[Napsnet] SPECIAL REPORT: The South Korean Laser Isotope Separation Experience

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NORTHEAST ASIA PEACE AND SECURITY NETWORK
***** SPECIAL REPORT *****

-“The South Korean Laser Isotope Separation Experience”
By Mark Gorwitz

October 18th, 2004

A world wide web version of this report can be found at:
http://www.nautilus.org/archives/pub/ftp/napsnet/special_reports/0441-ISIS.html

This paper published by the Institute for Science and International 
Security. Another copy may be found at: 
http://www.isis-online.org/publications/dprk/sklisword2.html

Nautilus invites your contributions to this forum, including any responses 
to this essay.

--------------------
CONTENTS
I.  Introduction
II.  Essay by Mark Gorwitz
III.  Notes by Mark Gorwitz
IV.  Nautilus invites your responses

I.  Introduction

Mark Gorwitz, Special Contributor to the Institute for Science and 
International Security (ISIS) Online, writes: “Taken as a whole, the open 
literature shows the extent of South Korean research in the area of laser 
isotope separation. Technology learned in one enrichment program has been 
successfully transferred to another. The undeclared uranium enrichment 
experiments have been the main beneficiary of such a transfer and are a 
proliferation concern that deserves to be thoroughly examined by the IAEA.”

The views expressed in this article are those of the author and do not 
necessarily reflect the official policy or position of the Nautilus 
Institute.  Readers should note that Nautilus seeks a diversity of views 
and opinions on contentious topics in order to identify common ground.

II.  Essay by Mark Gorwitz

-“The South Korean Laser Isotope Separation Experience”
by Mark Gorwitz

Introduction and Findings:

South Korea has been actively researching molecular and atomic vapor laser 
based isotope separation techniques since the early 1980s. These efforts 
have been geared towards supplying industrial, medical and civilian nuclear 
applications.

In September 2004, a number of news reports appeared concerning experiments 
conducted by researchers at the Korea Atomic Energy Research Institute 
(KAERI) using laser isotope separation to enrich and separate uranium 
isotopes. News sources reported that levels of enrichment averaged about 
10% and reached almost 80%. According to the South Korean government, about 
200 milligrams of enriched uranium were produced. South Korea reportedly 
used 3.5 kilograms of uranium as feed for the experiments.

According to a 2001 article written by Lawrence Livermore National 
Laboratory, an atomic vapor laser isotope separation program (AVLIS) "was 
regarded as too difficult a technology for a typical proliferating nation 
in the middle economic rank to utilize. However as technology advances, 
this will not remain so".(2) The current case of South Korea is 
illustrative of this. South Korea was able to successfully apply knowledge 
learned in earlier AVLIS efforts on elements such as gadolinium and 
ytterbium to an undeclared uranium enrichment program. Nucleonics Week 
quotes Dr. Jeffrey Eerkens, an expert in laser isotope separation methods, 
as saying, "on the basis of the experience that KAERI scientists apparently 
acquired using the laboratory equipment for separation of stable isotopes 
such as ytterbium or samarium, it would have been very straightforward for 
KAERI to have enriched a small quantity of uranium using the same 
equipment. The required frequency for U-235 is different but the technology 
is the same. If they knew how to enrich ytterbium, then they could enrich 
uranium".(3)

This report, which surveys the open literature, shows the extent of South 
Korean research in the area of laser isotope separation. The undeclared 
uranium enrichment experiments are a proliferation concern that deserve to 
be examined thoroughly by the IAEA.

Background:

South Korea's multiyear laser isotope separation research programs have 
involved researchers from KAERI, private industry and academia. The 
majority of the program's research efforts were clearly aimed at industrial 
and medical applications. Pertinent scientific research that has been 
carried out and reported in publicly available reports and the scientific 
literature was analyzed for this and an earlier report. Efforts in both the 
molecular and atomic vapor isotope separation schemes were looked at. 
Sources used were Chemical Abstracts, Engineering Abstracts, Nuclear and 
Energy Citation Abstracts, INSPEC and the European patent database.

The scientific literature concerning activities in the laser isotope 
separation area by South Korea has been fairly detailed, and open 
scientific publications of experiments detailing spectroscopy of uranium 
vapor date back to the early 1990s. Although most publications have been 
published in Korean and have not been translated, abstracts are usually 
available for those publications. A significant number of conference and 
scientific papers have also appeared in the English-language scientific 
literature. Lastly, a number of patents dealing specifically with laser 
isotope separation have been issued to South Korean researchers.

