Gamma detection during this quarter was accomplished with four high purity intrinsic germanium detectors. These detectors are each operated in a similar fashion. 

The analog signals generated by the detectors are passed to preamplifiers which enhance the signal-to-noise ratio and provide impedance matching before being passed to amplifiers. The amplifiers supply preamplifier power, amplify the signal from the preamplifiers, perform pulse shaping, and provide the signal to the analog-to-digital converter(ADC). Data are displayed and analyzed on a microcomputer using commercial software. The acquisition and analysis system is a Pentium PC and Canberra Model S500 Genie 2000 software running under the Windows NT operating system. Nuclear Instrumentation Module (NIM) equipment and Canberra Model 556 Acquisition Interface Modules (AIM) are used to interface the detectors to the computer. This multi-tasking system controls and acquires data from all detectors simultaneously and all data are analyzed using the same software system. The software provides background subtraction, efficiency correction, nuclide identification, interference correction, weighted mean activity (including negative values), uncertainty, and minimum detectable activity. The detectors are calibrated in the energy range from 88 to 1,836 keV at about 0.25 keV per channel. Some specifications of the detectors are given in Table.

Analysis of Charcoal Cartridges (Air Iodine)

Up to nine charcoal cartridges are screened simultaneously on a gamma spectrometer. Cartridges are placed in a Marinelli beaker in a reproducible fashion. The detector is calibrated for the least efficient position. Should ¹³¹I be observed in the initial counting and subsequent recount of a set, a screening procedure is employed to identify the sample or samples which demonstrate enhanced activity. The sample blank used for background subtraction appropriate to charcoal screening consists of 9 unused cartridges.

Analysis of Air Particulate Filter Composite

The 13-airborne particulate filters collected during the calendar quarter for each field station are aligned one in front of another, placed in a plastic petri dish, and then counted in a special calibrated geometry in which the composited filters are suspended about 2.54-cm above the detector surfaces.

Analysis of Water

A 500-mL or 1,000-mL Marinelli beaker is filled with the corresponding aliquot of sample. All samples are analyzed for 72,000 seconds (20-hours). To analyze these samples gamma spectrometers are calibrated using a 1.0 g/mL, 500-mL Marinelli beaker and a 1,000-mL Marinelli beaker mixed gamma standard prepared by a commercial vendor and traceable to NIST.

Analysis of Tissue Samples

Several different types of animal tissue samples are analyzed for the S.M. Stoller Corporation by the EAL. These include muscle, liver, and thyroid tissues. Typically, liver and muscle samples are of large enough quantity to fill a 500-mL Marinelli beaker. Hence, liver and muscle samples are counted while in a 500-mL Marinelli beaker geometry. To analyze these samples, gamma spectrometers are calibrated using a 1.0 g/mL, 500-mL Marinelli beaker mixed gamma standard prepared by a commercial vendor and traceable to NIST. Thyroid tissue samples are much smaller. Typically thyroid samples have a mass between 5 and 15 grams. The closest approximation to this geometry is that of a filter paper mounted in a petri dish which is placed near the surface of the detector. The filter paper used is a sealed mixed gamma standard prepared by a commercial vendor and traceable to NIST. All samples are analyzed for 72,000 seconds (20-hours).

Analysis of Milk Samples

Milk samples are provided in ample quantity to fill a 1,000-mL Marinelli beaker. All milk samples are prepared by removing a 1.0 ± 0.040-kg aliquot and placing it into a 1,000-mL Marinelli beaker for analysis. A simulated milk background was prepared by the EAL in order to obtain a consistent K-40 background activity. This was prepared by combining 2.25 g KCl with 1000-mL distilled water (pH=8.47, conductivity=0.0977 mS/cm). The simulated milk background has a pH of 5.88 and a conductivity of 267 mS/cm. The resulting K-40 activity of approximately 1,000 pCi is representative of the average K-40 activity observed in 1,000-mL of commercially purchased milk. To analyze these samples, gamma spectrometers are calibrated using a 1.0 g/mL, 1,000-mL Marinelli beaker mixed gamma standard prepared by a commercial vendor and traceable to NIST. All samples analyzed on Gamma Spectroscopy systems #1 thru #4 are analyzed for 72,000 seconds (20-hours). Count times for Gamma Spectroscopy systems #3 and #4 may change in the future due to MDA level requirements. As seen in Figure A.2.5 Gamma Spectroscopy system #3 was taken off-line and temperature cycled in an attempt to increase resolution. At the time of this report, the system was still undergoing service. See section 6.1 for further comment on this issue.

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Last modified: June 26, 2001