Speaker
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Summary
We investigated the influence of impurities on the VUV light yield in a tonne scale LAr target. In particular the following two processes are considered, the non-radiative destruction of excimers states, often referred-to as (impurity) quenching, and
secondly and more important the absorption of produced VUV scintillation light during its
propagation through the LAr bulk. By first principles both effects trace back to the presence of impurities in the argon.
The study of the VUV light yield in the tonne scale LAr detector ArDM resulted in a lower
than expected value of 0.5 m for the attenuation length of the liquid argon bulk to its own scintillation light. The result was found by means of a Bayesian variation technique and yielded systematic uncertainties on the order of 20%. We interpret this result with the presence of optically-active trace impurities in the LAr which are not filtered by the installed purification systems primarily designed to target O2 and H2O molecules.
This allowed us to conduct a combined analysis of our result with respect to the involved
photoabsorption cross sections, the lifetime of the slow scintillation component, as well as mass spectra taken on argon gas samples.
The results of the work presented here has pointed out that other trace elements than
the usually targeted water and oxygen molecules might affect the overall performance of a
liquid argon TPC. This observation will likely have some implications on the design and optimisation of light detection systems and/or on the liquid argon purification systems of future large LAr detectors, where scintillation light attenuation length in excess of meters will be desirable.