The Laser Spectroscopy Laboratory and the Laboratory for Quantum Optics, 
both located at the Korean Atomic Energy Research Institute (KAERI) in 
Taejon, are the lead laboratories for research in the laser isotope 
separation area. Support is provided in the spectroscopy area by the 
Department of Physics, Korea Advanced Institute of Science and Technology, 
Taejon, the Department of Physics, Sogang University, the Department of 
Physics, Chungnam University, and the Department of Physics, Yeungnam 
University. TEA CO2 laser research is done by the Department of Physics, 
Sogang University, Seoul. The Department of Physics, Kyungpook National 
University, Daegu, the Department of Mechanical Engineering, Chosun 
University, and the Department of Physics, Chonnam National University, 
Kwangju are carrying out copper vapor laser research. Support in the dye 
laser area has been provided by the Department of Physics, University of 
Ulsan, Ulsan. The Korea Research Institute of Standards and Science, 
Taejon, has provided support in the optics area.

Molecular Laser Isotope Separation (MLIS) Related Research:

The early South Korean efforts during the mid 1970's were directed towards 
the separation of isotopes of light elements by the multi-photon 
dissociation process. TEA CO2 lasers were developed for this purpose.

A 1981 KAERI document contained the following information about early laser 
isotope separation efforts:

"To achieve a separation of isotopes by multi-photon dissociation method, 
high power laser is needed. In our laboratory, a photo-ionized TEA CO2 
laser which has a power of 1 Mega Watt was successfully constructed. In TEA 
CO2 laser operation it is important for high power and high efficiency to 
establish a stable glow discharge uniformly, UV pre-ionization employing a 
trigger wire was used. The uniformity of discharge between electrodes 
depended on the capacity of storage capacitor. And the number of initial 
electrons effective in the main gap discharge depended on the position of a 
wire. The electrode shape, which has a Rogowski profile, was an important 
parameter for the uniform discharge. Optimum performance was obtained with 
1:1:8 ratio of CO2:N2:He at atmospheric pressure. Also a discharge tube (68 
cm in length) for a CW ion laser was made of quartz tubes. It consists of a 
small bored tube (5 mm i.d.), water jacket, gas return path, Brewster 
windows, and electrodes closed by a molybdenum foil seal. The 
current-voltage characteristics have been investigated up to current 
density of 66A/cm2 for the tube filled with 0.1 torr Xe."(4)

The effects of seeding on the stability and uniformity of the glow 
discharge in a TEA CO2 laser and well as the output were studied. 
Researchers at Sognag University studied the mechanism of decomposition of 
various additives and concluded that studying the decomposition products of 
tri-n-propylamine (current best quality seeding agent) will lead to a more 
effective seeding gas."

Some details of later TEA CO2 research efforts have been published. The 
following information was obtained from a 1986 paper: "Our experiment is 
achieved sing a double-discharge TEA CO2 laser with a hexagonal acrylic box 
having Brewster windows and a CO2 cell. A gold-coated total reflector with 
10-m radius of curvature and a ZnSe flat half mirror with 80% reflectivity 
mounted to the end of the cell are set 175 cm apart, forming an optical 
cavity. The electrodes of brass have a discharge length of 24 cm, a width 
of 2 cm, and a height of 3 cm. The pre-ionization is achieved using two 
tungsten trigger wires with 330-pF trigger capacitors. The laser is 
energized using a two-stage Marx-Bank generator with a 26-nF capacitor per 
stage, and the voltage to the circuit is set at 18 kV. The laser medium 
introduced into the amplifier is a mixture of CO2, N2, and He gas, of which 
the ratio is 2:1:10".

"The CO2 cell made of pyrex glass has a total length of 60 cm and a heating 
length of 35 cm. An anti-reflecting-coated ZnSe flat window blocks one end 
of the cell facing the cavity, and heating is done by a Ni-Cr wire and a 
transformer. Half of the CO2 gas flowing in the cell is exchanged per second".

"The laser operates a dominant 10.6-um P-20 and P-18 line in a low pressure 
of CO2 cell and a single-line operation of P-20 at more high pressure".(5)

Further information was contained in a 1985 KAERI document:

"Research on laser isotope separation of deuterium using Infrared 
Multi-Photon Absorption/Dissociation (IR MPA/D) and UV pre-dissociation 
were reviewed and several kinds of lasers were built for this purpose. A 
tunable TEA CO2 laser with power of about 10 MW was assembled and a HF 
chemical laser with output energy of 300 mJ was built. These lasers are not 
ready to be used as sources for IR MPA/D experiment yet. The TEA CO2 laser 
needs modification for more stable output and higher repetition rate and 
the HF chemical laser needs improvement for more output energy and 
tunability. Also a KrF excimer laser was built for UV pre-dissociation 
experiment, but requires modification for stable output".(6)

Some additional details describing the above lasers are available. The TEA 
CO2 laser made in 1983 was reassembled to increase the output power and 
tune the wavelength. The HF chemical laser was produced for IRMPD of 
molecules with an -OH structure. And lastly the KrF excimer laser was to be 
used to pump a dye laser to generate 345 nm radiation for UV 
pre-dissociation experiments.

