Measurements of Sub-Barrier Transfer Yields in SULFUR-32 + NIOBIUM-93, MOLYBDENUM(98,100) Reactions at 180 Degrees

The Rochester RMS was used to measure excitation functions for 180^circ sub -barrier one- and two-neutron pickup reactions for E _{rm lab} <= 106 MeV in ^{32}S + ^{93}Nb, ^ {98,100}Mo systems by detecting target -like recoils at 0^circ. The measured yields are for quasi-elastic transfer; final states were not identified. The RMS technique was chosen for its self-normalizing property which makes obtaining absolute cross sections straightforward. The distorted-wave Born-approximation (DWBA) computer code scPTOLEMY was used to obtain quantal predictions of the one-neutron pickup yields. The calculations were performed for several final states and summed (using the appropriate spectroscopic factors) to estimate the total quasi-elastic transfer yield. P scTOLEMY over-predicted the yield in each system by a factor of 2-3. Since DWBA calculations for heavy-ion reactions are known to have difficulty reproducing experimentally measured yields within a factor of two, this discrepancy is not surprising. Although the absolute yields were not reproduced by the calculations, the shape of the excitation function is well reproduced. No calculations were performed for two-neutron transfer due to the lack of reliable spectroscopic factors. The transfer probabilities are obtained directly from these measurements. Distances of closest approach were calculated using a proximity potential. The slopes of transfer probability vs distance of closest approach are in good agreement with the predictions obtained from semi-classical theory using binding energies, indicating the absence of a "slope anomaly." This is consistent with the prediction that diffractive effects, which may distort the measured slope, are minimized at backward angles and sub-barrier energies--the precise conditions under which these measurements were performed. Angle-integrated transfer cross sections were derived from the measured transfer probabilities by assuming the ions follow Rutherford trajectories. These derived yields are consistent with the hypothesis that fusion enhancements in previously measured fusion yields for the ^ {32}S + ^{98,100} Mo systems are due to transfer of a neutron pair with a large, positive Q value.