Yu. M. Sereda

WIDE APERTURE KINEMATIC SEPARATOR COMBAS REALIZED ON THE STRONG FOCUSING PRINCIPLE

Abstract The COMBAS large solid angle and high momentum acceptance and high-resolving kinematic separator has been created at the Flerov Laboratory of Nuclear Reactions, JINR, to efficiently collect extremely short-lived nuclei near the zero angle which are produced in intermediate energy fragmentation reactions with wide momentum and wide angular distributions. For the first time the M 1 M 2 M 3 M 4 F d M 5 M 6 M 7 M 8 F a magneto-optical configuration of the COMBAS separator has been realized on the strong focussing principle. The separation and trajectory analysis of particles by the separator are carried out by three parameters: the magnetic rigidity (B· ρ ), the energy loss difference in the degrader (Δ E /Δ x ) and the time-of-flight (Δ T ) of the analyzed particles. The COMBAS separator can be used efficiently both in the mode of a high- resolving spectrometer to study reaction mechanisms and in the mode of an in-flight separator in experiments on the synthesis and study of properties of short-lived exotic nuclei near the drip-lines.

3D magnetic measurements of the combined function magnets in separator COMBAS

Abstract The high-resolving wide aperture separator COMBAS has been designed and commissioned at the FLNR, JINR. Three-component magnetic measurements of all the magnets were performed. The measured data allow reconstruction of the 3D-distributions of the fields in all the magnets. 3D-maps are needed for particle trajectory simulations throughout the entire separator. The magnetic fields of analysing magnets, M 1 , M 2 , M 7 , and M 8 , contain quadrupole components of alternating sign that provide necessary beam focusing. All the magnets M 1 –M 8 , contain sextupole and octupole field components, which minimizes the second and third order aberrations. All this allowed one to increase their apertures, to form effectively a beam of the required sizes and to decrease the channel length.

Forward-angle yields of 2≤Z≤11 isotopes in the reaction of 18O(35A MeV) with 9Be

A systematic investigation of the forward-angle inclusive yields of 2≤Z≤11 isotopes produced in collisions of 18O projectile nuclei with a 9Be target in the Fermi energy region (35A MeV) is performed. The measurements were based on the use of the COMBAS double achromatic kinematical separator in the spectrometry mode at the Flerov Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research, FLNR (JINR, Dubna). The velocity, isotopic, and element distributions are presented. There is no unique mechanism that would explain the total set of results obtained in this experiment. A dominant role of low-energy reaction mechanisms is observed. The intensity of secondary beams of halolike nuclei 11Li, 12Be, and 14Be is determined.

Dissipative processes in peripheral heavy ion collisions at Fermi energies

Peripheral heavy ion reactions are of interest for the production of new isotopes. In this contribution we present an investigation of reactions of light projectiles O and Ne on Be and Ta targets at Fermi energies in the framework of transport theory. Transport theory describes dissipative (deep-inelastic) processes, where considerable amounts of mass and energy are exchanged. The data, on the other hand, also seem to include a more direct component with small energy loss. We separate the two components on the basis of the velocity distribution and compare the dissipative component to the transport calculations. The primary fragments of the transport calculation still have considerable excitation energies. For the comparison with experiment we take into account the secondary evaporation in a statistical model. This improves the qualitative agreement with the data.

Total reaction cross sections measurement for 6,8He and 8,9Li nuclei with energies of (25–45)/A MeV on natAl and natPb

Preliminary results of measurements of the total reaction cross sections σR for weakly-bound 6,8He and 8,9Li nuclei at energy range (25–45)/A MeV on 27Al and 208Pb targets are presented. The secondary beams of 6,8He and 8,9Li were produced by bombardment of the 11B (33 A MeV) primary beam on Be (89 mg cm–2) target and separated by COMBAS fragment-separator. In dispersive focal plane a horizontal slit defined the momentum acceptance as 1% and a wedge degrader of 200 μm Al was installed. The Bρ of the second section of the fragment-separator was adjusted for measurements in energy range (25–45)/A MeV. The secondary products were detected by a telescope consisting of two Si ΔE detectors 300, 1000 μm and E-detector, which consisted of nine CsI/Tl granules.

