Formation and decay of the compound nucleus studied in the reaction20Ne+27Al

Energy and velocity spectra, angular and mass distributions have been measured for evaporation residue products of the^{20, 22}Ne+²⁷Al system in the energy range ofE _L (²⁰Ne)=51 to 395 MeV and in the angular rangeθ=2° and 30°. Calculations with simple assumptions of the velocity and angular distributions of the evaporation residues are presented and compared to the data. The structure seen in the mass distributions, a competition between α-particle and nucleon evaporation in the deexcitation of the compound nucleus, is described well by calculations with the computer code CASCADE. The evaporation residues exhibit mass distributions varying systematically as a function of the excitation energy. The excitation function of the evaporation residue cross section is compared with theoretical models. At higher incident energies contributions of incomplete momentum transfer (incomplete fusion) are observed. A limitation for complete compound nucleus formation with following light particle evaporation is found.     

A comparative spectroscopic investigation of21Ne and21F by pickup reactions

The reactions²²Ne(d, t)²¹Ne and²²Ne(d, τ)²¹F have been investigated in a parallel experiment atE _d=52MeV. Energy spectra of tritons andτ particles have been measured up to excitation energies of 18MeV in both²¹Ne and²¹F nuclei. Thel values could be determined, and spectroscopic factors have been obtained by a DWBA analysis of the measured angular distributions. Together with the results from a preceding \({}^{22}Ne(\vec d, \tau ){}^{21}F\) experiment, several spins could be determined in²¹F. From a comparison of the triton andτ particle spectra,T=3/2 states in²¹Ne could be identified and on this basis the new information on spins of²¹F states could be transferred to their analog states in²¹Ne. The completeT _< andT _> parts of the (2s, 1d) strengths could be observed. The 1p strength is highly fragmented. Of theT _< part only 60% of the 1p1/2 and 40% of the lp3/2 strength could be found, but the totalT _> part of the 1p strength could be localized. The spectroscopic results for positive parity states are in qualitatively good agreement with shell model predictions.     

Giant resonance decays in 20Ne, 22Ne and interference effects with quasifree scattering processes

The charged particle (c) decay from excited states up to the giant quadrupole resonance (GQR) in ²⁰Ne has been studied in a kinematically complete ²⁰Ne(α, α′ c) coincident experiment at E_α = 155 MeV. Angular correlation functions and branching ratios are extracted for the α₀, α₁ and p₀ decay channels. The (α, α′ α₀) angular correlation functions are analysed in PWBA in terms of coherent interference with the quasifree scattering process leading to the same final states. Good fits to the data are achieved over a large range of excitation energies. Branching ratios have been extracted and compared to results of Hauser-Feshbach calculations. Above E_x = 12.5 MeV excitation energy a discrepancy was found between the experimentally observed α₀ branching ratios and the HF predictions. These results yield evidence for a direct α₀ decay mechanism of the split isoscalar giant quadrupole resonance in ²⁰Ne. Some results are presented also for a ²²Ne(α, α′ c) coincidence experiment. Qualitative comparison has been made between the general decay behaviour of the two Ne isotopes.     

Inelastic scattering of20Ne and12C on58Ni and the three-body continuum

Using beams of²⁰Ne at 291 and 392 MeV and¹²C at 300 MeV the inelastic excitation of collective modes in⁵⁸Ni has been studied with a Q3D magnetic spectrometer. In the analysis different contributions are unfolded from the spectra:1. single lines,2. the inelastic continuum,3. the three-body continuum, which mainly originates from one-nucleon pick-up to unbound states followed by the nucleon emission. Special care was taken to calculate the spectral shape and strength of this process. The population of unbound²¹Ne-states is measured in a neutron-²⁰Ne coincidence experiment. Coupled channel calculations have been performed to extract the deformation parameters for the inelastic states, including mutual excitation. For the giant quadrupole resonance 70 % of the energy weighted sum rule is found. The excitation of higher multipoles is calculated and extrapolated to higher incident energies (50 MeV/u). The importance of the in-elastic excitation as a “doorway process” to more complex interactions is discussed. It is found that especially the excitation of the projectile (²⁰Ne) takes a large fraction (20–50 %) of the incoming flux in the first step.     

Cross sections for 6Li production in the reactions 10B + 16O and 12C + 14N at low energies

Absolute cross sections have been measured for the reactions ¹⁰B(¹⁶O, ⁶Li)²⁰Ne and ¹²C(¹⁴N, ⁶Li)²⁰Ne at several energies in the range E_c.m. = 7.5–16.2 MeV, and 13.8–16.6 MeV, respectively. The predictions of Hauser-Feshbach calculations show generally good agreement with the experimental data without parameter variation. The consequences of an angular momentum cutoff in the entrance channel and in the compound nucleus are discussed.     

Analyses of inelastic proton scattering from 20Ne

Differential cross sections and analyzing powers for inelastic proton scattering to the 2⁺₁, 4⁺₁ and 6⁺₁ states of ²⁰Ne are analyzed using an antisymmetrized distorted wave approximation in direct reaction theory. The spectroscopy of these states was obtained by angular momentum projection from an axially symmetric minimal-energy Hartree-Fock intrinsic state.     

