Structure of 14C from 12C(t,p)

The reaction ¹²C(t, p)¹⁴C has been investigated with an 18 MeV triton beam. Twenty energy levels of ¹⁴C were identified up to 13 MeV excitation. Angular distributions were measured and compared with DWBA calculations. A previously unreported 0⁺ level at 9.75 MeV was observed; it undoubtedly corresponds to the second predominantly sd shell 0⁺ state in ¹⁴C. Additional spin and parity assignments have been made in the present work: 9.81 MeV, (1^?); 10.43 MeV, 2⁺; 10.50 MeV, (3^?); 10.74 MeV, 4⁺; 11.40 MeV, 1^?; 11.67 MeV, (1^?); 11.73 MeV, (5^?); 12.58 MeV, (2⁺, 3^?); 12.87 MeV. 2⁺, 3^?; and 12.96 MeV, (1^?); none of which had a definite spin and parity assignment previously. Our results confirm the previous information on the level structure of ¹⁴C below 8.5 MeV. The cross section for the unnatural parity state at 7.34 MeV, J^π = 2^?, is well reproduced by a two-step reaction calculation. The results are compared with the predictions of a weak coupling model.     

Measurements of the radiative widths of the first T = 1, 2+ state of 20Ne and their relationship to analog β-decays

The radiative widths for decays of the ²⁰Ne T = 1, 2⁺ (10.27 MeV) state were measured by resonance α-capture in the reaction ¹⁶O(α, γ)²⁰Ne. A special windowless gas-cell target yielded a low-background spectrum enabling six γ-branches to be observed with a Ge(Li) detector. The six branches correspond to decays from the 10.27 MeV level to the following levels: 2⁺(7.83 MeV), 2⁺(7.42 MeV), 3^?(5.62 MeV), 2^?(4.97 MeV), 2⁺(1.63 MeV) and 0⁺(g.s.). The branching ratios and radiative widths Γ_γ to these levels are: 7.83 MeV [(0.22 ± 0.06)%, 0.008 ± 0.002 eV], 7.42 MeV [(6.9 ± 0.4)%, 0.31 ± 0.04 eV], 5.62 MeV [(2.1 ± 0.2)%, 0.097 ± 0.014 eV], 4.97 MeV [(1.3 ± 0.1)%, 0.060 ± 0.008 eV], 1.63 MeV [(88.9 ± 0.5)%, 4.08 ± 0.43 eV] and 0.0 MeV [(0.64 ± 0.14)%, 0.029 ± 0.008 eV]. The radiative widths to the 1.63 MeV and 7.42 MeV levels are used to determine the CVC predictions of the weak magnetism form factors and their effects on certain β-decay observables are evaluated.     

States in Be studied through the reaction B(p, α)Be with 40 MeV protons

Spectroscopy of the α- and τ-particles from 40 MeV proton bombardment of ¹¹B reveals transitions to ⁸Be states up to 19 MeV. The ratio of excitation of the 16.6 MeV state to the 16.9 MeV state is 2.3 ± 0.4. The ⁸Be(4⁺) state is found at 12.5 MeV with a width of 4.0 ± 0.5 MeV.     

Spin assignment to the 9.026 MeV resonance level of36Ar using35Cl(p, γ)36Ar triple angular correlation measurements forE p = 533 keV

Triple correlation experiments have been performed to study the³⁵Cl(p, γ)³⁶Ar resonance for a proton energyE _p = 533 keV. From the analysis of the measured gamma ray angular distributions, due to the gamma decay of the 9.026 MeV resonance level in³⁶Ar via the 9.026 MeV→1.97 MeV→0 MeV double cascade, a spin value ofJ ₁ =2⁺ for the 9.026 MeV level and a mixing ratio of ?1.97 MeV∥L′₁ = 2∥9.026 MeV?/?1.97 MeV∥ L₁ = 1∥9.026 MeV? = ?0.055 + 0.022 for the 9.026 MeV→ 1.97 MeV transition could be derived.     

Study of even-mass Ba nuclei by the (p,t) reaction

The (p, t) reaction on the nuclei ^{134, 136, 138}Ba has been studied at a bombarding energy of 52 MeV. Angular distributions of emitted tritons were obtained between 6° and 60°. The following six negative-parity states were strongly excited by the (p, t) reaction: 5^?(2.121 MeV) and 7^?(2.482 MeV) in ¹³²Ba, 5^?(1.998 MeV) and 7^?(2.274 MeV) in ¹³⁴Ba, and 5^?(2.139 MeV) and 7^?(2.031 MeV) in ¹³⁶Ba. DWBA calculations using the code DWUCK successfully reproduce these angular distributions. The 0⁺ assignment to the 1.761 MeV level in ¹³⁴Ba is confirmed. Intensities of the (p, t) reaction for low-lying states are discussed.     

