Search

The fragmentation of neutron-rich Sn isotopes obtained from the fission of 238U projectiles at 950 MeV/u has been investigated at the FRagment Separator (FRS) at GSI. In order to study the feasibility of a two-step reaction scheme for the production of medium-mass neutron-rich nuclei, fragments with masses A ≈ 130 have been isotopically resolved in the first part of FRS and their fragmentation residues identified in the second part. [ABSTRACT FROM AUTHOR]

We propose a new formula for the saddle-to-scission time that is more general that the one based on Kramers' approach. Its validity and applicability is then studied in detail. Such a formula is useful for the evaluation of the fission time of very heavy nuclei. [ABSTRACT FROM AUTHOR]

Knowledge of the influence of nuclear shell structure on the survival probability of heavy compound nuclei against fission is important for a quantitative understanding of the production rates of spherical super-heavy elements (SHE). Fission probabilities of N = 126 isotones beyond astatine can be used as test cases for the production of spherical super-heavy elements, as those isotones possess a strong shell correction energy and are highly fissile. Here, we report on two new experimental approaches which probe the effect of the closed neutron shell at N = 126 on the competition between fission and particle evaporation using projectile fragmentation and electromagnetic-induced fission of radioactive beams. We conclude that these nuclei lose at least a great part of their stability against fission at low excitation energies and angular momenta - mostly due to the influence of collective contributions in the level density. Implications on the production of spherical SHE will be discussed. [Copyright &y& Elsevier]

Spallation reactions are considered as an optimum neutron source for nuclear waste transmutation in accelerator driven systems (ADS). They are also used to produce intense radioactive nuclear beams in ISOL facilities. A common problem to both applications is the characterisation of these reactions in terms of neutron and residual nuclei production. In this paper we review the most outstanding experimental programs recently undertaken to investigate spallation reactions and their implications in the production of radioactive nuclear beams. [Copyright &y& Elsevier]

Peripheral heavy-ion collisions at relativistic energy are proposed as a new experimental approach dedicated to nuclear dissipation studies and, in particular, to investigate transient effects which are responsible for the inhibition of fission at the beginning of the process. To extract reliable information from the data, an analytical approximation of the time-dependent fission decay width is used in connection with new experimental signatures of relaxation effects. [ABSTRACT FROM AUTHOR]

New data from GSI on the production-cross-section for fragmentation of the systems [sup 56]Fe+p and [sup 136]Xe+p at 1 A GeV revealed the appearance of even-odd staggering in the cross-section distribution for chains of isotopes with given N-Z. The staggering is strongly enhanced for the chain N=Z, it reduces as the production moves away from the N=Z chain, and it reverses for the most neutron-rich odd-A residues. These phenomena, observed in the residues of rather violent reactions, are related to structural effects in the level-densities below the particle-emission threshold. [ABSTRACT FROM AUTHOR]

This contribution discusses the characteristics of different types of nuclear reactions and the influence of the beam energy in view of future prospects for secondary-beam production. First, electronic interactions in the target are considered because they define the usable target thickness. Rather high beam energies are advantageous. Secondly, the nuclear-reaction aspects are discussed. Three reaction mechanisms provide the most promising prospects for the production of secondary beams. Fusion is best suited for the production of nuclei near the proton drip line and for the heaviest elements. Fission specifically populates mid-mass neutron-rich isotopes. Fragmentation and spallation reactions represent rather universal production mechanisms for both neutron-deficient and neutron-rich exotic nuclei, since the fluctuations in the N-over-Z ratio are very important. Due to these large fluctuations, this is the most promising reaction mechanism to reach extremely exotic nuclei over the whole mass range, if sufficiently high primary-beam intensities are available. In particular, it seems to be a unique process for the production of extremely neutron-rich isotopes of elements above the region of fission fragments. These considerations are verified with experimental data. In particular, a systematic overview on the production of residual nuclei in reactions at relativistic energies has been obtained in several experiments recently performed at GSI in inverse kinematics. They allow to give a rather realistic estimate on the prospects for secondary-beam production in next-generation facilities. [Copyright &y& Elsevier]

Abstract: Electromagnetic-induced fission of several neutron-deficient actinides and pre-actinides was studied at GSI Darmstadt by use of relativistic secondary beams. The characteristics of multi-modal fission of nuclei around 226Th are systematically investigated and interpreted as the superposition of three fission channels. Properties of these fission channels have been determined for 15 systems. A global view on the properties of fission channels including previous results is presented. The positions of the asymmetric fission channels are found to be constant in atomic number over the whole range of systems investigated. [Copyright &y& Elsevier]

Abstract: The conditions for the manifestation of transient effects in fission are carefully examined by analysing experimental data where fission is induced by peripheral heavy-ion collisions at relativistic energies. Experimental total nuclear fission cross sections of 238U at 1 A GeV on gold and uranium targets are compared with the predictions of a nuclear-reaction code, where transient effects in fission are modeled using different approximations to the numerical time-dependent fission-decay width: a new analytical description based on the solution of the Fokker–Planck equation and two widely used but less realistic descriptions, a step function and an exponential-like function. The experimental data are only reproduced when dissipation is considered. The influence of transient effects on the fission process, as well as the deduced value of the dissipation strength β, depends strongly on the approximation applied for the time-dependent fission-decay width. In particular, a meticulous analysis sheds severe doubts on the use of the exponential-like in-growth function. Finally, we investigate which should be the characteristics of experimental observables to be most sensitive to transient effects in fission. The pertinence of observables related to the excitation energy at saddle is discussed. [Copyright &y& Elsevier]

