38.1 Gamma Counting Equipment and Techniques The sensing element of the scintillation spectrometer was a 1+ ine-diamter, l-in. thick cylindrical crystal of NaI(Tl). The crystal was mounted with a powdered MgO reflector on the photo-cathode of a selected RCA 5819 photomultiplier tube. The voltage supplying the photomultiplier was well stabilized, being kent constant to a few tenths ver cent during a particular run. The output of the photo= multiplier was coupled to an Atomic 204-3 pulse amplifier. The linear high level output of the amplifier went to an Atomic 510 single channel pulse height analyzer, the outpout of which was recorded with a standard-type scaler. Background was reduced by using a 4 in. lead shield surrounding the NaI(T1l) crystal. Several grans of fallout, consisting largely of coral—like material, made up the sample to be analyzed. The material was ground to a powder and for the first series of runs a 0.0246 gram sample was used, The sample was placed about 9/16 in. from the face of the NaI(Tl) crystal. There was 1/8 in. of aluminum between the source and crystal to stop the high-energy beta rays coming from some of the decaying isotopes. A channel width of one volt was chosen for the pulse height analyzer as a compromise between good statistics and resolution. Data were obtained by moving the pulse height analyzer in ona volt steps over the whole pulse height spectrum, counting for a given length of time at each point. Before each run the pulse height dial of the spectrometer was calibrated for gnerey using the 0.511 Mev annihilation radiation from the decay of Na2* positrons. Data on each run were taken for the above ersrgy scale. In addition, the amplifier gain was increased by a factor of 4 and the spectrum rerun to examine the low energy end of the spectrum. The pulse height spectrum ob- tained 10 days after shot tim: is shown in Figs. 3624 and 325. 3e3e2 Procedure Used in Analysis ot Curves Analysis of th> experimental data is based on four facts: (i) the gamma decay schemes of mst isotopes are known with a reason- able degree of accuracy, (2) the shape of the spsctrum for any one isotope remains unchanged for varying amounts of the isotope, (3) the photo-peak of the highest energy gamma ina spectrum is not affected by any other reaction in the crystal, (.,) the area of a photo-peak is a valid measure of the anount of the gamma producing that peak. Additional aids in the assignment of specific photo-peaks to individual isotopes were found in decay data fromthe sample spectra, and the informtion covering the major contributing fission products at any given time after the fission of U235,15/ The photo~peals and part of the Compton distribution of the 1.6 Mev gamm ray of La appeared to be uncontaminated by other gamma rays. Lal40 is the 40 hr daughter of 12.8 day Bal40, According to the table of isotopes,17/ these two isotopes have peak gamma rays at 2651 and 3.00 Mev. The 1.6 Mev photo-peak suggested the possibility of normalizing the known scintillation counter spectrum of Bal40 and La!40 to that of the fallout sample. Then,by point—by-point subtrac- tion of the spectrum,one would remove the effect of the Bal40 and 76