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The time constraints on foreign body detection in food using Compton scattered X-rays were examined. An equation was developed to calculate the number of scatter counts detected per second, given the geometry of the sample and the X-ray system. This was used to calculate the minimum time required to detect a contaminant in a sample, given that a signal-to-noise ratio of at least 7·4 was required for production line applications. The predicted times required to detect a glass fragment in a cube of water ranged from 0·018 s for a 5 mm fragment to 280 s for a 1 mm fragment in a 10 cm cube, or respectively, 0·00036 and 5·5 s in a 5 cm cube. The detection of a 4 mm glass fragment in water, instant coffee and muesli using Compton scatter was demonstrated by experiment. The contrasts obtained between the glass and the samples were, respectively, 0·5, 2·7 and 2·6, and the signal-to-noise ratios were, respectively, 10·1, 23·4 and 24·3. The minimum measurement times implied by the experiments were similar to those predicted by theory. The glass fragment was detected with strong contrast in the experiments, but the measurement times for whole-sample scanning were slow compared to production line speeds in the food industry. The detection of glass from the mean energy of the scattered X-ray spectrum was also demonstrated. A high contrast of 9 was obtained for glass in water, but the measurements were more noisy than the corresponding photon counting measurements, giving a signal-to-noise ratio of only 3·3. |
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