Methods and related systems are described for use for making subterranean nuclear measurements. The system can include a plurality of elongated scintillator members each generating optical signals in response to ionizing radiation. Optical detector units can be optically coupled to at least one end of each elongated scintillator member so as to detect optical signals from each elongated scintillator member. The system can be suitable for permanent or semi-permanent deployment downhole. For example, the system can operate for more than six months in a subterranean deployment measuring cosmic radiation. The system can be suited to monitor density changes in subterranean regions of interest, for example, density changes brought about by steam injection as part of a steam assisted gravity drainage operation.