202 PERKINS, THOMAS, AND NIELSEN — << ao OQ + < — z 5 0 — Fig. 3—Spectra for *Co. 0 10 | | L 20 30 40 50 60 CHANNEL NUMBER curve, By either this method or the comparison shownin the figure, the background suppression with multidimensional analysis is three to five orders of magnitude lower. The sensitivity for the measurement of a given radionuclide is dependent on the counting efficiency of the instrument used as well as on the background counting rate and the Compton interference from other radionuclides, In Fig. 5 the efficiency for total absorption by either crystal for gamma rays originating in the sample is plotted as curve 2E. The curve 2E’ gives the efficiency for two coincidence gamma rays to be totally absorbed, one each by the two detectors. This curve thus gives the probability that a coincidence event in the sample will be stored in a total absorption peak (either one of the two) in the energy—energy plane. Although the curve also is plotted for the