Finding the depth of clicked movie clip

Hi All
I need help to find depth of one of the movie clips on the stage.

Movie clip exists in flash library and its associated with a class using linkage properties.Its instantiated as below

var atomSmall1:AtomSmall=new AtomSmall();

Mulitple duplicates are created on the screen. I need to find the depth of the movie clip when I click on it. Code given below is the handler for the movie clip.
Below is the code snippet I have used.

private function removeAtoms1(evt:Event):void {
 var targetName:String=String(;
 trace("Target Name is "; ///instance120 or keeps changing
 var target1:DisplayObject = getChildByName(targetName); 
 trace(target1)                          ////Prints NULL
 trace("Depth of the atom "+ getChildIndex(target1)); 

Below is the output of trace and error message

Target Name is instance208
TypeError: Error #2007: Parameter child must be non-null.
 at flash.display::DisplayObjectContainer/getChildIndex()
 at ChemistryActivity1View/::removeAtoms1()