1,445 research outputs found
Can Hyperfine Excitation explain the Observed Oscillation-Puzzle of Nuclear Orbital Electron Capture of Hydrogen-like Ions?
Modulated in time orbital electron capture (EC) decays have been observed
recently in stored H-like Pr and Pm ions.
Although, the experimental results are extensively discussed in literature, a
firm interpretation has still to be established. Periodic transitions between
the hyperfine states could possible lead to the observed effect. Both selected
nuclides decay to stable daughter nuclei via allowed Gamow-Teller transitions.
Due to the conservation of total angular momentum, the allowed EC decay can
only proceed from the hyperfine ground state of parent ions. In this work we
argue that periodic transitions to the excited hyperfine state (sterile) in
respect to the allowed EC decay ground state cannot explain the observed decay
pattern
Testing Chiral Dynamics in Pionic Atoms
The energy dependence of chirally expanded pi N isoscalar and isovector
amplitudes b_0(E) and b_1(E) respectively, for zero-momentum off shell pions
near threshold, is used to impose the minimal substitution requirement E -> E -
V_c on the properly constructed pion optical potential within a large-scale fit
to 100 pionic-atom data across the periodic table which also include the
recently established `deeply bound' pionic atoms of Pb and Sn. This fit cannot
be reconciled with the well known free-space values of the pi N threshold
amplitudes. In contrast, introducing the empirically known energy dependence
for on-shell pions leads to a better fit and to satisfactory values for the pi
N threshold amplitudes. The difference between these two approaches is briefly
discussed.Comment: 10 pages, 3 figures, submitted to PLB. Discussion section rewritten,
omitting an erroneous equation. Results and conclusions unchanged Accepted by
PL
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Spall And Dynamic Yielding Of Aluminum And Aluminum Alloys At Strain Rates Of 3X10(6) S(-1)
We have explored the role that grain size, impurity particles and alloying in aluminum play in dynamic yielding and spall fracture at tensile strain rates of similar to 3x10(6) We achieved these strain rates shocking the aluminum specimens via laser ablation using the Z-Beamlet Laser at Sandia National Laboratories. The high purity aluminum and 1100 series aluminum alloy produced very different spall strengths and nearly the same yield strengths. In contrast, various grain-sized Al + 3 wt. % Mg specimens presented the lowest spall strength, but the greatest dynamic yield strength. Fracture morphology results and particle analysis are presented along with hydrodynamic simulations to put these results in context. Impurity particles appeared to play a vital role in spall fracture at these fast strain rates. Alloying elements such as Mg seem to be the dominant factor in the dynamic yield results.Mechanical Engineerin
Observation of Non-Exponential Orbital Electron Capture Decays of Hydrogen-Like Pr and Pm Ions
We report on time-modulated two-body weak decays observed in the orbital
electron capture of hydrogen-like Pr and Pm
ions coasting in an ion storage ring. Using non-destructive single ion,
time-resolved Schottky mass spectrometry we found that the expected exponential
decay is modulated in time with a modulation period of about 7 seconds for both
systems. Tentatively this observation is attributed to the coherent
superposition of finite mass eigenstates of the electron neutrinos from the
weak decay into a two-body final state.Comment: 12 pages, 5 figure
Renormalization of the isovector amplitude in pionic atoms
The extraction of the isovector s-wave pi N amplitude from pionic atoms is
studied with special emphasis on uncertainties and their dependence on the
assumptions made regarding the neutron density distributions in nuclei and on
the size of the data base used . Only `global' analyses of pionic-atom data
reveal a discrepancy between the extracted isovector s-wave pi N amplitude
b_1=-0.108\pm0.007 m_\pi^{-1} and its free pi N counterpart
b_1^{free}=-0.0885^{+0.0010}_{-0.0021} m_\pi ^{-1}, where the uncertainty in
the neutron densities is included in the error analysis. The role of `deeply
bound' pionic atom states is discussed and the reason for failure of these
states to provide new information is explained.Comment: 17 pages, 4 figures, slightly extended, accepted by NP
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Laser-Induced Spall Of Aluminum And Aluminum Alloys At High Strain Rates
We conducted laser-induced spall experiments aimed at studying how a material's microstructure affects the tensile fracture characteristics at high strain rates (> 10(6) s(-1)). We used the Z-Beamlet Laser at Sandia National Laboratory to drive shocks and to measure the spall strength of aluminum targets with various microstructures. The targets were recrystallized, high-purity aluminum (Al-HP RX), recrystallized aluminum + 3 wt.% magnesium (Al-3Mg RX), and cold-worked aluminum + 3 wt.% magnesium (Al-3Mg CW). The Al-3Mg RX and Al-3Mg CW are used to explore the roles that solid-solution alloying and cold-work strengthening play in the spall process. Using a line-VISAR (Velocity Interferometer System for Any Reflector) and analysis of recovered samples, we were able to measure spall strength and determine failure morphology in these targets. We find that the spall strength is highest for Al-HP RX. Analysis reveals that material grain size plays a vital role in the fracture morphology and spall strength results.Mechanical Engineerin
Application of the RMF mass model to the r-process and the influence of mass uncertainties
A new mass table calculated by the relativistic mean field approach with the
state-dependent BCS method for the pairing correlation is applied for the first
time to study r-process nucleosynthesis. The solar r-process abundance is well
reproduced within a waiting-point approximation approach. Using an exponential
fitting procedure to find the required astrophysical conditions, the influence
of mass uncertainty is investigated. R-process calculations using the FRDM,
ETFSI-Q and HFB-13 mass tables have been used for that purpose. It is found
that the nuclear physical uncertainty can significantly influence the deduced
astrophysical conditions for the r-process site. In addition, the influence of
the shell closure and shape transition have been examined in detail in the
r-process simulations.Comment: to be published in Phys. Rev. C, 22 pages, 9 figure
Enhanced Production of Neutron-Rich Rare Isotopes in Peripheral Collisions at Fermi Energies
A large enhancement in the production of neutron-rich projectile residues is
observed in the reactions of a 25 MeV/nucleon 86Kr beam with the neutron rich
124Sn and 64Ni targets relative to the predictions of the EPAX parametrization
of high-energy fragmentation, as well as relative to the reaction with the less
neutron-rich 112Sn target. The data demonstrate the significant effect of the
target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi
energies. A hybrid model based on a deep-inelastic transfer code (DIT) followed
by a statistical de-excitation code appears to account for part of the observed
large cross sections. The DIT simulation indicates that the production of the
neutron-rich nuclides in these reactions is associated with peripheral nucleon
exchange. In such peripheral encounters, the neutron skins of the neutron-rich
124Sn and 64Ni target nuclei may play an important role. From a practical
viewpoint, such reactions between massive neutron-rich nuclei offer a novel and
attractive synthetic avenue to access extremely neutron-rich rare isotopes
towards the neutron-drip line.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Coulomb plasmas in outer envelopes of neutron stars
Outer envelopes of neutron stars consist mostly of fully ionized, strongly
coupled Coulomb plasmas characterized by typical densities about 10^4-10^{11}
g/cc and temperatures about 10^4-10^9 K. Many neutron stars possess magnetic
fields about 10^{11}-10^{14} G. Here we briefly review recent theoretical
advances which allow one to calculate thermodynamic functions and electron
transport coefficients for such plasmas with an accuracy required for
theoretical interpretation of observations.Comment: 4 pages, 2 figures, latex2e using cpp2e.cls (included). Proc. PNP-10
Workshop, Greifswald, Germany, 4-9 Sept. 2000. Accepted for publication in
Contrib. Plasma Phys. 41 (2001) no. 2-
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