607 research outputs found

    Extraction of level density and gamma strength function from primary gamma spectra

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    We present a new iterative procedure to extract the level density and the gamma strength function from primary gamma spectra for energies close up to the neutron binding energy. The procedure is tested on simulated spectra and on data from the Yb-173(He-3,alpha)Yb-172 reaction.Comment: 23 pages including 1 table and 7 figure

    Thermal and electromagnetic properties of 166-Er and 167-Er

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    The primary gamma-ray spectra of 166-Er and 167-Er are deduced from the (3-He,alpha gamma) and (3-He,3-He' gamma) reaction, respectively, enabling a simultaneous extraction of the level density and the gamma-ray strength function. Entropy, temperature and heat capacity are deduced from the level density within the micro-canonical and the canonical ensemble, displaying signals of a phase-like transition from the pair-correlated ground state to an uncorrelated state at Tc=0.5 MeV. The gamma-ray strength function displays a bump around E-gamma=3 MeV, interpreted as the pygmy resonance.Comment: 21 pages including 2 tables and 11 figure

    The SiRi Particle-Telescope System

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    A silicon particle-telescope system for light-ion nuclear reactions is described. In particular, the system is designed to be optimized for level density and gamma-ray strength function measurements with the so-called Oslo method. Eight trapezoidal modules are mounted at 5 cm distance from the target, covering 8 forward angles between theta = 40 and 54 degrees. The thin front dE detectors (130 micrometer) are segmented into eight pads, determining the reaction angle for the outgoing charged ejectile. Guard rings on the thick back E detectors (1550 micrometer) guarantee low leakage current at high depletion voltage.Comment: 6 pages, 8 figure

    Heat capacity and pairing transition in nuclei

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    A simple model based on the canonical-ensemble theory is outlined for hot nuclei. The properties of the model are discussed with respect to the Fermi gas model and the breaking of Cooper pairs. The model describes well the experimental level density of deformed nuclei in various mass regions. The origin of the so-called S-shape of the heat capacity curve Cv(T) is discussed.Comment: 6 pages + 8 figure

    Level density and gamma strength function in 162-Dy from inelastic 3-He scattering

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    Complementary measurements have been performed for the level density and gamma strength function in 162-Dy using inelastic 3-He scattering. Comparing these results to previous measurements using the 163-Dy(3-He,alpha) reaction, reveals that the measured quantities above 1.5 MeV do not depend significantly on the nuclear reaction chosen.Comment: 15 pages, including 7 figure

    Novel Techniques for Constraining Neutron-Capture Rates Relevant for r-Process Heavy-Element Nucleosynthesis

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    The rapid-neutron capture process (rr process) is identified as the producer of about 50\% of elements heavier than iron. This process requires an astrophysical environment with an extremely high neutron flux over a short amount of time (Ôł╝\sim seconds), creating very neutron-rich nuclei that are subsequently transformed to stable nuclei via ╬▓Ôłĺ\beta^- decay. One key ingredient to large-scale rr-process reaction networks is radiative neutron-capture (n,╬│n,\gamma) rates, for which there exist virtually no data for extremely neutron-rich nuclei involved in the rr process. Due to the current status of nuclear-reaction theory and our poor understanding of basic nuclear properties such as level densities and average ╬│\gamma-decay strengths, theoretically estimated (n,╬│n,\gamma) rates may vary by orders of magnitude and represent a major source of uncertainty in any nuclear-reaction network calculation of rr-process abundances. In this review, we discuss new approaches to provide information on neutron-capture cross sections and reaction rates relevant to the rr process. In particular, we focus on indirect, experimental techniques to measure radiative neutron-capture rates. While direct measurements are not available at present, but could possibly be realized in the future, the indirect approaches present a first step towards constraining neutron-capture rates of importance to the rr process.Comment: 62 pages, 24 figures, accepted for publication in Progress in Particle and Nuclear Physic

    Level densities and ╬│\gamma-ray strength functions in 170,171,172^{170,171,172}Yb

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    Level densities and radiative strength functions in 171^{171}Yb and 170^{170}Yb nuclei have been measured using the 171^{171}Yb(3^3He,3^3HeÔÇ▓╬│^\prime\gamma)171^{171}Yb and 171^{171}Yb(3^3He,╬▒╬│\alpha\gamma)170^{170}Yb reactions. New data on 171^{171}Yb are compared to a previous measurement for 171^{171}Yb from the 172^{172}Yb(3^3He,╬▒╬│\alpha\gamma)171^{171}Yb reaction. Systematics of level densities and radiative strength functions in 170,171,172^{170,171,172}Yb are established. The entropy excess in 171^{171}Yb relative to the even-even nuclei 170,172^{170,172}Yb due to the unpaired neutron quasiparticle is found to be approximately 2kBk_B. Results for the radiative strength function from the two reactions lead to consistent parameters characterizing the ``pygmy'' resonances. Pygmy resonances in the 170,172^{170,172}Yb populated by the (3^3He,╬▒\alpha) reaction appear to be split into two components for both of which a complete set of resonance parameters are obtained.Comment: 8 pages, 7 figure

    Level densities and ╬│\gamma-strength functions in 148,149^{148,149}Sm

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    The level densities and ╬│\gamma-strength functions of the weakly deformed 148^{148}Sm and 149^{149}Sm nuclei have been extracted. The temperature versus excitation energy curve, derived within the framework of the micro canonical ensemble, shows structures, which we associate with the break up of Cooper pairs. The nuclear heat capacity is deduced within the framework of both the micro canonical and the canonical ensemble. We observe negative heat capacity in the micro canonical ensemble whereas the canonical heat capacity exhibits an S-shape as function of temperature, both signals of a phase transition. The structures in the ╬│\gamma-strength functions are discussed in terms of the pygmy resonance and the scissors mode built on exited states. The samarium results are compared with data for the well deformed 161,162^{161,162}Dy, 166,167^{166,167}Er and 171,172^{171,172}Yb isotopes and with data from (n,╬│\gamma)-experiments and giant dipole resonance studies.Comment: 12 figure

    Level density and thermal properties in rare earth nuclei

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    A convergent method to extract the nuclear level density and the gamma-ray strength function from primary gamma-ray spectra has been established. Thermodynamical quantities have been obtained within the microcanonical and canonical ensemble theory. Structures in the caloric curve and in the heat capacity curve are interpreted as fingerprints of breaking of Cooper pairs and quenching of pairing correlations. The strength function can be described using models and common parameterizations for the E1, M1 and pygmy resonance strength. However, a significant decrease of the pygmy resonance strength at finite temperatures has been observed.Comment: 15 pages including 8 figures. Proceedings article for the conference Nuclear Structure and Related Topics, Dubna, Russia, June 6-10, 200
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