Measurement of neutron capture on ${}^{136}\mathrm{Xe}$

${}^{136}\mathrm{Xe}$ is a $0\nu \beta \beta$ decay candidate isotope, and is used in multiple experiments searching for this hypothetical decay mode. These experiments require precise information about neutron capture for their background characterization and minimization. Thermal and resonant neutron capture on ${}^{136}\mathrm{Xe}$ have been measured at the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center. A neutron beam ranging from thermal energy to greater than 100 keV was incident on a gas cell filled with isotopically pure ${}^{136}\mathrm{Xe}$. The relative neutron capture cross sections for neutrons at thermal energies and the first resonance at 2.154 keV have been measured, yielding a new absolute measurement of $0.238\pm 0.019$ b for the thermal neutron capture cross section. Additionally, the $\gamma$ cascades for captures at both energies have been measured, and cascade models have been developed which may be used by $0\nu \beta \beta$ experiments using ${}^{136}\mathrm{Xe}$.

Figure 1

Neutron flux measured by the ${}^6\mathrm{Li}$ and ${}^{235}\mathrm{U}$ neutron monitors. The flux is integrated over the regions indicated by the dark grey bars for calculation of our thermal and resonance cross sections. The gap in flux measurement using the ${}^{235}\mathrm{U}$ monitor is due to an energy region with resonances that make the flux difficult to evaluate.

Figure 2

(a) Summed event energy for pressurized xenon data at the 25 meV neutron energy window and for beam-off data (also called constant in time, CIT), including cluster multiplicities 2 through 5. The beam-off data have been scaled by live time. (b) Same as (a), but with evacuated target data rather than pressurized xenon data. (c) Summed event energy for pressurized xenon data and evacuated target data both at the 25 meV neutron energy window after the CIT backgrounds have been subtracted. The evacuated target spectrum has been scaled so that the counts in the 6 to 9 MeV region match the pressurized xenon data. (d) Summed event energy spectrum after the evacuated target data has been subtracted from the pressurized xenon data. For all panels, the $Q$ gate used in this analysis is indicated by black vertical lines.

Figure 3

Measured and simulated MSC spectra from the ${}^{136}\mathrm{Xe}$ capture cascade in the 25 meV neutron energy window for cluster multiplicities 2 through 5. The simulated spectra, shown in red, represent the dicebox realization best matching the experimental data.

Figure 4

Measured and simulated MSC spectra from the ${}^{136}\mathrm{Xe}$ capture cascade at the 2.154 keV neutron energy resonance for cluster multiplicities 2 through 5. The simulated spectra, shown in red, represent the dicebox realization best matching the experimental data.

Figure 5

Capture cascade $\gamma$-ray emission intensities from the dicebox realizations that best match the data as a function of excitation energy and $\gamma$-ray energy. These correspond to the red lines on the MSC plots in Figs. 3 and 4. (a) Cascade model for thermal neutron capture (b) Cascade model for 2.154 keV ${}^{136}\mathrm{Xe}$ neutron capture resonance. Intensities (color scale) are expressed as transitions per ${10}^6$ captures.

Figure 6

Comparison of various measurements and evaluations of the ${}^{136}\mathrm{Xe}(n,\gamma )$ cross section for thermal neutrons. Each row corresponds to a measurement (red circle) or evaluation (blue square), and includes the cross section in both plot and text. All cross sections are in barns. Measurements by Macnamara et al. (1950) [29] and Eastwood et al. (1963) [30] are less precise and are not included here. Most modern evaluations, such as TENDL-2014 [21, 22] and JEFF-3.2 [31] have the same thermal neutron cross section as ENDF/B-VII.1 [23], so we do not list them separately. JENDL-4.0 [28] is an exception to this. Information on the Turkevich et al. measurement comes from Ref. [25, 32]. Other results listed come from Ref. [19, 25, 26].