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.     

The lifetime of the 5.14 MeV state in22Ne

Using the delayed coincidence method and the reaction¹⁹F(α, pγ)²²Ne at 10.80 MeV alpha energy, the lifetime of the 5.14 MeV state in²²Ne is determined to beτ=0.84 nsec±10%.     

Investigation of two resonances in the18O(p,n)18F reaction

Excitation functions for the¹⁸O(p,n)¹⁸F reaction were measured at bombarding energiesE _p=4.6 to 6.6 MeV. In and near two resonances of the yield curves atE _p=5.622 and 6.061 MeV, angular distributions were measured with neutron time-of-flight techniques. The strong neutron decay to theT=1 state in¹⁸F and the similarity of the¹⁸O(p,n) and¹⁸O(n,n) yield curves give good evidence that the structures in the¹⁸O(p,n) yield curve arise from the formation ofT=3/2 states in¹⁹F. A two-level-analysis does not give satisfactory fits to the strongly asymmetric angular distributions.     

The 22Ne(d,n) 23Na reaction

Absolute differential cross sections are determined for 32 states from the ²²Ne(d, n) ²³Na reaction by the neutron time-of-flight method. A gaseous ²²Ne target was bombarded with 5.5 MeV deuterons and angular distributions taken from 0° to 160°. In addition yield curves were taken at a fixed angle of 10° in 0.5 MeV steps from 2.5 to 5.5 MeV. The analysis of both types of data used computer programs for DWBA and compound-nucleus calculations. With two exceptions and three additions the l_p values determined in the present experiment agree with those of a recent (τ, d) experiment on the same target nucleus. The two previous (τ, d) experiments show considerable differences in proton transfer strengths to various states. The present experiment agrees well with the one which showed generally lower strengths for individual states, and hence with an assumption of greater spreading of the single-particle strength. The implications of those results on the Nilsson-model scheme for ²³Na are discussed.     

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.     

The11B(p,p′)11B* reaction: An extended coupled-channels treatment

The predictions of a multi-configurational shell model continuum calculation for the¹¹B(p, p) and¹¹B(p,p′) reaction channels are discussed. In the calculated excitation function for the (p, p ₁) channel theT=1, 2^? resonances play a dominant role in the region of 22 MeV to 24 MeV excitation in¹²C. These model predictions are consistent with the known parent states in the¹²B system. Corroborative evidence is also obtained by comparing theory with the differential cross section data.     

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.     

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.     

Mixing of 2+ states in 20Ne

The broad 8.8 MeV 2⁺ state of ²⁰Ne is seen to be populated with appreciable strength [(2J + 1)S = 0.21] in the reaction ¹⁹F(τ, d)²⁰Ne, even though this strength would be zero in the simplest model. A model previously used to explain the mixing of 0⁺ states is found to work surprisingly well for the 2⁺ states.     

Investigation of22Na excited states

Fourteen²¹Ne(p, γ)²²Na resonances have been observed in the rangeE _p =300–1,300 keV. Theγ-decay of all these resonances has been investigated by means of a 38 cm³ Ge(Li) detector. Energies and branching ratios of several bound states have been determined. TheQ-value was determined as 6,738.5±1.7 keV. Lifetimes of seven states were determined with the Doppler-shift attenuation method. The observed upper limit (τ _m ≦4 fs) of the lifetime for the 4,071 keV state, regarded as the analogue of the third excited state in²²Ne, and the transition observed from this state to the 1,528 keV state do not support the proposed rotational band structure of the²²Na low-lying states.     

Sequential processes in the14N(τ,pα)12C* reaction

α-particle and proton spectra in coincidence with the 4.44 MeVγ-ray from the¹²C first excited state have been taken atE _τ MeV. Several states in¹³N and¹⁶O are seen to contribute to the decay sequenceα?p andp?α respectively. Estimates are given for the branching of these two decay modes and for a possible simultaneous break-up.     

Proton induced resonances on 21Ne

Excitation curves for the ${}^{21}\text{Ne}\text{(}\text{p}\text{, γ)}{}^{22}\text{Na}\text{,}{}^{21}\text{Ne}\text{(}\text{p, p}\text{′γ)}{}^{21}\text{Ne and}{}^{21}\text{Ne}\text{(}\text{p, p}\text{)}{}^{21}\text{Ne}$ reactions have been obtained for E_p = 0.4–1.6 MeV. Neon gas enriched to 92 % in ²¹Ne was recirculated in a differentially pumped gas target system. The fifteen previously reported (p, γ) resonances were established and nineteen new (p, γ) resonances found. Anomalies in the elastic scattering yield were observed for fourteen resonances. The reported state at E_x = 7278 ± 7 keV in ²²Na was resolved into a doublet separated by 1 keV. All unbound states in ²²Na, observed previously in other reactions, have been confirmed as resonance states in the energy range covered, with the exception of the E_x = 7942 ± 7 keV state. The new E_p = 663, 694, 1235, 1432 and 1543 keV resonances correspond to new unbound states in ²²Na. Excitation energies, γ-ray decay schemes, resonance widths and strengths as well as limits on J^π assignments are reported for all the resonances. From the Coulomb excitation of the E_x = 350 keV, first excited state in ²¹Ne a value of B(E2) = 0.014 ± 0.002 e² · b² is deduced. The astrophysical as well as the nuclear structure implications of the present results are discussed.     

