The spectroscopy and structure of the transitional nucleus219Ra

The interchange of two sets of spins in the level structure of²¹⁹Ra observed following the alpha decay of²²³Th and the suggestion that the ground state of²¹⁹Ra was not observed in the heavy ion reaction spectroscopy²⁰⁸Pb(¹⁴C, 3n), allow the correlation of these levels which were previously unconnected. The resulting level structure is interpreted in terms of and parity doublet bands which evolve from anomalous rotational structures into vibrational-like structures with alternating spins and parities. The level structure of²¹⁹Ra is successfully interpreted both in terms of octupole deformed (ε ₃=0.08) Nilsson levels and in terms of intermediate coupling using normal Nilsson levels with very strong octupole correlations. The levels in²¹⁹Ra are then compared to the corresponding levels in a series of isotopic and isotonic nuclei to trace the collapse of octupole-quadrupole deformed nuclear structure into the more degenerate shell-model spectroscopy.     

Study of low-lying states in151Eu via (n,n′γ) reaction

The levels of¹⁵¹Eu have been investigated in the (n,n′γ) reaction using nuclear reactor fast neutrons. The energies, intensities and angular distributions of theγ-rays have been measured with the Ge(Li) spectrometer. Four rotational bands with the following band heads and Nilsson configurations have been identified: ground state band, 5/2⁺ [402]; 21.5 keV, 7/2⁺[404]; 196.5 keV, 3/2⁺[411]; 260.5 keV, 5/2⁺[413]. The low spin states at 332.2 and 336.2 keV have been tentatively assigned to the l/2⁺[411] Nilsson orbital, but 522.8, 580.0 and 587.0 keV states to the 1/2⁺[420] Nilsson orbital. The negative parity levels at 353.7, 522.1 and probably 546.2 keV have been proposed basing on theh _11/2 proton state.     

An unusualM1 electromagnetic transition connecting octupole and quadrupole vibrations in179Hf

A detailed level scheme for¹⁷⁹Hf has been constructed on the basis of extensive (n, γ), (d, p) and (d, t) data. The low lying levels are grouped into 15 rotational bands of which 14 are classified by Nilsson quantum numbers or vibrational configurations. One of the most interesting results concerns a dominant transition matrix element which connects an octupole vibration built on the 1/2[510] Nilsson orbit with a quadrupole vibration built on the 9/2⁺ [624] Nilsson ground state. A simple interpretation of this unusual decay mode is offered in terms of microscopic wave functions for the vibrational states. This interpretation leads to reasonable estimates forE1 hindrance factors from the octupole vibration to lower lying quasi-particle levels.     

Nuclear levels in193Os

The previously completely unknown energy levels of¹⁹³Os have been investigated using the reactions¹⁹²Os(d, p)¹⁹³Os and¹⁹²Os(n, γ)¹⁹³Os. The neutron separation energy was measured to be 5583.5±2.0 keV. Most of the states below 500 keV can be qualitatively interpreted in terms of the Nilsson model; however the occurrence of four levels with significantl=1 transfer strength in this energy range is incompatible with that model. The Nilsson orbital systematics of the odd-A Os and W isotopes are compared.     

Levels of233Pa excited in helium-induced single-proton transfer reactions

The²³³Th(³He,d)²³³Pa and²³³Th(α, t)²³³Pa reactions were studied with 28.5-MeV³He and 30-MeV α-particles, respectively. The reaction results are interpreted in terms of a distorted-wave Born-approximation analysis. Transferred angular momenta are deduced from (α, t) and (³He,d) cross-section ratios,R=dσ(α, t)/dσ(³He,d). Theoretical cross sections, calculated in the framework of the Nilsson model with pairing and Coriolis interactions included, are in reasonable agreement with experiment. Rotational bands built on the following Nilsson proton configurations are identified in²³³Pa: 1/2_?[530↑], 3/2⁺[651↑], 1/2⁺[660↑], 5/2⁺[642↑], 5/2_?[5231↓], and 3/2_?[521↑].     

