Cologne FN Tandem Accelerator Research Articles

Investigation of γ softness: Lifetime measurements in Ru104,106

Lifetimes of the ${2}_{1}^{+}, {4}_{1}^{+}, {6}_{1}^{+}, {2}_{\ensuremath{\gamma}}^{+}$, and ${3}_{\ensuremath{\gamma}}^{+}$ states in $^{104,106}\mathrm{Ru}$ were measured by using the recoil-distance Doppler-shift technique and the Cologne Plunger device. Low-lying excited states in both nuclei were populated in a $^{104}\mathrm{Ru}(^{18}\mathrm{O},^{18}\mathrm{O})^{104}\mathrm{Ru}$* inelastic scattering and in a $^{104}\mathrm{Ru}(^{18}\mathrm{O},^{16}\mathrm{O})^{106}\mathrm{Ru}$ two-neutron transfer reaction using the Cologne FN Tandem accelerator. The experimental energy levels and deduced electromagnetic transition probabilities are compared in the context of $\ensuremath{\gamma}$ softness and the mapped interacting boson model with input from the microscopic self-consistent mean-field calculation using a Gogny interaction. The newly obtained results for the $\ensuremath{\gamma}$ band give a more detailed insight about the triaxial behavior of $^{104,106}\mathrm{Ru}$. The results will be discussed in the context of $\ensuremath{\gamma}$ soft and rigid triaxial behavior which is present in the neutron-rich Ru isotopes.

Development of the multi-purpose Cologne Compact Differential Plunger (CoCoDiff) for the measurement of nuclear level lifetimes with the Recoil Distance Doppler-shift method

A new 3-foil plunger device, the Cologne Compact Differential (CoCoDiff) plunger has been built and commissioned. Due to its compact size, it can be used together with many different spectrometers and auxiliary detectors. As a commissioning experiment, level lifetimes of the 21+ and the 41+ excited states of 50Cr have been measured, using the Differential Decay Curve method (DDCM). A derivation is given on how this method can be applied to a differential plunger measurement, in order to measure distances for lifetimes from two different regions of sensitivity at the same time. The commissioning experiment took place at the Cologne FN tandem accelerator, using the reaction 24Mg(32S,4p2n)50Cr. Lifetimes, deduced from this measurement, agree well with literature values from earlier measurements.

Lifetime measurements to investigate γ softness and shape coexistence in Mo102

Lifetimes of low-spin excited states in $^{102}\mathrm{Mo}$ populated in a $^{100}\mathrm{Mo}(^{18}\mathrm{O},\phantom{\rule{0.16em}{0ex}}^{16}\mathrm{O})^{102}\mathrm{Mo}$ two-neutron transfer reaction were measured using the recoil-distance Doppler-shift technique at the Cologne FN Tandem accelerator. Lifetimes of the ${2}_{1}^{+}$, ${4}_{1}^{+}$, ${6}_{1}^{+}$, ${0}_{2}^{+}$, ${2}_{\ensuremath{\gamma}}^{+}$, ${3}_{\ensuremath{\gamma}}^{+}$ states and one upper limit for the lifetime of the ${4}_{\ensuremath{\gamma}}^{+}$ state were obtained. The energy levels and deduced electromagnetic transition probabilities are compared with those obtained within the mapped interacting boson model framework with microscopic input from Gogny mean-field calculations. With the newly obtained signatures a more detailed insight in the $\ensuremath{\gamma}$ softness and shape coexistence in $^{102}\mathrm{Mo}$ is possible and discussed in the context of the $Z\ensuremath{\approx}40$ and $N\ensuremath{\approx}60$ region. The nucleus of $^{102}\mathrm{Mo}$ follows the $\ensuremath{\gamma}$ soft trend of the Mo isotopes. The properties of the ${0}_{2}^{+}$ state indicate, in contrast with the microscopic predictions, shape coexistence which also occurs in other $N=60$ isotones.

