A detailed description of the soil sample screening procedures is given in Tech Note 6. Briefly, the soil sample was placed in a petri dish, and the dish placed in a holder which maintained a distance of about 3 em from the top of the dish to the detector entrance plane. (In the counting laboratory the same distance was maintained.) The sample was counted for five minutes. An initial calibration was performed using two samples previously measured in the Raditation Counting Lab. A calibration factor of approximately 10 pCi per count/5 minutes, or 3,000 pCi/eps was obtained. A screening level of 20 counts per 5 minutes was adopted, corresponding to approximately 1.5 to 2 pCi/g of soil (typical samples were around 100 g). Counts due to 37Cs and ©°Co were noted, but no attempt made to quantify them. Tech Note 6.1 presents a statistical analysis comparing the IMP soil sample screening results to Radiation Lab gamma counting. The mean ratio of IMP/LAB is 1.05 + 0.35. Linear regression gives the equation IMP = 0.92 - LAB + 2.72, with a coefficient of determination (r“) of 0.88. This comparison was based on measurements made of the same petri dish samples. The good agreement is not surprising, as the same type detector was used for both IMP and LAB counting. To prepare for the Aomon Crypt excision project, further calibration was performed using a series of petri dishes standardized by the Radiation Counting Lab, and the Radiation Lab standards. To account for the effective area factor of various detectors, the technique finally adopted was to input to the soil sample measurement program the average pCi/g measured using a standard petri dish calibration source. The final program also allowed the input of sample percent moisture, so that pCi/g dry soil could be ealeulated (corresponding to the value determined by Radiation Lab analysis). Approximate calibration factors were also determined for 155zy, 137¢s, and 60¢Co, For the Aomon Crypt core drill samples, an analysis similar to that of Tech Note 6.1 was conducted. The IMP sample was a filtered aliquot from a sample can, with an assigned 23 percent by weight moisture content. If the IMP screening gave a value for 24. am greater than 25 pCi/g, the Radiation Lab dried the entire core drill sample and then prepared an aliquot for laboratory gamma counting. For 95 pairs of data (each pair taken from the same core sample) the mean ratio of IMP/LABis 1.23 + 0.54. Linear regression gives the equation IMP = 0.95 - LAB - 3.8, with a coefficient of determination (r2) of 0.96. The greater standard deviation in the ratio of IMP/Lab result is probably a reflection of the different aliquots counted and the difference between the assumed constant moisture content of 23 percent and the actual moisture content, which varied from 14 to 49 percent. 3.4 OPERATIONAL EXPERIENCE 3.4.1 IMP Operations The IMP performs in situ gamma ray spectroscopy to measure 241Am and other gamma emitters and thus functions as a mobile laboratory. Experience in operation of a sophisticated system such as the IMP under remote tropical conditions is limited. Thus this section has been included to discuss the operational problems and their solutions. It was desired to have two IMPs operational at all times. To achieve this, three complete IMPs were provided. It was also desired to have two operating detectors for two of these IMPs, and a third available on-atoll in the Radiation Laboratory for soil sample spectroscopy. Four detectors were procured to satisfy these requirements. The desired mode of operation was not always achieved, however, because of detector repair requirements. Operating conditions for the IMPs included high temperature, high humidity, and salt spray. Depending on the season, tropical rain storms and high winds were often encountered. During the project, several tropical storms caused major damage to the atoll. During these times, the IMPs were secured inside the IMP shed and the detectors lashed inside the IMP cab. Transportation between islands was by military landing eraft or amphibious LARCs. The landing eraft ramp angle was usually about 30 degrees. The LARC ramp angle ranged up to 60 degrees. Considerable shock and vibration was inherent in any boat operation, and sea conditions sometimes made a fast, rough embarkation mandatory. 104