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A comparison between gross and net count methods when searching for orphan radioactive sources
Thomas Hjerpe1, Christer Samuelsson
1Department of Radiation Physics, The Jubileum Institute, Lund University, University Hospital, SE-221 85 Lund, Sweden. thomas.hjerpe@radfys.lu.se
Health Physics
|January 30, 2003
Summary
Researchers evaluated mobile radiation detection methods for finding orphan radioactive sources. Subtracting natural background interference improved detection distance, but over-subtraction reduced it, highlighting optimal strategies for environmental monitoring.
Area of Science:
- Environmental Radiation Monitoring
- Nuclear Detection Systems
- Applied Geophysics
Background:
- Orphan radioactive sources pose environmental and security risks.
- Mobile detector systems are crucial for rapid environmental surveying.
- Effective detection relies on distinguishing artificial sources from natural background radiation.
Purpose of the Study:
- To evaluate different numerical strategies for detecting orphan radioactive sources using a mobile system.
- To compare a moving average (MB) method with background subtraction techniques.
- To determine the critical distance (CD) for detecting a 137Cs source under various conditions.
Main Methods:
- Utilized a carborne sodium iodide (thallium-activated) [NaI(Tl)] spectrometric system.
- Employed a 2 GBq Cesium-137 (137Cs) source as a test source.
- Compared a gross count rate moving average method against three background subtraction strategies.
Main Results:
- The critical distance (CD) for detecting the 137Cs source improved from 105 m to 130 m when Potassium-40 (40K) interference was subtracted.
- Subtracting only 40K interference enhanced the detection capability.
- Subtracting contributions from other natural gamma emitters reduced the ability to detect the 137Cs source.
Conclusions:
- Selective background subtraction, specifically targeting known interfering isotopes like 40K, can improve orphan source detection.
- Over-subtraction of natural background radiation can detrimentally affect the detection of artificial radioactive sources.
- Optimized numerical strategies are essential for effective mobile radiation source search operations.