Mario Villa

The new high resolution ultra small-angle neutron scattering instrument at the High Flux Reactor in Grenoble

In 1998, the combined Interferometer and Ultra Small Angle Neutron Scattering (USANS) instrument S18 at the 58 MW High Flux Reactor at the Institute Laue-Langevin in Grenoble (France) started operation. The instrument has been upgraded to allow more advanced neutron optics experiments for fundamental, nuclear and condensed matter physics. The new supermirror guide along with the two channel cut silicon perfect crystals, configured as an advanced high resolution Bonse-Hart camera, provides optimal intensity conditions. A large range of wavelengths is accessible by variations of the Bragg angle and by using different reflecting planes of a properly cut silicon monochromator block in combination with different channel-cut analyzer crystals. The fine adjustment analyzer system is achieved by an advanced piezo drive, which has an accuracy of 0.036 seconds of arc. The basic features of the USANS camera, which takes advantage of a new tail suppression method, and the results of various test measurements concerning intensities, long term stability and the accessible wavelength range will be presented. A high resolution position sensitive detector can either be used to analyze beam profiles or to perform neutron radiography experiments.

Ultra-small-angle neutron scattering studies of artificial lattices

Ultra-small-angle neutron scattering (USANS) with the use of perfect silicon crystals provides a resolution of the order of 10-5 A-1 in reciprocal space, which corresponds to rad in scattering angles and m structures in real space. From small-angle scattering by artificial lattices follows a unique test procedure for the related devices and techniques. Corresponding measurements were performed at the USANS facilities of the Atominstitut in Vienna and of the S18 instrument at the ILL. We observed diffraction patterns from samples being periodically structured in one and two dimensions. These measurements take advantage of the extended coherence function of the set-up and the high quality of the manufactured silicon sample lattices. Due to these characteristics up to 50 interference orders were obtained at the S18 instrument. Scattering from two-dimensional periodic structures was observed for different orientations of the sample which shows characteristic diffraction maps in reciprocal space.

Optimisation of a crystal design for a Bonse- Hart camera

Bonse-Hart double-crystal diffractometers (DCDs) with multi-bounce channel-cut crystals show rocking curves that depart dramatically from dynamical diffraction theory in their wings. The intrinsic background is many orders of magnitude higher than the predictions of dynamical diffraction theory. This effect was studied at the ultra-small-angle neutron scattering facility at the Atominstitut in Wien and at facilities in Grenoble, Julich and Villigen. The scattering intensity contains Bragg reflections from the front and the back faces, and thermal diffuse scattering from the internal volume. The aim of this study was to eliminate this contamination and develop a new crystal design which provides optimal resolution. Therefore different ways were tested. In the first step the contamination was eliminated by cutting a groove in the middle of the back plate of the channel-cut crystals and inserting a cadmium absorber in this groove. With this modification an additional suppression of the wings of the rocking curve of about one order of magnitude was achieved. After this, we developed a new design for a DCD. The concept for this new crystal design was to avoid the back reflection and the thermal diffuse scattering. The different steps on the way to produce these crystals are presented in a detailed way. The crystal preparation and the different instruments where these crystals have been tested are also described.

Neutron flux measurements at the TRIGA reactor in Vienna for the prediction of the activation of the biological shield

The activation of the biological shield is an important process for waste management considerations of nuclear facilities. The final activity can be estimated by modeling using the neutron flux density rather than the radiometric approach of activity measurements. Measurement series at the TRIGA reactor Vienna reveal that the flux density next to the biological shield is in the order of 10(9)cm(-2)s(-1) at maximum power; but it is strongly influenced by reactor installations. The data allow the estimation of the final waste categorization of the concrete according to the Austrian legislation.

Performance and comparison of gold-based neutron flux monitors

In two test series, liquid and solid gold-based neutron flux monitor materials were investigated with respect to the effects of neutron absorbers such as chlorine, scattering effects, and the dependence of the enhanced activation caused by the epithermal resonance integral. The liquid monitors were prepared from aqueous solutions of tetraamminegold(III) nitrate and tetrachloroauric(III) acid. The presence of chlorine-35 partly suppresses the activation of gold-197; this effect depends not only on the concentration of the absorber but also on the state of the neutron flux density monitor. Aqueous samples show greater relative losses than solid monitors. Neutron scattering occurs in hydrogen-rich sample matrices which is shown by the fact that cadmium-shielded aqueous samples show an over proportional activation. Hence, fast neutrons must be moderated to the epithermal energies covered by the resonance integral, which is characterized by much greater cross sections for the capture of neutrons. The insight of this study with respect to neutron scattering in hydrogen-rich matrices must be taken into account also for neutron activation analysis; sample and standard must have a similar matrix with respect to its neutron scattering properties, otherwise the effect of increased activation as well as of enhanced self-shielding are underestimated.

