26 FREILING, CROCKER, AND ADAMS Delayed-neutron emission has accounted for only seven cases— bromine in mass-chains 87, 88, 89, and 90 and iodine in chains 137, 138, and 139. The branching fractions for these decays are taken from the National Science Foundation—National Research Council Nuclear Data Sheets. A few other decays of this kind are supposed to occur among the fission products, but reliable data arenot available. It should be remembered that at least a few long-accepted halflife values may still be subject to rather drastic revision. A case in ‘point is the 50-min state of '!"Cd reported as long ago as 1940. It now appears’® that this half-life is closer to 3 hr. Serious revision of currently accepted experimental values for branching fractions is even more probable. In view of the uncertainties so far discussed, it is very difficult to estimate a time point after fission at which one could begin to take the details of the computer abundance lists seriously. In view of the fact that several of the crudely estimated half-lives are as long as a few minutes, we are inclined to be skeptical of abundanceslisted for times earlier than 15 to 30 min, Even at these times, the pertinent in- put data for the chain involved should be carefully scrutinized before an abundance figure is accepted. Gamma-photon Emission Data The program converts the abundance list to an activity list (by reference to the input half-lives). The activities are then considered one by one along with input gamma-photon emission data for the appropriate nuclide. These data speci: by energy all gamma and X-ray photons emitted by the nuclide and provide the number of each emitted per disintegration. This permits calculation of photon-emission rates, which may be stored by energy increments and eventually output as gamma-emission spectra or which may be converted to dose-rate or ionization-rate contributions for storage and accumulation. The gamma-photon emission data used in the program”? have been listed in detail in USNRDL-R&L-143. This list contains all data of reasonable reliability currently available for fission-product nuclides. The data cover about 180 of a total of about 690 nuclides contained in the program. As might be expected, most of the nuclides for which gamma-emission data are not available have relatively short halflives; many of them are very short indeed. In fact, data are available for all with half-lives greater than 10 hr, with the exception of about 25-year ‘'/sn. (The amount of this nuclide produced in uranium and plutonium fission is exceedingly small.) For nuclides with shorter half-lives, the situation is summarized in Table 2. However, an inspection of the activity predictions indicates that the missing gamma-~ ray data for nuclides with half-lives greater than 10 min are not im- portant since none of these nuclides ever contributes more than 1 or