Performance Tuning :: New Trace File

I need to take the database trace of each page of my web application. I am giving the following commands.

BEGIN
dbms_monitor.session_trace_enable(session_id=>122, serial_num=>NULL, waits=>true, binds=>true);
END;

Accessing the web application. Once it renders completely, I am executing the following command.

BEGIN
dbms_monitor.session_trace_disable(session_id=>122);
END;

In the user_dump_dest folder, I am expecting to see a new trace whenever I execute these commands. But the same trace file is getting updated. How do I make oracle to create a new trace for each iteration. I am using Oracle 11g Release on CentOS 5.x