K. Pham

Gamow-Teller strengths from (t,3He) charge-exchange reactions on light nuclei

Abstract Zero-degree cross sections have been measured for (t, 3 He) reactions on the light target nuclei 9 Be, 10 B, 11 B, 12 C, and 13 C at a bombarding energy of 127 MeV/A. The triton beam was produced by ( 4 He,t+p) breakup on a Be production target using a 4 He beam of 155 MeV/A. Dispersion matching was applied to obtain good energy resolution ( Δ E/E=0.002, Δ E=780 keV). The zero-degree (t, 3 He) cross sections are used to discuss Gamow-Teller strengths in 10 Be and 11 Be.

(t,3He) Reaction; New tool for studying spin-isospin excitations in neutron rich nuclei

Initial experiments were carried out to study spin-isospin transitions in nuclei using (t,3He) measurements. A triton beam was produced from the breakup reaction (4He,t-p) on a 9.25 g/cm2 Be production target using a 4He beam of 620 MeV. With a beam of 70 enA a yield of 381 MeV tritons with 4.5 × 106 particles/s was achieved. The first half of the A1200 beam transport system at NSCL was used to analyze the triton beam. The energy spread at the target position was estimated to be about 23 MeV. The second half of the A1200 was used to analyze the energy of 3He particles from the (t,3He) reaction. The dispersion matching method was applied to obtain good energy resolution despite the significant energy spread of the incident tritons. The achromatic properties of the A1200 system made it possible to obtain high-resolution (t,3He) spectra at θ = 0° with a resolution of ΔEE = 0.002 (ΔE = 780 keV).

Isovector giant resonances in 6He, 12B, 90Y, 120In, and 208Tl observed in the (7Li,7Be) charge-exchange reaction

Abstract The (7Li,7Be) and (7Li,7Beγ) reactions have been studied at bombarding energies of 50 A MeV on targets of 6Li, 12C, 90Zr, 120Sn, 208Pb, and polystyrene. The detection of Doppler-shifted γ-rays from excited 7Be permits the identification of the spinflip and non-spinflip characteristics. Transitions to over 20 discrete states and resonances have been observed. Angular distributions for 6Li and 12C are well described by microscopic one-step distorted-waves calculations with shell-model transition amplitudes. The transitions to the ground states of 6He and 12C exhibit significant Gamow-Teller strength, but ∼7% and ∼17%, respectively, of the calculated cross sections at θ ≈ 0° arise from the tensor interaction. A broad resonance at ∼5.6 MeV in the halo nucleus 6He with Jπ = (2+,1−,0+) seems to correspond to a structure predicted at this excitation energy containing soft modes of excitation with Jπ = 0+and/or 2+. A resonance in 12B at ∼4.5 MeV is described by a superposition of known 2− and 4− states, and another one at ∼7.5 MeV is dominated by a transition to a 2− state with weaker contributions from two 1− states (spin dipole and giant dipole). Four to six resonances each, mostly spin-dipole resonances, were identified in the final nuclei 90Y, 120In, and 208Tl. The non-resonant quasi-free charge-exchange reaction pbound(7Li,7Be)nfree was observed for all targets up to effective excitation energies of 50 MeV. The mechanism is related to that on free protons. The effective number of participating protons was determined.

Isovector giant resonances in 6He, 12B, 90B, 120In, and 208Tl

Abstract The (7Li,7Be) charge-exchange reaction has been studied at E( 7 Li)=350 MeV on targets of 6Li, 12C, 90Zr, 120Sn, and 208Pb. Coincidences with Doppler-shifted γ-rays from the de-excitation in flight of the first-excited state in 7Be were recorded to identify spinflip transitions. Numerous discrete states and giant resonances were observed. Angular distributions were obtained for 6Li and 12C targets. They are well described by microscopic finite-range distorted-waves calculations with theoretical shell-model transition amplitudes and a nucleon-nucleon interaction containing central and tensor components.

UNUSUAL ENERGY-DEPENDENCE OF THE TOTAL NUCLEAR-REACTION CROSS-SECTION FOR A SECONDARY ISOMERIC NUCLEAR BEAM (F-18(M), J(PI)=5(+), E(X)=1.1 MEV)

A primary O-17 beam has been used to produce a 22.3 MeV/nucleon F-18(m) isomeric secondary beam via a single nucleon transfer reaction on a carbon target. The total nuclear reaction cross sections for F-18(m) and F-18(g.s.) in silicon were measured in a stack of seven silicon solid-state detectors. The cross section for F-18(m) exhibits some unusual features at low energy not seen for F-18(g.s.) or other ions.

Fast γ-ray coincidence timing using a BGO detector array in a nuclear reaction experiment

Abstract The ( 7 Li, 7 Be) nuclear charge-exchange reaction has been investigated at E( 7 Li ) = 350 MeV . Coincidences were measured between the charged 7 Be reaction products in the focal plane of a magnetic spectrometer and 430-keV Doppler-shifted γ-rays from the decay in flight of excited 7 Be ejectiles. The latter were detected in a compact BGO detector array. Fast timing was used to optimize the separation between prompt and randoms events. The BGO output signals display the statistical distribution of the arrival times of individual photoelectrons from the photocathode in the photomultiplier tubes. A timing resolution of 6.5 ns relative to the accelerator RF was obtained. A simple model was successfully developed to simulate the BGO output signals with a statistical distribution of a predetermined number of photoelectrons modulated by the light output response of the scintillator and the single-photoelectron gain distribution and transit time through the photomultiplier tube. It is concluded that the timing resolution is primarily limited by the small number of photoelectrons.

Looking for the isoscalar giant dipole resonance in 208Pd using inelastic a scattering at and near O

Abstract The isoscalar giant dipole resonance (ISGDR) in 208 Pb has been investigated using inelastic scattering of 200 MeV α particles at and near 0° where the angular distribution of the ISGDR can be clearly differentiated from other modes. The “difference of spectra” technique was employed to separate the contribution from the high-energy octupole resonance (HEOR). Results from a preliminary analysis of the data are consistent with the expected positions of the ISGOR and HEOR, as well as their expected angular distributions near 0°.