u(E)az = energy dependent absorption coefficient of Xaz aluminun = thickness of aluminum which gamra rays must penetrate u(E)yaz = energy dependent absorption coefficient of NaI z if = effective solid angle the source subtends at the crystal XNaI = thickness of NaI crystal Ane I = the area of the photo=peak and A; = the total area of the pulse height distribution, then the number of cownts Npe in the photo-peak will be: E “al EB ‘ z . t It is assumed that Gis independent of energy which is only true to a good approximation. The absorption in NaI at low energies is mich 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 Theeu de~ tection efficiency. Use was madeo3f the 6 erimental Ape As a partial check, sources of Na? and Cgl37 were counFeaaae a Gel counter so as to get their relative source strengths. From the known ‘decay schemes the number of gamma rays per beta were determined 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 isotopese Products of the slow neutron fission of a U*35 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.éb report discusses the mthods of obtaining - Zr95-mH95, ogi! > and Cel4é_p,144 standards from the thermal neutron fission of U<3 93 it also discusses the Zr and Ce calibration proce= dure.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 gamma rays of Z2r?5 are 730 kev, those of Cel4l are 145 kev, and_Cel44 are 134 kev. The Ce gamm rays were pre~ dominantly due to Cel4) as it has a 33-day half lifee,compared with 282 days for Cel44,. To determine the amount of Cel4l present made of the known U<25 fission yields of 5.7 percent for Ce percent for Cel44, 19/ usé was 1 and 5.3 Experimentally the intensity of a given gamma ray was deter- mined through the area of its -‘photo-peak. The abscissa of the curve is in volts and the ordinate in counts per minute so that the area of the photo~peak is in the units of count-volts per minute. To obtain the correction factor for converting count-volts per minute to gamma 80