Study of the reaction238U (α, α′f) atE α=480 MeV

The excitation function of the fission probability P _E E _x) for²³⁸U has been measured in the reaction²³⁸U(α, α′ f) at 480 MeV bombarding energy. The reaction mechanism of this reaction is discussed for excitation energies belowB _nf , the threshold for second chance fission, and aboveB _nf up toE _x =37 MeV. In comparing with results from fission induced by photons and by particle transfer reactions the (α, α′f) reaction gives too low values for the fission probabilityP _f at excitation energies well aboveB _nE . The role of the quasi-elastic knock-out process in this reaction is discussed.     

Study of the reactions31P(α,p)34S and29Si(α,n)32S

The (α,p) reaction on³¹P in the energy rangeE _α=3.25 to 5.25 MeV and the (α,n) reaction on²⁹Si in the energy rangeE _α=3 to 4.8 MeV, have been used to determine nuclear properties of³⁵Cl and³³S respectively. Resonance energies, strengths and partial widths are reported. The analysis of angular distribution measurements yields spin values for 23 resonance levels in³⁵Cl and 14 levels in³³S.     

Investigation of 28Si levels with the (α, γ) and (p, γ) reactions

Particle energies have been measured for resonances in the ²⁷Al(p, γ)²⁸Si and ²⁴Mg(α, γ)²⁸Si reactions with an accuracy of 0.5 × 10^?4 and 1 × 10^?4, respectively. The E_p = 991.88 ± 0.04 keV²⁷Al(p, γ)²⁸Si resonance served as calibration point. From these data the Q-value of the reaction ²⁷Al(p, α)²⁴Mg has been determined as 1600.14 ± 0.21 keV. Excitation energies of ²⁸Si levels have been measured with the ²⁷Al(p, γ)²⁸Si reaction; the reaction energy is Q = 11584.5 ± 0.4 keV.Of 33 resonances observed in the ²⁴Mg(α, γ)²⁸Si reaction (E_α = 1.5–3.8 MeV), energies, strengths and γ-ray decay have been measured; five of these resonances had not been reported previously. The γ-ray angular distribution measurements at three resonances yield the resonance J^π values and the mixing ratios of the strongest transitions involved in the decay. The 10.38 MeV level has J^π = 3⁺, T = 1. The arguments on which T-assignments can be based are critically reviewed. These arguments are used to assign T = 1 character to ¹⁹ states of ²⁸Si.     

The reaction 2H(α,αp)n near threshold and excited states of 6Li

The reaction ²H(α,αp)n has been studied in a kinematically complete experiment between E_α=9.22 and 11.3 MeV (3.07 to 3.77 MeV in the CM system). Data at the highest energies are dominated by αp and αn final-state interactions and at the same time indicate the relevance of the broad state (J^π=2⁺, T=0) centered at 4.7 MeV in ⁶Li ignored heretofore in theoretical calculations. Possible effects due to three-body forces are discussed.     

The (d, 6Li) reaction on 12C, 16O, 24Mg, 40Ca and 58Ni at 54.25 MeV

The (d, ⁶Li) reaction was studied at E_d = 54.25 MeV on the target nuclei ¹²C, ¹⁶O, ²⁴Mg, ⁴⁰Ca and ⁵⁸Ni. The data were analyzed with finite-range DWBA calculations. The absolute values of the α-cluster spectroscopic factors and the target mass dependence of the relative S_α were in agreement with those in the (p, pα) reaction at E_p = 100 and 157 MeV. The theoretical calculations of the relative S_α were in better agreement with the experimental data at higher energy than at the lower energies.     

Stellar reaction rate of 20Ne(α, γ)24Mg

The reaction ²⁰Ne(α, γ)²⁴Mg has been investigated at E_α(lab) = 0.55–3.20 MeV. Neon gas enriched to 99.95% in ²⁰Ne was recirculated in differentially pumped gas target systems of the extended and quasipoint jet types. New resonances were found at E_α(lab) = 958, 1226, 1260, 1704 and 2277 keV, which correspond to known states in ²⁴Mg. Excitation energies, γ-ray decay schemes, γ-ray angular distributions, resonance widths and strengths as well as J^π and T-assignments are reported for all the resonances. Information on low-lying states in ²⁴Mg is also obtained. The nuclear and astrophysical aspects of the results are discussed.     

The 42Ca(α, γ)46Ti and 42Ca(α, p)45Sc cross sections and the mass-45 bottleneck

The yield of γ-rays from the reaction ⁴²Ca(α, γ)⁴⁶Ti has been measured as a function of bombarding energy over the range E_α = 3.42–5.62 MeV and from ⁴²Ca(α, pγ)⁴⁵Sc over the range E_α = 4.06–5.92 MeV, and the yield of protons from the reaction ⁴²Ca(α, p)⁴⁵Sc has been measured over the range E_α = 4.78–5.92 MeV. Cross sections for all three reactions have been extracted from the data and compared with global statistical-model calculations. The agreement is good. Thermonuclear reaction rates under stellar conditions appropriate for silicon burning are calculated and their significance for the approach to and bridging of the mass-45 bottleneck in the upward flow of nucleosynthesis is discussed.     

