Avoid postback on close of modaldialog

Hi,
I am opening a modaldialog using the below javascript from a page
  function Day_Summary(Dt, Emp_id, Emp_Name) {
         
            window.showModalDialog('frmDetails.aspx?Date=' + Dt + '&Emp_id=' + Emp_id + '&Emp_Name=' + Emp_Name, 'window', 'dialogHeight:565px;dialogWidth:625px;center:yes;help:no;resizable:no;status:no;');
            return false;
        }
When i close the modaldialog, my page is poasting back. how cna i avoid this?
pl help me on this

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