${}^{181}\mathrm{Ta}$($n,\gamma$) cross section and average resonance parameter measurements in the unresolved resonance region from 24 to 1180 keV using a filtered-beam technique

A new array of four Deuterated Benzene (${\mathrm{C}}_6{\mathrm{D}}_6$) detectors has been installed at the Gaerttner Linear Accelerator Center at Rensselaer Polytechnic Institute for the purpose of measuring neutron capture cross sections in the keV region. Measurements were performed on samples of ${}^{181}\mathrm{Ta}$ in the unresolved resonance region (URR) using a filtered-beam technique, by which a 30 cm iron filter was placed in a white-spectrum neutron beam to remove all time-dependent $\gamma$-ray background and all neutrons except those transmitted through resonance-potential interference “windows” in the iron. The resulting filtered beam was effectively a quasimonoenergetic neutron source, which was used for performing measurements on isotopes with narrow level spacings in the URR. The capture cross-section results obtained for two thicknesses of tantalum are in agreement with those documented in the JEFF-3.2 library, as are the average resonance parameters obtained via a fit to the data using the sammy-fitacs code.

Figure 1

${}^{181}\mathrm{Ta}$ ($n$,tot) and ($n,\gamma$) cross-section evaluations. ENDF/B-VII.1 exhibits significant anomalies compared to the other libraries [5].

Figure 2

Total ENDF/B-VII.1 cross section of elemental iron compared with energy-dependent neutron transmission through a 30 cm iron slab [5, 9].

Figure 3

The ${\mathrm{C}}_6{\mathrm{D}}_6$ detector system used in this work. The rectangular sample is positioned at the center of the four detector modules.

Figure 4

3D rendering of the mcnp geometry of the detector system and sample used in this work [5].

Figure 5

Selection of mcnp-generated response functions used in weighting function fitting. The dotted line shows the measured response to a ${}^{137}\mathrm{Cs}$ check source.

Figure 6

Raw TOF spectra of filtered-beam measurements. Counts in both ${\mathrm{B}}_4\mathrm{C}$ and Ta are observed at the Fe-transmission windows, and only ambient background is observed between the windows [5].

Figure 7

Filter window at 308 keV in the Ta-6 sample measurement. The lower and upper energy limits, as well as the centroid energy, are shown.

Figure 8

Count rates for the 2 mm tantalum sample, the ambient background, and the in-beam background as a function of TOF [5].

Figure 9

Normalization of the ${}^{181}\mathrm{Ta}$ saturated resonance at 4.28 eV to the ${\mathrm{B}}_4\mathrm{C}$ measurement. Both measurements have $Y_{\mathrm{expt}}\approx 1$ in the outlined region [5]. The small depressions in the ${\mathrm{B}}_4\mathrm{C}$ TOF spectrum in the vicinity of the tantalum resonances result from the partial removal of neutrons by the tantalum photoneutron target of the LINAC.

Figure 10

Capture depth correction was implemented in sammy code to improve the calculation of the capture yield in thick, highly attenuating samples such as tantalum. The corrected yield exhibits closer agreement with the experimental data than the default sammy output.

Figure 11

Self-shielding and multiple scattering correction factors ($k_s$) for 2 and 6 mm tantalum samples were calculated using both mcnp and sesh. The simpler sesh results agree with the more detailed mcnp simulations.

Figure 12

Comparison of the signal-to-background ratio for the 2 mm tantalum sample in filtered and unfiltered configurations.

Figure 13

Comparison of the experimental results of this work with past tantalum capture measurements and evaluations. Data are in excellent agreement with prior measurements [5].

Figure 14

Comparison of the experimental results of this work (and the corresponding sammy-fitacs fit) to existing evaluations.