Velocity Filter SHIP Research Articles

Study of evaporation residues produced in reactions of207,208Pb with50Ti

Evaporation residues produced in heavy-ion fusion-reactions of50Ti and207Pb,208pb have been investigated. They were separated from the projectile beam by the velocity filter SHIP and identified after implantation into position-sensitive surface-barrier detectors, where their α decay or their decay by spontaneous fission was measured. The production of weakly excited compound nuclei in reactions of projectiles and targets close to the doubly magic48Ca and208Pb was confirmed by the observation of the 1n deexcitation channel in the reaction50Ti +208Pb.

Fusion-fission and neutron-evaporation-residue cross-sections in 40Ar- and 50Ti-induced fusion reactions

For the reactions 40Ar+ 165Ho. 169Tm, 174Yb. 115Lu, 176–180Hf, 181Ta, 208Pb and 50Ti + 208Pb, 209Bi the cross sections for the fusion-fission process were determined by measuring energy and time-of-flight of the reaction products. In addition, the neutron-evaporation-residue cross sections were measured by using the velocity filter SHIP. A σ versus 1/E analysis of the fusion-fission cross sections is used to determine fusion barriers and fusion radii. The evaporation- residue cross sections are used to extract in an approximate way barriers for compound-nucleus formation. These barriers are found to agree with the fusion barriers determined from the fission cross sections. For all systems investigated the neutron-evaporation-residue cross sections reach their maximum close to the fusion barrier as calculated from the Bass potential.

Calculation of a dipole magnet for the velocity filter ship and computer simulation of fusion experiments

Abstract At the velocity filter SHIP most of the observed evaporation residues are identified, at present, by alpha spectroscopic methods. The possibility of a mass separation of unretarded fusion products via a deflection magnet coupled to SHIP in such a way that focusing in angle and velocity is achieved for a certain mass over charge ratio, is discussed. The calculations were carried out with the ion-optical computer program GIOS. In combination with a Monte Carlo program to calculate the characteristics of the evaporation residues after leaving the target, it is possible to create realistic particle spectra in the focal plane of the separator. The improvement of mass resolution by measuring the time-of-flight and the magnetic beam rigidity of the reaction products is also discussed. A computer program which simulates ion trajectories through gas-filled systems allows the study of mass resolution of a gas-filled dipole after SHIP.

The velocity filter SHIP, performance and survey of current experiments

The velocity filter SHIP is used to investigate unslowed evaporation residues from heavy ion fusion reactions. The main subjects of investigation are α emitting and spontaneously fissioning nuclei very far from stability with halflives as short as microseconds and formation cross-section down to the nanobarn region. The efficiency of the filter is calculated by a Monte Carlo method taking into account momentum transfer from nucleons and α particles evaporated from the excited compound system as well as energy loss, scattering in the target, and the ionic charge distribution. The calculations are compared with experimental results. For the irradiation of low melting point targets with high beam currents a rotating target wheel has been developed. Light evaporation residues are identified by an ionisation chamber. α emitting and spontaneously fissioning nuclei are investigated by implantation into surface barrier detectors. The high background rejection and sensitivity of detection are demonstrated for the case of the identification of Fm and element 104.

A high voltage deflection condenser operated under high vacuum conditions

Two electrostatic deflection condensers with plane electrodes of 50×60 cm 2 surface area are operated under high vacuum conditions up to voltages of 620 kV across a gap of 15 cm. The condensers are operated in parallel by common power supplies. They are installed at the velocity filter SHIP at GSI Darmstadt.

The efficiency of the velocity filter ship for unslowed evaporation residues

The efficiency of the velocity filter SHIP is calculated for unshowed reaction products from heavy ion fusion reactions. The ionic charge distribution of the reaction products due to stripping of the outer electrons, the angular and velocity distributions due to neutron, proton, and/or α-particle emission of the excited nucleus as well as by scattering and energy loss in the target are considered. For the ion optical calculations phase-space transformation as well as the Monte Carlo method are used. The results of the calculations are compared with experimental results.

