R. E. Warner

Coulomb dissociation of 11Li.

Kinematically complete measurements for Coulomb dissociation of [sup 11]Li into [sup 9]Li+2[ital n] were made at 28 MeV/nucleon. The [ital n]-[ital n] correlation function suggests a large source size for the two-neutron emission. The electromagnetic excitation spectrum of [sup 11]Li has a peak, as anticipated in low-energy dipole resonance models, but a large post-breakup Coulomb acceleration of the [sup 9]Li fragment is observed, indicating a very short lifetime of the excited state and favoring direct breakup as the dissociation mechanism.

New results for reaction cross sections of intermediate energy α-particles on targets from 9Be to 208Pb

Abstract Reaction cross sections for α -particles have been measured for 9 Be, 12 C, 16 O, 28 Si, 40 Ca, 58,60 Ni, 112,116,120,124 Sn, and 208 Pb at 117.2, 163.9 and 192.4 MeV and for the lighter nuclei also at 69.6 MeV. The results are compared with predictions from optical model calculations using phenomenological, global and double-folded optical potentials. Comparisons are also made with predictions using the Glauber model approach suggested by Bertsch, Brown and Sagawa.

Cross section for the astrophysical 14C(n, γ) 15C reaction via the inverse reaction

The 14C(n, γ)15C reaction is important in neutron-induced CNO cycles of stellar evolution phases beyond the main sequence. The chain of reactions in primordial nucleosynthesis in the neutron-rich environment of an inhomogeneous big bang also involves the 14C(n, γ)15C reaction. We have used a beam of 15C ions at E/A = 35 MeV to measure cross sections for 15C breakup on targets of C, Al, Zn, Sn, and Pb. The Coulomb part of the breakup cross section was determined as a function of decay energy. By the principle of detailed balance, the neutron capture cross section was then determined as a function of neutron energy. This excitation function rises with energy to 7 μbarns at 300 keV and falls to 2 μbarns at 1.2 MeV. In the stellar-burning region, our cross section exceeds a previous measurement by a factor of 3. Neither the shape nor the magnitude of the excitation function agrees with those of theoretical calculations based on the direct capture model. The reaction rate is calculated reliably up to ~T9 = 5. In the temperature range of inhomogeneous big bang models, our rate exceeds the rate of the 14C(p, γ)15N reaction for T9 < 1.2.

A comparison of α-transfer and α-decay in the lead region

Abstract Differential cross sections have been measured for the 204, 207, 208 Pb( 16 O, 12 C) 208, 211, 212 Po and 209 Bi( 16 O, 12 C) 213 At ground state transitions at a bombarding energy of 93 MeV; the cross section leading to the first excited state of 212 Po was also measured. These data were quantitatively compared with the corresponding α-decay data via R -matrix theory using the same target-plus-α nuclear potential. Good agreement between the absolute reduced widths determined from α-transfer and α-decay suggests that in the transfer reactions, as in α-decay, an α-particle in its ground state is transferred in a one-step process.

Production and use of 6He, 7Be, 8Li, 12B and metastable nuclear beams

Abstract A low energy (few MeV/nucleon), modest flux ( 10 4 −10 7 s ) radioactive nuclear beam (RNB) facility has been in operation for approximately three years at the University of Notre Dame Van de Graaff accelerator. This facility utilizes a compact superconducting solenoid lens, designed at the University of Michigan, with adjustable apertures to produce momentum-analyzed secondary beams via the direct transfer and other methods. Useable beams of 6He, 7Be, 8Li, 12B, 18F and to our knowledge the first isomeric beam, 18mF, have been produced and a first generation of RNB experiments has been successfully completed.

