Enroute Controller described an alleged loss of separation event when RADAR contact was lost between two IFR helicopters that were appropriately separated prior to the RADAR loss; the facility determination indicating that non-RADAR separation was needed immediately.

Narrative

Aircraft X & Aircraft Y where 2 of 3 CH47's en route IFR at 050. They were RADAR separated and both under RADAR control and in trail; estimating around 8-10 miles in trail. When they got Southeast of ABC VOR we lost RADAR on Aircraft Y because of limited RADAR coverage in the area. We advised the aircraft that RADAR contact was lost; advised Aircraft X that he could expect the same thing and decided to climb to 070 for better RADAR coverage. We later picked up Aircraft Y back on RADAR. I came in the next day; and before the end of the day my Supervisor advised me that Quality Assurance considered what had happened as an Operational Error because the 2 aircraft were not 'non-RADAR' separated and I needed 10 minutes or 20 miles between the two the instant Aircraft Y dropped off RADAR coverage. If this is considered an operational error; then my recommendations would be to put every single aircraft we work on that sector below 7;000 FT on a non-RADAR route and non-RADAR separated regardless if we have them on RADAR or not.