Excitation of 1+ AND 2− states in the neon isotopes

High resolution inelastic proton scattering is used to study the excitation of 1⁺ states in ²⁰Ne and ²²Ne. By comparing these results with those from gamma-ray fluorescence measurements, the strong role of the orbital contribution in the electromagnetic transitions, predicted by the theory, is confirmed. Shell model predictions of the 1⁺ strength are compared to the measured strength in ²⁰Ne and ²²Ne. The (p, p′) excitation of 2⁻ states in ²⁰Ne is compared to (π⁻, γ) and (e, e′) results.     

The16O(α, γ)20Ne direct capture reaction at low energies

The¹⁶O(α, γ)²⁰Ne direct capture cross section has been calculated in a microscopically founded cluster model which reproduces simultaneously both the correct binding energies and the deformations of the²⁰Ne bound states. Theα+¹⁶O scattering states are derived from a microscopically derived local potential. The astrophysicalS-factor is found to increase linearly with energy in the energy rangeE _cm≈0.4–2 MeV and might therefore be determinable experimentally.     

Study of form factors of elastic and inelastic electron scattering by 20Ne

The elastic electron scattering form factor and also the form factor of inelastic scattering accompanied by nuclear transitions to excited states of the ground rotational band are calculated within the Sp_z(2, R) model reproducing the dynamics of quadrupole nonsphericity of ²⁰Ne. The Sp_z(2, R) model is shown to describe correctly the experimental dependence of the elastic and inelastic (for the 0⁺ to 2⁺ state transition) form factors of the momentum transferred and, consequently, to remove some difficulties experienced by other dynamic models.     

Improved lower limits on neutral fermion masses

Present lower limits on the mass of a neutral fermion N coupled with full strength to an electron via the right-handed weak current are set only by the absence of the decay K⁺→Ne⁺: m(N)?0.5 GeV/c². It is shown that searches for the decays τ→ν_τNe or F→Ne, followed by N→eπ, can raise the lower bound on m(N) to well over 1 G+-eV/c² if no signal is seen in present or soon-to-be-acquired data. Estimates for the branching ratios for these processes are given. If N couples to e cos φ + μ sin φ with full strength, reference to φ may be eliminated by summing over e and μ in all final states.     

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.     

20Ne(p,p0)20Ne and states of 21Na

Cross-section and analyzing power angular distributions have been measured for ²⁰Ne(p, p)²⁰Ne and ²⁰Ne(p, p₁)²⁰Ne¹(1.63 MeV) for proton energies between 3.7 and 7.9 MeV. The measurements were made in 25 keV intervals between 3.7 and 4.4 MeV, and in 10 keV intervals over most of the region between 4.4 and 7.9 MeV. A phase-shift analysis of the elastic-scattering data has yielded resonance parameters for thirty-three levels in ²¹Na in the excitation energy region 6.0–9.9 MeV. Some of the strong even-parity resonances can be understood within the framework of the Nilsson model or the shell model. These resonances are also predicted by a macroscopic coupled-channels calculation involving rotational excitation of the 2⁺ and 4⁺ states of ²⁰Ne.     

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.     

K+ production in a cascade model for high-energy nucleus-nucleus collisions

The K⁺ production is studied for the p + NaF, Ne + NaF, Ne + Pb systems at 2.1 GeV/A in the frame of a 3-dimensional cascade model. Owing to the small elementary production cross sections, the K⁺ production is calculated perturbatively. Two kinds of production processes are introduced: baryon-baryon collisions leading to three-particle final states, and pion-nucleon collisions leading to two-body final states. The time evolution of the two processes is studied. The integrated K⁺ cross sections are in good agreement with the experimental data. The contribution of the πN induced mechanism is of the order of 25% for Ne + NaF, but increases with the size of the system. Scaling properties are discussed. A simple rescattering model is used to calculate the invariant cross section for the Ne + NaF case. Good agreement with experiment is obtained, except at forward angles.     

The (3He,n) reaction on 16O and 18O

The (³He, n) reaction on ¹⁶O and ¹⁸O has been used to study low-spin states in ¹⁸Ne and ²⁰Ne up to E_x ≈ 8 and 20 MeV, respectively. The measured neutron angular distributions have been analysed using DWBA. By a comparison with shell-model calculations in the (s, d) shell it is found that most of the two-proton transfer strength can be explained within that shell. Important contributions, however, from the (f, p) shell in low-lying negative parity states are also present.     

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.     

