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In this paper we present the results of the analysis of 7r + -He 4 elastic scattering at an incident pion laboratory momentum of 610 MeV/c. The data for this investigation were obtained from 17 500 frames exposed in the Argone National Laboratory-Carnegie Institute of Technology 10-in. He 4 bubble chamber 1 at the Argonne National Laboratory.Events were measured on conventional digitized projectors and processed through geometry (NP54) and kinematic fitting (GRIND) programs. To insure good measurements, fiducial volume and beam track criteria were imposed in selecting the data. Acceptable events were required to have beam momentum in the range 550-650 MeV/c to avoid as much as possible the variations of energy-dependent parameters and still have a statistically significant sample of events. The mode of the beam distribution was 610 MeV/c. The total number of events that satisfied the elastic criteria was 869, of which 194 had a discernible recoil track and were fitted as a four-constraint (4C) class, while the rest were 1C fits. The x 2 cuts for both topologies were based on a 1% confidence level. Because of the large scanning losses involved with small-angle scattering, we did not accept events with 6> lab < 10°. This cut eliminated the background from 7r + -JU + + ^L, decays and at the same time made the contribution of the Coulomb amplitude vanishingly small. The events in each individual bin of the differential cross section (Fig. 1) were corrected for scanning biases by requiring isotropic distributions for the azimuthal angle of the interaction plane. Since the muon contamination of the beam was unknown, we have normalized our total cross section to 111.4 mb, the result of the counter experiment of Chavanon et al. 2 The experimental elastic cross section is then 30.7 mb.Two models were used in fitting the experimental results: a strong-absorption model suggested by Palevsky's analysis of p-He 4 elastic scattering, 3 and the multiple-scattering model. 4 (a) Strong-absorption model. 5 -In the strongabsorption model the elastic-scattering ampli-tude is given bywhere 07 is the Coulomb phase shift (taken equal to zero), and 77/ the nuclear reflection coefficient which is parametrized to give a closedform expression for the differential cross section. In this analysis we used for the parametrization of YJI the Woods-Saxon functional form irtt) 6 :(2) r) .={(!-€ )g(l) + e} + i \idg/dlThe parameters L 0 and f are related, respectively, to the nuclear radius R and spatial diffuseness d of the interaction region byThe parameter fi represents the effect of the real part of the nuclear phase shift while e is 100| (4)FIG. 1. Differential cross section for 7r + -He 4 elastic scattering. Curve A is the impulse approximation, curve B is the multiple-scattering model, and curve C is the strong-absorption model, 822

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