The velocity filter ship, a separator of unslowed heavy ion fusion products

The velocity filter SHIP installed at the UNILAC (GSI Darmstadt) is described. It is used for the investigation of neutron deficient nuclei generated by heavy ion induced fusion. The two stage filter separates unretarded reaction products emerging from a thin target with high efficiency and low background of primary beam particles. For reaction products covering an angular range of 3° in horizontal and vertical directions and a velocity range of 10% the transmission is between 30% and 60%. The separation time is of the order of 10 −6s. High suppression of primary particles by factors of 10 7-10 11 permits the investigation of reaction products in the direction of the primary beam. α-active reaction products have been implanted into a surface barrier detector and identified by their decay. A description of the ion optical principles, technical details and the performance of SHIP is given.

Alpha decay properties of new protactinium isotopes

Evaporation residues produced in the reactions 40Ar + 181Ta and 40Ar + 184W were separated from the primary beam by the velocity filter SHIP and detected by a Δ E-E counter telescope. The technique of delayed coincidences was applied to individually identify the reaction products implanted into a Si surface barrier detector by their subsequent α-decays. The previously unknown nuclei 215Pa and 218Pa were identified by their known daughter decays. 215Pa was found to decay with E α = 8.085±0.015MeV and t 1/2 = 14± 2 3ms. For 218Pa two α-lines were found at 9.535±0.015MeV and 9.614±0.020MeV with a corresponding half-life of 120 +40 -20μs. The half-life of the 8.33 MeV α-decay of 217Pa. was determined to be 4.9 +0.6 -0.4 ms. Furthermore a new isomer was found in 217Pa which decays with E = 10.16±0.02MeV and t 1/2 = l.6 +1.0 -0.5ms. The decay of this isomer is tentatively interpreted as being hindered by an angular momentum of about 10ħ of the emitted α-particle. The ground state α-decay energies of these very proton-rich isotopes agree well with systematics but deviate considerably from theoretical mass predictions.

Alpha decay studies of very neutron deficient isotopes of Hf, Ta, W, and Re

Very neutron deficient isotopes in the Hf-Re region were produced by bombarding targets of107, 109Ag,nat, 108, 110Pd, and103Rh with58Ni accelerated by the linear accelerator UNILAC at GSI. After separation from the projectile beam by the velocity filter SHIP, the fusion products were implanted with their full recoil energy into a silicon surface barrier detector. The subsequent alpha decay of the implanted ions was measured with the same detector. In some of the experiments the evaporation residues were implanted into a position sensitive silicon surface barrier detector. With a newly developed position and time correlation technique parent daughter relationships, half lives, andα branching ratios of a large number of isotopes could be determined. In the investigated reactions the eleven new isotopes161–164Re,160W,157–161Ta, and156Hf could be identified. The measuredQ α values, half lives, and a branching ratios are discussed. Two new high energyα. transitions of (7,408±10) keV and (7,804±15) keV were also found with half lives of 2.7 ms and 0.52 ms, respectively.A tentative mass and atomic number assignment givesA=153–156 andZ=70–72. The energies of the excited isomeric states are between 2 MeV and 3 MeV. From the systematics of alpha ground state transitions in even nuclei a hindrance of 105 can be deduced for both transitions, possibly indicating orbital angular momenta on the order of 10ħ for the alpha emitting states. The isomers are proposed to belong to shell model isomers beyond146Gd.

Dependence of the evaporation residue production on the excitation energy of the compound nucleus in the reaction40Ar+164Dy

With the velocity filter SHIP at the heavy ion accelerator UNILAC the energy dependence of the evaporation residues production in the reaction40Ar+164Dy was investigated. The results show a much smaller shift of the (204Po, xn) excitation functions with respect to theoretical predictions than has been reported by other authors.