Details are also available of hydrogen isotope separation experiments. Both 
deuterium and tritium separation were looked at. For deuterium, the 
literature was reviewed and a state of the art report issued 
(KAERI/AR-248/84).(7)

The absorption coefficient of CDF3 neat and in the presence of CHF3 was 
determined. The effects of added inert gas on CDF3 absorption was also 
studied. Actual photochemical energy requirements for deuterium separation 
by multi-photon dissociation were calculated and compared to the H2S/H2O 
system.

A 1990 paper reports on the selective photo-dissociation of 32SF6 by a TEA 
CO2 laser. The following details are provided: The molecular selective 
photo-dissociation of 32SF6 by IRMPD in natural SF6 was investigated using 
a pulsed TEA CO2 laser. The TEA CO2 laser consisted of 2 stage Marx bank 
power supply and for uniform discharge, it was designed to use the UV 
pre-ionization generated by corona discharge along the surface of glass 
sheets on both electrodes. The laser gave multimode energy of about 1.7 
J/pulse, pulse width (FWHM) of less than 100 ns and overall efficiency of 
about 9.6%. The laser beam (10P(20) line) was focused into the reaction 
cell filled with reactant mixture, SF6/NO or SF6/H2; the 32SF6 absorbs the 
laser photons through multi-photon absorption and dissociates, selectively. 
The study focused on the relative reaction selectivities of 32SF6 under 
influence of additives, such as NO and H2, which were expected to act as a 
radical scavenger of free radical energy, generated from SF6 dissociation. 
The relative reaction selectivity was determined by measuring the unreacted 
34SF6/32SF6 ratios using a mass spectrometer".(8)

The following further information was contained in a 1991 KAERI document:

"For IRMPA/D studies, we measured IR fluorescence emitted from 
vibrationally excited DF* or HF* produced in IRMPD of CDF3/CHF3 using an IR 
monochromator and InSb IR detector. We could detect the ir fluorescence of 
HF from the sample mixture CDF3 /CHF3 to which the CDF3 was selectively 
irradiated. This means that the molecular selective excitation of CDF3 in 
CDF3/CHF3 did not give molecular selective dissociation of CDF3 because of 
the fast intermolecular vibrational energy transfer from the excited state 
CDF3 to the ground state CHF3. This technique will play an important role 
for measuring isotope selectivity in IRMPD of CDF3/CHF3. We developed data 
analysis technique for the laser fluorometer to improve analytical speed 
and accuracy. We calculated fluorescence intensity at time zero using two 
values obtained by integration of two intervals on the time-resolved 
fluorescence signal. Applying this method, we could eliminate any 
interference effects from quenching elements or temperature fluctuations of 
samples, effectively".(9)

Laser Development:

Copper vapor laser research, which is an integral part of any atomic vapor 
laser isotope program, has been ongoing since the mid 1980's. The early 
foreign assisted research was done at the University of New Mexico and the 
lead Korean researcher K. Im was on leave from Chonnam National 
University.(10) Another scientist, N. Sung, after completing work at the 
University of New Mexico on transverse-discharge copper vapor lasers 
returned to the Korea Advanced Energy Research Institute. The results 
published on this research claimed that the transverse-discharge copper 
vapor lasers (CVL) could be developed for practical applications.

In a 1987 paper, N. Sung stated: "The copper-vapor laser (CVL) has recently 
received a great deal of attention as a practical device for applications 
in dye-laser pumping for isotope separation..."(11) A research contract 
from Lawrence Livermore National Laboratory provided part of the funding 
for this effort.

By 1987 researchers at Kyungpook National University had succeeded in 
developing a 1.5 W, 5 kHz repetition rate laser using xenon as the inert 
buffering gas. Researchers at Chonan and Chosun Universities are also 
involved in the development of copper vapor lasers useful for pumping dye 
lasers.

