Euroball Cluster Detector Research Articles

Lifetime measurements of excited states in Cr55

Excited states in $^{55}\mathrm{Cr}$ have been populated via the fusion-evaporation reaction $^{48}\mathrm{Ca}(^{11}\mathrm{B},p3n){\phantom{\rule{0.16em}{0ex}}}^{55}\mathrm{Cr}$ at a beam energy of 32 MeV. The Cologne plunger device surrounded by a $\ensuremath{\gamma}$-ray detector array was employed to determine lifetimes with the recoil-distance Doppler-shift method. $\ensuremath{\gamma}$ rays were observed by one Ge EUROBALL cluster detector and five HPGe detectors. $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence data were analyzed with help from the differential decay-curve method, and precise lifetimes for the first excited states were extracted from the $5/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}3/{2}^{\ensuremath{-}}$ and the $9/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}5/{2}^{\ensuremath{-}}$ transitions. Reduced transition strengths $B(\ensuremath{\sigma}\ensuremath{\lambda})$ were determined and compared to shell-model calculations employing four interactions KB3G, FPD6, GXPF1A, and GXPF1Br. The calculations were also performed for the $N=31$ isotonic chain from $^{53}\mathrm{Ti}$ to $^{61}\mathrm{Zn}$. Inspection of the wave functions as well as particle-plus-rotor model calculations allow for a detailed understanding of the excited states and the reduced transition strength in $^{55}\mathrm{Cr}$. The interactions GXPF1A and GXPF1Br reproduce well the experimental findings in $^{55}\mathrm{Cr}$ and in other isotones.

Conceptual design of a high resolution Ge array with tracking and imaging capabilities for the DESPEC (FAIR) experiment

We present results of Monte Carlo simulations for the conceptual design of the high-resolution DESPEC Germanium Array Spectrometer (DEGAS) proposed for the Facility for Ion and Antiproton Research (FAIR) under construction at Darmstadt, Germany. The project is carried out in three phases, although only results for the two first phases will be addressed in this work. The first phase will consist of a re-arrangement of the EUROBALL cluster detectors previously used in the RISING campaign at GSI. The second phase is based on coupling AGATA-type triple-cluster detectors with EUROBALL cluster detectors in a compact geometry around the active ion implantation target of DESPEC.

Low-spin electromagnetic transition probabilities inCd102,104

Lifetimes of low-lying states in $^{102,104}\mathrm{Cd}$ were determined by using the recoil distance Doppler-shift technique with a plunger device and a Ge array consisting of five HP Ge detectors and one Euroball cluster detector. The experiments were carried out at the Cologne FN Tandem facility using the $^{92}\mathrm{Mo}$($^{12}\mathrm{C}$,$2n$)$^{102}\mathrm{Cd}$ reaction at 41 MeV and the $^{94}\mathrm{Mo}$($^{12}\mathrm{C}$,$2n$)$^{104}\mathrm{Cd}$ reaction at 42 MeV. The differential decay curve method in coincidence mode was employed to derive the lifetime of the first ${2}^{+}$ state in both nuclei and the lifetime of the ${4}^{+}$ state in $^{104}\mathrm{Cd}$. The corresponding reduced $E2$ transition probabilities have been studied within the framework of the nuclear shell model.

E2 transition probabilities inTe114: A conundrum

Lifetimes in $^{114}\mathrm{Te}$ were determined using the recoil distance Doppler-shift technique with a plunger device coupled to five HP Ge detectors enhanced by one Euroball cluster detector. The experiment was carried out at the Cologne FN Tandem facility using the $^{93}\mathrm{Nb}$($^{24}\mathrm{Mg},p2n$) reaction at 90 MeV. The differential decay curve method in coincidence mode was employed to derive lifetimes for seven excited states, whereas the lifetime of an isomeric state was obtained in singles mode. The resulting $E2$ transition probabilities are shown to be very anomalous in comparison with the vibrational energy spacings of the ground-state band.

Enhanced neutron pair transfer and collective excitations in the system 206Pb + 118Sn at barrier energies

At energies below the Coulomb barrier, neutron transfer and Coulomb excitation have been measured in a very heavy asymmetric nuclear system, in 206Pb + 118Sn. These are semi-magic nuclei showing super-fluid properties. Particle-γ coincidence techniques using 5 Euroball Cluster detectors (EB), combined in a set-up with the Heidelberg-Darmstadt NaI Crystal Ball (CB), have been used. Position-sensitive detectors allowed the observation of scattering processes covering angles from 110 up to 150 degrees. The fragments are identified via the known γ-decays of the lowest excited states using the high resolution of EB. Using the unique feature of the set-up with the CB, transfer to well-defined final channels with known quantum numbers is selected using the high-efficiency multiplicity filter of the CB with no second γ-ray, i.e. without feeding. The data are analysed using the semi-classical approach and transfer probabilities are obtained. Coulomb excitation has been analysed using known transition probabilities. The enhancement is deduced for the two-neutron transfer populating the low-lying super-fluid 2+ states in 120Sn and 116Sn, while the 2n transition remains in the ground state for the 20NPb nuclei. Large enhancements up to EF ≃ 103 are observed. This is the first observation of neutron pair transfer enhancement for a heavy nuclear binary system with super-fluid properties with experimentally separated levels. The calculations with microscopic 2-neutron wave functions, with configuration mixing over six shell model configurations and using the coupled reaction channels approach, reproduce well the observed probabilities and the enhancement.