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.     

The 12C(13C, α)21Ne reaction: High-spin states,resonances and coherence widths

Excitation functions were measured for states of ²¹Ne populated by the ${}^{12}\text{C}\text{(}{}^{13}\text{C}\text{, α)}$ reaction over the bombarding energy range E_lab = 18.2–32.0 MeV (18.4–27.0 MeV) at θ_lab = 7°(25°) in in 200 keV steps, and average coherence widths of states and the moment of inertia of the compound nucleus ²⁵Mg were obtained from these excitation functions. A statistical analysis of these data was performed. Angular distributions for states in ²¹Ne to 10 MeV in excitation energy were measured at θ_lab = 7°, 18°, 28° and 43° at bombarding energies from 29.0 to 31.0 MeV in 400 keV steps. These data along with Hauser-Feshbach predictions allow us to suggest spins for some states as well as to suggest possible candidates for rotational bands in ²¹Ne.     

Spins and parities of hole states in25Na and25Mg

Angular distributions of differential cross sections and vector analyzing powers were measured forl=0, 1 and 2 transitions induced by the \(^{26} Mg\left( {\overrightarrow d ,\tau } \right)^{25} Na\) reaction at 52 MeV. The following spins and parities of final states in²⁵Na were deduced:J ^π =1/2⁺ (1.069 MeV), 3/2⁺ (2.202 MeV), 5/2⁺ (2.914 MeV), 1/2^? (3.995 MeV) and 3/2^? (5.190, 5.690, 6.549 and 7.603 MeV). The DWBA analysis of proton and neutron pick-up spectra obtained from a parallel measurement of the²⁶Mg(d, τ)²⁵Na and²⁶Mg(d, t)²⁵Mg reactions allows the identification ofT=3/2 analog states in²⁵Mg. Interpretation in terms of the Nilsson model of energies and spectroscopic factors of the first 1/2^? and 3/2^? hole states observed in proton pick-up reactions from even 1d _5/2- shell nuclei indicates a close correspondence of the final state deformations with those of the first excited 2⁺ states in the target nuclei.     

Lifetime measurements of the3 P 2 and4 P 5 2/0 states in He- and Li-like titanium and chromium

Ti and Cr ions at specific energies of 3.6 MeV/N and 5.0 MeV/N have been excited by passing them through thinC foils. The emission ofK-x rays andK-Auger electrons have been investigated using a flat crystal spectrometer and an electrostatic cyclindrical mirror analyzer. The lifetimes of the (1s2p)³ P ₂ and (1s2s2p)⁴ P _5/2 states have been determined by applying the time-of-flight technique. The experimental resultsτ(Ti²⁰⁺,³ P ₂)=(404±40)ps,τ(Cr²²⁺,³ P ₂)=(215±35)ps,τ(Ti¹⁹⁺,⁴ P _5/2)=(236±12)ps, andτ(Cr²¹⁺,⁴ P _5/2)=(140±8)ps are compared with calculated lifetimes.     

Transition probabilities and the structure of some negative-parity states in126,130Xe and132Ba

The lifetimes of several negative-parity states in^126,130Xe and¹³²Ba have been determined by means of the generalized centroid-shift method. The reactions^124,128Te(α,2n) and¹²²Sn(¹³C, 3n) have been used. Following results were obtained:T _1/2(2758 keV)=1.3±0.2 ns in¹²⁶Xe,T _1/2(2060 keV)=0.20±0.10 ns,T _1/2(2104 keV)=0.50±0.10 ns,T _1/2(2376 keV)=0.30±0.10 ns andT _1/2(2973 keV)=4.6±0.4 ns in¹³⁰Xe as well asT _1/2(2120 keV)=0.40 _?0.10 ^+0.20 ns in¹³²Ba. A systematics of the B(E2; 7 _? ¹ ?5 ₁ ^? ) values in theN=76 nuclei is presented. Electric dipole and quadrupole transition rates are discussed in terms of octupole and quadrupole collectivity. The structure of the 5 ₁ ^? and 7 ₁ ^? states is considered. Nuclear reactions:^124,128Te(α, 2 _n ),E=26 MeV,¹²²Sn (¹³C, 3n),E=53 MeV; measuredE _γ I _γ , γ-r.f. DeducedT _1/2, B(σL) in^126,130Xe and¹³²Ba. Ge detectors. Generalized centroid-shift analysis.     

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.     

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.