Investigation of the mechanism of inelastic alpha-particle scattering on 28Si nuclei by the method of angular αγ correlations at E α = 30.3 MeV

The angular dependence of the differential cross sections for alpha-particle scattering on ²⁸Si nuclei and double-differential cross sections for the reaction ²⁸Si(α, αγ)²⁸Si at E _α = 30.3 MeV is measured for the case of alpha-particle emission angle between 20° and 160° and the excitation of low-lying states of the ²⁸Si nucleus (0⁺, ground state; 2⁺ state at 1.78 MeV; 4⁺ state at 4.62 MeV; 0⁺ state at 4.96 MeV; and 3^? + 4⁺, 6.88 MeV + 6.89 MeV). The spin-tensor components of the density matrix for the 2⁺ state at 1.78 MeV and the 4⁺ state at 4.62 MeV in the ²⁸Si* nucleus are reconstructed in a modelindependent way. Seven rank-6 components are reconstructed for the 3^? state at 6.88 MeV. Orientation features of ²⁸Si* are determined. The experimental data in question are compared with the results of the calculations performed under the assumption of the collective-excitation mechanism and by the coupled-channel method.     

Der Zerfall des In109m (0,21 sec)

In^109m(0,21±0,01 sec) was produced from natural Ag and enriched Ag¹⁰⁷ byα irradiation and investigated using scintillation spectrometers and coincidence techniques. The mass assignment follows from the excitation function for the (α, 2n) reaction on Ag¹⁰⁷. The following results were obtained: energy of the isomeric level (2,11±0,02) MeV, determined directly by summing up the cascade transitions in a well-type scintillator; four gamma transitions with energies (0,680±0,015) MeV, (0,405±0,015) MeV, (1,035±0,030) MeV and (1,435±0,020) MeV, respectively. The proposed decay scheme contains the isomeric level (19/2⁺) at 2,11 MeV and a further level (13/2⁺) at 1,435 MeV which decays either by the 1,435 MeV transition (80±3)% or by a 0,405–1,035 MeV cascade (20±3)% via an (11/2⁺) level at 1,035 MeV to the 9/2⁺-ground state of In¹⁰⁹.     

Analysis of inelastic scattering of 3He on 13C at 37.9 MeV

The differential cross section of elastic and inelastic scattering of ³He ions on the ¹³C nucleus has been measured at an energy of 37.9 MeV. By fitting the shape of the measured angular distribution of the elastic scattering the parameters of the optical model have been found. These parameters have been used for the DWBA calculations of angular distributions corresponding to excitations of the ¹³C levels 3.09(1/2⁺) MeV, 3.68(3/2^–) MeV, 3.85(5/2⁺) MeV, 7.55(5/2^–) MeV, 8.84(1/2^–) MeV and 11.85(7/2⁺) MeV. Several sets of transition densities calculated with different effective nucleon-nucleon interactions and six different M3Y interactions between a projectile and target nucleon have been tested. Analysis of the inelastic scattering of 135 MeV protons on ¹³C is also presented, which enabled us to obtain additional information about the tested structure models.     

Investigation of the structure of lower excited states of the 28Si nucleus in α scattering at Eα = 30.3 MeV

The differential cross sections of the ²⁸Si(α, αγ)²⁸Si reaction at E _α = 30.3 MeV, calculated according to the coupled channel method (CCM) with the FRESCO program, are presented for a wide α-particle scattering region with the excitation of the lower ²⁸Si states (0⁺, ground; 2⁺, 1.78 MeV; 4⁺, 4.62 MeV; 0⁺, 4.96 MeV; 3⁻ + 4⁺, 6.88 MeV + 6.89 MeV). The coupling factors are calculated in the Nilsson model.     

Elastic and inelastic scattering of 185 MeV protons from 12C

The polarization of 185 MeV protons in elastic scattering and in the excitation of the 2⁺ state at 4.44 MeV and the 0⁺ state at 7.65 MeV in ¹²C has been measured in the angular region 2°–60°. Optical model calculations are performed for the elastic scattering. Angular distributions for the inelastic scattering from the 2⁺ state at 4.44 MeV and the 3^? state at 9.64 MeV are calculated in the distorted-wave impulse approximation (DWIA) as well as in the distorted-wave Born approximation (DWBA).     

