16O Nuclei Research Articles

Momentum distribution of Δ-isobars in light nuclei with closed shells

The wave function of the Δ-isobar configuration for light nuclei with closed shells, which is obtained for a transition potential caused by the exchange between the π-and ρ-mesons is used to find the momentum distribution of the Δ-isobar in the 4He, 16O, and 12C light nuclei.

Nuclear rainbow in the elastic scattering of 16O nuclei on carbon isotopes

An investigation of refractive effects in heavy-ion scattering is continued. For the elastic scattering of 16O nuclei on a 13C target at E(16O) = 132 MeV, the differential cross sections are measured for the first time in addition to previous measurements for targets from the carbon isotopes 12C and 14C. Airy structures that are similar for all isotopes and which have close cross sections are observed and are found to be consistent with the energy systematics of Airy minima that were obtained previously. The volume integrals of the real and imaginary parts of the optical potentials found in the present study are nearly identical for all of the isotopes considered here and are also in good agreement with the systematics obtained previously.

NMR Evidence for Higher-Order Multipole Order Parameters inNpO2

We report a microscopic investigation of multipolar order parameters in the ordered state of NpO2 conducted via 17O NMR on a single crystal. From the angular dependence of hyperfine fields at 17O nuclei, we have obtained clear evidence for the appearance of field-induced antiferro-octupolar as well as field-induced antiferro-dipolar moments below T0 = 26 K. We have also observed oscillatory spin-echo decay, which is well understood in terms of small electric field gradients created by antiferro-quadrupolar ordering. This reveals that the quadrupolar order parameter is directly observable by means of NMR. The present NMR studies provide definitive support for a proposed longitudinal triple-q type octupolar-quadrupolar ordering model for NpO2.

Dose‐volume delivery guided proton therapy using beam on‐line PET system

Proton therapy is one form of radiotherapy in which the irradiation can be concentrated on a tumor using a scanned or modulated Bragg peak. Therefore, it is very important to evaluate the proton-irradiated volume accurately. The proton-irradiated volume can be confirmed by detection of pair annihilation gamma rays from positron emitter nuclei generated by the target nuclear fragment reaction of irradiated proton nuclei and nuclei in the irradiation target using a positron emission tomography (PET) apparatus, and dose-volume delivery guided proton therapy (DGPT) can thereby be achieved using PET images. In the proton treatment room, a beam ON-LINE PET system (BOLPs) was constructed so that a PET apparatus of the planar-type with a high spatial resolution of about 2 mm was mounted with the field of view covering the isocenter of the beam irradiation system. The position and intensity of activity were measured using the BOLPs immediately after the proton irradiation of a gelatinous water target containing 16O nuclei at different proton irradiation energy levels. The change of the activity-distribution range against the change of the physical range was observed within 2 mm. The experiments of proton irradiation to a rabbit and the imaging of the activity were performed. In addition, the proton beam energy used to irradiate the rabbit was changed. When the beam condition was changed, the difference between the two images acquired from the measurement of the BOLPs was confirmed to clearly identify the proton-irradiated volume.

Proton emission induced by polarized photons

The proton emission induced by polarized photons is studied in the energy range above the giant resonance region and below the pion emission threshold. Results for the 12C, 16O and 40Ca nuclei are presented. The sensitivity of various observables to final state interaction, meson exchange currents and short range correlations is analyzed. We found relevant effects due to the virtual excitation of the $\Delta$ resonance.

High resolution γ-ray spectroscopy of flares on the east and west limbs of the Sun

A new generation of Ge-based high-resolution gamma-ray spectrometers has allowed accurate measurements to be made of the profiles, widths and energies of the gamma-ray lines emitted in the impulsive phases of solar flares. Here we present measurements in two flares of the energies of the de-excitation lines of 12C and 16O at 4.4 and 6.1 MeV respectively by the Ge spectrometer SPI on board INTEGRAL, from which Doppler shifts are derived and compared with those expected from the recoil of 12C and 16O nuclei which were excited by the impacts of flare-accelerated ions. An anomalous Doppler measurement (in terms of recoil theory) has been reported by the Ge spectrometer RHESSI in a flare near the east limb, and explained by a tilt of the magnetic field lines at the footpoint of a magnetic loop away from the vertical, and towards the observer. This might be interpreted to imply a significant difference between the Doppler shifts on the east and west limbs, if it is a general phenomenon. SPI observed both east and west limb flares and found no significant difference in Doppler shifts. We also measured the shapes and fluences of these lines, and their fluence ratio to the 2.2 MeV line from the capture of flare-generated neutrons. Analyses of both quantities using thick-target models parametrized by solar physical and geometric quantities suggest that the two flares studied here also have magnetic fields tilted towards the observer, though the significance of the measurements is not high.

