calibrated HgeO3 source was used. This use of the calibrated standards allowed a constant check to be made on the stabil- ity of the equipment (see Appendix I). ANALYSIS OF DATA The pulse height spectrum of a monoenergetic gamma ray consists of a total absorption peak and continuous "Compton" -. 4 distribution. Mev, If the energy of the gamma ray is above 1.02 pair production begins to take place and one of two secondary pesks is contributed to, according to whether one or both of the annihilation quanta escape from the crystal. The peaks all have the shape of normal distribution functions or "Gaussians" because of the statistical nature of the photoelectric effect and the secondary-emission phenomenon inherent in the operation of a photomultiplier tube. If two incident gamma energies are present, the lower is superimposed additively on the continuous Compton dis- tribution of the higher. An example of the idealized addition of three different monoenergetic gamma rays is snown in Fig. 3. As more gamma energies are added the resulting confusion becomes progressively worse. higher energies are above 1.02 Mev, (If the the presence of annihilation radiation escape peaks further complicates the spectrum and thus the analysis.) 10