u(E)ay = energy dependent absorption coefficient of alumi nun = thickness of aluminum which gamma rays must Xaz . penetrate ulE yar = energy dependent absorption coefficient of g if XNaI Ane NaI = effective solid angle the sovrce subtends at the crystal = thickness of NaI crystal = the area of the photo~peak and Ay = the total area of the pulse height distribution, then the number of counts Noe in ths photo-peak will be: Noe © Ape Noe eee. vera (3.23) Ay 4 It is assumed that J is independent of energy which is only true to a good approximation. The absorption in NaI at low energies is much greater than at high energies so that the gamma rays are ab~ sorbed largely near the incident face of the crystal. This results in an increased solid angle over that for the higher energies. Tests indicate that this effect is of little importance in the analysis of the present data. The above formula has been used to determine the Thecur detection efficiency. Use was made gf the experimental A CUrve es As a partial check, sources of Na? and csi37 were aeed na G-M counter so as to get their relative source strengths. From the known decay schemes the number of gamma rays per beta were Ccstermined and an efficiency curve plotted which was in excellent agreement with the above cirvee Absolute calibration of the spectrometer was attempted in order to perform absolute analysis for various isotopes. Products of the slew neutron fission of a U235 sample that had been recently ir- radiated at Brookhaven National Laboratory were available. The neu- tron flux was known and it was possible to calculate the yield of the various isotopes. The Project 2.6baigPort discusses the methods of obtaining Zr95—_mb95, Ciil, and Cel44—pr144 standards from the thermal neutron fission of U3 2; it also discusses the Zr and Ce calibration procedure.12/ The samples were mounted under the same conditions as the fallout samples (described in Sec. 3.2) and gamma spectra were taken for the known sources. The gamm rays of 2r%5 are 730 kev, those of Cel41 are 145 kev, and Cel44 are 134 kev. The Ce gemmrays werepre- dominantly due to Cel41 as it has a 33-day half life compared with 282 days for Cel44,. To determine the amount of Cel4l vresent, use wag made of the known U225 fission yields of 5.7 percent for Ce percent for Cel44, 19/ 1 and 5.3 Experimentally the intensity of a given gamma ray was deter- mined through the area cf its photo=peake. The abscissa of the curve is in volts and the ordinate in counts per minute so that the area of the photc~peak is in the units of count-volts per minute. To obtain the correction factor for converting count-wolts per itinute to gamma 80