A density functional study of 17O, 14N and 2H electric field gradient tensors in the real crystalline structure of α-glycine

A density functional theory (DFT) study was carried out to calculate 17O, 14N and 2H electric field gradient (EFG) tensors in accurate neutron diffraction structures of α-glycine at 288 and 427 K. B3LYP is the used method and 6-311+G ⁎ and 6-311++G ⁎⁎ are the basis sets in the calculations of EFG tensors at the sites of 17O, 14N and 2H nuclei in the monomer and the octameric cluster of α-glycine at two temperatures. Quadrupole coupling constants and asymmetry parameters are the converted parameters of calculated EFG tensors to experimentally measurable ones. The calculated results of monomer and the target molecule in octameric cluster reveal that hydrogen-bonding interactions play an important role in the crystalline structure of α-glycine where the results of the target molecule in octameric cluster are in good agreement with the experiments.

Gamow shell model and realistic nucleon-nucleon interactions

We present a new and efficient method to obtain a Gamow shell-model basis and matrix elements generated by realistic nucleon-nucleon interactions. We derive a self-consistent Hartree-Fock potential from the renormalized N3LO interaction model. The corresponding Gamow one-body eigenstates are generated in a plane wave basis in order to build a Gamow shell-model set of basis states for the closed shell nuclei 4He and 16O. We address also the problem of representing a realistic nucleon-nucleon interaction in a two-particle Berggren basis in the laboratory frame. To achieve this, an expansion of matrix elements of the residual nucleon-nucleon interaction in a finite set of harmonic oscillator wave functions is used. We show that all loosely bound and narrow resonant states converge surprisingly fast. Even broad resonances in these two-particle valence systems converge within a reasonable number of harmonic oscillator functions. Examples of 6He and 18O Gamow shell-model calculations using 4He and 16}O as closed shell cores are presented. This procedure allows Gamow shell-model calculations to be performed with all realistic nucleon-nucleon interactions and with either momentum or position space representations for the Gamow basis. The possibility to remove the center of mass spuriosity of Gamow shell-model nuclear states with this method is also discussed. Perspectives for nuclear structure calculations of dripline nuclei are outlined.

Ground states of medium-heavy doubly-closed-shell nuclei in correlated-basis function theory

The correlated basis function theory is applied to the study of medium-heavy doubly closed shell nuclei with different wave functions for protons and neutrons and in the jj coupling scheme. State dependent correlations including tensor correlations are used. Realistic two-body interactions of Argonne and Urbana type, together with three-body interactions have been used to calculate ground state energies and density distributions of the 12C, 16O, 40Ca, 48Ca and 208Pb nuclei.

An α-Particle Model for 16O: Is There a New Four-Body Scale?

The 16O nucleus is modelled as a system of four α-particles. It is shown that a Hamiltonian that includes established two- and three-body forces reproducing the α-α phase shifts, the 8Be resonance, and the first and second 0+ states of 12C overbinds the 16O nucleus. To rectify this a novel four-body force is introduced, which then leads to a good agreement with the low-lying experimental spectrum of 16O.

Cluster breakup of 18O and 22Ne

The 14C + α breakup of 18O has been studied at energies of Ebeam = 136 MeV and 148.5 MeV, using a 200 µg cm−2 thick Be target. Several states belonging to the proposed octupole deformed band, previously associated with the 8.04 MeV, 1− and 8.22 MeV, 2+ states, have been populated through inelastic excitation. A candidate for the 7− member was not found, providing evidence that the negative parity component of the band terminates at the 5− member. This is shown to be consistent with the Nilsson description of the systematics in the 16O, 18O and 20Ne nuclei. In addition, several states at Ex > 13 MeV in 18O have been measured to α-decay for the first time. The α-transfer reactions 9Be(18O, 12C 10Be)5He, 9Be(18O, 13C 9Be)5He and 9Be(18O, 18O 4He)5He were also measured; however, little or no evidence for the population of states in 22Ne was found.