Alpha spectroscopic factors for 6Li, 7Li, 9Be and 12C from the (p, pα) reaction at 296 MeV

Abstract Three-body breakup cross sections and analyzing powers for the 6Li, 7Li, 9Be and 12 C ( p ,pα) reactions were measured at an incident energy of 296 MeV. Data were analyzed using the plane wave impulse approximation (PWIA) and the distorted wave impulse approximation (DWIA) and compared with previous studies. DWIA calculations reproduce shapes of projected spectra and analyzing power distributions fairly well with the exception of the 12 C ( p ,pα) 8 Be reactions. Extracted spectroscopic factors for 6Li, 9Be and 12C are larger than those found in the previous studies. In contrast, extracted spectroscopic factors for 7Li agree with the previous work. This work suggests that the spectroscopic factor for 6Li is ∼ 0.8, independent of incident energies and reaction types.

Comparison of the noncoplanar 6Li(p, pd)4He reactions at 120 and 200 MeV

Abstract The 6 Li(p, pd) 4 He reaction was studied at 200.2 MeV, at the quasi-free angle pair ( θ p , θ d ) = (54°, −48.9°), for noncoplanarity angles φ from 0° to 28°. 6 Li αd spectroscopic factors of 0.84 and 0.76 are deduced from our coplanar data at this energy and 120 MeV, respectively, for ground-state 2S Woods-Saxon wave functions. A recent microscopic three-body calculation predicts spectroscopic factors from 0.70 to 0.75; using the ground-state wave functions from this study, we deduce a factor of 0.76 from the 200 MeV data. DWIA calculations fit the measured integrated cross sections versus φ for spectator momenta P α ≲ 100 MeV/ c at both bombarding energies, but underpredict them for larger P α . Momentum form factors were better reproduced with 1S αd cluster wave functions for a soft-core bound-state potential than with the 2S Woods-Saxon wave functions, but the former wave functions generate unphysically large (∼1.25) spectroscopic factors.

Astrophysical reaction rate for the Li-8(n,gamma)Li-9 reaction

An attempt was made to measure the excitation function of the cross section for the Li-8(n,gamma)Li-9 reaction by performing the inverse reaction Li-9(gamma,n)Li-8, with the equivalent photons in the electric field of nuclei in a Pb target providing the gamma rays for the reaction. The energy spectrum of lithium nuclei in coincidence with neutrons had no discernible peak where any beam-velocity Li-8`s would be located. Statistically, a Gaussian-shaped Li-8 peak could have been present with 30+/-29 counts, which we interpreted as consistent with zero, with a two-standard-deviation upper limit of 87 counts. Using the fact that neutron capture on Li-8 must be dominantly s-wave capture, and applying detailed balance, we obtained, with E in eV, sigma(n,gamma)>930E(-1/2) mu b. The corresponding limit on the astrophysical reaction rate is >790 cm(3) mol(-1) s(-1). Theoretical predictions of the reaction rate have exceeded our upper limit by factors of 3-50.

Final-state interactions in the 2H(d, pp)nn reaction

Absolute coincidence cross sections for the 2H(d, pp)nn reaction were measured at Ed = 23.15 MeV by detecting both protons at a small included angle to observe events with small p-p and n-n relative energies (epp and enn). Evidence for a double final-state interaction was obtained at(θ1 = 18δ, θ2 = 36°) and (25°, 43°) where the yields were elevated above phase space at low ennand depressed below phase space at low epp. Watson-Migdal predictions, using accepted values of the effective range parameters for the singlet p-p and n-n interactions, gave reasonably good fits to the data except for the enn spectrum at (25°, 43°) which required an n-n scattering length ann < −50 fm. Acceptable fits were also obtained with a double distorted-wave theory using computed p-p and n-n scattering wave functions, a Gaussian transition potential and Slaus-Van Oers deuteron wave functions.

COINCIDENCE STUDY AND DWBA ANALYSIS OF THE

Abstract Absolute cross sections for coincidence detection of d + d, and of d + p, from the reaction 3 He+d → d+d+p initiated by 26.8 MeV helions, were measured at geometries in which two of the outgoing particles emerged with small relative velocity and therefore exhibited a strong final-state interaction. Good DWBA fits to most of the observed spectra were obtained using either one or two distorted waves to describe the relative motions in the final state. Woods-Saxon optical parameters for the p-d interaction are given. No enhancement resulting from sequential decays of unbound 3 He levels was observed.