Measurements of the ${}^{169}\mathbf{Tm}(n,2n){}^{168}\mathbf{Tm}$ cross section from threshold to 15 MeV

Measurements of the ${}^{169}\mathrm{Tm}(n,2n){}^{168}\mathrm{Tm}$ cross section have been performed via the activation technique at 13 energies between 8.5 and 15.0 MeV. The purpose of this comprehensive data set is to provide an alternative diagnostic tool for obtaining subtle information on the neutron energy distribution produced in inertial confinement deuterium-tritium fusion experiments at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. The ${}^{169}\mathrm{Tm}(n,2n){}^{168}\mathrm{Tm}$ reaction not only provides the primary 14-MeV neutron fluence, but also the important down-scattered neutron fluence, the latter providing information on the density achieved in the deuterium-tritium plasma during a laser shot.

Figure 1

Available literature data and evaluations for the ${}^{169}\mathrm{Tm}(n,2n){}^{168}\mathrm{Tm}$ cross section from threshold to 12.5 MeV. The horizontal error bars illustrate the spread of neutron energies used in the experiment.

Figure 2

$\gamma$-ray energy spectra, expanded around the 200-keV region, obtained with a HPGe detector after 5 h of irradiation with a $11.91\pm 0.11$ MeV neutron beam. The activated sample spectra is shown in blue and the background spectrum, measured before irradiation, is shown in red.

Figure 3

Partial level scheme for decay of ${}^{168}\mathrm{Tm}$, showing all transitions measured in the present work. All energies are given in keV. Data taken from Ref. [15].

Figure 4

Measured decay curves of the (a) ${}^{168}\mathrm{Tm}$ 198-keV activity after irradiation with 12.41-MeV neutrons and (b) ${}^{196}\mathrm{Au}$ 355-keV activity after irradiation with 12.90-MeV neutrons. The data are fit to an exponential decay (red curves).

Figure 5

Measured efficiency data and fit (red curve) using a mixed $\gamma$-ray source positioned 5 cm away from the face of a 55% relative efficiency HPGe detector. The efficiency was fit to a function of the form ${\sum }_{i=0}^3{\epsilon }_{i}ln{\left(E\right)}^i$, where ${\epsilon }_i$ are free parameters.

Figure 6

${}^{169}\mathrm{Tm}(n,2n){}^{168}\mathrm{Tm}$ cross-sectional data in comparison to evaluations and previous data. The horizontal error bars indicate the neutron energy spread for the present experiment.

Figure 7

The present data compared with the TENDL-2015 output of the TALYS calculation. The calculation with the default parameters is shown by the solid black curve, with the shaded region showing the range of results achieved by varying the parameters.