Measurement of the ${}^{209}\mathbf{Bi}(n,4n){}^{206}\mathbf{Bi}$ and ${}^{169}\mathbf{Tm}(n,3n){}^{167}\mathbf{Tm}$ cross sections between 23.5 and 30.5 MeV relevant to reaction-in-flight neutron studies at the National Ignition Facility

At the National Ignition Facility, experiments are being performed to measure charged-particle stopping powers in the previously unexplored warm dense plasma regime. These measurements are done using reaction-in-flight (RIF) neutrons from an inertial confinement fusion system. RIF neutrons are produced with a continuum of energies up to 30 MeV. By making activation measurements utilizing threshold reactions for neutrons in the energy range of $15<E_n<30\mathrm{MeV}$, the number of RIF neutrons can be determined and from this the stopping power of the deuterium and tritium ions that produced the RIF neutrons can be inferred. Currently, the ${}^{169}\mathrm{Tm}(n,3n){}^{167}\mathrm{Tm}$ reaction has been used. However, in an effort to provide a secondary complimentary measurement, efforts are underway to make use of the ${}^{209}\mathrm{Bi}(n,4n){}^{206}\mathrm{Bi}$ reaction, with a threshold of 22.5 MeV. The cross sections were measured at the 10 MV tandem Van De Graaff accelerator at the Triangle Universities Nuclear Laboratory with quasimonoenergetic neutrons between 23.5 and 30.5 MeV, where few previous measurements have been made. Cross-section data are compared to calculations and other available measurements.

Figure 1

Simulated neutron spectrum from a high-foot implosion at NIF.

Figure 2

Schematic of gas cell and tritiated target assembly for neutron product at TUNL.

Figure 3

Evaluated cross-section data used for flux determination from reference reactions overlaid with the NTOF spectra normalized for comparison. TOF spectra shown are uncorrected for neutron detector efficiency or resolution. The vertical line is the location of the ${}^{209}\mathrm{Bi}(n,4n){}^{206}\mathrm{Bi}$ reaction threshold.

Figure 4

Calculated neutron energy distribution using the cross sections and angular distribution of the ${}^3\mathrm{H}(d,n){}^4\mathrm{He}$ reaction [6] averaged over the geometry of the measurement.

Figure 5

Present data for the ${}^{209}\mathrm{Bi}(n,4n){}^{206}\mathrm{Bi}$ reaction, along with existing experimental data up to 50 MeV.

Figure 6

Present data for the ${}^{209}\mathrm{Bi}(n,4n){}^{206}\mathrm{Bi}$ reaction, along with existing experimental data up to 32 MeV in log-scale for comparison.

Figure 7

Present data along with existing experimental data and evaluations for the ${}^{169}\mathrm{Tm}(n,3n){}^{167}\mathrm{Tm}$ reaction.