The (18O, 20Ne) reaction on the even Ni isotopes and the mass of 62Fe

The (¹⁸O, ²⁰Ne) reaction on the even Ni isotopes has been studied at 63.0 MeV with ΔE-E time-of-flight telescopes. From the measured ground-state Q-value for the ⁶⁴Ni(¹⁸O, ²⁰Ne)⁶²Fe reaction, ?1.97±0.20 MeV, a mass excess ?58.87±0.20 MeV is obtained for the ⁶²Fe nucleus. This result is in good agreement with a recent measurement of the β-endpoint energy. Angular distributions for the transitions to the Fe ground states, leaving ²⁰Ne in its ground and 1.63 MeV 2⁺ excited state, yield relative spectroscopic strengths in fair agreement with DWBA calculations based on simple shell-model estimates.     

Recoil-distance measurements of g-factors for 24Mg(21+) and 20Ne(21+)

Time-differential recoil-into-vacuum measurements have been performed with a plunger on the first-excited I^π = 2⁺ states of ²⁴Mg and ²⁰Ne. The states were populated by the reactions ¹²C(¹⁶O, α)²⁴Mg and ¹²C(¹²C, α)²⁰Ne. The measured anisotropy of the α-γ angular correlation was greatly increased by means of a vertical slit on the annular particle detector. Values of $\text{¦g¦= 0.51 ± 0.02}\text{and}\text{0.54 ± 0.04}$ have been deduced for the ²⁴Mg and ²⁰Ne g-factors, respectively. The mean lives of these states have been determined as τ_m = 2.09 ± 0.13 ps and 0.8 ± 0.2 ps, respectively. Various theoretical calculations are discussed and compared with the measured g-factors.The analysis of the measurement also yields values for the populations of electronic states contributing to the hyperfine interaction. For ²⁰Ne the populations of the different electronic configurations are compared with the results of a separate time-integral measurement, in which the correlations were measured for each ionic state separately. Large fractions of two-electron excited states are found to contribute.     

The g-factor of the yrast 4+ state in 20Ne from transient-field precession measurements

Transient field precessions of the first excited 2⁺ and 4⁺ states in ²⁰Ne nuclei traversing polarized gadolinium have been measured. The deduced g-factor of the 4⁺ state, g = + 0.08(20), agrees with our earlier reported value of g = ? 0.10(19) measured in iron by the same technique. The significant reduction in the value of the $\text{g-}\text{factor}\text{,}\text{g}\text{?}\text{= ? 0.01(14)}$, relative to that of the 2 ⁺ state, g = + 0.54(4), is incompatible with the pure T = 0 character expected for low-lying states in this self-conjugate nucleus. In addition, the lifetime of the 4⁺ state has been measured to be τ = 95(13) fs, in agreement with a previous measurement.     

Yrast and high-spin states in 22Ne

High-spin states in ²²Ne have been investigated by the reactions ¹¹B(¹³C, d)²²Ne and ¹³(¹¹B, d)²²Ne up to $\text{E}{}^1\text{～- 19}\text{MeV}$. Yrast states were observed at 11.02 MeV (8⁺) and 15.46 MeV (10⁺) excitation energy. A backbending in ²²Ne is observed around spin 8⁺. The location of high-spin states I ≦ 10 is discussed in terms of the rotational band structure, Strutinsky-type calculations, and pure shell-model predictions.     

Research on neutron-rich nuclei in the region of the nuclear shells N=20 and N=28

The results of the joint experiments carried out by the Dubna-GANIL (France) and the Dubna-RIKEN (Japan) collaborations aimed at synthesizing new isotopes close to the neutron shells N=20 and N=28 and at studying their properties are presented. Gamma-spectroscopic methods were used to study low-lying states in ^30,32Mg, ^26–28Ne, ²²O, and ¹⁸C. The ratios E(4⁺)/E(2⁺) were determined. A direct method was used to measure the masses of 20 nuclides located between the shells N=20 and N=28. The decay properties were determined for ³⁰Ne and ^26,27,29F. Information obtained in this way suggests the existence of a deformation close to the neutron shell N=20. The results of experiments devoted to searches for the doubly magic nucleus ²⁸O are also presented. Only the upper limit on the cross section for its production was deduced, which can be taken as evidence of its instability.     