Selection of cold transfer and enhanced neutron-pair transfer in the 206 Pb + 118 Sn reaction

The neutron transfer in a very heavy asymmetric nuclear system, in 206 Pb + 118 Sn has been measured using particle- $\gamma$ coincidence techniques with position-sensitive detectors, 5 EUROBALL-Cluster detectors (EB) and the Heidelberg-Darmstadt NaI-CRYSTAL BALL (CB). The fragments are identified via the known $\gamma$ -decays of the lowest excited states using the high resolution of EB. Using the unique feature of the set-up with the CB, transfer to well-defined final channels with known quantum numbers is selected using the high-efficiency multiplicity filter of the CB with no second $\gamma$ -ray, i.e. without feeding. The enhancement in the two-neutron transfer is deduced, for population of the low-lying super-fluid 2 + states in 120 Sn and 116 Sn, while the 2n-transition remains in the ground state for the recoiling 20X Pb-nuclei. Large enhancements up to EF $\simeq 10^3$ are observed. This is the first observation of neutron pair-transfer enhancement for a heavy nuclear binary system with superfluid properties with experimentally separated levels.

Transition probabilities and isospin structure in theN=Znucleus46V

Picosecond lifetimes in ${}^{46}\mathrm{V}$ and ${}^{46}\mathrm{Ti}$ were determined using the recoil distance Doppler-shift technique with a plunger device coupled to a setup of five HP Ge detectors enhanced by one EUROBALL CLUSTER detector. The experiment was carried out using the ${}^{32}\mathrm{S}{(}^{16}\mathrm{O},pn)$ reaction at 38 MeV at the Cologne FN TANDEM facility. The differential decay curve method in coincidence mode was employed to derive lifetimes for five excited states in each nucleus. The resulting $E2$ transition probabilities are compared with existing shell model calculations and a comparison within the $T=1$ isospin triplet is given. Absolute $E1$ strengths of the ${2}^{\ensuremath{-}}$ decay in ${}^{46}\mathrm{V}$ are discussed.

Change of the dipole strength distributions between the neighbouring γ-soft nuclei 194Pt and 196Pt

A nuclear resonance fluorescence experiment with two highly efficient EUROBALL Cluster detectors has been performed on the γ-soft nucleus 194Pt. Dipole excitations were observed between 2 and 4 MeV excitation energy. They are tentatively interpreted as the main fragments of the scissors mode based on the measured excitation strengths and a comparison to microscopic calculations in the framework of the quasiparticle random phase approximation (QRPA). The data indicate large differences to the neighbouring isotope 196Pt: a doubling of the observed dipole strength and a shift of the energy centroid by about 600 keV. None of the currently available models is able to reproduce these features consistently in both nuclei.

Magnetic and collective rotation in79Br

Excited states of the nucleus ${}^{79}\mathrm{Br}$ were investigated via the reaction ${}^{76}\mathrm{Ge}{(}^{7}\mathrm{Li},4n)$ at a beam energy of 35 MeV. Coincidence data of emitted \ensuremath{\gamma} rays were measured with an arrangement of six EUROBALL CLUSTER detectors. The $E2$ bands built on the ${9/2}^{+}$ and ${3/2}^{\ensuremath{-}}$ states were extended up to $J=37/2$ at $E\ensuremath{\approx}8.8 \mathrm{MeV}.$ The $M1$ band starting with a ${15/2}^{\ensuremath{-}}$ state at 2.6 MeV was observed up to $J=(29/2)$ at $E=6.4 \mathrm{MeV}.$ Crossover $E2$ transitions within this band were observed for the first time. Mean lifetimes of 17 levels were deduced using the Doppler-shift-attenuation method. The $M1$ band can be described within the tilted-axis-cranking model on the basis of the tilted three-quasiparticle configuration $\ensuremath{\pi}{(g}_{9/2}) \ensuremath{\nu}{(g}_{9/2}) \ensuremath{\nu}(\mathrm{fp})$ which has a triaxial shape. This band appears as a mixed case including contributions of both magnetic and collective rotation.

