Spontaneous fission and α-decay of 253−257 104 and 258 106 were investigated in irradiations of 204,206,208 Pb with 50 Ti and 209 Bi with 51 V, respectively. New spontaneous fission activities were identified and assigned to 253 104, 254 104, and 258 106. The half-lives were measured as T 1/2 = (48 −10 +14 ) μs for 253 104, T 1/2 = (23 ± 3) μs for 254 104, and T 1/2 = (2.9 −0.7 +1.3 ) ms for 258 106. No indication for α-decay of any of these isotopes was found. For the α-decay branching ratios bα limits corresponding to bα ≤ 0.1 for 253 104, bα ≤ 0.015 for 254 104, and bα ≤0.2 for 258 106 were obtained. These results prove a reduced fission probability for nuclei with neutron numbers N=152 up to Z=106 and a steep decrease of the fission half-lives for neutron numbers N < 152 up to element 104. α-decay data of 257 104 and 255 104 have been improved. An isomeric state decaying by α-emission was identified in 257 104 and attributed to a low lying 11/2- [725] state. A small α-decay branch for the even — even nucleus 256 104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even — even nuclei up to 264 Hs (Z=108), the heaviest even — even nucleus identified so far.

The history of the discovery of the six elements Z = 107 ∓ 112, bohrium, hassium, meitnerium, darmstadtium, roentgenium, and copernicium goes back to the early 1960s. An experimental method to separate and identify rare nuclear reaction products, the recoil separation, was developed and optimised for beams of fission products at European research reactors. Chemical elements beyond the then first transactinides (Z = 104), which owe their stability to the internal structure of atomic nuclei, were predicted theoretically. A big brother of the shell-stabilised nucleus 208Pb, a spherical magic nucleus at Z = 114∓126 and N = 184, might reach lifetimes long enough to be detected. In the seventies, hunting superheavy elements (SHE) was on the agenda of nuclear chemistry. Could the Periodic Table of Elements be extended to Z = 120, and is the order of electrons in the atom still following the laws established for lighter elements? In Germany, the heavy ion accelerator (UNILAC) was built by Christoph Schmelzer and his team at GSI, Darmstadt. SHE and UNILAC met the recoil separators in 1968, and SHIP (Separator for Heavy Ion reaction Products) was ready together with the first UNILAC-beams in 1976. Recoil separation is orders of magnitude more sensitive, selective, and faster than earlier methods used to synthesise elements up to seaborgium, Z = 106. The experimental paradigm we introduced opened the world of SHEs. At SHIP we discovered and investigated the elements Z = 107∓112 in the years 1980–2000. Our laboratory was the world champion during this time. Today our experimental method is used worldwide in the search for SHEs, but the leadership went to the Russian laboratory JINR in Dubna, which extended the Periodic Table by 6 more elements to Z = 118, the candidate for the next rare gas.

More than 600 residual nuclei, formed in the spallation of 136Xe projectiles impinging on deuterium at 500 AMeV of incident energy, have been unambiguously identified and their production cross sections have been determined with high accuracy. By comparing these data to others previously measured for the reactions 136Xe + p at 1 AGeV and 136Xe + p at 500 AMeV we investigated the role that neutrons play in peripheral collisions and to understand the energy dissipation in frontal collisions in spallation reactions.

More than six hundred nuclei produced in the fragmentation of Xe-136 projectiles at 500 A MeV on a liquid deuterium target were identified using inverse kinematics at the GSI Fragment Separator (FRS). These data are relevant for understanding of spallation reactions.

A research program on spallation reactions in inverse kinematics has been performed at GSI, Darmstadt, taking advantage of the relativistic heavy-ions beams available from GSI accelerators and the high-resolution magnetic spectrometer, used to identify the reactions products in-flight and to determine their kinematical properties.In this paper, we report the results obtained up to now on the spallation reaction 136 Xe on protons, focusing on 500 and 200 AMeV energies.

Isotopic production cross sections and momentum distributions of more than 1400 residual nuclei produced in the collision of 238 U(1AGeV)+ 2 H have been measured. The experiment was performed at GSI where we could take profit of the inverse kinematics technique and the high-resolution magnetic spectrometer FRS to identify in atomic and mass numbers all the produced nuclei. The identification technique challenges for the case of 238 Ua re reviewed. Some features of 238 U residues are discussed, including the access to information of fission dynamics.

