A detailed field-ion microscope (FIM) study has been made of the vacancy structure of a (220) platelet created by a single 30 keV W ion in a platinum-U.O at.$ gold alloy; the specimen was maintained at kO K (below substage II-) during the-9 in-situ irradiation at ^2x10 torr. Prior to the pulsed field-evaporation dissection of the specimen at Uo K it was warmed isochronally to 100 K (above substage II). The (220) platelet was found to consist of 31 vacant lattice sites, lying in four (220) planes, and clustered in a disc-shaped region which is "^20 A in diameter. If only first nearest-neighbor lattice sites are considered then the distribution of cluster sizes is as follows: (l) two monovacancies; and (2) one jumbo vacancy cluster containing 29 vacancies. The range of the vacancy concentration within the (220) vacancy platelet is ^35 to ^4 at.$. Employing the modified Kinchin-Pease equation it was calculated that the displacement efficiency (K) for this platelet is 0.12. It is suggested that the prismatic dislocation loops lying on {220} type planes, observed by transmission electron microscopy, in ion or fast-neutron irradiated platinum can form as a result of the direct collapse of {220} type vacancy platelets.