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.     

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.     

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.     

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⁺).     

The cross section of the7Be(n,p)7Li reaction at 24.5 keV neutron energy

The 24.5 keV neutrons induced reaction with the⁷Be radioactive nucleus was studied. The value of σ_np=(18±4)b was obtained for the⁷Be(n, p)⁷Li reaction cross section.     

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.     

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.     

Measurements of a partial cross section for the reaction 58Ni(n, γ0)59Ni

The partial cross section for radiative neutron capture accompanied by gamma transitions to the ground state of the ⁵⁹Ni nucleus was measured as a function of energy by a new neutron-spectrometry method that employed the shift of a primary gamma transition in response to a change in the energy of the captured neutron. The reaction ⁷Li(p, n)⁷Be was used as source of neutrons for the present measurements. The protons that induced this reaction were accelerated by a Van de Graaff electrostatic generator to energies exceeding the reaction threshold by 60 keV, in which case an appropriate geometry of the experiment permitted irradiation of the sample under study with neutrons whose energy ranged between 10 and 120 keV. The partial widths of some resonances and radiative strength function for hard primary M1 gamma transitions were determined in addition to the above cross sections.     

The proton capture reactions on21Na and22Na under hydrogen burning conditions

We have updated and recalculated the²¹Na(p, γ)²²Mg and²²Na(p, γ)²³Mg reaction rates for temperatures 10⁷–10⁹ K relevant for explosive hydrogen burning in stellar environment. We find the²¹Na(p, γ)²²Mg rate lower than recently suggested. Considering various uncertainties in spin and parity assignments of the low energy²²Na+p resonances we present upper and lower limits for the²²Na(p, γ)²³Mg reaction rate which, however, allows for a meaningful discussion of the consequences of the new rates on the Ne - Na cycle under various stellar conditions.     

The reaction 23Na(n, γ)24Na studied with unpolarized and polarized thermal neutrons

The γ-radiation produced by thermal neutron capture in a natural Na sample has been investigated. Of the 158 γ-rays ascribed to the ²³Na(n, γ)²⁴Na reaction, 143 have been placed in a ²⁴Na decay scheme accounting for 100(2) % of the total primary strength. The reaction Q-value amounts to 6959.42 ± 0.08 keV. The data resulted in spin assignments for four and spin restrictions for six levels.The circular polarization of 14 γ-rays from the capture of polarized neutrons has been measured. The contribution of the J^π = 2⁺ channel in thermal capture was determined to be below 5 %.     

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.     

22Ne(a,n)25Mg reaction in CONe mantle of massive stars at high temperatures

The effectiveness of²²Ne (a, n)²⁵Mg reaction as a neutron source is examined at high temperatures 0.8?T ₉?3.0 (T ₉ is the temperature in units of 10⁹ K). An assembly consisting of¹²C,¹⁶O, and²²Ne is considered at the end of helium burning in some massive stars. Alpha particles necessary for this neutron producing reaction are assumed to be due to reactions involving¹²C and¹⁶O nuclei. Assuming²²Ne as the only neutron absorber, the number density of neutrons is calculated. The mantles outside the cores of massive stars are possibly the physical sites for these reactions.     

Nuclear levels and spectroscopy of191Os

Thirty-nine states, all except two of which are new, have been observed up to 1535 keV in¹⁹¹Os using the reactions¹⁹⁰Os(d,p)¹⁹¹Os,¹⁹²Os(d, t)¹⁹¹Os with 12 MeV deuterons and magnetic analysis with photographic emulsions of the outgoing particles, and¹⁹⁰Os(n, γ)¹⁹¹Os with thermal neutrons and three modes of gamma detection in the energy range from 40 keV to ～6 MeV. The neutron separation energy was determined as 5758.5±2.0 keV in agreement with the value of the (d, p) reaction. TheQ value for the (d, t) reaction was determined as ?1265±15 keV. Spins and parities are assigned for most of the states below 800 keV. The states below 463 keV are shown to be qualitatively but not quantitatively consistent with the expectations of the Nilsson model. Nilsson systematics of the 1/2^?: ¦510¦ and 3/2^? ¦512¦ orbitals in odd-A Os isotopes with increasing neutron number can be understood in terms of decreasing deformation. Anomalously large (d, p) and (d, t) cross sections populating 5/2^? and 3/2^? states at 134 and 142 keV, respectively, are explained as a tendency to restore the cross sections expected for a spherical nucleus. Evidence for triaxiality in¹⁹¹Os is observed in the form of a 5/2^? state at 273 keV which appears to be a rotational state built on the 9/2^? ground state.     

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.     