Triaxiality in the mid-shell nucleus Pd112

Lifetimes of low-spin excited states in $^{112}\mathrm{Pd}$ were measured using the recoil-distance Doppler-shift technique. The nucleus of interest was populated in a $^{110}\mathrm{Pd}(^{18}\mathrm{O},^{16}\mathrm{O})^{112}\mathrm{Pd}$ reaction using the Cologne FN Tandem accelerator. Three lifetimes of ground-state band members and one lifetime of the $\ensuremath{\gamma}$ band were measured. From these lifetimes reduced transition probabilities were extracted and compared to interacting boson model, $\ensuremath{\gamma}$-soft calculations, and Davydov calculations. The lifetime of the ${2}_{\ensuremath{\gamma}}^{+}$ gives some insights on the nuclear shape and structure of the $\ensuremath{\gamma}$ band. The deduced transition rates show an indicator for a rigid triaxial nucleus as well as more indicators for a $\ensuremath{\gamma}$-soft nucleus.

Tests of collectivity in Zr98 by absolute transition rates

Lifetimes of low-spin excited states in $^{98}$Zr were measured using the recoil-distance Doppler-shift technique and the Doppler-shift attenuation method. The nucleus of interest was populated in a $^{96}$Zr($^{18}$O,$^{16}$O)$^{98}$Zr two-neutron transfer reaction at the Cologne FN Tandem accelerator. Lifetimes of six low-spin excited states, of which four are unknown, were measured. The deduced $B(E2)$ values were compared with Monte Carlo shell model and interacting boson model with configuration mixing calculations. Both approaches reproduce well most of the data but leave challenging questions regarding the structure of some low lying states.

Lifetime measurement of excited states in Ce144 : Enhanced E1 strengths in a candidate for octupole deformation

A lifetime measurement of excited states in $^{144}\mathrm{Ce}$ using the $^{142}\mathrm{Ce}(^{18}\mathrm{O},^{16}\mathrm{O})$ reaction with a beam energy of 67 MeV and the recoil distance Doppler-shift method was performed at the Cologne FN Tandem accelerator. Lifetimes of the three lowest yrast states in $^{144}\mathrm{Ce}$ have been measured as well as for the ${3}_{1}^{\ensuremath{-}}$ state and an effective lifetime of the ${4}_{2}^{+}$ state. Reduced $E2$ transition strengths determined using these results have been compared to predictions from recent shell-model calculations. From the interband transitions reduced $E1$ strengths could be determined, which are strongly enhanced.

The first (53Mn/55Mn) isotopic ratio measurements at the Cologne FN-Tandem Accelerator

A new accelerator mass spectrometer was successfully designed for the Cologne FN accelerator to enlarge the measurement capability of CologneAMS to medium mass radionuclides. With this AMS-system (53Mn/55Mn) isotopic ratios are measurable at a 10 MV tandem accelerator in the range of 10-9 to 10-10 for the first time, with a detection limit of CL=5.0×10-12. A high 53Cr suppression in the (53Mn/55Mn) isotopic ratio measurement was achieved by using the optimal silicon nitride degrader foil thickness in combination with an electrostatic analyzer (ESA). The new system opens the field of research for CologneAMS users to the medium mass region of cosmogenic nuclides, like 53Mn for geological applications of long term erosion, exposure and burial dating and 60Fe for nuclear astrophysical models. The presented definition of an error normalized sensitivity Sx allows an inter laboratory comparison of isotopic ratio measurements.

A dedicated AMS setup for medium mass isotopes at the Cologne FN tandem accelerator

AMS measurements of medium mass isotopes, e.g. of 53Mn and 60Fe, are gaining interest in various fields of operation, especially geoscience. Therefore a dedicated AMS setup has been built at the Cologne 10MV FN tandem accelerator. This setup is designed to obtain a sufficient suppression of the stable isobars at energies around 100MeV.In this contribution we report on the actual status of the new setup and the first in-beam tests of its individual components. The isobar suppression is done with (dE/dx) techniques using combinations of energy degrader foils with an electrostatic analyzer (ESA) and a time of flight (ToF) system, as well as a (dE/dx),E gas ionization detector. Furthermore, the upgraded ion source and its negative ion yield measurement for MnO− are presented.

