Measurement of the ${}^{240}\mathrm{Am}(n,f$) cross section using the surrogate-ratio method

The ${}^{240}\mathrm{Am}(n,f$) cross section has been measured for the first time above 4 MeV, using the surrogate-ratio method over the neutron energy range of 200 keV to 14 MeV. The reactions ${}^{243}\mathrm{Am}(p,tf$) and ${}^{238}\mathrm{U}(p,tf$), which proceed through the fissioning excited nuclei ${}^{241}\mathrm{Am}$* and ${}^{236}\mathrm{U}$*, were used as surrogates for the desired ${}^{240}\mathrm{Am}(n,f$) and ${}^{235}\mathrm{U}(n,f$) reactions. The experiment was fielded using the STARLiTeR detector system with a recently commissioned VME-based data acquisition system. The 38.4-MeV proton beam used in these measurements was provided by the K150 cyclotron at the Texas A&M Cyclotron Institute. The measured ${}^{240}\mathrm{Am}(n,f$) cross section disagrees with many of the most recent evaluations, and a reevaluation is recommended.

Figure 1

Cross-section view of the silicon telescope (not to scale). The detector arrangement is cylindrical with respect to the beam axis. The gray regions represent the physical dimension of the silicon layer. The dashing on each side of the silicon represents the geometric subdivision of the contacts.

Figure 2

A diagram illustrating how the triton particle energy in the reaction ${}^{243}\mathrm{Am}(p,tf$) relates to the neutron energy in the reaction ${}^{240}\mathrm{Am}(n,f$). The bottom bar represents charged particle energies, the middle bar represents nuclear excitation energy, and the top bar represents equivalent neutron energy. $S_n$ is the neutron separation energy of ${}^{241}\mathrm{Am}$, and $Q$ value refers to the $Q$ value of the ${}^{243}\mathrm{Am}(p,t$) reaction. The telescope limit (dashed line) represents the maximum triton energy that the telescope can stop for the angle ranges of interest.

Figure 3

The raw spectrum of outgoing particles $p$, $d$, and $t$, identified using the silicon telescope. Plotted are the results of the range-energy relation for all hydrogen isotopes that were identified as passing through a specific ring of the $\Delta \mathrm{E}$ detector from the ${}^{243}\mathrm{Am}$ target. The $y$ axis represents the threshold energy for the given particle type passing through the $\Delta \mathrm{E}$ detector. The three discrete states seen in the tritons, which have the largest range-energy value shown, originate from ($p,t$) reactions on carbon and oxygen contaminants in the target.

Figure 4

Projected range-energy spectrum for a specific ring in the ${}^{243}\mathrm{Am}(p,tf$) data set, illustrating the fitting procedure. Events in each region and theoretical contributions of each particle type were separated by solving a linear equation to extract the triton spectrum.

Figure 5

The gated fission spectrum for ${}^{243}\mathrm{Am}(p,tf$) events from all rings for the entire ${}^{243}\mathrm{Am}$ data set, compared to the raw fission spectrum for the ${}^{243}\mathrm{Am}$ target without gates. The raw fission spectrum has been scaled down by a factor of 43 for comparison. The large low-energy peak in the raw spectrum is suspected to result from ${}^{12}\mathrm{C}(p,p^{\prime }\alpha$) in the ${}^{\mathrm{nat}}\mathrm{C}$ backing.

Figure 6

The particle-fission timing spectrum shows a gated spectrum with a prompt region where tritons and fission fragments from the same event are detected. Also shown in the plot is the raw ungated data, scaled down by a factor of 20, for comparison. The peak at later times in the raw spectrum is suspected to result from ${}^{12}\mathrm{C}(p,p^{\prime }\alpha$) in the ${}^{\mathrm{nat}}\mathrm{C}$ backing.

Figure 7

The fission energy versus particle-fission timing, with PID and RT1 gates applied. The smearing near the lower energy edge of the fission fragment distribution is suspected to result from ${}^{12}\mathrm{C}(p,t\alpha$) in the ${}^{\mathrm{nat}}\mathrm{C}$ backing.

Figure 8

$\mathrm{PFT}+\mathrm{RT}1+\mathrm{RT}2$ gated spectrum of outgoing particles $p$, $d$, and $t$, identified using the silicon telescope. Plotted are the results of the range-energy relation for all hydrogen isotopes that were identified as passing through a specific ring of the $\Delta \mathrm{E}$ detector from the ${}^{243}\mathrm{Am}$ target. The $y$ axis represents the threshold energy for the given particle type passing through the $\Delta \mathrm{E}$ detector. No carbon or oxygen contamination is visible, owing to the PFT gate. The $\mathrm{RT}1+\mathrm{RT}2$ gates reduce the amount of scattered beam visible in the plot.

Figure 9

The spectrum of ${}^{243}\mathrm{Am}(p,tf$) events, plotted as a function of equivalent neutron energy.

Figure 10

The spectrum of ${}^{238}\mathrm{U}(p,tf$) events, plotted as a function of equivalent neutron energy.

Figure 11

The calculated compound nucleus formation cross sections for ${}^{240}\mathrm{Am}$ and ${}^{235}\mathrm{U}$.

Figure 12

The ${}^{240}\mathrm{Am}(n,f$) cross section from this work compared to the existing data measured using the surrogate method up to 4 MeV equivalent neutron energy [26, 28].

Figure 13

The ${}^{240}\mathrm{Am}(n,f$) cross section from this work compared to the existing estimates in endl2009, jendl-4.0, endf/B-VII.1, endl2011, rosfond-2010, and cendl-3.1.