Measurement of the total neutron cross section on argon in the 20 to 70 keV energy range

The cross section for neutron interactions on argon is an important design and operational parameter for a number of neutrino, dark matter, and neutrinoless double beta decay experiments which use liquid argon as a detection or shielding medium. There is a discrepancy between the evaluated total cross section in the 20 to $70\mathrm{keV}$ neutron kinetic energy region given in the ENDF database and a single measurement conducted by an experiment with a thin target (0.2 atoms/b) optimized for higher cross sections. This gives rise to significant uncertainty in the interaction length of neutrons in liquid argon. This discrepancy is now resolved by new results presented here from the Argon Resonance Transport Interaction Experiment (ARTIE), a thick target experiment (3.3 atoms/b) optimized for the small cross sections in this energy region.

Figure 1

Configuration of the beam line elements. (1) Mark III LANSCE target and moderator. (2) Cadmium filter for suppression of thermal neutrons. (3) $\approx 30\text{m}$ beam pipe where carbon and aluminum filters were mounted. (4) Brass collimators. (5) ARTIE target. (6) $\approx 30\text{m}$ beam pipe. (7) ${}^6\mathrm{Li}$-glass detector at $64\text{m}$ followed by beam dump.

Figure 2

The event rates and energy for a LAr and GAr filled target as a function of TOF. Filled points are the extracted background rates.

Figure 3

The measured transmission of argon compared to EXFOR [14] and ENDF. The points are the central values and the bars represent the statistical uncertainty. The shaded grey regions are the total uncertainty, including systematics. For most energies, statistical error bars are smaller than the symbols.

Figure 4

Neutron-argon total cross section as a function of energy. Also shown are the EXFOR evaluation and the ENDF evaluation smeared by the ARTIE energy resolution.