7.2 Introduction

The possibility of using the slight differences that exist in the absorption spectra of isotopes of an element for isotope separation has been recognized ever since isotopes were discovered. The first reported successful photochemical separation of isotopes was that of Kuhn and Martin [K4], who dissociated C03sCl2 molecules in natural phosgene by light of 281.618-nm wavelength from an aluminum spark, which happened to be the correct wavelength. The first photochemical separation of isotopes on a practical scale was that of mercury isotopes. In one example [Zl], light from a mercury arc containing a preseparated mercury isotope was used to excite the same isotope in natural mercury vapor and cause it to form HgO with water vapor also present. Ibis method is not generally applicable to other elements because it makes use of the especially simple character of the mercury spectrum, with few, widely spaced lines.

Invention of the laser provided the intense, monochromatic, tunable light source needed to make photochemical isotope separation applicable to all elements, at least on a laboratory scale. The promise of this method was recognized as early as 1965 by Robieux and Auclair [Rl], who were issued the first patent on it. Since the pioneering experiments of Tiffany et al. [Tl] on bromine isotopes in 1966, an enormous amount of work has been done with lasers, with small-scale separation reported for most elements.

This text can describe only briefly the incomplete information publicly available on laser separation of uranium isotopes. For a more detailed discussion of the history and principles of laser isotope separation, reference may be made to the review articles of Letokhov and Moore [LI] and Aldridge et al. [A2], and to Farrar and Smith’s report on uranium [FI].

Two general methods have been proposed for separating uranium isotopes. In the photoionization method to be discussed in Sec. 9.2, MSU in uranium metal vapor is ionized selectively and then separated from unionized 23®U by deflection in electric or magnetic fields. In the photochemical method, to be described in Sec. 9.3, MSUFS in UF6 vapor is excited selectively and caused to react chemically to produce a solid lower fluoride, which is then separated from unreacted 23*UF6 vapor.