Electrons In Liquid Xenon Research Articles

Search for the Migdal effect in liquid xenon with keV-level nuclear recoils

The Migdal effect predicts that a nuclear recoil interaction can be accompanied by atomic ionization, allowing many dark matter direct detection experiments to gain sensitivity to sub-GeV masses. We report the first direct search for the Migdal effect for M- and L-shell electrons in liquid xenon using nuclear recoils produced by tagged neutron scatters. Despite an observed background rate lower than that of expected signals in the region of interest, we do not observe a signal consistent with predictions. We discuss possible explanations, including inaccurate predictions for either the Migdal rate or the signal response in liquid xenon. We comment on the implications for direct dark matter searches and future Migdal characterization efforts. Published by the American Physical Society 2024

Scintillation decay-time constants for alpha particles and electrons in liquid xenon.

Understanding liquid xenon scintillation and ionization processes is of great interest to improve analysis methods in current and future detectors. In this paper, we investigate the dynamics of the scintillation process for excitation by O(10keV) electrons from a 83mKr source and O(6MeV) α-particles from a 222Rn source, both mixed with the xenon target. The single photon sampling method is used to record photon arrival times in order to obtain the corresponding time distributions for different applied electric fields between about 0.8Vcm-1 to 1.2kVcm-1. Energy and field dependencies of the signals, which are observed in the results, are discussed.

A measurement of the mean electronic excitation energy of liquid xenon

Detectors using liquid xenon as target are widely deployed in rare event searches. Conclusions on the interacting particle rely on a precise reconstruction of the deposited energy which requires calibrations of the energy scale of the detector by means of radioactive sources. However, a microscopic calibration, i.e. the translation from the number of excitation quanta into deposited energy, also necessitates good knowledge of the energy required to produce single scintillation photons or ionisation electrons in liquid xenon. The sum of these excitation quanta is directly proportional to the deposited energy in the target. The proportionality constant is the mean excitation energy and is commonly known as W-value. Here we present a measurement of the W-value with electronic recoil interactions in a small dual-phase xenon time projection chamber with a hybrid (photomultiplier tube and silicon photomultipliers) photosensor configuration. Our result is based on calibrations at mathcal {O}(1{-}10,{hbox {keV}}) with internal {^{37}hbox {Ar}} and {^{83text {m}}hbox {Kr}} sources and single electron events. We obtain a value of W={11.5}{} , ^{+0.2}_{-0.3} , mathrm {(syst.)} , hbox {eV}, with negligible statistical uncertainty, which is lower than previously measured at these energies. If further confirmed, our result will be relevant for modelling the absolute response of liquid xenon detectors to particle interactions.

Measurement of the drift velocity and transverse diffusion of electrons in liquid xenon with the EXO-200 detector

The EXO-200 Collaboration is searching for neutrinoless double beta decay using a liquid xenon (LXe) time projection chamber. This measurement relies on modeling the transport of charge deposits produced by interactions in the LXe to allow discrimination between signal and background events. Here we present measurements of the transverse diffusion constant and drift velocity of electrons at drift fields between 20~V/cm and 615~V/cm using EXO-200 data. At the operating field of 380~V/cm EXO-200 measures a drift velocity of 1.705$_{-0.010}^{+0.014}$~mm/$\mu$s and a transverse diffusion coefficient of 55$\pm$4~cm$^2$/s.

Ab initio electron scattering cross-sections and transport in liquid xenon

Ab initio fully differential cross-sections for electron scattering in liquid xenon are developed from a solution of the Dirac–Fock scattering equations, using a recently developed framework (Boyle et al 2015 J. Chem. Phys. 142 154507) which considers multipole polarizabilities, a non-local treatment of exchange, and screening and coherent scattering effects. A multi-term solution of Boltzmann’s equation accounting for the full anisotropic nature of the differential cross-section is used to calculate transport properties of excess electrons in liquid xenon. The results were found to agree to within 25% of the measured mobilities and characteristic energies over the reduced field range of 10−4–1 Td. The accuracies are comparable to those achieved in the gas phase. A simple model, informed by highly accurate gas-phase cross-sections, is presented to improve the liquid cross-sections, which was found to enhance the accuracy of the transport coefficient calculations.

