Resonance ionization spectroscopy on Yb and No at SHIP

For the investigation of the completely unknown atomic level structure of the element nobelium a novel experimental procedure has been developed. It is based on Radiation Detected Resonance Ionization Spectroscopy (RADRIS) which had to be improved and modified to accommodate the prerequisites at SHIP. It can be applied to elements like nobelium produced at rates of a few ions per second. The ionized fraction of the fusion product beam from SHIP, is stopped in an argon buffer gas cell, collected onto a tantalum catcher filament from which the radioactive species are re-evaporated as atoms. Then the atoms are resonantly ionized with laser beams and the ions subsequently transported by electrical fields to a semiconductor PIPS detector which detects the resonance ionization by the α decay of the fusion product. This method yields significant efficiency gains compared with earlier attempts to perform resonance ionization directly on the neutral part of the stopped fusion products. Test experiments were performed with the isotope Yb, which is supposed to have an atomic level structure similar to nobelium. It was produced by the fusion reaction Ag(Cr,p3n)Yb. For the on-line resonance ionization of Yb a periodic measurement sequence was applied. During an accumulation period the incoming fusion product ions are guided to a catcher filament by suitable electrical fields. After the beam is turned off the electrical fields are changed so that ions created in the buffer gas close to the filament are transported to an α detector. Then the catcher filament is heated by a short current pulse. The evaporated atoms are ionized by laser beams which are activated at the same time. The laser ions are transported to the detector where their α-energy spectrum is recorded. The resonance ionization of Yb was performed with a two-step excitation scheme (λ1 = 398.9 nm, λ2 = 399.6 nm). Laser scans were performed for both excitation steps. For this, α-energy spectra were recorded for different laser frequencies. Higher α activities were detected when the laser frequency was in resonance with the atomic level. Thus, resonance enhancement factors between 20 and 30 were observed, see Fig. 1. This result demonstrates that the detection of atomic levels in fusion products from SHIP with the RADRIS method is possible. A total RIS efficiency of ǫ RIS = 0.8 % has been determined under consideration of all experimental parameters [1]. After successful completion of this test experiment, the search for atomic levels in nobelium has been started in a recent beam time last summer. We sought for the 5f7s7p 1P1 term which was predicted by a MultiFigure 1: Atomic resonance lines of a two-step excitation scheme in the form of Yb α decay rate.