W.R. Falk

Three-nucleon transfer on 12C: The 12C(α, p)15N reaction at 96.8 MeV

Abstract Angular distributions of protons from the 12C(α, p)15N reaction have been measured over the angular range from 10–70° at an α-particle bombarding energy of 96.8 MeV. Well defined particle groups are observed up to an excitation energy of 18 MeV in 15N. The relatively small number of states excited implies a selectivity both in the reaction mechanism and in structure effects. DWBA calculations using a semi-microscopic three-nucleon form factor have been performed using several different sets of wave functions. Good agreement in the ratio σexp/σth is obtained for most states using the 15N wave functions of de Meijer. The strongest state in the (α, p) spectrum is observed at 15.397 MeV in 15N and DWBA calculations give good agreement for a 13 2 + assignment. This state has been observed only in other three-nucleon transfer reactions involving heavy ions. The recent discovery of a 9 2 + state at 10.693 MeV in a p+14C resonance measurement is supported by our analysis.

Investigation of microscopic form factors for the (p, α) reaction☆

Abstract Microscopic form factors for the three-nucleon transfer (p, α) reaction are described, with special emphasis on their comparison with conventional cluster form factors. Considerable differences in shape of these form factors is found, especially in the surface region. An extension of the earlier zero-range descriptions of the microscopic form factor to include finite-range effects, is formulated and tested. Center of mass corrections, the α-particle size parameter, and non-local corrections in the single-particle states, all have a strong influence on the shape of the microscopic form factor. The implications of these observations for the extraction of nuclear structure information is discussed, especially as this affects the commonly used procedure of replacing the microscopic form factor with a cluster form factor, in the actual DWBA calculation.

Analyzing power of the pd → tπ+ reaction at 305, 330, 375 and 400 MeV

Abstract Analyzing power measurements of the pd → tπ+ reaction are reported at incident proton energies of 305, 330, 375 and 400 MeV over the angular range from 68° to 145.5° in the center of mass. The results are compared with earlier measurements obtained at 400 and 500 MeV. The analyzing power at 375 MeV exhibits a distribution unlike that observed at lower energies. The analyzing power at 400 MeV, and for large pion emission angles, is in significant disagreement with other results, for the same energy, reported in the literature.

The differential cross section for proton-proton elastic scattering at 90° c.m. between 300 and 500 MeV

Abstract The absolute differential cross section for proton-proton elastic scattering has been measured at 90° c.m. for 300, 350, 400, 450 and 500 MeV. The statistical uncertainty of the measurements is 0.5% with an additional systematic normalization uncertainty of 1.8%. The results are compared to phase-shift analyses.

T = 12and32 negative-parity states in 11B with the 14C(p, α)11B reaction

Abstract The 14 C(p, α) 11 B pickup reaction has been studied at a proton energy of 41.9 MeV. Differential cross sections to the ground state and 13 excited states (including members of the isospin T = 3 2 multiplet) were measured from 10° to 155° (lab). The experimental results show that only negative-parity states are strongly excited. This suggests that pickup strongly dominates over knockout as the primary reaction mechanism. A finite-range DWBA analysis of the data was carried out using cluster form factors, and the deduced spectroscopic factors compared with the shell-model calculations of Kurath.

Exchange terms in the optical model

Abstract A number of existing proton elastic scattering differential cross section and polarisation angular distributions for 16 O, 28 Si, 40 Ca, 58 Ni, 66, 68 Zn, 120 Sn and 208 Pb at several incident energies have been analyzed using an optical-model potential in which the real central and absorptive potentials are multiplied by “exchange terms” of the form [1+ C r ,(−1) l ] and [1+ C i (−1) l ] respectively. Since the major effect of these terms is to be found at backward angles, a number of differential cross section angular distributions for 16 O, 66,68 Zn and 208 Pb have been extended to θ lab = 175°. Small improvements in the quality of the fits to experimental data were obtained with C r , C i ≠ 0. The behaviour of the coefficients C r and C i as functions of incident proton energy is discussed.

