137 research outputs found
Spectroscopy of Ne for the thermonuclear O()Ne and F()O reaction rates
Uncertainties in the thermonuclear rates of the
O()Ne and F()O reactions
affect model predictions of light curves from type I X-ray bursts and the
amount of the observable radioisotope F produced in classical novae,
respectively. To address these uncertainties, we have studied the nuclear
structure of Ne over MeV and MeV using
the F(He,t)Ne reaction. We find the values of the
4.14 and 4.20 MeV levels to be consistent with and
respectively, in contrast to previous assumptions. We confirm the recently
observed triplet of states around 6.4 MeV, and find evidence that the state at
6.29 MeV, just below the proton threshold, is either broad or a doublet. Our
data also suggest that predicted but yet unobserved levels may exist near the
6.86 MeV state. Higher resolution experiments are urgently needed to further
clarify the structure of Ne around the proton threshold before a
reliable F()O rate for nova models can be determined.Comment: 5 pages, 3 figures, Phys. Rev. C (in press
Recent results on heavy-ion direct reactions of interest for 0νββ decay at INFN - LNS
Neutrinoless double beta decay of nuclei, if observed, would have important
implications on fundamental physics. In particular it would give access to the effective neutrino
mass. In order to extract such information from 0νββ decay half-life measurements, the
knowledge of the Nuclear Matrix Elements (NME) is of utmost importance. In this context the
NUMEN and the NURE projects aim to extract information on the NME by measuring cross
sections of Double Charge Exchange reactions in selected systems which are expected to
spontaneously decay via 0νββ. In this work an overview of the experimental challenges that
NUMEN is facing in order to perform the experiments with accelerated beams and the research
and development activity for the planned upgrade of the INFN-LNS facilities is reported.Consejo Europeo de Investigación (ERC), Fondo Europeo Programa de investigación e innovación Horizonte 2020 de la Unión n.º 714625)
Extending the Hoyle-State Paradigm to 12 C + 12 C Fusion
Carbon burning is a key step in the evolution of massive stars, Type 1a supernovae and superbursts in
x-ray binary systems. Determining the 12C þ 12C fusion cross section at relevant energies by extrapolation
of direct measurements is challenging due to resonances at and below the Coulomb barrier. A study of the
24Mgðα; α0Þ24Mg reaction has identified several 0þ states in 24Mg, close to the 12C þ 12C threshold, which
predominantly decay to 20Neðground stateÞ þ α. These states were not observed in 20Neðα; α0
Þ20Ne
resonance scattering suggesting that they may have a dominant 12C þ 12C cluster structure. Given the
very low angular momentum associated with sub-barrier fusion, these states may play a decisive role
in 12C þ 12C fusion in analogy to the Hoyle state in helium burning. We present estimates of updated
12C þ 12C fusion reaction rates
Second T = 3/2 state in B and the isobaric multiplet mass equation
Recent high-precision mass measurements and shell model calculations~[Phys.
Rev. Lett. {\bf 108}, 212501 (2012)] have challenged a longstanding explanation
for the requirement of a cubic isobaric multiplet mass equation for the lowest
isospin quartet. The conclusions relied upon the choice of the
excitation energy for the second state in B, which had two
conflicting measurements prior to this work. We remeasured the energy of the
state using the reaction and significantly disagree
with the most recent measurement. Our result supports the contention that
continuum coupling in the most proton-rich member of the quartet is not the
predominant reason for the large cubic term required for nuclei
Two-neutron transfer reaction mechanisms in C(He,He)C using a realistic three-body He model
The reaction mechanisms of the two-neutron transfer reaction
C(He,He) have been studied at 30 MeV at the TRIUMF ISAC-II
facility using the SHARC charged-particle detector array. Optical potential
parameters have been extracted from the analysis of the elastic scattering
angular distribution. The new potential has been applied to the study of the
transfer angular distribution to the 2 8.32 MeV state in C, using
a realistic 3-body He model and advanced shell model calculations for the
carbon structure, allowing to calculate the relative contributions of the
simultaneous and sequential two-neutron transfer. The reaction model provides a
good description of the 30 MeV data set and shows that the simultaneous process
is the dominant transfer mechanism. Sensitivity tests of optical potential
parameters show that the final results can be considerably affected by the
choice of optical potentials. A reanalysis of data measured previously at 18
MeV however, is not as well described by the same reaction model, suggesting
that one needs to include higher order effects in the reaction mechanism.Comment: 9 pages, 9 figure
Neutron occupancies and single-particle energies across the stable tin isotopes
The occupancies and vacancies of the valence neutron orbitals across the
stable tin isotopic chain from have been determined. These
were inferred from the cross sections of neutron-adding and -removing
reactions. In each case, the reactions were chosen to have good
angular-momentum matching for transfer to the low- and high- orbitals
present in this valence space. These new data are compared to older systematic
studies. The effective single-neutron energies are determined by combining
information from energy centroids determined from the adding and removing
reactions. Two of the five orbitals are nearly degenerate, below , and
approximately two MeV more bound than the other three, which are also
degenerate.Comment: 11 pages, 10 figure
Study of proton-unbound states in relevant for the reaction in novae
Background: The nucleosynthesis of several proton-rich nuclei is determined
by radiative proton-capture reactions on unstable nuclei in nova explosions.
