116 research outputs found
Consolidating the concept of low-energy magnetic dipole decay radiation
We have made a thorough study of the low-energy behaviour of the -ray
strength function within the framework of the shell model. We have performed
large-scale calculations spanning isotopic and isotonic chains over several
mass regions, with the purpose of studying the systematic behavior of the
low-energy enhancement (LEE) for transitions. There are clear trends in
the calculations: From being all but absent in the lowest mass region, the LEE
becomes steeper and more pronounced as the mass number increases, and for a
given mass region it further increases towards shell closures. Moreover, the
LEE is found to be steeper in regions near doubly-magic nuclei where proton
particles couple to neutron holes. These trends enable us to consolidate
several previous works on the LEE into a single, consistent concept. We compare
the inferred trends to the available experimental data from the Oslo method,
and find suppport for the systematic behaviour. Lastly we have compared the
calculations to strength functions compiled from discrete, experimental
lifetimes, and find excellent agreement; the discrete data are consistent with
a LEE, and indicate that the slope varies as function of mass number.Comment: 11 pages, 13 figure
Shell Evolution towards Ni 78: Low-Lying States in Cu 77
The level structure of the neutron-rich Cu77 nucleus is investigated through β-delayed γ-ray spectroscopy at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. Ions of Ni77 are produced by in-flight fission, separated and identified in the BigRIPS fragment separator, and implanted in the WAS3ABi silicon detector array, surrounded by Ge cluster detectors of the EURICA array. A large number of excited states in Cu77 are identified for the first time by correlating γ rays with the β decay of Ni77, and a level scheme is constructed by utilizing their coincidence relationships. The good agreement between large-scale Monte Carlo shell model calculations and experimental results allows for the evaluation of the single-particle structure near Ni78 and suggests a single-particle nature for both the 5/21- and 3/21- states in Cu77, leading to doubly magic Ni78. © 2017 American Physical Society
Statistical γ -decay properties of Ni 64 and deduced (n,γ) cross section of the s -process branch-point nucleus Ni 63
Particle-γ coincidence data have been analyzed to obtain the nuclear level density and the γ-strength function of Ni64 by means of the Oslo method. The level density found in this work is in very good agreement with known energy levels at low excitation energies as well as with data deduced from particle-evaporation measurements at excitation energies above Ex≈5.5 MeV. The experimental γ-strength function presents an enhancement at γ energies below Eγ≈3 MeV and possibly a resonancelike structure centered at Eγ≈9.2 MeV. The obtained nuclear level density and γ-strength function have been used to estimate the (n,γ) cross section for the s-process branch-point nucleus Ni63, of particular interest for astrophysical calculations of elemental abundances. © 2016 American Physical Society
Level densities and thermodynamical properties of Pt and Au isotopes
The nuclear level densities of Pt and Au below the
neutron separation energy have been measured using transfer and scattering
reactions. All the level density distributions follow the constant-temperature
description. Each group of isotopes is characterized by the same temperature
above the energy threshold corresponding to the breaking of the first Cooper
pair. A constant entropy excess and is observed in
Pt and Au with respect to Pt and Au,
respectively, giving information on the available single-particle level space
for the last unpaired valence neutron. The breaking of nucleon Cooper pairs is
revealed by sequential peaks in the microcanonical caloric curve
Completing the nuclear reaction puzzle of the nucleosynthesis of 92Mo
One of the greatest questions for modern physics to address is how elements
heavier than iron are created in extreme, astrophysical environments. A
particularly challenging part of that question is the creation of the so-called
p-nuclei, which are believed to be mainly produced in some types of supernovae.
The lack of needed nuclear data presents an obstacle in nailing down the
precise site and astrophysical conditions. In this work, we present for the
first time measurements on the nuclear level density and average strength
function of Mo. State-of-the-art p-process calculations systematically
underestimate the observed solar abundance of this isotope. Our data provide
stringent constraints on the NbMo reaction rate,
which is the last unmeasured reaction in the nucleosynthesis puzzle of
Mo. Based on our results, we conclude that the Mo abundance
anomaly is not due to the nuclear physics input to astrophysical model
calculations.Comment: Submitted to PR
Nuclear level densities and ray strength functions of Sn isotopes studied with the Oslo method
The Sn isotopes have been studied with (),
(), and () reactions to extract the nuclear
level densities (NLDs) and -ray strength functions (GSFs) of these
nuclei below the neutron separation energy by means of the Oslo method. The
experimental NLDs for all three nuclei demonstrate a trend compatible with the
constant-temperature model below the neutron separation energy while also being
in good agreement with the NLDs of neighboring Sn isotopes, obtained previously
with the Oslo-type and neutron evaporation experiments. The extracted
microcanonical entropies yield entropy of a valence neutron
in both Sn and Sn. Moreover, the deduced microcanonical
temperatures indeed suggest a clear constant-temperature behavior above
3 MeV in Sn and above 4.5 MeV in Sn. We
observe signatures for the first broken neutron pairs between 2 and 4 MeV in
all three nuclei. The GSFs obtained with the Oslo method are found to be in
good agreement below the neutron threshold with the strengths of Sn
extracted in the () Coulomb excitation experiments.Comment: 13 pages, 9 figure
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