WPF Datagrid Column Collection Changed Event

I use this code to add or remove column from datagrid. each column header I have mouse enter and leave event. For new column I also would like to add the same event handler after inserting to datagrid.

private void Columns_CollectionChanged(object sender, System.ComponentModel.CollectionChangeEventArgs e)
{
if (e.Action == CollectionChangeAction.Add)
{
int columnPosition = (this.Columns.Count - 1);
DataGridTextColumn column = new DataGridTextColumn();
column.Header = (e.Element as DataColumn).ColumnName;
column.Binding = new Binding(string.Format("[{0}]", column.Header.ToString()));
this.Columns.Insert(columnPosition, column);

DataGridColumnHeader columnHeader = DataGridHelper.GetColumnHeader(this, columnPosition);
if (columnHeader != null)
{
columnHeader.MouseEnter += new MouseEventHandler(ColumnHeader_MouseEnter);
columnHeader.MouseLeave += new MouseEventHandler(ColumnHeader_MouseLeave);
}
SetAutomappingOnOff = false;
}
else if (e.Action == CollectionChangeAction.Remove)
{
DataColumn column = e.Element as DataColumn;
DataGridColumn toRemove = (from DataGridColumn dc in this.Columns
where dc.Header != null && dc.Header.ToString() == column.ColumnName
select dc).First();
this.Columns.Remove(toRemove);
SetAutomappingOnOff = false;
}
}

But columnHeader always returns null even though I can see that object is created and added to datagrid.

Pls help me.

Thanks
Dee