482 research outputs found
Electric dipole response of 208Pb from proton inelastic scattering: constraints on neutron skin thickness and symmetry energy
The electric dipole (E1) response of 208Pb has been precisely determined by
measuring Coulomb excitation induced by proton scattering at very forward
angles. The electric dipole polarizability, defined as inverse energy-weighted
sum rule of the E1 strength, has been extracted as 20.1+-0.6 fm^3. The data can
be used to constrain the neutron skin thickness of 208Pb to
0.168(+-0.009)_expt(+-0.013)_theo(+-0.021)_est fm, where the subscript "expt"
refers to the experimental uncertainty, "theor" to the theoretical confidence
band and "est" to the uncertainty associated with the estimation of the
symmetry energy at the saturation density. In addition, a constraint band has
been extracted in the plane of the symmetry energy (J) and its slope parameter
(L) at the saturation density.Comment: 6 pages, 8 figures, revised manuscript submitted to special volume of
Eur. Phys. J. A on symmetry energ
Electric and magnetic dipole modes in high-resolution inelastic proton scattering at
Inelastic proton scattering under extreme forward angles including
and at energies of a few hundred MeV has been established as a new
spectroscopic tool for the study of complete dipole strength distributions in
nuclei. Such data allow an extraction of the electric dipole polarizability
which provides important constraints parameters of the symmetry energy, which
determine the neutron skin thickness and the equation of state (EOS) of
neutron-rich matter. Also new insight into the much-debated nature of the pygmy
dipole resonance (PDR) is obtained. Additionally, the isovector spin-M1
resonance can be studied in heavy nuclei, where only limited experimental
information exists so far. Together with much improved results on the isoscalar
spin-M1 strength distributions in nuclei, these data shed new light on
the phenomenon of quenching of the nuclear spin response. Using dispersion
matching techniques, high energy resolution (
full width at half maximum, FWHM) can be achieved in the experiments. In
spherical-vibrational nuclei considerable fine structure is observed in the
energy region of the isovector giant dipole resonance (IVGDR). A quantitative
analysis of the fine structure with wavelet methods provides information on the
role of different damping mechanisms contributing to the width of the IVGDR.
Furthermore, level densities can be extracted from a fluctuation analysis at
excitation energies well above neutron threshold, a region hardly accessible by
other means. The combination of the gamma strength function (GSF) extracted
from the E1 and M1 strength distributions with the independently derived level
density permits novel tests of the Brink-Axel hypothesis underlying all
calculations of statistical model reaction cross sections in astrophysical
applications in the energy region of the PDR.Comment: 52 pages, 64 figures, review article submitted to Eur. Phys. J.
Isoscalar and isovector dipole excitations: Nuclear properties from low-lying states and from the isovector giant dipole resonance
Abstract This review paper concerns the research devoted to the study of the properties of dipole excitations in nuclei. The main focus is on questions related to isospin effects in these types of excitations. Particular attention is given to the experimental and theoretical efforts made to understand the nature and the specific structure of the low-lying dipole states known as the Pygmy Dipole Resonance (PDR). The main experimental methods employed in the study of the PDR are reviewed as well as the most interesting theoretical aspects. The main features of the experiments and of theoretical models are reported with special emphasis on the reaction cross sections populating the dipole states. Results are organized for nuclei according to different mass regions. The knowledge of the isovector dipole response as well as its low energy part is important in order to deduce the nuclear polarizability as accurate as possible. This issue is discussed in this paper together with the connection with the neutron skin and the nuclear equation of state. The important role played by the dipole response to deduce other physical quantities of general interest is discussed in the last two chapters. One concerns the level density and the other the isospin mixing in nuclei at finite temperature and its relation with beta decay
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
Study of nuclear correlation effects via 12C(p,n)12N(g.s.,1+) at 296 MeV
We report measurements of the cross section and a complete set of
polarization observables for the Gamow--Teller reaction at a bombarding
energy of 296 MeV.
The data are compared with distorted wave impulse approximation calculations
employing transition form factors normalized to reproduce the observed
beta-decay value.
The cross section is significantly under-predicted by the calculations at
momentum transfers 0.5 .
The discrepancy is partly resolved by considering the non-locality of the
nuclear mean field. However, the calculations still under-predict the cross
section at large momentum transfers of 1.6 .
We also performed calculations employing random phase approximation response
functions and found that the observed enhancement can be attributed in part to
pionic correlations in nuclei.Comment: 5 figures, submitted to Phys. Lett.
Complete set of polarization transfer coefficients for the reaction at 346 MeV and 0 degrees
We report measurements of the cross-section and a complete set of
polarization transfer coefficients for the reaction at a
bombarding energy = 346 MeV and a reaction angle =
.
The data are compared with the corresponding free nucleon-nucleon values on
the basis of the predominance of quasi-elastic scattering processes.
Significant discrepancies have been observed in the polarization transfer
, which are presumably the result of the three-proton =
3/2 resonance.
The spin--parity of the resonance is estimated to be , and the
distribution is consistent with previous results obtained for the same reaction
at = 48.8 MeV.Comment: 4 figures, Accepted for publication in Physical Review
Thin Ice Target for O(p,p') experiment
A windowless and self-supporting ice target is described. An ice sheet with a
thickness of 29.7 mg/cm cooled by liquid nitrogen was placed at the target
position of a magnetic spectrometer and worked stably in the O
experiment at MeV. Background-free spectra were obtained.Comment: 14 pages, 4 figures, Nucl. Instr. & Meth. A (in press
- …