A serial rapist accused of sexually assaulting a Rikers Island jail guard last month while locked up on rape and robbery charges kept detailed notes about his sick crimes, an NYPD detective testified Tuesday.

Dexter Bell — convicted in 2008 of raping a 17-year-old girl — had a handwritten note on him with dates and times of two rapes he committed nearly 12 hours apart on April 11, 2013, Detective Jonathan Cruz said during a pretrial hearing in Brooklyn Supreme Court.

Bell, 26, allegedly attacked correction officer Samantha Moscoso on April 26 while being moved from a high-security facility on Rikers.