Neutron-induced charged-particle reactions in 6Li and 9Be

Angular distributions of tritons from the ⁶Li(n, t)⁴He reaction were measured at E_n = 7.25, 6.77, 6.57, 5.24 and 4.71 MeV. Angular distributions of douterons from respectively, the ⁶Li(n, d)⁵He two-body breakup reaction were measured at E_n = 6.77 and 6.57 MeV, and of protons from the ⁶Li(n, p)⁶He reaction at E_n = 6.77, 5.24 and 4.71 MeV. All these reactions in ⁶Li were analyzed as direct interaction in the formalism of the distorted wave Born approximation. The optical model for the nuclear interaction was found to apply reasonably well to nuclei as light as ⁴He, ⁵He, ⁶He and ⁶Li. In addition, ⁶Li as an alpha-deuteron cluster gives the best bound-state wave function to describe the experimental angular distribution of tritons. The excitation functions at forward angles of the ⁶Li(n, t)⁴He, ⁶Li(n, d)⁵He and ⁶Li(n, p)⁶He reactions were measured for incident neutron energies between 4.4 and 7.3 MeV. It is found that the ⁶Li(n, d)⁵He two-body breakup reaction has a threshold at about E_n = 5.3 MeV. Angular distributions at E_n = 18.3 MeV for tritons and protons from the ⁹Be(n, t)⁷Li and ⁹Be(n, p)⁹Li, respectively, were also measured.     

Absolute cross sections for the 6Li(p, 3He)4He reaction at energies below 1 MeV

Total cross sections and angular distributions in the ⁶Li(p,³He)⁴He reaction have been measured over the energy range E_p = 100?700 keV. The extrapolation of the cross section to the energy region which is of interest in controlled thermonuclear reactors is given. The values of the “astrophysical S-function” are deduced from the cross sections.     

Giant-resonance excitation by 130 MeV 3He

Energy spectra of 130 MeV ³He scattered from ²⁴Mg, ⁹⁰Zr, ¹²⁰Sn and ²⁰⁸Pb have been measured. The spectra exhibit a pronounced giant resonance (GR) structure in the excitation energy region around $\text{E}{}_{\mathrm{<mtext>x</mtext>}}\text{≈ 63A}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}\text{MeV}$. The GR angular distributions as well as those corresponding to the first 2⁺ levels in ²⁴Mg and ¹²⁰Sn have been obtained for angles 7° ≦ θ_L ≦ 35°. Distorted wave calculations using the optical model parameters fitting the elastic data resulted in good fits to the 2⁺ levels. For ²⁴Mg, ⁹⁰Zr and ¹²⁰Sn the shapes of the GR angular distributions are well fitted by L = 2 curves alone. However, for ²⁰⁸Pb both L = 2 and L = 4 curves give fits of comparable quality.     

Alpha-heavy-ion angular correlations from 28Si + 12C

Alpha particles have been measured in coincidence with heavy recoil nuclei from the ²⁸Si + ¹²C reaction. At E_lab = 87 MeV angular correlations for alphas between 15° and 55° and heavy ions at angles ?9°, ?12° and ?15° have been taken. An excitation function of coincidence events with θ_α = 30° and θ_HI = ?12° has been measured for 84 MeV < E_lab < 91.5 MeV. The results are well described by a statistical-model calculation for compound nucleus decay. No evidence is found for additional processes.     

The 6He emission accompanying the spontaneous fission of 252Cf

The results of a multiparameter investigation of the ⁶He emission accompanying the spontaneous fission of ²⁵²Cf are presented. The energy spectrum and the yield of the ⁶He particles are found to be in accord with previous measurements, but their angular distribution is observed to be narrower at 13° ± 3° (FWHM) than the value of ? 32° deduced in a previous investigation.Comparisons of the experimental with published calculated energy spectra and angular distributions for ³H, ⁴He and ⁶He particles are shown to provide evidence for a compact scission configuration in ternary fission.     

A study of the 14C(6Li, 6He)14N reaction at 93 MeV

The reaction ¹⁴C(⁶Li, ⁶He)¹⁴N was investigated with 93 MeV ⁶Li ions in an angular interval of 7–26°. Angular distributions were analysed for the four most intense groups of ⁶He nuclei, corresponding to transitions to the ground (1₁⁺) and the excited (1₂⁺, 2₁^?, 4₁^?) states of ¹⁴N. In the theoretical analysis a mechanism of the spin-isospin excitation was suggested in the DWBA frame with the finite range of interaction and recoil in the light system (⁶Li⁶He) taken into account. In the calculations both shell-model wave functions and transition densities obtained in the theory of finite Fermi systems (FFS) were used. From the comparison between theory and experiment the Landau-Migdal force constant g′ is estimated in order to obtain some information on the degree of nuclear proximity to the threshold of pion condensation.     

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.     

The reactions 50,52Cr(d, 6Li) 46,48Ti at Ed = 651 MeV

The ^50,52Cr(d, ⁶Li) ^46,48Ti reactions have been studied at E_d = 65 MeV bombarding energy. Angular distributions of outgoing Li particles were measured for final states in ^46,48Ti nuclei from 15° to 50° (lab). These were compared with zero-range and finite-range DWBA calculations in an α-cluster pick-up approximation to obtain relative α-spectroscopic factors.     