Helium burning of22Ne

The²²Ne(α, γ)²⁶Mg and²²Ne(α, n)²⁵Mg reactions were investigated forE _α(lab) from 0.71 to 2.25 MeV. Neon gas enriched to 99% in²²Ne was recirculated in a differentially pumped gas target system of the extended type. Theγ-ray transitions were observed with Ge(Li) detectors and the neutrons with³He ionization chambers. A previously known resonance at E_R(lab)=2.05 MeV was verified and 15 new resonances were found in the energy range covered, with the lowest at E_R(lab)=0.83 MeV. Information on resonance energies, widths, strengths,γ-ray branching ratios, as well asJ ^π assignments, is reported. The energy range investigated corresponds to the important temperature range ofT ₉ from 0.3 to 1.4 (10⁹ K), for which the astrophysical rates were determined for both reactions. The results show that the ratios of the rates for²²Ne(α, n)²⁵Mg and²²Ne(α, γ)²⁶Mg are significantly smaller than the previously adopted values, e.g., by at least a factor of 60 nearT ₉=0.65. Thus, the²²Ne(α, n)²⁵Mg reaction will likely play a smaller role as a neutron source fors-process nucleosynthesis, than has frequently been assumed.     

Untersuchungen zur Struktur des Kerns23Ne

Low-lying states of²³Ne up to about 3 MeV excitation energy have been investigated by studying the reactions²²Ne(d, p)²³Ne (E _d =4–6 MeV) and²³Na(n, p γ)²³Ne(E _n=8–9 MeV). From the (d, p) data,l-values and spectroscopic factors for the transferred neutron have been extracted by DWBA analyses. From the (n, p γ) data,γ-ray branchings and possible spin assignments have been derived. The results are used to discuss the applicability of the Nilsson model and the excited core model to the nucleus²³Ne.     

Quadrupole transitions in 20Ne

Untruncated shell model calculations in the 1s-0d shell using Chung-Wildenthal effective interactions have been performed for ²⁰Ne and the electron scattering form factors for elastic scattering and for the excitation of 2₁⁺, 2₁⁺ and 2₃⁺ states have been examined. Saxon-Woods wave functions and a phase-shift analysis have been used. It is shown that within the limited space of the 1s-0d shell, it is possible to obtain vastly different momentum transfer dependence for the electro-excitation of ΔJ^π = 2⁺ transitions. The 2₁⁺ and 2₂⁺ states are seen to be adequately described by the wave functions obtained in this calculation, but 2₃⁺ is not described well.     

The influence of correlations on the odd-mass Nuclei21Ne and25Mg

Excitation energies and electromagnetic properties of the low energy spectra of²¹Ne and²⁵Mg have been calculated using the Multi-Configuration-Hartree-Fock (MCHF) model. BothT=0 andT=1 pairing correlations are found to be simultaneously important in those odd mass nuclei. Furthermore, though axial symmetry was requested, quite good agreement with the experimental data is reached.     

DSA lifetime measurements in21Ne at high recoil velocity

States in²¹Ne up to 5 MeV excitation energy have been populated using the inverted reaction²H(²⁰Ne,pγ). The Doppler shift attenuation (DSA) analysis of thepγ coincidence spectra taken in a Ge(Li) detector at 45° and 135° and an annular silicon surface barrier detector near 0° yielded the lifetimes of 8 states in²¹Ne. Due to the large recoil ofπ _i/c～ 4% three new lifetimes were determined for the short lived levels at 2.80, 4.68 and 4.73 MeV, namely 10±4 fs, 16±4 fs and 10±4 fs, respectively. The results are compared with rotational and shell model calculations.     

Search for highly deformed shape isomers in 28Si, 24Mg and 20Ne linked by successive alpha decays

Measurements of the ¹²C(¹⁶O,²⁰Neα)α, ¹²C(¹⁶O,αα)²⁰Ne, ¹²C(¹⁶O,⁸Be)²⁰Ne and ¹²C(¹⁶O,¹⁶Oα)⁸Be reactions have been performed at E_c.m. = 27.0 MeV. For decays proceeding through resonant states in the intermediate ²⁴Mg nucleus, angular correlation measurements have enabled spin assignments to be made and the dominant entrance channel angular momentum to be determined. The results are inconsistent with recent similar experiments which were interpreted as arising from successive α-decays from deformed shape isomeric states in ²⁸Si and ²⁴Mg.     

Properties of levels in22Ne

Levels up to 9.3 MeV excitation in²²Ne were studied with the¹⁹ F(α, pγ) ²²Ne reaction atE _α = 12 MeV. Level energies, branching ratios, mean lifetimes, spins and mixing ratios were obtained for a number of levels. Relevant results are compared with many-particle shell-model calculations.     

