AVLIS

AVLIS experiment at the Lawrence Livermore National Laboratory

AVLIS ( A tomic V apour L aser I sotope S eparation ) is the abbreviation for a process for isotope separation with lasers , whereby the isotopes are first converted into the vapor phase in atomic form.

When using the nuclear energy of uranium ( uranium enrichment ), the process is used to enrich the thermally fissile ²³⁵ U isotope. Another type of laser enrichment is the molecular method (see MLIS ). Both processes have not achieved any industrial importance.

The basic principle of the AVLIS process is that the atoms of an isotope mixture (e.g. uranium isotopes ) are selectively ionized. After the ionization of one isotope ( ²³⁵ U) it can be separated from the non-ionized atoms of the other isotope ( ²³⁸ U) by acceleration in an electric field. The method was originally developed in the Lawrence Livermore National Laboratory (USA). A similar variant pursued in France is called SILVA (Séparation Isotopique par Laser de la Vapeur Atomique d'Uranium).

In the case of atoms with higher mass numbers , interactions between the valence electrons lead to manifold splitting of the energy levels and make the term scheme very confusing. Uranium has six valence electrons. The term schema is correspondingly complex with around 900 identified levels to date.

In the laser cutting process developed in Livermore, uranium is first vaporized in a furnace at 2500 K. The atomic ²³⁵ U present in the steam is excited into the continuum with a laser light beam tuned to certain wavelengths of the ²³⁵ U spectrum. The ²³⁵ U emits an electron and can be sucked off as an ion on a negatively biased metallic collector. The isotope shift between ²³⁵ U and ²³⁸ U results from the different size of the two nuclei and is around 0.005 nm at a wavelength of around 600 nm. The laser must therefore be tuned very precisely and constantly.

In terms of apparatus, the process consists of the laser system and the separator system. The laser system contains dye lasers , which can be tuned to the desired frequency and which are pumped with copper vapor lasers . The separator system consists of the evaporator furnace, in which the metallic uranium is converted into the vapor phase, and the collectors, on which the positively charged ²³⁵ U ions are deposited.

A particularly critical point is the vapor density in the fan-shaped atomic beam that leaves the furnace. The excitation energy and also the ion charge can be easily transferred from one atom to the other by collisions, which impairs the separation effect. The uranium vapor density should therefore not exceed a value of about 10 ¹³ / cm³. The density restriction due to the charge exchange can be reduced if the ions are extracted from the interaction zone with a strong electric field. Another difficulty arises from the thermal ionization of uranium in a hot furnace. If only a fraction of 0.1% of the ²³⁸ U is ionized, the ²³⁵ U / ²³⁸ U ratio in the end product cannot exceed the value 5, even if 70% of the ²³⁵ U is ionized.

literature

Petr A. Bokhan, Vladimir V. Buchanov, Nikolai V. Fateev, Mikhail M. Kalugin, Mishik A. Kazaryan, Alexander M. Prokhorov, Dmitrij E. Zakrevskii: Laser Isotope Separation in Atomic Vapor . Wiley-VCH , Berlin 2006, ISBN 3-527-40621-2