Logo of the Physikalisch-Technische Bundesanstalt

Measurement of cross sections for the elastic and inelastic scattering of neutrons on lead, bismuth and tantalum in the energy range from 2 MeV to 4 MeV using the 15N(p,n) reaction as neutron source


In the past 20 years, cross sections for the elastic and inelastic scattering of neutrons have been determined at PTB's time-of-flight spectrometer with high accuracy for a great number of elements. With the D(d,n) reaction, which has so far been used for these experiments, neutrons in the energy range from 6 MeV to 15 MeV can be produced. With the 15N(p,n) reaction, a source for monoenergetic neutrons is now available which enables the determination of cross sections for neutron energies below 6 MeV. In 2008, the analysis of the measurements of the elastic and inelastic scattering carried out within the scope of the PhD thesis of E. Pönitz was concluded for natPb, 209Bi and 181Ta. Lead and bismuth are candidates for spallation neutron sources and coolants in accelerator-driven subcritical reactors for the transmutation of actinides from spent nuclear fuel. Tantalum is also a candidate for spallation targets as well as an alloying component for structural materials for the fusion reactor ITER. For these materials, reliable cross sectional data is required. Especially the neutron transportation in spallation targets is strongly dependent on the inelastic scattering cross sections in the energy range from 1 MeV to 4 MeV.

The differential cross sections are determined by detection of the elastically and inelastically scattered neutrons using the time-of-flight method, and the simulation of the time-of-flight spectra by means of the Monte Carlo Code STREUER. The cross sections used in the simulation, which mostly originate from the nuclear data library ENDF/B-VI, are iteratively improved until the calculation and the experiment agree with each other as well as possible. The realistic simulation of the time-of-flight spectra as well as the precise knowledge of the detector characteristics examined in detail by PTB enables a highly precise determination of scattering cross sections. Figure 1 shows a time-of-flight spectrum (red) measured with a lead sample of natural isotopic composition as well as calculated spectra for comparison. Besides the complete time-of-flight spectrum (black), also sub-spectra for the individual isotopes and levels (blue) can be calculated. Thereby, a separate determination of the inelastic scattering cross sections is also possible for time-of-flight peaks that are not completely separated.

Figure 1 : Time-of-flight spectrum of the neutrons scattered at a lead sample with a natural isotope composition. The red histogram shows the experimental data and the black histogram shows the result of the Monte Carlo simulation with repetitively improved cross sections. The blue histograms show the calculated time-of-flight spectra calculated for the inelastically scattered neutrons under stimulation of the individual energy states.

Integral scattering cross sections were determined by fitting the differential cross sections with a Legendre polynomial. Figure 2 shows the cross sections for the inelastic scattering with excitation of the first level as well as the cross sections from the nuclear data libraries and other experiments for comparison. A very good agreement with the data by Ramström et al. (AE1975) can be seen. The comparison with the data of Smith et al. (ANL1969) shows a good agreement at 4 MeV, but a deviation at a neutron energy of 2.71. The uncertainties achieved within the scope of this study are clearly smaller than in the earlier experiments, and some are even smaller than the uncertainties established during a precision measurement which has recently been carried out at the IRMM in Geel/Belgium by Mihailescu et al. (IRMM 2008). The cause for the discrepancies could not be clarified so far, but might be due to the different measuring procedures. In the experiment carried out by Mihailescu et al., the cross sections were determined by detecting the photons emitted by the excited nuclei, which requires an exact knowledge of the gamma cascades especially in the case of higher neutron energies.

Figure 2 : Angle-integrated inelastic scattering cross section for the first excited state of 209Bi. The symbols show the results of measurements for which cross sections have been determined by detecting the scattered neutrons. The histogram (IRMM 2008) shows a measurement for which the photons emitted after an inelastic scattering were detected, and the inelastic scattering cross section was calculated from the photo emission cross section using nuclear models. The lines represent cross sections from nuclear data libraries.

The data determined at PTB contribute considerably to improving the experimental data situation and enable a comparison of the cross sections determined by different measuring methods. Moreover, they permit the verification and improvement of the nuclear data libraries based on nuclear model calculations.