Polarization and phase shifts in Be(p, p)Be from 0.8 TO 2.7 MeV and the structure of B

Angular distributions of the analyzing power in Be have been measured to an accuracy of about ±0.03 at 21 energies from 0.9 to 2.7 MeV with a target thickness of 20 keV at 1 MeV. These data and the cross section measurements of others are reproduced well in the region from 0.8 to 1.6 MeV by a set of phase shifts that vary reasonably with energy. The ³S₁, ⁵S₂, ⁵P₁, and ⁵P₂ phases suffice to describe the data, although channel spin and s-d mixing are required. Three levels satisfy the data: at 0.980±0.010 MeV are two levels, a 1⁺ of width 0.10 MeV and a 2⁻ of width 0.11 MeV; at 1.37±0.02 MeV is a 1⁻ level of width 0.30 MeV. The 1⁺ and 1⁻ states do not agree with earlier assignments. Outside of the energy region from 0.8 to 1.6 MeV a satisfactory set of phases could not be obtained, owing to inadequate data. These regions were studied in the analysis, however, and the results are discussed.     

On the identification of the nuclear forward glory in heavy-ion scattering

We present a simple method which enables us to identify the occurence of a forward glory in heavy ion scattering data. The method is successfully applied to the elastic scattering data of ¹²C (65 MeV), ¹³C (60 MeV), ¹⁵N (85 MeV) and ¹⁶O (75 MeV) on ²⁸Si. Received: 2 July 1999 / Revised version: 3 December 1999     

Spin-dependent effects in inelastic scattering

The differential cross sections for the inelastic scattering of 10 MeV, 19.6 MeV, 30.4 MeV, 40 MeV and 49.35 MeV protons to the 2⁺ state (1.409 MeV) in ⁵⁴Fe and of 19.6 MeV protons to the 2⁺ state (0.846 MeV) in ⁵⁶Fe are analyzed in conjunction with the available data on the asymmetries and spin-flip probability amplitudes. The scattering amplitudes for both one step (valence plus core polarization) and two step (intermediate resonance) processes are evaluated using an antisymmetrized distorted wave approximation. Collective model representations for both the one step (core polarization) and two step (intermediate resonance) processes are used, and included are the effects of deforming the full Thomas spin-orbit potentials. The one step processes are fixed by the analyses of the scattering of 30.4, 40 and 49.35 MeV protons, with the core polarization contributions being constrained by the B(E2) values for the γ-ray deexcitation of the 2⁺ states. The analyses of the 19.6 MeV data demonstrates the need for an extra (two step) contribution to the reaction process and are consistent with the virtual formation of an L = 3 giant resonance. The 10 MeV data most certainly demonstrate compound nucleus effects but could also have some strength due to the virtual formation of an intermediate L = 2 giant resonance. The resonance parameters are consistent with recent information concerning the mass variation of giant resonances.     

Cluster states in the neutron excess nucleus 22Ne

High-spin states of the ²²Ne nucleus in the excitation energy range of 15–30 MeV have been studied. The angular correlation method has been used to determine the spins of excited states. A number of new states with high angular momenta—20.0 MeV (9^?), 20.7 MeV (11^?), 21.6 MeV (9^?), 22.2 MeV (12⁺), and 25.0 MeV (9^?)—have been revealed. They are intensely populated in the reaction ¹⁴C(¹²C, α₁)²²Ne* → α₂ + ¹⁸O and correspond to the rotational bands of various structures.     

Untersuchung von Kernniveaus des16O zwischen 14 und 21 MeV Anregungsenergie durch unelastische Elektronenstreuung

The investigation of¹⁶O-levels by electron scattering (60 MeV maximum energy) was continued up to the region of the Giant Resonance yielding spins, parities, transition probabilities and transition radii for oneE0-(14.00 MeV), fourE1- (17.2, 19.0, 19.5 and 20.95 MeV), oneM1-(16.21 MeV), twoE2-(15.15 and 16.46 MeV) and twoM2- (19.04 and 20.36 MeV) transitions. The behaviour of one resonance at 18.5 MeV is not very well understood, and the spin and parity assignments of two other levels at 16.8 (3⁺) and 17.6 (2^?) MeV remain doubtful. Excitation energies and total widths as obtained from the experimental spectra are presented.     

The 12C(α, γ)16O reaction and stellar helium burning

The cross section for the reaction ¹²C(α, γ)¹⁶O has been measured for a range of c.m. energies extending from 1.41 MeV to 2.94 MeV, by using ¹²C targets of high isotopic purity, large NaI(T1) crystals, and the time-of-flight technique for the suppression of prompt neutron background and time-independent background. Gamma-ray angular distributions were measured at c.m. energies of 2.18, 2.42, 2.56 and 2.83 MeV. By means of theoretical fits, which include the coherent effects of the 1^? states of ¹⁶O at 7.12 MeV, 9.60 MeV, and those at higher energies, the electric-dipole portion of the cross section at astrophysically relevant energies has been determined. A three-level R-matrix parametrization of the data yields an S-factor at E_c.m. = 0.3 MeV, S(0.3 MeV) = 0.14^+0.14_?0.12 MeV · b. A “hybrid” R-matrix optical-m parameterization yields S(0.3 MeV) = 0.08^+0.05_?0.04 MeV · b. This S-factor is of crucial importance in determining the abundances of ¹²C and ¹⁶O at the end of helium burning in stars.     