The study of the angular distributions of tritons from the reaction162Dy (d,t)161Dy at 12·1 mev

The angular distributions of tritons from the reaction¹⁶²Dy (d, t)¹⁶¹Dy were measured. The experimental results were analysed by a Distorted Waves Born Approximation method and the transferred angular momenta were determined for the stronger transitions. The experimental relative intensities of transitions at 90° were compared with those obtained from the Nilsson's model calculations including Coriolis and pairing effects. Eight rotational bands, built on the 5/2+[642], 5/2?[523], 3/2?[521], 1/2?[521], 11/2?[505], 3/2?[532], 1/2?[530] and 7/2+[404] Nilsson states were established. Some new information about the strongly mixed even parity 1/2+[400], 1/2+[660], 3/2+[402] and 3/2+[651] Nilsson states is presented.     

Beta decay of 157Dy

The decay of ¹⁵⁷Dy(T_1/2 = 8.14(4)h) is investigated. Spectra of γ-rays, internal conversion electrons and γ ? γ coincidences are measured. The 983.50 and 991.70 keV transitions show E0 admixtures. Our experimental results made it possible to extend substantially the decay scheme. From the coincidence relationships we established four new excited states of ¹⁵⁷Tb at 883.32, 895.06, 925.29, 970.38 keV and confirmed the 357.66 keV state. The levels at 1160.71 and 1318.62 keV are introduced from the energy balance. Tentative evidence is given for the existence of five new levels at 252.58, 327.65, 658.36, 793.52 and 922.69 keV. We assign Nilsson’s quantum characteristics to all introduced levels. The structure of the excited states of ¹⁵⁷Tb is discussed in terms of current nuclear models.     

Evidence for rotational bands in103Nb

The nucleus¹⁰³ Nb ₆₂ has been studied through the ß^? decay of the fission product¹⁰³Zr at LOHENGRIN and JOSEF. The energies of the lowlying levels and the γ transitions indicate rotational bands based on the Nilsson proton configurations [422 5/2⁺] [303 5/2^?] and [301 3/2^?] at 0, 164 and 248 keV, respectively. The measured half-lives of (4.7±0.5) ns and (2.0±0.6) ns of the levels at 164 and 248 keV are consistent with the Nilsson-model estimates including pairing.     

The decay of 1.4 min173Er

A 1.4 ± 0.1 min activity which is assigned to β^?-decay of ₆₈ ¹⁷³ Er has been produced with 14–15 MeV neutrons through the reaction¹⁷⁶Yb(n, α)¹⁷³Er. Its decay has been studied with Ge(Li) detectors and several (γ)(γ) as well as (γ)(β) coincidence arrangements. Eight gamma rays with energies 94.2, 116.14, 118.6, 122.40, 192.8, 199.2, 800.8 and 895.2 keV were assigned to the decay of¹⁷³Er. The proposed level scheme of the daughter nuclide ₆₉ ¹⁷³ Tm contains levels at 0, 2.5, 118.6, 124.9, 317.7, 411.9 and 1212.8 keV. The 317.7 keV level is an isomeric state with a measured half-life of 10±3 μs. This 10 μs isomer and the 411.9 keV level are thought to be the band head and 9/2^? rotational member of 7/2^? [523] Nilsson state, respectively. The levels at 0, 2.5, 118.6 and 124.9 keV are interpreted as 1/2⁺, 3/2⁺, 5/2⁺ and 7/2⁺ members of the 1/2⁺ [411] band and the level at 1212.8 keV as the 9/2^? [514] Nilsson state. Some systematic considerations and theoretical transition probability calculations are also included.     

Conversion electron measurement in the β<Superscript>-</Superscript>-decay of <Superscript>151</Superscript>Pr

The β^--decay of ¹⁵¹Pr produced by the thermal neutron-induced fission of ²³⁵U has been studied using an on-line isotope separator. From an internal-conversion electron measurement with a Si(Li) detector, K-conversion coefficients were obtained for 20 γ-transitions. Spins and parities of 6 excited levels in ¹⁵¹Nd were newly determined from the deduced multipolarities: even parities for the 543 and 627 keV levels, odd parities for 250 and 599 keV, (3/2, 5/2)⁺ for 685 keV, and (1/2, 3/2)⁺ for 880 keV. The level structure was compared with the rotation-vibration coupling Nilsson model. Received: 28 June 2002 / Accepted: 31 October 2002 / Published online: 25 February 2003 RID="a" ID="a"e-mail: ykojima@hiroshima-u.ac.jp Communicated by J. ?yst?     