A 1978 paper provided details of a tunable dye laser pumped by a pulsed N2 
gas laser suitable for spectroscopic experiments: "The dye laser is 
transversely pumped by the focused 3371 AA emission line of a pulsed N2 gas 
laser. The N2 laser's peak power output is 1 MW at the repetition rate up 
to 100 pulses/sec. The dye laser output power is 1.36 kW at 2.5 ns pulse 
width, the spectral bandwidth is 3.3 x10-2/AA centered at 5900 AA, the 
emission wavelength is variable over the visible range by choosing one of 
seven dyes such as PPO, PBBO, POPOP, 4-methyl umbelliferone, Rhodamine 6G, 
Rhodamine B and cresyl violet perchlorate. Using the present tuning method, 
with the holographic grating, the prism and the etalon, the tunable dye 
laser with a short cavity, 15 cm in length is realized".(12)

A KrF excimer laser for pumping a dye laser was developed in the early 
1980's. The 345 nm generated laser radiation was to be used for UV 
predissociation experiments. Problems, however, arose in assembling the 
grating required for the dye laser.

Considerable work has also been done on the development of XeCl excimer 
lasers. Experiments have focused on the development of controllable long 
pulse lasers using a simple capacitor-discharge circuit. An injection 
locked laser with a modified-branch conformal unstable resonator was also 
developed and characterized. Lastly, a UV-preionized discharge-pumped laser 
was developed. The following design details were provided: "A maximum 
output energy of 96mJ/pulse, pulse duration of 20 ns (FWHM) and beam cross 
section of 7*17 mm2 have been extracted from the gas mixture of HCl/Xe/He = 
0.2/3.1/96.7% in total pressure of 3.5 atm, which was pumped with 26 kV of 
charging voltage."

Frequency has been controlled in pulsed dye lasers by the use of a volume 
holographic transmission grating along with a tuning mirror. The purpose of 
the grating was to maximize energy conversion and to minimize linewidth 
distortion. A Nd:YAG laser was used to pump a dye cell containing Rhodamine 
6G. And the following conclusion was reached: "The energy conversion 
efficiency is greatly improved compared with that of the grazing incidence 
type, and the linewidth is reduced compared with that of the Hansch type". 
The Hansch type was used in early Israeli pulse dye lasers.

Argon ion pumped dye laser research has been conducted by T.S.Kim of the 
Department of Physics, University of Ulsan, in collaboration with the 
University of Rochester, Rochester, New York. These experiments involved 
measuring the photon-number fluctuations in a single-mode dye jet laser. 
Argon ion pumped lasers are known to have been used by many groups in 
atomic vapor spectroscopic studies. Recent experimental results in this 
area have been reported by Israeli researchers.

Ti:Sapphire lasers suitable for spectroscopic studies have been developed 
by KAERI. The following details have been provided concerning this laser: 
"We have constructed a self-seeded Ti:sapphire laser oscillator by using a 
dual-cavity configuration that consists of a Littman configuration cavity 
and a partially reflecting feedback mirror. This configuration can be 
decomposed with two kinds of cavity, a grazing-incidence cavity and a 
standing-wave cavity. The former behaves as an injection seeder and the 
latter as a slave oscillator. This Ti:sapphire laser system is capable of 
delivering a continuously tunable laser pulse with a narrow linewidth. 
Injection at the laser emission region of the free-running Ti:sapphire 
laser resulted in essentially complete energy extraction."(13)

Recent work has also been reported on the development of an extended-cavity 
violet diode laser that has been used to perform spectroscopic studies on 
samarium.(14) Earlier experiments done on samarium had used a commercial 
single-mode tunable diode laser. Similar experiments were also done on 
ytterbium and gadolinium.

Atomic Vapor Laser Isotope Separation (AVLIS) Related Research:

South Korea has had a very active program in the atomic vapor laser isotope 
separation area for many years. Both American and Russian researchers have 
provided scientific support to the South Korean program. American support 
has been in the area of laser development and Russian support has been in 
the AVLIS area. This AVLIS program has mainly aimed at separating isotopes 
of the lanthanide elements for use as burnable poisons in nuclear reactors, 
has been under development for a number of years, and has focused on the 
metals erbium, gadolinium, samarium, and ytterbium. This work has been a 
partial collaborative effort with the General Physics Institute, Moscow. 
Support has been provided by Russian researchers in both the theoretical 
and experimental areas.

Arisawa, a leading Japanese laser isotope researcher (JAERI) stated: "that 
tunable lasers for AVLIS application require high repetition rate, high 
average power, high reliability and high stability. Ti:sapphire laser or 
F-center laser could be candidates, but each needs a good pumping source 
and nonlinear crystal for converting the wavelength from fundamental IR 
wavelength to visible one at high efficiency. A direct application of the 
diode laser as a tunable source might be promising in the future, if 
wavelength range coverage, average power and price are satisfied".

