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International audience; Absolute cross sections, energy spectra, and angular distributions have been measured for $^{1,2,3}$H, $^{3,4,6}$He, $^{6,7,8,9}$Li and $^{7,9,10}$Be isotopes produced in 1.2 GeV proton-induced spallation reactions with targets between Al and Th. Results of simulation calculations with the intra-nuclear cascade code INCL2.0 coupled to the statistical model GEMINI are in good agreement with these data, as to charged-particle evaporation, mean excitation energy, and mean linear momentum transfer. The pre-equilibrium emission of composite particles, not accounted for in these simulations, however, typically contributes to the total production of composite particles by 40–60% for $^{2}$H and $^{3}$He, 20–40% for $^{3}$H, 5–20% for $^{4}$He, and about 15–25% for Li and Be. The composite pre-equilibrium particles together carry off a mean energy of up to 50 MeV, i.e., about 30% compared to the mean energy released by particle evaporation. For deuterons, pre-equilibrium emission is shown to be well described by surface coalescence while definitely other mechanisms are required for $^{4}$He and heavier clusters.

Two 4π detectors for neutrons and charged particles have been combined for the investigation of GeV-proton and antiproton induced spallation reactions. It is a large neutron scintillator tank with a high-granularity charged-particle detector in its central vacuum chamber. Together they register the whole, neutral and charged, de-excitation cascade of the excited reaction complex as well as heavier reaction products, i.e. intermediate-mass and fission fragments and heavy residues. Part I of our communication describes the neutron detector Berlin neutron ball (BNB): Its spherical tank is filled with 1.5 m 3 scintillator liquid, loaded with 0.4 wt % Gd. BNB recognizes each reaction in the target with a low inelasticity threshold of 2.5 MeV ee and records event-wise the multiplicity of evaporated neutrons with ≈75% efficiency. Part II [12] describes the charged particle detector Berlin silicon ball (BSiB) assembled from 151 independent Si-detectors and six ancillary four-fold telescopes, considers its detection efficiency for various particle species and provides experience gained from the joint operation of both detectors, BNB and BSiB, as a filter on the excitation energy dissipated event-by-event in the spallation reaction.

A combination of two 4π-detectors for neutrons and charged particles is employed for the event-by-event determination of the thermal excitation energy of nuclei produced in spallation reactions. Here the highly efficient charged particle detector BSiB is described while the neutron detector is presented in the preceding paper (Jahnke et al., Nucl. Instr. and Meth. A, this issue). The Berlin Silicon Ball (BSiB) consists of 162 500 μm thick silicon detectors forming a self-supporting complete sphere with a diameter of 20 cm . By means of time of flight and energy H, He, IMFs, FFs, and HRs are identified. The lower detection thresholds are about 2 MeV for all detectors and about 0.5 MeV for 16 selected detectors in the forward direction. In addition, 6 ancillary four-fold detector-telescopes are employed with isotopic resolution up to C, and Z-identification up to Na. The geometric efficiency of BSiB has been determined experimentally to 80.4±1.0% for the standard configuration with 151 mounted detectors. Efficiency losses due to energy thresholds, target shadow, multiple hits and reaction kinematics are calculated for protons, helium, fission fragments, and heavy residues released in 1.2 and 2.5 GeV proton-induced spallation reactions on Fe and Au. The results are supported by experimental data for binary fission, which have been analysed as a function of excitation energy.

Absolute production cross sections have been measured simultaneously for neutrons and light charged particles in 0.8-2.5 GeV proton induced spallation reactions for a series of target nuclei from aluminum up to uranium. The high detection efficiency both for neutral and charged evaporative particles provides an event-wise access to the amount of projectile energy dissipated into nuclear excitation. Various intra nuclear cascade plus evaporation models have been confronted with the experimental data showing large discrepancies for hydrogen and helium production.

Reactions of 1.2 GeV antiprotons with Cu and Ag targets have been studied. It was found that in such reactions, excitation energies are generated in excess of the total binding energy of the nuclei involved. Accordingly, one observes in these reactions the onset of vaporization and multifragmentation processes. In the p + Cu reaction, the vaporization is observed to set in at about 7.5 MeV/nucleon. However, its probability remains relatively small and below 15% up to the highest excitation energies identified in the present studies (approximately 15 MeV/nucleon). The threshold excitation energy for multifragmentation processes, defined as the detection of at least 3 intermediate mass fragments, is observed at about 4 MeV/nucleon for both studied reactions. Multifragmentation is seen to account for no more than approximately 5% and 20% of the corresponding inclusive yield for Cu and Ag targets, respectively.

