28-30 November 2019
C-CUBE, Kyushu University Chikushi Campus 九州大学筑紫キャンパス総合研究棟 (C-Cube)
Asia/Tokyo timezone
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九州大学筑紫キャンパス総合研究棟 (C-Cube) - 3階広場

High-Energy Measurement of the Neutron Capture Cross Section of $^{237}$Np



Primary authors



Precise nuclear data for neutron capture reactions of minor actinides (MA) have become a primary research topic for their preponderant role in the study and design of transmutation methods in nuclear waste management. Accurate nuclear data are necessary in order to examine their production and long term burn-up characteristics. Present nuclear evaluated data libraries can be used in the early stages of the design of transmutation nuclear systems. Nonetheless, the exhaustive engineering designs and safety assessments require more precise and complete nuclear data and a significant reduction in terms of their uncertainties.

$^{237}$Np possesses a long half-life of 2.14 x 10$^{6}$ years and it is one of the most abundant MA present in spent nuclear fuel. 237Np is also one of the main components of the Accelerator-Driven Systems (ADS) core, a sub-critical reactor facility for nuclear transmutation. Currently available nuclear data for the neutron capture cross section of $^{237}$Np is an important contributor to the ADS criticality uncertainty. JENDL-4.0 includes uncertainties from 6% up to 10% in the region of interest for the core design, from 0.5 to 500 keV, much higher than the requirements of less than 5%. Hence, it is essential to accurately determine the neutron capture cross section at such energy range along with the resonance parameters for examining the nuclear transmutation of $^{237}$Np.

The neutron capture cross section measurements were performed using the Accurate Neutron Nucleus Reaction Measurement Instrument (ANNRI) at the Materials and Life Science Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). A Time of Flight (TOF) method using a NaI(Tl) detector was employed for this measurement and the data were analyzed based on a pulseheight weighting technique in order to derive a neutron capture yield.

A capture sample of $^{237}$Np with a mass of 200 mg was used for the measurements. The sample consisted of 227 mg of neptunium dioxide (NpO$_{2}$) powder together with 624.5 mg of Al powder, used as a binder, and it possessed an activity of 5 MBq. The neutron spectrum was reconstructed using the 478 keV gamma-rays from the $^{10}$B (n, αγ) $^{7}$Li reaction with a boron sample containing enriched 10B up to 90%.

A final value for the capture cross section of $^{237}$Np is presented using two normalization techniques. The capture data was normalized at the first resonance using JENDL-4.0 and also using the total neutron flux obtained from a $^{197}$Au sample measurement in which the first resonance was completely saturated. This measurement results were complemented with calculations using CCONE code to estimate the PH spectrum under the detection threshold.

Alongside these results, an analysis of the high-energy capture cross-section with CCONE was performed. An evaluated cross-section is presented using the experimental results.