Low-energy Calibration of XENON1T with an Internal $^{37}$Ar Source

TL;DR

This paper reports on the use of an internal $^{37}$Ar source for low-energy calibration of the XENON1T detector, validating its response near the detection threshold and confirming the detector's understanding in that energy region.

Contribution

It introduces $^{37}$Ar as an effective internal calibration source for multi-tonne xenon detectors, providing precise measurements at low energies and validating detector models.

Findings

Photon and electron yields at 2.82 keV match predictions.

Electron yield at 0.27 keV measured accurately.

Calibration confirms detector understanding near threshold.

Abstract

A low-energy electronic recoil calibration of XENON1T, a dual-phase xenon time projection chamber, with an internal $^{37}$Ar source was performed. This calibration source features a 35-day half-life and provides two mono-energetic lines at 2.82 keV and 0.27 keV. The photon yield and electron yield at 2.82 keV are measured to be (32.3$\pm$0.3) photons/keV and (40.6$\pm$0.5) electrons/keV, respectively, in agreement with other measurements and with NEST predictions. The electron yield at 0.27 keV is also measured and it is (68.0$^{+6.3}_{-3.7}$) electrons/keV. The $^{37}$Ar calibration confirms that the detector is well-understood in the energy region close to the detection threshold, with the 2.82 keV line reconstructed at (2.83$\pm$0.02) keV, which further validates the model used to interpret the low-energy electronic recoil excess previously reported by XENON1T. The ability to efficiently remove argon with cryogenic distillation after the calibration proves that $^{37}$Ar can be considered as a regular calibration source for multi-tonne xenon detectors.