Shuffler bias corrections using calculated count rates Page: 2 of 5
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SHUFFLER BIAS CORRECTIONS USING CALCULATED COUNT RATES
Jon R. Hurd, Phillip M. Rinard, and Faye Hsue
Los Alamos National Laboratory
Los Alamos, NM USA 87545
Los Alamos National Laboratory has two identical shufflers that have been calibrated with a dozen
U308 certified standards from 10 g 235U to 3600 g 235U. The shufflers are used to assay a wide
variety of material types for their 5U contents. When the items differ greatly in chemical
composition or shape from the U308 standards a bias is introduced because the calibration is not
appropriate. Recently a new tool has been created to calculate shuffler count rates accurately, and
this has been applied to generate bias correction factors. The tool has also been used to verify the
masses and count rates of some uncertified U308 standards up to 8.0 kg of 235U which were used to
provisionally extend the calibration beyond the 3.6 kg of 235U mass when a special need arose.
Metallic uranium has significantly different neutronic properties from the U308 standards and
measured count rates from metals are biased low when the U308 calibration is applied. The
application of the calculational tool to generate bias corrrections for assorted metals will be
described. The accuracy of the calculational tool was verified using highly enriched metal disk
standards that could be stacked to form cylinders or put into spread arrays.
Two identical shufflers at Los Alamos National Laboratory are routinely used to assay the 235U
masses of a wide variety of inventory items. Yet the only certified calibration standards available
are a set of U308 materials with 92.41% enrichments within identical cans; the 235U masses range
from 10 to 3600 g. While these are excellent standards, there are many inventory items with masses
greater than 3600 g 235U and the standards cannot be expected to accurately represent metals,
carbides, aluminum alloys, or even oxides with different moisture contents or container sizes.
With only U308 standards to use, bias in inventory measurements will be very common. Moisture
and carbon raise assay results through energy moderation. Metals have reduced assay masses
because of increased self-shielding. Aluminum alloys will have assay results biased high because
the uranium has reduced self-shielding and the aluminum provides some moderation.
The fabrication of standards more suitable for even a small fraction of all the inventory materials is
unlikely because of the large number of materials and the fabrication expense. The alternative is to
apply an accurate calculational bias correction technique. Such a technique has been developed and
its use is described in this report.1,2
The calculations that are used for bias correction are described in detail in Ref. 1. The shuffler's
operation is mathematically simulated by the calculations. The Monte Carlo code MCNP is used to
calculate the probability that a neutron from the shuffler's 252Cf source will induce a fission in the
item of interest; neutron multiplication within the item is included. For a source that moves during
an irradiation, as done in the Los Alamos shufflers, calculations are done at several stationary 252Cf
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Rinard, Phillip M.; Hurd, J. R. (Jon R.) & Hsue, F. (Faye). Shuffler bias corrections using calculated count rates, article, April 1, 2001; United States. (digital.library.unt.edu/ark:/67531/metadc930163/m1/2/: accessed September 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.