Electroluminescence and charge multiplication in liquid xenon with a VCC-like Microstrip Plate

TL;DR

The paper investigates electroluminescence and charge multiplication in liquid xenon using a VCC-like microstrip plate, aiming to enable high-gain, single-phase noble-liquid detectors for dark-matter and neutrino experiments. It demonstrates EL and CM with a plate consisting of $2\mu m$-wide anode strips on SCHOTT S8900 glass at a $2~\mathrm{mm}$ pitch, using an $^{241}$Am alpha source and PMT readout; the initial light yield is $\sim 460$ photons per drifting electron, decaying to $$(27.0\pm 3.1)$$ photons/e, while the estimated charge gain is $\lesssim 5$ at $\Delta V_{ab}=5$ kV. The results, compared with model predictions and prior MSGC work, indicate substrate effects and modeling limitations; degradation of the light yield is attributed to charging up or polarization of the glass, motivating exploration of lower-resistivity and VUV-transparent substrates to achieve larger, more stable yields. The study suggests that VCC-based single-phase detectors could offer scalable advantages for DM and neutrino experiments, warranting further materials and geometry optimization to realize robust high-gain EL/CM in LXe.

Abstract

We report on the first observation of electroluminescence and charge amplification with a Virtual Cathode Chamber (VCC) microstrips plate immersed in liquid xenon. Both were observed in an intense non-uniform electric field in the vicinity of 2-$μ$m narrow anode strips deposited, with a 2~mm pitch, on a semiconductive glass substrate (S8900), with a cathode film on its backside. An initial light yield of $\sim$460 VUV photons per drifting electron was measured, which degraded within tens of minutes stabilizing at (27.0~$\pm$~3.1)~photons per electron. The electroluminescence was accompanied by electron multiplication with an estimated charge gain $<$10. Further investigations are necessary to understand and mitigate the light yield degradation phenomenon. We expect other substrate materials, including VUV-transparent ones, to provide large stable photon yields, compatible with our model predictions. The VCC configuration has demonstrated great potential in single-phase noble-liquid detectors, particularly for dark-matter searches, neutrino physics and other fields.

Figures (13)

• Figure 1: Comparison of the field line patterns of a MSGC (left) and a VCC (right) in liquid xenon calculated with COMSOL$^{\textrm{\textregistered}}$comsol. The anode strip (a) voltage is 5 kV, the cathode strips (c) are grounded in the MSGC configuration, the backplane is also grounded, and the voltage of the drift electrode (2 mm above strips) is -300 V. The anode-strip width was 8 $\mu$m with a pitch of 1 mm in both calculations, while the width of the cathode strips for the MSGC was set to 400 $\mu$m. The left panel was redrawn and adapted from Martinez-Lema_2024_microstrips.
• Figure 2: Comparison of different microstructures in liquid xenon in what concerns secondary light emission. Left: extent of the region along the shortest field line for which the electric field strength is above the electroluminescence threshold measured in Aprile:2014ELthreshold. Right: predicted photon yield as a function of the anode voltage. The experimental MSGC result (black cross) was obtained at the highest applicable anode-to-cathode potential. The MSGC, COCA-COLA and VCC-8$\mu$m curves are reproduced from Martinez-Lema_2024_microstrips and correspond to plates with 1 mm pitch and 8 $\mu$m-wide anode strips. VCC-2$\mu$m (blue lines) corresponds to the geometry used in this study (2 mm pitch and 2-$\mu$m-wide anode strips). This curve follows the same normalization as the other ones. For further details the reader is referred to Martinez-Lema_2024_microstrips.
• Figure 3: Left: VCC plate mounted on a FR4 support: 13 parallel 2$\mu$m wide strips are connected to a square frame. Two stainless steel clamps provide connection to the external circuit using a multiple-folded aluminium foil to avoid damage to the metal coating. Right: Microphotograph of an anode strip at its junction point with the surrounding frame (indicated by an arrow on the left panel).
• Figure 4: Schematic drawing of the experimental setup. Scintillation photons generated at the interaction site can be reflected from the microstrip plate and reach the PMT, providing the S1 signal. The delayed electroluminescence pulse is generated by the ionization electrons in the vicinity of the strips. The S2 signal results from both photons that reach the PMT directly and those reflected from the strip surface.
• Figure 5: A typical PMT waveform from the VCC in LXe taken at $\Delta V_{\textrm{ab}}\xspace=5.0$ kV. A primary scintillation pulse (S1, zoomed inset axis) at around $t\approx4~\mu$s is followed by a delayed electroluminescence (S2) signal at $t\approx8~\mu$s. Electrode potentials: $V_{\textrm{m}}\xspace=0$, $V_{\textrm{a}}\xspace=+3.75$ kV, $V_{\textrm{b}}\xspace=-1.25$ kV.