A dedicated AMS setup for 53Mn/60Fe at the Cologne FN tandem accelerator

Following demands for AMS measurements of medium mass isotopes, especially for 53Mn and 60Fe, we started to build a dedicated AMS setup at the Cologne FN tandem accelerator. This accelerator with a maximum terminal voltage of 10MV can be reliably operated at a terminal voltage of 9.5MV which corresponds to energies of 93–102MeV for 60Fe or 53Mn beams using the 9+ or 10+ charge state. These charge states can be obtained by foil stripping with efficiencies of 30% and 20%, respectively. Energies around 100MeV are sufficient to effectively suppress the stable isobars 60Ni and 53Cr by (dE/dx) techniques using combinations of energy degrader foils and dispersive elements like electrostatic analyzers and time of flight (TOF) systems as well as (dE/dx)E ion detectors.In this contribution we report on the actual status of the AMS setup and discuss details and expected features.

Study of vibrational signatures in 102Ru

Abstract A Mo 100 ( α , 2 n ) Ru 102 fusion evaporation experiment, performed at the HORUS cube spectrometer at the Cologne FN-Tandem accelerator, is presented and discussed. Via the analysis of γγ coincidences, it was possible to extend the level scheme with 40 new levels and 81 decays. Several new spins and multipole mixing ratios were determined using the γγ angular correlation technique. In addition, a DSAM analysis was performed and 14 new effective lifetimes were determined in the femtosecond range. Thus, several effective transition strengths were determined. Investigating the phononic structure, candidates for the quadrupole–octupole phonon multiplet 2 1 + ⊗ 3 1 − were found. The data is compared to calculations within the Interacting Boson Model (IBM-1) in the U(5) limit, the O(6) limit as well as a transitional class U(5)–O(6). The low spin level energies show characteristics of multiphonon excitations and point towards 102Ru resembling an anharmonic vibrational nucleus. The B ( E 2 ) transition strengths do not exactly match this picture, and are described in a better way using the U(5)–O(6) calculation.

Hg196and the “magical quartet” of the extendedUν(6/12)⊗Uπ(6/4) supersymmetry

We present the results of a $\ensuremath{\gamma}\ensuremath{\gamma}$ angular correlation experiment investigating the nucleus $^{196}\mathrm{Hg}$ and compare these with a theoretical description of $^{196}\mathrm{Hg}$ within the ${\mathrm{U}}_{\ensuremath{\nu}}(6/12)\ensuremath{\bigotimes}$${U}_{\ensuremath{\pi}}(6/4)$ extended supersymmetry. To populate excited $^{196}\mathrm{Hg}$ states, we used the Cologne FN Tandem accelerator inducing the reaction $^{194}\mathrm{Pt}$$(\ensuremath{\alpha},2n)^{196}\mathrm{Hg}$ and analyzed the $\ensuremath{\gamma}$ decays of levels up to an excitation energy of $2.4$ MeV with the HORUS cube spectrometer. The new results on this mercury isotope allow a comparison between the experimental data and the supersymmetrical predictions and show good agreement. This way we can add $^{196}\mathrm{Hg}$ as a fifth supermultiplet member to the so-called magical quartet consisting of $^{194,195}\mathrm{Pt}$ and $^{195,196}\mathrm{Au}$.

First measurement of lifetimes in the yrast band ofPd100

Lifetimes of the yrast states up to ${12}^{+}$ and of a negative parity ${5}^{\ensuremath{-}}$ state in $^{100}\mathrm{Pd}$ were measured by means of the recoil distance Doppler shift technique. The experiment was performed at the Cologne FN Tandem accelerator with the Cologne Plunger using the reaction $^{92}\mathrm{Mo}$$($$^{11}\mathrm{B}$,$p2n)$ to populate excited states in $^{100}\mathrm{Pd}$. Lifetimes were determined by analyzing the transitions in coincidence mode employing the differential decay curve method. The obtained $E2$ transition strengths were compared to predictions from collective models and the shell model.

