Isotopic molybdenum total neutron cross section in the unresolved resonance region

Accurate isotopic molybdenum nuclear data are important because molybdenum can exist in nuclear reactor components including fuel, cladding, or as a high yield fission product. High-resolution time-of-flight neutron transmission measurements on highly enriched isotopic metallic samples of ${}^{95}\mathrm{Mo}$, ${}^{96}\mathrm{Mo}$, ${}^{98}\mathrm{Mo}$, and ${}^{100}\mathrm{Mo}$ were performed in the resonance energy range from 1 to $620\mathrm{keV}$. The measurements were taken with the newly developed modular ${}^6\mathrm{Li}$-glass transmission detector positioned at the 100-m experimental flight station. In the unresolved energy region (URR), new comprehensive methods of analysis were developed and validated in order to obtain accurate neutron total cross-section data from the measurement by correcting for background and transmission enhancement effects. Average parameters and fits to the total cross section for ${}^{95}\mathrm{Mo}$ were obtained using the Hauser-Feshbach statistical model code fitacs, which is currently incorporated into the sammy code. The fits to the experimental data deviate from the current evaluated nuclear data file/B-VII.1 isotopic Mo evaluations by several percent in the URR.

Figure 1

Detailed three-dimensional computer model of the midenergy ${}^6\mathrm{Li}$-glass neutron detector array.

Figure 2

Decomposition of the background shapes observed by the ${}^6\mathrm{Li}$-glass detector during measurements.

Figure 3

New ${}^{100}\mathrm{Mo}$ experimental data compared to previous data upon which the current major evaluations (endf, jeff, and jendl) are based.

Figure 4

Example of the Fröhner and Larson correction [18] on elemental molybdenum transmission data with a thick sample.

Figure 5

Exponential fit to the final ${}^{95}\mathrm{Mo}$ transmission enhancement correction obtained from different sesh iterations $(i=1–3)$.

Figure 6

Test for intermediate structure in the ${}^{95}\mathrm{Mo}$ experimental cross-section data based on the Pappalardo autocorrelation function [34].

Figure 7

${}^{95}\mathrm{Mo}$ sammy fit compared to data and evaluation. The fit to the new experimental data is slightly lower than the current evaluations across most of the graphed energy range.

Figure 8

${}^{95}\mathrm{Mo}$ theoretical capture cross-section results from the sammy analysis based on the total cross-section data compared to the latest endf evaluation. The experimental values plotted consist of those available in the exfor database and data measured by Koehler [41], which also appear in [42].