how to get the id of a checkbox in a GridView

I have a checkbox and appears on each line of the GridView and I want to find the id of the checkbox that has been ticked.

Code for GridView:

<asp:GridView  style="width:75%"  
                        ID="gvCVRTDetails" 
                        ShowHeaderWhenEmpty="true"
                        CssClass="tblResults" 
                        runat="server" 
                        OnRowDataBound="gvCVRTDetails_RowDataBound"                             
                        DataKeyField="ID" 
                        AutoGenerateColumns="false"
                        allowpaging="false"
                        AlternatingRowStyle-BackColor="#EEEEEE">
                        <HeaderStyle CssClass="tblResultsHeader" />
                        <Columns>
                            <asp:BoundField DataField="Description" HeaderText="Checklist Items"></asp:BoundField> 
                            <asp:TemplateField HeaderText ="Checked?" ItemStyle-HorizontalAlign="Center">
                                <ItemTemplate>
                                         <asp:CheckBox ID="chkChecked" runat="server" onchange=UpdateCheckedBox(this,this.value)></asp:CheckBox>
                                </ItemTemplate>
                            </asp:TemplateField>
                        </Columns>
                    </asp:GridView>

I tried using onchange=UpdateCheckedBox(this,this.value) and onchange=UpdateCheckedBox(this.id,this.checked) but it doesn't return the id.

I added a simple function so I can check the id of the checkbox:

function UpdateCheckedBox(id, checked) {
        alert(id);
        alert(checked);
    }

If I use this.id the alert message is blank. If I use this is says [object htmlSpanElement]. Using this.checked or this.value just returns undefined