Have Permanent GC but divorced now--Need help

[ita_kk]

Hi,
things didn't go well between me and my wife after filing I-751(Renewal of temp GC from Marriage) in Feb and I got Permanent GC in June 09 for 10 years and we got divorced finally in Sept 09..Now Do I have to let Immigration know about it??can i go out of Country and come back without any issues if I don't let Immigration know about the divorce??
Thanks
ITA_KK