Fast-neutron-induced fission cross section of ${}^{242}\mathrm{Pu}$ measured at the neutron time-of-flight facility $n\mathrm{ELBE}$

The fast-neutron-induced fission cross section of ${}^{242}\mathrm{Pu}$ was measured at the neutron time-of-flight facility $n$ELBE. A parallel-plate fission ionization chamber with novel, homogeneous, large-area ${}^{242}\mathrm{Pu}$ deposits on Si-wafer backings was used to determine this quantity relative to the IAEA neutron cross-section standard ${}^{235}\mathrm{U}(n,f)$ in the energy range of 0.5 to 10 MeV. The number of target nuclei was determined from the measured spontaneous fission rate of ${}^{242}\mathrm{Pu}$. This helps to reduce the influence of the fission fragment detection efficiency on the cross section. Neutron transport simulations performed with geant4, mcnp6, and fluka2011 are used to correct the cross-section data for neutron scattering. In the reported energy range the systematic uncertainty is below 2.7% and on average the statistical uncertainty is 4.9%. The determined results show an agreement within 0.67(16)% to recently published data and a good accordance to current evaluated data sets.

Figure 1

Experimental setup. The ELBE electron beam comes from the lower left side and is guided to the photoneutron source. A fraction of the isotropically emitted neutrons passes a collimator and enters the low-scattering experimental area. The incident neutron flux was measured with the ${}^{235}\mathrm{U}$ fission chamber H19. Fast neutron-induced fission events of ${}^{242}\mathrm{Pu}$ were recorded with the fission chamber PuFC. Picture not to scale.

Figure 2

Scheme of the electronic setup and the data acquisition system. The output signals of the charge-sensitive (ns) preamplifiers are split to determine the timing and the collected charge. Pulse heights of the H19 signals are acquired with an ADC after getting shaped by a spectroscopic amplifier, whereas the charge of the eight PuFC channels (only one is shown here) is determined by a QDC. The production of a fast trigger makes the use of a timing-filter amplifier in the timing branch of the H19 necessary. The second output signal is converted to a logical signal by an in-house developed discriminator (CFD/LED). The logical signals are used to determine the timing in a time-to-digital converter (TDC) and to produce a trigger for the data acquisition in an FPGA.

Figure 3

Charge spectrum of channel no. 1 in the PuFC (a) and pulse height spectrum of the H19 (b). The leading-edge triggered QDC and ADC values of the chambers are shown in black, the constant-fraction triggered ones in blue or red, respectively. The colored areas indicate regions of pulse heights and charges related to fission fragments.

Figure 4

Left: Detected time-of-flight spectrum $N$ before (in black) and after (in blue for PuFC channel no. 1 and in red for H19) background subtraction. The horizontal red and blue lines indicate a constant extrapolation of the background induced by spontaneous fission events and room-return neutrons. Right: Background subtracted neutron energy distribution calculated using the time-of-flight spectrum shown on the left side.

Figure 5

Compilation of measured and evaluated partial half-lives for the spontaneous fission (SF) of ${}^{242}\mathrm{Pu}$. The experimental data were taken from [30, 32]. The reevaluation of this data by Bé et al. [31] has a slightly higher uncertainty. In blue, the weighted average of all listed values is shown including the latest measurement of Salvador-Castiñeira et al. [32]. The blue shaded area marks the combined standard uncertainty of this value (1.3%).

Figure 6

Correction factor for the detection inefficiency $I$ of fission fragments in the PuFC due to linear and angular momentum transfer according to the Carlson model [33].

Figure 7

Energy to time-of-flight-correlation of the last PuFC deposit in the neutron beam calculated using geant4.10.1. On the right-hand side (a) only events are drawn which have been scattered at least once, whereas on the left-hand side (b) all neutrons passing the actinide target are shown. The bin content of each histogram was multiplied columnwise with the ${}^{242}\mathrm{Pu}$ fission cross section at the respective neutron energy $E_n$ to be proportional to the fission rate. Structures off the diagonal are caused by elastic and inelastic scattering on the target backing (mostly ${}^{28}\mathrm{Si}$) and stainless steel windows of the fission chamber (mostly ${}^{56}\mathrm{Fe}$).

Figure 8

Correction factor $C$ for neutron scattering derived from geant4 (blue), mcnp6 (red) and fluka2011 (green). This plot shows the maximum effect by comparing the first target of the H19 (b) with the last target [PuFC, channel no. 1 (a)] in the neutron beam. The confidence intervals shown here as transparent ribbons correspond to the $1\sigma$ statistical uncertainty.

Figure 9

Ratio of neutron-induced and spontaneous fission rate without (red) and with (blue) correction for neutron scattering. Without the correction, the fraction of the neutron-induced fission rate drops exponentially with the number of fission chamber channels, whereas the spontaneous fission rate stays constant. Note that channel no. 8 corresponds to the deposit closest to the neutron source while channel no. 1 is the farthest from it, thus having the largest absorption correction.

Figure 10

Neutron-induced fission cross section of ${}^{242}\mathrm{Pu}$ relative to the one of ${}^{235}\mathrm{U}$. The $n$ELBE data are shown in blue together with selected EXFOR data of Tovesson et al. [10], Staples et al. [42], and Weigmann et al. [29]. Within their statistical uncertainties, there is a good agreement of the presented data set with the data of Tovesson. Small deviations from the Weigmann and Staples data are clearly visible.

Figure 11

Neutron-induced fission cross section of ${}^{242}\mathrm{Pu}$. The $n$ELBE data are shown in blue together with selected EXFOR-data of Tovesson et al. [10], Staples et al. [42], Weigmann et al. [29] and Salvador-Castiñeira et al. [8]. Within their total uncertainties, there is a good agreement of the presented data set with the data of Tovesson. Small deviations from the Weigmann data and the measurement of Salvador-Castiñeira are clearly visible.

Figure 12

Residuals of the discussed data sets shown in Fig. 11 with respect to the ENDF/B-VIII.0 evaluation. The error bars plotted here only represent the statistical uncertainty of the measurements. The used color code is identical to that in Fig. 11.

Figure 13

Average deviations of JEFF-3.3 and selected EXFOR data sets with respect to ENDF/B-VIII.0. The weighted average has been determined by fitting a constant to the residuals shown in Fig. 12. Error bars indicating both the statistical and the total uncertainty are drawn for each data point. With the exception of the Weigmann et al. and Matei et al. data, all recent measurements tend on average to 4.1(15)% smaller cross sections compared to ENDF/B-VIII.0.

Figure 14

Comparison of the $n$ELBE data with nuclear model calculations from talys1.8 and empire3.2.