The two-neutron transfer reactions (O, O) with O, C and Mg targets

Excitation functions, angular distributions and differential ranges were measured for the ²⁶Mg(¹⁸O, ¹⁶O)²⁸Mg reaction at ¹⁸O beam energies of 20–45 MeV. Excitation functions only were measured for the reactions ¹⁴C(¹⁸O, ¹⁹O)¹³C, ¹⁴C(¹⁸O, ¹⁶O)¹⁶C, ¹⁴C(¹⁸O, ²⁰O)¹²C, ¹⁴C(¹⁸O, ¹⁵N)¹⁷N and ¹⁸O(¹⁸O, ¹⁹O)¹⁷O, ¹⁸O(¹⁸O, ¹⁶O)²⁰O, ¹⁸O(¹⁸O, ¹⁵N)²¹F at ¹⁸O beam energies of 13–41 MeV. We have identified these as direct reactions in which a single neutron, a two-neutron cluster, a deuteron and a triton are transferred between projectile and target.

The cross sections for two-neutron transfer reactions were found to be relatively high and those for the ¹⁸O+¹⁸O and the ¹⁴C+¹⁸O reactions were higher than the ones of single-neutron transfers over most of the energy range.

Attempts were made to apply the theory of Buttle and Goldfarb for single-neutron transfer to the case of two-neutron transfer in the ²⁶Mg(¹⁸O, ¹⁶O)²⁸Mg reaction below the Coulomb barrier. It is shown that for those reactions for which the assumptions, implicit in the model, are valid, good agreement is obtained with experiment. We also tried to apply the diffraction model of Dar and Kozlovsky to the calculation of the angular distribution of these reactions. A good fit to the experimental results could be obtained if quite different sets of parameters were used in the calculations for the two bombarding energies.     

Search for parity violation in the 1081 keV gamma transition of F

The weak neutral current results in a parity-violating one-pion-exchange potential in nuclei. A search for this force has been carried out with the 1081 keV γ-radiation of ¹⁸F, which is expected to be circularly polarized. Excited ¹⁸F nuclei were produced with the ¹⁶O(³He, p)¹⁸F reaction and the γ-ray polarization was measured with a multiple transmission polarimeter system. The experimental result P = (− 1.0 ± 1.8) × 10⁻³ is smaller than recent theoretical estimates.     

Magnetic moments of light nuclei

Magnetic moments of the nuclei ¹⁵N, ¹⁵O, ¹⁷O, ¹⁷F and ¹⁹F are investigated. The importance ; of configuration mixing as well as meson-exchange currents is discussed in detail. Meson-exchange currents are treated in the one-boson exchange limit. Short-range N-N correlations are obtained from solutions of the Bethe-Goldstone equation for doubly closed shell nuclei by the use of realistic N-N potentials which ensure a correct treatment of tensor correlations.     

The relativistic stripping model of (p, π) reactions in the Δ resonance region

The relativistic stripping model is presented for the (p, π⁺) reaction in the Δ-resonance region. The Dirac impulse approximation and a Dirac-Hartree calculation are used for the proton nucleus interaction and neutron bound state wave function respectively. The Δ-hole model is employed for the pion-nucleus interaction. This model is applied to the reaction ¹⁶O(p, π⁺)¹⁷O(g.s.).     

Electro-pion production on O to bou nd states of the residual nuclei

Differential cross sections for the reactions 16O(γ, π−π+−)16F16F to the sum of the four lowest lying states in 16F and 16N have been measured as a function of angle for pions with a kinetic energy of 30 MeV. The extracted ratios R = σ(γ, π−)/σ(γ, π+), the first ones to discrete final states as a function of angle, are in fair agreement with results obtained for the nucleon. For positive pions the energy dependence of the cross section has been measured at the angles of 45° and 90°. Distorted wave impulse approximation calculations fail to describe the energy dependence.     

Heavy-ion reactions induced by O, O and F ions

Excitation curves have been determined from γ-ray yield measurements for heavyion reactions induced by E_lab = 12–30 MeV ¹⁶O, ¹⁸O and ¹⁹F ions incident upon thick targets of ⁹Be, ¹⁰B, ¹¹B, ¹²C and ²³Na. The yields of radioactive decay products with half-lives greater than one second were measured; hence a large number of the outgoing reaction channels could be observed. The preponderance of heavy reaction products suggests compound-nucleus formation as the dominant reaction mechanism. Statistical-model calculations with a spin-dependent level density have been performed, in which the nuclear moment of inertia was treated as a parameter. Many of the results can be explained satisfactorily with a nuclear moment of inertia 0.55 to 0.7 of the rigid body value.     