Extracting the Weinberg angle at intermediate energies

A recent experiment by the NuTeV collaboration resulted in a surprisingly high value for the weak mixing angle sin 2 θ W . The Paschos–Wolfenstein relation, relating neutrino cross sections to the Weinberg angle, is of pivotal importance in the NuTeV analysis. In this work, we investigate the sensitivity of the Paschos–Wolfenstein relation to nuclear structure aspects at neutrino energies in the few GeV range. Neutrino–nucleus cross sections are calculated for 16O and 56Fe target nuclei within a relativistic quasi-elastic nucleon-knockout model.

Correlation between NQR parameters and residue size of aliphatic amino acids and their dimers

Nuclear quadrupole coupling constants (NQCC), χ, and asymmetry parameters, η, of 2D, 14N and 17O nuclei have been calculated for aliphatic amino acids and their dimers using MP2/6-311++G** method to shed some light on the differences between the structural parameters in the aliphatic amino acids and their dimers. For this purpose, electric field gradient (EFG) at the sites of quadrupolar nuclei have been calculated and evaluated for each compound. A correlation is observed between the calculated NQCC parameters and the conformational structures of the compounds, showing that extraction of structural data from the NQR spectra might be promising. Our results showed that 17O NQCC of terminal carboxylic acid and 14N NQCC of the terminal amino groups are, respectively, the least and the most sensitive parameters to the variation of the size of the residue. It is found also that conformation of R (i.e. values of the dihedral angles) plays a very effective role in the determination of the values of the calculated NQCC parameters. Sensitivity of the NQR parameters to the changes in the conformational structure is significantly greater (nearly 20-fold) than that to the changes in the other structural parameters such as bond lengths.

Pulsed EPR Investigations of Systems Modeling Molybdenum Enzymes: Hyperfine and Quadrupole Parameters of Oxo-17O in [Mo17O(SPh)4]-

Ka band ESEEM spectroscopy was used to determine the hyperfine (hfi) and nuclear quadrupole (nqi) interaction parameters for the oxo-17O ligand in [Mo 17O(SPh)4]-, a spectroscopic model of the oxo-Mo(V) centers of enzymes. The isotropic hfi constant of 6.5 MHz found for the oxo-17O is much smaller than the values of approximately 20-40 MHz typical for the 17O nucleus of an equatorial OH(2) ligand in molybdenum enzymes. The 17O nqi parameter (e2qQ/h = 1.45 MHz, eta approximately = 0) is the first to be obtained for an oxo group in a metal complex. The parameters of the oxo-17O ligand, as well as other magnetic resonance parameters of [Mo 17O(SPh)4]- predicted by quasi-relativistic DFT calculations, were in good agreement with those obtained in experiment. From the electronic structure of the complex revealed by DFT, it follows that the SOMO is almost entirely molybdenum d(xy) and sulfur p, while the spin density on the oxo-17O is negative, determined by spin polarization mechanisms. The results of this work will enable direct experimental identification of the oxo ligand in a variety of chemical and biological systems.

Rare Gas Effects on Hyperfine Coupling Constants of BO, AlO, and GaO

Using density functional theory methods and large basis sets, we calculated hyperfine coupling constants (HFCCs) for the (11)B, (17)O, (27)Al, and (69)Ga nuclei of the radicals BO, AlO, and GaO (XO), embedded in 2-14 rare gas (Rg) Ne and Ar atoms. Kr atoms were included for AlO. The distance of the Rg atoms from XO was varied from 4 to 12 bohr. Matrix effects cause A(iso)(X) to increase, accompanied by decreases in A(dip)(X) and A(dip)(O), while A(iso)(O) remains close to zero. Changes are largest for AlO, slightly smaller for GaO, and very small for BO, in line with the molecular polarizabilities. Observed changes of A(iso)(X) and A(dip)(X) for BO in Ne matrixes and for AlO in Ne, Ar, and Kr matrixes are reproduced in complexes with 12 Rg atoms at distances of 5-6 bohr or 14 Rg atoms at distances of 6-7 bohr. For GaO, experimental data are available only in Ne matrixes. Theoretical results obtained for HFCCs of (17)O could not be verified due to insufficient experimental information. Estimates of HFCCs in matrixes not yet experimentally studied and for GaO in the gas phase have been made. Due to the interaction with rare gas atoms, p-spin density on the X and O atoms of XO is converted into s-spin density on X, thereby causing an increase (in magnitude) of A(iso)(X), accompanied by decreases in A(dip) of X and O. The higher polarizability of XO along the bond axis is reflected in complexes that have axial Rg atoms showing larger changes in HFCCs than comparable complexes without axial Rg atoms.