Possible usage of Cherenkov photons to reduce the background in a 136Xe neutrino-less double-beta decay experiment

One of the main backgrounds in the search for 136Xe nutrino-less double-beta decay (0νββ) is the signal from Compton scattering of photons with energy around the decay endpoint at 2.458MeV. Electrons in liquid xenon emit scintillation light at 178nm. Liquid xenon being extremely transparent to ultra violet light it is in principle possible to discriminate one particle events (such as the Compton background) from two particle events (double-beta decay signals) by the amount of Cherenkov radiation emitted. The identification of the Cherenkov photons may be performed by looking at the different time structure of the signal with respect to the scintillation, by selecting photons with wavelengths larger than the typical Xenon scintillation light, and by the different emission topology. A proof-of-principle study of this approach is presented here together with preliminary studies on possible detectors for the two light components at different wavelengths.

Measurement of the scintillation yield of low-energy electrons in liquid xenon

We have measured the energy dependence of the liquid xenon (LXe) scintillation yield of electrons with energies between 2.1 and 120.2 keV, using the Compton coincidence technique. A LXe scintillation detector with a very high light detection efficiency was irradiated with $^{137}\mathrm{Cs}$ $\ensuremath{\gamma}$ rays, and the energy of the Compton-scattered $\ensuremath{\gamma}$ rays was measured with a high-purity germanium detector placed at different scattering angles. The excellent energy resolution of the high-purity germanium detector allows the selection of events with Compton electrons of known energy in the LXe detector. We find that the scintillation yield initially increases as the electron energy decreases from 120 to about 60 keV but then decreases by about 30% from 60 to 2 keV. The scintillation yield was also measured with conversion electrons from the 32.1 and 9.4 keV transitions of the $^{83m}\mathrm{Kr}$ isomer, used as an internal calibration source. We find that the scintillation yield of the 32.1 keV transition is compatible with that obtained from the Compton coincidence measurement. On the other hand, the yield for the 9.4 keV transition is much higher than that measured for a Compton electron of the same energy. We interpret the enhancement in the scintillation yield as due to the enhanced recombination rate in the presence of Xe ions left from the 32.1 keV transition, which precedes the 9.4 keV one by 220 ns, on average.

Anisotropic diffusion of electrons in liquid xenon with application to improving the sensitivity of direct dark matter searches

Electron diffusion in a liquid xenon time projection chamber has recently been used to infer the z coordinate of a particle interaction, from the width of the electron signal. The goal of this technique is to reduce the background event rate by discriminating edge events from bulk events. Analyses of dark matter search data which employ it would benefit from increased longitudinal electron diffusion. We show that a significant increase is expected if the applied electric field is decreased. This observation is trivial to implement but runs contrary to conventional wisdom and practice. We also extract a first measurement of the longitudinal diffusion coefficient, and confirm the expectation that electron diffusion in liquid xenon is highly anisotropic under typical operating conditions.

Observation of excimer luminescence from electron-excited liquid xenon

This letter presents an experimental study of vacuum-ultraviolet luminescence from low-energy electron-excited liquid xenon. Our experimental results provide strong support for the theory by [E. B. Gordon, V. V. Khmelenko and O. S. Rzhevskii, Chem. Phys. Lett. 217, 605 (1994)] which considers the rare-gas-atom excitations to be the main reason for the electron drift-velocity saturation occurring at high-electric fields in the condensed rare gases. Furthermore, our results prove the existence of hot electrons in liquid xenon and show that electron-excited liquid xenon could serve as the active medium for a novel type of excimer laser as proposed by [E. B. Gordon, O. S. Rzhevsky, and V. V. Khmelenko, Quantum Electron. 24, 227 (1994)].

Asymptotic Field Dependence of Approach to Saturation of Drift Velocity of Electrons in Liquid Xenon

Abstract The high electric field saturation drift velocity v sat of electrons in liquid xenon near the triple point is 3 × 103 m/s. This value is recovered if (Vsat d) κsat τsat =D. Here D is the self-diffusion coefficient of liquid xenon, τsat is the lower limit of phonon oscillation periods, and κsat is an electron energy-loss probability characteristic of scattering of field-driven electrons by the atoms of the liquid medium. Given the form of the saturation drift velocity, the asymptotic field (E) dependence of the approach to the drift velocity saturation is then explored by expanding κ(Eτ(E) in the form κ(Eτ(E) = κsat τsat + A 1/E γ + higher-order terms. From experimental results of Huang and Freeman, γ is found to be 4/3. This form cannot, of course, be extended into regimes of much lower electric field where a further physical mechanism contributes to the electron mobility.