Test of the critical point symmetry X(5) in the mass A = 180 region

Based on the energy spectra and relative transition probabilities 176,178,180Os turned out to be very promising candidates for testing the features of the critical point symmetry X(5). In order to also test absolute transition probabilities predicted in the framework of the X(5) symmetry lifetime measurements were performed at the Cologne FN tandem accelerator. The lifetimes of the first 2+ states in 176,178,180Os were measured using electronic timing and in the case of 178Os with the recoil distance Doppler shift (RDDS) technique using an 16O-induced fusion evaporation reaction. In addition, a coincidence RDDS measurement was performed at the Laboratori Nazionali di Legnaro with the GASP spectrometer using the 154Sm(29Si,5n)178Os reaction. The deduced transition quadrupole moments agree well with the X(5) predictions, thus further supporting an X(5) structure for 178Os. The experimental data for 178Os are compared to calculations in the framework of the interaction boson model (IBM) and of the general collective model (GCM).

Investigation of low spin states inCr48with the MINIBALLγ-ray spectrometer

Low spin states in the self-conjugate even-even nucleus $^{48}\mathrm{Cr}$ were investigated using the MINIBALL $\ensuremath{\gamma}$-ray spectrometer. At the FN TANDEM accelerator in Cologne the $^{46}\mathrm{Ti}(^{3}\mathrm{He},n)$ reaction was used for the measurement of $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences for an excitation function from 7 to $12\phantom{\rule{0.3em}{0ex}}\text{MeV}$ beam energy. Seventeen excited states were observed, nine for the first time by means of $\ensuremath{\gamma}$-ray spectroscopy, and new spin assignments were made. No excited states apart from the ground band were observed below $3.4\phantom{\rule{0.3em}{0ex}}\text{MeV}$. First comparisons with different model calculations are given.

Experimental evidence for56Ni-core breaking from the low-spin structure of theN=Znucleus2958Cu29

Low-spin states in the odd-odd $N=Z$ nucleus ${}^{58}\mathrm{Cu}$ were investigated with the ${}^{58}\mathrm{Ni}(p,n\ensuremath{\gamma}{)}^{58}\mathrm{Cu}$ fusion evaporation reaction at the FN-TANDEM accelerator in Cologne. $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidences, $\ensuremath{\gamma}\ensuremath{\gamma}$-angular correlations, and signs of $\ensuremath{\gamma}$-ray polarizations were measured. Seventeen low-spin states below 3.6 MeV and 17 new transitions were observed. Ten multipole mixing ratios and 17 $\ensuremath{\gamma}$-branching ratios were determined for the first time. New detailed spectroscopic information on the ${2}_{2}^{+}$ state, the isobaric analogue state (IAS) of the ${2}_{1}^{+},T=1$ state of ${}^{58}\mathrm{Ni},$ makes ${}^{58}\mathrm{Cu}$ the heaviest odd-odd $N=Z$ nucleus with known ${B(E2;2}^{+},T=\stackrel{\ensuremath{\rightarrow}}{1}{0}^{+},T=1)$ value. The ${4}^{+}$ state at 2.751 MeV, observed here for the first time, is identified as the IAS of the ${4}_{1}^{+},T=1$ state in ${}^{58}\mathrm{Ni}.$ The new data are compared to full $\mathrm{pf}$-shell-model calculations with the GXPF1 residual interaction and to calculations within a ${\mathrm{pf}}_{5/2}$ configurational space with a residual surface $\ensuremath{\delta}$ interaction. The role of the ${}^{56}\mathrm{Ni}$ core excitations for the low-spin structure in ${}^{58}\mathrm{Cu}$ is discussed.

