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.

M. Jung, F. Bosch, K. Beckert, H. Eickhoff, H. Folger, B. Franzke, A. Gruber, P. Kienle, O. Klepper, W. Koenig, C. Kozhuharov, R. Mann, R. Moshamnler, F. Nolden, U. Schaaf, G. Soff, P. Spadtke, M. Steck, Th. Stohlker, and K. Summerer Gesellschaft fur Schwerionenforschung (GSl), D 6100 -Darmstadt, Germany (Received 20 July 1992) Bound-state P decay was observed for the first time by storing bare 'QDy + ions in a heavy-ion storage ring. From the number of 'QHo66+ daughter ions, measured as a function of the storage time, a half-life of 47-+4 d was derived. By comparing this result with reported half-lives for electron capture (EC) from the Ml and M2 shells of neutral '[jHo, bounds for both the QEc value of neutral ')/Ho and for the electron-neutrino mass were set.

The feasibility of the spectroscopy of dynamically ionized electrons (positrons) from heavy-ion collisions at intermediate energies, e.g. Pb+Pb at 60 A·MeV has been studied. We propose a magnetic toroid spectrometer for lepton spectroscopy in an energy range between 5 and 50 MeV. Special emphasis was laid on large solid angles, on broad-band characteristics and on a good suppression of secondary events. The device is a versatile compact-size instrument for lepton detection in in-beam experiments at a moderate energy resolution of ΔE/E ≈ 4%.

A brief introductory survey of γ-ray detector arrays and in-beam electron spectrometers developed during the last three lustra is followed by a broad discussion of the general requirements for single and multiple in-beam e-γ spectrometers. A detailed analysis is made of a few important tools for reducing the low-energy electron background (i.e. simultaneous E-Bρ selection, true and random coincidence rates, EM static fields around the target region). This survey and discussion are illustrated by the results of several in-beam e-γ measurements dealing with nuclear spectroscopic investigations of a few medium and heavy nuclei.

A compact detection system for heavy ions scattered in collisions at the Coulomb barrier is presented. This system, consisting of four identical, low-pressure Parallel Plate Avalanche Counter (PPAC) modules with two sensitive layers each, was built to operate in an ultra-high-vacuum environment inside the EPoS II solenoid spectrometer at GSI Darmstadt. The detector covers polar angles between 20° and 70° with respect to the beam axis, and about 80% of 2π in azimuthal angle. Segmented cathodes and a delay-line read-out allow for a determination of both angles with a precision of δΘ≈0.7° in polar and δΦ≈1.5° in azimuthal angle, respectively. The system has been proven to be capable of handling instantaneous rates of up to 5×105 detected ions per second per module. It neither exhibits the degradation of detection efficiency nor loss in resolution over a 500 h period of a 6 MeV/u238U+181Ta measurement at average luminosities of 8×1027cm−2s−1.

A brief introductory survey of γ-ray detector arrays and in-beam electron spectrometers developed during the last three lustra is followed by a broad discussion of the general requirements for single and multiple in-beam e-γ spectrometers. A detailed analysis is made of a few important tools for reducing the low-energy electron background (i.e. simultaneous E-Bρ selection, true and random coincidence rates, EM static fields around the target region). This survey and discussion are illustrated by the results of several in-beam e-γ measurements dealing with nuclear spectroscopic investigations of a few medium and heavy nuclei.

The progressively improved GSI accelerators provide beams of heavy ions from energies of 0.05-2000 A.MeV at high particle intensities now. Therefore, a wide variety of common and new heavy-ion target techniques had to be installed and developed during the past 25 years to prepare and characterize self-supported or backed heavy-ion-targets of chemical elements and compounds from hydrogen las polyethylene) to uranium. The thickness ranged from 2 x 10(-6) to 20 g/cm(2) for beam spots of about 5 mm in diameter. Homogeneity, surface structure or individual shape had to be adapted to the needs of each experiment. Special setups were required for targets of poisonous materials, of highly enriched stable isotopes or those of radioactive species in minute amounts. The capability of thin-layer technologies was as well applied to prepare and measure stripper foils or various high-vacuum deposits for experimental or accelerator purposes.The development of different rotating target wheels and control mechanisms was enforced by the continuous change of target qualities during high-intensity heavy-ion bombardments of up to 10(13) particles/s at Coulomb energies. The mass production of complete target systems, especially for the experiments EPOS, ORANGE and SHIP, required conditional improvement of target relevant parameters. For relativistic energies up to 2000 A.MeV at the FRS, an automated water-cooled assembly with 75 positions has been developed and installed as production target.The brief historical review can touch only aspects of targetry at GSI. A few typical examples are included. (C) 1999 Elsevier Science B.V. All rights reserved.

A few accelerated ion beam requirements for in-beam e-gamma spectroscopy are briefly reviewed as well as several features of the MP Tandem accelerator of IPN-Orsay and the accelerated ion-beam transport devices leading to the experimental area of in-beam e-gamma spectroscopy. In particular, the main capabilities of the ion-sources, the ion pulsing system, the ion stripping and stabilizing devices as well as the versatility of the ion beam transport system are discussed from the point of view of the different kinds of in-beam e-gamma experiments performed in that area. (C) 1998 Elsevier Science B.V. All rights reserved.

The main features of nuclear targets, recoil ion catchers and reaction chambers used in nuclear spectroscopic investigations involving in-beam multi-e-gamma spectrometers are discussed. The relative importance of the F-ray background due to the accelerated ion-target and the recoil-ion-target interaction is estimated. Its impact on the prompt low-energy electron measurements is stressed. Finally a few general features of the interplay between accelerated ion beams, targets and recoil ion catchers particularly relevant for these measurements are broadly discussed and illustrated with typical examples of in-beam e-gamma studies. (C) 1998 Elsevier Science B.V. All rights reserved.