Monte Carlo investigation of electron-impact ionization in liquid xenon.

The electron-energy-distribution function, drift velocity, and ionization rate for electrons in liquid xenon have been calculated using a kinetic model for electron transport. The electron-scattering rates have been obtained with the Van Hove approach in which the liquid is characterized by the dynamic structure function S(q,\ensuremath{\omega}). The threshold field for ionization ${\mathit{E}}_{\mathit{t}}$ (such that the ionization coefficient \ensuremath{\alpha}\ensuremath{\ne}0 for E>${\mathit{E}}_{\mathit{t}}$) is found to be approximately 400 kV/cm. This value for ${\mathit{E}}_{\mathit{t}}$ as well as those for \ensuremath{\alpha} are in good agreement with experiment. In particular, the minimum at high fields in the experimental \ensuremath{\alpha} vs E curve is also present in the calculated results. These results serve to elucidate the kinetics of electron avalanches in liquids.

Measurement of attenuation length of drifting electrons in liquid xenon

To realize a long attenuation length of drifting electrons in liquid xenon, a purification system which consists of Oxisorb, molecular sieves and a Zr-V-Fe alloy getter has been constructed. A dual type gridded ionization chamber is used for the measurement of the attenuation length. An attenuation length longer than 2 m is achieved in the purified liquid xenon.

Hall mobility of electrons in liquid xenon

Reports the Hall mobility of electrons injected into liquid xenon whose thermodynamic state is varied along the liquid-vapour coexistence line from the triple point to 272 K. Contrary to what would be expected from transport theory, when omega c tau <1, the Hall angle is not a linear function of the magnetic field. This is particularly important near the mobility maximum, and is interpreted as arising from interference effects. Their importance is a consequence of the fact that, to first order, at the mobility maximum, the electron-phonon interaction is zero while elastic scattering by large density fluctuations persists. Hot electron effects support this interpretation. Beyond 272 K no Hall signal could be detected. Although this is an indication that hopping conductivity is dominant, it is shown that the eventual localized states cannot be associated with electrons trapped into density fluctuations associated with the pure fluid.

Motion of electrons in liquid xenon: Evidence for non-Boltzmann transport.

This letter reports the Hall mobility of electrons injected into liquid xenon whose thermodynamic state is varied along the liquid-vapor coexistence line from the triple point to 275 K. Contrary to what would be expected from transport theory, when ${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$\ensuremath{\tau}1, the Hall angle is not a linear function of the magnetic field. This is particularly important near the mobility maximum and is interpreted as arising from interference effects. Their importance is a consequence of the fact that at the mobility maximum, the electron-phonon interaction is zero while elastic scattering by large density fluctuations persists.

Measurements of the lifetime of conduction electrons in liquid xenon

Abstract A gridded ionization chamber with a gap of 5.9 cm was used to study the drift of conduction electrons in liquid xenon. Electrons were generated by cosmic rays traversing the active volume of 0.6 liter. To be sensitive to the effect of electron attachment by electronegative impurities, long drift distances, or equivalently, long drift times, were achieved by reducing the electric field strength. Data were recorded for the range 7 V/cm ≤ E

Hot Electron Energy Exchange in Liquid Hot Electron Energy Exchange in Liquid Xenon

Values of the energy exchange frequency vk for excess electrons in liquid xenon have been estimated from equilibrium field-dependent drift mobilities and from the relaxation of the radiation induced conductivity following a short ionizing pulse. Good agreement between the two methods is found if the mobility is taken to be dependent on the inverse first power of the electron energy, rather than the inverse square root as suggested by theory. The thermal value of vk is 5±1x108 s-1 with a close to direct square root dependence on electron energy being indicated.

Band–Nonband Mobility Transition for Extra Electrons in Liquid Xenon

The mobility of electrons in xenon increases from 1080 cm2/Vs at 168 K to a maximum of 3900 cm2/Vs at 220 K, then decreases as the temperature is increased further, reaching 21 cm2/Vs at the critical temperature, 290 K. The pressure on the sample was the vapor pressure of the liquid and the field strength was 16 V/cm. Electron scattering appears to be gas-like in the liquid near the critical region, while electron transport occurs in conduction bands in the liquid at lower temperatures.