Measurement of the LCLS-II dark current using the LDMX Trigger Scintillator Prototype

The Light Dark Matter eXperiment (LDMX) is a proposed fixed-target missing momentum search for sub-GeV thermal relic dark matter. LDMX aims to probe thermal dark matter targets with 1016 electrons on target. Such an approach requires a high-repetition rate, low-current beam, with an average of one electron on target per event. These requirements are well-suited to the DArk Sector Experiments at LCLS-II (DASEL) facility, which will take advantage of the unused RF buckets between LCLS-II bunches to produce a well-defined low-current beam with a 26.9 ns bunch spacing. This document describes the results of a measurement of dark current in the Sector 30 transfer line (S30XL) of the LCLS-II beam, using a prototype of the LDMX trigger scintillator (TS) subsystem.

💡 Research Summary

The paper reports a precise measurement of the dark‑current background present in the Sector 30 transfer line (S30XL) of the LCLS‑II accelerator, using a prototype of the Light Dark Matter eXperiment (LDMX) trigger scintillator (TS). LDMX aims to search for sub‑GeV thermal relic dark matter by delivering 10¹⁶ electrons on target, which requires a low‑current (∼ pA), high‑repetition‑rate (∼ 37 MHz) beam. The DArk Sector Experiments at LCLS‑II (DASEL) facility provides such a beam by exploiting unused RF buckets between the main FEL bunches, delivering electron bunches spaced by 26.9 ns. However, the level of “dark current” – electrons that populate the empty RF buckets – was not known with sufficient precision, and this uncertainty directly impacts the design of the LDMX laser and spoiler system.

Detector prototype
The TS prototype consists of twelve 2 × 3 × 30 mm EJ‑200 plastic scintillator bars arranged in two staggered rows of six. Each bar is read out by a 2 × 2 mm² Hamamatsu S13360‑2050VE SiPM. The bars are spaced 0.3 mm in the horizontal (x) direction and 2 mm in the beam (z) direction, with enhanced specular reflector (ESR) films separating them, and a light‑tight housing enclosing the whole assembly. This geometry ensures that an electron passing through the gap of one layer will be captured by the opposite layer, maximizing detection efficiency.

Readout electronics and timing
SiPM signals are digitized by a CMS HCal‑style readout module based on the QIE11 ASIC. The QIE integrates the SiPM current over each 37.14 MHz clock cycle, delivering an 8‑bit ADC value and a 6‑bit TDC. Four operational phases run in parallel, providing dead‑time‑free digitization with a dynamic range of ≈ 400 pC; input‑current shunts can extend this range by a factor of twelve. The SiPM gain (≈ 180–200 fC/PE) limits the usable range, but the dark‑current signal is well within this limit. The TDC subdivides the 26.9 ns sampling period into 50 bins, giving a timing resolution of 0.538 ns.

Synchronization to the accelerator is achieved with a custom Zynq‑based board (zCCM) that recovers the 186 MHz LCLS‑II reference and generates the 37.14 MHz clock for the front‑end. The zCCM also handles slow control (power, I²C) and distributes fast control signals. Trigger and data acquisition are managed by an Advanced Processor (APx) board, originally developed for the HL‑LHC CMS upgrade. Three trigger modes are implemented: a simple threshold trigger, a synthetic periodic trigger for SiPM characterization, and a “kicker” trigger that is locked to the 10 Hz kicker signal that diverts dark‑current bunches into the S30XL line.

Calibration
SiPMs were operated at 53 V (3 V over‑voltage). Calibration runs without beam showed distinct single‑photo‑electron peaks, allowing extraction of gain (≈ 180–200 fC/PE) and pedestal (≈ 20 fC). A light leak broadened the noise distribution, but the signal peak (≈ 16 000 fC) remained well separated.

Dark‑current measurement
During data taking the kicker was set to divert dark‑current bunches at 10 Hz. The TS was triggered on the kicker timing and recorded 128 samples (3.44 µs) per trigger. Runs with the kicker disabled demonstrated negligible background (≤ 1 MIP‑like signal per hour), corresponding to a background current of ≈ 20 aA. With the kicker active, the accumulated charge waveforms displayed a clear region of activity between samples 30 and 60, matching the ≈ 700 ns flat‑top of the kicker. An additional isolated pulse at sample 75 was observed both with and without the kicker and is attributed to occasional upstream electrons.

A 20 PE threshold (well above the noise floor) was applied to identify hits. The hit rate as a function of bar number fell to zero at the edges, confirming that the beam spot (a few millimetres in diameter) was fully contained within the scintillator array both horizontally and vertically. Small beam steering (± 1 cm) in the vertical direction did not change the rate, while horizontal steering shifted the hit‑rate profile as expected, further confirming the beam profile measurement.

Quantifying the dark current
Two complementary approaches were used:

1. Charge integration – The total charge in the kicker window (samples 30‑60) was summed for each bar and fitted with a Poisson‑convolved Landau model. The fit yielded an average of λ_tot ≈ 6–7 electrons per kicker flat‑top, i.e. ≈ 0.05 e⁻ per 26.9 ns bunch. Given the ≈ 700 ns flat‑top, this corresponds to a dark‑current of ≈ 1.5 pA.

2. Electron counting – Peaks above 20 PE were tagged as individual electrons, clustered across adjacent bars occurring in the same 26.9 ns sample, and counted. The distribution of counted electrons per kicker window was well described by a Poisson with λ ≈ 7, in agreement with the charge‑integration result. The probability of two electrons falling in the same 26.9 ns bin is only ≈ 2.7 %, so double‑counting effects are negligible.

Timing structure
TDC information revealed seven distinct peaks spaced by 5.38 ns, matching the 186 MHz RF gun frequency. The peak widths are dominated by the TDC resolution, indicating that intrinsic RF‑gun timing jitter is sub‑dominant. The beam position, inferred from the bar number of hits, drifts during the kicker ramp (≈ 150 ns) and stabilizes for ≈ 570 ns, consistent with the expected kicker timing.

Stability
Dark‑current measurements were repeated in several 6–8 hour runs over a month. Apart from periods when the kicker was intentionally disabled, the rate of >20 PE hits remained stable within statistical fluctuations, demonstrating that the dark‑current background is reproducible and does not drift significantly over time.

Conclusions and impact
The prototype TS successfully measured the LCLS‑II dark‑current level at ≈ 1.5 pA, corresponding to an average of 0.05 electrons per 26.9 ns bunch. The detector’s timing resolution (≈ 0.5 ns), charge resolution, and spatial coverage were sufficient to resolve individual electrons and map the beam profile. These results provide the quantitative input needed for LDMX’s laser‑generation and spoiler design, confirming that the parasitic DASEL beam can deliver the ultra‑low‑current, high‑repetition‑rate electron stream required for a sub‑GeV missing‑momentum dark‑matter search. The demonstrated methodology—combining SiPM‑based scintillation, QIE11 readout, and FPGA‑based timing synchronization—offers a scalable solution for the full LDMX trigger scintillator system and for other experiments that need precise monitoring of low‑intensity, high‑frequency beams.