A detector system measures radioactive material. A fluid path receives at least one aliquot of radiopharmaceutical. The fluid path locates the aliquot within a positioner formed with a concave configuration. A detector is located at an axial distance from the concave surface and determines the level of radioactivity of the aliquot. Alternatively, the fluid path may be less concave and a variable attenuator may be placed between the fluid path and detector. The variable attenuator may have a concavity that is based on the concavity of the fluid path so that the detector's ability to read the radioactivity is optimized. A method for forming an aliquot of radiopharmaceutical in a concave fluid passage. Positioning a detector located a distance from the concave surface to optimize reading spectral energy of the aliquot and activity is determining activity regardless of the position of the aliquot in the passage.