Devices, methods and related systems are described for measuring a property of a fluid including density in a subterranean environment. A device includes a pressure housing having one or more window formed in the pressure housing, and a flow device arranged in the pressure housing for the fluid to flow through the flow device. Further, a radiation source mounted within the pressure housing approximate a first source window configured to generate particles into the fluid. A detector supported by the pressure housing and positioned approximate a first detector window of the one or more window, the first detector window located between the detector and the flow device. The detector can be a solid state beta particle detector with a wide band gap such as the diamond detector, and the radiation source can be a beta particle source such as a strontium 90 source.