Particle identification in intermediate heavy-ion collisions, using a modern 4 pi detector which contains several active layers, relies on a parametrisation or numerical integration of the energy loss in thick layers of detector material for different ions. Here an analytical solution applicable over an energy range of a few MeV up to a 100A MeV and for ions up to at least Z = 8 is presented. Also, the consequences for time-of-flight measurements (TOF) in detectors behind several thick layers of detector material are discussed. The solution is applied to the data of the Huygens detector, which uses a TPC (dE/dx) and plastic scintillators for particle identification(E and TOF or dE/dx and TOF). (C) 1999 Elsevier Science B.V. All rights reserved.

Reaction cross sections and production cross sections for neutrons, hydrogen, and helium have been measured for 1.2, 1.8 GeV p+Fe, Ni, Ag, Ta, W, Au, Pb and U and are compared with different intra-nuclear-cascade- combined with evaporation-models. Agreement for neutrons and considerable differences for light charged particles are observed between experiment and calculation as well as between different models. The discrepancies are associated with specific deficiencies in the models. The exclusive data measured with two 4π-detectors for neutron and charged particle detection allowed furthermore a systematic comparison of observables characteristic of different stages of the temporal evolution of a spallation reaction: inelastic collision probability, excitation energy distribution, pre-equilibrium emission, and inclusive production cross sections.

Energetic antiproton-nucleus interactions allow to build up thermal excitation energies exceeding 800 MeV in nuclei of masses close to 200. We found that for these high excitation energies the fission process still remains a very significant exit channel. At excitation energies close to 800 MeV the fission process approaches 20–50% of the cross section for Ho, Au and U-like nuclei depending on the fission events selection criteria. The average intermediate mass fragment multiplicity at such high excitation energies is equal to about 1 and alpha particle multiplicity is equal to about 5 for all three reactions independently of the exit channel, including fission. A transition towards the multifragmentation is not observed.

ORION is a sectorized 4 pi neutron detector: part of it being used as a time-of-flight spectrometer. In a sector, six phototubes collect the light created during the neutron slowing down. The amount of charge integrated by each one strongly depends on the impact position of the neutron. Several tests have been performed with cosmic rays to study in detail the relation between the collected charge distribution and the particle impact position. The results are presented and compared with a simulation based on simple assumptions of light propagation in this particular medium. The detailed analysis has revealed a rather good resolution, better than 5 cm, and at the same time, regions of ambiguity where the correct position could not always be extracted. A new detector prototype, TOURNESOL, allowing one to overcome the present limitations is under construction. (C) 1998 Elsevier Science B.V. All rights reserved.

The neutron detection performances of the 4 pi ORION counter were investigated with monoenergetic neutron beams delivered by the Louvain-la-Neuve facility. Two responses are supplied by such a counter: a prompt response, arising from the slowing down of neutrons within the liquid scintillator contained in the detector, and a delayed response corresponding to the subsequent capture of the neutrons by Gd nuclei: with which the detector is loaded. Two experimental methods were used, allowing one to probe the responses for different neutron energies. Despite the large size of the detector, the prompt response is observed to be very similar to that exhibited by small cells. For both responses, a reasonable overall agreement is found between the data and the predictions of Monte-Carlo simulations. The timing resolution. of the order of 3 ns, makes this detector suitable for time-of-flight methods in applications requiring a high detection efficiency while accepting a moderate energy resolution. (C) 1998 Elsevier Science B.V. All rights reserved.

The formation and decay of hot nuclei generated in the interaction of light projectiles (475 MeV and 2 GeV protons and 2 GeV He-3) on a series of targets (Ag-107, Au-197, Bi-209, and U-238) are studied with an apparatus combining the efficient detection of neutrons in 4 pi sr and an accurate characterization of light charged particles, intermediate-mass fragments (IMF's), and fission fragments. A two-step approach with an intranuclear cascade process for modeling the initial off-equilibrium phase of the collision followed by a classical step-by-step evaporation-including fission competition-is used to reproduce the data. It is inferred from the model, which is found to reproduce several data sensitive to heat, that nuclei with temperatures exceeding T =5 MeV are produced for a sizable part of the events, thus giving the opportunity to study the behavior of hot nuclei free from strong collective excitations which generally accompany nucleus-nucleus collisions. Most of the observed features related to particle emission or more specifically to particle evaporation are rather well accounted for by the model calculation. The evaporationlike IMF emission is generally rather weak, and does not show any rapid onset at the highest excitation energies as would have been expected in a genuine thermal multifragmentation process. Binary fission of the U-like target is shown to be a fairly probable channel at most excitation energies. Some of the characteristics of the fission channel are satisfactorily reproduced, but not all. [S0556-2813(98)02605-3].

