Non-statistical structures in12C(15N,4He)23Na reaction

The data on the? _lab=7° excitation functions of¹²C(¹⁵N,⁴He)²³Na reaction betweenE _cm=9.42 and 17.33 MeV for 28 states upto an excitation energy of 8.940 MeV in²³Na have been subjected to statistical analysis. In addition to statistical fluctuations, the results of the analysis indicate the existence of non-statistical structures atE _cm=10.66, 10.93, 11.38, 12.62, 13.16, 15.32 and 16.18 MeV.     

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⁺).     

Investigation of high spin states in23Na and23Mg and the12C+12C resonance atE c.m.=19.3 MeV

The resonance in the¹²C+¹²C system atE _c.m.=19.3 MeV has been studied in the neutron and proton decay channels. From(p, γ), (n, p) and (n, γ) coincidence measurements high spin states could be localized inA=23 nuclei, in²³Mg up to possibly 21/2⁺. These states were resonantly populated. The present data favorJ ^π=17/2⁺ for the 9.61 MeV state in²³Mg and therefore also for the probable parent state at 9.81 MeV in²³Na. AJ ^π=12⁺ assignment to the resonance is supported and evidence was found that the resonant excitation of the lowest 15/2⁺ and 17/2⁺ states in mass 23 nuclei is caused by a change of the intrinsic structure of the Yrast levels aroundJ ^π=15/2⁺ towards larger deformation.     

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.     

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.     

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.     

On the decay of compound nuclei following alpha-particle and12C induced reactions

Multiple coincidence rates have been measured using a detector system consisting of a Ge(Li) spectrometer and eight NaI(Tl) or eight liquid scintillators. Reactions induced byα-particles with energies of 51–55 MeV and 118 MeV¹²C ions are studied. The data are analysed to give the first and second central moments of the distribution of the number ofγ-rays feeding individual levels in the final nuclei. When these numbers are compared to spin distributions calculated with the statistical model code GROGI the relative importance of dipole and quadrupole deexcitation modes can be ascertained. In particular, in the¹²²Te(α, 4n)¹²²Xe reaction theγ-decay prior to the entry into the ground band is well described as a statistical process proceeding to 50% by dipole and 50% by quadrupole radiation. In the¹⁶⁶Er(α,4n)¹⁶⁶Yb and¹⁹²Os(α,4n)¹⁹²Pt reactions the relative amount of quadrupole radiation is larger and it seems that the dipole and quadrupole decay takes place via separate cascades. In the¹⁶⁴Dy(¹²C, 7-8n) reactions the average multiplicity is independent of spin, suggesting that the nucleus forgets the spin of the entry state before the process enters into the ground band. In the¹⁷⁶Yb(¹²C, 8n)¹⁸⁰Os reaction, finally, the nucleus definitely retains memory of the entry state during the decay. In this last case the multiplicity measurement is combined with aγ-ray singles measurement to give an average excitation energy prior to theα-decay and the average moment of inertia characterising the decay of the high-spin states.     

Neutron branching in the reaction 12C + 12C

The cross section for the reaction ¹²C(¹²C, n)²³Mg has been measured in the energy range E_c.m. = 3.54?4.94 MeV by counting the delayed γ-rays from ²³Mg decays (half-life = 11.57 sec), and a theoretical model has been employed to extrapolate the results to threshold (E_c.m = 2.60 MeV). By combining these results with previous measurements of the reactions ¹²C(¹²C, p)²³Na and ¹²C(¹²C, α)²⁰Ne, the neutron branching ratio in the energy interval from threshold to 8 MeV is deduced, and a thermal average is computed that should be valid for use in astrophysical environments characterized by temperatures in the range (0.5–5) × 10⁹ °K. The neutron branching at temperatures appropriate to hydrostatic carbon burning in stars (T ≈ 109 °K) is found to be much smaller than previously estimated.     

The12C+12C reaction at subcoulomb energies (I)

The reaction¹²C+¹²C has been studied in the energy rangeE _cm=2.45–6.15 MeV byγ-ray spectroscopy. Gamma-ray transitions from a large number of excited states in²⁰Ne,²³Na and²³Mg were observed, which show strong and rapid yield variations. When the influence of the Coulomb barrier is removed, these structures appear superimposed on a flat reaction yield, which does not show a strong increase at low energies, in contrast to previous work. These results obviate the need for the hypothesis of absorption under the barrier at least down toE _cm=2.45 MeV. The nuclear and astrophysical aspects of the data are discussed.     

