78 research outputs found
Observation of low- and high-energy gamow-teller phonon excitations in nuclei
Y. Fujita et al. ; 5 pags. ; 3 figs. PACS numbers: 24.30.Cz, 25.55.Kr, 27.40.+zGamow-Teller (GT) transitions in atomic nuclei are sensitive to both nuclear shell structure and effective residual interactions. The nuclear GT excitations were studied for the mass number A=42, 46, 50, and 54 >f-shell> nuclei in (He3, t) charge-exchange reactions. In the Ca42→Sc42 reaction, most of the GT strength is concentrated in the lowest excited state at 0.6 MeV, suggesting the existence of a low-energy GT phonon excitation. As A increases, a high-energy GT phonon excitation develops in the 6-11 MeV region. In the Fe54→Co54 reaction, the high-energy GT phonon excitation mainly carries the GT strength. The existence of these two GT phonon excitations are attributed to the 2 fermionic degrees of freedom in nuclei. © 2014 American Physical Society.This work was in part supported by MEXT, Japan (Grants No. 13002001, No. 15540274, and No. 18540270); MICINN, Spain (Grant No. FPA200806419-C02-01). Y. F. and B. R. acknowledge the support of the Japan-Spain collaboration program by JSPS and CSIC.Peer Reviewe
Charged-current neutrino-208Pb reactions
We present theoretical results on the non flux-averaged
and
reaction cross sections, obtained within the charge-exchange
Random-Phase-Approximation. A detailed knowledge of these cross sections is
important in different contexts. In particular, it is necessary to assess the
possibility of using lead as a detector in future experiments on supernova
neutrinos, such as OMNIS and LAND, and eventually detect neutrino oscillation
signals by exploiting the spectroscopic properties of . We discuss
the present status on the theoretical predictions of the reaction cross
sections.Comment: 5 pages, latex, 3 figures. added discussion on present status,
Submitted to Phys.Rev.
In-beam γ -ray spectroscopy of Fe 68 from charge exchange on Co 68 projectiles
Excited states in the neutron-rich nucleus Fe68 were populated using a Be9(Co68, 68Fe+γ)X charge-exchange reaction at 95 MeV/u. The new γ-ray transitions reported here for the first time complement data from β-decay studies and nucleon knockout reactions. In comparison to shell-model calculations with the LNPS effective interaction, two candidate states for the 61+ level emerge. The distinct population pattern of excited states and the magnitude of the cross section, σinc=0.51(6) mb, make this reaction a promising one for future in-beam γ-ray spectroscopy. Reaction calculations with nuclear structure input from a new, locally optimized Hamiltonian, f7j4a, together with general considerations for heavy-ion-induced charge-exchange reactions appear consistent with most of the observations, although challenges remain
Backward-angle photoproduction of mesons on the proton at = 1.5--2.4 GeV
Differential cross sections and photon beam asymmetries for
photoproduction have been measured at = 1.5--2.4 GeV and at the
scattering angles, --1 cos --0.6. The
energy-dependent slope of differential cross sections for -channel
production has been determined. An enhancement at backward angles is found
above = 2.0 GeV. This is inferred to be due to the -channel
contribution and/or resonances. Photon beam asymmetries have been obtained for
the first time at backward angles. A strong angular dependence has been found
at 2.0 GeV, which may be due to the unknown high-mass resonances.Comment: 12 pages, 4 figures, submitted to PL
LENDA, a Low Energy Neutron Detector Array for experiments with radioactive beams in inverse kinematics
The Low Energy Neutron Detector Array (LENDA) is a neutron time-of-flight
(TOF) spectrometer developed at the National Superconducting Cyclotron Lab-
oratory (NSCL) for use in inverse kinematics experiments with rare isotope
beams. Its design has been motivated by the need to study the spin-isospin
response of unstable nuclei using (p, n) charge-exchange reactions at
intermediate energies (> 100 MeV/u). It can be used, however, for any reaction
study that involves emission of low energy neutrons (150 keV - 10 MeV). The
array consists of 24 plastic scintillator bars and is capable of registering
the recoiling neutron energy and angle with high detection efficiency. The
neutron energy is determined by the time-of-flight technique, while the
position of interaction is deduced using the timing and energy information from
the two photomultipliers of each bar. A simple test setup utilizing radioactive
sources has been used to characterize the array. Results of test measurements
are compared with simulations. A neutron energy threshold of < 150 keV, an