The 17O(p, α)14N reaction: Physics and astrophysics

The nucleus ¹⁸F has been studied using the ¹⁷O(p,α)¹⁴N reaction over the energy range E_p = 400 to 1400 keV with thin Ti¹⁷O₂ targets. New resonances at E_p = 525, 688, 1015, 1276 and 1338 keV have been discovered. The strengths for all resonances in this energy range have been measured and the entire cross section in this region has been fitted using the R-matrix formalism of Lane and Thomas. Some discussion is presented concerning the values of the boundary condition parameters used in the R-matrix fit and two sets of results are presented corresponding to two different treatments of the boundary conditions.The ¹⁷O(p, α)¹⁴N cross section has been extrapolated to astrophysical energies using an R-matrix calculation incorporating the partial widths obtained from the data from E_p = 400 to 1400 keV. These results are compared with previous ¹⁷O(p, α)¹⁴N calculations and the ¹⁷O(p, γ)¹⁸F direct capture calculation of Rolfs and Rodney.     

Spectroscopy of 32P (II)

Spins, parities and transition probabilities of levels in ³²P at E_x > 3.5 MeV have been determined with the ²⁹Si(α, pγ)³²P reaction at bombarding energies of E_α = 12.80, 12.93 and 16.30 MeV. Proton-gamma angular correlation experiments and DSA lifetime measurements lead to six unambiguous spin assignments and to many spin limitations. The measured mixing and branching ratios yield many transition strengths for dipole and electric quadrupole transitions. Five, sofar unknown, energy levels are reported. A doublet at 4.03 MeV excitation energy and a high-spin state (E_x = 4.27 MeV; J^π = 5^?) were observed. Shell-model predictions have been compared to the present experimental results.     

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.     

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.     

Mechanism of the 6Li(6Li, 2α)α reaction from 36 to 46 MeV

The principal mechanism of the ⁶Li(⁶Li, 2α)α reaction for E₀ = 36 to 46 MeV is the formation and sequential decay of ⁸Be levels near E_x = 11, 17, and 20 MeV. In contrast to previous results obtained at lower bombarding energies, the cross section for the direct reaction involving a spectator α-particle is, under the most favorable conditions, only one third of that for excitation of these ⁸Be levels.     

Total reaction cross section for 12C + 16O below the Coulomb barrier

The energy dependence of the total reaction cross section, σ(E), for ¹²C + ¹⁶O has been measured over the range E_c.m. = 4–12 MeV, by detecting γ-rays from the various possible residual nuclei with two large NaI(Tl) detectors placed close to the target. This technique for measuring total reaction cross sections was explored in some detail and shown to yield reliable values for σ(E). Although the principal emphasis of this work was placed on obtaining reliable cross sections, a preliminary study has been made of the suitability of various methods for extrapolating the cross section to still lower energies. The statistical model provides a good fit with a reasonable value for the strength function, 〈γ²〉/〈D〉 = 6.8 × 10^?2, over the range E_c.m. = 6.5–12 MeV, but predicts cross sections which are much too large for E_c.m. < 6.5 MeV. Optical model fits at low energies are especially sensitive to the radius and diffuseness of the imaginary component of the potential and, since these are still poorly known at present, such extrapolations may be wrong by orders of magnitude. A simple barrier penetration model gives a moderately good fit to the data and seems to provide the safest extrapolation to lower energies at the present time. It is clear, however, that our knowledge of the heavy-ion reaction mechanism at low energies is incomplete, and that cross-section measurements at still lower energies are needed to establish the correct procedure for extrapolating heavy-ion reaction cross sections to low energies.     

A study of 28Si(α, γ)32S resonances below Eα = 3.83 MeV

The γ-decays of eleven resonances in the ²⁸Si(α,γ)³²S reaction below E_α = 3.83 MeV have been studied using a large Ge(Li) detector. Results for branching ratios differ considerably from previous NaI work. The previous discrepancy in radiative strengths for the 2.61 MeV resonance is explained by this data. The strengths of the first five resonances at E_α = 1.77, 1.99, 2.19, 2.37 and 2.42 MeV appear to be (39 ± 13)% lower than previously reported. Spin-parities of l^?, 2⁺ and 2⁺ have been assigned to the levels at 8.50, 8.69 and 8.86 MeV respectively. The radiative width of the E_p = 1.467 MeV, J^π = 3^? resonance in the ³¹P(p,γ)³²S reaction has also been measured.     

