Protean Instrument's systems with dual phosphor detectors, ASC-950-DP and MPC-900-DP, now include a cosmic guard detector option. A guard detector should be used whenever the requirement for lower beta background exists. A guard detector lowers beta background by helping to eliminate the interference in a beta measurement caused by cosmic events or background counts. Cosmic background counts are eliminated using electronic rejection in conjunction with a cosmic (guard) detector. The guard detector is of similar construction as the primary sample detector and is mounted such that it will intercept any cosmic radiation seen by the sample detector. Any event detected in the sample detector which is coincident with an event detected in the guard detector is rejected.

The end result of the optional guard detector is a reduced beta background which increases the performance of the instrument dramatically. A system utilizing a guard detector typically reaches the required MDA in approximately half the counting time of a system without a guard detector.