Low energy resonances in21Ne(p,γ)22Na examined with a high energy resolution ion beam

TheE _R=126, 272 and 291 keV resonances in the²¹Ne(p, γ)²²Na reaction have been investigated with a high-energy-resolution ion beam. TheE _R=272 keV resonance was found to consist of two states separated by (888+5) eV, where the lower (higher) energy member is a high-spin (low-spin) state. All four resonances have widths less than a few eV, which is an improvement of nearly two orders of magnitude below previously reported limits. The influence of atomic effects on the determination of the correct value for the resonance energy is examined.     

Search for low-energy resonances in 21Ne(p, γ)22Na and 22Ne(p, γ)23Na

The reactions ²¹Ne(p, γ)²²Na and ²²Ne(p, γ)²³Na have been investigated at E_p(lab) = 70–355keV. Neon gas enriched to 91% in ²¹Ne and to 99% in ²²Ne was recirculated in a differentially pumped gas target system of the extended-static and quasi-point supersonic jet type. For ²¹Ne(p, γ)²²Na, new resonances were found at E_p = 126, 272, 291 and 352 keV. The 291 keV resonance corresponds to a new unbound state in ²²Na. Excitation energies, γ-ray decay schemes, resonance widths and strengths as well as J^π assignments are reported for all the resonances. Information on low-lying states in ²²Na is also obtained. Of the 9 expected resonances in ²²Ne(p, γ)²³Na none has been observed. Upper limits on their ωγy strengths are presented. The astrophysical as well as the nulcear structure aspects of the results are discussed.     

A study of the excited states of23Na from the22Ne(p, γ)23Na reaction

The gamma decays of 19²²Ne(p, γ)²³Na resonances in the range ofE _p =400?1300 keV have been investigated by means of a 38 cm³ Ge(Li) detector. Branching ratios of the resonances and bound states were determined. Gamma-ray energy measurements yield the energies of 19 bound states and the value of 8794.0±1.5 keV forQ. The existence of the 3913 keV doublet state was studied through the (p, γ) reaction. No evidence for such a doublet was found.     

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.     

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⁺).     

Isobaric analogue states excited in the59Co(p, γ)60Ni reaction

The⁵⁹Co(p, γ)⁶⁰Ni reaction has been investigated in the proton energy regionE _p=1365–2150 keV. Decay schemes and branching ratios have been determined for ten resonances, five of which have been identified as possible analogues or fragments of analogues of the ground state (5⁺) and the 58.6 keV (2⁺), 277.1 keV (4⁺), 288.4 keV (3⁺), and 435.7 keV (5⁺) levels in⁶⁰Co. At eight of the resonances most of the decay seems to go via a group of states with an excitation energy of 5–9 MeV. The investigated analogue states give a Coulomb displacement energy of 9118±7 keV.     

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.     

Internal electron-positron pair production from electric monopole transitions

The yield curve of the reaction⁵⁶Fe(p,γ)⁵⁷Co has been measured over the energy rangeE _p =1,300–1,900 keV and the decay of nine resonances has been investigated. For twelve of the resonances the strengths have been determined. The angular distributions of the gamma rays have been recorded for resonances atE _p =1,599, 1,623, 1,643 and 1,649 keV, giving spin-parity assignmentsJ ^π=3/2^? for all four resonances. The resonances atE _p =1,623, 1,643 and 1,649 keV have been identified as the split analogue resonances of the 367 (J ^π=3/2^?) states in⁵⁷Fe. TheM1 transition strengths to the corresponding antianalogue states have been measured and compared with theoretical predictions.     

