An apparatus, system and method are disclosed for estimating a property of a fluid downhole, the apparatus including but not limited to a carrier that is conveyable in a borehole; a test cell carried by the carrier for capturing a fluid downhole; a fluid channel immersed in the fluid downhole, the fluid channel having a first wall and a second wall, wherein the first wall faces the second wall; at least on charged particle source placed at location along the first wall of the fluid channel; and at least one charged particle detector placed at a location along the second wall of the fluid channel, wherein the at least one radioactive detector is in positioned to be in particle communication with the at least one of the charged particle source.