Tungsten layers of 1–6 mg/cm2 with diameters of 10 mm are needed as heavy-ion targets and as flat or domed windows for in-beam ion sources. They are prepared by a high-vacuum evaporation-deposition process carried out in a computer-controlled apparatus. The tungsten, placed in the water-cooled crucible of an electron-beam gun, is gradually heated and evaporates at about its melting point (Tm = 3643 K). The tungsten is deposited onto 9 mg/cm2 copper backings which are preheated by the thermal radiation from the evaporant. The deposition rate is controlled by a quartz crystal monitor. After the high-vacuum deposition process, the copper backings are removed by selective etching and the self-supported tungsten foils are characterized by their areal mass. The features of the commercially available evaporation unit used are discussed by way of this deposition process.

Spontaneous fission and α-decay of253−257104 and 258106 were investigated in irradiations of 204,206,208Pb with 50Ti and 209Bi with 51V, respectively. New spontaneous fission activities were identified and assigned to 253104, 254104, and 258106. The half-lives were measured as T1/2 = (48 −10 +14 ) μs for 253104, T1/2 = (23 ± 3) μs for 254104, and T1/2 = (2.9 −0.7 +1.3 ) ms for 258106. 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 253104, bα ≤ 0.015 for 254104, and bα ≤0.2 for 258106 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 257104 and 255104 have been improved. An isomeric state decaying by α-emission was identified in 257104 and attributed to a low lying 11/2- [725] state. A small α-decay branch for the even — even nucleus 256104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even — even nuclei up to 264Hs (Z=108), the heaviest even — even nucleus identified so far.

The cross sections are measured for the evaporation channel of the decay of the compound nucleus Pa-221 formed in the complete-fusion reactions involving Mg-24 and V-51 ions. The measurements are performed for excitation energies ranging between 40 and 80 MeV. Throughout the excitation-energy range being investigated, these cross sections are found to be much smaller in the reaction with V-51 ions than in the reactions with Mg-24 ions. This result is at odds with the generally accepted concepts presuming that complete fusion is the dominant process in the evolution of a dinuclear system formed at small impact parameters and at projectile energies exceeding the fusion barrier.

The new element 112 was produced and identified unambiguously in an experiment at SHIP, GSI Darmstadt. Two decay chains of the isotope277112 were observed in irradiations of208Pb targets with70Zn projectiles of 344 MeV kinetic energy. The isotope decays by emission of α particles with a half-life of (240 −90 +430 )µs. Two different α energies of (11,649±20) keV and (11,454±20) keV were measured for the two observed decays. The cross-section measured in three weeks of irradiations is (1.0 −0.4 +1.8 ) pb.

In inelastic scattering of 22 MeV deuterons from an isotope-separated target of $^{178}\mathrm{Hf}$ containing about 2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ nuclei in the isomeric ${16}^{+}$ state we observed rotational excitation to the ${17}^{+}$ state at an excitation energy with respect to the isomeric state of 356.5\ifmmode\pm\else\textpm\fi{}0.4 keV and weak evidence for the ${18}^{+}$ state at 737\ifmmode\pm\else\textpm\fi{}2 keV. We compared the differential cross sections with coupled-channel calculations and with scattering from $^{178}\mathrm{Hf}$ (${0}^{+}$) and $^{177}\mathrm{Hf}$ (7/${2}^{\mathrm{\ensuremath{-}}}$). \textcopyright{} 1996 The American Physical Society.

The investigation of e+e− pairs emitted in heavy-ion collisions at the Coulomb barrier has been continued at GSI Darmstadt with the redesigned spectrometer EPoS II. Due to its enlarged efficiency for the detection of e+e− pairs the reproducibility of the narrow line structures previously observed in the e+e− sum-energy spectra with the EPOS I spectrometer could be tested using a highly improved statistical database. No lines have been observed with the new data sets when applying the same selection criteria as in the old data. Our measurements give upper limits for the cross-sections of these lines, which are a factor of up to 10 smaller than the cross-sections deduced from the EPOS I data.

The E1 e+e− decay of the 17.2 MeV level in 12C, and the M1 e+e− decay of the 17.6 MeV level in 8Be have been studied in a search for possible signals of short-lived neutral bosons with masses between 5 and 15 MeV/c2. Whereas for the E1 decay at large correlation angles no deviation is found from internal pair conversion (IPC), surprisingly the M1 angular correlation deviates from IPC at the 4.5σ level.

The four new neutron deficient isotopes246Md (T1/2=(1.0±0.4) s),245Md (T1/2=(0.35 −0.16 +0.23 ) s),242Es (T1/2=(16 −4 +6 ) s) and241Es(T1/2=(8 −4 +6 ) s) were produced in fusion reactions of40Ar ions with209Bi targets at bombarding energies of E p =4.78, 4.93 and 5.12 A*MeV. The assignment was based on evaporation residue-α-α time and position correlations. A spontaneous fission activity of T1/2=(0.90±0.25) ms was observed at a projectile energy of E p =5.12 A*MeV and assigned to245Md.

In inelastic scattering of 22 MeV deuterons from an isotope-separated target of Hf-178 containing about 2x10(13) nuclei in the isomeric 16(+) state we observed rotational excitation to the 17(+) state at an excitation energy with respect to the isomeric state of 356.5+/-0.4 keV and weak evidence for the 18(+) state at 737+/-2 keV. We compared the differential cross sections with coupled-channel calculations and with scattering from Hf-178(0(+)) and Hf-177(7/2(-)).