Results were published in 1990 on selective photo-ionization of magnesium 
atoms. The abstract presented the following information: "A spectroscopic 
study of photoelectrons arising from non-resonant multi-photon ionization 
of magnesium atoms in a high intensity laser field is performed 
experimentally. Both the 532 nm and 1064 nm excitations in the intensity 
region of 1010-13 W/cm2 are used for single and double ionization. The 
emphasis is placed on the photoelectron spectra and their variations with 
laser wavelength, intensity, and polarization. Also, the ionization process 
of doubly charged ions which can be produced either by a stepwise process 
or by the simultaneous removal of two electrons is discussed".(15)

The effects of the AC-Stark shifts on the selective resonant ionization of 
both lithium (Li) and Strontium (Sr) were studied. The abstract of a 1993 
paper reads: "The authors show that the ionization rate and the isotope 
selectivity become sensitive functions of the wavelength and intensity of 
the laser due to the AC-Stark shifts of the energy levels involved in the 
two-photon resonant three-photon ionization of Li and Sr atoms. They also 
examined the optimum conditions for isotope separation."(16)

An April 1991 report entitled "Development of nuclear fuel: Development of 
laser spectroscopic technology in nuclear industry"(17), details multi-step 
photo-ionization experiments on mercury and the development of a computer 
code (MCDF) for calculating the transition probabilities of mercury atoms.

he May 1992 progress report states that the following research efforts were 
carried out that year: "multi-step photo-ionization spectroscopy of mercury 
atom carried out by 3-color 3-step ionization scheme, selective 
photo-ionization using polarization spectroscopy, design and construction 
of an ion separator chamber and theoretical study for spectroscopic 
parameters of mercury".(18)

The January 1993 progress report states "that laser atomic spectroscopic 
study on actinium element has been performed in many areas of 
spectroscopy." The report goes on to further state "In spectroscopic 
experiment, first and second ionized states for actinium element are 
identified and the most efficient scheme for actinium element is 
identified. In addition, the corrosion problem for filament material due to 
the heating of the actinium element has been studied".(19)

Another 1993 report entitled "Magnetic field effect on selective 
photo-ionization", states "the magnetic field effects on selective 
photo-ionization of the atoms of the lanthanides have been examined in a 
point of view of the enhancement of the efficiency of selective 
photo-ionization".(20)

In 1994 details on the optogalvanic spectroscopy of uranium, thorium and 
rubidium using diode lasers were published. Using commercially available 
sources, detailed experimental results were later published for erbium, 
gadolinium, lanthanum, lutetium, and samarium.(21) Further optogalvanic 
results were presented for uranium in a 1999 paper.(22) A reading of this 
and other papers shows that research in the optogalvanic spectroscopy area 
has been ongoing since the early 1990s. Brazilian researchers have openly 
reported similar research as part of their uranium laser isotope separation 
program. In a 1999 review of the Brazilian uranium laser isotope 
separation, it was stated that the optogalvanic effect "is a powerful and 
inexpensive technique for investigation of atomic and molecular species, 
and is particularly useful in the spectroscopy of refractory elements, like 
uranium"(23). Brazilian researchers using optogalvanic spectroscopy were 
able to measure two photon absorption assignments, lifetimes of uranium 
excited states and superelastic relaxation rates. South Korean scientists 
were able to obtain similar information from their experiments.

Argon/hollow cathode and sealed tube sources as sources of uranium vapor 
for spectroscopic experiments later gave way to resistively heated sources. 
A high temperature oven was later developed to generate vapor of the 
element being studied.
Hollow cathode sources were another means of measuring multi-photon 
absorption spectroscopy in uranium vapor used by KAERI. These experiments 
used commercial argon buffer gas sources obtained from Cathodeon, Ltd. 
Collisions between the buffer gas and the metallic vapor can cause problems 
in observing certain excited transition states. However, by careful choice 
of experimental technique, they were able to obtain useful information on 
photo-ionization and lifetime states of uranium and other elements such as 
samarium and ytterbium.

