Measurement of neutron capture cross sections of Pd-107 at J-PARC/MLF/ANNRI
Introduction
Long-lived fission products (LLFPs) produced in nuclear fission reactors are problematic for future nuclear waste management. Nuclear transmutation of LLFPs into stable or short-lived nuclides by neutron capture reaction is an attractive option to reduce the environmental burden in the geological disposal of nuclear waste (Takano and Ikegami, 2002).
Palladium-107 (half-life: 6.5 × 106 y, fission yield for U-235: 1% and Pu-239: 3%) is one of the most important LLFPs. Its neutron capture cross section is necessary for the study of LLFP transmutation systems. However, there are only a few previous measurements for the capture cross section of 107Pd. Singh et al. carried out the neutron capture and transmission measurements using an electron-accelerator neutron source of the Rensselaer Polytechnic Institute Gaerttner Laboratory (Singh et al., 1978). The resonance parameters of 34 resonances below 700 eV were determined. Macklin made neutron capture measurements at the Oak Ridge Electron Linear Accelerator facility (Macklin, 1984). He determined the resonance parameters of 130 resonances up to 3.5 keV and the average capture cross sections in the energy region from 3 to 600 keV. Nakamura et al. performed thermal neutron capture measurement at the 8-MW Los Alamos Omega West Reactor with a prompt γ ray spectroscopic method, and determined a lower limit of the thermal neutron capture cross section (Nakamura et al., 2007). The evaluated cross sections in JENDL-4.0 and ENDF/B-VII.1 above 3 eV are close together because the evaluated data are based on mainly the Macklin's results. However a large difference exists between the evaluations in the lower neutron enegies. It comes from the difference of the adopted thermal capture cross section values. Recently, in order to improve the reliability of the cross section data,the capture cross section of 107Pd was measured with a large Ge detector array in the neutron energies from the thermal to a few hundred eV by the time-of-flight (TOF) method at the Japan Proton Accelerator Research Complex (J-PARC) (Nakamura et al., to be published). In the present work, we measured the capture cross section of 107Pd with a faster NaI(Tl) detector system installed in the J-PARC beam line to extend the high energy limit of measurement.
Section snippets
Experiments
Experiments were carried out at the Materials and Life Sceinece Experimental Facility (MLF) of J-PARC. Neutrons were produced via the spallation reaction induced by a 3-GeV proton beam impinging on a mercury target of MLF. The accelerator was operated at a repetition rate of 25 Hz. The proton beam power on the spallation target was 120 kW. The produced neutrons were moderated in liquid hydrogen moderators cooled at 19 K, thereby providing neutron beams having wide energy neutron spectrum (
Incident neutron energy spectrum
The incident neutron energy spectrum was determined from counts of 478 keV γ-rays from the 10B(n, )7Li reaction. TOF spectra of gated counts in the 478 keV peak region were made for the boron, 208Pb and blank runs. Sample-independent and sample-scattered neutron backgrounds were estimated from the TOF spectra of 208Pb and blank runs and subtracted from the boron TOF spectrum. The cross section data of the 10B(n, )7Li reaction in JENDL-4.0 (Chiba, 2010) was used to convert the detected
Results
The derived relative capture cross sections of 107Pd normalized to the value at the 44 eV resonance peak in JENDL-4.0 is shown in Fig. 4. The previous measurements (Macklin, 1984, Nakamura et al., 2007), and the evaluated cross sections of JENDL-4.0 (Iwamoto and Shibata, 2010, Shibata et al., 2011) and ENDF/B-VII.1 (JNDC FP Nuclear Data W.G., 2011) are also shown.
In the thermal energy region, JENDL-4.0 is smaller than the present results by 30%, and ENDF/B-VII.1 is smaller by 80%. JENDL-4.0
Conclusion
The measurements of neutron capture cross sections on 107Pd were performed by means of the TOF method in neutron energies ranging from the thermal to keV energy regions with an NaI(Tl) spectrometer of ANNRI in J-PARC. The present cross section was normalized to the JENDL-4.0 evaluated value at 44 eV resonance. Comparison of the present results with previous experimental data and evaluations of JENDL-4.0 and ENDF/B-VII.1 was made. In the thermal energy region, both the previous data and the
Acknowledgment
This work was supported by JSPS KAKENHI Grant Number 22226016.
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