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AN EXPERIMENTAL DETERMINATION OF FIDLER SCANNING EFFICIENCY AT SPECIFIC SPEEDS

Marianno, C. M.; Higley, K. A.; Moss, S. C.; Palmer, T. S.

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Abstract— As part of mass remediation efforts across the country some radiation detection systems are now being used in conjunction with data logging and positioning system technology. These systems can be used in the scanning mode, simultaneously recording both count rate and position. Following data analysis, hot spots can be identified and remediation efforts for that particular area can commence. This technique has been used for nearly a decade and has had success in accelerating preliminary remediation work while also reducing potential clean up costs. However, little work has been completed on how the sensitivity of these detection systems are affected when used with this technology because while the intrinsic efficiency of the detector is constant, scanning efficiency can vary depending on data sampling time and scanning speed. To better understand scanning efficiency for a detector attached to such a system, a device was developed which moved soil at a constant speed underneath a Field Instrument for Detecting Low Energy Radiation (FIDLER). Count rate was measured every 2 s as a 241Am source passed under the detector at speeds ranging from ∼10 cm s−1 to 100 cm s−1. A surface source and a buried source were both examined. Experimental detection efficiency was calculated and compared to Monte Carlo generated results. For the surface source, the efficiency dropped to a value of approximately 1% at 100 cm s−1. At the same speed, the buried source had a detection efficiency of 0.1%, primarily due to attenuation of the low energy photon in the soil. It was also noted that the response time of the meter affected the scanning efficiency. With a response time set at 1 s, higher average efficiencies were recorded but with a large standard deviation from the mean. Higher response time setting had the effect of reducing the variability of the reading but also reducing efficiency.

*Bechtel NV, Suitland, MD 20752 Oregon State University, Corvallis, OR 97331.

(Manuscript received 17 August 2000;

revised manuscript received 13 May 2002, accepted 2 September 2002)

For correspondence or reprints contact: C. M. Marianno, 2969 Marsh Hawk Drive, Waldorf, MD 20603, or email at mariancm@ nv.doe.gov.

© 2003 by the Health Physics Society