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- Kock, Peder, et al.
(författare)
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A real-time statistical alarm method for mobile gamma spectrometry-Combining counts of pulses with spectral distribution of pulses
- 2012
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Ingår i: Nuclear Instruments & Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment. - : Elsevier BV. - 0167-5087 .- 0168-9002. ; 681, s. 55-60
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Tidskriftsartikel (refereegranskat)abstract
- A well-founded decision needs to take into account as much information from a sample as possible. In gamma spectrometry, the number of photons and their energy are the two quantities readily accessible to the physicist and both should be used in order to increase the power of a statistical test. While the problem of counts of pulses has been much studied the problem of spectral distribution of pulses has been generally overlooked. This work presents a statistical test combining tests on count rate and tests on spectral distribution. The proposed method is shown to have an acceptable false positive rate and, when compared with two other test statistics found in the literature, greater power. (C) 2012 Elsevier B.V. All rights reserved.
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| 2. |
- Kock, Peder
(författare)
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Orphan source detection in mobile gamma-ray spectrometry - Improved techniques for background assessment
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hazardous radioactive sources out of regulatory control are referred to as orphan sources. Focussing on gamma-emitting orphan sources, this thesis describes methods that can be used in mobile gamma-ray spectrometry to perform real-time discrimination between an actual source signal and the signal due to the natural radiation background. Two categories of methods for orphan source detection are presented: visualisation and statistical hypothesis testing. Visualisation leaves the interpretation to the operator, whereas the hypothesis testing aims at answering the question: is there a signal or not? The methods are shown to work well for purely stochastic background data. The visualisation method gives a homogeneous representation of large data sets, which enhances the capability to discern significant positive deviations. The hypothesis testing method is shown to exhibit false alarms at the expected rate. Furthermore, by taking into account the spectral distribution in a hypothesis test, the power of the test was increased when compared with reference methods from the literature. Systematic background radiation variations can be disclosed by using data from airborne gamma-ray spectrometry (AGS) surveys. Validation of AGS data at the ground level by the use of terrestrial surveys yielded a strong positive correlation between the two data sets for background activity of uranium and thorium. By deriving a step function from AGS data, the systematic background variations were successfully modelled at the ground level. This approach was shown to reduce the number of excess false positives in an area with strong background gradients. In addition to random and systematic background radiation fluctuations, germanium spectrometers can have problems with microphonic noise due to the vibrations that inevitably will occur in mobile gamma-ray spectrometry. The additional noise can be expected to degrade the resolution of the detector. The level of resolution degradation was found to be 0-19%, depending on the speed of the vehicle.
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