D. Guerreau

Production of neutron-rich nuclei at the limits of particles stability by fragmentation of 44 MeVu 40Ar projectiles☆

Abstract In-flight identification of fragments from 44 MeV u 40 Ar emitted at 0° provides the first experimental observation of 23 N , 29 Ne and 30 Ne . On the other hand 18 B , 21 C and 25 O are found to be particle-instable.

Production and identification of new neutron-rich fragments from 33 MeV/u86Kr beam in the 18≦Z≦27 region

The new nuclei47Ar,57Ti,59,60V,61,62Cr,64,65Mn,66,67,68Fe,68,69,70Co haye been produced by fragmentation of a86Kr beam and identified through time of flight andΔE×E measurements. The tentative observation of56Ti,57,58V,60Cr and65Fe by other groups is confirmed.

Evaporation barriers for4He indicate very extended forms for many emitting nuclei

We have analyzed a large set of mean energies and angular anisotropies for evaporative4He emission to obtain barriers to evaporation,B. These exit channel barriers are often substantially smaller than the corresponding empirical s-wave fusion barriersE0. The differences (E0-B) are interpreted as indicators of the extent of distortion of the emitters. These distortions have in turn been characterized by the deformation parameter for a spheroid α20. ForZ=80 the dependence ofB or {α}20 on spin is somewhat suggestive of the superdeformed shapes predicted by the liquid drop model. ForZ>70 significant distortions are indicated for emitters of both large and small spin.

β-Delayed neutron emission of the isotopes 20C, 40,41,42P, 43,44S☆

Abstract Very neutron-rich isotopes have been produced by the fragmentation of 55 MeV/u 48 Ca projectiles from GANIL onto a 181 Ta target. The nuclei were separated by means of the doubly achromatic spectrometer LISE and implanted into a semiconductor telescope for identification in Z and A . Their β-delayed neutron radioactivity is observed by means of a 4π β -n coincidence detector. The results include first measurements of the half-life T 1 2 and the neutron emission probability P n for the isotopes 20 C, 40,41,42 p and 43,44 S. They are compared to current theoretical predictions.

Zero degree measurements of isotopic distributions in 44 MeV/u 86Kr-induced reactions for the production of nuclei far from stability

Abstract The velocity and isotopic distributions of nuclei produced in 44 MeV/u 86Kr-induced reactions on 27Al, 103Rh and 197Au targets are investigated by means of the magnetic spectrometer LISE. These distributions are compared with two different models, and indicate that no real projectile fragmentation occurs, but rather highly dissipative processes such as incomplete fusion or deep-inelastic transfer. Consequently, the yields of nuclei far from stability is much lower relative to those observed with lighter projectiles such as 40Ar at the same energy.

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Emission of4He in the reaction 334 MeV40Ar+238U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the4He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed4He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the4He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average4He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.

Products of 3≦Z≦9 from long-lived intermediates in the reactions 336 MeV40Ar+Ag,154Sm and Au

Angular and energy distributions have been measured for products of 3≦Z≦9 from reactions of 336MeV40Ar+107,9Ag/154Sm/197Au. The c.m. differential cross sections and mean energies increase as one moves from ≈100deg. to ≈170deg. similar to those for fission or evaporation-like emission. Most characteristics mimic those observed for4He, but the cross sections are smaller by a factor of as ≈1/300 for each reaction studied. A semiclassical analysis has been used to estimate the effective barriers to emission and root-mean-square spins of the emitters.

Studies of4He emission in both fusion-like and inelastic reactions of 340-MeV40Ar+238U

The fission-fragment angular correlation technique has been used in conjunction with a position-sensitive avalanche detector to identify the fusion-like and inelastic collisions of 340-MeV40Ar+238U. For each of these two reaction types, we have characterized the correlated4He emission by measuring three-fold coincidences between two fission fragments and the light charged particle. We find an abundance of both evaporation-like and direct4He emission, each of which is associated with both fusion-fission and sequential-fission processes. Several comparison tests of both the coincidence and singles data strongly indicate that very little4He emission is due to evaporation from accelerated fission fragments in this system. The evaporative4He emission appears instead to have a strong contribution originating from the composite system prior to fission. The forward angle4He emissions, characterized by prominent forward-peaking and relatively high energies, suggest an origin associated with the very early stages of the collision.

Results on two-, three-, and four-body events from the100Mo+100Mo and120Sn+120Sn collisions aroundE/A=20 MeV

Events with 2, 3 and 4 heavy-fragments (A≧20) have been detected in the reactions100Mo+100Mo atE/A =18.7, 23.7 MeV and120Sn+120Sn atE/A=18.4 MeV. The experiments were performed with an array of 12 detectors which together covered a large fraction of the forward hemisphere and allowed a high detection efficiency for these events. Masses and energies of all fragments have been reconstructed by means of an improved version of the kinematic coincidence method. The probabilitiesP3 andP4 of producing 3- and 4-body events were found to depend mainly on the dissipated energy rather than on the bombarding energy, thus indicating that their origin lies more in the decay properties of the excited fragments than in the dynamics of the interaction. Emission of light particles from the composite system is shown to become more relevant with increasing bombarding energy and may explain the drop of theP3 andP4 curves at high energy losses. Small deviations of theP3 andP4 curves at 23.7A · MeV from those at lower bombarding energies were used to estimate the amount of a possible pre-equilibrium light particle emission as a function of impact parameter.

Analysis of the sequential fission observed in collisions of100Mo +100Mo and120Sn +120Sn around 20 AMeV

Events with 2, 3 and 4 heavy fragments (A≥20) detected in the reactions100Mo +100Mo at 18.7, 23.7 A·MeV and120Sn +120Sn at 18.4 A·MeV were analyzed by means of an improved version of the kinematic coincidence method. The phase-space distributions prove that 3- (and possibly 4-) body events predominantly originate from a two-step mechanism and are compatible with the hypothesis of a binary deep-inelastic interaction followed by the further fissionlike decay of one (or both) of the primary fragments. The characteristics of the fission step — mass asymmetry, relative velocity, in-plane and out-of-plane angles — have been reconstructed for the 3-body events and indications are found that nonequilibrium effects at the end of the deep-inelastic phase may influence the fissionlike decay.