Projectile fragmentation at Fermi energies with transport simulations

Projectile fragmentation at Fermi energies is an important method to produce radioactive beams for the study of isospin asymmetric nuclear matter. Fragmentation is usually parametrized successfully by empirical phase space models. In this contribution we apply a microscopical method, semiclassical transport theory, to study in detail the reaction mechanism of the fragmentation process. We apply it to experimental data of 18O on 181Ta at E/A = 35 MeV measured in Dubna. We calculate consistently the excitation energy of the primary fragments and take into account their decay by a statistical model. It is found that the dissipative part of the fragment spectra is well described by transport theory. However, there are in addition important direct and collective contributions.

Total reaction cross sections and neutron-removal cross sections of neutron-rich light nuclei measured by the COMBAS fragment-separator

Preliminary results of measurements of the total reaction cross sections σR and neutron removal cross section σ-xn for weakly bound 6He, 8Li, 9Be and 10Be nuclei at energy range (20-35) A MeV with 28Si target is presented. The secondary beams of light nuclei were produced by bombardment of the 22Ne (35 A MeV) primary beam on Be target and separated by COMBAS fragment-separator. In dispersive focal plane a horizontal slit defined the momentum acceptance as 1% and a wedge degrader of 200 μm Al was installed. The Bρ of the second section of the fragment-separator was adjusted for measurements in energy range (20-35) A MeV. Two-neutron removal cross sections for 6He and 10Be and one –neutron removal cross sections 8Li and 9Be were measured.

Radioactive nuclear beams of COMBAS facility

The basic ion-optical characteristics of the luminosity and the high-resolution of kinematic separator COMBAS realized for the first time on the strong focusing principle are presented. The developed facility allows to separate the high-intensity secondary radioactive beams in a wide range of mass numbers A and atomic numbers Z which are produced in heavy ion reactions in the energy range of 20 ≤ E ≤ 100 MeV/A (Fermi energy domain). Two distinct detector systems such as realized Si strip detector telescope and the promising development of the three dimension time-projection chamber are discussed. Program of the investigations of nuclear reaction mechanisms at intermediate energies of 20–100 MeV/A, measurement of the radii of unstable nuclei, study of the cluster structure of light nuclei near the nuclear drip-line and search of 26,28O resonances in exchange reactions is proposed. The upgrading of experimental facility by the integration of COMBAS separator with the Ion Catcher is discussed.

A secondary beam separator: A combination of the COMBAS fragment separator with the ion catcher

The development of an experimental facility based on the high-luminosity COMBAS fragment separator and a fast ion catcher is discussed. The main characteristics of the COMBAS fragment separator and the ion catcher determining the advantages of the proposed combination are presented. The developed facility is expected to allow production of secondary radioactive beams with a quality higher than the quality of beams obtained using the in-flight separation technique. It is planned that the facility will be used in a tandem with a post-accelerator for producing single-isotope and monochromatic high-intensity secondary radioactive beams in a wide range of mass numbers A and atomic numbers Z. A list of perspective scientific problems requiring high-intensity and high-quality secondary beams of exotic nuclei is proposed.

Transport description of dissipative heavy ion collisions at Fermi energies

Peripheral collisions of heavy ions at Fermi energies are a powerful tool for producing new isotopes far from stability line. The mechanism of these reactions has features between direct and dissipative reactions, as is seen from the velocity distributions of the projectile-like fragments, which are peaked near the beam velocity and have a long tail to the smaller velocities. In this report we treat these heavy ion collisions in a Boltzmannlike transport approach, which describes the evolution of the phase space distribution function under the influence of a mean field and a two-body collision term. It is used to calculate the primary fragments after the collision phase, which are, however, still highly excited. The de-excitation of the fragments is calculated in a statistical model. We calculate isotope distributions and velocity spectra for the reaction of 40Ar as a projectile on targets of 9Be and 181Ta at E/A = 57 MeV and find reasonably good agreement with experimental data.