sum probes are tungsten shielded to attenuate gamma background and still be user friendly for those with longevity bilt into there survey schemes. but, did eye miss sumthing? like the source material to be shielded? there's alot of different shieldings available for different radiations. these range from the simple single material to multi-ply composites. source dependent, don't you know, as different materials have different effects on different radiations. iffen cost ain't that much of a factor, comparatively speaking, you can knock the energy right out of a source with a relative thin layer of shielding if you ply it correctly. or you can load up on the gross material iffen ya gots bean counters up yer ask.