One important requirement for every experiment running over long periods of time is the detector’s stability. In the XENON1T experiment we can monitor the time evolution of the response to interactions happening inside the TPC over a wide range of energies and for both gamma and alpha particles of known energies. For this purpose we exploit mono-energetic lines of different nature:
83mKr (9, 32 and 41 keV gamma lines)
The lowest energy calibration lines are from decays of the metastable krypton isotope 83mKr, which is regularly (approximately every 2 weeks) injected in the LXe target to calibrate the spatial dependency of detector’s response. 83mKr decays to stable state by emitting two gammas of 32.2 and 9.4 keV. In some cases the two gammas can be emitted so close in time that cannot be resolved (the half-life of the second transition is 157 ns) resulting thus in a combined gamma line of 41 keV.
131mXe and 129mXe (164 and 236 keV gamma lines)
Metastable isotopes of xenon are activated during calibrations of the detector’s response to low energetic nuclear recoils, which are performed using external neutron sources such as a DD fusion neutron generator or a 241AmBe source. The activated xenon isotopes 131mXe and 129mXe decay to stable state emitting a 164 and 236 keV gamma respectively. Their half-life is in the order of 10 days and such gamma lines are therefore present only for few weeks after any neutron calibration.
High energy gamma lines (1.8, 2.2, 2.6 MeV)
We can also monitor the signals of gamma lines in the MeV energy range which are originated from the (very low) radioactivity of detector’s materials. In particular, we study the gammas from 214Bi (1.8 and 2.2 MeV) and 208Tl (2.6 MeV). Even if the statistics of such energy lines is not very large, they allow a monthly monitoring of signals stability throughout the science run.
222Rn (5.5 MeV alphas)
The radioactive 222Rn is a noble gas that permanently emanates from the detector components into the LXe reservoir. The activity of its alpha decay to 218Po in our detector is of ~13 µBq/kg producing a clear energy line at 5.5 MeV with enough statistics to monitor the detector’s response at this energy on a daily basis.
The prompt scintillation light signal (called S1) detected after energy depositions of known energy gives us a measure of the light yield, in units of detected photoelectrons (PE) per keV. Therefore, we measure the light yield for all the energy lines mentioned above at different times during the science run. The big XENON1T detector proofs its remarkable stability as the S1 signals stay flat over time for all the different sources.
If we look at the relative deviation from the average light yield of the various energy lines, we find that their time trends are in pretty good agreement. The stability level is observed at 0.2% for both the 41 keV gamma and the 5.5 MeV alpha, spanning over almost 1 year time range. This is an excellent demonstration of how smoothly the XENON1T detector operated during the dark matter search data acquisition. Stay tuned!