A 4 pi charged particle detector array with a low velocity threshold has been used to detect the products from reactions induced by Ar-36 on Al-27 at energies ranging from 55 to 95 MeV/u. Well characterized events were selected and sorted as a function of the impact parameter. Two methods were used for sorting these events with respect to their impact parameters and three methods were compared to determine the reaction plane. The transverse momentum analysis has been found to be the best method to extract the direction of the reaction plane for this system and for the experimental set-up used here. The energy of vanishing flow for central collisions has been found to be around 90-95 MeV/u. The azimuthal distributions of mid-rapidity particles exhibit a preferential in-plane emission and no squeeze-out effect.

In Ar-36 + Al-27 reactions at bombarding energies E/A from 55 to 95 MeV, the mass and excitation energy of the quasi-projectile have been reconstructed at all impact parameters. Up to excitation energies E*/A of similar to 5-6 MeV, the mean kinetic energies of Z = 1 to Z = 9 fragments are in agreement with values predicted by sequential statistical decay, Above, a collective expansion energy is needed that grows with E*/A, reaching 1 to 2.2 MeV/u, depending on the de-excitation process, at E*/A similar to 10 MeV. It decreases with the fragment charge, The dominant role of excitation energy supports the scenario of thermal expansion.

Formation and decay of hot nuclei have been studied in Zn-64 + Ti-nat collisions between 35 and 79 MeV/nucleon. The mass and excitation energy of excited quasiprojectiles are reconstructed from the kinematical characteristics of their decay products. In central collisions, excitation energies larger than 10 MeV/nucleon are reached. Comparisons with theoretical predictions indicate that a fraction of the excitation energy is associated with an isotropic radial flow.

A 4 pi multidetector array with a high efficiency has been used ia detect the charged products from reactions induced by Ar-36 on Al-27 at energies ranging from 55 to 95 MeV/u. Well characterized events were selected and sorted as a function of the impact parameter. Fusion or full stopping events have a very small cross section. Dissipative binary collisions accompanied by pre-equilibrium (mid-rapidity) emission, and followed by de-excitation of the quasi-projectile and quasi-target, were found to dominate for all impact parameters. The mass of the quasi-projectile has been reconstructed from its products and found to be slightly below the projectile mass. Isotropic emission in its rest frame has been observed at all energies and impact parameters, indicating that equilibrated nuclei may have been formed. The excitation energy of the quasi-projectile, determined via calorimetry, and the slope of kinetic energy spectra of alpha-particles were found to increase with incident energy and with decreasing impact parameter. Excitation energies per nucleon above II MeV are reached in a sizable cross section (70 mb). The mass distribution and kinetic energy spectra of emitted particles and fragments are essentially governed by the excitation energy, with a weak dependence on the impact parameter. The evolution of dissipative processes from deep inelastic collisions to the participant-spectator regime is discussed.

The lifetime, τ, with respect to multifragmentation of highly excited nuclei is deduced from the analysis of strongly damped Pb + Au collisions at 29 MeV/u. The method is based on the study of space-time correlations induced by “proximity” effects between fragments emitted by the two primary products of the reaction and gives the time between the reseparation of the two primary products and the subsequent multifragment decay of one partner. A lifetime significantly longer than the interaction time is found. It is compared with time scales corresponding to different types of dynamical instabilities which may be responsible for nuclear disassembly.

Nucleus-nucleus reaction mechanisms at incident energies below 10 MeV/u are governed by mean field effects. Nucleon-nucleon collisions begin to become important as the energy increases above the Fermi energy. This evolution manifests itself experimentally in the modification of the directed flow of the emitted participant (mid-rapidity) nucleons. In the reaction plane, it is expressed by the flow parameter [1]. Another aspect lies in the azimuthal distribution of particles relative to the reaction plane. At high energies, a maximum in the direction perpendicular to the reaction plane on both sides was named squeeze-out-effect [2]. Conversely, below 100 MeV/u, maxima in the reaction plane have been observed for the light systems of Ar + V : rotation-like effect [3]. It was studied as a Function of energy, impact parameter and emitted particle mass in the collisions of Ar-40 on Al-27 at energies from 36 to 65 MeV/u [4]. The latter study has been extended up to 95 MeV/u and measurements have been made for the Zn-64 + Ni-58 system at 5 energies from 35 to 79 MeV/u.

