SHORT LIVED FISSION PRODUCT GAMMA RADIATION By W. Zopen and T. A. Love Qak Ridge National Laboratory, Oak Ridge, Tennessee Fission-product: gamma-rays are defined as those gamma-rays emitted byfission-product. nuclei, either primary or their daughters, at times measurably later than the fission event. Most of the available information sbout these gamme-rays is the result of radiochemical experiments which, by their nature, tend to discriminate against short-lived activity. As one is generally interested in the gross fissionproduct gamma-ray spectrum one has had to construct such a spectrum from the known emitters since experimental evidence was available only for times in excess of 17 hours efter the fission event..? One would expect this synthetic spectrum to be in error at short times after fission, up to perhaps an hour,due to lack of information on nuclides of short half-life. Experiments were therefore undertaken at ORNIL to measure the gross fission-product gamma-ray spectrum at short times, ik e., starting at about | second, after fission. This paper will present preliminaryresults obtained so far. To investigate the energy spectrum and time behavior of the gross fission-product gammaray mixture we exposed small samples of hizhly enriched uranium for short periods in the ORNL Graphite Reactor and withdrew them rapidly to @ position in front of the spectrometer. Sample sizes varied from about 2 mg to about 32 mg and exposure times varied from about 0.7 second to about 64 seconds. The experimental arrangement allowed us to measure either the time behavior of different energy groups, or detailed energyspectra. The experimental results ere summarized in 'D, H. Peirson, AERE-EL/R-155 (Nov. 8, 1954). 4Germagnoli and Mongini, Energta Nucteare §, 32 (1858). Figures 1, 2, and 3 for the two phases of the experiments. It should be emphasized that these results are preliminary only, based on a rather crude analysis of the data which is currently being refined. The time behavior of 6 energy groups, cov~ ering the range from 0.28 Mev to 5.0 Mav, is shown in Figure 1 for times after fission between1.25 seconds and 1,600 seconds. These curves were integrated to obtain the number of photons/fission and the energy/fission car~ ried off by fission-product gamma-rays in the time range and energy range mentioned above. The results are shown in Table 1. Taste 1 Photon energy range Numberof CM oy phe bons/fis- 0.28-0.51.....---. 0.51-1.12 1.12-1.62. 1,62-2,30. 2.3-3.5_ ~ 3.6~5.0..-.-----.- 0. 1, . - Total... ---- sion 747 225 452 235 198 067 Average energy "] Energy/fis- sion ‘Wiow) 0. 395 » 815 1.37 1. 96 2.9 4, 25 0, + 2,924 |__...-.--- 295 998 619 461 875 285 3, 233 Detailed energy spectra taken at 10 different times after fission are presented in Figures 2 and 3. The peaks shown represent merely an attempt by the authors to indicate some of the fine structure. No errors have been computed on the experimental points at this time, so that this fine structure is still somewhat uncertain. It should be noted, however, that peaks tend to appear on successive curves, thus lending some credence to their existence. The 26