W. Michaelis

Two-quasiparticle states in 168Er

Abstract The level structure of 168Er and the de-excitation mechanism in this nucleus have been studied by radiative capture of thermal neutrons in samples of natural erbium which give a cross-section contribution of 89.8±2.3 % for 167Er. Interference from the 9.1±2.1 % capture contribution of 166Er was well known from a separate study with enriched 166Er samples. Highresolution measurements of the γ-ray spectrum have been performed using the Karlsruhe Ge(Li) anti-Compton spectrometer below 2 MeV and a Ge(Li) double-escape spectrometer above 5.16 MeV. The high accuracy of the data allows the construction of a considerably extended transition diagram up to 2 MeV excitation energy. A large number of new levels has been assigned to specific configurations and their superimposed rotational bands. Conclusive evidence is given that the previously known states at 1094 keV and 1542 keV are the two-quasiparticle neutron levels [633 ↑ + 524 ↓]nn and [633↑−521↓]nn, respectively. Some aspects of the corresponding log ft values are discussed in detail. Rotational bands at 1354 keV, 1569 keV and 1542 keV have to be identified with the Kπ = 1−,2− and 3− octupole vibrational bands. The properties of the Kπ = 1− state are presumably close to those of the neutron level [512 ↑–633↓]nn. The 3− rotational member of the Kπ = 0− octupole band probably occurs at 1914 keV. The simple model of pairing plus state-independent octupole force provides a surprisingly good microscopic description of the octupole excitation energies. The first Kπ = 0+ band has been well established to occur at 1217 keV. Within the model of pairing plus quadrupole and spin-quadrupole force one obtains the interaction constants kq = 5.3 and kt = 8.8 (in units of A − 1 3 h ω 0 ). The neutron level [521↓ + 642 ↑]nn has been tentatively assigned to a rotational band at 1893 keV. The properties of some other more or less collective states are discussed. No positive evidence has been found for the occurrence of two-quasiparticle proton levels below 1820 keV excitation energy. The neutron separation energy was determined to be 7771.24 ±0.48 keV.

Untersuchung angeregter kernzustände in 68Zn durch neutroneneinfang in 67Zn

Abstract The electromagnetic transitions between excited nuclear states of 68 Zn resulting from thermal neutron capture in 67 Zn have been investigated by four different external neutronbeam experiments at the Karlsruhe reactor FR 2. Enriched samples of 89.55% 67 Zn and samples of natural ZnO were used as targets. The γ-spectra recorded with high-resolution Ge(Li) anti-Compton and Ge(Li) pair spectrometers revealed more than 400γ-transitions as originating from 68 Zn. Coincidence measurements were performed with a Ge(Li)-NaI(Tl) detector system. The results lead to a considerably extended transition diagram of 68 Zn with 49 energy levels many of which were previously unknown both in their existence and in their de-excitation mode. In addition angular correlation measurements were carried out on 7 prominent cascades in the lower part of the decay scheme. The level positions, multipole mixing ratios and γ-branching ratios are discussed in the framework of the vibrational and unified collective models. The neutron-separation energies of the zinc isotopes with masses 65, 67, 68 and 69 were determined to be 7979.2 ± 0.8, 7052.4 ± 0.7, 10198.10 ± 0.50 and 6482.2 ± 0.9 keV respectively. The thermal neutron capture cross sections were derived to 0.85 ± 0.20 b for 66 Zn and 6.9 ± 1.4 b for 67 Zn.

Untersuchung der anregungszustände von 58Fe über die gammastrahlung nach dem einfang thermischer neutronen

Abstract The neutron capture γ-radiation from the reaction 57 Fe(n, γ ) 58 Fe has been studied using enriched samples of 57 Fe (90.7%) as external targets at the Karlsruhe reactor FR-2. Energies and intensities of 100 transitions were determined by applying a Ge(Li) anti-Compton device in the energy range 0.2-2.7 MeV and a five-crystal Ge(Li)-pair spectrometer in the energy range 2–11 MeV. Cascade relationships were found by two-parameter measurements using both a 34 cm 3 Ge(Li) 7.6 × 7.6 cm NaI(Tl) coincidence apparatus and a system with two 10.2 × 12.7 cm NaI(Tl) detectors. A decay scheme is proposed with 24 levels and 70 transitions corresponding to 97 % of the intensity of all 58 Fe lines observed. The spins of 12 levels could be assigned by γγ-angular correlation measurements, a great part of which are consistent with (t, p) results of Cohen 1 ). Multipole mixing ratios of 12 transitions were determined. The excited states are discussed in the framework of vibrational models with little success, and are compared with recently calculated 2 ) shell-model states showing quite good agreement. The binding energy of the last neutron in 58 Fe has been found to be 10043.1 ± 1.0 keV as a weighted mean value from 17 cascades.

The excited states of 62Ni studied by the (n, γ) reaction

Abstract The electromagnetic transition modes and level structure in 62 Ni have been investigated via the radiative capture of thermal neutrons in 61 Ni. Enriched samples of 92.11 % 61 Ni were used as external targets in filtered reactor neutron beams. High-energy precision has been obtained by using Ge(Li) anti-Compton and Ge(Li) pair spectrometers. Coincidence relationships were found with a Ge(Li)-NaI(Tl) detector system, spin sequences and multipole mixing ratios have been deduced by measurements with a NaI-NaI angular correlation apparatus. The experimental data allow the construction of a considerably extended transition diagram up to 5 MeV with new spin and parity assignments for a large number of levels. The neutron separation energy of 62 Ni was determined to be 10596.2±1.5 keV. Basing on intensities and δ-values, ratios of reduced transition rates can be given in some cases. The experimental level scheme and transition branching ratios are compared with results of several shell-model calculations. The agreement of excitation energies is fairly good up to 3.2 MeV.