The heavy-ion reaction channels of the system 16O + 16O

The reaction channels of the system ¹⁶O + ¹⁶O with outgoing heavy particles from lithium to magnesium have been measured using a ΔE-E telescope. Excitation functions from 49 to 65 MeV at θ_Lab = 30° and angular distributions from θ_Lab = 10° (20°) to 50° at E_Lab = 51.5 MeV are presented for the strong transitions. The excitation function of the ¹²C-²⁰Ne (4.25 MeV) channel shows a pronounced regular cross structure with peaks at 52 and 60 MeV. A selective excitation of certain states in the inelastic scattering and the ¹²C-²⁰Ne channel is observed; the yields of the other heavy-ion channels being weaker by at least one order of magnitude. An explanation of this phenomenon is given by considering the angular momentum matching between entrance and exit channels. Furthermore it is shown that no strong dependence of the cross sections on the transferred angular momentum or on the nuclear structure of the final states is observed. Possible implications of these results on the reaction mechanism are discussed.     

Spectroscopy of intermediate 16F states with the reaction 14N(3He,np)15O

Excited states of ¹⁶F have been investigated with the reaction ¹⁴N(³He, np)¹⁵O at E = 10.5 and 12 MeV in kinematically complete experiments. Proton groups corresponding to the decays of intermediate ¹⁶F states were observed at various angles with counter telescopes in time coincidence with the associated neutrons detected at θ_n^lab = 0° with a time-of-flight spectrometer. Excitation energies and decay widths Γ_p0 of these states have been extracted from the proton spectra. Lower limits for the orbital angular momentum in the decay channel and for the spin of the states have been deduced from the obtained angular correlations. By comparison with the reaction ¹⁴N(³He, pp)¹⁵N measured at E = 13 MeV, pairs of $\text{T = 1}{}^{16}\text{F}$ parent/¹⁶O analog states have been identified. J^π assignments and shell-model configurations are discussed on the basis of the selectivity of the reactions measured.     

Investigation of states in 30P via the 30Si(3He,t)30P reaction at 30 MeV

The ³⁰Si(³⁰He, t)³⁰P reaction has been measured for about 100 levels in ³⁰P with E_x < 8.8 MeV. Little selectivity in the population of states has been observed. For 75 levels angular distributions have been analysed using a “fingerprint method” by determining the L-value from a comparison in shape with transitions to states with known J^π. For possible mixed L-transitions a dominance of the higher L-value is observed for almost all cases. Coulomb displacement energy calculations utilizing shell-model wave functions have been used to identify T = 1 states.     

Direct processes in the6Li(α, d)8Be reaction

The angular distributions of deuterons from the ⁶Li(α, d)⁸Be reaction corresponding to the 0⁺ ground state and the 2.9 MeV (2⁺) state in⁸Be have been measured at E_α = 17.3, 23.3 and 25.1 MeV. The excitation functions for this reaction have been measured at seven emission angles in the region from 12 up to 25 MeV. The experimental angular distributions are compared with theoretical predictions obtained using DWBA stripping calculations and the plane wave approximation (PWA) for various direct processes. It is shown that the experimental angular distributions can be described throughout the angular region using the simple mechanisms associated with the break-up of the ⁶Li target nuc clusters and heavy-particle stripping. The interference of these processes is taken into account.     

Fusion and nonfusion phenomena in the 6Li + 40Ca reaction at 156 MeV

Reaction products from ⁶Li-induced reactions on ⁴⁰Ca at 156 MeV have been studied using the dE × E identification as well as the inclusive γ-ray method. The complete fusion cross section has been found to be σ_F = 67 ± 20 mb. The Z-distribution of fusion evaporation residues is compared with statistical model predictions. The Z-spectrum of reaction products shows a maximum at 15 ? Z ? 20, probably due to transfer and to incomplete fusion. It is suggested that the small value of the fusion cross section is due to the strong competition of ⁶Li break-up processes.     

The elastic scattering of 3He on 4He at medium energies

The differential cross section of the elastic scattering ³He(⁴He, ⁴He)³He has been measured at center of mass energies E_c.m. between 28 and 44 MeV and in the c.m. angular range of 20° to 160°. The ³He polarization P at E_c.m. = 42 MeV and θ_c.m. = 132° was determined in a double scattering experiment. The analysis of the cross section data with the optical and the cluster model has been described previously. The phase shifts obtained in the cluster model calculations were used as starting values in a phase shift analysis. The resulting final real phase shifts and elasticity parameters give good fits to the cross section data. At 44 MeV the elasticity parameters show a pronounced odd-even dependence on angular momentum which had been found already in the case of the real phase shifts. The result of the polarization experiment |P| < 0.22 is consistent with cluster model predictions.     

