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A gaseous position sensitive detector originally designed to operate with X-rays has been modified for using in thermal neutron tomographic systems. This modification included the replacing of the standard argon–methane mixture by 3He-enriched helium. Due to its low-density, helium should be submitted to a relatively high pressure in order to increase its stopping-power for the products emerging from the 3He(n,p)3H nuclear reaction. High pressure however, creates tightness problems. To mitigate this difficulty some heavy gases have been mixed to helium, reducing thus the range of the particles. In this work, three gases have been studied: pure 3He-enriched helium, and its mixture with argon–methane and with propane. For each case the parameters governing the performance of a detector coupled to a tomographic system, such as spatial resolution, linearity and response homogeneity have been determined. Besides that, the modulation transfer function and the distance resolution curve for a tomographic system incorporating a detector filled with those three different gases have been measured. The best spatial resolution – around 800 μm – has been obtained with a helium–propane mixture. This kind of detector is intended to equip thermal neutron tomographic systems, to perform non-destructive assay, exploiting thus the capability of this particle to pass through heavy materials where a X-ray tomography would not work properly. For an equivalent resolution, a tomographic system using a detector of this kind would require a much shorter acquiring time with regard to the first generation systems, since a sample translation is no longer necessary. |
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