How to get Header Template Checkbox value from Datagrid

protected void DataGrid1_ItemCreated(object sender, DataGridItemEventArgs e)
{
    if (e.Item.ItemType == ListItemType.Header)
    {

    }
}
protected void DataGrid1_ItemDataBound(object sender, DataGridItemEventArgs e)
{
    if (e.Item.ItemType == ListItemType.Header)
    {
        if (((CheckBox)e.Item.FindControl("CheckBox2")).Checked == true)
        {
            Response.Write("<script>alert('Checked')</script>");
        }
        else if (((CheckBox)e.Item.FindControl("CheckBox2")).Checked == false)
        {
            Response.Write("<script>alert('NOT Checked')</script>");

        }
    }
}

protected void CheckBox2_CheckedChanged(object sender, EventArgs e)
{
    if (((CheckBox)sender).Checked == true)
    {
                 Response.Write("<script>alert('Checked')</script>");
    }
    else
    {
        Response.Write("<script>alert('NOT Checked')</script>");

    }
}

<asp:DataGrid ID="DataGrid1" runat="server"
    DataSourceID="SqlDataSource1" onitemcreated="DataGrid1_ItemCreated"
          onitemdatabound="DataGrid1_ItemDataBound"  ClientIDMode="Static" >
          <Columns>
              <asp:TemplateColumn HeaderText="test">
                  <HeaderTemplate >
                      <asp:CheckBox ID="CheckBox2" runat="server" ClientIDMode="Static"
                          oncheckedchanged="CheckBox2_CheckedChanged" />

                  </HeaderTemplate>
              </asp:TemplateColumn>
          </Columns>
    </asp:DataGrid>