204 i RADIOLOGICAL CLEANUP OF ENEWETAK ATOLL Since plutonium is an alpha emitter, and since thereis no efficient Way to detect and measure alpha contamination in soil over large areas, the IMP system was designed to detect gamma radiation from Am-24l—a daughter product of plutonium—in the soil. The detection was done by meansofa planarintrinsic detector made of germanium. The detector was suspended approximately 6 meters above the surface of the earth using a retractable boom mounted on the rear of the van. The germanium detector was cooledby liquid nitrogen. Other equipment on board the IMP included a high voltage power supply, amplifier, analyzer, calculator, printer, and tape recorder. Sensitive electronics equipment wasinstalled in an enclosed space in which temperature control was maintained by a small, i : ; t ; self-contained, air-conditioning system mounted on the IMP. Gamma spectra from the detector were analyzed and recorded. The average concentration of Am-241 in the top 3 centimeters of soil within the detector’s field of view (a 2l-meter diametercircle) was determined from the 60 kilo-electron Volt (keV) readings. Radiation at 60 keV is the most prominent line of the spectrum of americium andis, therefore, the best indicator of intensity of radiation and quantity of americium. At a few selected points where IMP readings were made, soil samples were taken for analysis in the Enewetak Radiation Laboratory. The concentrationsof ii Pu-238, -239, and -240 and of Am-241 were determined from thesesoil samples and the ratios of plutonium to americium derived. Conversion factors then permitted estimates of plutonium and total transuranic concentrations in the soil to be calculated from the americium measurements madeby the IMP.5! To survey a large area, such as oneoftheislands, the IMP traveled from point to point along a surveyed grid, making a measurement at each grid intersection. Soil samples were taken at intersection points and analyzed for plutonium-americium ratio. Data from the entire area werestatistically analyzed, and lines (isopleths) were drawn on maps throughpoints having the same numerical values of average concentrations of either plutonium -or total transuranics. The isopleths were based on the 70 percent upper bound; i.e., the probability is at least 0.7 that the true average concentration is no greater than the upper bound. After soil was removed, the process was repeated to determine the concentration values of the newly exposed surfaces. Figure 4-ll is a schematic diag-am of the measuring-analyzing-recording system in operation. The IMP system had the advantages of being mobile and of providing quick answers to questions concerning the plutonium concentrations in a particular area. Once a ratio between americium and plutonium or total transuranic elements had been established for a large area, the only time required to obtain a concentration was that neededto reach the point being