Analyzing power and cross section distributions of the knockout reaction 208 Pb„ p ¢ ,2p … 207 Tl at an incident energy of 202 MeV R. Neveling, 1 A. A. Cowley, 1 G. F. Steyn, 2 S. V. Fo ¨ rtsch, 2 G. C. Hillhouse, 1 J. Mano, 3 and S. M. Wyngaardt 1 1 Department of Physics, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa 2 iThemba Laboratory for Accelerator Based Sciences, P.O. Box 72, Faure 7131, South Africa 3 Department of Electrical Engineering and Computer Science, Osaka Prefectural College of Technology, Osaka 572-8572, Japan ~Received 20 February 2002; published 3 September 2002! Exclusive measurements of the cross section and analyzing power for the 208 Pb( p W ,2p ) 207 Tl proton knockout reaction at 202 MeV are presented for three quasifree angle pairs. Energy-sharing cross section distributions are found to be in excellent agreement with distorted wave impulse approximation ~DWIA! calculations, yielding spectroscopic factors that are in reasonable agreement with ( e , e 8 p ) studies and theoretical expecta- tions. The measured analyzing powers are, however, in significant disagreement with results of standard DWIA calculations that utilize the free nucleon-nucleon interaction. Analyzing power calculations are furthermore found to be insensitive to variations in the distorting potentials, different descriptions of the bound state, different energy prescriptions of the two-body interaction, and nonlocality effects. Agreement between theory and experiment is shown to improve only when the density dependence of the nucleon-nucleon interaction is incorporated within the DWIA. DOI: 10.1103/PhysRevC.66.034602 PACS number~s!: 24.50.1g, 24.70.1s, 25.40.Hs, 27.80.1w I. INTRODUCTION The description of quasifree proton scattering by means of the distorted wave impulse approximation ~DWIA! theoreti- cal framework @1# has proven successful in predicting angle- and energy-sharing correlation cross sections over a wide energy range ~76 – 600 MeV! for light and medium mass tar- gets up to 40 Ca @2–9#. Recently, in a study of the 208 Pb( p ,2p ) 207 Tl reaction at an incident energy of 200 MeV @10#, it was shown that the DWIA can also accurately predict cross sections for proton knockout from a heavy target, yielding spectroscopic factors that are in good agreement with results from ( e , e 8 p ) studies. This success, despite the severe distortion effects due to the heavy target nuclei, dem- onstrates the validity of the theoretical treatment of the pro- ton distortions within the DWIA framework, at least as far as the ability to predict cross sections is concerned. Predictions of the analyzing power of quasifree proton knockout reactions for light to medium mass targets are, however, known to be problematic. Although some success has been achieved for the spin observables for quasifree ( p ,2p ) scattering at 200 MeV on 16 O and 40 Ca for both relativistic and nonrelativistic DWIA calculations @11–13#, it is also known that experimental energy-sharing analyzing power for the very light targets 3,4 He at 200 MeV is substan- tially reduced compared to calculations @14,15#. Likewise, Carman et al. @16# illustrated a similar discrepancy in the angle-integrated exclusive data for 12 C( p ,2p ) at 200 MeV. For proton knockout from a 16 O target at 500 MeV, Miller et al. @17# observed that the analyzing power data are signifi- cantly reduced in comparison with non-relativistic or relativ- istic DWIA calculations, particularly for knockout of the 1 s 1/2 protons. Measurements of proton knockout at 392 MeV from the s 1/2 states in 6 Li, 12 C, and 40 Ca made by Hatanaka et al. @18# revealed a similar reduction that is a monotoni- cally increasing function of the averaged density at which the reaction experiences its major contribution. This strongly suggests the existence of a medium effect on the nucleon- nucleon interaction, as would be intuitively expected. On the other hand, Miller et al. @17# showed that, al- though the inclusion of density-dependent interactions in the DWIA calculations improved agreement with the data, it still does not resolve the discrepancy satisfactorily. This result supports the conclusion of Noro et al. @19#, who showed that the distinct density dependence of the reduction in analyzing power in 6 Li, 12 C, and 40 Ca is only qualitatively reproduced when incorporating similar density-dependent DWIA calcu- lations as used in Ref. @17#. In order to investigate the origin of the overprediction of the analyzing power, it is useful to compare the observed phenomenon with the situation for different target nuclei, especially for s-state knockout. For s-state knockout the re- lation between the spin observables of the ( p ,2p ) reaction and that of free nucleon-nucleon scattering is expected to be a relatively simple one, whereas the effective polarization of the bound nucleons ~the so-called Maris effect @20#! makes the relation somewhat complicated in the case of l 0 states @18#. The prominence of s-state knockout in the case of a 208 Pb target would then make it an ideal candidate for further study. Since previous cross section results @10# suggested that the available theoretical framework could successfully be applied to knockout from heavy target nuclei, a high- resolution coincidence measurement of energy-sharing cross section and analyzing power of the 208 Pb( p W ,2p ) 207 Tl knock- out reaction at 202 MeV was undertaken with the aim to separate the 3 s 1/2 ground state from the first three excited states of 207 Tl. The following section ~Sec. II! elaborates on the details related to the experimental arrangement and the data analy- sis. Details of the theoretical calculations are given in Sec. III. The results are presented in Sec IV, followed by the summary and conclusion in Sec. V. PHYSICAL REVIEW C 66, 034602 ~2002! 0556-2813/2002/66~3!/034602~13!/$20.00 ©2002 The American Physical Society 66 034602-1