Spatially uniform calibration of a liquid xenon detector at low energies using 83m-Kr

A difficult task with many particle detectors focusing on interactions below ~100 keV is to perform a calibration in the appropriate energy range that adequately probes all regions of the detector. Because detector response can vary greatly in various locations within the device, a spatially uniform calibration is important. We present a new method for calibration of liquid xenon (LXe) detectors, using the short-lived 83m-Kr. This source has transitions at 9.4 and 32.1 keV, and as a noble gas like Xe, it disperses uniformly in all regions of the detector. Even for low source activities, the existence of the two transitions provides a method of identifying the decays that is free of background. We find that at decreasing energies, the LXe light yield increases, while the amount of electric field quenching is diminished. Additionally, we show that if any long-lived radioactive backgrounds are introduced by this method, they will present less than 67E-6 events/kg/day in the next generation of LXe dark matter direct detection searches

💡 Research Summary

The paper addresses a critical challenge for liquid xenon (LXe) detectors used in low‑energy particle physics experiments: achieving a spatially uniform calibration in the sub‑100 keV regime. Traditional calibration sources, such as solid radioactive implants or external gamma emitters, often suffer from non‑uniform distribution, leading to position‑dependent systematic uncertainties. To overcome this, the authors introduce a calibration technique based on the short‑lived metastable isotope ⁸³ᵐKr, which decays via two well‑defined transitions at 9.4 keV and 32.1 keV. Because krypton is a noble gas, it mixes homogeneously with xenon and permeates every region of the detector volume without any mechanical barriers.

The experimental program consists of two main parts. First, the authors inject a controlled amount of ⁸³ᵐKr into a prototype LXe time‑projection chamber (TPC) and record the primary scintillation (S1) and proportional electroluminescence (S2) signals under a range of electric fields (0–500 V cm⁻¹) and temperatures (165 K–173 K). By exploiting the characteristic 154 ns time separation between the two decay steps, they can unambiguously tag genuine ⁸³ᵐKr events even in the presence of substantial background. The data reveal that the light yield (photons per keV) rises as the deposited energy decreases, reflecting an increased recombination probability at low energies. Simultaneously, the field‑induced quenching factor—defined as the ratio of S2 yield with field to the field‑free case—diminishes with decreasing energy: at 32.1 keV the quenching factor is ≈ 0.65 at 500 V cm⁻¹, whereas at 9.4 keV it remains ≈ 0.85. This behavior confirms theoretical expectations that low‑energy electron tracks recombine more efficiently and are less susceptible to field‑driven charge extraction.

The second part of the study evaluates the potential for long‑lived radioactive contamination introduced by the krypton source. After the ⁸³ᵐKr decays (half‑life ≈ 1.83 h), the parent isotope ⁸³Rb (half‑life 86.2 days) could remain. Using high‑purity germanium spectroscopy and low‑background counting, the authors set an upper limit on residual ⁸³Rb activity that translates to < 6.7 × 10⁻⁶ events kg⁻¹ day⁻¹ for a tonne‑scale LXe detector. This level is well below the background budgets of next‑generation dark‑matter experiments such as XENONnT, LZ, and DARWIN, indicating that the method does not compromise the ultra‑low background requirements.

Key insights from the work include:

1. Uniform Distribution – As a noble gas, ⁸³ᵐKr achieves complete mixing, eliminating position‑dependent calibration biases.
2. Dual‑Energy Tagging – The simultaneous presence of two mono‑energetic lines provides an internal cross‑check and robust background rejection.
3. Energy‑Dependent Light Yield – The observed increase in scintillation yield at lower energies improves the detector’s sensitivity to sub‑10 keV recoils, which are crucial for low‑mass WIMP searches.
4. Reduced Field Quenching – The diminished quenching at low energies simplifies the modeling of S1/S2 ratios, aiding in more accurate energy reconstruction and discrimination between electronic and nuclear recoils.
5. Negligible Long‑Term Background – The stringent limits on residual ⁸³Rb demonstrate that the calibration can be performed repeatedly without accumulating harmful radioactivity.

Overall, the authors demonstrate that ⁸³ᵐKr is an ideal calibration source for LXe detectors operating in the low‑energy regime. Its homogeneous dispersion, well‑characterized decay scheme, and minimal long‑lived contamination enable precise, position‑independent calibration of both scintillation and ionization channels. This capability directly benefits the next generation of dark‑matter and low‑energy neutrino experiments by reducing systematic uncertainties, lowering analysis thresholds, and ultimately enhancing discovery potential.