The recent experiments at FLNR, Dubna, demonstrated that cross-sections to produce SHEs by 48 Ca-induced reactions on actinide targets increase beyond Z = 111, reach a maximum of 5 pb at Z = 114 and fall below the 1 pb level at Z = 118. A scenario is proposed to understand the findings within the frame of former experimental results of heavy-element production and theoretical predictions about the stability of the nuclides concerned. New ingredients introduced are: 1) to shift the next proton shell beyond Pb from Z = 114 to Z = 122; 2) the isotopes of the elements Z = 112 to Z = 118 are deformed and their nuclei have oblate shapes; 3) the fission barriers around the next nucleus with doubly closed shells 306 184 122 are larger than the neutron separation energies and reach values in the range of 10MeV. The ascent of the flat top at 306 184 122 is described by the proposed scenario, which likewise excludes reaching the doubly closed shell region at the top by today’s experimental methods.

Fission fragments of 1 GeV/nucleon U-238 nuclei interacting with a deuterium target have been investigated with the Fragment Separator at GSI (Darmstadt) by measuring the isotopic production cross sections and fragment velocities. The combination of these two observables enabled to disentangle the high- and low-energy fission modes. Specific properties of these two modes were analyzed; in particular, the charge distribution of the fissioning projectile-like prefragments could be determined for the first time.

Complete fusion reactions (xn-channels) using actinide targets are observed for values of the effective fissilities x eff ∼ 0.80 in the sub-pb range of production cross sections. The elements produced at this limit are Z = 108–112. Beyond complete fusion, heavier elements might still be produced by reaction mechanisms releasing part of the nuclear charge before an equilibrated compound system might have been reached. Precompound Charged particle Emission (PCE) is proposed as a possible mechanism following complete fusion. A scheme delivering isotopes of elements Z = 110–115 is discussed, and experimental evidence for such a process is presented. Compound systems, the atomic numbers of which are smaller than in complete fusion reactions, might be produced in 48Ca induced reactions on actinides with larger cross sections than those at the limits of complete fusion. Besides complete fusion, the PCE-mechanism should be considered as an alternative to interpret the 48Ca-induced reactions on actinides.

Fission fragments of 1A GeV{sup 238}U nuclei interacting with a deuterium target have been investigated with the Fragment Separator (FRS) at Gesellschaft fuer Schwerionenforschung (GSI) by measuring their isotopic production cross sections and velocities. Results, along with those obtained recently for spallation-evaporation fragments, provide a comprehensive analysis of the spallation nuclear productions in this reaction. Details about the experimental performance, data reduction and results are presented.

We present an extensive overview of production cross sections and kinetic energies for the complete set of nuclides formed in the spallation of Xe-136 by protons at the incident energy of 1 GeV per nucleon. The measurement was performed in inverse kinematics at the GSI fragment separator. Slightly below the Businaro-Gallone point, Xe-136 is the stable nuclide with the largest neutron excess. The kinematic data and cross sections collected in this work for the full nuclide production are a general benchmark for modeling the spallation process in a neutron-rich nuclear system, where fission is characterized by predominantly mass-asymmetric splits.

The spallation residues produced in the bombardment of 56}Fe at 1.5, 1.0, 0.75, 0.5 and 0.3 A GeV on a liquid-hydrogen target have been measured using the reverse kinematics technique and the Fragment Separator at GSI (Darmstadt). This technique has permitted the full identification in charge and mass of all isotopes produced with cross-sections larger than 10^{-2} mb down to Z=8. Their individual production cross-sections and recoil velocities at the five energies are presented. Production cross-sections are compared to previously existing data and to empirical parametric formulas, often used in cosmic-ray astrophysics. The experimental data are also extensively compared to different combinations of intra-nuclear cascade and de-excitation models. It is shown that the yields of the lightest isotopes cannot be accounted for by standard evaporation models. The GEMINI model, which includes an asymmetric fission decay mode, gives an overall good agreement with the data. These experimental data can be directly used for the estimation of composition modifications and damages in materials containing iron in spallation sources. They are also useful for improving high precision cosmic-ray measurements.

The production of light and intermediate-mass nuclides formed in the reaction 1H+238U at 1 GeV was measured at the Fragment Separator (FRS) at GSI, Darmstadt. The experiment was performed in inverse kinematics, shooting a 1 A GeV 238U beam on a thin liquid-hydrogen target. 254 isotopes of all elements in the range from Z=7 to Z=37 were unambiguously identified, and the velocity distributions of the produced nuclides were determined with high precision. The results show that the nuclides are produced in a very asymmetric binary decay of heavy nuclei originating from the spallation of uranium. All the features of the produced nuclides merge with the characteristics of the fission products as their mass increases.