Multipole mixing ratios in130Ba

The multipole mixing ratios of $\ensuremath{\gamma}$ transitions among the low-spin states of ${}^{130}\mathrm{Ba}$ have been studied by means of a $\ensuremath{\gamma}\ensuremath{\gamma}$ angular correlation analysis. The excited states of ${}^{130}\mathrm{Ba}$ were populated in the $\ensuremath{\beta}$ decay of ${}^{130}\mathrm{La}$ obtained with the ${}^{120}\mathrm{Sn}{(}^{14}\mathrm{N},4n)$ reaction at 65 MeV. The $\ensuremath{\gamma}$ ray spectra were measured off-beam with the Osiris-8 detector array, which was enhanced by one Euroball cluster detector, at the University of Cologne. We obtained precise and unique values for the multipole mixing ratios of 18 transitions.

Depopulation of the Jπ = 9- isomer in 180Ta to the Jπ = 1+ ground state by Coulomb excitation

In-beam Coulomb excitation of the exotic odd-odd nucleus 180Ta has been studied by using a 136Xe beam and a setup consisting of five EUROBALL CLUSTER detectors and the Darmstadt-Heidelberg Crystal Ball array. Spectroscopic information on the extremely rare 180Ta is obtained from the comparison between an enriched (3.6±0.3% 180Ta) and a natural tantalum target. Possible evidence for a depopulation from the long-lived high-spin Jπ = 9- isomer to the short-lived Jπ = 1+ ground state is searched for by different methods. The decay of low-K bandheads, which are nanosecond isomers, towards the ground-state band can be demonstrated in delayed spectroscopy. A γγ coincidence analysis provides indications of K = 5 in-band transitions. Finally, when the Crystal Ball is used as an energy and γ multiplicity filter, signals of decay into the K = 0 band are observed.

New states and $ \boldsymbol{\gamma}$ -transitions in Bi observed in the (He, d $ \boldsymbol{\gamma\gamma}$ )-subcoulomb-reaction

Single-particle states as well as collective states in the natural Bismuth isotope 209Bi were investigatedusing two Euroball Cluster detectors at the Cologne FN-tandem accelerator. The states were populated by the subcoulomb proton stripping reaction 208Pb(3He,d*γγ)209Bi at 20.5 MeV beam-energy. Forty-eight γ-transitions and 33 levels were observedfor the first time. Gamma-transitions up to an energy of 6.0 MeV were detected. The energy of the observed states were determined to an accuracy of about 0.3 keV.

γ Spectroscopy in the superdeformed minimum of 240Pu

The γ decay of excited states built on the superdeformed shape isomer 240 f Pu was measured for the first time with an array of EUROBALL CLUSTER detectors in coincidence with delayed fission decays using the 238U( α,2n) reaction. The strongest γ transitions can be assigned to the out-of-band decays of three excited superdeformed rotational sequences: A K π =2 − and K π =1 −(2 −) band with respective bandhead energies of 806.2 keV and 836.0 (846.8) keV decaying to the ground state band of 240 f Pu, and a K π =2(1) band at 1360.9 (1344) keV populating mainly the K π =2 − (806.2 keV) band.

Is the 4.742 MeV state in 88Sr the 1− two-phonon state?

A nuclear resonance fluorescence experiment on 88Sr has been performed with bremsstrahlung of 6.7 MeV endpoint energy. The γ-ray linear polarisation has been measured with a EUROBALL CLUSTER detector used as a Compton polarimeter. The results indicate positive parity for the J= 1 state at 4.742 MeV in 88Sr, in contrast to the previous interpretation as a 1− two-phonon (2+1⊗ 3−1) state and in conflict with the predictions of the quasiparticle-phonon model. On the basis of such calculations the 1+ state at 3.486 MeV may be considered as the 1+1 one-phonon state and the very strong 1+1→ 0+1 deexcitation as proton spin-flip 2p1/2→ 2p3/2 transition.

The 3701 keV neutron g 9/2 single-particle state in 57Ni

The neutron g(9/2) single-particle state in Ni-57 has been unambiguously identified in a combined measurement of the linear polarization, angular distribution, and angular correlation of gamma rays following the fusion-evaporation reaction Si-28(S-32,2pn)Ni-57 at a beam energy of 90 MeV. The linear polarization was measured with a EUROBALL cluster detector and a prototype of an encapsulated six-fold segmented hexaconical Ge-detector. The spin of the 3701 keV level in Ni-57 was confirmed to be I = 9/2 and its parity determined to be positive.