Excitation of nuclear giant resonances in neutrino scattering off nuclei

The cross sections for neutrino scattering off the 12C and 16O nuclei are calculated within the framework of the continuum Random Phase Approximation. A model to consider also the final state interactions is developed. Total charge-conserving and charge-exchange cross sections for both electron neutrinos and antineutrinos have been calculated up to projectile energies of 100 MeV. The sensitivity of the cross sections to the residual interaction and to the final state interactions is investigated. A direct comparison between neutrino and electron scattering cross sections calculated under the same kinematic conditions is presented. We found remarkable differences between electromagnetic and weak nuclear responses. The model is applied to describe cross sections of neutrinos produced by muon decay at rest and in supernovae explosions.

Dirac Optical Potential of p-14Be andPronounced ``Long-Tail'' Shape**The project supported by National Natural Science Foundation of China under Grant No. 10125521, the 973 State Major Basic Research and Development Program of China under Grant No. G2000077400, the CAS Knowledge Innovation Project KJCX2-SW-N02, and the Research Fund of Higher Education underGrant No. 20010284036

The Dirac optical potential for p-14Be elastic scattering isevaluated by the relativistic impulse approximation. Each ofthe real part and the imaginary part of the potential shows apronounced ``long tail'' for the proton elastic scattering fromhalo nucleus 14Be compared with the potentials for protonscattering from its adjacent nuclei 12C and 16O, whichdo not have halo structures. This kind of ``long tail'' phenomenonsuggests another signature for halo nuclei.

A dependence of inclusive pion double charge exchange

The A dependence of the forward cross section for inclusive pion double charge exchange on nine target nuclei from 6Li to 209Bi at T0 = 0.59 GeV, as well as the cross section for 6Li, 7Li, and 12C nuclei at T0 = 0.59, 0.75, and 1.1 GeV, was measured with the 3-m magnetic spectrometer of the Institute of Theoretical and Experimental Physics (ITEP, Moscow). The resulting A dependence is well described within the model involving two sequential single charge exchanges and taking into account the renormalization of the amplitude for pion single charge exchange in a nucleus. A relatively weak energy dependence of the cross section for the 6Li, 7Li, and 12C nuclei agrees with the analogous dependence obtained previously for the 16O nucleus, but it contradicts the predicted sharp decrease in the cross section within the model involving two sequential single charge exchanges. This result provides an additional piece of evidence that the contribution from the mechanism of inelastic Glauber rescattering is significant at T0 ≳ 0.6 GeV.

Nuclear magnetic dipole moments from NMR spectra

In the procedure applied to extract the magnetic dipole moments of bare nuclei from NMR spectra of molecules the shielding of nuclei by electrons has been often described in a very crude manner. We determine the moments for 13C, 14N, 15N, 17O, 19F, 31P and 33S using more accurate shielding constants. Our gas phase NMR data are next used to improve the accuracy of the moments of 13C, 17O, 19F and 33S nuclei. We present a new, systematic analysis of the results, based on the relations between the nuclear magnetic dipole moments, the resonance frequencies and the shielding constants.

Elastic and inelastic scattering of 800-MeV protons on 16O and 20Ne nuclei

Differential cross sections and polarization observables for the elastic and inelastic scattering of 800-MeV protons on 16O and 20Ne nuclei are calculated on the basis of the theory of multiple diffractive scattering and the α-cluster model involving dispersion. The single-particle nucleon-density distributions obtained within the α-cluster model involving dispersion are used in the calculations. The differential cross sections and polarization calculated for elastic and inelastic p16O and p20Ne scattering are compatible with available experimental data. The spin-rotation functions calculated for elastic p16O and p20Ne scattering within the independent-nucleon model differ qualitatively from their counterparts calculated within the α-cluster model involving dispersion.