user control unload

I have page that has radio button list rblColumns. On each radio button click I loaded user control into table on same page like this:
LoadControl(rblColumns.SelectedValue.ToString() + ".ascx");
I want to trap when user control has been unloaded, before new control loading. Unload and Disposed events in user control page does not firing. I add body tag to the user control page html code and add onunload event. Not firing.
Please help.
Thanks 


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