Neutron backscatter is a a proactive sensor that operates on the prescence of radiaton. It reads the density of the foam and ,therfore, lets you know the proper time to anti-foam.
Gamma is more re-active and works on the principal of the absence of radiation and, therefore, has difficulty in defining the type of foam. The best solution is a combination of using neutron backscatter (three to four units) on a drum with gamma level at the top (typically ten feet) for foam over prevention. This combination also allows for monitoring closer to the chord line.