K.P. Lieb

Gamma ray energies and 36Cl level scheme from the reaction 35Cl(n, γ)

Abstract The γ-ray spectrum emitted after thermal neutron capture in 35Cl has been studied by use of the crystal and pair spectrometers installed at the ILL high flux reactor. We identified about 400 transitions in this reaction 326 of which were placed into the 36C1 level scheme; several new states were found. The level energies up to 3.5 MeV were measured with a precision of 5–20 eV relative to the 412 keV 198Au standard, those above 3.5 MeV with a precision of 10ppm. The neutron binding energy was determined to be EB = 8579.68(9) keV.

Characterization of laser-nitrided iron and sputtered iron nitride films

Laser-nitriding may be a promising technique for substituting conventional nitriding processes. We have irradiated pure iron with pulses of an excimer laser and achieved high nitrogen contents in a thin surface layer. We found that the nitrogen is dissolved into γ-Fe, leading to a large amount of retained austenite. This was also verified by X-ray diffraction (XRD) measurements. Three subspectra can be resolved in the Mössbauer spectra (CEMS) for this nitrogen austenite. The nitrogen concentration can be calculated in terms of site occupation, indicating a content as high as 16(1) at%, which is consistent with the results of Rutherford backscattering spectroscopy (RBS), resonant nuclear reaction analysis (RNRA) and Auger electron spectroscopy (AES) measurements. This is more than the solubility limit for γ-Fe(N). By reactive magnetron-sputtering it is possible to produce thin iron nitride films of various stoichiometries. We report on the production ofε-FexN and FeNy films. These films were again characterized by CEMS, RBS, RNRA (15N(p, αγ)) and XRD. Forε-FexN, produced in the range 2⩽x⩽3 with medium nitrogen flows during reactive sputtering, the Mössbauer spectra can be well resolved in terms of different iron sites, enabling an accurate calculation of the nitrogen content. For high nitrogen flows during sputtering a phase FeNy withy>0.5 is produced. This phase is not reported in the Fe-N phase diagram.

Frozen target measurements of the 430 keV 15N(p,αψ) resonance

Because of the extensive use of the 15N(p,αψ) 12C resonance reaction for nitrogen and hydrogen depth profiling near solid surfaces, we have remeasured the energy and width of the 430 keV resonance to be Er=429.57(9) keV and Γ=124(17) eV, using a frozen solid nitrogen target.

Dopper shift attenuation lifetime measurement in 54Cr following thermal neutron capture

Abstract The double crystal spectrometer GAMS4 in combination with the ILL high flux reactor has been used to determine the lifetimes of the 2620 KeV 2 + 2 , 3074 KeV 2 + 3 and 3720 KeV (1, 2) + states in 54 Cr. The initial recoil energy of about 0.5 KeV imparted by the primary γ-radiation after thermal neutron capture in 53 Cr produces Doppler broadened line shapes of the secondary transitions. The large 2 + 3 →2 + 1 M1 strength of B (M1)=0.39(6) μ 2 N suggests the 2 + 3 state to be mixed symmetry character within the interacting boson model IBM-2.

Nuclear structure studies of high-spin states in 79Rb and 79Kr

Abstract Fluorine and carbon induced fusion-evaporation reactions were used to excite high-spin states in 79 Rb and 79 Kr. The level scheme of 79 Rb was established on the basis of neutron multiplicity measurements, γγ coincidence data and excitation functions. From recoil distance and Doppler shift attenuation lifetime measurements, E2 strengths of the positive-parity yrast states in 79 Kr ( up to I π = 21 2 + ) and the favored and unfavored states in 79 Rb ( up to I π = 25 2 + ) were derived. An interpretation of these data in terms of the asymmetric rotor-plus-particle model and the interacting boson fermion model (with or without separate proton and neutron bosons) is proposed. Energies and lifetimes of a second stretched band in 79 Rb (probably with negative parity) were also measured.

