The Benefits of Laser Isotope Separation (LIS)
- Dr. Mark Raizen, University of Texas at Austin
- Dr. Francis Slakey, American Physical Society
Our planet contains vast natural resources, still largely untapped. These resources hold the promise of detecting and treating cancer, saving energy, making new materials, and advancing basic science. What are these valuable resources? Where can they be found? How can we make them available?
The answer to the first question is that the resources are rare isotopes of the elements. The answer to the second question is easy: these isotopes are literally in our midst, within the elements that make up our planet. The third question is the crux of the matter; isolating rare isotopes of elements has been extremely difficult because they have nearly the same physical and chemical properties as other, more common, isotopes of the same element. This is the reason that many rare isotopes are the most expensive commodity on earth, with a price that can be over one thousand times that of gold! This prohibitive cost severely limits the exploration of new applications and therapies.
Here are just two examples of rare isotopes that could be widely used if only they were less expensive : Nickel-64, a stable isotope with a natural abundance of only 1 percent. It can be converted in a medical accelerator to Copper-64 which is a short lived radio-isotope with great promise for PET scans and cancer therapy. Calcium-48 is a stable isotope with a natural abundance of 0.2 percent. It is used as a diagnostic for osteoporosis in women, bone development in children, and for a basic physics experiment that may determine the mass of the neutrino.
The only method for separating such isotopes dates back more than eighty years. This method, known as the Calutron, relies on electron ionization of atoms, and separation by the charge-to-mass ratio. Although first used in the 1930s for separating uranium, they were replaced by the gas centrifuge which is limited mostly to that element. The Calutrons remained as general purpose, though inefficient, isotope separators. Today, these machines are only operating in Russia, with an obsolete technology that is facing imminent shut-down. Without an alternative approach, most rare isotopes will not be available in the future at any price. The looming shortage of crucial isotopes is a national priority, as indicated by a 2009 report of the Nuclear Science Advisory Committee to the Department of Energy, "Isotopes for the Nation's Future."
I recommend this report to anyone with an interest in the scope and uses of stable and radio-isotopes. One topic discussed in this report is laser isotope separation. Although isotopes are almost identical in every manner, the wavelengths of the atomic transitions of different isotopes are slightly shifted from one another.
This "isotope shift" makes it possible to excite only one isotope with a narrow-band laser, leaving the others unaffected. The common wisdom until now has been that one must use lasers to selectively ionize the desired atoms. However, it turns out that in order to have a large probability for ionization, very high laser power at multiple colors is required. The scale is so large that it required a government effort, with one dedicated goal: laser isotope separation of uranium. This effort was ultimately terminated in 1999, mainly due to the high cost and complexity of the lasers, and to the best of my knowledge is not being pursued. Laser separation of a molecular compound of uranium is still being pursued commercially by GE-Hitachi. I have followed this work from a distance, and always felt there must be a solution which would be simple and cost-effective for the many smaller-scale isotopes that are needed. It came from an unexpected direction.
Over the past few years, my research has focused on developing general methods for controlling the motion of atoms in gas phase. The successful realization of these methods uses single-photons to control the magnetic state of each atom, followed by magnetic manipulation. It has brought to reality a thought experiment by James Clerk Maxwell from 1870 known as Maxwell’s Demon. This work is reviewed in an article that I wrote for Scientific American, "Demons, Entropy and the Quest for Absolute Zero," published in the March 2011 issue. I realized that these very same methods can also be used for efficient isotope separation with low-power solid-state lasers, a paradigm shift from ionization. We are pursuing this avenue with a proof-of-principle experiment, soon to be completed. This will then be applied commercially towards production of important medical isotopes, where the need is most urgent. In fact, this could save your life!
Updated July 13, 2012 5:30 PM