FALLOUT MEASUREMENTS BY TOTAL-ABSORPTION GAMMA-RAY SPECTROSCOPY ROBERT S. FOOTE Texas Instruments Incorporated, Dallas, Texas ABSTRACT The growth and the decay of gamma-ray-producing radionuclides in the field near Dallas, Tex., have been measured since Dec, 22, 1962. All data have been resolved to provide the effective surface amounts of the fission-produced radionuclides as well as the natural gamma-ray emitters. A discussion of the data-reduction procedure is included which shows the capability of the method to account for the entire field gamma-ray spectrum. Initial fallout measurements of fission debris from the 1964 test by Communist China are included. METHODS AND MEASUREMENTS Gamma-ray-producing radionuclides in the field near Dallas, Tex., have been determined and monitored by the use of gamma-ray spectroscopy since Dec. 22, 1962. The objectives of these measurements have been (1) to demonstrate the exactness and consistency with which gamma-ray spectral data can be resolved from field data, (2) to op- timize data-handling techniques, and (3) to follow the change of nuclear fallout debris in the field. A large, near-total gamma-ray-absorption NalI(T1l) crystal that is 11/4, in. in diameter and 4 in. thick and multichannel pulse-height- analysis equipment consisting of a 400-channel analyzer operating in the 200-channel mode have been used to measure the gamma-ray energy spectrum of the terrestrial gamma radiation. A 40-min multichatinel pulse-height-analyzer live time was the counting-time period 183