Hi Mahesh,
Please configure the SRX as below.
user@host#
set system syslog file traffic-log any any
user@host# set system syslog file traffic-log match "RT_FLOW_SESSION"
Along with that you have to configure the session-init or session-close according to your requirement.
Below link help you to get the more details.
http://kb.juniper.net/InfoCenter/index?page=content&id=KB16509&smlogin=true
Regards,
Deepak