International Reactor Dosimetry File 2002 - IAEA Publications

III.2. Q 0 VALUES
Only the values that are marked as ‘reliable’ in the k 0 database are
included in the intercomparison (see also Table III.2). As specified in Refs [III.2,
III.12], most of the measured data originate from two laboratories: the
WWWR-SM reactor at the Central Research Institute for Physics, Budapest
(labelled KFKI) and the THETIS reactor at the Institute for Nuclear Sciences,
Gent (labelled INW). The Mughabghab recommendation is defined as the
ratio of the resonance integral to the thermal cross-section, and the uncertainty
is the sum of relative uncertainties. Comparison of the ratio for metastable
products is valid if the assumption can be made that the branching ratio is
independent of energy. Evaluated data files that give explicitly the excitation
functions for metastable states support this assumption. The ratio values
derived from evaluated data files are calculated as the ratio of the resonance
integral (see below) and the thermal cross-section in the same file. The
following nuclides exhibit discrepancies:
— 55 Mn: The capture reaction is considered standard in the k 0 NAA, and the
resonance integral and thermal cross-section by Mughabghab were
adopted for the k 0 database. The Q 0 value is reduced by 0.5% if the most
recent Mughabghab recommendation for the thermal capture crosssection
is used. Direct measurements show good consistency [III.2]
(KFKI: 1.035 ± 4.5%, INW: 1.097 ± 3.9%, 1.077 ± 3.3%, 1.041 ± 3.9%),
with a mean value of 1.062 ± 2.8%, where the uncertainty is the standard
deviation. The maximum spread of any measurement from the mean does
not exceed 3.3%. Since there is reasonably good agreement in the
thermal cross-sections and Q 0 values from different sources, the
resonance parameters in evaluated data files are suspect.
— 58 Fe: There is an extremely large discrepancy of more than 30% between
the Q 0 value from the k 0 database and the Mughabghab recommendation.
Resonance integrals in old publications might be susceptible to the
natural abundance of 58 Fe, which was not known accurately for a long
time. Direct measurements of Q 0 by the cadmium ratio method are not
sensitive to the detector efficiency or the abundance. Direct measurements
at several facilities show good consistency [III.2] (KFKI: 0.979 ±
2.1%, INW: 0.981 ± 1.9%, 0.975 ± 1.6%, 0.954 ± 2.9%), therefore they
may be considered reliable. The Q 0 value derived from the JENDL-3.3
file shows less than 5% discrepancy from the value in the k 0 database.
— 59 Co: The value in the k 0 database was adopted from the literature and is
in agreement with the Mughabghab recommendation. Direct measurements
support a somewhat lower value [III.2].
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