function beta=betfds(r1,r2,rv1,rv2,gamma)
%  calculate coagulation kernel for diff sed use fractal kernel. 
%  input - r1,r2 radius  cm 
%          gamma   shear in 1/s
%  output- beta coagulation kernel
%                         11 jy 93

%      -  incorporate LI&Logan results by
%		using     a) beta rectilinear * (r1/r2)^power
global ratrirg

clds=0.984;		% power for ds
ratio=min(r1./r2,r2./r1);

rvvec=[rv1;rv2];
vsv=setvel([r1;r2],rvvec);
betds=abs(vsv(1,:)-vsv(2,:)) .* pi .* (ratrirg*(r1+r2)) .^ 2;


beta=betds .*ratio .^ clds;