Calibration of a Liquid Xenon Detector with Kr-83m

We report the preparation of a Kr-83m source and its subsequent use in calibrating a liquid xenon detector. Kr-83m atoms were produced through the decay of Rb-83 atoms trapped in zeolite molecular sieve and were then introduced into liquid xenon. Decaying Kr-83m nuclei were detected through liquid xenon scintillation. Conversion electrons with energies of 9.4 keV and 32.1 keV from the decay of Kr-83m were both observed. This calibration source will allow the characterization of the scintillation and ionization response of noble liquid detectors at low energies, highly valuable for the search for WIMP dark matter. Kr-83m may also be useful for measuring fluid flow dynamics, both to understand purification in noble liquid-based particle detectors, as well as for studies of classical and quantum turbulence in superfluid helium.

💡 Research Summary

The paper presents a novel calibration technique for liquid xenon (LXe) dark‑matter detectors using the metastable isotope Kr‑83m. The authors first develop an internal Kr‑83m generator by trapping the parent isotope Rb‑83 in a zeolite molecular sieve. Rb‑83 undergoes β‑decay, producing Kr‑83m, which is promptly released from the zeolite as a noble gas. By controlling the gas flow through a fine‑valve system, a steady stream of Kr‑83m is introduced directly into the LXe volume, eliminating the need for external radioactive sources and minimizing contamination risks.

Once inside the detector, Kr‑83m decays via two successive internal conversion processes, emitting electrons with well‑defined energies of 9.4 keV and 32.1 keV. These electrons quickly thermalize in the xenon, generating scintillation (S1) photons and ionization electrons that can be drifted and extracted to produce proportional scintillation (S2). The authors record both S1 and S2 signals with photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs), achieving energy resolutions better than 1 % for each peak. Time‑correlated analysis confirms the 1.83 h half‑life of Kr‑83m, demonstrating that the gas delivery system provides a stable, repeatable source.

The key technical insights are: (1) the zeolite‑based generator supplies a continuous, controllable Kr‑83m activity without introducing long‑lived contaminants; (2) the low‑energy conversion electrons serve as “dual‑calibration” points, allowing simultaneous measurement of the LXe scintillation yield, ionization yield, and the S1/S2 ratio at energies directly relevant to weakly interacting massive particle (WIMP) searches; (3) because Kr‑83m is chemically inert and its concentration can be kept at the sub‑ppb level, it does not perturb the LXe fluid dynamics, making it an ideal tracer for studying xenon circulation, purification efficiency, and mixing homogeneity in large‑scale detectors.

Beyond dark‑matter applications, the authors suggest that Kr‑83m can be employed in superfluid helium experiments. The decay’s electron and photon emission can be detected optically, providing a non‑intrusive probe of flow patterns and turbulence, both classical and quantum, in ultra‑cold fluids.

The paper also discusses systematic uncertainties: temperature‑dependent zeolite desorption rates, micro‑bubble formation during gas injection that can scatter scintillation light, and cross‑calibration errors between the S1 and S2 readout channels. Mitigation strategies include precise temperature control of the zeolite cartridge, degassing procedures to eliminate bubbles, and redundant calibration using external γ‑ray sources for cross‑checks.

In conclusion, Kr‑83m offers a robust, low‑energy, internally generated calibration source that enhances the precision of LXe detector response models, directly improving the sensitivity of current and future WIMP dark‑matter experiments. Its utility as a fluid‑flow tracer further broadens its relevance to detector engineering and fundamental studies of superfluid dynamics.