How to obtain the field derivatives with respect to radius

[physicsxf]

Dear all,
I have two questions:

1. How to use FindBounce to obtain the field derivatives with respect to radius in the trajectory of the bounce?
2. FindBounce gives a list of radii (R). I find that R is not given uniformly in the trajectory. In some regions, the value of field decreases to zero suddenly. How to let FindBounce to uniformly give the R and the corresponding the value of field?
   Best