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[en] The experimental proof for the existence of proton-neutron mixed symmetry states has been shown in several, mostly stable nuclei in the 50< A<150 mass region in the last years by analysing M1 transitions to fully symmetric states, which are the main experimental signatures for the existence of these states. Since information about mixed symmetry states in unstable nuclei is very sparse, experiments have been performed at the Tandem accelerator of the Institut fuer Kernphysik in Cologne to identify the first mixed symmetry state 2+ms,1 in several unstable N=48,52 isotones, but absolute transition strengths could not be obtained until now. An additional experiment was performed at the IKP in Cologne using the 61Ni(3He,2nγ)62Zn reaction to identify the first mixed symmetry state in 62Zn. Utilizing the high efficiency HORUS cube spectrometer, which allows the analysis of γγ angular correlations and the determination of lifetimes via the DSA Method, absolute transition strengths in 62Zn could be obtained, by which a fragmentation of the mixed symmetry state 2+ms,1 turned out to be the case. The results have been complemented to the systematical trends in the Z=30 Zinc isotopes and a predicted loss of collectivity in the lighter Zn isotopes up to 58Zn could be verified.
[en] In this new approach to lifetime measurements via Doppler attenuated line shapes, the spectra of a feeding f and a deexciting transition d of the level of interest are used to determine the lifetime without any lineshape analysis of the feeding transition (direct or indirect). Similarly to the DDC method, the decay function λdnd(t) of the deexciting transition is determined. The feeding of the level is included via the spectral difference of the two successive decays. Consequently, the determined lifetime is the real lifetime. After transforming both transitions into the same energy region, their spectral difference D(vθ) = Sd(vθ)-Sf(vθ) = ∫0∞(∂Pθ(t,vθ))/(∂t)nd(t) dt, is solved for nd(t). Dividing nd(t) by the decay function λdnd(t) should yield a constant τ value for the level lifetime as a function of the time t. After the development and test of the procedure in 2015, it is now applied for the first time. Two level lifetimes are determined in 86Sr for the 2+2 and the 2+3 levels.
[en] In the analysis of coincidence RDDS experiments one uses the Differential Decay Curve (DDC) Method to determine lifetimes of excited states. Experiments with small recoil velocities, thus small Doppler shifts, enforce the use of narrow coincidence gates to determine peak intensities. This results in a loss of statistics. As an alternative to the application of gates, we present the fit of 2-dimensional functions to the γγ coincidence data. This approach has been studied on data taken in a RDDS measurement for the ground state band of 168Yb. The 18O(154Sm,4n)168Yb* fusion evaporation reaction was induced by an 80 MeV ion beam of the tandem accelerator facility in Cologne. The target was mounted in the Cologne coincidence plunger device. Lifetimes from the 41+ to the 101+ states have been extracted. The method is discussed and the results are compared to the CBS rotor model in the context of centrifugal stretching.
[en] The lifetime measurement for the excited states in the neutron-rich nucleus 13B has been performed, at the FN tandem facility in the University of Cologne, by the Doppler-shift attenuation method with the 7Li(7Li,p)13B reaction. Anomalously hindered strength was found for the transition between the second excited states at 3.53 MeV and the ground state. A comparison with the modern shell model calculations suggests the spin and parity (Jπ) of 3/2- for the 3.53-MeV state with the hindered transition strengths to the ground state with Jπ=3/2-, providing a consistent picture for the shape coexistence in the neutron-rich isotope 13B with N = 8. Experimental results are presented and discussed in terms of the shell evolution in the light neutron-rich nuclei around N = 8.
[en] The nuclei 176,178,180Os are known to show the characteristic features of the critical point symmetry X(5). This symmetry was introduced by F. Iachello and describes nuclei at the critical point of deformation phase transition from a vibrator to axial rotor. A rapid change in deformation is expected when the proton number is changed, because the neutron number is close to mid-shell. Therefore we performed measurements to determine the lifetimes of low lying states in 180Pt which is a neighbor of the X(5) type Os nuclei, from which absolute transition probabilities can be deduced directly. In this contribution we will report on a Recoil distance Doppler shift experiment which was performed at the JYFL, Jyvaeskylae (Finland). Furthermore the results of the 168Yb(16O,4n)180Pt experiment using the fast timing technique to determine the lifetime of the rather longlived first 2+ state at the IKP, University of Cologne (Germany) are presented. The experimental results will be discussed in the framework of the Interacting Boson Model and compared to a General Collective Model calculation.
[en] In the series of Ne isotopes it becomes apparent that for Ne the drastic increase in B(E2; 20) strength, compared to the double-magic O, is difficult to be reproduced by modern configuration-mixing models or shell model calculations without changing parameters, such that they deviate drastically from those describing neighbouring nuclei. Because of this discrepancy, we remeasured lifetimes in Ne and Ne using the Doppler-shift attenuation method. The experiments were performed at the Cologne FN Tandem accelerator in October 2017 using the Be(O,n)Ne and Be(O,n)Ne reactions with a 0.9 Be target on a 2.7 Mg backing at multiple beam energies between 30 and 38 MeV. The stopping powers of the reaction products in target and backing were measured at the Cologne Accelerator Mass Spectrometer in March 2018. For the line shape analysis an improved version of DESASTOP was used, which utilizes a Monte-Carlo simulation of the stopping process of the recoil nuclei. Detailed discussion of the used analysis method as well as results will be presented and compared to the systematics of light nuclei.
[en] In this investigation lifetimes in 20Ne were determined using the Doppler-shift attenuation method. The experiment was performed at the Cologne FN-Tandem accelerator in October 2017 using a 16O(9Be,nα)20Ne reaction with a 0.9 (mg)/(cm2) Be target on a 2.7 (mg)/(cm2) Mg backing at 4 beam energies between 30 and 38 MeV. For the line shape analysis an improved version of DESASTOP was used. A detailed discussion of the used analysis method as well as preliminary results will be presented and compared to the systematics of light nuclei. In the series of Ne isotopes it becomes apparent that for 20Ne and 22Ne the increase in B(E2; 2+→0+) strength, compared to more neutron rich Ne isotopes, cannot be reproduced by shell model calculations. Because of this discrepancy we were motivated to revisit this topic experimentally.
[en] Absolute transition matrix elements are valuable observables in nuclear-structure physics since they are directly related to the nuclear wave functions. A key ingredient to determine transition matrix elements is the measurement of lifetimes of excited states. In a recent experiment, we extracted the lifetimes of 30 excited states of the low-abundant isotope "9"6Ru utilizing the Doppler-shift attenuation method (DSAM) in an inelastic proton-scattering experiment and taking advantage of the proton-γ coincidence technique. In contrast to the DSAM technique following inelastic neutron scattering, which was frequently performed to extract comprehensive lifetime information in the sub-picosecond regime, the (p,p"'γ) reaction requires a much less amount of target material and is thus especially suited to investigate low-abundant isotopes. In this contribution, the (p,p"'γ) method for lifetime measurements is presented and the results of recent experiments on "9"6Ru, "9"4Zr, and "1"1"2","1"1"4Sn are shown.
[en] Recently, a new technique to determine lifetimes of excited states from a few to hundreds of femtoseconds has been established in Cologne: the Doppler-shift attenuation method (DSAM) using pγ coincidences. The SONIC rate at HORUS setup with its 14 HPGe detectors and several silicon detectors enables to extract the centroid-energy shifts from proton-gated γ-ray spectra, yielding lifetime values that are independent of feeding contributions. This contribution introduces this (p,p'γ) DSAM technique and show the results of measurements on 96Ru, 98Ru, 112,114Sn, and 94Zr.