Energy levels of 18Ne from the 20Ne(p, t)18Ne reaction

Abstract Angular distributions of tritons from the 20Ne(p, t)18Ne reaction have been measured at a proton bombarding energy of 42.6 MeV. In addition to the excitation of the established states at 1.887, 3.376, 3.576 and 3.616 MeV, strong groups were also seen at 4.53, 5.10, 6.28 and 9.17 MeV. The 4.53 MeV state is found to have a spin-parity of 1−. The 5.10 MeV group is probably a doublet comprised of a 2+ and a 3− state. A tentative 4+ assignment is given to the 6.28 MeV state. DWBA calculations have been made using the finite-range two-nucleon transfer code of Nelson and Macefield. Various wave functions describing 18Ne and 20Ne, taken from the work of Arima et al., Wildenthal et al., Kuo and Zuker, were used in calculating the spectroscopic amplitudes for transitions to the various final states in 18Ne. The sensitivity of the DWBA results to the binding energy of the single-particle states is discussed.

Heavy-ion reactions induced by 16O, 18O and 19F ions

Excitation curves have been determined from γ-ray yield measurements for heavyion reactions induced by Elab = 12–30 MeV 16O, 18O and 19F ions incident upon thick targets of 9Be, 10B, 11B, 12C and 23Na. The yields of radioactive decay products with half-lives greater than one second were measured; hence a large number of the outgoing reaction channels could be observed. The preponderance of heavy reaction products suggests compound-nucleus formation as the dominant reaction mechanism. Statistical-model calculations with a spin-dependent level density have been performed, in which the nuclear moment of inertia was treated as a parameter. Many of the results can be explained satisfactorily with a nuclear moment of inertia 0.55 to 0.7 of the rigid body value.

Spin-dependence and coherent summations in two-nucleon transfer reactions☆

Abstract The differential cross sections of the 19 F(p, 3 He) 17 O and 19 F(p, t) 17 F reactions leading to the ground and first three excited states have been measured at an incident energy of 42.4 MeV. The polarization analysing powers of two of the (p, 3 He) reactions have been measured at a higher energy of 49.5 MeV with a polarized incident beam. The experimental results have been analysed in terms of conventional DWBA theory, and the importance of interference terms arising from the proper coherent summations contained in the definition of the differential cross section and analysing power, have been investigated. It is found that the interference term arising from the coherent summation over the transferred spin S is significant and should not normally be neglected.

Tensor polarisation of 12C(15.11 MeV, Jπ = 1+) in the 13C(p,d)12C(→12C g.s. + γ) reaction at Ep = 41.3 MeV

Abstract Angular correlation measurements have been made between the deuterons and the 12 C g.s. product nuclei in the reaction 13 C ( p,d ) 12 C ∗ (15.11 MeV , 1 + )(→ 120 C g.s. + γ) at deuteron scattering angles of 40°, 50°, 60°, and 75°, and a proton bombarding energy of 41.3 MeV. These measurements yield information concerning the k = 2 polarisation tensors ( t 2q ) of the excited 12 C nucleus produced in the direct (p, d) reaction. Cross-section angular distributions to the strongly excited g.s., 4.43, 12.71, 15.11 and 16.11 MeV states are also reported. Monte Carlo simulations of the laboratory distribution functions associated with the individual t kq were performed, incorporating the effects of finite geometry (beam spot and detector sizes), beam divergence, energy loss and multiple scattering of the 12 C nuclei in the target. Finite range DWBA calculations were performed describing the polarisation tensors and cross sections, assuming a 1p neutron pickup. DWBA calculations for both conventional (elastic) and adiabatic d + 12 C potentials were performed. Also, the effects of spin-orbit and the T r and T l tensor potential were studied. The results confirmed that the “standard” DWBA formalism using an elastic d + 12 C potential is inadequate for describing the recoil nucleus polarization. The use of adiabatic potentials, which arise from three-body effects, produced an improvement in the description of the extracted data. The DWBA calculations showed only a modest sensitivity to inclusion of spin-orbit and the T r or T l tensor terms in the deuteron optical potential.