RESONANCE REGION EVALUATION OF 16 O FOR CRITICALITY SAFETY AND REACTOR APPLICATIONS

. The intent of this paper is to present the resolved resonance evaluation of 16 O in the energy range from thermal to 6 MeV. The newness of the present work is that recent cross-section data for the 16 O(n,α) 13 C reaction taken at the GELINA time-of-flight facility and transmission data obtained at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR ) were included in the evaluation. The evaluation was carried out with the SAMMY code. The evaluation was used to calculate critical benchmark experiment sensitive to the 16 O cross sections.


Introduction
Normalization issues of existing 16 O(n,α) 13 C reaction cross section have generated renewed interest on measuring the (n,α) cross section data. The Nuclear Energy Agency High Priority Nuclear Data Request List demonstrate the interest on accurate 16 O(n,α) crosssection data for practical applications. [1] Recent experimental data measurements, namely transmission and (n,α), have motivated revising the 16 O resonance region evaluation. Issues with the normalization of the (n, α) cross sections have been investigated and a very dependable measurement has been carried out at the GELINA facility at the Joint Research Center (JRC) Geel from the energy threshold 2.354 MeV to 9 MeV. Additionally, transmission measurements were done at the nELBE time-of-flight (TOF) facility at Helmholtz-Zentrum Dresden -Rossendorf (HZDR). Experimental data used in previous evaluations were also considered in the evaluation. The resonance evaluation was performed in the energy range from 0 eV to 6 MeV using the reduced R-matrix Reich-Moore methodology of the computer code SAMMY resulting in a set of resonance parameters (RPs) that describes well the experimental data used in the evaluation. The recent transmission measurements and the (n,α) cross section data are well reproduced. The RPs were converted to the evaluated nuclear data file (ENDF) format using the R-Matrix Limited format option LRF=7 that allows adding information that could not be accommodated in other ENDF representation of RPs. The intent of the paper is to describe the procedures used in the evaluation of the RPs and the use of the RPs in calculations of critical benchmark experiments. Preliminary results for Pu-SOL-THERM-041 (PST-041) configurations listed in the International Criticality Safety Benchmark Evaluation Project (ICSBEP) handbook indicates an improvement on the average keff values with calculated keff being consistent with the benchmark keff, discrepancies on keff not exceeding the combined effect of experimental and Monte Carlo uncertainties. The PST benchmark consists of series of 40 experiments carried out at the Valduc research center in response to criticality safety needs. In particular, for the PST-041 series the changes in keff values indicate that these benchmarks are sensitive to the (n,α) cross section of 16 O and that the new measurements were essential to improve the benchmark results.

Experimental data
Resolved resonance evaluation of 16 O cross section using the code SAMMY [2] in the energy range from thermal to 6 MeV has been reported in Reference [3] which addresses concerns with regard thermal capture and scattering cross section data, coherent scattering data and present an attempt to quantify the normalization issue in connection to the (n,α) cross section. Two sets of resonance parameters, named low and high, from which the calculated (n,α) cross sections differed by ~30 %. From these sets of resonance parameters benchmark calculations of systems sensitive to the (n,α) cross section were carried out. Improvements on the keff, in comparison with results of (n,α) cross section evaluation available in the ENDF/B-VII.1 are observed with a better performance to the set of resonances named low. After the study presented in Reference [3], there had been attempt to unveil the normalization (n,α) cross section issue. As a result, measurements of (n,α) cross section were done at the time-of-flight (TOF) GELINA facility at the JRC from the energy threshold 2.354 MeV to 9 MeV [4]. In addition, transmission measurements at the nELBE TOF facility at HZDR were also done in the energy range 100 keV to 10 MeV. [5] The experimental data sets used in the SAMMY evaluation process are displayed in Table  1. Listed in Table 1 are the experimental data used in Reference [3] and the somehow recent (n,α) cross section measurements performed at GELINA and nELBE.

Data evaluation
The experimental data indicated in Table 1 were evaluated with the SAMMY code, and in addition to the RPs resonance parameter also covariance (RPCs) were also derived. Up to the (n,α) energy threshold (2.354 MeV) each resonance level is represented by the energy of the resonance Er, gamma width Γγ, and the neutron width Γn. Above the threshold an additional channel to represent the (n,α) reaction is added to each energy level with the width Γα. The experimental data are well represented with the RPs in conjunction with the reduced R-matrix Reich-Moore formalism. The RPs resulting from the evaluation include 54 resonances with 3 energy bound levels and 16 resonance levels above 6 MeV. Capture and scattering cross sections at thermal (0.0253 eV) obtained with calculation using the evaluated resonance parameters are listed in Table 2. Also included in Table 2 are the capture resonance integral and the coherent scattering length. The calculated values listed in the Atlas of Neutron Resonances (ANR) [6] are also listed in Table 2. The quantities listed in Table 2 are the thermal capture and scattering cross sections γ, and s, the coherent scattering length acoh, and the capture resonance integral Iγ, which is defined as integral from 0.5 eV to 20 MeV with a 1 $ weighting spectrum. One caveat regarding the ANR scattering cross section is that it is related to the value calculated at the 0 K temperature. The value calculated with the RP at 0 K is 3.765 ± 0.025 b. The uncertainties included in the values calculated based on the RP are that generated with RPC. The good agreement between the coherent scattering derived with the resonance parameters with experimental values is the result of a careful determination of the energy bound levels. They were determined according to the excitation energy levels of the compound nucleus 17 O, that is the n+ 16 O interaction. The SAMMY fitting of four experimental data displayed in Table 1 is shown in Fig.  1. Two transmission data are displayed in Fig. 1, namely the data taken at GAERTTNER RPI linear accelerator (the very bottom picture), the most recent transmission data taken at the German laboratory nELBE. The (n,α) data are the Harissopulos data with a 30 % normalization and the data taken at GELINA by Urlass. The SAMMY fitting of the data shown in Table 2  It should be mentioned that the resonance parameters are also used to generate angular dependent cross section data as well as data uncertainties calculated with the resonance parameters covariance.

Benchmark results
The 16 O evaluation presented in this paper has been used for benchmark calculations of systems sensitive to the 16  Only results for the JEFF-4T1 library are reported in this paper. The impact of the new oxygen evaluation was tested by including the new oxygen evaluation in the JEFF-4T1 library. The characteristics of the benchmark cases along with the keff results are reported in Table 3. Figure 2 also plots the keff results versus the energy corresponding to average lethargy of neutrons causing fission (EALF). The experimental uncertainty at the 1s level is indicated using red dotted lines.
The comparison of JEFF-4T1 results with those where oxygen is replaced by the old evaluation in JEFF-4T1 library indicates an improvement on the average keff values. Indeed, calculated keff are more consistent with the benchmark keff, since the differences on keff do not exceed the combined effect of experimental and Monte Carlo uncertainties.

Conclusions
This paper presents a new 16 O resolved resonance evaluation in the energy range from thermal to 6 MeV using the Reich-Moore approach of the SAMMY code. Recent transmission and (n,α) cross section data were included in the evaluation together with data used in previous evaluation. The resulting set of resonance parameters reproduces the experimental data very well. The impact of using the new evaluation on benchmark calculations has been verified for critical benchmark systems sensitive to 16