read xml value as byte

public void Page_Load(object e, EventArgs s)
    {
        try
        {
            XmlDocument doc = new XmlDocument();
            byte[] chunk;
            string xmlmessage;
            xmmessagel = "<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
    "<chunk xmlns:dt=\"urn:schemas-microsoft-com:datatypes\" dt:dt=\"bin.base64\">SSdtIGp1c3QgYSBsaXR0bGUgYml0IGNhdWdodCBpbiB0aGUgbWlkZGxlDQpMaWZlIGlzIGEg" +
    "bWF6ZSBhbmQgbG92ZSBpcyBhIHJpZGRsZQ0KSSBkb24ndCBrbm93IHdoZXJlIHRvIGdvIEkg" +
    "Y2FuJ3QgZG8gaXQgYWxvbmUgSSd2ZSB0cmllZA0KQW5kIEkgZG9uJ3Qga25vdyB3aHkNCg0K" +
    "U2xvdyBpdCBkb3duDQpNYWtlIGl0IHN0b3ANCk9yIGVsc2UgbXkgaGVhcnQgaXMgZ29pbmcg" +
    "dG8gcG9wDQonQ2F1c2UgaXQncyB0b28gbXVjaA0KWWVhaCwgaXQncyBhIGxvdA0KVG8gYmUg" +
    "c29tZXRoaW5nIEknbSBub3QNCg0KSSdtIGEgZm9vbA0KT3V0IG9mIGxvdmUNCidDYXVzZSBJ" +
    "IGp1c3QgY2FuJ3QgZ2V0IGVub3VnaA0KDQpJJ20ganVzdCBhIGxpdHRsZSBiaXQ=</chunk>";
            doc.LoadXml(xmlmessage);
            XmlNode node = doc.SelectSingleNode("chunk");
            chunk = node.Value ;// this is my error
            Response.Write("sucsess");
        }
        catch (Exception x)
        {
            Response.Write("error"+x);
        }

    }

can somebody help me to solve the problem,
thx ..

regards
novhard