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R-values [electronic resource].

Published
Washington, D.C. : United States. Dept. of Energy, 2009.
Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy.
Physical Description
PDF-file: 4 pages; size: 0.3 Mbytes
Additional Creators
Lawrence Berkeley National Laboratory, United States. Department of Energy, and United States. Department of Energy. Office of Scientific and Technical Information
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Summary
I'll try to keep this short and simple. R{sub LANL} = (beta cpm of X{sub exp} on system 'A')/ (beta cpm of ⁹⁹Mo{sub exp} on system 'A')/ (beta cpm of X on system 'A', from thermal on ²³⁵U)/ (beta cpm of ⁹⁹Mo on system 'A', from thermal on ²³⁵U). As I understand it, the above equation is the historical (as well as current) way of determining R-values using data from beta counting at LANL. The ratio in the denominator, a little 'r', is the 'baseline' or 'calibration' value for a specific beta detector. Over time, if the detector 'drifts' one would see a variation in this 'r' during a thermal calibration measurement. This baseline is what LANL likes to track to monitor specific detector performance - this is not relevant to LLNL where gamma detection is used for determining R-values. LANL states that uncertainty is only dependent upon the count statistics for the isotopes measured. If one tries to convert this to an atom basis, the uncertainties will increase due to the incorporation of the uncertainties in the nuclear data used to convert the cpm to atoms. LLNL switched to gamma detection methods in the 1970s thus replacing our beta counting effort. The equation below is how we have since determined R-values. The numerator ratios atom values of isotopes that are determined by measuring gamma cpm (usually? using several peaks per isotope) and then converting to particle decay in dpm using detector efficiency for each peak and the appropriate branch ratio for each gamma emission. Isotope decay is then converted to atoms using specific activity, mass or volume?, and Avogadro's number. The denominator is simply the ratio of published, cumulative fission product chain yields for isotopes produced in a thermal irradiation on 235U - values of England & Ryder are used by LLNL for the NTNF program. Uncertainties in LLNL R-values are dependent upon gamma counting statistics as well as the nuclear data for each isotope. R{sub LLNL} = (Atoms of X{sub exp})/(Atoms of ⁹⁹Mo{sub exp})/(Cumulative Fission Chain Yield of X, from thermal on ²³⁵U)/(Cumulative Fission Chain Yield of ⁹⁹Mo, from thermal on ²³⁵U). The next page tabulates fission chain yields and 'atoms per gram' amounts measured in a recent NTNF Thermal Calibration. The R-values in the table are calculated using the LLNL method of determining R. The measure of success is demonstrated by how close to 1.00 the R-value is when determined during a Thermal Calibration. A value of 1.00 is the desired value. In the example below, only four isotopes lie outside of 1.00 by more than 3 percent. These are the four isotopic measurements that obviously need to be improved.
Report Numbers
E 1.99:llnl-tr-411152
llnl-tr-411152
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Other Subject(s)
Note
Published through SciTech Connect.
03/03/2009.
"llnl-tr-411152"
Roberts, K.
Funding Information
W-7405-ENG-48
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