Supersymmetry of identical bands in 171,172Yb supported by lifetime data

Abstract Precision lifetimes of six states in the ground-state bands of 171Yb have been measured using the recoil distance method following Coulomb excitation with a detector of the Euroball Cluster type at the FN-Tandem accelerator in Cologne. From these and previous data on branching and mixing ratios from Canberra, we found, that the transition quadrupole moments for corresponding E2 transitions in 171,172Yb are almost equal. This gives strong support for a weakly broken supersymmetry in 171,172Yb. On the other hand, the neighbor pair of nuclei 171Tm and 172Yb shows no supersymmetry demonstrating the importance of an even pseudoorbital angular momentum as condition for supersymmetry.

Decay properties of low-lying collective states in 132Ba

The decay properties of low-lying collective states in 132Ba were studied by means of γ spectroscopy following the β-decay of the 2 − ground state ( T 1/2=4.8 h) and a 6 − isomer ( T 1/2=24.3 min) of 132La. The lanthanum nuclei were produced at the Cologne fn tandem accelerator using the reaction 122Sn( 14N, 4n) 132La. The γγ coincidences and singles spectra were measured with the osiris-cube spectrometer. Beside ground and quasi-gamma band many other low-lying states were observed. The γγ angular correlations were analyzed to assign spins to the excited states, and to determine the multipolarities of the depopulating the γ transitions. We also confirmed the expected dominant E2 character of transitions in the quasi-gamma band and from the quasi-gamma to the ground band but with a certain deviation: the decay 6 + 2→6 + 1 shows an unexpected large M1 fraction. Also the decays of the third, fourth and fifth 2 + states are dominated by M1 radiation. A level at 1660 keV could be identified as the 0 + 3 state in 132Ba. The experimental data are compared with calculations using the proton–neutron interacting boson model (IBM-2). Good agreement is reached in the vicinity of the O(6) limit.

Toward isovector M1 transitions in odd-odd N = Z nuclei

IsovectorM1 transitions between low-lying T=1 and T=0 states in odd-odd N = Z nuclei are discussed. The data on low-spin states in the odd-odd nuclei 46V and 50Mn investigated with the 46Ti(p, nγ)46V and 50Cr(p, nγ)50Mn fusion evaporation reactions at the FN-TANDEM accelerator in Cologne are reported. A simple explanation of the enhancement of the M1 transitions is given in terms of quasideuteron configurations. The fragmentation of the strong M1 transitions is shown to be due to the coupling of the two-particle configurations to the rotating core.

Quasideuteron configurations in 46V and 58Cu

The data on low spin states in the odd-odd nuclei 46V and 58Cu investigated with the 46Ti(p,ngamma)46V, 32S(16O,pn)46V and 58Ni(p,ngamma)58Cu reactions at the FN-TANDEM accelerator in Cologne are reported. The states containing large quasideuteron components are identified from the strong isovector M1 transitions, from shell model calculations and from experimental data for low-lying states.

Low spin structure of theN=Zodd-odd nucleus2550Mn25

Low spin states in the odd-odd nucleus ${}^{54}\mathrm{Co}$ were investigated with the ${}^{54}\mathrm{Fe}(p,n\ensuremath{\gamma}{)}^{54}\mathrm{Co}$ fusion evaporation reaction at the FN-TANDEM accelerator in Cologne. $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidences, $\ensuremath{\gamma}\ensuremath{\gamma}$-angular correlations, and Compton asymmetries were measured. 28 low spin states were observed. 19 for the first time, eight new spin assignments and six new parity assignments were made, and seven $\ensuremath{\gamma}$-branching ratios were measured for the first time. $M1$ and $E2$ transition strengths between low-lying states in ${}^{54}\mathrm{Co}$ were calculated in the shell-model framework. The experimental branching ratios and multipole mixing ratios indicate the existence of very strong isovector $M1$ transitions.