Hydrogen burning of 20Ne and 22Ne in stars

The ²⁰Ne(p, γ)²¹Na capture reaction has been studied in the energy range E_p = 0.37–2.10 MeV. Direct-capture transitions to the $\text{332 (}\text{5}\text{2}{}^{\mathrm{+}}\text{)}\text{and}\text{2425}\text{keV}\text{(}\text{1}\text{2}{}^{\mathrm{+}}\text{)}$ states have been found with spectroscopic factors of C²S(1d) = 0.77±0.13 and C²S(2s) = 0.90±0.12, respectively. The high-energy tail of the 2425 keV state, bound by 7 keV against proton decay, has also been observed in the above energy range as a subthreshold resonance. The excitation function for this tail is consistent with a single-level Breit-Wigner shape for a γ-width of Γ_γ = 0.31±0.07 eV at E_x = 2425 keV. The extrapolation of these data to stellar energies gives an astrophysical S-factor of S(0) = 3500 keV · b. Two new resonances at E_p = 384±5 and 417± 5 keV have been observed with strengths of ωγ = 0.11±0.02 and 0.06±0.01 meV, corresponding to the known states at and 2829 keV (presumably $\text{9}\text{2}{}^{\mathrm{+}}$), respectively. For the known E_p = 1830 keV resonance, a strength of ωγ = 1.0± 0.3 eV and a total width of Γ = 180± 15 keV were found. Branching ratios as well as transition strengths have been obtained for these three states. The Q-value for the ²⁰Ne(p, γ)²¹Na reaction (Q = 2432.3 ± 0.5 keV) as well as excitation energies for many low-lying states in ²¹Na have been measured. No evidence was found for the existence of the state reported at E_x = 4308±4 keV.In the case of ²²Ne(p, γ)²³Na, direct-capture transitions to six final bound states have been observed revealing sizeable spectroscopic factors for these states. The astrophysical S-factor extrapolated from these data to stellar energies, is S(0) = 67 ± 12 keV · b.The astrophysical as well as the nuclear structure aspects of the present results are discussed.     

Investigation of the gamma decay of excited states in25Al

The gamma decay of the low-lying energy levels (E _x <4MeV) in²⁵Al has been studied through the²⁴Mg(p, γ)²⁵Al reaction using seven resonances in the proton energy rangeE _p +200?1700 keV. Ge(Li) detectors and isotopically separated targets have been used. Energies and branching ratios of several excited states have been measured. TheQ-value of the reaction was determined to be 2271.3±0.8 keV. Lifetimes, investigated by the Doppler-shift attenuation method and γ-ray angular distributions have been determined using theE _p +823, 1201, 1490 and 1660 keV resonances of the²⁴Mg(p, γ)²⁵Al reaction.     

Selective population of the lowest-lying negative parity states in23Na in the12C(12C,p)23Na reaction

The¹²C(¹²C,p)²³Na reaction was investigated in the vicinity of the Coulomb barrier. The lowest-lying negative parity states in²³Na were found to be preferentially populated at energies for which correlated resonances exist in the excitation functions of theα, p, n andγ exit channels of the¹²C+¹²C reaction. If these negative parity states are assumed to consist of ap _1/2 hole coupled to a²⁰Ne quartet state, this selectivity can be explained with the existence ofα-particle doorway states.     

New22Ne(α,n)25Mg-resonances at very low energies relevant for the astrophysical s-process

The ²²Ne(α,n)²⁵Mgreaction is thought to be one of the main neutron sources for the astrophysical s-process. Thus cross section data for this reaction have been obtained from near the threshold (E _α≈570keV) up toE _α=2100 keV using the 4 MV DYNAMITRON accelerator at Stuttgart, the windowless gastarget system RHINOCEROS and a 4π neutron detector. Two new resonances have been observed atE _α=623 ± 6 and 838 ± 6 kev, which dominate the reaction rate at T₉<0.3.Possible background reactions are discussed.     

Proton transitions in22Na(n,p)22Ne reaction

The nuclear reactions induced by thermal and 2 keV neutrons on²²Na radioactive nucleus were studied. For the thermal neutrons a more accurate value of²²Na (n,p)²²Ne reaction cross section was obtained. A weak proton transition to the²²Ne ground state was observed. The upper limits of cross sections for thermal neutron induced (n,∞) reaction and (n, p) reaction with 2 keV neut·rons are given.     

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.     

Search for low-energy resonances in25Mg(p,γ)26Al

The reaction²⁵Mg(p,γ)²⁶Al has been investigated atE _p (lab)=80-350keV. The existence of the resonance atE _p =317keV has been verified and new resonances have been found atE _p =198, 255 and 304 keV. TheE _p = 304keV resonance represents a new compound state in²⁶A1. Information on branching ratios,Ωγ values, total widths andJ ^π assignments for the observed resonances as well as upper limits on themy strengths for the expected resonances atE _p =97 and 134 keV are given. The astrophysical aspects of the data are discussed in the light of the renewed interest in the precise location of²⁶A1 nucleosynthesis.     

