Isotopically Selective Multiphoton Dissociation of UF6 in Static Gas Cell at -35°C

Abstract

In molecular laser isotope separation of uranium (MLIS), the compound UF6 is a natural choice since it is the most familiar material in conventional uranium enrichment processes. The UF6 has the sufficient chemical stability and the highest vapor pressure of all known uranium compounds. However, the studies on MLIS have not been as successful as those on AVLIS (atomic vapor laser isotope separation) until recently owing to the lack of the suitable infrared lasers that can be tuned precisely to the absorption center of the 235UF6ν3 band near 16 pm.Many attempts have been made to develop the 16 μm lasers. Continuously tunable 16 μm radiation was obtained using an optical parametric oscillator(1)(2). Since the available energy was very small, however, the second laser for dissociation of the vibrationally excited UF6 was needed. The groups at Los Alamos National Laboratory and Centre d'Etudes Nucleaires at Saclay both utilized the UV laser for this purpose. But it was found(3)(4) that the use of the UV laser deteriorates the isotopic selectivity because even unexcited UF6 absorbs the UV laser very strongly. Although infrared multiphoton excitation without using UV lasers was expected to yield better selectivity, no high-power gas laser had the capability of tuning the laser frequency to the center of the 235UF6 16 μm band until the advent of the p-H2 Raman laser. For example, the CO2-pumped CF4 laser was found to be successful in dissociating UF6 only in the region between 610 and 620 cm-1(5), which corresponds to the lower frequency tail of the absorption of 238UF6.Immediately after the proposal(6) and the following successful demonstration(7), it was recognized that p-H2 Raman laser is the most suitable 16 μm laser which is capable to tune over the region of 235UF6 absorption. It is a scalable gas laser based on well established CO2 laser technology and on the very efficient stimulated rotational Raman scattering conversion in a gaseous medium.Using this p-H2 Raman laser, Rabinowitz et al.(8) demonstrated infrared two-color multiphoton excitation to dissociate UF6 selectively at ambient temperature. Although their choice of all-infrared excitation scheme itself is more advantageous than IR-UV excitation scheme, the selectivity was not very high probably owing to the fact that UF6 was not cooled with an expansion nozzle. They reported a selectivity of 1.20. Since the amount of UF6 treated in their experiment was small, they developed a complicated on-line detection system for the isotopic selectivity measurement.It is always of great interest from a practical viewpoint whether the selectivity observed at the time immediately after dissociation is deteriorated by the secondary reactions or not. In this note, an attempt was made to irradiate UF6 contained in a static gas cell with one-frequency p-H2 Raman laser and to obtain the selectivity based on the stable compound remaining in the cell after irradiation.