利用18O(6Li,d)22Ne反应研究22Ne Eα=470 keV共振态的性质

慢速中子俘获过程（s过程）是合成比铁重元素的重要途径之一。22Ne（α,n）25Mg反应是大质量AGB星中s过程主要的中子源,其中的22Ne主要通过14N（α,γ）18F（β+）18O（α,γ）22Ne反应链合成。该反应链中关键反应18O（α,γ）22Ne在天体物理感兴趣能区的截面非常低,其天体反应率主要来自于22Ne α分离阈附近低能共振态的贡献,但目前相关能级的共振参数严重缺失。在HI-13串列加速器的Q3D磁谱仪上,通过测量18O（6Li,d）22Ne反应的角分布,利用DWBA分析确定了22Ne分离阈附近共振能级E_α=470 keV的自旋宇称为0+,为后续计算18O（α,γ）22Ne的天体反应率打下了基础。About a half of the abundances of elements heavier than iron comes from the so-called slowneutron capture process (s-process) in Asymptotic Giant Branch (AGB) stars, with the 22Ne(α, n)25Mg reaction as one of the main neutron sources. In the beginning phase of AGB thermal pulse, 22Ne is produced by the 14N(α, γ)18F(β+)18O(α, γ)22Ne reaction sequence, in which the 18O(α, γ)22Ne reaction plays a key role. While the reaction rate of the 18O(α, γ)22Ne is mainly affected by several resonant states lying closely to the α threshold in 22Ne, up to now, the relevant 22Ne parameters are fragmentary in the energy region corresponding to the typical temperatures of s-process. The direct measurement of the 18O(α, γ)22Ne reaction rate is extremely difficult due to the very low cross section. In this work, we investigated the 22Ne resonant states via the 18O(6Li, d)22Ne reaction at the Beijing HI-13 tandem accelerator of China Institute of Atomic Energy. Based on the DWBA analysis, preliminary results showed that the spin-parity of 22Ne E_α=470 keV resonant states was assigned as 0+, which would make contributions to subsequent calculation for the reaction rate of the 18O(α, γ)22Ne.     

The N(p,γ)O low-energy S-factor

The ¹⁴N(p,γ)¹⁵O low-energy S-factor is analyzed using the R-matrix model. We find that the g.s. contribution is less than previously reported. The S-factor is mainly given by the 6.79 MeV state contribution which is determined by its asymptotic normalization constant (ANC). Consequently, the S-factor at zero energy is lower by a factor of 1.7 compared to the values given in recent compilations. This result may affect the nucleosynthesis and time scale evolution in massive stars. New measurements of the ¹⁴N(p,γ)¹⁵O cross section over a wide energy range, and especially at low energies, are highly desirable. Significant improvement could be also obtained from the ANC measurement of the 6.79 MeV state.     

Photoparticle production in solid state detectors used for neutron dosimetry in the presence of photons

Photon-induced neutron, proton and alpha particle production in polyethylene and CR-39 has been estimated for the photon energy range of 2–30 MeV, using our previously established methods and photonuclear cross-section data for hydrogen, carbon and oxygen. The rarer isotopes of the constituents of CR-39 and polyethylene, namely ²H, ¹³C, ¹⁷O and ¹⁸O, have been taken into account. Neutrons and protons are produced in polyethylene and CR-39 for photon energies above 2.2 MeV, the (γ, np) threshold for ²H. Photoparticles produced in these materials may need to be taken into consideration when using them for neutron dosimetry in the presence of photons in this energy range, especially when the neutron flux is several orders of magnitude less than that of the photons.     

The (p, t) and (p, τ) reactions on O at 39.8 MeV

Differential cross sections have been measured at forward angles for (p, t) and (p, τ) transitions from ¹⁷O to the ⁻ ground states and lowest-energy ⁻ states in the ¹⁵O and ¹⁵N mirror nuclei. The data are compared with DWBA calculations using simple single-particle and single-hole wave functions. When the (p, t) and (p, τ) transitions are considered separately, the calculated and experimental ratios of the ⁻ integrated cross sections to the ⁻ integrated cross sections agree to within 30 %; however, the ratios of (p, τ) cross sections to the mirror state (p, t) cross sections are calculated to be about twice as large as actually measured. This experimentally observed reduction of the (p, τ) cross section relative to the (p, t) cross section can possibly be attributed to interference between the S = 0 and S = 1 components of the (p, τ) transitions.     

Polarization of neutrons from the O(d, n)F reaction

The polarizations of neutrons from the ¹⁶O(d, n₀)¹⁷F and ¹⁶O(d, n₁ ¹⁷F reactions have been measured at 30° (lab) in steps of approximately 0.15 MeV from E_d = 3.96 to 5.35 MeV. Polarization angular distributions have been obtained at 3.96 and 5.35 MeV. It is determined that the random phase approximation for the scattering amplitudes is not appropriate for the compound nucleus contributions. The analysis indicates probable interference between the compound nucleus and direct interaction reaction mechanisms.     

Complete structure determination of the astrophysically important nucleus Na below the proton threshold

The fusion–evaporation reaction ¹⁰B(¹²C, 2n) was used to make the first observation of in-beam γ decays from the astrophysically important nucleus ²⁰Na, lying adjacent to the proton drip-line. All states below the proton threshold in ²⁰Na were populated and identified in the experiment. These include new levels, previously unresolved levels, and states located with improved energy precision. The level structure of ²⁰Na, and its γ transitions, are compared to the mirror partner ²⁰F measured simultaneously in this experiment. In particular, a high degree of energy stability is found for all negative parity states. These results are discussed in the context of the nuclear shell model.     

