IN SITU VAN FUNCTION DOE/ERSP PROCEDURENO,2 APPROVED: L DATE DRAFTED: 27 September 1977 3 October 1977 by Paul B. Dunaway (ERSP Manager) Introduction The in situ van's primary function during the Enewetak cleanup operation is the determination of plutonium concentration within the top few centimeters of soil. This is done by measuring the 60 keV gamma ray from 241am (a daughter of 241pu). Conversion factors have been established to convert the 241Am photopeak count rate data into 2414m concentration in the ground. These data are then converted into plutonium concentrations using plutonium to americium ratios established from soil sample data. Since the free path for 60 keV gamma rays in soil is approximately 2.5 cm, the detector only "sees" down through the top 3 to 5 centimeters. Other techniques must be used to look for Pu contamination buried below the top few centimeters. IL Instrum entation The in situ van itself is a Thiokol "IMP" which is a small, lightweight, tracked vehicle purchased especially for its ability to operate in soft sand. The IMP has been modified and equipped to be a fully self-contained mobile data acquisition and reduction laboratory. Power is provided by a 4 kW Onan generator mounted in front of the vehicle. The back part (rear cabin) of the IMP contains the electronics and is air conditioned to provide the required temperature and humidity controls. Gamma radiation from the ground is detected by a planar intrinsie germanium detector mounted on the end of a retractable boom located at the rear of the IMP. The detector has a surface area of 19 square centimeters, is 1.6 em thick and has a gausian resolution of 840 eV FWHM (i.e., full width, half maximum of the gausian photo peak curve) at 122 keV. In its normal operating position the detector face is 740 cm above the ground. A thin 1/2" lead collimating cone mounted on the detector limits the field of view for 60 keV gamma rays to a 21 meter diameter circle. Signals from the preamplifier (mounted on the detector) are fed inside the IMP to a 4096 multichannel analyzer. Data from the analyzer can be stored on a cassette tape for future data reduction or can be transferred into a Hewlett Packard 9831 calculator for immediate processing. A printer is available for hard copy output. TIL Operational Procedures Prior to making any measurements the detector system is calibrated to 375 eV per channel (approximately 1500 keV full scale) using a combination 60Co, 187Cs and 241Am calibration source. The calibration is checked periodically and any gain shift is corrected. (Maintaining power to the preamplifier and amplifier on a 24-hour-a-day basis has minimized gain shift problems.) The IMP is moved from location to location with the boom fully retracted and the detector securely fastened. At a measurement point the boom is extended to its full length and then inclined at an angle of 20 degrees away from the IMP. After completing the measurement(a typical acquisition time of 900 seconds) the boom is retracted and the detector secured for movement to the next measurement location, The total time required for each measurement sequenceis typically 25 to 30 minutes. IV. Data Reduction While the detector is being secured and the IMP moves to the next location, data from the previous measurement is normally processed on the HP-9831 calculator. The calculator has several software options available. The data from any portion of the spectrum can be printed or plotted-normally the first 200 channels are printed and the 241Am, 137Cs and 60Co portions of the spectrum plotted out. automatic peak search routine identifies the 24lam, 137Cs and 60Co photopeaks spectrum, and then calculates the concentration (in pCi/g) for each isotope. An within the The entire spectrum may be plotted and a large number of isotopes identified and quantified using another software routine at the A-2-1