Level lifetimes for the candidate chiral doublet bands of 80Br were extracted by means of the Doppler-shift attenuation method. The absolute transition probabilities derived from the lifetimes agree well with the M1 and E2 chiral electromagnetic selection rules, and are well reproduced by the triaxial particle rotor model calculations. Such good agreements among the experimental data, selection rules of chiral doublet bands and theoretical calculations are rare and outstanding in researches of nuclear chirality. Besides odd-odd Cs isotopes, odd-odd Br isotopes in the A≈ 80 mass region represent another territory that exhibits the ideal selection rules expected for chiral doublet bands.

Excited states of the nucleus Br-79 were investigated via the reaction Ge-76(Li-7,4n) at a beam energy of 35 MeV. Coincidence data of emitted gamma rays were measured with an arrangement of six EUROBALL CLUSTER detectors. The E2 bands built on the 9/2(+) and 3/2(-) states were extended up to J=37/2 at Eapproximate to8.8 MeV. The M1 band starting with a 15/2(-) state at 2.6 MeV was observed up to J=(29/2) at E=6.4 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 pi(g(9/2)) nu(g(9/2)) nu(fp) which has a triaxial shape. This band appears as a mixed case including contributions of both magnetic and collective rotation.

The low-energy dipole strength distributions in 165Ho and 169Tm have been studied in nuclear resonance fluorescence experiments at the S-DALINAC utilizing a Euroball Cluster detector. In the energy interval between 2.5 and 4.0 MeV, where the scissors mode is expected, 35 ground state transitions could be observed in 165Ho and 53 in 169Tm. Assuming M1 character for all these transitions corresponds to ΣB(M1) ↑= 1.54(23), μN2 and ΣB(M1) ↑= 2.15(63), μN2 for 165Ho and 169Tm, respectively. A statistical analysis is applied to the measured spectra which is capable of reconstructing the complete strength, including contributions of transitions below the observation limit, with the ratio between M1 and E1 excitations and their total strengths taken as an average over the even-mass neighbours. This results in summed M1 transition strengths of ΣB(M1) ↑= 3.54−0.95+0.75, μN2 (165Ho) and ΣB(M1) ↑= 3.36−0.57+1.00μN2 (169Tm). The large variations of the summed dipole strength and the fragmentation in other deformed odd-mass nuclei can be reproduced by the same statistical ansatz. The total low-lying M1 strength in heavy deformed odd-mass nuclei is in agreement with the findings in the neighbouring even-mass nuclides as well as with sum-rule predictions.

Excited states of the nucleus Y-87 were investigated via the reaction Se-80(B-11,4n) at 45 MeV. gamma rays were detected with the six-detector array OSIRIS CUBE. The level scheme of Y-87 has been extended up to J(pi) = 33/2((-)) at E approximate to 7 MeV. Mean lifetimes of eight levels have been deduced using the Doppler-shift-attenuation method. The structure of Y-87 has been interpreted in terms of the shell model. The calculations performed in the model space pi(0f(5/2),1p(3/2),1p(1/2),0g(9/2)) nu(1P(1/2), 0g(9/2)) well reproduce experimental excitation energies and transition strengths in Y-87, especially the large B(M1) values of up to 1.8 W.u. between yrast states with J >21/2. Structural differences between Y-87 and the isotone Rb-85,discussed on the basis of the shell-model calculations.

A first level scheme of ${}^{68}\mathrm{Se}$ with five states up to 4.2 MeV excitation energy and tentative spins ${6}^{+}$ and ${7}^{\ensuremath{-}}$ has been established from \ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\gamma} coincidences measured with six EUROBALL Cluster detectors in close geometry. The level scheme is interpreted in comparison with neutron deficient nuclei of the $A\ensuremath{\approx}70$ region and cranking model calculations. In a similar experiment a triple coincidence between the ${}^{72}\mathrm{Kr}$ transitions 710, 611, and 792 keV has been proven.

