Search Results (6 total)

Unpolarized cross sections and vector target analyzing powers for the ⁷Li→(π⁺,π^+′p) proton knockout reaction were measured using a vector polarized ⁷LiH target and a 240 MeV π⁺ beam at the πM1 channel of PSI. Typical target polarizations were >30% for ⁷Li. Coincident π⁺-p data are presented for three emitted pion angles (60°, 85°, and 108°). For each π⁺ angle coincident data with adequate statistics were obtained for three proton angles near the quasifree π⁺-p angle. The π⁺ angles were chosen to emphasize and isolate contributions to the target analyzing powers from the two-body π-nucleon interaction with a polarized nucleon whose polarization resulted from either the target polarization or from the distortion-induced effective polarization. The data are compared with factorized-amplitude distorted-wave impulse approximation (DWIA) calculations. The unpolarized cross sections are rather well described by these calculations. However, for all three angles the target analyzing powers are substantially reduced from predictions of conventional DWIA calculations. This result suggests a rather strong spin dependence in the Δ-nucleus spreading potential.

The exclusive ¹⁶O(γ,pn)¹⁴N_0,1,2,… reaction was measured for the photon energy range E_γ=98.5 to 141.0 MeV with an excitation energy resolution of 2.8 MeV. Protons were detected at 76° and 82° and coincident neutrons were detected at the opening angle for quasifree deuteron photodisintegration. Only the T=0, 1⁺, 3.95 MeV state in the residual ¹⁴N nucleus was significantly populated. This corresponds to a preferred L=0 angular momentum transfer to the recoil nucleus as is expected for quasifree kinematics. The absence of significant population to other discrete states is evidence that absorption on correlated proton-neutron pairs with an angular momentum transfer of L>~2 is largely suppressed for the low recoil momentum range of the current measurement. As a consequence of this fact, and the absence of unnatural parity states, only absorption on proton-neutron pairs in relative motion l=0,2 is expected for all possible shell model couplings. No significant population of the T=1 state at 2.31 MeV is consistent with previous measurements, demonstrating that absorption on proton-neutron pairs in an isospin triplet is suppressed. The results of the measurement to discrete states in the residual ¹⁴N nucleus are compared to microscopic calculations which include contributions above that of the mean field due to the short range and tensor parts of the nuclear potential. Clear separation in the contribution of (1p)^-2 and (1p)^-1(1s)^-1 proton-neutron pairs is seen to occur at an excitation energy of 20±2 MeV. Significant population of the (1p)^-1(1s)^-1 continuum above 20 MeV indicates that absorption on L=1 nucleon pairs is also important near quasifree kinematics.

Cross-section measurements of the ⁹Be,¹²C(α,2α) reaction at 580 MeV bombarding energy are presented. The data are compared with distorted-wave impulse approximation calculations. The agreement between theory and experiment suggests a dominance of the quasifree knockout mechanism. The extracted α-particle spectroscopic factors are in reasonable agreement with theory and proton-induced knockout reactions, unlike measurements for the (α,2α) reaction at energies <~140 MeV, but still show significant angular dependence.

We have measured the analyzing power for elastic scattering of π⁺ from a target of polarized ¹H. Data were taken for incident pion beam energies of 165 and 240 MeV at several pion scattering angles. The current data generally agree with previously existing measurements of A_y for this reaction and also with results of the SAID phase-shift analysis program. In most cases the new data are of higher precision than previously existing data. © 1996 The American Physical Society.

Angular distributions and total cross sections for the ⁴He(π⁺,pp)²H reaction have been measured with small relative uncertainties at incident energies of T_π⁺=64, 87, 114, 162, 217, 278, and 327 MeV. The results strongly support the quasideuteron absorption model for pion absorption on two nucleons, which is found to contribute only ≊50% to the total absorption cross section near the Δ resonance. All essential reaction channels of pion absorption near the Δ resonance on heavier nuclei seem to be present in ⁴He, but not in ³He. Any nuclear-density-related increase of pion absorption in ⁴He relative to ²H is <50% and no binding energy effect is found.

Differential and integrated cross sections for the reaction ⁴He(π⁺,pp) at T_π=114 and 162 MeV are reported. These are the first results for this reaction using a large solid angle detector covering ≊55% of 4π sr. Directly measured integrated cross sections for absorption on two-nucleon pairs are σ_abs=25.7±1.8 mb at 114 MeV and σ_abs=21.0±1.5 mb at 162 MeV. These cross sections, when corrected by a factor of 1.4 final state interaction effects, account for about 50% of the total absorption cross section for all final-state reaction channels at both energies. The data are compared with Monte Carlo simulations of the reaction process using a quasideuteron absorption model for the two-nucleon absorption process in a plane wave impulse approximation. The consequences for the extraction of the two-nucleon absorption cross section by imposing different conditions on the data are explored.