ACR MDT ARR DESCENDED THROUGH THE ALT OF SMA OVERFLT BEFORE LATERAL SEPARATION EXISTED RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative

I WAS WORKING APCH CTL; THE ONLY PERSON CERTIFIED ON APCH ON DUTY AT THE TIME. APCH POS STARTED TO GET BUSY--NON RADAR. SMA X WAS OVERFLYING FROM SE V132 TO V138; NWBND AT 12000'; NON DME. MDT Y WAS INBND ON V81 FROM S. SMA Z HOLDING AT CRIMS AT 80 AS MDT Y WOULD OVERTAKE HIM; MDT Y WAS DSNDED TO 13000'. I SHOULD HAVE HAD DEN CENTER TAKE MDT Y TO 11000' BELOW SMA X OVERFLT AT 12000'; BUT WAS TOO LATE. I CHKED SEVERAL TIMES WITH CENTER BUT DIDN'T HAVE THEM PAST EACH OTHER. TOLD MDT Y I WOULD HAVE TO KEEP HIM AT 13000' UNTIL TFC PASSED THE VOR. TOLD CENTER N SECTOR THIS. THEIR PATHS CROSSED AT HAMET INTXN. THEN I WAS THINKING THAT V132 WOULD PASS W OF HORSE OUTBND (E) ABOVE AND SMA X WOULD CONTINUE NWBND. I MADE A MISTAKE AS V132 CROSSES OVER HORSE AND I DIDN'T ALLOW ENOUGH SEP. I WASN'T ON TOP OF SITUATION AND PICTURED SOMETHING THAT WASN'T RIGHT. THE C/A WENT OFF AT CENTER. WHEN THEY CALLED LATER ABOUT IT I HAD A FEELING THAT I HAD MADE A MISTAKE AND AFTER RECHKING REALIZED THAT I HAD. NO EVASIVE ACTION WAS TAKEN. NOT REALIZED UNTIL AFTER EVERYTHING HAD OCCURRED.