[en] 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

No abstract available

[en] Short communication

[en] The Centro Brasileiro de Pesquisas Fisicas (CBPF) is producing Position Sensitive X-ray Detectors (PSD). Such gaseous detectors, permit to measure the number of photons hitting the detector and their positions. These features open the possibility to use these detectors in computerized tomography (CT) to perform low cost non-destructive analysis, in small objects or samples. To evaluate this feasibility, several tomographic images of objects and standards were taken at different conditions. Homogeneous samples with known attenuation coefficients were measured to calibrate the system and determine parameters for the quantitative evaluation of the tomographic images. This paper presents the results obtained with an 1-dimension PSD filled with Argon (10% CH₄) at 2 atm, provided with a 80 x 2 mm collimator and a 272 μm thick carbon window. An X-ray ampoule of 25 kV maximum anode tension, under 1 mA, producing a fan-like beam, was used as radiation source. (author)

[en] A position sensitive gas detector originally designed for X-ray has been modified to detect thermal neutrons addressing its utilization in tomography. This work describes the modifications done for this purpose, including the incorporation of neutron-to-charged particle converter materials, and the measurements carried out to evaluate the detector performance for each of them. The first converter studied was a natural gadolinium foil placed close to the detector window, producing an internal conversion electron from the neutron gadolinium interaction, which causes an ionization in the argon-methane filling gas. Another converter studied was high-enriched ³He, replacing both the gadolinium foil and the argon-methane gas. ³ He fulfills the main requirements for thermal neutron detection such as high absorption cross section - 5330 barns - low efficiency for gamma-ray conversion and a good charge amplification factor. The ³He(n,p)³H reaction produces a 573 keV-proton and a 191 keV-tritium which are emitted practically into opposite directions, since the neutron itself, due to its very low energy (0.025 eV) does not contribute significantly to the momentum balance. Tomographic systems for thermal neutrons incorporating the developed detector, present some advantages with regard to the earlier 1^st generation equipment, such as a better resolution and a reduction in the image acquiring time. (author)