Alpha-particle unstable states in12C

Alpha particle unstable states in¹²C have been investigated using the three reactions¹⁴N(d, α′)¹²C,¹⁴N(d, α′α)⁸Be and¹²C(α, α′)¹²C. Excitation cross sections and angular distributions have been measured for the reactions¹⁴N(d, α′) at 52 MeV and¹²C(α, α′) at 90 MeV. For a few angle pairs the α-decay of excited states in¹²C have been observed in a¹⁴N(d, α′α) correlation measurement. The reactions selectively excite onlyT=0 states. A previously undetected level with a large α-decay width (Γ=1.2 MeV) has been observed at 15.62 MeV excitation. This level shows up clearly in both reactions and is further distinguished from the nearby 14.08 state in the correlation measurement because of the distinctly different energy distributions of the decay products. On the basis of a particle angular distributions the 15.62 MeV level was assigned spin and parity 4⁺ and the level at 14.08 was assigned 3^?. The latter differs from the value suggest by earlier work. Comparison with DWBA calculations indicates that angular distributions of all other prominent levels are in agreement with their earlier assignments. Two levels at 19.20 and 20.30 MeV (both Γ?0.4 MeV) and three further levels at 21.81, 22.7 and 24.24 MeV also decay predominantly by α-emission.     

Gammadecay of quasimolecular resonances in the12C+12C system

We have studied the heavy-ion radiative capture reaction¹²C(¹²C,γ)²⁴Mg forE _cm =4.7–6.0 MeV. Transitions to the ground-, first and unresolved second and third excited states in the final nucleus²⁴Mg have been observed with cross sections as low as 1 nb/sr. Forγ ₁ two strong resonance-like structures at 4.9 and 5.0 MeV were found correlated in energy with established 2⁺ resonances. Statistical model calculations cannot account for the observed yield. The branching ratioΓ _γ /Γ associated with theγ ₁ decay channel of the 5 MeV resonance was estimated to be 1.1·10^?5 yielding aγ-ray strength of 0.8 eV. The experimental result is in agreement with calculations based on the generator coordinate method where broad barrier resonances are viewed as short lived states of quasimolecular nature.     

Spin and parity assignments for 23Na levels from the 19F(α, p)22Ne reaction

Resonances were found in the 120° and 160° excitation functions for the ¹⁹F(α, p_o)²²Ne reaction between E = 2.5 and 5.0 MeV corresponding to ²³Na levels at excitation energies between 12.56 and 14.51 MeV. Twenty-one on-resonance angular distributions were analysed with single-level and two-level theory to extract ²³Na spin and parity information. The results of an earlier experiment were analysed by the same procedure, extending the diagnostics down to a ²³Na excitation energy of 11.55 MeV. The analysis incorporated an optical potential for the α-particle consistent with previous channel-spin-$\text{1}\text{2}$ reaction studies and α-induced reaction data.     

Measurement of spin-alignment of excited12C2+ nuclei in interactions of16O with12C usingγ-decay in flight

Measuring energy spectra of nuclei afterγ-decay of excited states in flight the spin alignment of¹²C₂₊ states has been measured. Inelastic scattering,¹⁶O(¹⁶C,¹²C₂₊)¹⁶O and the reaction¹²C(¹⁶O,¹²C₂₊)¹⁶O leading to¹²C₂₊ (4.43 MeV) state have been studied. Characteristic line shapes of the¹²C₂₊ peak were observed using a Q3D magnetic spectrometer. The magnetic substate (m-states) population has been deduced from the spectra as function of reaction angle. A comparison of the measuredm-state population with reaction models shows that the first reaction is consistent with inelastic scattering although discrepancies remain. Discrepancies are also obtained if the reaction¹²C(¹⁶O,¹²C₂₊)¹⁶O is interpreted using a FRDWBA transfer calculation. At least 1/3 of the cross section can be attributed toα-transfer. A calculation which couples transfer and inelastic scattering channels seems to be necessary.     

Structures in the excitation functions for compound reactions in the12C+14C system

Excitation functions for the elastic scattering of¹²C on¹⁴C and the reactions¹⁴C(¹²C, α)²²Ne,¹⁴C(¹²C,t)²³Na and¹⁴C(¹²C,d)²⁴Na have been measured in the vicinity of the Coulombbarrier. Strong fluctuations of the differential cross sections as a function of incident energy are observed in the α-particle, triton and deuteron channels. The total yield in the three different channels shows correlated structures at energiesE _c.m.=6.8, 7.2 and 8.3 MeV. This phenomenon is similar to the structures observed in the¹²C+¹²C reactions and indicates the possible presence of resonances in the entrance channel.     