A rotational model description of F

Rotational-particle coupling (RPC) model calculations have been performed for the positive parity states of ¹⁹F using a deformed oscillator potential (Nilsson) and a deformed Woods-Saxon potential. The energy spectra and spectroscopic factors (for the ¹⁸O(³He, d)¹⁹F reaction) are very similar for both calculations and in good agreement with experimental values and with recent full intermediate coupling shell-model calculations.     

Range profiles of 6–10 MeV N ions implanted in silicon

Implantation profiles of 6 to 10 MeV ¹⁵N ions in crystalline silicon have been investigated. Measurements of the profiles at depths from 4 to 7 μm were rendered possible by combining the depth profiling of the ¹⁵N atoms through the ¹⁵N(p, γ) ¹²C reaction and the exfoliation of the surface layer of the samples, accomplished by high dose ⁴He ion bombardment. In this way the range profiles, measured at the exfoliated crater bottom, could be obtained accurately without uncertainties due to straggling of the probing proton beam. The range parameters are compared to those of Monte Carlo calculations using the electronic stopping powers given by J.F. Ziegler, J.P. Biersack and U. Littmark [The Stopping Powers and Ranges of Ions in Matter, Vol. 1 (Pergamon, New York, 1985)]. Projected ranges were observed to be 5%–10% larger and range stragglings 27%–6% smaller than those predicted by Monte Carlo calculations along with the adopted stopping power parametrization.     

Effective charges and radial integrals in A = 17 nuclei

Radial integrals involving single-particle states in ¹⁷O have been determined from sub-Coulomb heavy-ion transfer reactions. Corresponding integrals in ¹⁷F have been inferred. Neutron and proton effective charges have been calculated. It is suggested that radial matrix elements obtained by the present technique be used as a consistent method for extracting effective charges from experimental data, and as starting points for microscopic calculations of effective charges.     

Unusual observation of nitrogen chemical shift anisotropies in tetraalkylammonium halides by 14N MAS NMR spectroscopy

¹⁴N Magic-angle spinning (MAS) NMR spectra for a number of polycrystalline, symmetrical tetraalkylammonium halides with short alkyl chains (C₂H₅– to n-C₄H₉–) have been recorded following a careful setup of the experimental conditions. Analysis of the spectra demonstrates the presence of ¹⁴N chemical shift anisotropies (CSAs) on the order of |δ_σ|=10–30 ppm along with ¹⁴N quadrupole coupling constants in the range of 10–70 kHz. The magnitude and sign of the CSAs determined from ¹⁴N MAS NMR are confirmed by recording and analysis of the corresponding slow-speed spinning (500–650 Hz) ¹⁵N CP/MAS NMR spectra. Most interestingly, it is observed experimentally and demonstrated theoretically and by simulations, that these CSAs are reflected in the spinning sideband (ssb) intensities of the ¹⁴N MAS spectra at much higher spinning speeds than can be applied to retrieve the corresponding ¹⁵N CSAs from the ssb pattern in the ¹⁵N CP/MAS spectra.     

The C(Li, t)O reaction at 20 MeV

States in ¹⁶O op to an excitation energy of 16.9 MeV were observed from the ¹³C(⁶Li, t)¹⁶O reaction at 20 MeV. Differential cross sections were obtained from θ_lab = 15° to 105° for the triton groups corresponding to the states in ¹⁶O at 6.13, 6.92, 7.12, 8.87, 9.85, 10.35 and 11.09 MeV.     

Mesonic and non-mesonic decay widths of ΛC

Results are reported on the precise measurement of the lifetime and the weak-decay branching ratios with charged particle(π⁻ and proton) emission from _Λ¹²C formed via ¹²C(π⁺,K⁺) reaction. The π⁻ mesonic decay width is consistent with the calculations including the pion wave function distortion effect. We observed large contribution of the neutron-stimulated decay widths, Γ(Λ + n → n + n), in the non-mesonic weak decay, which cannot be explained by the current theoretical calculations.     

Solid-state 17O NMR study of the electric-field-gradient and chemical shielding tensors in polycrystalline gamma-glycine

We report the first experimental determination of the carboxylate oxygen electric-field-gradient (EFG) and chemical shielding (CS) tensors in polycrystalline γ-glycine. Analysis of magic-angle spinning (MAS) and stationary ¹⁷O NMR spectra of [¹⁷O]-γ-glycine obtained at 9.4, 14.1, 16.4, and 18.8 T yields the magnitudes of the ¹⁷O EFG and CS tensors and the relative orientations between the two tensors. Extensive quantum chemical calculations at both the restricted Hartree–Fock and density functional levels have been performed to present the absolute tensor orientations in term of the molecular frame. We have demonstrated that ¹⁷O NMR tensor information could be unambiguously derived by the multiple field analyses of stationary ¹⁷O NMR spectra.