Isotopic production cross sections and momentum distributions of 602 residual nuclei produced in the collision of $^{238}\mathrm{U}$($1A$ GeV) with deuterium have been measured. These data are relevant for a better understanding of spallation reactions for use as neutron sources for accelerator-driven systems or to produce radioactive nuclear beams. Access to primary residue production makes it possible to study the main reaction mechanisms involved: intranuclear cascade, particle evaporation, and fission. The characteristics of the reaction investigated and the high fissility of the $^{238}\mathrm{U}$ and the dinucleon projectile system are discussed and compared with other available experimental data.

The production cross sections of fragmentation–evaporation residues in the reaction Pb+p at 500AMeV have been measured using the inverse-kinematics method and the FRS spectrometer (GSI). Fragments were identified in nuclear charge using ionisation chambers. The mass identification was performed event-by-event using the Bρ–TOF–ΔE technique. Although partially-unresolved ionic charge states induced an ambiguity on the mass of some heavy fragments, production rates could be obtained with a high accuracy by systematically accounting for the polluting ionic charge states. The contribution of multiple reactions in the target was subtracted using a new, partly self-consistent code. The isobaric distributions are found to have a shape very close to the one observed in experiments at higher energy. Kinematic properties of the fragments were also measured. The total and the isotopic cross sections, including charge-pickup cross sections, are in good agreement with previous measurements and models. The data are discussed in the light of previous spallation measurements, especially on lead at 1 GeV.

The accurate prediction of formation cross sections of isotopes produced in spallation and fission reactions at intermediate energies is of extreme importance for the design of ADS and radioactive-ion-beam facilities. The predictive power of the computational tools is determined by the description of the reaction mechanisms involved. The physics involved in such reactions is still a subject of research, and precise experimental data are needed to test the reliability of theoretical models [1]. Residual nuclei from a U beam at 1⋅A GeV on Be [2] and Pb [3] have already been produced in inverse kinematics at the FRS [3]. In this report, some preliminary results on reactions of proton and deuteron with U at 1⋅A GeV are described. U has been chosen as a reference material to investigate spallation and fission of fissile material at intermediate energy. Some attractive peculiarities of the in-flight separation are that fragments can be measured before they decay, the whole isotropic distribution can be obtained for every element, the velocity of the identified isotope can be precisely evaluated and used to deduce the reaction mechanism which generated that isotope (in this way, fission and fragmentation events can be disentangled). Although data have been taken for most of the produced residual nuclei from Z=6 to Z=93, only the formation cross sections and velocity distributions in fragmentation reactions and the velocity features of very light masses produced in strongly asymmetric fission (6<Z<20) have been analysed up to now. In figure 1, the isotopic distributions of Yb (Z=70), Hg (Z=80), At (Z=85), U (Z=92), generated in the fragmentation of 1A GeV U in H2 and D2 targets are represented. Due to the heavier mass of the target nucleus, in the case of deuterium the higher energy transferred to the U nucleus leads to a longer evaporation chain and thus to a higher production of lighter residual masses. The drastic discrepancies between the data and a recent empirical systematics [4], which represents the actual knowledge on this field, demonstrates the important progress achieved by these new results. Concerning the kinematics, the velocity of the fragments is lower than that of the beam due to momentum transfer in the collision. The surviving nucleus can then reach a stable configuration by an evaporation process or by fissioning. If the surviving nucleus fissions, the two fragments will run apart in opposite directions in the frame of the fissioning nucleus; the centre-ofmass velocity of the two ions must be equal to the velocity of the compound nucleus from which they were generated, thus slightly negative. This can be clearly observed in figure 2. There, the velocity of light elements produced in fission reactions is presented in the beam frame. The picture must be observed keeping in mind that, due to the limited angular acceptance of the spectrometer, fragmentation products can be almost entirely transmitted whilst only backward and forward emitted fission fragments succeed to pass through. Counts collected in the picture include a contribution from the titanium target windows. From data taken with a thin titanium target we deduced that the nuclei with the extreme velocity values are mostly due to the interaction of uranium with protons, while the central part is mostly due to the interaction of uranium with titanium. The velocities of very asymmetric fission fragments originated from the dichotomy of compound nuclei with masses around 180 to 240, estimated with a theoretical formula [3], perfectly overlap to the counts which fill the external wings of the distribution reported in fig. 2. Moreover, as already noted, the average value of the velocity of these two wings is consistent with the slightly negative value of the surviving nuclei observed in the fragmentation process. This allows us to interpret the counts in the two wings as very asymmetric fission products. So, neutron-rich light masses (like sodium), whose formation was already observed in direct kinematics in interactions of high-energetic protons with U [5] [6], are produced in very asymmetric fission processes. The high value of the A/Z-ratio of formed isotopes is another sign that fission is the responsible production mechanism, since fragmentation generally produces isotopes on the neutron-deficient side of the betastability valley.