intrinsic time (position) resolution of \sim 400 ps (\sim 6 cm) and an
efficiency > 20 % for neutrons below 4 MeV have been obtained.Comment: Version accepted for publication in Nucl. Instr. Methods A. Revised
text, 2 new figures added (one in section 4 and one in section 7
High-resolution study of Gamow-Teller excitations in the Ca42(He3,t)Sc42 reaction and the observation of a “low-energy super-Gamow-Teller state”
Y. Fujita et al.; 15 pags.; 6 figs.; 7 tabs.; PACS number(s): 21.10.Hw, 25.55.Kr, 27.40.+z, 25.40.Ep© 2015 American Physical Society. To study the Gamow-Teller (GT) transitions from the Tz=+1 nucleus Ca42 to the Tz=0 nucleus Sc42, where Tz is the z component of isospin T, we performed a (p,n)-type (He3,t) charge-exchange reaction at 140 MeV/nucleon and scattering angles around 0. With an energy resolution of 29 keV, states excited by GT transitions (GT states) could be studied accurately. The reduced GT transition strengths B(GT) were derived up to the excitation energy of 13 MeV, assuming the proportionality between the cross sections at 0 and B(GT) values. The main part of the observed GT transition strength is concentrated in the lowest 0.611-MeV, Jπ=1+ GT state. All the other states at higher energies are weakly excited. Shell-model calculations could reproduce the gross feature of the experimental B(GT) distribution, and random-phase-approximation calculations including an attractive isoscalar interaction showed that the 0.611-MeV state has a collective nature. It was found that this state has all of the properties of a >low-energy super-Gamow-Teller state.> It is expected that low-lying Jπ=1+ GT states have T=0 in the Tz=0 nucleus Sc42. However, T=1 states are situated in a higher energy region. Assuming an isospin-analogous structure in A=42 isobars, analogous T=1, 1+ states are also expected in Ca42. Comparing the Ca42(He3,t)Sc42 and Ca42(p,p′) spectra measured at 0, candidates for T=1 GT states could be found in the 10-12-MeV region of Sc42. They were all weakly excited. The mass dependence of the GT strength distributions in Sc isotopes is also discussed.Y.F. also acknowledges the support of MEXT, Japan under
Grants No. 18540270, No. 22540310, and No. 15K05104.
Y.F. and B.R. are grateful for the support of the Japan-Spain
collaboration program by JSPS and CSIC; A.A., E.E.A., and
B.R. are thankful for the support of Spanish Ministry under
Grants No. FPA2005-03993, No. FPA2008-06419-C02-01,
and No. FPA2011-24553. G.S. acknowledges the support of
TUBITAK, Turkey under Research Scholarship No. BIDEB
2214. J.M.D., C.J.G., R.M., G.P., and R.G.T.Z. are grateful for
the support of the US NSF under Grants No. PHY-0606007 and
No. PHY-0822648 (JINA). M.C., J.G., and A.K. acknowledge
the support of the OTKA Foundation, Hungary, under Grant
No. K106035. This work was in part supported by the RIKENCNS
joint research project on large-scale nuclear-structure
calculations.Peer Reviewe
Direct proton decay of the isoscalar giant dipole resonance in 208Pb
The excitation and subsequent proton decay of the isoscalar giant dipole
resonance (ISGDR) in Pb have been investigated via the
Pb(Tl reaction at 400 MeV. Excitation
of the ISGDR has been identified by the difference-of-spectra method. The
enhancement of the ISGDR strength at high excitation energies observed in the
multipole-decomposition-analysis of the singles
Pb() spectra is not present in the excitation
energy spectrum obtained in coincidence measurement. The partial branching
ratios for direct proton decay of ISGDR to low-lying states of Tl have
been determined and the results are compared with predictions of continuum
random-phase-approximation (CRPA) calculations.Comment: 12 pages, 4 figures; accepted for publication in Physics Letters
Forward coherent -meson photoproduction from deuterons near threshold
Differential cross sections and decay asymmetries for coherent -meson
photoproduction from deuterons were measured for the first time at forward
angles using linearly polarized photons at = 1.5-2.4 GeV. This
reaction offers a unique way to directly access natural-parity Pomeron dynamics
and gluon exchange at low energies. The cross sections at zero degrees increase
with increasing photon energy. The decay asymmetries demonstrate a complete
dominance of natural-parity exchange processes, showing that isovector
unnatural-parity -meson exchange is small. Nevertheless the deduced cross
sections of -mesons from nucleons contributed by isoscalar t-channel
exchange processes are not well described by the conventional Pomeron model.Comment: 14 pages, 6 figures. Final published versio
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