An ultralow background multiphoton detector device for radioisotopes (20) has a background of about one count per day and can detect one attomole of material. Two opposed gamma and X-ray photon detectors each preferably include a scintillation crystal (22) and a photomultiplier tube (26). Sandwiched between the detectors are a separator (32) and a sample holder (34) for a sample (36) labelled with a radioisotope such as I125. Each detector converts emissions from the radioisotope into electric output pulses in a base (30) which pulses 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 in a composite shield (55). Detectors operating on different principes and sample holders of different configurations may be used. The device may further be constructed to resolve and provide images of two-dimensional sample arrays.
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