Another paper presented details of numerical calculations. The following 
abstract was given: "The authors present results of numerical calculations 
obtained through solving integral-differential equations for the electron 
density matrices sigma11 and sigma22 and the ionization rate P in 
two-photon resonant three-photon ionization of Li and Sr, as well as the 
selectivity S for isotope separation. They also compare their results with 
the predictions from quasi-stationary solutions based on the rate 
approximation. Their numerical results for P and S show that the 
quasi-stationary solutions are valid at very high and very low laser 
intensities when the isotope shift is large as in the case with 4s of 6Li 
and 7Li, and that the validity is rather limited in the cases with small 
isotope shift as in 5p/2/1/S of 88Sr and 90Sr". These calculations were 
performed on a CRAY C90 YMP supercomputer.

Information on the selectivity of photo-ion extraction for isotopes of the 
rare earth elements has been published. The abstract presented the 
following details: "An analysis of selectivity of three-stage 
photo-ionization for isotopes of rare-earth atoms with (1-2) GHz isotopic 
shifts in the absence of polarization effects is performed. Because of 
field broadening, a sufficiently high selectivity eta > or = 100 is 
achieved only for low average laser intensities, I < or = 10 mW/cm2. The 
excitation of ions produced in photo-ionization of atoms by an electric 
field is investigated. The dependence of the selectivity on ion and gas 
densities, as well as on parameters of the external field, is calculated".(24)

Experimental details were presented in 1995 on actual three-photon 
polarization spectroscopy of ytterbium vapor. The General Physics 
Institute, Moscow, was thanked for useful discussions. The following 
experimental details were provided: "The first (Lambda Physik FL3002/E) and 
second (Lumonics HD-300) dye lasers were pumped by the second harmonic of a 
Nd:YAG laser (Lumonics HY750). The wavelength of dye laser 1 was 555.648 
nm, and that of dye laser 2 was 581.067 nm. Ytterbium atoms are excited to 
the intermediate state, 4f13(2F7/2)6s26p3/2 (J=2) by these two dye laser 
pulses. Atoms in the intermediate state are excited to the autoionizing 
state by the third laser (Lumonics HD-300), which was pumped by the third 
harmonic of the Nd:YAG laser. The wavelength of the third laser was scanned 
from 430 to 660 nm using the dye Rhodamine 640, 610, and 590 and Coumarine 
540A, 500, 480, 460, and 440".

"The laser have pulse durations of about 8 ns and are pulsed at the rate of 
10 Hz. The laser pulses, which are used for the excitation of intermediate 
states, arrived at the chamber simultaneously. Approximately 8 ns later, 
the third laser pulse arrived at the chamber, to avoid the two-photon 
process. All lasers were incident to the chamber, with an angle of less 
than 2° between them".

"The linear polarization of lasers was improved by a Glan polarizer placed 
at the exit of each laser. Circular polarizations of the first and second 
lasers were made by lambda/4 waveplates. For the third laser, we used 
lambda/2 or lambda/4 Fresnel rhomb phase retarders. Each polarizer was 
placed in front of the chamber".

"The linewidth of the exciting laser was 5 GHz, and those of the other 
lasers were 3 GHz. The wavelengths of the third laser were calibrated by 
recording the optogalvanic signals from an Ar-Yb hollow cathode lamp 
(Cathodeon) simultaneously with the ion signal. The air wavelengths of the 
Ar lines listed in the NBS table were converted into vacuum wavelengths 
before evaluating the energy levels of the autoionizing states".

"An atomic vapor of Ytterbium (Yb) was generated by heating pure Yb metal 
(99.9%) in a tantalum oven. Atoms were collected by two circular apertures 
placed before the electrodes of a time-of-flight mass spectrometer (TOF 
MS). 1 us later, atoms were ionized by three dye lasers, and the ions were 
extracted by applying a voltage pulse of 200-V/cm amplitude to the 
electrode for 10 us to reduce the perturbation due to a dc electric field. 
The extracted ions were analyzed by TOF MS (flight length: 1.5 m). Because 
we used natural Yb, a mass analyzer was needed for angular momentum 
identification. The ion signals were integrated by a boxcar (Stanford 
SR245) and stored in a computer".

"When an autoionizing state has a broad linewidth, the peak positions and 
the intensity of the ion signals are affected by the variation of the dye 
laser energy. Thus the laser energy was also recorded as a function of 
wavelength, and ion signals normalized by the laser energy were used for 
analysis. The intensity of the third laser was attenuated with a neutral 
density filter enough to avoid depletion broadening of the line profile".

