New Method for the Separation of Isotopes

REHOVOT, Israel -- February 23, 1997 -- A Weizmann Institute of Science researcher has turned a 70-year-old textbook concept in quantum mechanics into a practical method for separating isotopes, different "versions" of the same element.

The technique, designed by Dr. Ilya Averbukh of the Chemical Physics Department, promises to open the door to fast and effective separation of isotopes for such fields as chemical and pharmaceutical industries, genetic engineering, scientific research and medical diagnosis, in which radioactive isotopes can be introduced into the body to examine the shape and function of a diseased organ.

Traditional mechanical methods for separating isotopes, such as those based on the use of centrifuges, are relatively slow and inefficient. In the past 25 years, highly effective laser methods for isotope separation have appeared, but their use is limited by the need to precisely fine-tune the laser beam to one particular isotope, which renders these approaches quite expensive.

The new Weizmann Institute method -- whose first experimental application has just been reported in Physical Review Letters and reviewed in Physics Today -- combines the advantages of these two approaches, namely, the universality of mechanical separation with the efficiency of the laser techniques.

The method is referred to as "wavepacket technology" because it makes use of wavepackets, a quantum theory concept describing particular states in which electrons, atoms or molecules can be found. This concept can be applied to isotope separation because different isotopes are clearly distinguishable by the motion patterns of their wavepackets.

"Although wavepackets were first described at the dawn of quantum mechanics more than 70 years ago and appear in all physics textbooks, they have only been intensively studied experimentally in the past decade," Averbukh says.

"And now the new isotope separation method offers what, to the best of my knowledge, is the first practical application of this concept."

The wavepacket approach is based on a concept developed within the framework of the quantum theory, one of the best proven but least intuitive scientific theories.

This theory makes, for example, the mind-boggling statement that the location of a particle, or its quantum state, is not a certain fact but a matter of probability. Hence from the perspective of quantum physics, an electron moving in orbit around the nucleus of an atom appears to be a sort of "probability cloud" spread over a relatively large area, and not, as classical physics maintains, a particle whose position is clear-cut.

Introducing some "sanity" into this seeming blurring of reality, Austrian physicist Erwin Schroedinger in 1926 formulated a concept intended to "reconcile" the quantum theory's view of particles with the classical model.

According to this concept, different quantum states of a particle -- each of which can be visualized as a wave -- can be combined to form a "wavepacket." And when the waves in the packet come together to form a single peak, for a certain time interval this packet behaves like a classical electron, atom or molecule that obeys the laws of Newtonian mechanics.

As befits the strange world of quantum physics, wavepackets lose shape, spread and disintegrate over time. But in 1989 Averbukh, together with his colleague Dr. Naum Perelman (both working in the former Soviet Union at the time), expanded this scenario of Schroedinger's.

They discovered that, amazingly enough, wavepackets are able to reappear, sometimes as several smaller identical "probability clouds." Each isotope follows its own cyclic pattern of such quantum "revivals," whose periodicity serves as its unique "identity card."

Now Averbukh, who has worked at the Weizmann Institute since immigrating to Israel in 1991, has developed a method for separating isotopes using their "revival" identity. First, an extremely short laser pulse is applied to a mixture of different isotopes, exciting them and causing them to form wavepackets.

At the precise moment when the wavepackets of the two isotopes are identifiably different, a second laser pulse is applied to ionize one isotope but not the other. The ionized isotopes are then simply pulled out of the mixture using an electric field. This method, in effect, creates an isotope separation "machine" operating at the level of a single molecule or atom.

In the study reported in Physical Review Letters (77, 3518, 1996), conducted by Averbukh with scientists from the Steacie Institute for Molecular Sciences in Ottawa, Canada, the wavepacket approach was successfully applied in a laser lab to separate two different isotopes of bromine. The Canadian team included Dr. Marc J.J. Vrakking, Dr. D.M. Villeneuve and Dr. Albert Stolow.

The researchers believe that apart from isotope separation, wavepacket technology can also be used to control chemical reactions and in a variety of other applications. For example, it may eventually be applied to the development of ultrafast switches that turn on and off a trillion times a second -- one thousand times faster than the fastest existing switches.

Yeda Research & Development Co. Ltd., the Weizmann Institute's technology transfer organization, has filed a patent application for the wavepacket isotope separation approach.