To turn on trace for a specific event.
exec dbms_system.set_ev(<SID>, <serial#>, <event>, 8, '');
Example: Enable trace at level 8 for session id 10046
exec dbms_system.set_ev(12345, 543211, 10046, 8, '');

— To turn off the tracing:
exec dbms_system.set_ev(<SID>, <serial#>, <event>, 0, '');
Example: exec dbms_system.set_ev( 1234, 56789, 10046, 0, '');