu foil during 6 days in a 1.5*10¹⁵ n.cm^-2.s^-1 neutron flux in an experimental port of the High Flux Reactor of the ILL, followed by a 150 days cooling time. A similar non-irradiated natural Lutetium foil from the same sample used for the isomer target was used as blank target. The measurement consists in counting the thermal neutrons for various configurations when the Orphee neutron beams impinge or not on the ^177mLu target and when the ^177mLu target is replaced with the natural Lu target. The data analysis is in progress and preliminary results confirm the existence of the INNA reaction

[en] Fission and γ-emission probabilities induced by transfer or inelastic scattering reactions with light projectile nuclei are very valuable quantities for constraining the models that describe the de-excitation of heavy nuclei. We have developed an experimental set-up that allows us to simultaneously measure fission and γ-emission probabilities. The measurement of the γ-emission probability at excitation energies where the fission channel is open is challenging due to the intense background of γ rays emitted by the fission fragments. We discuss the procedure to subtract such a background and the constraints that this subtraction and other experimental conditions put on the set up. We show that our set-up complies with these constraints. (authors)