Coincidence Doppler shift lifetime measurements in 73,74Se and 74Br using the EUROBALL Cluster cube array

Subpicosecond lifetimes of high spin states in the rotational nuclei 73Se, 74Se and 74Br have been measured using the 58Ni +19F compound reaction and the Doppler Shift Attenuation method. Six EUROBALL Cluster detectors arranged in cube geometry allowed us to select the relevant transitions in γγ coincidence mode. The high counting statistics achieved in this setup also facilitated the determination of average sidefeeding times, which were found to agree rather well with the results of Monte-Carlo calculations of the particle and γ-ray evaporation process. The deduced quadrupole strengths and deformations are compared with the results of previous measurements and the predictions of Cranked Shell Model calculations. In 74Br, a large and constant prolate deformation of β2= 0.37(1) was found for the presumed 4+ and 3− two-quasiparticle bands.

Resolved dipole strength below theE1giant resonance in138Ba

The electromagnetic dipole response of ${}^{138}\mathrm{Ba}$ was measured up to 6.7 MeV excitation energy in a photon scattering experiment. Two Euroball Cluster detectors were used to detect the scattered photons under 94 and 132 degrees. The Cluster at 94 degrees served as a Compton polarimeter. The total observed dipole cross section is in good agreement with previous tagged photon data, but in the present experiment the transition strength and the electric character of most of the transitions has been determined on a state-by-state basis. The data show a concentration of $E1$ strength around 6 MeV. For one of the strongest excitations $M1$ character is suggested. The results of model calculations using the quasiparticle phonon model (QPM) agree with the observed electric dipole strength distribution.

First identification of excited states in the N = Z nucleus 70Br

Excited states in the Tz= 0 nucleus 70Br have been investigated using the reaction 58Ni(16O,1p3n). γ rays were detected with one EUROBALL CLUSTER detector and three single HPGe detectors. Charged particles and neutrons were registered with the Rossendorf silicon ball and six modules of the EUROBALL neutron wall, respectively. The identification of γ transitions in 70Br is based on the analysis of γγ-proton-neutron coincidences. A level scheme of 70Br has been established for the first time. It shows a multiplet-like structure of probably isospin T= 0 while T= 1 isobaric analogue states are not observed.

Lifetime study of particle-hole excitations in the semimagic nucleus94Ru

The recoil-distance Doppler-shift technique was employed to determine lifetimes of high-spin states in the semimagic nucleus ${}^{94}\mathrm{Ru}$. The nuclei were populated using the reaction ${}^{58}\mathrm{Ni}{(}^{40}\mathrm{Ca},4p)$ at a beam energy of 145 MeV, and the $\ensuremath{\gamma}$ radiation from their decay was detected in six EUROBALL cluster detectors. A total of 23 reduced transition probabilities and limits for fifteen further transitions were extracted and compared to large-scale shell model calculations, considering different configuration spaces and residual interactions. The information deduced on transition strengths turned out to be essential for the correct assignment of the calculated to the experimental excited states. The results indicate that the ${13}_{2}^{\ensuremath{-}}$ (6919 keV), ${14}_{1}^{\ensuremath{-}}$ (7970 keV), and the ${15}_{1}^{\ensuremath{-}}$ (8133 keV) levels have pure proton $\ensuremath{\pi}{(f}_{5/2}{)}^{\ensuremath{-}1}$ $\ensuremath{\pi}{(g}_{9/2}{)}^{5}$ configurations, whereas all other excited states above 6.3 MeV are built from a neutron ${g}_{9/2}\ensuremath{\rightarrow}{d}_{5/2}$ excitation across the $N=50$ shell closure, coupled to up to six valence protons. Strong $M1$ transitions were found in a stretched dipole cascade within the sequence of neutron core-excited states at positive parity, while the strengths of the transitions between core-excited and pure proton states were proven to be small, similar as in ${}^{95}\mathrm{Rh}$.

Strong enhancement of two-neutron transfer in the system 206Pb+118Sn

One and two neutron transfer has been measured in the heaviest asymmetric nuclear system with semi magic nuclei showing superfluid properties, in 206Pb+118Sn collisions at an energy well below the Coulomb barrier with scattering orbits covering the largest angles. Particle-γ coincidence techniques using 5 Euroball-Cluster detectors (EB) combined in a set-up with the Heidelberg-Darmstadt NaI-Crystal Ball (CB) have been used. Transfer channels are identified with EB via their known γ-decays of the lowest excited states. Using the unique feature of the set-up with the CB, transfer to well defined final states with known quantum numbers (without feeding) are selected using the high efficiency multiplicity filter of the CB (no second γ-ray). The data are analysed using the semiclassical approach and transfer probabilities are obtained. The enhancement for the two-neutron transfer populating the low lying superfluid 2+ state in 120Sn (and 116Sn), while the Pb-branch is in the groundstate is deduced by comparison with the strongest single neutron transfer transition. Large enhancements (EF ≃ 103) are observed. This is the first direct measurement of enhancement for a heavy nuclear binary system with experimentally separated levels suggesting a strong contribution from superfluid pair transfer.