The results of in-beam investigations of 113 Sn using the (p,n), (p,3n), (α,n) and (α,2n) reactions are summarized. Excited states have been identified until E x = 4715 MeV and J π = (27/2t -). For a large number of levels mean lifetimes τ have been determined with the DSA method. For the J k = 25/2+ state at E x = 4059 MeV, τ = 1.0(4) ns has been measured with the γ-RF method. The experimental results are compared with the predictions of shell-model calculations. Most of the positive-parity states may be considered as one-or three-quasiparticle neutron excitations of the 2d5/2, 1g7/2, 3s1/2 and 2d3/2 shells, the negative-parity states as the coupling of one 1h11/2 neutron to the two- or four-quasiparticle neutron excitations in the even-mass 112Sn core. For the 25/2+ isomer the three-quasiparticle neutron configuration ν(h 11 2/2 g 7 2/−1 ) has been proposed on the basis of a shell-model analysis using the mass-formula formalism. The experimentally observed yrast states in 50 113 Sn63 are compared with the corresponding states in the valence mirror nucleus 63 145 Eu82 giving remarkable similarities although the parameters for the shell-model calculations differ considerably. The analysis of nearest-neighbour spacing distributions of experimentally obtained 5/2+ states in 113Sn does not allow definite conclusions about regularity or chaos.

Excited states of the nuclei 122,126,130Te were populated via the (γ, γ′) reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL CLUSTER detector and a single HPGe detector. In all investigated nuclei two or three prominent dipole transitions were identified at Eγ ≈ 3 MeV. The corresponding low-lying J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict at about 3 MeV one 1− state arising from the coupling of the first quadrupole and the first octupole phonon, and one 1+ state arising from the coupling of the first and the second quadrupole phonon, where the latter has isovector character. Such an excitation mode can be considered as an analogue of the scissors mode in vibrational nuclei. The calculated transition strengths are compatible with experimental ones within a factor of about 1.5.

The EUROBALL Cluster detector is composed of seven encapsulated Ge detectors in a common cryostat with a total volume of 2000 ccm of HP Gemanium. The development and the performance of the Cluster detector is summarized. Up to six Cluster detectors were used in pre-EUROBALL experiments at the S-DALINAC Darmstadt, at the tandem-postaccelerator facility of the MPI-K Heidelberg and at the UNILAC at GSI. Examples of these experiments - the excitation of dipole modes with (γ, γ′)-reactions and the first coincidence spectroscopy of the N=Z-nucleus 68Se with a CLUSTER Cube - are discussed. The development of segmented encapsulated Ge detectors for a MINIBALL at the radioactive beam facility REX-ISOLDE has been launched. The status of the project is presented.

The E1 response of the semi-magic nucleus 140Ce below the particle threshold was measured in a (γ,γ′) experiment utilizing the new Euroball Cluster detector at the S-DALINAC. While the energy averaged data are in good agreement with tagged photon results, here they are resolved for the first time into 54 individual transitions. A quasiparticle-phonon model calculation including up to three-phonon configurations compares well to the extracted strength distribution. The interference between one- and two-phonon contributions is essential for a quantitative reproduction.

The gamma decays of high spin states in Rb-77 populated in the reaction Ca-40(Ca-40,3p) Rb-77 were studied with the EUROGAM I gamma-ray spectrometer at Daresburp Laboratory, in conjunction with the recoil mass separator. The data were used to extend or establish three positive parity and five negative parity bands in Rb-77. Levei lifetimes and sidefeeding times were then measured with the NORDBALL gamma-ray spectrometer, using the Doppler shift attenuation method, A very detailed level scheme including many weakly populated states was established and the bands were observed over large spin ranges. The deduced lifetimes demonstrate the high collectivity of the excited states and are consistent with their having large prolate deformations. The structures of the bands and their crossings are discussed in relation to the large shell gap at prolate deformation beta(2) approximate to 0.4.

The fusion-evaporation reaction Ni-58(S-32,2 alpha p) has been used to study the neutron-deficient isotope Y-81. Multiple particle gamma-ray coincidences have been detected by the GAMMASPHERE array combined with the MICROBALL charged-particle detector system. Gamma-ray spectra with an improved resolution have been achieved from an event-by-event determination of the nucleus recoil momentum, thus allowing a precise Doppler-shift correction. In this way a resolution enhancement by a factor 2 was obtained for a 1 MeV gamma line. During the analysis an E-gamma-E-gamma matrix Bs well las an E-gamma-E-gamma-E-gamma cube have been used to extend the previously known level scheme to higher spin (I approximate to 57/2) and excitation energy (E-x approximate to 17 MeV). More than 100 new gamma rays and 80 new levels have been added to the level scheme and six new bands have been established. The interpretation of these bands in terms of the cranking model and their comparison with similar bands in neighboring nuclei is discussed.

