oracle blocking sessions

Question

I am trying to investigate some concurrency issues on Oracle 11 RAC. So far the most helpful tool I have is "blocking sessions" view of the Enterprise Manager, which tells me which query is blocking everyone else and what it is waiting for. However, to take advantage of that view, I need to catch the problem as it is happening. So I'm looking for a way to query oracle for historic data which would give me data similar to what "blocking sessions" screen has to offer in Enterprise Manager.