function beta=betrds(r1,r2,rv1,rv2,gamma)
%  calculate coagulation kernel for differential sed use rectilinear kernel. 
%  input - r1,r2 radius- cm 
%  output- beta coagulation kernel
%                         11 jy 93
global ratrirg

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