Abstract: The time-dependent flux over the fission barrier of an excited nucleus under the influence of dissipation is investigated. Characteristic features of the evolution of the amplitude of the probability distribution and the velocity profile at the fission barrier are derived. Analytical results are compared to numerical Langevin calculations and used to develop a new analytical approximation to the solution of the Fokker–Planck equation for the time-dependent fission-decay width. This approximation is shown to be more realistic than previously proposed descriptions, which were widely used in the past. [Copyright &y& Elsevier]

A new method to estimate the angular transmission in zero-degree magnetic spectrometers is presented. This method is based on a parameterisation of the angular aperture of the spectrometer for any possible value of the magnetic rigidity of the transmitted particles. This parameterisation of the angular aperture together with a description of the kinematics of the reaction mechanism allows to determine the angular transmission analytically, avoiding tedious Monte-Carlo calculations. The analytical solutions are implemented for residual nuclei produced in fission, projectile–fragmentation and fusion–evaporation reactions. [Copyright &y& Elsevier]

A method to access the complete identification in atomic number Z and mass A of fragments produced in low-energy fission of actinides is presented. This method, based on the use of multinucleon transfer and fusion reactions in inverse kinematics, is applied in this work to reactions between a 238U beam and a 12C target to produce and induce fission of moderately excited actinides. The fission fragments are detected and fully identified with the VAMOS spectrometer of GANIL, allowing the measurement of fragment yields of several hundreds of isotopes in a range between A~80 and ~160, and from Z~30 to ~64. Complete isotopic yield distributions of fragments from well defined fissioning systems are made available. Together with the precise measurement of the fragment emission angles and velocities, this technique gives further insight into the nuclear-fission process. [ABSTRACT FROM AUTHOR]

Abstract: Gamma-decay probabilities of 173Yb and 176Lu have been measured using the surrogate reactions 174Yb(3He,αγ)173Yb* and 174Yb(3He,pγ)176Lu*, respectively. For the first time, the gamma-decay probabilities have been obtained with two independent experimental methods based on the use of C6D6 scintillators and Germanium detectors. Our results for the radiative-capture cross sections are several times higher than the corresponding neutron-induced data. To explain these differences, we have used our gamma-decay probabilities to extract rather direct information on the spin distributions populated in the transfer reactions used. They are about two times wider and the mean values are 3 to 4 ℏ higher than the ones populated in the neutron-induced reactions. As a consequence, in the transfer reactions neutron emission to the ground and first excited states of the residual nucleus is strongly suppressed and gamma-decay is considerably enhanced. [Copyright &y& Elsevier]

Abstract: Recently, a series of dedicated inverse-kinematics experiments performed at GSI, Darmstadt, has brought an important progress in our understanding of proton and heavy-ion induced reactions at relativistic energies. The nuclear reaction code ABRABLA that has been developed and benchmarked against the results of these experiments has been used to calculate nuclide production cross-sections at different energies and with different targets and beams. These calculations are used to estimate nuclide production rates by protons in thick targets, taking into account the energy loss and the attenuation of the proton beam in the target, as well as the low-energy fission induced by the secondary neutrons. The results are compared to the yields of isotopes of various elements obtained from different targets at CERN-ISOLDE with 600MeV protons, and the overall extraction efficiencies are deduced. The dependence of these extraction efficiencies on the nuclide half-life follows a simple pattern in many different cases. A universal function is proposed to parameterize this behavior in a way that quantifies the essential properties of the extraction efficiency for the element and the target–ion-source system in question. [Copyright &y& Elsevier]

Abstract: The production cross-sections of proton-rich protactinium and thorium isotopes have been investigated for the fragmentation of 1 A GeV 238U in a beryllium target at the SIS/FRS facility at GSI Darmstadt. The experimental results are compared with the predictions of an abrasion–ablation model of nuclear fragmentation yielding good overall agreement. Weak evidence for first observation of the new isotopes 208Th and 211Pa is presented. [Copyright &y& Elsevier]

Abstract: Fission fragments from GeV 238U ions impinging a hydrogen target are investigated by using the fragment separator FRS for magnetic selection of reaction products including ray-tracing, and ΔE-ToF techniques. On the basis of the kinematics of the fragments, binary fission can be assigned as production process up to very heavy fragments. Fission fragments are separated and identified as far as 184Re for the first time. Cross sections are measured and found in the range of 500 μb to 5 μb. Isobaric cross sections decrease smoothly with increasing masses. Beyond tungsten, fission fragments are hidden in the more abundant spallation evaporation residues. 283 isotopes of elements gadolinium to rhenium in the mass range 147 to 184 are observed and cross sections measured. All the fragments are formed by fission of excited parent nuclei following emission of a cascade of secondary neutrons. [Copyright &y& Elsevier]