Lock UI position to circle edge

Hi, I'm currently coding a game where we need to manipulate UI elements. I quickly explain the game.

To answer question the player needs to make a symphony by placing notes on different circle. A shockwave spawn in center of the screen and grow up over time. When the shockwave hit the "notes" it record the symphony and compare to answer. If it fits with an answer the answer is selected.

I hope this is enough clear. So what I need is to manipulate the "notes" (bot left corner) and snap them to the different circle.

I managed to move the UI element using OnPointerEnter and Exit but I still have some problems using it. When the pointer is on a note (checking the tag) I can click to move the UI and click again to release it. The problems is sometime my pointer is on the parent of the note and I cant pass trought it. Even whith a RectTransform with minimum size sometimes my pointer detects the parent and not the notes.

Any idea to manipulate easily UI Elements ?

My second problem is the biggest. I would like my notes to be fixed on the closest circle when I release them. I really have no idea how to do this.

If you have any question, do not hesitate :)

PS : I am on the 2018.2.8f1 unity version