Oliver Forstner

Measurement of the stellar cross sections for the reactions 9Be(n,?)10Be and 13C(n,?)14C via AMS

We started a series of experiments, aiming at obtaining neutron-capture cross sections from measurements of the production of both long-lived nuclides 10Be and 14C, in the energy region of interest for astrophysical applications. Neutron-rich scenarios lead to the production of the long-lived radionuclides 10Be (t1/2 = 1.51 Myr) and 14C (t1/2 = 5730 yr) via neutron-capture reactions on 9Be and 13C, respectively. Data for both neutron-capture reactions are sparse and disagree in the astrophysical relevant energy region between 10 and 250 keV. Since (n,γ) cross sections in this mass and energy range are expected to be very small (some tens of µbarn), the combination of the activation technique and accelerator mass spectrometry (AMS) offers the powerful experimental approach which is necessary for such investigations. Beryllium oxide and enriched 13C graphite samples have been irradiated in a quasi-stellar neutron spectrum of kT = 25 keV at Forschungszentrum Karlsruhe. The subsequent AMS measurements were performed at the VERA (Vienna Environmental Research Accelerator) laboratory. The first AMS results confirm the expected low cross sections of about 10 µbarn for both reactions.

11Be(beta-p), a quasi-free neutron decay?

We have observed beta(-)-delayed proton emission from the neutron-rich nucleus Be-11 by analyzing a sample collected at the ISOLDE facility at CERN with accelerator mass spectrometry (AMS). With a branching ratio of (8.3 +/- 0.9). 10(-6) the strength of this decay mode, as measured by the B-GT-value, is unexpectedly high. The result is discussed within a simple single-particle model and could be interpreted as a quasi-free decay of the Be-11 halo neutron into a single-proton state.

Depletion of the excited state population in negative ions using laser photodetachment in a gas-filled RF quadrupole ion guide

The depopulation of excited states in beams of negatively charged carbon and silicon ions was demonstrated using collisional detachment and laser photodetachment in a radio-frequency quadrupole ion guide filled with helium. The high-lying, loosely bound 2D excited state in C− was completely depleted through collisional detachment alone, which was quantitatively determined within 6%. For Si− the combined signal from the population in the 2P and 2D excited states was only partly depleted through collisions in the cooler. The loosely bound 2P state was likely to be completely depopulated, and the more tightly bound 2D state was partly depopulated through collisions. 98(2)% of the remaining 2D population was removed by photodetachment in the cooler using less than 2 W laser power. The total reduction of the excited population in Si−, including collisional detachment and photodetachment, was estimated to be 99(1)%. Employing this novel technique to produce a pure ground state negative ion beam offers possibilities of enhancing selectivity, as well as accuracy, in high-precision experiments on atomic as well as molecular negative ions.

Precise measurement of the neutron capture reaction 54Fe(n,γ)55Fe via AMS

The measurement of cross sections relevant to nuclear astrophysics has become one main research topic at the VERA (Vienna Environmental Research Accelerator) facility. The technique applied, accelerator mass spectrometry (AMS), offers excellent sensitivity for the detection of long-lived radionuclides through ultra-low isotope ratio measurements. We discuss the potential and preliminary results of ongoing precision measurements of neutron-capture cross sections of

Isobar Selective Laser Photodetachment In Trace Element Analysis

We are investigating the possibility to use laser photodetachment of negative ions as an isobaric selective filter in accelerator mass spectrometry (AMS). If successful, this method can be used to obtain higher sensitivity realized through better selectivity by suppression of molecular and/or elemental isobaric interference in different investigations using ultra rare isotopes in the 10−13 range and below.

Beam purification by photodetachment (invited).

Ion beam purity is of crucial importance to many basic and applied studies in nuclear science. Selective photodetachment has been proposed to suppress unwanted species in negative ion beams while preserving the intensity of the species of interest. A highly efficient technique based on photodetachment in a gas-filled radio frequency quadrupole ion cooler has been demonstrated. In off-line experiments with stable ions, up to 10(4) times suppression of the isobar contaminants in a number of interesting radioactive negative ion beams has been demonstrated. For selected species, this technique promises new experimental possibilities in studies on exotic nuclei, accelerator mass spectrometry, and fundamental properties of negative atomic and molecular ions.

Commissioning of the REX-ISOLDE linac

The radioactive beam experiment (REX-ISOLDE) is now in the commissioning phase. REX-ISOLDE is a pilot experiment for testing a new concept of post acceleration at ISOLDE/CERN. The concept of REX-ISOLDE using a Penning trap-EBIS combination as a charge breeder and a compact linac for post acceleration requires detailed tests of all components in order to get a high efficiency. First measurements with REXTRAP, the transfer beam line and the REXEBIS have been done as well as first acceleration of stable beams from the EBIS with use of REXRFQ.