One such reaction is , which links the
NeNa and MgAl cycles in oxygen-neon (ONe) novae.
Purpose: To extract resonance
strengths from a study of proton-unbound states in , produced
via the Mg(He,) reaction.
Methods: A beam of ions at 50.7 MeV was used to produce
the states of interest in Al. Proton-triton angular correlations were
measured with a QDD magnetic spectrometer and a silicon detector array,
located at iThemba LABS, South Africa.
Results: We measured the excitation energies of the four lowest
proton-unbound states in Al and place lower-limits on
values for these four states. Together with USD-C shell-model calculations of
partial gamma widths, the experimental data are also used to determine
resonance strengths for the three lowest
resonances.
Conclusions: The energy of the dominant first
resonance is determined to be keV, with a resonance
strength meV
Characterization of the proposed 4-α cluster state candidate in O 16
The O16(α,α′) reaction was studied at θlab=0 at an incident energy of Elab=200 MeV using the K600 magnetic spectrometer at iThemba LABS. Proton decay and α decay from the natural parity states were observed in a large-acceptance silicon strip detector array at backward angles. The coincident charged-particle measurements were used to characterize the decay channels of the 06+ state in O16 located at Ex=15.097(5) MeV. This state is identified by several theoretical cluster calculations to be a good candidate for the 4-α cluster state. The results of this work suggest the presence of a previously unidentified resonance at Ex≈15 MeV that does not exhibit a 0+ character. This unresolved resonance may have contaminated previous observations of the 06+ state
Isoscalar giant monopole strength in Ni, Zr, Sn and Pb
Inelastic -particle scattering at energies of a few hundred MeV and
very-forward scattering angles including has been established as a
tool for the study of the isoscalar giant monopole (IS0) strength distributions
in nuclei. An independent investigation of the IS0 strength in nuclei across a
wide mass range was performed using the facility at iThemba
Laboratory for Accelerator Based Sciences (iThemba LABS), South Africa, to
understand differences observed between IS0 strength distributions in previous
experiments performed at the Texas A\&M University (TAMU) Cyclotron Institute,
USA and the Research Center for Nuclear Physics (RCNP), Japan. The isoscalar
giant monopole resonance (ISGMR) was excited in Ni, Zr,
Sn and Pb using -particle inelastic scattering with
MeV beam and scattering angles
and . The K magnetic spectrometer at iThemba LABS was used to
detect and momentum analyze the inelastically scattered particles. The
IS0 strength distributions in the nuclei studied were deduced with the
difference-of-spectra (DoS) technique including a correction factor for the
data based on the decomposition of cross sections in previous
experiments. IS0 strength distributions for Ni, Zr, Sn
and Pb are extracted in the excitation-energy region MeV.Using correction factors extracted from the RCNP experiments, there is
a fair agreement with their published IS0 results. Good agreement for IS0
strength in Ni is also obtained with correction factors deduced from the
TAMU results, while marked differences are found for Zr and Pb.Comment: 12 pages, 10 figures, regular article submitted to PR
α Clustering in Si 28 probed through the identification of high-lying 0+ states
Background: Aspects of the nuclear structure of light α-conjugate nuclei have long been associated with nuclear clustering based on α particles and heavier α-conjugate systems such as C12 and O16. Such structures are associated with strong deformation corresponding to superdeformed or even hyperdeformed bands. Superdeformed bands have been identified in Ca40 and neighboring nuclei and find good description within shell model, mean-field, and α-cluster models. The utility of the α-cluster description may be probed further by extending such studies to more challenging cases comprising lighter α-conjugate nuclei such as Mg24, Si28, and S32. Purpose: The purpose of this study is to look for the number and energy of isoscalar 0+ states in Si28. These states are the potential bandheads for superdeformed bands in Si28 corresponding to the exotic structures of Si28. Of particular interest is locating the 0+ bandhead of the previously identified superdeformed band in Si28. Methods: α-particle inelastic scattering from a Sinat target at very forward angles including 0 has been performed at the iThemba Laboratory for Accelerator-Based Sciences in South Africa. Scattered particles corresponding to the excitation energy region of 6 to 14 MeV were momentum-analysed in the K600 magnetic spectrometer and detected at the focal plane using two multiwire drift chambers and two plastic scintillators. Results: Several 0+ states have been identified above 9 MeV in Si28. A newly identified 9.71 MeV 0+ state is a strong candidate for the bandhead of the previously discussed superdeformed band. The multichannel dynamical symmetry of the semimicroscopic algebraic model predicts the spectrum of the excited 0+ states. The theoretical prediction is in good agreement with the experimental finding, supporting the assignment of the 9.71-MeV state as the bandhead of a superdeformed band. Conclusion: Excited isoscalar 0+ states in Si28 have been identified. The number of states observed in the present experiment shows good agreement with the prediction of the multichannel dynamical symmetry
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