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Rapid Isotopic Screening

Summary:

We have exploited the built-in functionality of a secondary ion mass spectrometer to provide the capability to screen the enrichment levels of a large number of uranium-bearing particles that are dispersed on a flat substrate. The isotopic measurement protocol is set for a measurement time of about 2.5 minutes per particle so 100 particles can be screened in about 4 hours. This project provides a means to determine whether a uranium enrichment facility has produced a product outside of its declared limits.

Description:

The Cameca IMS-1270 is a high sensitivity large radius magnetic sector secondary ion mass spectrometer that is routinely used for measurements of isotopic compositions of materials in localized regions. We have exploited the built-in functionality of the instrument to provide the capability to screen the enrichment levels of a large number of uranium-bearing particles that are dispersed on a flat substrate. The first step is to store the locations of particles by manually scanning the sample stage stepper motors while the sample is under ion bombardment and observing the secondary ion image display with the spectrometer tuned to a major uranium isotope. When a particle image is centered in the display, indicating that it is aligned with the optical axis of the spectrometer, its location coordinates are stored on the instrument control computer. The primary ion current is adjusted to yield a measurement precision of about 1 % on the 235U/238U ratio for a typical particle. The isotopic measurement protocol is set for a measurement time of about 2.5 minutes per particle so 100 particles can be screened in about 4 hours. After storing the coordinates of as many particles as one chooses to survey, an automated sequence of isotopic measurements of all particles in the list is invoked with the “CHAIN” command. After the data acquisition is complete the raw isotopic data are processed using previously developed routines to correct for mass bias and changing signal intensities. It is often convenient to plot the isotopic data after it has been sorted monotonically by isotopic ratio. The accompanying figure shows the results of a test run of 100 particles of highly enriched uranium. Two of the particles fell outside the range of the other 98 that appeared to be drawn from a homogeneous set. When the two anomalous particles were remeasured with a conventional isotopic procedure the results of the initial screening were confirmed.

Major Accomplishments:

Rapid isotopic screening of a collection of particles by secondary ion mass spectrometry has been developed and validated

Isotopic survey of 100 enriched Uranium particles ordered by increasing U-235 abundance.

Start Date:

October 1, 2007

End Date:

September 30, 2008

Lead Organizational Unit:

mml

Facilities/Tools Used:

Cameca IMS-1270 Secondary Ion Mass Spectrometer

Staff:

David Simons

Jack Fassett (contractor)

Contact

David Simons
301-975-3903
david.simons@nist.gov