Investigation of the 6Li(d, α)4He reaction between 1.5 and 11.5 MeV

The differential cross section σ₀(θ) and the analysing powers T₁₁(θ), T₂₀(θ), T₂₁(θ) and T₂₂(θ) of the reaction ⁶Li(d, α)⁴He have been measured for twelve energies between 1.5 and 11.5 MeV at c.m. angles between 3.5° and 90°. The results were fitted with Legendre polynomials. The energy dependence of the resulting coefficients indicates resonance-like behaviour at several energies corresponding to excitation between 22 and 32 MeV in ⁸Be. The reaction ⁶Li(d, α)⁴He shows excellent features as an analyser for deuteron vector and tensor polarization over the whole energy range investigated.     

The reaction 14C(6Li, 6He)14N and the distribution of Gamow-Teller strength

We find that the correlation between the L=0 component of the (⁶Li, ⁶He) reaction and the GT matrix element holds down to a very low level of GT strength, and that essentially all (90%) of the GT strength lies in the 3.95 MeV state of ¹⁴N.     

The elastic scattering of polarized 3He by 3He and excitation in the 6Be system

Angular distributions of the polarization in the elastic scattering of ³He by ³He have been measured at eight energies between 18 and 33 MeV, corresponding to excitation of ⁶Be between 20.5 and 28 MeV. The measurements were made using the 33 MeV polarised ³He beam at the University of Birmingham Radial Ridge cyclotron and a small gas target. The data have been analysed in terms of real and complex phase shifts. The deduced phase shift energy dependence cannot be associated with a single isolated level in ⁶Be, however an application of the two-level R-matrix formula reveals some broad L = 3 structures around E_x = 25 MeV.     

A study of the 35Cl(p, γ)36Ar reaction and the energy levels of 36Ar

Proton energies and strengths of (p,γ) and (p, p₁) resonances of the ³⁵Cl + p reaction were determined for E_p = 0.4?3.0 MeV and 1.9?3.0 MeV, respectively. The γ-decay of 84 resonances was studied with a 40 cm³ Ge(Li) detector. The branching ratios and excitation energies of 38 bound levels were determined. A new bound level was observed at E_x = 8472.0 ± 1.0 keV. Doppler-shift attenuation experiments yielded lifetimes of 20 bound levels. Spins and/or parities for bound levels and resonances were derived on the basis of observed transition strengths.     

The 19F(p,d)18F reaction studied at Ep = 19.3 MeV

The ¹⁹F(p, d)¹⁸F reaction was used at E_p = 19.3 MeV to study the level properties of ¹⁸F up to 6 MeV excitation energy. Angular distributions were measured over the angular range θ_lab = 10°–70° with an overall resolution of 35 keV. The experimental data were analyzed by DWBA theory to obtain l_n and C²S_l values, which were compared with existent theoretical nuclear structure calculations.     

The decay of isoscalar giant resonances in 24Mg and 40Ca

In this paper we present data on the charged-particle decay of the isoscalar 2⁺ strength between 10 and 20 MeV excitation energy (E_x) in ²⁴Mg and ⁴⁰Ca. The isoscalar strength was excited by inelastic scattering of 120MeV α-particles at 14° and 12.5° for ²⁴Mg and ⁴⁰Ca, respectively. The charged particles originating from the decay were detected in coincidence with the α′ particles at several angles in the scattering plane. J^π assignments of the decaying states were made on the basis of the angular correlation pattern of the α₀ decay to the ground state of ²⁰Ne and ³⁶Ar, respectively, using a DWBA calculation for the m-state population of the decaying state.For ⁴⁰Ca, about 40% of the E2 EWSR is found to be located in the interval E_x = 13.5 ± 1.5 MeV, which is similar to what has been found from previous inelastic scattering experiments at E_x = 18 ± 2 MeV, but much more than such experiments located in the region E_x = 12–15 MeV. The difference for the region E_x < 16 MeV is due to the fact that from our α₀ angular correlation pattern we conclude that virtually no continuum is excited in the (α, α′) process up to E_x = 16 MeV while all previous inelastic hadron scattering experiments assumed such a continuum to be present. The E2 strength distribution for ⁴⁰Ca thus obtained is very different from what previous theoretical calculations predict. For ²⁴Mg about 30% of the E2 EWSR is present in the interval 12.5 ? E_x ? experiments. 16.5 MeV which again is about twice as much as deduced from previous inelastic scattering The observed branching ratios are compared with calculated ones assuming statistical decay. Reasonable agreement was obtained for ⁴⁰Ca, but for ²⁴Mg especially the α₀-decay branch and to a lesser extent also the p₁ one are much stronger than the statistical calculations predict, indicating that especially the α₀ decay occurs mainly in a non-statistical way.A similar conclusion can be drawn from the behaviour of the forward-backward asymmetry in the angular correlations of the decay particles as a function of the excitation energy FBA(E_x). For ⁴⁰Ca, FBA(E_x) for all decay channels increases smoothly on the average once E_x is above a well-defined threshold, which is due to the onset of knock-out processes. For ²⁴Mg, however, the FBA(E_x) for the α₀ shows a large fluctuation as a function of E_x, indicating an interference process between semi-direct decay and knock-out processes.