text: In attempt to reduce the high nuisance alarm rates of first generation RPMs, the Advanced Spectroscopic Portal (ASP) program was called into life. Some of the portal monitors evaluated for this purposes are based on NaI(Tl) scintillating crystals. These devices, having better energy resolution than PVT, were supposed to reduce nuisance alarm rates by distinguishing threats from benign sources on the basis of the detected gamma radiation spectra. ASPs based on NaI(Tl) had a cost several times that of first generation RPMs. To date, NaI(Tl) based ASPs have not been able to demonstrate significantly better performance than PVT based RPMs.
  The ASP program was canceled in 2011 after continued problems, including a high rate of false positives and difficulty maintaining stable operation.