Main channels of the decay of the giant dipole resonance in the 20,22Ne nuclei and isospin splitting of the giant dipole resonance in the 22Ne nucleus

Data published in the literature on various photonuclear reactions for the ^20,22Ne isotopes and for their natural mixture are analyzed with the aim of exploring special features of the decay of giant-dipole-resonance states in these two isotopes. With the aid of data on the abundances of the isotopes and on the energy reaction thresholds, the cross sections for the reactions ^20,22Ne[(γ, n)+(γ, np)] and ^20,22Ne[(γ, p)+(γ, np)] are broken down into the contributions from the one-nucleon reactions (γ, n) and (γ, p) and the contributions from the reactions (γ, np). The cross sections for the reactions ^20,22Ne(γ, n)^19,21Ne and ^20,22Ne(γ, p)^19,21F in the energy range E _γ=16.0–28.0 MeV and the cross sections for the reactions ^20,22Ne(γ, np)^18,20F in the energy range E _γ=23.3–28.0 MeV are estimated. The behavior of the cross-section ratio r=σ(γ, p)/σ(γ, n) for the ²²Ne nucleus as a function of energy is analyzed, and the isospin components of the giant dipole resonance in the ²²Ne nucleus are identified. The contributions of the isospin components of the giant dipole resonance in the ²²Ne nucleus to the cross sections for various photonuclear reactions are determined on the basis of an analysis of the diagram of the excitation and decay of pure isospin states in the ²²Ne nucleus and in nuclei neighboring it, which are members of the corresponding isospin multiplets. The isospin splitting of the giant dipole resonance and the ratio of the intensities of the isospin components are determined to be ΔE=4.57±0.69 MeV and R=0.24±0.04, respectively.     

Thermal neutron induced charged particle reactions in radioactive targets of37Ar,109Cd,125Xe,127Xe,and132Cs

Thermal neutron induced charged particle reactions in a radioactive target of³⁷Ar have been studied. Upper limits of the cross-sections for the (n, α) reaction in radioactive targets of¹⁰⁹Cd,^{125, 127}Xe, and¹³²Cs have been obtained. The isotopically pure targets were produced at the ISOLDE facility at CERN and irradiated with thermal neutrons at the high flux reactor of the Institute Laue-Langevin in Grenoble. Charged particles from (n, p) and (n, α) reactions in³⁷Ar were observed with cross-sections of 69±14b and 1970 ±330b, respectively. TheQ-values for these reactions were determined to be 1600 ±12 keV and 4630±7 keV, in agreement with existing mass data. The branching ratio Γ_α/Γ_p of the³⁷Ar capturing state was found to be 28.5±2.7. An upper limit of the cross-section for the (n, γα) reaction in³⁷Ar was obtained.     

The14N(n,p)14C reaction cross section for thermal neutrons

The¹⁴N(n,p)¹⁴C reaction cross section has been measured for thermal neutrons, making use of the neutron beam of the IBR-30 pulsed booster of the JINR, Dubna. Thermal neutrons were identified by a time-offlight method. Solid adenin targets were utilized. An ionization chamber was used for detecting the charged particles. The cross section was found to beσ _np ^th =(1.83±0.07)b.     

Cross section of the reaction 6Li(p, γ)7Be

The cross section of the reaction ⁶Li(p, γ)⁷Be has been measured using Ge(Li) γ-ray spectrometers for proton bombarding energies E_p from 200 keV to 1200 keV. At E_p = 800 keV, the total (p, γ) integrated cross section is found to be 3.1 ± 0.4 μb. The cross section adopted from consideration of this and previous measurements is in good agreement with that predicted from the known thermal neutron cross section for ⁶Li(n, γ)⁷Li on the assumption that properties of mirror direct capture reactions can be well described by optical potentials that use the same parmeter values for the two reactions.     

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.     

Resonant absorption of the γ-radiation from the22Ne(p, γ)23Na reaction and the effect of rutherford scattering in the target on the width of the absorption curves

Gamma rays of energy 9.404 and 9.700 MeV were obtained from the reaction²²Ne(p, γ)²³Na at proton energies of 640 and 950 keV respectively and were resonantly absorbed in sodium metal. Isotopically separated²²Ne targets on nickel backings were used. The following level widths for²³Na were obtained: The 9.404 MeV level: Γ=65 ±40eV Γ_γ=2.2±0.7eV. The 9.700 MeV level: Γ=9.5 _?1.0 ^{+4.5 eV} Γ_γ=5.6 _?0.8 ^{+1.2 eV} . The disagreement between the results for the 9.404 MeV level and the previously determined values can probably be ascribed to proton scattering in the window of the gas target used in the latter measurement.