Z TT eee ee RE le eal le ae pe Alam 229 a single emission step, or if there are successive steps that involve only states of ordinary lifetimes {ie., of the order of 10—*® sec). On the other hand, the effect of the same concentration of impurity can be much greater if the emission scheme involves a metastable state, or a state with a long effective lifetime because of radiation trapping, and if the ionization potential of the impurity is less than the energy of excitation of this state. Then the Jesse effect will be in com- petition with the associative ionization process; each process will yield an ion, and the two kinds of ions will be indistinguishable to an ionization chamber. Until it is proved otherwise, it is reasonable to assume that both types of radiative decay, the fast and the slow, will occur for the states involved in the associative ionization of the rare gases. After the various parts of the ionization chamber had been cleaned, a great deal of filing and polishing was required before they could be reassembled, because clean stainless steel surfaces in intimate contact tend to “gall” or “cold weld.” The design of the shutter in front of the alpha source of the large cham- ber was modified for the same reason. It had operated satisfactorily until after the chamber had been completely assembled for use with the alpha source in place. Most of the gas leaks that were found by checking the entire apparatus were of types that can be re- paired easily and permanently, eg. by resoldering a pinhole or retightening a flange. In general, leaks that occur in components that are to be stressed from _ time to time, or that may possibly be stressed, are much more serious. Modifications were made in the apparatus to reduce the probability of having recurring leaks in the shutter manipulator of the large chamber, in the flexible connection between the small chamber and the gas supply and evacuation system, and in the connection between the precision pressure gage and the gas system. Carbon films of 50 »g/cm? and 6.60 mm diameter were found to be too fragile for the intended use as a filter for atomic or aggregate particles ejected from the alpha source in the large chamber. A search revealed that films of nickel and of alumina with stopping powersimilar to that of the carbon could be obtained readily at the Laboratory. Either type of film appeared to have much better mechanical strength than the carbon type. A nickel film of 0.10» thickness was chosen because thefilter forms part of the wall of the ionization chamber, and, therefore, high electrical conductivity is desirable. At this writing, all components of the equipment have been checked, everything is ready for the final assembly and the measurement of ion currents early in the new fiseal year. REFERENCE 1. Schultz, Harvey A. Argonne National Laboratory Radiological Physies Division Annual Report, July 1967—June 1968. ANL-7489, pp. 100-101. BIO-ENVIRONMENTAL STUDIES BEHAVIOR OF FALLOUT 87Cs IN AQUATIC AND TERRESTRIAL ENVIRONMENTS P. F. Gustafson, 8. S. Brar, D. M. Nelson, and 8S. E. Muniak Fallout ™Cs in terrestrial systems was found to be highly dependent upon fallout rate, where total deposition seems more important in the aquatic case. In both situations a general downward trend has been evident since 1964-1965. The biological importance of "Cs was first noted in 1955 by Miller and Marimelli,“) who detected this radionuclide in human subjects serving as controls for the whole-body counting portion of the Argonne Radium Toxicity Program. The metabolic activity of cesium stems from its chemical similarity to potassium, a necessary nutrient element. Like potassium, cesium goes to soft tissue In man, primarily muscle, and, therefore, has a relatively short biological half- time (~100 days in adults). Over the years since 1955, 187Cs has been examined in a wide variety of media: air, precipitation, soil, plants, animals, and man. Empirical relationships between atmospheric inventory, deposition, and up- take by man have led to the construction of mathematical fallout models for both 48°Cs and Sr, These models have shown a reasonable capability for predicting the radiological consequences of nuclear debris globaliy dispersed from a stratospheric source.