236-Uranium RIMS

The mixture of naturally occurring uranium isotopes contains only very few U-236 atoms; the isotopic abundance in comparison to U-238 is lower than 1E-12. U-236 is produced predominantly by neutron capture of U-235 in nuclear reactors. Therefore, material from nuclear facilities can show a significantly increased amount of U-236, higher by many orders of magnitude than the natural value.

Formation of Uranium-236 in Nuclear Reactors - click for bigger version

Thus, sensitive determination of U-236 can distinguish a possible anthropogenic uranium contamination from natural uranium.

Vacuum Chamber for Detection of Uranium-236
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Refined studies about nuclear fuel migration in the environment could become possible by direct determination of U-236. Particularly, the dynamics of the formation of soluble U(VI) species (which are accessible to living organisms) out of anthropogenic uranium sources might be studied. Primarily interesting samples can be taken from places where larger amounts of nuclear fuel in the form of inert U(IV) particles were released into the environment, e.g. near to the Chernobyl nuclear power plant.
The method of High Resolution Resonance Ionization Mass Spectrometry (HR-RIMS) has been developed for the selective determination of long-lived radioisotopes. This method is the only one besides the experimentally elaborate Accelerator Mass Spectrometry that can fulfill the outstanding requirements concerning detection sensitivity, isobar suppression and isotopic selectivity needed for ultra trace determination of U-236.

During the project period, HR-RIMS will be adapted and tested for the determination of U-236. This mainly includes detailed laser spectroscopic studies on uranium and investigations on an efficient atomization procedure for analytical samples. Furthermore, the method has to be specified concerning isotopic selectivity, detection efficiency, precision and reproducibility and real analytical samples will be measured.

Analysis of Three-Dimensional Laser Spectra
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Curriculum Vitae of Uranium Project

July 2002 start of project

planning phase, literature studies

development and testing of blue diode lasers for HR-RIMS

improvement of laser stabilization routine

October 2002 first use of a blue diode laser for HR-RIMS

construction of a test chamber for HR-RIMS, using a Balzers quadrupole mass spectrometer (QMS)

selection of appropriate first excited level for resonance ionization of uranium at 24066 cm-1

acquisition of blue laser diode, construction of extended cavity diode laser

January 2003 first uranium ions in the Balzers QMS test chamber, using electron impact ionization

first atomization studies for aqueous uranium samples

March 2003 first single resonant laser ionization of U-238 in the HR-RIMS test chamber using blue diode laser as first step, UV light from argon ion laser as ionization step

takeover and initial operation of a Coherent MBR-110 titanium sapphire ring laser system and a Spectra Physics Wavetrain resonant frequency doubling cavity

June 2003 investigation of five possible first excited levels with frequency-doubled titanium sapphire ring laser system concerning precise level energies and saturation powers

acquisition of laser tables

planning and construction of new final vacuum chamber for HR-RIMS of Uranium

takeover and initial operation of high performance ABB Extrel QMS

development and construction of a new laser stabilization system for continuous Lasers with Wavelengths between 400 and 900 nm:

  • construction of ultra stable high finesse Invar-spaced Fabry Perot interferometer
  • development of highly linear piezo drive electronics
  • development of novel spectroscopy-free calibration routine for Fabry Perot interferometer
March 2004 first double resonant laser ionization of U-238 using new final vacuum chamber with ABB Extrel QMS, blue diode laser as first step, Ti:S laser as second step (36174 cm-1), 514 nm line from argon ion laser as ionization step
April 2004 investigation of various possible excited levels around 36100-36400 cm-1: J value assignments, precise level energy determination
May - June 2004 research visit at Pacific Northwest National Laboratory, Richland (WA), USA, work with Dr. Bruce A. Bushaw on triple resonant autoionization of uranium isotopes:

  • autoionization (AI) spectra of U-238
  • determination of suitable excitation path for triple resonant autoionization, second excited level at 36127 cm-1, AI resonance at 49971 cm-1 with FWHM of 50 MHz
  • specification of selected excitation path concerning U-235 hyperfine structure and U-234 isotope shifts

fitting hyperfine structure data

September 2004 analysis of autoionization spectra, first implementation of K matrix theory for complex asymmetric lineshape fitting
December 2004 construction of 829 nm NIR Diode laser for excitation 36127 cm-1 level

simultaneous stabilization of three lasers:

  • blue diode laser at 415 nm
  • NIR diode laser at 829 nm
  • Ti:S laser at 722 nm
January 2005 first triple resonant laser ionization of U-238 at Mainz using above-mentioned excitation path
February 2005 repeat of U-234 isotope shift measurement
first resonance ionization of U-236, measurement of isotope shifts in all three excitation steps
coming soon...
February 2005 measurement and optimization of absolute detection efficiency