Browsing the senate gov site, it states how the pensions work using calculations based on years of experience.
They would lose their retirement annuity if convicted of crimes like treason or espionage.
The golden number of years of service is FIVE, but the time served can be combined with any previous federal service or additional service subsequent to service as a member of the senate or representatives.
That's the official lingo.
Unofficially, hearsay and behind the scenes, there are numerous loopholes and benefits.