Joab. geie. oy at Bia Fa ee. 4.2 PHYSICAL AND CHEMICAL NATURE OF THE FALL-OUT Preshot planning indicated that either liquid or dry fall-out, and probably a mixture of poth, would occur. Upon examination of the collected material, especially that which was protected in the differential fall-out collectors, it was concluded that the fall-out was primarily particulate in a dry or semidry state. Several rain showers, apparently over small areas, occurred at shot time and during the fall-out period and caused a fall-out mixture of rain and particulate. This type of fall-out was probably the source of contamination of the survey helicopter (No, 28). hr au ‘ pithy “th wf ong BNE beads on, 4.2.1 Physical Description of Particulate The particulate, as preserved in its state of arrival, was white and either sphericalor irregular in shape (Fig. 4.2); many particles were flaky. Measurements on the larger particles showed their density to be between 1 and 3 g/cu cm. None of these particles were hollow. The floats which constituted the lagoon stations had many particles attached to their surfaces. These particles were also white, but their physical nature was entirely different from that of particles collected in the differential fall-out collectors. The particles were hemispherical and very firmly attached to the float surfaces. Many were hollow, and in some cases their internal structure consisted of a series of concentric shells (Figs. 4.3 and 4.4). These particles were located everywhereon the life floats, including horizontal and vertical surfaces, horizontal surfaces below the water line, the undersides of the horizontal surfaces, and even on the manila line running under water between the life float and its anchoring drum. Figures 4.5 and 4.6 show sections of the decking from the life float located at Station 540.20. The top deck was disturbed by recovery personnel and is not representative of the particle distribution, yet the great accumulation of particles was obvious on the protected second deck even though this deck was spaced only 1.6 in. below the first and completely covered by it. The particles at- tached to the underside of the first deck are not shown. No explanation is offered for the ability of these particles to seek such well-protected surfaces for deposition; however, it is suggested that, since the surfaces were intermittently wet from sea wash, the wetting aided in retaining the particles. 4.2.2 Chemical Composition of the Particulate Spectrographic analysis of the fall-out particulate is shown in Table 4.1. This fall-out was taken from the float decking and had been exposed to sea water, The main cation constituents are calcium and magnesium. Table 4.2 shows the results of X-ray-diffraction analysis of ai seven samples from the differential fall-out collectors; these samples were protected from the time of their arrival and consequently were not exposed to sea water or the atmosphere. There is a lack of magnesium here, with the exception of the sample taken at Station 540.14. The particles which were collected in the differential fall-out collectors contained no hydrated calcium sulfate; however, a petrographic analysis of the particles taken from the float sections shows positive evidence of hydrated calcium sulfate. The presence of this material, as well as the preponderance of hollow and quasi-hollow particles on the float decking and their tenacious adherence to the decking, is accounted for by the following theory.* It is reasonable to suppose that the fall-out particles originated as calcium oxide, rapidly changing to calcium hydroxide with the formation of a very thin layer of calcium carbonate on their outer surfaces, Generally the radioactivity was irregularly distributed throughout the particles. In some cases there was a tendency for the activity to be concentrated near the surface of the particle. *Developed by Charles E. Adams, U. S. Naval Radiological Defense Laboratory (NRDL). This work will be published at a later date. 35