Band mixing in 167Er

Abstract The level structure of 167 Er and the de-excitation mechanism m this nucleus have been studied by radiative capture of thermal neutrons in 166 Er. High-resolution measurements of the gamma-ray spectrum have been performed using a Ge(Li) anti-Compton spectrometer in the low-energy region and a Ge(Li) pair spectrometer for the high-energy transitions. Coincidence relationships have been obtained from measurements with a Ge(Li)-NaI(T1) coincidence apparatus. The target was Er 2 O 3 enriched to 95.6 % in 166 Er thus corresponding to a cross-section contribution of (68.8 +5.0 −6.4 ) % for 166 Er. More than 350 gamma-ray lines have been detected in the spectrum. Isotope assignment has been achieved by comparison of the spectra with those obtained from a sample of natural erbium. The high accuracy of the data and the coincidence results allow the construction of a transition diagram up to 1.5 MeV excitation energy. A large number of the observed energy levels have been assigned to specific configurations and their superimposed rotational bands. Detailed analysis of the data suggests considerable mixing between Nilsson states and the quadrupole vibrations Q 22 , Q 2−2 and Q 20 . Theoretical calculations have been performed which take into account pair correlations, quasiparticle-phonon interaction, rotation-vibration interaction and Coriolis coupling. It is shown that neglect of pair correlations leads to unreasonable matrix elements. While quasiparticle-phonon interaction and Coriolis coupling have decisive influence on the level structure and the de-excitation mechanism, the effects due to rotation-vibration interaction are in general small. The model predicts the energy and structure of individual levels, branching ratios for gamma-ray transitions, multipole mixtures and partial gamma-ray half-lives. In most cases, good agreement is achieved between the theoretical calculations and the experimental results. The neutron separation energy was found to be 436.15 ± 0.48 keV.

Energy levels of 98Mo excited in the (n, γ) reaction

Abstract The γ-ray spectra following thermal neutron capture in 97 Mo have been studied using Ge(Li) and NaI(Tl) detectors in various arrangements at the Karlsruhe research reactor FR-2. The target consisting of metallic molybdenum powder enriched to 92.8 % in 97 Mo was irradiated in external beam geometry. Energies and intensities of 247 transitions in the energy range 0.15 to 2.30 MeV and 4.9 to 8.7 MeV have been determined. Coincidence relationships measured with a Ge(Li)-Ge(Li) coincidence set-up were of considerable aid in constructing a level scheme for 98 Mo, which includes 29 excited states and 99 transitions. The analysis of angular correlation measurements of five cascades yielded the spin values of the levels involved in these cascades and the multipole mixtures of the linking transitions. The insufficiency of existing shell-model predictions for Mo isotopes is pointed out. Analogies with the properties of some states in the neighbouring 96 Mo isotope, as recently investigated by the present authors, are found. The discussion of the deduced levels in terms of a new collective model leads to the interpretation of 98 Mo as a nucleus in the transition region between vibrating spherical and rotating deformed nuclei.

Winkelverteilungsmessungen an γ-kaskaden aus der reaktion 57Fe(n, γ)58Fe

Abstract Within a study of the gamma radiation following thermal neutron capture in enriched 57 Fe(cross section 2.5 b), angular correlations of gamma-ray cascades have been measured with NaI(TI) scintillation detectors. By using a 2 × 1024 channel dual ADC connected to an on-line computer, more than 30 angular correlations could be determined simultaneously. Systematic errors arising from the coincident background under the peaks and from the complex structure of the capture spectrum were eliminated by application of the double-window technique. An extensive set of data has been obtained which allows the mutually consistent determination of a large number of level spins and multipole mixtures of the transitions observed in 58 Fe. The contributions of the two possible capture state spins (0 − and 1 − ) to the intensities of the primary transitions have been estimated for about 83 % of the observed total intensity. Conclusions about the level structure and capture mechanism are briefly discussed.

Untersuchung Der Anregungszustande Des 96Mo Kernes über Die Reaktion 95Mo(n, γ)96Mo

Abstract The gamma radiation following thermal neutron capture in 95 Mo has been studied using an enriched sample of MoO 3 in external target geometry at the Karlsruhe reactor FR 2. Energies and intensities of the gamma rays in the energy ranges 0.15 to 2.70 MeV and 4.87 to 8.90 MeV have been determined with Ge(Li) diodes in Compton-suppression and singles mode. Cascade relationships were studied with a Ge(Li)-Ge(Li) coincidence setup. A level scheme of 96 Mo is proposed which includes 37 levels and 138 gamma transitions. Gamma-gamma angular correlation measurements with an apparatus consisting of two NaI(T1) detectors were used to determine the spin values of 7 levels and the mixing ratios of the involved transitions. The most striking result is the observation of a second 2 + level in the region of the two-phonon states. This level is separated only by 2.3 keV from the well known 4 + state at 1628 keV. A level at 1148 keV appears to have spin and parity 0 + . The results are compared with earlier shell-model calculations and are discussed in terms of collective nuclear motions.