2.2 El algoritmo de exploración Random
3.3.2 Procedimiento para el cálculo de los puntos objetivo
The work presented in this thesis has involved investigations in two different, though not unrelated, aspects of low energy positron physics, namely: slow e+ moderator development and Ps beam production and interaction with atomic gases. In this chapter the results and conclusions from each of these studies are assessed and future possibilities are discussed.
Interest in single crystal moderators of thicknesses in the order of 100s to 10000s of
A
has arisen for a number of reasons. The employment of a slow e+ moderator in the transmission mode eliminates the source shadow problem associated with the backscattering geometry and intercepts a larger proportion of the flux than a grid arrangement. It allows the use of simpler linear optics in the remoderation stages of electrostatic transport systems. Furthermore, the longitudinal energy distribution of the ejected e+ is likely to be narrower than those from the vane, grid or cone configurations as the surface of emission is normal to the beam axis. However, the use of thin film moderators is not without difficulties. Defects in the crystal structure can arise from handling of the fragile films and, moreover, the annealing procedure must be implemented with care as damage can occur from hot spots and evaporation. As discussed in Chapters 2 and 3, work on the performance of thin single crystal films has been carried out in vacuums of >10"® Torr with in situ heating facilities. Following Lynn et al (1985) and Schultz et al (1986), the present study undertook to develop an annealing technique and subsequently, to investigate the moderator properties of thin single crystal W(100) and Ni(100) foils of thicknesses 1000-18000A in vacuum conditions of =10-7 Torr, similar to those of most gas scattering experiments.A simple annealing procedure was employed, using resistive heating in a low vacuum environment of 6xl0-2 Torr. After transfer in air to the test system, the maximum efficiencies obtained were (8.8±1.2)xl0-4 for a 2000A W(100) foil and (6.5±1.0)xl0“4 for a 5000A Ni(100) sample. The longitudinal energy distribution from both W and Ni single crystals were found to be smaller than that from a W mesh annealed in a similar manner under the same conditions. The FWHM of the positrons emitted from
compared to 2.8eV from the W mesh.
The maximum efficiencies determined are in good agreement with those from the study of Gramsch et al (1987), using a somewhat more complex annealing technique in higher vacuum conditions (10~8 Torr), but are a factor of 2.5-4.5 lower than those predicted from calculations. Moreover, the variation of the efficiency with thickness is also inconsistent with that computed. This is suggestive of the presence of defects and/or contaminants in the bulk and on the surface of the samples. However, efficiencies matching those from calculations based on perfect crystalline structure with no contamination cannot be expected since the procedure employed in the present study involved handling of the foils and exposure to air. On the subject of contamination, annealing in low vacuum may have been of benefit as it is known that carbon, the main contaminant in both the Ni and W foils, can be removed by the formation of gaseous oxides at the surface (Becker et al> 1961). The workfunction determined in the present study did not indicate the identity of the surface adsorbate(s), although carbon and oxygen are the most likely candidates. Bulk and surface analysis tools are required to further elucidate and quantify the effects of contamination. Nevertheless, the aim of this project was fulfilled and thin single crystal W and Ni foil moderators were demonstrated to produce slow e+ in vacuum conditions similar to those of most gas scattering studies.
Extensions of this study can be made to include other single crystal transition elements. It is noted that the majority of the metals that have been observed to emit slow positrons are either of body centred or face centred cubic structures. Tungsten, the most efficient metallic moderator discovered to date, has a body centred cubic (BCC) structure. It may be of interest to examine other BCC crystals such as tantalum, molybdenum, chromium and vanadium which all possess melting points lower than W and so may facilitate greater temperature control in the annealing procedure. In contrast to Cu, W and Ni moderators perform efficiently following exposure to air as these metals are resistant to substantial oxide build up on the surface. Cr and, particularly, V are also known to exhibit such a resistance to oxidation (Nordlander and Ronay, 1987). Metallic moderators emitting e+ with the narrowest energy distribution in non-UHV conditions are those with the smaller positron workfunctions, such as Ni, Cu and Pt which all have a face centred cubic (FCC) structure. Therefore, it may be of interest to study iridium which possesses
a FCC structure and is more dense than W. Nieminen and Hodges (1978) have calculated the positron workfunction of Ir to be -1.4eV which is comparable to that determined experimentally for Ni by Gullikson et al (1985). Furthermore, Dale et al
(1980) found that polycrystalline Mo and Ir emitted e+ with yields approximately 80% and 65%, respectively, of that from W in the same study. On the subject of Ni, the magnetic property of this element in relation to the e+ yield and energy distribution obtained was not conclusively determined. The magnitude of the magnetism differs with face orientation, for example, N i(lll) possesses the largest proportion of magnetically aligned domains. Therefore, further investigations, such as the comparison of yields from various crystal orientations of Ni in magnetic and electrostatic systems, may reveal the effect of this property.
The original motivation of this work was to develop a transmission mode moderator for use in gas scattering investigations, therefore the next step might be to utilise the foils in experiments which may benefit from the generation of slow e+ beams with narrow energy spreads. These include the measurements of excitation, ionisation and differential scattering cross-section from molecular gases. Beams of widths in the order of 10s to 100s of meV are required for molecular targets due to the presence of vibrational and rotational levels and these may be achieved by remoderation and filtering. Additionally, investigation of the higher excitation states in atomic gases and studies into the behaviour of the individual cross-sections at thresholds, such as the onset of Ps formation, also require systems with high resolution.
The second part of the work discussed in this thesis concerned the production and employment of energy-controllable Ps beams. The technique developed by Laricchia
et al (1988b) was used to generate a timed Ps beam which, following modifications, allowed the identification of excited state Ps produced in e+ collisions with He and Ar gases. Ps and Ps*, formed in the beam in the energy range 7-4leV (Ps energy), were quantified in 4eV portions and the maximum yields of n=l Ps were obtained at «35eV in both gases with the n=2 component exhibiting a gradual increase with energy. The highest ground state Ps yields, following correction for decay in flight, were (0.28±0.02) in He and (0.108±0.006) in Ar per scattered e+ per steradian per second. The ratios of Ps* to the total (Ps+Ps*) yields were also found to increase with energy to maximum values of (33±3)% in He and (42±4)% in Ar. A comparison with theory (Mandal et al, 1980, Khan et al, 1984, 1985) was made which indicated the dominant presence of the long lived metastable S state in the excited Ps