How to disable a Parent window when the Child pop-up window opens in C#.net

My Code:

using System;

using System.Data;

using System.Configuration;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Web.UI.HtmlControls;

using System.Data.SqlClient;
public partial class _Default : System.Web.UI.Page 
{dbUtils dbu = new dbUtils(); 
//SqlDataAdapter dataAdapter;

//DataSet dSet;

//SqlCommand cmd;

//int value;

//string Temp,var;
protected void Page_Load(object sender, EventArgs e) 
{if (Page.IsPostBack == false) 
{
session();
}
 
 
}public void session() 
{if (Session["sess"] == null) 
{string popupWindow = null; 
string testScript = "window.open('Default2.aspx','win','width=500,height=200,scrollbars= yes')";
string scriptID = "popup"; 
string myScript = buildScript(scriptID, testScript);
if (!Page.IsStartupScriptRegistered(scriptID)) 
{
Page.RegisterStartupScript(scriptID, myScript);
}
}
}string buildScript(string id, string output_page) 
{string myScript = "
"; 
myScript += "