Nonstatistical structures in the excitation functions of20Ne(16O,12C)24Mg and20Ne(16O,20Ne)16O reactions

The data on the excitation functions of²⁰Ne(¹⁶O,¹²C)²⁴Mg,²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(1.37, 2⁺),²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(4.12, 4⁺+4.24, 2⁺) +²⁰Ne(¹⁶O,¹²C^*(4.44, 2⁺))²⁴Mg,²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(6.01, 4⁺+6.43, 0⁺),²⁰Ne(¹⁶O,²⁰Ne)¹⁶O,²⁰Ne(¹⁶O,²⁰Ne^*(1.63, 2⁺))¹⁶O, and²⁰Ne(¹⁶O,²⁰Ne^*(4.25, 4⁺))¹⁶O reactions atθ _lab=13° fromE _c.m.=22.8 to 38.6 MeV have been subjected to a statistical analysis comprising of the calculations of the distribution of cross sections, deviation functions, cross-correlation functions, summed excitation functions, cross-channel correlation coefficients and coherence widths. The analysis confirms the existence of nonstatistical structures atE _c.m.=24.6, 27.8, 31.7 and 35.5 MeV, and identifies a new structure of the same nature atE _c.m. =25.6 MeV.     

Studies on strongly damped components in relatively light heavy ion reaction systems

Strongly damped components have been studied in relatively light heavy ion reaction systems;²⁰Ne(E _lab=93, 120 and 146MeV)+⁵⁰Cr,²⁰Ne(E _lab=146MeV)+⁵⁴Cr and²⁰Ne(E _lab=146MeV)+⁹²Mo,¹⁰⁰Mo. The kinetic energy, angular and charge distributions have been observed for those products. The yields of symmetric-mass-splitting products for²⁰Ne+⁵⁰Cr were found about three times larger than those for²⁰Ne+⁵⁴Cr. There was also observed a similar difference in the cross sections of the symmetric-mass-splitting products between²⁰Ne+⁹²Mo and²⁰Ne+¹⁰⁰Mo reactions. In order to explain the bombarding energy dependence of the cross sections of symmetric-mass-splitting products by the transport theory, it was found necessary to assume that the mean life of the composite nucleus was dependent on the bombarding energy. However, the target isotope dependence of the cross sections could not be explained by such an assumption. They could be partly explained by fission calculations.     

The generator coordinate method as an approximation to the shell model: A comparison on 24Ne

The applicability of the generator coordinate method as an approximation to the complete shell model diagonalization is tested in the sd shell on ²⁴Ne. We use the quadrupole moment and the pairing energy as generator coordinates and choose as generating function a constrained Hartree-Bogoliubov solution projected on the subspace of good angular momentum and good proton and neutron number. The spectra obtained by the generator coordinate method and the complete diagonalization show good overall agreement. Also we draw some conclusions about the nature of some low-lying states in ²⁴Ne, interpreted in terms of vibrations.     

Two-step mechanism in the 22Ne(p,t) reaction

The forward-angle differential cross sections have been measured for the ²²Ne(p,t) reaction leading to the 4.97 MeV, 2^? state in ²⁰Ne. Comparison of the data with the results of calculations shows that the two-step mechanism responsible for exciting this state is (p-p′-t) rather than (p-d-t).     

Proton-induced direct capture on 21Ne and 22Ne

The direct capture process in the reactions ²¹Ne(p, γ)²²Na and ²²Ne(p, γ)²³Na has been investigated at E_p = 0.3–1.6 MeV using neon gas enriched to 91 % in ²¹Ne and to 99 % in ²²Ne, respectively. The gas was recirculated in a differentially pumped gas target system of the extended-static and quasi-point supersonic jet type. For ²²Ne(p, γ)²³Na, the direct capture process has been observed to several final states in ²³Na up to E_x = 8.83 MeV excitation energy. The deduced spectroscopic factors C²S are in fair agreement with the corresponding values from stripping reactions. The capture transition into the ²³Na ground state exhibits broad structures, which resemble Ericson fluctuations. The data remove the previously reported discrepancies in C²S for the ²³Na ground state. The excitation functions for the ²¹Ne(p, γ)²²Na reaction are dominated by broad and intense resonances, which hampered the measurement of the direct capture process. The nuclear and astrophysical aspects of the results are discussed.     

Complete fusion in a light heavy-ion reaction from 2.5 to 20 MeV/nucleon

Mass distributions of evaporation-residue-like fragments have been measured using a time-of-flight system for the reaction ²⁰Ne + ²⁶Mg in the energy range of 51 to 400 MeV bombarding energy for ²⁰Ne. A good mass resolution allowed for the separation of the evaporation residues and fragments from two-body reactions like e.g. damped processes. The residue distributions were compared with evaporation calculations. The analysis of velocity spectra measured at bombarding energies of 85–395 MeV showed incomplete momentum transfer for evaporation-residue-like fragments at higher energies. Statistical-model calculations and Monte Carlo methods applied to the calculation of the velocity spectra have been used to extract the complete-fusion cross section.