Atomic beams /Actinide elements / Resonance ionization mass spectroscopy / Volatilization
SunmiaryFor ultratrace analysis of actinide Clements and studies of their atomic propcrties with resonance ionization mass spectroscopy (RIMS), efficient and stable sources of actinide atomic beams are required. The thermodynamics and idnetics of the evaporation of actinide elements and oxides from a variety of metals were considered, including diffusion, desorption, and associative desorption. On this basis various sandwich-type filaments were studied. The most promising system was found to consist of tantalum as the backing material, an electrolytically deposited actinide hydroxide as the source of the element, and a titanium covering layer for its reduction to the metal. Such sandwich sources were experimentally proven to be well suited for the production of atomic beams of plutonium, curium, berkelium and califomium at relatively low operating temperatures and with high and reproducible yields.
By multistep resonance ionization with lasers, followed by mass spectrometry, the first ionization potential of neutral atoms can be measured with quantities of only about one nanogram. The ionization potentials of americium (5.9736(3) eV) and of curium (5.9914(2) eV) were thus determined for the first time. The figure on the right shows a plot of the number of ions N versus the total photon energy Etot of three laser beams, which are essential for the experiment. The arrow points to the ionization threshold.
Die Messungen wurden in verlingerten und versiegelten 5-mm-NMR-Rohrchen ausgefuhrt, die eine Losung des Polycyclus in [DJTHF, eine geringe Menge von Benzol als internen Standard und eine mit Dichlormethan gefullte Kapillare als externen Standard enthielten. Nach der Messung der Diffusionskoeffizienten der ungeladenen Probe wurde das 5-mm-Rohrchen umgedreht, urn einen Kontakt zwischen der Losung und dem Metalldraht (Li, Na) oder Metallspiegel (Kalium) zu ermoglichen; dabei wurde das polycyclische System zu dem entsprechenden geladenen System reduziert. Alle lonen ergaben ihre bekannten 'H-NMR-Spektren. Alle Diffusionsmessungen wurden mit einem Bruker-500-MHz-AM-NMR-Spektrometer durchgefuhrt, das rnit einer Z-Gradienten-Einheit und einem inversen Probenkopf fur 5-mm-NMR-Rohrchen ausgerustet war. Die gepulsten Gradienten lagen im Bereich von 0 bis 41.57 Gcm-'. Die gesamte Echozeit betrug typischerweise 124ms, d und D dauerten 2 bzw. 62 ms. Alle Diffusionskoeffizienten sind Mittelwerte von mindestens drei Experimenten. Es wurden nur Daten rnit einem Korrelationskoeffizienten (R') von mehr als 0.998 berucksichtigt.
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