Performance Tuning :: Identify Blocking Sessions

My statspack reports are sometimes showing figures such as this from the Foreground Wait Events section:

Avg %Total
%Tim Total Wait wait Waits Call
Event Waits out Time (s) (ms) /txn Time
---------------------------- ------------ ---- ---------- ------ -------- ------
PL/SQL lock timer 1,430 100 6,982 4883 0.0 18.0
enq: TM - contention 3 0 6,438 ###### 0.0 16.6
db file sequential read 310,385 0 1,765 6 3.5 4.6
TCP Socket (KGAS) 12,593 13 180 14 0.1 .5
db file scattered read 23,673 0 87 4 0.3 .2
log file sync 19,409 0 82 4 0.2 .2
ksfd: async disk IO 1,680 0 29 17 0.0 .1
[code].....

The TM lock that occurred 3 times appears to be disastrous.Historically, this could be caused by missing indexes on foreign key columns.

How can I be alerted when this event occurs, so that I can do some real-time investigation into the sessions and the SQL that hit it? I suppose I could schedule a job to query v$lock/dba_waiters/dba_blockers every few minutes, but is there a better way? Any standard edition scripts for this?