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No abstract available

[en] The Recoil Distance Doppler-Shift (RDDS) method is a well established method to measure lifetimes of excited nuclear states in the pico-second range. In an RDDS experiment, nuclei of interest, produced in a nuclear reaction, recoil out of a thin target foil and fly with a velocity distributed around a mean velocity (anti v)/(c). Usually only the mean velocity (anti v)/(c) is considered in the lifetime determination. In cases where the velocity distribution becomes broad it affects the shape and positions of the shifted components of the peaks. In this contribution we present details on our investigation of the effects caused by broad velocity distributions and point out limits outside of which corrections for the lifetime analysis become important. Further, an estimate for the width of typical velocity distributions, observed in recent RDDS experiments with a rather low (anti v)/(c) of about 0.5%, will be shown.

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[en] The Recoil-Distance Doppler-Shift (RDDS) technique is a well established method to measure lifetimes of excited nuclear states in the pico-second range. In standard RDDS experiments at non-relativistc beam-energies, the velocities of the emerging recoils are usually distributed narrowly around a mean velocity v = left angle v right angle _v. Under these circumstances, the effect of the velocity distribution is neglectable and the assumption that all nuclei move with the average velocity is justified. In this poster we investigate the influence of broader velocity distributions on lifetimes determined using the standard lifetime analysis-method DDCM. This can be observed, e.g., in experiments with thick targets. In particular, it is shown that the effect of the velocity distribution on the deduced lifetime is minimised at the maximum amplitude of the derivative of the decay function.

[en] The Doppler shift attenuation method (DSAM) is the method of choice for the measurement of lifetimes of nuclear excited states in the sub-picosecond region. DSAM experiments employ thin production targets mounted on a thick stopper foil (backing) in which the recoiling nuclei of interest slow down and stop in a well-defined manner. The emission of γ rays during this time leads to the observation of a Doppler-broadened lineshape whose details are sensitive to the lifetime and the time evolution of the population of the level of interest. The APCAD analysis code (Analysis Programm for Continuous-Angle DSAM) is a versatile tool for the analysis of γ-ray lineshapes. The code simulates the slowing-down history of the recoiling ions and fits the distinct lineshapes to the observed spectra. Excited states in ²⁰Ne and ⁴⁷V were populated in fusion-evaporation reactions with the tandem accelerator of the IKP, University of Cologne to test and benchmark the analysis procedures with APCAD. Different stopping approximations (SRIM and GEANT4) are investigated and the impact of nuclear and electronic stopping powers on the fitted lineshapes is studied in detail. Results of these variations as well as a first results for the lifetime of the 4⁺ state of ⁴⁸Cr are presented.

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[en] For lifetime measurements of excited nuclear states with the recoil distance Doppler-shift (RDDS) method and the plunger target foils with very good surface qualities, i.e., roughnesses in the micrometer range, are often essential. Therefore, these targets are typically stretched over highly precisely made cones and careful estimates on the target temperature in the beam spot are required to avoid the formation of bumps in these foils due to beam-induced heating up caused by the energy transfer. However, when performing RDDS experiments with heavy ion beams with energies of several MeV/u an additional effect must be considered: the sensitivity of several (metallic) target materials to the electronic stopping power (S_e) of swift heavy ions causing structural changes. This effect is known in material science since several years, but in RDDS experiments it was only observed in very recent experiments by our group where extreme wrinkles appeared in some target foils with amplitudes of the order of 100 μm. Such can hamper a RDDS measurement completely. Based on the results and predictions in we discuss these observations with respect to the choice of target materials including the use of multilayer targets.

[en] The new AMS system at the Cologne 10 MV FN accelerator is finalized in its first stage. The system has been designed for the measurement of medium mass isotopes, especially for ⁵³Mn and ⁶⁰Fe. It provides the opportunity to use several detector systems in different combinations: degrader foils with an electrostatic analyzer, a 4 m time-of-flight system, a 135 gas filled magnet and different gas ionization detectors. The whole system was tested with measurements of stable ion beams and isotopic ratios. During the test measurements of (⁴¹Ca/⁴⁰Ca) isotopic ratios without passive absorbers a detection limit lower 5.3 x 10^-13 was reached. A high ⁵³Cr suppression in the (⁵³Mn/⁵⁵Mn) isotopic ratio measurement with the electrostatic analyzer was reached by using the optimal silicon nitride degrader foil thickness. We will present the first (⁵³Mn/⁵⁵Mn) ratio measurements in a range from 10^-9 to 10^-10, and background spectra measured with blank sample material. Transmission tests show an overall efficiency of 0.0033%, including sputter efficiency, charge state yield, transmissions and software gates.

[en] The recoil distance Doppler-shift (RDDS) method is a very valuable technique for measuring lifetimes of excited nuclear states in the picosecond range to deduce absolute transition strengths between nuclear excitations independent on the reaction mechanism. Dedicated plunger devices were built by our group for measurements with this method for a broad range of beam energies ranging from few MeV/u up to relativistic energies of the order of 100 MeV/u. Those were designed to match the constraints defined by state-of-the art γ-ray spectrometers like AGATA, Galileo, Gammasphere. Here we give an overview about recent experiments of our group to determine transition strengths from level lifetimes in exotic nuclei where also recoil separators or mass spectrographs were used for an identification of the recoiling reaction products. The aim is to learn about phenomena like shape phase coexistence in exotic regions and the evolution of the shell structure far from the valley of stability. We also review new plunger devices that are developed by our group for future experimental campaigns with stable and radioactive beams in different energy regimes, e.g., a plunger for HIE-ISOLDE.