523 research outputs found
Search for weak M1 transitions in Ca with inelastic proton scattering
The spinflip M1 resonance in the doubly magic nucleus Ca, dominated by
a single transition, serves as a reference case for the quenching of
spin-isospin modes in nuclei. The aim of the present work is a search for weak
M1 transitions in Ca with a high-resolution (p,p') experiment at 295 MeV
and forward angles including 0 degree and a comparison to results from a
similar study using backward-angle electron scattering at low momentum
transfers in order to estimate their contribution to the total B(M1) strength.
M1 cross sections of individual peaks in the spectra are deduced with a
multipole decomposition analysis. The corresponding reduced B(M1) transition
strengths are extracted following the approach outlined in J. Birkhan et al.,
Phys. Rev. C 93, 041302(R) (2016). In total, 29 peaks containing a M1
contribution are found in the excitation energy region 7 - 13 MeV. The
resulting B(M1) strength distribution compares well to the electron scattering
results considering different factors limiting the sensitivity in both
experiments and the enhanced importance of mechanisms breaking the
proportionality of nuclear cross sections and electromagnetic matrix elements
for weak transitions as studied here. The total strength of 1.19(6)
deduced assuming a non-quenched isoscalar part of the (p,p') cross sections
agrees with the (e,e') result of 1.21(13) . A binwise analysis above
10 MeV provides an upper limit of 1.62(23) . The present results
confirm that weak transitions contribute about 25% to the total B(M1) strength
in Ca and the quenching factors of GT and spin-M1 strength are
comparable in fp-shell nuclei. Thus, the role of of meson exchange currents
seems to be neglible, in contrast to sd-shell nuclei.Comment: 11 pages, 9 figures, revised analysis with oxygen contamination
remove
Q-phonon description of low lying 1^- two-phonon states in spherical nuclei
The properties of 1^-_1 two-phonon states and the characteristics of E1
transition probabilities between low-lying collective states in spherical
nuclei are analysed within the Q-phonon approach to the description of
collective states. Several relations between observables are obtained.
Microscopic calculations of the E1 0^+_1 -> 1^-_1 transition matrix elements
are performed on the basis of the RPA. A satisfactory description of the
experimental data is obtained.Comment: 16 pages, 2 figures, 9 table
E2 strengths and transition radii difference of one-phonon 2+ states of 92Zr from electron scattering at low momentum transfer
Background: Mixed-symmetry 2+ states in vibrational nuclei are characterized
by a sign change between dominant proton and neutron valence-shell components
with respect to the fully symmetric 2+ state. The sign can be measured by a
decomposition of proton and neutron transition radii with a combination of
inelastic electron and hadron scattering [C. Walz et al., Phys. Rev. Lett. 106,
062501 (2011)]. For the case of 92Zr, a difference could be experimentally
established for the neutron components, while about equal proton transition
radii were indicated by the data. Method: Differential cross sections for the
excitation of one-phonon 2+ and 3- states in 92Zr have been measured with the
(e,e') reaction at the S-DALINAC in a momentum transfer range q = 0.3-0.6
fm^(-1). Results: Transition strengths B(E2;2+_1 -> 0+_1) = 6.18(23), B(E2;
2+_2 -> 0+_1) = 3.31(10) and B(E3; 3-_1 -> 0+_1) = 18.4(11) Weisskopf units are
determined from a comparison of the experimental cross sections to
quasiparticle-phonon model (QPM) calculations. It is shown that a
model-independent plane wave Born approximation (PWBA) analysis can fix the
ratio of B(E2) transition strengths to the 2+_(1,2) states with a precision of
about 1%. The method furthermore allows to extract their proton transition
radii difference. With the present data -0.12(51) fm is obtained. Conclusions:
Electron scattering at low momentum transfers can provide information on
transition radii differences of one-phonon 2+ states even in heavy nuclei.
Proton transition radii for the 2+_(1,2) states in 92Zr are found to be
identical within uncertainties. The g.s. transition probability for the
mixed-symmetry state can be determined with high precision limited only by the
available experimental information on the B(E2; 2+_1 -> 0+_1) value.Comment: 14 pages, 5 figures, submitted to Phys. Rev. C, revised manuscrip
Strong fragmentation of low-energy electromagnetic excitation strength in Sn
Results of nuclear resonance fluorescence experiments on Sn are
reported. More than 50 transitions with MeV were
detected indicating a strong fragmentation of the electromagnetic excitation
strength. For the first time microscopic calculations making use of a complete
configuration space for low-lying states are performed in heavy odd-mass
spherical nuclei. The theoretical predictions are in good agreement with the
data. It is concluded that although the E1 transitions are the strongest ones
also M1 and E2 decays contribute substantially to the observed spectra. In
contrast to the neighboring even Sn, in Sn the
component of the two-phonon quintuplet built on top of
the 1/2 ground state is proved to be strongly fragmented.Comment: 4 pages, 3 figure
Fragmentation and systematics of the Pygmy Dipole Resonance in the stable N=82 isotones
The low-lying electric dipole (E1) strength in the semi-magic nucleus 136Xe
has been measured which finalizes the systematic survey to investigate the
so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with
the method of nuclear resonance fluorescence using real photons in the entrance
channel. In all cases, a fragmented resonance-like structure of E1 strength is
observed in the energy region 5 MeV to 8 MeV. An analysis of the fragmentation
of the strength reveals that the degree of fragmentation decreases towards the
proton-deficient isotones while the total integrated strength increases
indicating a dependence of the total strength on the neutron-to-proton ratio.
The experimental results are compared to microscopic calculations within the
quasi-particle phonon model (QPM). The calculation includes complex
configurations of up to three phonons and is able to reproduce also the
fragmentation of the E1 strength which allows to draw conclusions on the
damping of the PDR. Calculations and experimental data are in good agreement in
the degree of fragmentation and also in the integrated strength if the
sensitivity limit of the experiments is taken into account
Population of isomers in decay of the giant dipole resonance
The value of an isomeric ratio (IR) in N=81 isotones (Ba, Ce,
Nd and Sm) is studied by means of the ( reaction.
This quantity measures a probability to populate the isomeric state in respect
to the ground state population. In ( reactions, the giant dipole
resonance (GDR) is excited and after its decay by a neutron emission, the
nucleus has an excitation energy of a few MeV. The forthcoming decay
by direct or cascade transitions deexcites the nucleus into an isomeric or
ground state. It has been observed experimentally that the IR for Ba
and Ce equals about 0.13 while in two heavier isotones it is even less
than half the size. To explain this effect, the structure of the excited states
in the energy region up to 6.5 MeV has been calculated within the Quasiparticle
Phonon Model. Many states are found connected to the ground and isomeric states
by , and transitions. The single-particle component of the wave
function is responsible for the large values of the transitions. The calculated
value of the isomeric ratio is in very good agreement with the experimental
data for all isotones. A slightly different value of maximum energy with which
the nuclei rest after neutron decay of the GDR is responsible for the reported
effect of the A-dependence of the IR.Comment: 16 pages, 4 Fig
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