essa yey oo fC etgne, Ce mneadBed ayy, ‘ r a RR SERT:CTED Te cRoEs eeAT S ROR ( 0018933 of this nuclide to the number of atoms of a reference substance (usually Mo?) in the sample divided by the same ratio for the products of thermal of Ue35, neutron fission/ Atoms which do not separate from the reference sub- . stance have R-values appropriate for the type of detonation, while enrichment or depletion are manifested by positive or negative deviations from the characteristic value, Knowledge of the causes and mechanism of fractionation is still largely incomplete at the present time, One effect that seems to be indi- cated by the available data may occur in the isobaric chains near mass numbers 90 and 140 which contain rare gas nuclides as prominent chain members, These have half-lives and independent fission yields such that they comprise a considerable fraction of the total chain yield during the period when the environmental material is condensing. If the rare gas atoms which collide with the liquid drops of environmental material are not held by strong forces, as appears probable, the particles formed at this stage will be depleted in the nuclide chains in question, A variety of types of particles have been observed in the local fallout at previous test series (References 8,9,10,11, 12, 13). shots in the Pacific they have been mainly of three kinds: spherical solids and fragile agglomerated flakes, For land surface irregular grains, The grains were not, in general, uniform throughout, but consisted of leyers or shells of calcium 17 S @S6SR2EPRESTRBRT A Ri geecrerAcSo 2a 2].fy