SU(3) coupling schemes for odd-odd nuclei in the interacting boson – fermion – fermion model with both odd proton and odd neutron in natural parity orbits

The SU^BFF(3) dynamical symmetry limits of interacting boson – fermion – fermion model are identified and they are appropriate for heavy deformed odd – odd nuclei for configurations with both the odd proton and odd neutron occupying all the natural parity orbits in the corresponding valence shells. There are three symmetry limits and their correspondence with two quasi-particle (proton-neutron) Nilsson configurations is established; one of the limits mixes both Nilsson n_z's and Λ's and other two limits mix only Nilsson Λ's. The ¹⁹¹Ir (d,t) ¹⁹⁰Ir single nucleon transfer spectroscopic strengths are well described by one of the symmetry limits that mixes only Nilsson Λ's. Received: 22 June 1998     

The deformedN=60 nucleus 41 101 Nb

The ^– decay of¹⁰¹Zr has been investigated at the fission-product separators JOSEF and LOHENGRIN. The half-life of¹⁰¹Zr has been determined to 2.5(1) s and a level scheme for¹⁰¹Nb has been established from ray singles as well as X/3- and — coincidence measurements. Conversion coefficients for transitions in¹⁰¹Nb and level half-lives between 10 ps and 2 ns have been determined. Three rotational bands are identified among the low-lying levels with band heads at 0 keV, 206 keV and 208 keV. The bands are probably based on the Nilsson configurations [422 5/2⁺], [301 3/2^–] and [303 5/2^–], respectively. The deformation has been determined to _q=0.40(4) and 0.41(8) for the ground state band and the band based on the 206 keV level from the half-lives of the first and second excited members of these bands. This shows that the rapid onset of deformation at N=60 which is typical for the A=100 region of neutron-rich nuclei, takes also place in the Nb isotopes. Nilsson model calculations describe the experimental data well, especially the several determined transition probabilities including those for E1 transitions from the 206 and 208 keV band heads to the ground state.Dedicated to Prof. Dr. P. Armbruster on the occasion of his 60th birthday     

Lifetime measurements in99Sr and100Zr

The lifetime of the first excited state in⁹⁹Sr has been determined by the delayed γ-γ coincidence method. The value of t_/2=0.58(9) ns corresponds to a deformation parameter ß=0.38(4). The deduced z> value confirms the previously proposed ν[411]3/2 Nilsson assignment for the⁹⁹Sr ground state. In addition, lifetime values for the 2⁺ (t_1/2=0.40(8) ns) and 0⁺ (t_1/2=5.36(23) ns) states in¹⁰⁰Zr are obtained, which deviate from older measurements and indicate a larger deformation of ß=0.39(4).     

Gammaspektrum nach Neutroneneinfang in Eu152(12,4a) und Niveauschema von Eu153

Using a source of Eu ₂ ¹⁵¹ O₃ the (n, γ) spectrum of Eu¹⁵² and Eu¹⁵³ was investigated by means of the Risø bent crystal spectrometer. A total of 57 transitions were fitted into a level scheme of Eu¹⁵³. The rotational bands of the Nilsson states 5/2⁺ [413], 5/2^? [532], and 3/2⁺ [411] were found up to the spin values 15/2, 13/2, and 11/2 respectively. The 13/2 member of the 3/2⁺ [411] band is suggested at 716.15 keV. The interpretation of these levels is confirmed by the comparison of the experimental branching ratios with theoretical predictions and by considerations concerning the spin dependent population of these levels.     

Systematics of deformed states around doubly-magic <Superscript>40</Superscript>Ca

The low-lying rotational bands of A = 36-48 nuclei are consistently explained by starting from the recently discovered, superdeformed intrinsic state of ³⁶Ar as the core, filling successively the first three Nilsson orbits above the Fermi border. The critical single-particle energies were obtained from experimental data as were the residual interactions in the parametrization of Brink and Kerman. Implicit are the rearrangement energies due to configuration-dependent equilibrium deformations. The binding energies of 20 experimental bandheads were used to derive the parameters while another 38 bandheads were subsequently predicted and identified almost completely. The Racavy expression reduced by 20% reproduces or predicts the values of the deformation parameter . The empirical Nilsson model amended by -vibrational and rotation-aligned bands accounts completely for the multi-particle excitations from the N = 2 into the N = 3 major shell which are not accessible by shell-model calculations. In the case of ⁴⁰Ca a spectrum of 42 states below E _x = 8 MeV is explained.PACS: 21.60.Ev Collective models - 21.10.Dr Binding energies and masses - 27.30. + t - 27.40. + z      

