P. L. Marston
N. H. Sun
Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814
Thin spherical shells filled with a liquid having a low speed of sound c[sub i] are known to have enhanced high-frequency backscattering. It is generally recognized that when the acoustic refractive index N=c/c[sub i] is between [radical 2] and 2, the enhancement is associated with a two-chord off-axis backscattered ray. A geometrical analysis of the amplitude diverges since the Gaussian curvature of the outgoing wave front vanishes. In the present research, a physical-optics analysis of glory ray scattering [P. L. Marston and D. S. Langley, J. Acoust. Soc. Am. 73, 1464--1475 (1983)] was applied to the ray of interest. Attention was limited to neutrally buoyant spheres and shell effects were neglected in both the analysis and the partial-wave series (PWS) calculations. The form function contribution is proportional to (ka)[sup 1/2], where a is the sphere radius and k=(omega)/c. For large ka, the analysis gives the major contribution to the PWS calculations except for N near [radical 2] and 2. In this range for N the contribution has a concise analytical approximation that depends on the ray impact parameter b = a(N/2)[radical 4-N[sup 2]]. [Work supported by ONR.] [sup a)]Present address: EXP Group, Inc., 44063 Fremont Blvd., Fremont, CA 94538.