200 PERKINS, THOMAS, AND NIELSEN ~ Nol(Tl} ANTICOINCIDENCE SHIELD . [~~ SAMPLE LOCATION h Nal(Tl) PRINCIPAL DETECTORS (4 IN. THICK AND 6 IN. IN DIAMETER) PURE Nal CRYSTAL LIGHT PIPE PHOTOTUBES Fig. 1—Detection system for multidimensional gamma-ray spectrometer. detectors, the event is stored on the corresponding X or Y axis of the memory. When two photons are emitted simultaneously, each interacting with a separate detector, the event is stored in an energyenergy plane at a point uniquely characteristic of the two energies. The spectra of “Cu and Co illustrate (Figs. 2 and 3) the manner of storage. The normal spectra of these radionuclides as seen by one of the detectors are shown for comparison. With such a detection system, the unique decay characteristic of many radionuclides is readily resolvable. This permits a far greater sensitivity for detection and direct measurement of many minor radioactive constituents. In addi- tion, a much lower backgroundis obtained. , In Fig. 4 the background of a 4-in.-thick 6-in,-diameter singlecrystal detector without a light pipe or anticoincidence shielding is compared with that of the two-crystal multidimensional detector sys- tem in which the two detectors are separated by a 1-in.-thick lead