Permanent magnetic field-prism polarizer for perfect crystal neutron interferometers

Abstract Spin-dependent bi-refringence of neutrons upon passage through the air gap of a prismatically shaped permanent magnet yoke is used to split a thermal neutron beam in two polarized sub-beams with slightly different directions. This method is ideal to polarize the neutrons within perfect crystal interferometers without loss of intensity. Using a sequential arrangement of two such prisms a splitting larger than twice the rocking-curve width of a perfect crystal Mach–Zehnder-type neutron interferometer has been achieved, yielding a beam polarization of about 97%.

High-intensity power-resolved radiation imaging of an operational nuclear reactor

Knowledge of the neutron distribution in a nuclear reactor is necessary to ensure the safe and efficient burnup of reactor fuel. Currently these measurements are performed by in-core systems in what are extremely hostile environments and in most reactor accident scenarios it is likely that these systems would be damaged. Here we present a compact and portable radiation imaging system with the ability to image high-intensity fast-neutron and gamma-ray fields simultaneously. This system has been deployed to image radiation fields emitted during the operation of a TRIGA test reactor allowing a spatial visualization of the internal reactor conditions to be obtained. The imaged flux in each case is found to scale linearly with reactor power indicating that this method may be used for power-resolved reactor monitoring and for the assay of ongoing nuclear criticalities in damaged nuclear reactors.

The status and patterns of nuclear education in an anti-nuclear environment, Austria

This paper describes the current status and basic patterns of nuclear education in Austria and highlights the Nuclear Knowledge Management (NKM) activities of Atominstitute (ATI) through national and international networks. This institute hosts a TRIGA Mark II research reactor for research, training and education purposes. Austria has no nuclear power plant in operation because of the legislation prohibiting nuclear power production within its boundaries. Therefore, its nuclear education focuses on safety issues, i.e. environmental, health and engineering. This paper presents the increasing trend of students in nuclear subjects since 1995. The atomic institute is playing an active role in NKM through the European Nuclear Education Network (ENEN) association. Being the closest facility to the International Atomic Energy Agency (IAEA) headquarter, it also facilitates the international training and educational activities regarding nuclear science and technology.

Dihydrogen gas emission of a 250 kWth research reactor

The Vienna TRIGA pool-type reactor emits tiny gas bubbles at 250kW. They add up to a total volume of ∼2.4L in 7h of daily operation. The bubbles consist of nitrogen (72.5vol%), hydrogen (17.2%), oxygen plus argon (12.0%) and carbon dioxide (0.23%). The emission of constituents of air is caused by degassing of dissolved air in the hot regions of the reactor. Hydrogen results from neutron-induced radiolysis of the cooling water. This emission should be kept in mind for reasons of fire protection even for low-power reactors.

Ultra‐small‐angle neutron scattering from dense micrometre‐sized colloidal systems: data evaluation and comparison with static light scattering

Ultra-small-angle neutron scattering (USANS) probes the same q regime as static light scattering (LS), making USANS an additional tool for the study of structures between 100 nm and 10 μm. Dense oil-in-water emulsions, which have already been characterized intensively by light scattering, are investigated in this study as a model system using USANS. The two basic problems of such scattering studies are the following: on the one hand, one has to use different scattering theories for USANS and LS, and on the other hand, in both cases one has to deal not only with particle interactions but also with multiple-scattering effects. For neutron scattering it is always possible to use the simpler Rayleigh-Debye-Gans (RDG) theory instead of the Lorenz-Mie theory, which generally describes light scattering from micrometre-sized globular objects. The samples have different contrasts in neutron and light scattering, such that only low-contrast (close to index match) LS data can be interpreted by the RDG theory. The data evaluation is performed by means of the generalized indirect Fourier transformation (GIFT) method, which enables the simultaneous calculation of the form and structure factors. The results are discussed and compared with those from light scattering experiments, taking into account the advantages of both methods. The effect of multiple scattering and its influence on data evaluation is also examined. Data evaluation by applying the GIFT method works well for both neutron and light scattering data, with results of comparable quality. The advantages of light scattering are the fast data acquisition and the large number of data points. USANS, on the other hand, covers a wider q range and the problem of multiple scattering is not as severe as for light scattering, but still must not be neglected.