Comparison of tomographic systems for X-ray and thermal neutrons

Abstract

In this work, tomographic images of the same object have been taken with 25 keV X-rays and thermal neutrons (E=0.025 eV) aiming to demonstrate that thermal neutron tomography in some cases is a complementary technique to the X-ray tomography, such as in the examination of hydrogen-bearing compounds wrapped in a metallic matrix for instance. The capability of the neutron to pass through metallic materials such as lead, stainless steel and aluminium, allows to inspect encapsulated plastic explosives and visualize their inner structure like density variations, voids and alien materials, which are important features for the quality control of the final product. To obtain the images, a 3 rd generation tomographic system with a Position Sensitive Detector has been developed. For X-rays this proportional detector was provided with an 8cm long carbon window, and filled with Ar − CH 4 under a pressure of 2 atm. The X-ray beam was supplied by an ampoule with a tungsten anode manufactured by IPRJ/UERJ. For neutron detection the carbon window has been replaced by aluminium, and the filling-gas by 3 He enriched helium, acting simultaneously as neutron converter and ionization gas. The Argonauta reactor at the Instituto de Engenharia Nuclear / CNEN was used as neutron source and furnishes a thermal neutron flux of 4.5 × 10 5 n · cm −2 · s −1 at its main channel outlet, where the tomographic system was installed. I IntroductionThe radiographic method via transmission is based on the attenuation of the radiation through the matter. On this basis, the inner physical structure of an object can be examined by measuring the transmitted radiation intensity with a proper detector. The attenuation coefficient of the materials depends upon the nature and energy of the incident radiation, and thus, the more suitable radiation and energy will depend upon the involved materials. Important parameters which should be taken into account are the size and composition of the object under inspection, the kind and level of the information expected to be supplied by the image, and the cost of the procedure. Thanks to the absence of electrical charge, neutrons and X-rays can penetrate deeper into matter than charged particles, being thus more suitable to examine thick metallic objects. Since neutrons and X-rays interact with matter in intrinsically different fashions, the materials will exhibit different attenuation coefficients for these radiations, furnishing hence different information about the object under analysis. II DetectorThe gaseous Position Sensitive Detector -PSD, described in this work operates at the proportional region and is constituted by a gas chamber containing a multi-strip flat cathode. A wire placed between this cathode and the detector window acts as anode like in the conventional detectors. The electron avalanche produced near the anode induces a charge distribution in the cathode strips, which are connected to a delay line. The electronic signal produced by that charge distribution would propagate in both directions toward...