An experiment with the Huygens detectors has been performed at GANIL (France). These Huygens detectors comprise two detectors: a central detector, which consists of a time projection chamber (TPC) surrounded by a plastic scintillator barrel, and a CsI(Tl) wall in the backward region. In addition, in the forward hemisphere a plastic hodoscope (‘MUR’) is used in conjunction with the forward plastics of the Huygens plastic scintillator barrel. In this paper first results will be presented.

The azimuthal distributions of light particles relative to the reaction plane have been measured for several bins of experimentally estimated impact parameters in the reactions of Zn-64 + Ni-58 at energies between 35 and 79 MeV/u. An in-plane enhancement for mid-rapidity Z= 1, 2, 3 particles is observed at low incident energy but gradually evolves to out-of-plane enhancement (squeeze-out effect) with increasing energy. This evolution depends on the impact parameter in a way similar to the flow parameter. The energies for this system at which the azimuthal distribution is uniform are lower than the corresponding balance energies.

Hot-nuclei fusion properties have been studied for the Ar + Ag system at 50 and 70 MeV/u bombarding energy. The set up was very well suited to the analysis of sequential evaporation leading to an evaporation residue since the residue was detected in coincidence with all the corresponding light charged particles by use of a 4π- multidetector. An event by event analysis was performed in order to distinguish between particles evaporated from the fusion nucleus and others. It was possible to reconstruct the initial excitation energy of the fusion nucleus by adding the contributions of all the charged decay products and by estimating the neutron contribution. It was found that the excitation energy distributions obtained at 50 and 70 MeV/u were quite similar. Very large values are reached in both cases (∼ 600 MeV). This result means that the standard statistical decay observed at limited excitation is still observed for very hot nuclei. The corresponding temperatures were measured from the kinetic-energy spectra slopes. Slightly larger values were observed at 70 MeV/u. This result is consistent with a slightly smaller mass of the fusion nucleus obtained in this case. In a last aspect of the experiment where many particles events were analysed in the 4π set up, it turned out that the highest involved excitation energies can lead either to a multifragment emission, or to a standard sequential statistical decay.

Neutral pion production has been studied in the $^{36}\mathrm{Ar}$${+}^{27}$Al reaction at 95 MeV/nucleon with the aim to get a quantitative estimate of its impact-parameter dependence. A near 4\ensuremath{\pi} multidetector has been used to detect both the gamma rays originating from the ${\mathrm{\ensuremath{\pi}}}^{0}$ decay and the associated charged particles. The charged particle multiplicity has been used in the present analysis as a global variable to extract the impact parameter scale. A comparison with a Boltzmann-Nordheim-Vlasov calculation, whcih takes into account the effect of pion reabsorption in the nuclear medium, has been performed.

The production of neutral pions in the reaction [sup 16]O+[sup 27]Al at 94 MeV/nucleon was studied with a multidetector, which includes 180 BaF[sub 2] modules. Kinetic energy spectra for several laboratory angles were measured. The total cross section for neutral pion production was deduced. Results were compared with previous findings on charged pions from the same reaction at the same energy and with the prediction of a dynamical model based on the numerical solution of the Boltzmann-Nordheim-Vlasov equation.

The central part of the light-particle detection unit for the GANIL multidetector is a spherical (80 cm radius) shell of 2 mm thick NE 102A plastic scintillator. It spans the range of polar angles from θ = 30° − 150° and the azimuthal range θ = 0° − 360°, and it is segment into 72 individual Δθ = 60° and Δθ = 10° wide detection modules. Each module is viewed by a photomultiplier at each end which provide information on the particle impact position, atomic number Z and velocity. The performance of the modules was studied with an α-source and during several experiments with heavy-ion beams from 25 up to 85 MeV/amu.

International audience; This report summarizes the experiments relative to Super-Heavy Element studies done at GANIL — CEA — CNRS since 1999. It also gives an overview of future experiments and opportunities offered by SPIRAL 2 and LINAG beams in a medium term.