Derp 3/2-Analogzustand in51V

In an investigation ofT=7/2 analogue states in⁵¹V the⁵⁰Ti(p, γ) excitation curve has been measured for proton bombarding energies 1280–1480 keV and 2340–2660 keV. From the (p, γ) resonances 29 new virtual levels in the region of 9316–9510 keV excitation energy in⁵¹V were determined. The strong resonance atE _p=1 371 keV has been identified as the isobaric analogue state of the⁵¹Ti ground state by determining spin and parity of this resonance to be 3/2^?. There is no evidence for a strong analogue resonance in⁵¹V corresponding to the 1.16 MeV _{p 1/2} state in⁵¹Ti. The γ-decay of the _{p 3/2} analogue state has been studied by measuring branching ratios and angular distributions of primary γ-transitions with a Ge(Li) detector.M1E2 mixing ratios have been determined for these transitions. The total width of the resonance for γ-decay is found to be Γ_γ=1.6±0.4 eV. New bound levels in⁵¹V have been introduced at 3576, 4651 and 4661 keV excitation energy. TheJ ^π values of the 3085, 4770, and 4863 keV states are determined to be 5/2^?, 5/2^?, 3/2^?, respectively. The analogue-antianalogueM1 transition strength is found to be considerably reduced compared to the situation ins-d shell nuclei.     

Helium burning of 18O

The ¹⁸O(α, γ)²²Ne capture reaction has been studied at E_α = 0.6?2.3 MeV. The known resonance at E_α = 2.20 MeV has been established and fourteen new resonances have been found in the energy range covered. The E_α = 1.16, 1.32, 1.45, 1.53, 1.87, 1.96 and 2.15 MeV resonances correspond to resonances observed previously in the ¹⁸O(α, n)²¹Ne reaction. The E_α = 0.77, 1.25 and 1.27 MeV resonances represent new compound states in ²²Ne. Information on branching ratios, ωγ values and total widths is reported. Transition strength arguments and analyses of γ-ray angular distribution data together with results from previous work resulted in the most likely J^π assignments for the resonances.The E_α = 1.66 and 1.78 MeV resonances are good candidates for the two J^π = 8⁺ states predicted at E_x ≈ 11–12 MeV and are probably members of the K^π = 0⁺ (ground state) and K^π = 2⁺ rotational bands in ²²Ne.The investigated energy range of E_α together with that of previous work corresponds to stellar temperatures of T = (0.3?4.3) × 10⁹ K. The astrophysical reaction rate determined from these data is compared with predictions based in part on the nuclear optical model. The rate is also compared with that of the competing ¹⁸O(α, n)²¹Ne reaction.     

Investigation of gamma ray transitions and lifetimes in35Cl

Fortyeight resonances of the³⁴S(p, γ)³⁵Cl reaction in the proton rangeE _p=1–2 MeV have been observed. Gamma decay of twentyeight of these was investigated by means of a 55 cm³ Ge(Li) detector, and the energies and branching ratios of several bound states have been determined. TheQ-value was found to be 6367.0 ± 1.5 keV. The lifetimes 150±20, 400±140, 19±4, 14±2, 145±50, 20±5, 20±4, 42±4 and 7±3 fs were determined with the DSA method for the states at 1219, 1763, 2694, 3003, 3944, 3967, 4059, 4178 and 4882 keV, respectively. The experimental transition strengths were found to be in agreement with those given by manyparticle shell-modell calculations.     

Gamma-decay modes of Ne

The lifetimes of the 3376 and 3576 keV states in ¹⁸Ne have been investigated by the Doppler-shift recoil-distance method using the ¹⁶O(³He, n)¹⁸Ne reaction. The 3376 keV level has τ_m = 4.4 ± 0.6 ps and the 3576 keV level has τ_m < 6 ps. In addition the ground state branch of the 3616 keV level has been observed to be (12.5 ± 2.5) %. Furthermore it was found that δ(3616 → 1887) = +0.03 ± 0.09. These results are compared with theoretical predictions.     

Alpha decay of isobaric analogue states in24Mg and28Si

Theα-decay of isobaric analogue states (which are forbidden by isospin selection rules) excited in²⁴Mg and²⁸Si through preton capture by²³Na atE _p=677 and 739 keV and by²⁷Al atE _p=295, 327 and 405 keV, respectively, have been studied using solid state track detectors. The ratio ofα-decay widths of the two resonance states in²⁴Mg to the state at 1.632 MeV (2⁺) in²⁰Ne yields the value 0.065 for the mixing parameterε and the value 4.01 keV for the Coulomb matrix element responsible for the isospin mixing in²⁴Mg. In²⁸Si the measurement of theα-decay widths of the three resonance states resulted in the determination of the proton, gamma and alpha partial widths which comprise the total width of the resonance states. Limits have been set for the value of the two mixing parameters involved in this case. Upper limits of 16 and 39 keV have been obtained for the Coulomb matrix elements responsible for the isospin mixing in²⁸Si.