Excited states of the nuclei 122Te, 126Te and 130Te were populated via the (γ, γ') reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL-CLUSTER detector and a single detector. In all nuclei several dipole transitions were identified at energies around 3 MeV. The lowest corresponding J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict one 1− state arising from the coupling of the first quadrupole and the first octupole phonon and one 1+ state arising from the coupling of the first and the isovector second quadrupole phonon at about 3 MeV. The calculated transition strengths are compatible with experimental ones.

The semi-magic nucleus 89Y has been investigated in a (γ,γ′) experiment at an endpoint energy of the bremsstrahlung of E0 = 7 MeV. The scattered photons have been detected with a EUROBALL Cluster detector. The observed excitations are discussed in the framework of the shell model and a coupling of the unpaired proton to multi-phonon structures in the doubly even neighbours. Transitions above 4.7 MeV are considered to have E1 character. Around 6.3 MeV an unusually large concentration of E1 strength is found. Its origin is likely to correspond to similar structures in the 88Sr and 90Zr isotones which can be interpreted to result from the constructive interference of strong two-phonon amplitudes with weak admixtures from the low-energy tail of the giant dipole resonance.

Spherical shell-model calculations have been performed in the configuration space (s12h92i132) and (p12p32f52i132) for protons and neutrons, respectively, in order to interpret the sequences of strong dipole transitions found in neutron-deficient Pb isotopes. Regular dipole bands are found if several high-j protons and high-j neutron holes are interacting with neutrons in the low-spin (fp) orbitals. The calculated B(M1) values are in the order of several μN2 for the ΔJ = 1 transitions, and the crossover E2 transitions are very weak. The mechanism generating the dipole bands is found to be the same as in the tilted axis cranking mean-field description.

Six rotational bands up to energies Ex=24.7 MeV and spinsjπ=(79/2−) have been identified in109Sn using the GAMMASPHEREγ-detector array. Four of the bands show smoothly decreasing dynamic moments of inertia at rotational frequenciesℏω>0.6 MeV. The bands arise at medium spins from a coupling of a valence d5/2, g7/2 or h11/2 neutron to the deformed 2p2h proton excitation of the Z=50 core108Sn. At very highℏω these bands show the typical behaviour of smoothly terminating bands, i.e. a gradual alignment of the angular momenta of the valence particles and holes corresponding to a transition from high collectivity to noncollective states.

Neutron particle-hole states in the doubly magic nucleus 208Pb were investigated using a Euroball Cluster detector and five usual HPGe detectors at the Osiris cube spectrometer in Cologne. 126 γ-ray transitions with energies up to 7.2 MeV could be detected 14 for those for the first time. The states were excited with the 208Pb(p,p'γ)208Pb reaction via isobaric analogue resonances in 209Bi at Ep = 16.5, 17, 17.45, 17.5 and 18.5 MeV and though the subcoulomb 207Pb(d, pγ)208Pb reaction at Ed = 10 MeV. For most of the excited states assignments for the particle and/or the hole component of the configuration could be made, 22 of these are new.

A nuclear resonance fluorescence experiment with one of the newly developed highly efficient EUROBALL cluster detectors has been performed on the {gamma}-soft nucleus {sup 196}Pt. Magnetic dipole excitations were observed between 2 and 3.5 MeV excitation energy. They are interpreted as the main fragments of the scissors mode based on the measured excitation strengths and branching ratios. A strong {gamma} decay, which we believe to be an {ital E}2 decay of the scissors mode to the 2{sup +}{sub 1} state, is observed. It allows an extraction of the effective boson quadrupole charges in the interacting boson model. {copyright} {ital 1996 The American Physical Society.}

In-beam spectroscopic investigations of Sn-109 have been performed using the reactions Mn-55(Ni-58, 3pn) and Cd-106(alpha, n) with E(58Ni) = 240 MeV and E(alpha) = 23 MeV, respectively. An extended level scheme of Sn-109 is presented showing high-spin states up to E(x) approximate to 8 MeV with J(pi) = (41/2(+)). The problem of the Sn-109 ground state has been solved identifying a 12.8 keV transition deexciting the 7/2(+) state to the 5/2(+) g.s. A half-life of T-1/2 = 7(1) ns has been measured for the 17/2(+) state at E(x) = 2114 keV. The experimental data are compared with the predictions of shell-model calculations.