The role of partly Pauli-forbidden states in nucleus-nucleus elastic scattering

The¹²C(⁶Li,d)¹⁶O reaction at 28 MeV and 34 MeV bombarding energy are analized in Zero Range Distorted Wave Born Approximation (ZR-DWBA) utilizing microscopic four-particle transfer form factors. Configuration mixing is taken into account utilizing the Zuker, Buck and McGrory wave functions for¹⁶O. Good agreement is found for the angular distributions. For the relative cross sections to the different excited states, acceptable agreement is found for the 1^? state (7.12 MeV) and 4⁺ (10.56 MeV), while the model clearly underestimates the transition to the 2⁺ state (6.92 MeV).     

Charge exchange reaction 14C(6Li, 6He)14N

Angular distributions of the charge exchange reaction ¹⁴C(⁶Li, ⁶He)¹⁴N leading to the 1⁺ ground state and 3.95 MeV 1⁺, and 5.20 MeV 2^? excited states at the 34 MeV incident beam energy were analyzed and measured. The 62 MeV data of Goodman et al. were also reanalyzed. The direct one-step charge exchange caused by the spin-isospin dependent term in the two-body interaction can account well for the observed data. The strength of spin-isospin dependent effective interaction (gaussian form with a range parameter of 1.8 fm) was extracted to be 18.5 MeV.     

201 MeV proton excitation of giant resonances in 208Pb: Macroscopic and microscopic analysis

The region of the giant resonances in ²⁰⁸Pb has been investigated by inelastic scattering of 201 MeV protons. To test the analysis, angular distributions were measured for the low-lying 3^?, 5^?, 2⁺ and 4⁺ collective states. The giant isoscalar quadrupole resonance (ISGQR) is split into two structures, one at 9.0 MeV with a full width at half-maximum Γ = 1.0 MeV, the other one at 10.6 MeV (Γ = 2.0 MeV), with fine structures at 8.9, 9.3, 10.1, 10.6 and 11 MeV. A macroscopic analysis using the distorted-wave Born approximation (DWBA) leads for the low-lying collective levels, as well as for the ISGQR, to transition probabilities too small by a factor of two, compared with those obtained in other reactions. Microscopic analysis using the distorted-wave impulse approximation (DWIA), with three different sets of random phase approximation (RPA) transition densities, is in very good agreement with the data. At forward angles, in the 12 to 16 MeV excitation energy region, a strong resonance at 13.5 MeV (Γ = 3.6 MeV) is accounted for by the Coulomb excitation of the isovector giant dipole resonance (IVGDR); at larger angles the results are compatible with the excitation of the isoscalar monopole resonance (ISGMR) located at 13.9 MeV (Γ = 2.6 MeV).A resonance located at 21.5 MeV (Γ = 5.7 MeV) appears as the superposition of an isovector quadrupole resonance (IVGQR) excited by Coulomb interaction and a resonance of multipolarity L = 1 ΔT = 0 (ISGDR “squeezing mode”).     

The excitation function for the 89Y(n, 2n)88Y reaction

Activation cross sections for the ⁸⁹Y(n, 2n)⁸⁸Y reaction at neutron energies between 12.6 and 17.8 MeV have been measured by using the mixed-powder method and γ-ray detection by a Ge(Li) spectrometer. Using the ²⁷Al(n, α)²⁴Na reaction for monitoring the neutron flux, the measured cross-section values for the ⁸⁹Y(n, 2n)⁸⁸Y reaction were found to be 331±32 mb, 603±58 mb, 820±79 mb, 1040±100 mb, 1072±103 mb, 1172±112 mb, 1221±117 mb and 1218±117 mb at the respective incident neutron energies of 12.6±0.1 MeV, 13.3±0.1 MeV, 14.0±0.4 MeV, 14.9±0.3 MeV, 15.1±0.3 MeV, 16.0±0.4 MeV, 16.7±0.5 MeV and 17.8±0.7 MeV. The measured values are compared with the experimental values of others and with the theoretical values obtained from calculations based on the statistical model for the formation of a compound nucleus and its subsequent emission of neutrons.