Single-Proton states of159Ho studied with the (p,α) reaction spectroscopic evidence for the unusual coriolis strengths in159, 161, 163Ho nuclei

The proton-hole character of states in¹⁵⁹Ho has been studied using the^{162, 164}Er(p, α)^{159, 161}Ho reactions with 17 MeV protons. The shapes of angular distributions measured for both nuclei were compared, with good agreement in most cases, to tritoncluster-transfer DWBA calculations. The deviations obtained and the experimental normalization factors deduced for the¹⁶⁴Er(p, α)¹⁶¹Ho reaction by comparisons to (t, α) cross sections indicate a need for a revised treatment of the mechanism of the (p, α) reaction on deformed nuclei. By comparisons to the well-known states in¹⁶¹Ho, nuclear structure factors were obtained for many levels in¹⁵⁹Ho. The Nilsson assignments for the 3/2⁺[411], 5/2^?[532] and 5/2⁺[413] bands were confirmed and the lowest rotational members of these bands were established. The anomalous strengths of the strongly mixed positive parity states in^{159, 161, 163}Ho are discussed in the frame-work of the Nilsson model with the pairing and effective Coriolis strengths included. The Coriolis strengths giving best fits to excitation energies also improve the agreement of theory with the experimental nuclear structure factors. Rotational spacings of the 5/2⁺[413] and especially those of the 5/2^?[532] bands in^{159, 161, 163}Ho show mixmg effects unexplainable by the present calculations.     

Penning-trap-assisted study of 115Ru beta decay

The beta decay of ¹¹⁵Ru has been studied by means of Penning-trap-assisted beta and gamma spectroscopy at the IGISOL facility. The level scheme of ¹¹⁵Rh has been substantially extended and compared with the level systematics of lighter rhodium isotopes. Tentative candidates for three states of the deformed K = 1/2 band have been suggested. The beta-strength distribution of the beta decay of ¹¹⁵Ru differs from the beta decays of ^{111, 113, 113m}Ru isotopes due to non-observation of the 3-quasiparticle states in ¹¹⁵Rh. The decay properties of ¹¹⁵Ru indicate a spin-parity of (3/2⁺ for its beta-decaying ground state. In addition, possible Nilsson states as well as the shape and spin transitions in odd neutron-rich ruthenium isotopes are discussed.     

In-beam γ-ray spectroscopy with GASP

The performance of the multi-detector array GASP is described in detail with emphasis on the improvement due to utilization of ancillary detectors. GASP has been running for three years producing interesting new data on different nuclear structure topics. The study of the decay-out of superdeformed bands in the A=130 mass region is presented. The sudden disappearance of the superdeformed bands in the odd^{133, 135, 137}Nd isotopes is explained by the Total Routhian Surface calculations through a change of the nuclear shape which is microscopically related to the transfer of the valence neutron from a N=6 to a N=4 Nilsson orbital. A transient fieldg-factor measurement in the superdeformed band of¹³³Nd will also be presented. A meang-factor of $\bar g = 0.31(8)$ has been determined at the I*=41/2⁺ superdeformed state. The experimental value is compared with theoretical predictions and supports the assignment of the superdeformed band to thev[660]1/2⁺ Nilsson intruder orbital.     

Renaissance of the Nilsson-model approach to light nuclei: The case of the A = 26 system

The farthest reaching interpretation of a light nuclear system in terms of the Nilsson-model is obtained for A = 26. The T = 1 and T = 0 levels of either parity in ²⁶Mg and/or ²⁶Al are shown to be grouped in more than 35 rotational bands together containing some 55 levels in ²⁶Mg and 170 in ²⁶Al. The Nilsson-model configurations and deformations of bands are deduced from measured spectroscopic factors and electromagnetic transition rates. The experimental data are amended by shell-model calculations in the complete s-d basis space based on the universal s-d Hamiltonian. The rotational structure of positive-parity states is completely contained in the shell-model results. The Nilsson diagram gives a natural explanation of all bands. An important facet is the observation of particle-hole configurations with large K which arise from the complete alignment of particle and hole angular momenta along the major axis of deformation.