High spin states in103Rh

High spin states in¹⁰³Rh have been studied using the¹⁰⁰Mo(⁷Li, 4n) reaction. Standard in-beam measurements involving singlesγ-ray,γ-γ-t coincidences, angular distribution and linear polarization measurements have been performed. A perturbed band based on a 9/2⁺ state is interpreted as ag _9/2 coupled band. A sequence built on a 1/2^? state is related to thep _1/2 state. At higher excitation energy, in the range 2.2–3.7 MeV, the strongΔI=1 cascade observed, connecting negative parity levels, probably arises from states including three quasi-particle configurations.     

The 12C + 12C sub-coulomb fusion cross section

The ¹²C + ¹²C fusion cross section has been studied over the energy range 2.46 ≦ E_c.m. ≦ 5.88 MeV. The yields of the γ-rays emitted from the first excited states of ²³Na and ²⁰Ne, following ²⁴Mg compound nucleus decay via proton- and α-emissions, were measured using a Ge(Li) detector. The fusion cross section was obtained by normalizing these yields to previously reported ¹²C(¹²C, p)²³Na and ¹²C(¹²C, α)²⁰Ne cross sections. The data indicate that the cross section below 3.5 MeV is dominated by two or more resonances, and that the average trend in this energy region does not show the absorption-under-the barrier features of the optical model. For astrophysical extrapolations to lower energies, the new results are consistent with the extrapolation proposed by Fowler, Caughlan and Zimmerman.     

Two-quasiparticle bands in126Ba

High spin states of¹²⁶Ba have been populated via the¹¹⁶Sn (¹³C, 3n) reaction and were investigated by inbeam gamma-ray spectroscopy. Partially NaJ (Tl)-BGO anti-compton shields were used. Two positive parity band crossings were established. The yrast band was observed up to a tentative 18⁺ level. The gamma-bands and two band structures of negative parity were found. In addition a tentative 3^? state has been seen. Two-quasiparticle Routhians are constructed from the one-quasiparticle Routhians of the bands in the odd neighbouring isotopes and isotones and are compared to the ones observed in¹²⁶Ba. This leads to ah _11/2 g _7/2-neutron structure of the strongly coupled negative parity bands and a two-quasiproton structure of the decoupled bands of negative parity. The branching and mixing ratios for the neutron bands are analyzed.     

The reaction12C(α,p 0)15N around 18 mev excitation energy of16O

Proton angular distributions of the reaction¹²C(α, p ₀)¹⁵N have been measured at excitation energies from 17.5 to 18.7 MeV. The shape of the angular distributions changes strongly betweenθ _cm = 26°–162°. The integrated cross section shows a smooth behaviour in contrast to the (α, n) mirror reaction. The results can be explained by an interference of the known 18.18 MeV, 2⁺ state with a strong isospin mixed 18.1, 3^? state in the¹⁶O compound nucleus.     

On the coexistence of oblate and prolate deformed bands in83Zr

Deformation parameters of the positive parity yrast band and negative parity bands in⁸³Zr are deduced from lifetimes andE2/M1 mixing ratios. Lifetimes of high spin states have been determined from recoil distance Doppler shift and Doppler shift attenuation measurements using the⁵⁴Fe(³²S,2pnγ) ⁸³Zr reaction. Ten lifetimes and five lifetimes limits were determined. The positive parity band, built on theg _9/2 K=5/2 orbital has an average deformation ¦β ₂¦=0.28(2), and shows a reduction ofE2 transition strengths in the observed backbend region at I^π≈21/2⁺. In contrast, theE2 strengths in the negative parity states show a steady increase up to I^π≈=15/2^?. These states are more strongly deformed than the positive parity states (¦β ₂¦=0.33(3)). TheE2/M1 mixing ratios show that the negative parity band hasK=3/2 and is prolate, and favour oblate deformation for the positive parity yrast band. In theK=1/2^? band theE2 strength of the 7/2^?→3/2^? transition yields a deformation ¦β ₂¦=0.26(5). The band structure is compared with calculations within the Hartree-Fock Bogoliubov cranking model.     

26g,mAl production cross sections from the 23Na(α, n)26Al reaction

Cross sections have been determined for the production of ^{26g, m}Al from the ²³Na(α, n) reaction. Total ²⁶Al production cross sections were obtained from measurements of the thick-target neutron yield. ^26mAl cross sections were measured using an activation technique. ^26gAl cross sections were deduced by subtracting the ^26mAl cross sections from the total (α, n) cross sections. The principle of detailed balance has been applied to the low-energy data to obtain cross sections for the astrophysically interesting ^26gAl(n, α₀)²³Na reaction. These results are compared with the results of Hauser-Feshbach calculations.