Circular collision detection

I have a project where I\'m emulating a lottery machine. The balls currently bounce inside a rectangular box, but I want them to bounce inside a circle. All the code is there, the bouncing of the balls, etc, even the collision with rectangular box. I need someone to modify the rectangular box collision to circular collision. YOu will see in the attachment a green circle. The balls need to bounce INSIDE that circle. Any questions, please don\'t hesitate to ask!