"As a result of this experiment, 17 autoionizing states were found in the 
investigated energy range from 50400 to 58000 cm-1. The line profiles of 
the autoionizing states were nearly symmetric, and some states had very 
narrow linewidths comparable to those states observed by Bekov. The results 
of angular momentum identification required revision of electronic 
configuration assignments of the states investigated by Borisov. We propose 
that some autoionizing states with large excitation cross sections be used 
for efficient photo-ionization".(25)

In 1996, results were presented on the two-photon selective 
photo-ionization of ytterbium. Both ion yields and selectivity were 
reported for 168Y. They claimed "that the selectivity for 168Y was 
increased higher than 20000 when the laser was blue-detuned to the most 
efficient position".(26) For the coherent excitation this experiment used a 
commercial single-mode dye laser (Lumonics, HyperDYE SLM) pumped by a 
frequency-doubled Nd:YAG laser (Lumonics, HY-750). Ionization was done 
using a broadband dye laser (Lumonics, HD-500) pumped by a 
frequency-tripled Nd:YAG laser. Ytterbium vapor was generated from a 
natural source using resistive heating.

The effect of laser intensity on the selective three-step photo-ionization 
of 168Yb has also been studied and it was noted that a noticeable change in 
isotope abundance of 168Yb occurred when the laser intensity was varied.

A later paper added the following comment: "Some scheme of Yb has the 
possibility to yield high efficiency in selective ionization of an 
isotope".(27) Using a diode-pumped solid-state laser and three-color dye 
lasers 20 mg of 25.8% enriched 168Yb was produced.

Further detailed information on the spectroscopy of atomic vapors is 
contained in a series of April 1998 reports. The first report focuses on 
studies of the "thermodynamic properties of metallic atoms in both gas and 
beam states". The second report focuses on ytterbium and presents details 
on experiments on fluorescence in dense atomic vapor, single color 
two-photon resonant three-photon ionization and the production of a 
high-temperature oven and its spectroscopic application. A third report 
provides information on the hyperfine coupling constants. And the in last 
report of the April series detailed studies were reported on both the flux 
and velocity of resistively heated atomic beam sources "in order to control 
the population of a specific state so that we can increase the efficiency 
of photo-ionization". These are some of the most important parameters 
involved in laser isotope separation.(28)

Similar two and three-color photo-ionization experiments have been reported 
for erbium, gadolinium and samarium in a series of scientific papers. 
Additional details of experimental work involving samarium were presented 
in an April 2000 report.(29) Using a high temperature oven, KAERI 
scientists were able to generate samarium vapor having an atomic density of 
8x1014 atoms/cm3.

A very recent review on the Indian uranium laser isotope separation program 
states "that after establishing that uranium isotopes can be selectively 
ionized, we had to wait for some time to plan an isotope collection 
experiment. It was not possible to carry out the experiment unless a major 
improvement was made regarding uranium vapor generation".(30) Any 
large-scale enrichment program would use an e-beam source for heating and 
vaporizing material and this is not beyond the current capability of South 
Korea. KAERI scientists have been issued two patents in the area of e-beam 
heating in recent years.

At least three patents dealing specifically with laser isotope separation 
have been issued to scientists from the Korea Atomic Energy Research 
Institute (KAERI). One patent deals with feedback control methods, another 
deals with isotope separation of lanthanum or actinium by diode laser, and 
the third deals with an isotope separation scheme for thallium. These 
patents serve to illustrate the level of competence that South Korea has 
achieved in the area of laser isotope separation.(31)

Conclusion:

Taken as a whole, the open literature shows the extent of South Korean 
research in the area of laser isotope separation. Technology learned in one 
enrichment program has been successfully transferred to another. The 
undeclared uranium enrichment experiments have been the main beneficiary of 
such a transfer and are a proliferation concern that deserves to be 
thoroughly examined by the IAEA.

