Measurements of neutron-induced fission cross sections for ${}^{209}\mathrm{Bi},{}^{\mathrm{nat}}\mathrm{Pb},{}^{208}\mathrm{Pb},{}^{197}\mathrm{Au},{}^{\mathrm{nat}}\mathrm{W}$, and ${}^{181}\mathrm{Ta}$ in the intermediate energy region

Neutron-induced fission cross-section ratios ${}^{\mathrm{nat}}\mathrm{Pb}∕{}^{209}\mathrm{Bi},{}^{208}\mathrm{Pb}∕{}^{209}\mathrm{Bi},{}^{197}\mathrm{Au}∕{}^{209}\mathrm{Bi},{}^{\mathrm{nat}}\mathrm{W}∕{}^{209}\mathrm{Bi},{}^{181}\mathrm{Ta}∕{}^{209}\mathrm{Bi}$, and ${}^{209}\mathrm{Bi}∕{}^{238}\mathrm{U}$ have been measured in the 30–180 MeV energy range using the neutron beam facility at The Svedberg Laboratory in Uppsala. The ${}^7\mathrm{Li}(p,n)$ reaction was employed as a neutron source. The fission fragments were detected by thin-film breakdown counters. Cross sections at specific energies were determined using unfolding techniques with respect to the excitation function and the neutron spectra, the latter obtained from recent measurements and an evaluation. The absolute fission cross sections were obtained using the standard ${}^{238}\mathrm{U}(n,f)$ cross section. The ${}^{\mathrm{nat}}\mathrm{W}(n,f)$ and ${}^{181}\mathrm{Ta}(n,f)$ cross sections have been measured for the first time. The results for ${}^{209}\mathrm{Bi}(n,f),{}^{\mathrm{nat}}\mathrm{Pb}(n,f),{}^{208}\mathrm{Pb}(n,f)$, and ${}^{197}\mathrm{Au}(n,f)$ cross sections have been compared with available literature data. A universal easy-to-use parametrization has been suggested for all measured cross sections. The common features of subactinide neutron-induced fission cross sections are found to be similar to those of the proton-induced fission data.

Figure 1

An overview of the neutron beam facility.

Figure 2

Neutron spectra from the ${}^7\mathrm{Li}(p,n)$ reaction at 0° for the peak energy of 98, 133, and 160 MeV. The filled circles represent measurements at the TSL neutron facility [35, 39] in Irradiation position 2. The open symbols represent data from other facilities: the Indiana University Cyclotron Facility [40, 41] (shown as triangles and diamonds, respectively) and the RIKEN facility [42] (shown as squares). The lines represent the neutron spectrum calculations discussed in the text. All spectra are normalized so that the area under the high-energy peak is unity.

Figure 3

Calculations of neutron spectra (left panel) for the incident proton energies of 69.1 (a), 96.8 (d), 136.7 (g), and 177.3 MeV (j) used in the present experiment for 0° (solid lines) and 4° (dashed lines). The middle panel (b, e, h, k) represents experimental (symbols) and calculated (lines) distributions of ${}^{238}\mathrm{U}$ fission events induced by neutrons at a flight path of about 10 m at 0° (in Irradiation position 2). The right panel column (c, f, i, l) represents experimental (symbols) and calculated (lines) distributions of ${}^{238}\mathrm{U}$ fission events induced by neutrons at a flight path of about 2 m at 4° (in Irradiation position 1).

Figure 4

The design of an experimental chamber.

Figure 5

A schematic view of the electronics and the data acquisition system.

Figure 6

The calculated fission fragment detection efficiency for the TFBC and a sample of the unit area, versus incident neutron energy.

Figure 7

Energy dependences of the ${}^{209}\mathrm{Bi}∕{}^{238}\mathrm{U},{}^{208}\mathrm{Pb}∕{}^{209}\mathrm{Bi}$, and ${}^{197}\mathrm{Au}∕{}^{209}\mathrm{Bi}$ proton-induced fission cross-section ratios. The filled symbols represent the results of the present work. The open symbols represent results extracted from a compilation of literature experimental data [9].

Figure 8

The TOF spectrum of ${}^{238}\mathrm{U}$ neutron-induced fission events induced by neutrons from the $7\mathrm{Li}(p,n)$ reaction with the peak neutron energy of 173.3 MeV and its decomposition. The symbols represent experimental data of the present work. The dashed curve represents calculated TOF distribution of fission events induced by the continuum part of the neutron spectrum. The light-gray area represents fission events induced by high energy peak neutrons. The events in the dark-gray area originate from high energy continuum neutrons with energy above about 60 MeV. The black area corresponds to fission events induced by ”wrap-around” neutrons with energy lower than 60 MeV, which are produced by previous proton beam micropulses.

Figure 9

Absolute neutron-induced fission cross sections of ${}^{209}\mathrm{Bi}$. The scale of the vertical axis is logarithmic and linear in the upper and lower panels, respectively, in order to show the behavior of the cross section in the different energy regions.

Figure 10

Neutron-induced fission cross-section ratios ${}^{\mathrm{nat}}\mathrm{Pb}∕{}^{209}\mathrm{Bi},{}^{208}\mathrm{Pb}∕{}^{209}\mathrm{Bi},{}^{197}\mathrm{Au}∕{}^{209}\mathrm{Bi},{}^{\mathrm{nat}}\mathrm{W}∕{}^{209}\mathrm{Bi}$, and ${}^{181}\mathrm{Ta}∕{}^{209}\mathrm{Bi}$ versus incident energy. The results of the present study are shown as filled squares. The open squares and circles show data deduced from the results of Shcherbakov et al. [19] and Staples et al. [17, 18], respectively.

Figure 11

Absolute neutron-induced fission cross sections of ${}^{\mathrm{nat}}\mathrm{Pb},{}^{208}\mathrm{Pb},{}^{197}\mathrm{Au},{}^{\mathrm{nat}}\mathrm{W}$, and ${}^{181}\mathrm{Ta}$. The results of the present study are shown as filled squares. Crosses represent our earlier data for ${}^{208}\mathrm{Pb}$ [21, 22]. Filled triangles represent recent data of Nolte et al. for ${}^{\mathrm{nat}}\mathrm{Pb}$ [20]. Open squares, circles, and triangles represent data of Shcherbakov et al. [19], Staples et al. [17, 18], and Vorotnikov et al. [14, 15], respectively. Results of Reut et al. [12] and Dzhelepov et al. [13] are shown as diamonds, with horizontal error bars that represent the energy spread of the neutron beam. The lines represent parametrizations of the present work (see the text).