on electrons in the 16 micron band, which could be efficiently produced using CO2 lasers which were emerging in the late 1960s. However, the transitions in this area were closely spaced which made it difficult to select due to Doppler broadening, requiring the vapour to be cooled with a complex expansion system. The introduction of lasers working at tunable frequencies, typically dye lasers, allowed the selection of more convinenent excitations. Modern systems typically use the 238U absorption peak of 502.74 nanometers shifts to 502.73 nm in 235U. The AVLIS system consists of a vaporizer and a collector, forming the separation system, and the laser system. The vaporizer produces a stream of pure gaseous uranium. Laser excitation The laser commonly used is a two-stage tunable pulsed dye laser usually pumped by a copper vapor laser; the master oscillator is tunable, narrow-linewidth, low noise, and highly precise. Its power is significantly increased by a dye laser amplifier acting as optical amplifier. Three frequencies ("colors") of lasers are used for full ionization of uranium-235. For AVLIS in other elements, such as lithium, tunable narrow-linewidth diode lasers are used. Commercialization and international significance In the largest technology transfer in U.S. government history, in 1994 the AVLIS process was transferred to the United States Enrichment Corporation for commercialization. However, on 9 June 1999 after a $100 million investment, USEC cancelled its AVLIS program. AVLIS continues to be developed by some countries and it presents some specific challenges to international monitoring. Iran is now known to have had a secret AVLIS program. However, since it was uncovered in 2003, Iran has claimed to have dismantled it. Brief history The history of AVLIS, as recorded in the open refereed literature, began in the early-mid 1970s in the former Soviet Union and the United States. In the US, AVLIS research was mainly carried out at the Lawrence Livermore National Laboratory although some industrial laboratories were early players. Tunable laser development for AVLIS, applicable to uranium, has also been reported from several countries including Pakistan (1974), Australia (1982-1984), France (1984), India (1994), and Japan (1996). See also Australian Atomic Energy Commission Calutron Chemical reaction by isotope selective laser (CRISLA) Gaseous diffusion List of laser articles Separation of isotopes by laser excitation (SILEX) Nuclear fuel cycle Nuclear power References External links USEC News Release Cancelling AVLIS Report on Iranian AVLIS program Oxford Research Group report on Iran's nuclear activities. Laser