A. Metz

On the excitation energy of the ground state in the third minimum of U-234

Abstract The 233 U(d,pf) 234 U reaction has been studied with high energy resolution. The observed fission resonances in the excitation energy range of 4.75 ≤E ∗ ≤ 5.40 MeV were described as members of rotational bands with rotational parameters characteristic to the hyperdeformed nuclear shape (ℏ 2 /2 θ =2.1±0.2 keV). Information on the K values of the bands has been obtained from fission fragment angular distribution measurements. The level density of the most strongly excited J =3 states has been compared to the prediction of the back-shifted Fermi-gas formula and the energy of the ground state in the third minimum has been estimated to be E III = 3.1 ± 0.4 MeV.

Excited superdeformed Kπ=0+ rotational bands in β-vibrational fission resonances of 240Pu

The intermediate structure of fission resonances of 240 Pu was observed with an experimental energy resolution of 7 keV in the excitation energy region of E ∗ = 3.8–5.6 MeV using the 239 Pu(d, pf) 240 Pu reaction. Two-vibrational resonance groups centered at E ∗ = 4.6 MeV and 5.1 MeV, and attributed to the excitation of three and four β-phonons, were resolved into individual substates, which could be assigned to the low-spin members of K π = 0 + superdeformed (SD) rotational bands. In the region of the lower E ∗ = 4.6 MeV resonance individual moments of inertia of six well separated bands could be extracted for the first time with values of Θ/¯

Interplay of quasiparticle and phonon excitations in 181Hf observed through (n,γ) and (d→,p) reactions

Abstract Nuclear levels of 181 Hf were investigated in the range up to 3xa0MeV excitation energy by (n, γ ) and (d,p) reactions. Over 170 levels and about 390 γ -transitions were established most of them for the first time. 25 rotational bands were identified. Comparison of the results of the two reactions yields information on the fine structure in the fragmentation of Nilsson strength. The states below 2xa0MeV with the most complete spectroscopic information were interpreted in terms of the Quasiparticle Phonon Model (QPM). Excitation energies, electromagnetic transition rates, γ -branchings and spectroscopic factors are discussed in connection with their possible structure.

Study of 121Te with the reaction

Abstract The structure of the 121 Te nucleus has been studied with the 122 Te ( d , t ) reaction at E d =24.0 MeV. About 100 excited states have been observed up to an excitation energy of 2.65 MeV, and for most of them spin and parity values are proposed. A theoretical analysis based on the Interacting Boson–Fermion Model-1 is presented for the whole isotopic chain from 119 Te to 129 Te. The evolution of both positive and negative parity low-lying levels in these nuclei is reasonably well described with an essentially constant boson–fermion interaction. A detailed comparison of the calculations with experimental data is presented for 121 Te. The level scheme up to about 1.5 MeV excitation is reasonably well accounted for. Possible indications of states with “intruder” character are discussed.

A high intensity Stern-Gerlach polarized hydrogen source for the Munich MP-Tandem laboratory using ECR ionization and charge exchange in cesium vapor

The 14 year old Lamb-Shift hydrogen source of the Munich Tandem laboratory is presently replaced by a newly developed Stern-Gerlach type atomic beam source (ABS) with electron-cyclotron-resonance (ECR) ionization and subsequent double charge exchange in a supersonic cesium vapor jet target. The atomic beam source provides an intensity of 6.4*1016u200aatoms/sec of polarized hydrogen and of about 5*1016u200aatoms/sec of polarized deuterium. Beam intensities larger than 100 μA were observed for positive H+ and D+ ion beams after ECR ionization and intensities larger than 10 μA for negative D− ion beams in three magnetic substates.

Hyperdeformation and clusterization in the actinide region

The 233U(d,pf)234U, and the 235U(d,pf)236U reactions have been studied with high energy resolution. The observed fission resonances were described as members of rotational bands with rotational parameters characteristic of hyperdeformed nuclear shapes. Information on the K values of the bands and for the J values of the band members has been obtained from fission fragment angular distribution measurements. The level density of the most strongly excited states has been compared to the prediction of the back-shifted Fermi-gas formula and the energy of the ground state in the third minimum has been estimated for 234U. The fission fragment mass distribution of the hyperdeformed states in 236U has also been measured. The width of the mass distribution, coincident with the hyperdeformed bands, is significantly smaller than the ones obtained in coincidence with background regions below and above the resonances, which suggests a pear-shaped di-nuclear configuration of 236U in the third well of the potential barrier.

Nuclear structure of 181Hf studied in (n, γ) and (d, p) reactions

The level structure in 181Hf is studied by using (n, γ) and (d, p) reactions. Gamma-gamma coincidences permit an unambiguous placement of the majority of gamma transitions in the level scheme up to 1.8 MeV, while the (d, p) angular distributions and vector analyzing powers are used to deduce spin-parity assignments. Forty-three levels are grouped into 12 rotational bands built on one-quasiparticle and quasiparticle-plus-vibrational states. The proposed level structures are in qualitative agreement with earlier quasiparticle-phonon-model calculations.

Detailed spectroscopy in the superdeformed second minimum of 240Pu

Spectroscopic studies in the superdeformed shape isomer of 240Pu using γ-spectroscopy, conversion electron spectroscopy and transmission resonance spectroscopy have been performed. In a high-resolution and high-efficiency γ-spectroscopy experiment the out-of-band decays of several excited superdeformed rotational sequences with K=2− and 1− could be identified together with evidence for a weakly populated 0− octupole band. Surprisingly, no low-lying collective quadrupole excitations could be observed. Complementary information could be obtained in conversion electron measurements in coincidence with isomeric fission, resulting in the first identification of the lowest s-vibrational K=0+ band. For all rotational bands the variation of the moment of inertia with spin could be studied. A predominant population of negative parity states in the second well could be observed, which can be explained by the selective population and depopulation of the second minimum. Complementary transmission resonance measurements have been performed, yielding new information on the fine structure of high-lying (s-)vibrational multi-phonon states. A new method could be established to determine the excitation energy of the fission isomer ground state from measured level densities.

SINGLE PARTICLE EXCITATIONS IN 129BA FROM A (D, T) REACTION STUDY

Abstract The 130Ba( d , t) 129Ba reaction was studied with 25 MeV deuterons and excellent energy resolution. About 50 excited states have been observed up to Ex = 2.0 MeV and for 42 of them transferred l values and/or Jπ values were assigned on the basis of the measured angular distributions and asymmetries. The level scheme of 129Ba, the neutron pickup strength distribution and the known electromagnetic decay scheme are compared with results of calculations with the interacting boson-fermion model. These simple multi-shell IBFM-1 calculations describe well the main features of the experimental data.

Nuclear spectroscopy of odd Te isotopes

The odd-A Te isotopes, 119–131Te, have been investigated via (d,p), (p,p), (d,t), (3He,α) and (n,γ) reactions. The application of polarized deuterons in the (d,p) and (d,t) reaction allowed us to construct essentially complete level schemes with spin and parities up to about 2÷3u200aMeV. On the other hand, the measurement of γ−γ coincidences in the (n,γ) reaction enable us to place most of the γ transitions in the decay schemes of the odd Te isotopes. The experimental systematics of both positive and negative parity low-lying states for the whole odd Te chain are reasonably well described with Interacting Boson-Fermion Model-1 with an essentially constant boson-fermion interaction. The combination of the neutron transfer reactions and (n,γ) reaction helps us to understand the origin of the anomalous population of the long-lived isomers in odd-A Te isotopes. The evidence for the direct process mechanism in 127Te, 129Te and 131Te was observed.