Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Separation of Isotopes by Laser Excitation

  1. May 31, 2006 #1
  2. jcsd
  3. May 31, 2006 #2
  4. Jun 1, 2006 #3


    User Avatar
    Science Advisor


    Actually this technology was developed in the early '70s at Lawrence Livermore
    National Laboratory:

    http://www.llnl.gov/50th_anniv/decades/1970s.htm [Broken]

    [Scroll down to the entry for the year 1973 ]

    The process is called AVLIS - Atomic Vapor Laser Isotope Separation.

    Here's how it works. The equations of Quantum Mechanics that describe how
    electrons "orbit" atoms contains in one of its terms the mass of the electron.
    However, since the electron orbits a nucleus of finite mass; this is a "two-body"
    problem - and one has to use the so-called "reduced mass" of the electron -
    m = m*M / (m + M) where "m" is the electron mass, and "M" is the mass of the

    Therefore, the mass of the nucleus "M" enters into the equations that define the
    orbits of the electrons. When you do the calculations, you will find that the
    "ionization potential" - that is the amount of energy you have to add to the outermost
    electron to kick it free and form a charged ion - is very slightly less for U-235 than it
    is for U-238.

    You then need a VERY precise tunable laser. LLNL developed a tunable dye laser
    pumped by copper vapor lasers. You tune the laser to a frequency so that the laser
    photons have enough energy to ionize U-235; but not enough energy to ionize U-238.

    You send this laser zig-zagging through a chamber of UF6 gas or vaporized U and
    the laser will ionize the U-235, but not the U-238. The U-235 can then be deflected
    electrostatically by a charged plate into a separate stream from the U-238.

    To learn more about AVLIS, tunable laser applications, and a spin-off from this
    work called the "laser guide star" - a powerful new tool for astronomers first deployed
    on the 3m Shane telescope at Lick Observatory - but now in use at largest telescopes
    in the world - the Keck telescopes on Mauna Kea in Hawaii; see:


    Dr. Gregory Greenman
    Last edited by a moderator: May 2, 2017
  5. Jun 1, 2006 #4
    I have a figure in one of my texts that shows that light of a lower wavelength is used for U-235. Does this not correspond to a higher energy for ionization for U-235 than for U-238?
  6. Jun 1, 2006 #5


    User Avatar
    Science Advisor


    You may be correct. I forget whether U-235 absorbs at a lower frequency - in which
    case it has a lower ionization potential; or at a lower wavelength [ higher frequency ],
    in which case it has a higher ionization potential.

    Dr. Gregory Greenman
  7. Jun 1, 2006 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    The 'shorter' wavelength corresponds to a slightly higher ionization energy, but not by much.

    According to Wikipedia - The absorption lines of 235U and 238U differ slightly; for example, the 238[/sup]U absorption peak shifts from 5027.4 Angstroms to 5027.3 Angstroms in 235U.

    According to the UWisc NE notes, the wavelength used to ionized U-235 is 5915 Å, so I'll have to find other references to clarify.

    Some discussion here - http://fti.neep.wisc.edu/neep423/FALL99/lecture8.pdf

    Contrary to what other sources say, USEC took over ALVIS from LLNL then abandoned it.

    ref: http://www.llnl.gov/str/News1097.html


    Other interesting data - Table of Ionization Energies (eV) of Atoms and Ions -
    Last edited by a moderator: Apr 22, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook