An ultralow background multiphoton detectors device for radioisotopes has a background of about one count per day. Two opposed gamma and X-ray photon detectors each preferably include a scintillation crystal and a photomultiplier tube. Sandwiched between the detectors are a separator and a sample holder for a sample labelled with a radioisotope such as I. sup.125. The detectors convert emissions from the radioisotope into electric output pulses which are then subjected to signal processing for pulse shape analysis, discrimination of coincident from non- coincident events, and quantification of the radioisotope. Detector materials and components are selected to minimize background, and are shielded from external radiation. The device may further be constructed to resolve and provide images of two dimensional sample arrays.
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