High-spin states in112Cd from the110Pd(α, 2n γ)-reaction

The decay scheme of¹¹²Cd has been studied by γ-ray spectroscopy with the¹¹⁰Pd(α, 2nγ) reaction. Spin and parity assignments were made on the basis of coincidence measurements, angular distributions and excitation functions. Twelve new energy levels were established. A vibrational-like positive-parity band up to spin 8⁺ and a negativeparity side band 5^?, (7^?), (9^?) were found. For the negative-parity states a (v_j, vh_11/2) configuration is suggested. Nuclear Reactions:¹¹⁰Pd(α,2nγ),E _α =15–22.5 MeV; measuredE _γ ,E _γ , (E,E _γ ),E _γ (θ),γ-γ- coincidence;¹¹²Cd deduced levels,J; enriched target.     

High-spin states in27Al

The²⁴Mg(α, p γ) reaction has been studied atα-particle energies of 13, 15, and 15.2 MeV. Using method II of Litherland and Ferguson spin assignments or restrictions thereof could be made to the²⁷Al levels atE _x =5500 keV (I=11/2, 7/2), 6950 keV (I=11/2, 7/2), 7226 keV (I=9/2), 7286 keV (I=9/2–13/2), 7399 keV (I=11/2, 7/2) and 7443 keV (I=13/2, 9/2). Using the Doppler-shift attenuation method a lifetimeτ=20–50 fs was obtained for theE _x =7399 keV level and a limitτ<20 fs for the levels atE _x =7443, 7286, 7226, 6950, 6718, and 6287 keV, respectively. Further spin assignments in²⁷Al were obtained by a study of the²⁶Mg(p, γ) reaction at the E_p=2174 keV resonance. Fromγ-ray angular distributions the resonance spin was determined as I=9/2 (previous assignmentI=7/2) and the spin of theE _x =5959 keV level was determined asI=7/2. Tentative spin assignments were made to the levels atE _x =6537 keV (I=7/2) and 6718 keV (I=9/2). From the combined evidence of the²⁴Mg(α, p) and²⁶Mg(p, γ) reactions the spin of the 6287 keV level was found to beI=7/2. The results are compared with shell model calculations and the Nilsson model.     

Resonances in low energy40Ca(α, α)-scattering and quasimolecular band in44Ti

The elasticα-scattering from⁴⁰Ca has been studied betweenE _α=7.035 MeV and 7.405 MeV. Angular distributions and excitation functions have been measured in 5 keV steps. The data display a resonance atE _α=7.26 MeV which by its width and angular correlation is clearly distinguished from strong Ericson fluctuations in this region. From the analysis of the angular distributions and excitation functions a spinJ ^π=1^? is assigned to this resonance. The resonance parameters areΓ _α=31 KeV,Γ _total=51keV. By its spin and excitation energy this state is interpreted as a member of a quasimolecular band in⁴⁴Ti the higherl-members of which are known from⁴⁰¹Ca(α, α) scattering betweenE _α=18 MeV and 50 MeV. The experimental data are compared with recent microscopic calculations within a fully antisymmetrized reaction theory.     

212Th,a new isotope

The new isotope²¹²Th has been found to decay withE _α = (7.802 ±0.010) MeV andt _1/2 =(30 _?10 ⁺²⁰ ms. The decay energies of the previously observed isotopes²¹³Th and²¹⁴Th could be confirmed. The measured half-lives are in agreement with calculated values.     

A study of the (α, t) reaction on 19F, 27Al, 51V and 59Co

The angular distributions of tritons from the (α, t) reaction on ¹⁹F, ²⁷Al, ⁵¹V and ⁵⁹Co nuclei corresponding to the (0⁺) ground states and (2⁺) excited states in the final nuclei have been measured in the angular range between 15° and 170° at α-particle energies of 25 MeV. For reactions on ²⁷Al and ⁵¹V nuclei, the differential excitation functions have also been obtained at different angles of outgoing tritons at E_α from 20 to 25 MeV. The experimental angular distributions are analysed by the DWBA approximation on the assumption of a nucleon stripping mechanism. The analyses of the present results and the data obtained earlier for the (α, t) reaction on the 1 p shell nuclei, A ? 30, reveal that the distinguishing feature of the reaction under study is the presence of backward angle peaks in the reaction cross section, which appear to be associated with exchange processes. For the (α, t) reaction on the heavier nuclei (A > 30), the dominant mechanism is nucleon stripping.     

Angular distributions of neutron polarization from the 14C(p,n)14N and 11B(α, n)14N reactions and R-matrix analysis oF 15N in the excitation-energy range between 11.5 and 12.5 MeV

Angular distributions of neutron polarization from the ¹⁴C(p, n)¹⁴N and ¹¹B(α, n)¹⁴N reactions have been studied for the particle energies E_p = 1.788, 2.025, 2.272 and 2.450 MeV, and E_α = 2.049 MeV. The polarization was derived from the left-right asymmetry induced by elastic scattering from ⁴He. Together with existing measurements of angular distributions and total cross sections for several reaction channels leading to ¹⁵N with an excitation energy between 11.5 and 12.5 MeV, these data were used to deduce from R-matrix analysis a set of resonance parameters for the ¹⁵N levels in this energy range.