Coulomb excitation and lifetime measurements of the 21+ states in 76, 82, 84Kr

Abstract A mean lifetime of τ = 35 ± 3 ps of the 2+1 state in 76Kr has been measured with the recoil distance method via the reaction 63Cu(19F, α2n)76Kr. The B(E2; 0+1 → 2+1) values and lifetimes of the 2+1 states in 82, 84Kr have been measured via Coulomb excitation using the 1.4 MeV/A UNILAC krypton beams. The intensities of the γ-rays from the Coulomb excited levels of 82, 84Kr were interrelated with those of the target nuclei 27Al, 64, 66Zn and 70, 72, 74, 76Ge and yielded the values B( E 2; 0 + 1 → 2 + 1 = 0.255±0.009 and 0.122 ± 0.005 e 2 · b 2 for 82, 84 Kr , respectively. In turn, these B(E2) values and the (E2; 0+1 → 2+1 values of the even Ge and Zn isotopes from the literature were used in a Doppler-shift attenuation analysis to obtain experimentally lacking electronic stopping power for Kr ions slowing down in Al, Zn and Ge. for Ge ions in Ge and for Zn ions in Zn.

Study of electromagnetic transition strengths in 81Rb

Abstract High-spin states in 81 Rb were excited in the fusion-evaporation reaction 65 Cu( 19 F, p2n) 81 Rb. Their lifetimes were determined by means of the recoil distance and Doppler-shift attenuation techniques. Deduced B (E2) and B (M1) transition strengths are discussed in the framework of the asymmetric rotor-plus-particle model.

The yrast bands in 77Kr and 76Kr

Abstract High-spin states of both parities in 77 Kr have been studied in the reaction 63 Cu( 16 O, pn). On the basis of γγ- and nγ-coincidence experiments, we extended the yrast bands up to spin I π = 25 2 + and 21 2 t- . Doppler-shift attenuation and recoil-distance measurements have been performed for some fifteen states. In addition, lifetimes of some yrast states in 76 Kr have been determined in the reaction 63 Cu( 16 O, p2n). Transition energies and E2 strengths in 76 Kr have been interpreted with the IBA-2 and different collective and microscopic triaxial rotor models, those in 77 Kr with the triaxial rotor plus quasiparticle model.

Side-feeding times and level lifetimes measured in the reaction 54Fe(32S, 3p)83Y

Abstract We have performed recoil-distance and Doppler-shift-attenuation lifetime measurements in the reaction 54Fe(32S, 3p)83Y at 103–107 MeV beam energy. By analyzing the Doppler-broadened line shapes of the 964 keV 21 2 + → 17 2 + yrast transition in coincidence with feeding and depopulating transitions, the average side-feeding time and lifetime of this state have been deduced as τF = 0.6(2) ps and τ = 0.62(10) ps, respectively. A similar analysis of the 811 keV 17 2 + → 13 2 + transition gave values of τF = 0.65(25) ps and τ = 1.5(2) ps, respectively for the 17 2 + yrast state. Feeding times of the higher transitions in the positive-parity yrast band have been estimated from their Dopplerbroadened lineshapes measured in coincidence with depopulating transitions only. The feeding times are compared with the results of Monte Carlo calculations of the time development of the continuum γ-ray flux, and a damping width of 2–3 MeV has been deduced. The measured level lifetimes support the regular band structures with the deformation β2 = 0.33 previously inferred from the transition energies.

The MINIBALL array

Abstract MINIBALL a highly efficient, position sensitive γ -spectrometer is under construction for upcoming radioactive-beam experiments. In contrast to high spin arrays the total full energy peak efficiency and not the resolving power is optimized with a factor of two larger efficiency for γ -ray energies up to 10 MeV and for low to medium γ -ray multiplicity events. Cluster detectors, incorporating three or four Ge crystals, will be employed to provide high flexibility and to accomodate the various demands of the experiments.