607 research outputs found

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

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

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

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

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

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

The rapid-neutron capture process ($r$ 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 ($\sim$ seconds), creating very neutron-rich nuclei that are
subsequently transformed to stable nuclei via $\beta^-$ decay. One key
ingredient to large-scale $r$-process reaction networks is radiative
neutron-capture ($n,\gamma$) rates, for which there exist virtually no data for
extremely neutron-rich nuclei involved in the $r$ 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 ($n,\gamma$) rates may vary by orders of magnitude and
represent a major source of uncertainty in any nuclear-reaction network
calculation of $r$-process abundances. In this review, we discuss new
approaches to provide information on neutron-capture cross sections and
reaction rates relevant to the $r$ 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 $r$ process.Comment: 62 pages, 24 figures, accepted for publication in Progress in
Particle and Nuclear Physic

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

Level densities and radiative strength functions in $^{171}$Yb and $^{170}$Yb
nuclei have been measured using the
$^{171}$Yb($^3$He,$^3$He$^\prime\gamma$)$^{171}$Yb and
$^{171}$Yb($^3$He,$\alpha\gamma$)$^{170}$Yb reactions. New data on $^{171}$Yb
are compared to a previous measurement for $^{171}$Yb from the
$^{172}$Yb($^3$He,$\alpha\gamma$)$^{171}$Yb reaction. Systematics of level
densities and radiative strength functions in $^{170,171,172}$Yb are
established. The entropy excess in $^{171}$Yb relative to the even-even nuclei
$^{170,172}$Yb due to the unpaired neutron quasiparticle is found to be
approximately 2$k_B$. Results for the radiative strength function from the two
reactions lead to consistent parameters characterizing the ``pygmy''
resonances. Pygmy resonances in the $^{170,172}$Yb populated by the
($^3$He,$\alpha$) 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 $^{148,149}$Sm

The level densities and $\gamma$-strength functions of the weakly deformed
$^{148}$Sm and $^{149}$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 $^{161,162}$Dy,
$^{166,167}$Er and $^{171,172}$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

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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