Structure of the mirror nuclei21Ne and21Na

Theγ-decay of levels in²¹Ne up to 10 MeV excitation energy has been investigated byn — γ coincidence measurements initiated with the¹⁸O(α, nγ) reaction at 12, 13, 14.5 and 15.4 MeV bombarding energies. Spin(-parity) assignments of excited states are obtained by combining then — γ angular correlation measurements performed atE _α=11, 11.82 and 13.6 MeV with a consideration of lifetimes, neutron penetrabilities of the unbound states, and information from the mirror nucleus²¹Na. The resulting values of E_x[keV]?J ^π are as follows: 4525-5⁺, 4686-3⁺, 5431-7⁺, 5549-3⁺, 5819-7^?, 6175-7⁺, 6268-9⁺, 6550-9, 6639-9, 7006-7⁺, 7041-9, 7356-7 or 9, 7422-11^(?), 7648-7⁺, 7981-11 or (7⁺), 8154-9, 8240-11, 8664-9^? or 11 or 13^?, 9401-13^?, 9867-13^? or 15⁺, 9941-13^? or 15 or 17⁺. The assignment of mirror levels in²¹Ne —²¹Na has been extended to the 6175 keV level of²¹Ne. Excitation energies, electromagnetic properties, Gamow?Teller matrix elements and spectroscopic factors of positive parity states are compared with the results of shell-model calculations which employ a unifieds—d shell Hamiltonian and the unrestricted configuration space of the 0d _5/2 —1s _1/2—0d_3/2 shell. Collective properties contained in shell model wave functions are explored up to the termination of bands atJ=17/2 or 19/2. The spectrum of intruder states in²¹Ne is observed to begin with a 5628 keV,J ^π=7/2⁺ state. The 7422, 8664 and 9401 keV levels are assigned as members of previously established negative-parity rotational bands.     

Angular distribution measurements of gamma-rays from the22Ne(p, γ)23Na reaction

Angular distribution measurements of gamma-rays from the²²Ne(p, γ)²³Na reaction on theE _p =897, 1006, 1091 and 1278 keV resonances have been made. Spin and parity assignments for several bound states are presented together with multipole mixing ratios for transitions from resonance and bound states. The level at 6617 keV (7/2⁺) is proposed to be a member of the rotational band based on the Nilsson orbit 5(K ^π=5/2⁺).     

Polarization effects in ionizing thermal energy collisions of laser-excited Ne(3p 3 D 3) atoms with Ar atoms

Using transverse and longitudinal excitation of a collimated metastable Ne(3s ³ P _2.0) beam with average velocities of 500, 800, and 1,200 m/s by means of a single mode dye laser on the²⁰Ne(3s ³ P ₂→3p ³ D ₃) transition, we have investigated ionizing collisions of polarized Ne(3s ³ P ₂) and Ne(3p ³ D ₃) atoms with Ar atoms. The product electrons were energy analyzed with high resolution (9–25 meV). The resulting Ne(3p ³ D ₃) electron spectra exhibit a strong dependence on the three types of laser polarization (π _∥,π_⊥, σ_?), chosen to prepare the excited atoms. In contrast, the Ne(3s ³ P ₂) spectra are only weakly dependent on polarization. Detailed model calculations have been carried out for the Ne(3p)+Ar cross sections, using computed excited-state potential curves, semi-empirical ionic potentials, and local autoionization width functions. A semiclassical closecoupling method is applied to describe the evolution of the polarized collision system in the coupled entrance channels. It is found that a single autoionization widthΓ(R) is not sufficient to describe the measured polarization effects properly. The dependence ofΓ on the initial and final state is expressed in terms of few reduced electronic transition matrix elements, which are determined by comparison of measured and calculated total cross sections and Ar⁺(² P _3/2)/Ar⁺(² P _1/2) branching ratios for ionizing collisions of the various Ne(3pJ=1,2,3) multiplet states with Ar. The matrix elements corresponding to Ar(3pσ)→Ne(2pσ) electron transfer during autoionization are found to dominate, but Ar(3pπ)→Ne(2pπ) transfer has also to be included. The resulting calculated electron spectra reproduce the measured polarization effects in a semi-quantitative way.     

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.     

Spin assignment of the 8.555 MeV state in28Si

Angular correlations between the gamma rays following the decay of the 8.555 MeV state in²⁸Si, known to have natural parity, were measured. The spin assignment of this state, found in this experiment, isJ ^π=6⁺. Energy sequence of the lowestJ ^π=2⁺, 4⁺ and 6⁺ states in²⁸Si compares well with the energy sequence of the ground state rotational bands in²⁰Ne and²⁴Mg.