III.  Notes by Mark Gorwitz

1. This review was adopted from a 1996 report entitled: Second Tier Nuclear 
Nations: Laser Isotope Separation Programs, written by the author. Further 
scientific references may be found in the Open Source Scientific References 
bibliography associated with the current article.
2. For proliferation concerns over laser isotope separation see: S.A. 
Erickson, Nuclear Proliferation Using Laser Isotope Separation - 
Verification Options, UCRL-JC-145343, October 2001
3. Nucleonics Week, September 13, 2004, South Korea's KAERI optics lab used 
dye lasers to separate U-235
4. D.W. Suh, Apparatus for the Isotope Separation, KAERI/RR-256/80
5. Chil-Min Kim, Hot Band Effect of the Vibrational Transitions Lines in a 
Tunable CO2 Laser, New Physics (Korean Physical Society), Vol. 26, p101-6, 
1986
6. C.J. Kim, Development of Applied Optical Techniques Using Lasers, 
KAERI/RR-437/84
7. No Author Listed, Isotope Separation of Deuterium Using Lasers, 
KAERI/AR-284/84
8. Jang-Soo Shin, Molecular Selective Photodissociation of 32SF6 by Pulsed 
TEA CO2 Laser, Korean Applied Physics, Vol. 3, p60-6, 1990
9. C.J. Kim, Development of Applied Optical Techniques Using Lasers, 
KAERI/RR-437/84
10. K. Im, Transverse-Discharge Copper-Vapor Laser, IEEE Journal of Quantum 
Electronics, Vol. QE-21, p1747-8, 1985
11. N. Sung, Stimulated Emission in Optically Pumped Atomic-Copper Vapor, 
Optics Letters, Vol. 12, p885-7, 1987
12. Hwan Suh, Tunable Dye Laser Excited by a Pulsed Nitrogen Gas Laser, New 
Physics (Korean Physical Society), Vol. 18, p8-14, 1978
13. Do-Kyeong Ko, Self-Seeding in a Dual-Cavity-Type T-Sapphire Laser 
Oscillator, Optics Letters, Vol. 20, p710-2, 1995
14. Jae Ihn Kim, Frequency-Stabilized High-Power Violet Laser Diode with a 
Ytterbium Hollow-Cathode Lamp, Optics Letters, Vol. 28, p245-7, 2003
15. Dalwoo Kim, Laser Intensity and Polarization Effects on the Multiphoton 
Ionization of Magnesium Atoms, Journal of the Korean Physical Society, Vol. 
23, p191-8, 1990
16. Wang Young Oh, Effect of AC-Stark Shifts on the Selective Resonant 
Ionizations, New (Korean Physical Society), Vol. 33, p24-30, 1993
17. J.M. Lee, Development of Nuclear Fuel. Development of Laser 
Spectroscopic Technology in Nuclear Industry, KAERI/RR-1007/91
18. J.M. Lee, Development of Laser Spectroscopic Technology in Nuclear 
Industry, KAERI/RR-1117-92
19. H.K. Cha, Study on Laser Atomic Spectroscopy, KAERI/RR-1166/92
20. J.M. Lee, Magnetic Field Effect on Selective Photoionization, 
KAERI/TR-367/93
21. S.C. Lee, Optogalvanic Spectroscopy of U, Th, and Rb Using Diode 
Lasers, Journal of the Korean Chemical Society, Vol. 38, p34, 1994
22. E.C. Jung, Specific Behaviors of Dynamic Optogalvanic Signals of an 
Argon Hollow Cathode Discharge, Optics Communications, Vol. 161, p149-55, 1999
23. C. Schwab, etal., Laser Techniques Applied to Isotope Separation of 
Uranium, Progress in Nuclear Energy, Vol. 33, p217-64, 1998
24. Jongmin Lee, Selectivity of Photoion Extraction for Isotopes of 
Rare-Earth Elements, Laser Physics, Vol. 4, p1132-8, 1994
25. Jong-Hoon Yi, Autoionization States of the Ytterbium Atom by 
Three-Photon Polarization Spectroscopy, Physical Review A, Vol. 51, 
p3053-7, 1995
26. Hyunmin Park, Selective Photoionization of the Ytterbium Atom by 
Coherent Two-Photon Excitation, Physical Review A, Vol. 53, p1751-55, 1996
27. Hyunmin Park, Isotope Separation of Yb-168 Stable Isotope for 
Low-Energy Gamma-Ray Sources, Undated report
28. Study on Thermodynamic Properties of Metallic Vapor, KAERI-CM-197/97
The Spectroscopy in the Atomic Vapor, KAERI-CM-207/97 The Development of 
Rydberg Filed Ionization Method and Its Application to the Infinitesimal 
Quantity Analysis, KAERI-CM-208/97
Physical and Chemical Studies of Atomic Beams, KAERI-CM-213/97
29. Development of Atomic Spectroscopy Technologies - The Characteristics 
of Laser Beam Propogation in Resonant and Near-Resonant Atomic Media, 
KAERI-CM-385/98
30. P. Ramakoteswara Rao, Laser Isotope Separation of Uranium, Current 
Science, Vol. 85, p615-33, 2003
31. Cha Hyeing Gi, Real-Time Feedback Control Method of Dye Laser 
Wavelengths Using Mass Composition Signal Generated From Laser Isotope 
Separation Process, KR2003041656, 2003
Lee Jong Hun, Isotope Separation Device of Lanthanum or Actinium by Diode 
Laser, KR2003051485, 2003
Jeong Do-Young, Method for Isotope Separation

IV.  Nautilus invites your responses

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