Measurement of the ${}^{169}\text{Tm}(n,3n){}^{167}\mathrm{Tm}$ cross section and the associated branching ratios in the decay of ${}^{167}\text{Tm}$

The cross section for the ${}^{169}\text{Tm}\left(n,3n\right){}^{167}\text{Tm}$ reaction was measured from 17 to 22 MeV using quasimonoenergetic neutrons produced by the ${}^2\text{H}\left(d,n\right){}^3\text{He}$ reaction. This energy range was studied to resolve the discrepancy between previous $\left(n,3n\right)$ cross-section measurements. In addition, the absolute $\gamma$-ray branching ratios following the electron-capture decay of ${}^{167}\text{Tm}$ were measured. These results provide more reliable nuclear data for an important diagnostic that is used at the National Ignition Facility to estimate the yield of reaction-in-flight neutrons produced via the inertial-confinement-fusion plasma in deuterium-tritium capsules.

Figure 1

Overlay of a simulated NIF neutron-energy distribution ($y$ axis on left side) and the ${}^{169}\text{Tm}\left(n,3n\right){}^{167}\text{Tm}$ cross section ($y$ axis on right side). Only reaction-in-flight neutrons have enough energy to produce ${}^{167}\text{Tm}$.

Figure 2

${}^{169}\text{Tm}$($n,3n)$ cross-section data [4, 5, 6] from the EXFOR database.

Figure 3

Energy spectrum of the irradiated holmium foil measured using a planar HPGe detector.

Figure 4

Relevant decay scheme properties for the electron-capture decay of ${}^{167}\text{Tm}$. All energies are given in keV [10].

Figure 5

Calculated energy distribution of neutrons striking the foil at the 21.5-MeV settings. The spread in neutron energies is primarily caused by kinematic effects from the extended geometry of the foil.

Figure 6

(a) Time-of-flight spectrum for the settings used to produce 20.5-MeV neutrons. Additional measurements were performed with ${}^2\text{H}$ removed from the gas cell to help identify features noted in the plot. (b) Neutron-energy spectra determined from time-of-flight measurements. The spectra are normalized by area in the quasimonoenergetic peak and are shown with equal energy bin widths. The time-of-flight measurement at the 21.5-MeV settings was performed under timing conditions that resulted in greater broadening of the highest-energy neutrons.

Figure 7

Experimental results from this work compared with the previous data summarized in EXFOR and evaluations [22, 26].