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

This paper presents the results of measuring the dark current in the LCLS-II Sector 30 transfer line using a prototype LDMX trigger scintillator, a critical step toward validating the facility's suitability for the proposed Light Dark Matter eXperiment's low-current beam requirements.

The Gist

The Big Picture: Hunting for Invisible Ghosts

Imagine scientists are trying to find a very shy, invisible ghost called Dark Matter. To catch it, they built a giant trap called LDMX (Light Dark Matter eXperiment).

To make this trap work, they need a very specific type of "net" made of electrons. They don't want a flood of electrons (like a firehose); they want a gentle, steady drip—literally one electron at a time. If too many electrons hit the target at once, the "ghost" gets lost in the noise.

The Problem: The "Leaky Faucet"

The scientists planned to use a massive particle accelerator called LCLS-II to create this gentle drip. However, LCLS-II is designed to shoot huge bursts of electrons to make X-rays for other experiments.

Between these huge bursts, the machine isn't perfectly quiet. It has a "leaky faucet" called Dark Current. This is a tiny, unwanted trickle of extra electrons that the machine accidentally shoots out when it's supposed to be resting.

The Question: How big is this leak? Is it a tiny drip (good) or a steady stream (bad)? If the leak is too big, it ruins the delicate LDMX experiment.

The Solution: The "Scintillator Sandwich"

To measure this leak, the scientists built a special detector prototype called a Trigger Scintillator (TS). Think of this detector as a high-tech sandwich made of:

• The Bread: 12 strips of special plastic that glow (scintillate) when hit by an electron.
• The Filling: Tiny light sensors (SiPMs) that act like super-sensitive eyes, counting every single photon of light the plastic emits.

They installed this sandwich in a side-tunnel (Sector 30) of the accelerator just to "listen" to the leak before the main experiment starts.

How They Measured the Leak

The scientists turned on the accelerator's "kicker" magnet. Imagine a conductor waving a baton to tell the electrons when to leave the main line and go down the side tunnel.

1. The Timing: The main machine fires huge bursts of electrons every 1.1 microseconds. But between those bursts, there are empty slots. The "kicker" diverts the unwanted "leak" electrons into these empty slots.
2. The Measurement: The sandwich detector watched the side tunnel for 5.5 hours.
   • Method A (The Bucket): They collected all the light energy (charge) from the glow and measured the total volume.
   • Method B (The Counter): Because the leak was so small, they could actually count the individual electrons one by one, like counting raindrops hitting a roof.

The Results: A Very Small Leak

The results were excellent news for the LDMX experiment:

• The Leak is Tiny: The dark current was measured to be between 0.8 and 1.7 picoamperes.
  • Analogy: If the main electron beam is a raging river, this dark current is less than a single drop of water falling from a leaf every second.
• The Timing is Perfect: The detector confirmed that the electrons arrive in a very neat, organized pattern, exactly when the scientists predicted.
• The "Ghost" is Safe: Because the leak is so small and predictable, the LDMX team knows they can build their experiment to ignore this background noise. They can confidently hunt for dark matter without worrying about the machine's "leaky faucet" ruining the data.

The "Aha!" Moment

The paper also revealed something cool about the machine itself. By looking at when the electrons arrived, they saw a pattern repeating every 5.38 nanoseconds. This confirmed that the machine's internal clock (the RF gun) is ticking with incredible precision, like a Swiss watch.

Summary

In short, the scientists built a super-sensitive "light sandwich" to check if their particle accelerator was leaking unwanted electrons. They found the leak was incredibly small and well-behaved. This proves that the accelerator is ready to be used as the perfect, quiet background for the LDMX experiment to hunt for the universe's most elusive particles.

Technical Summary

1. Problem Statement

The Light Dark Matter eXperiment (LDMX) is a proposed fixed-target experiment designed to search for sub-GeV thermal relic dark matter. To achieve its physics goals, LDMX requires a unique beam configuration: a high-repetition-rate, low-current electron beam with an average of one electron per event ($10^{16}$ electrons on target).

The proposed solution utilizes the DArK Sector Experiments at LCLS-II (DASEL) facility at SLAC. This facility plans to extract "unused" RF buckets between the main LCLS-II Free Electron Laser (FEL) pulses to create a dedicated low-current beam. However, these empty buckets are populated by dark current (stray electrons) from the RF gun.

• The Challenge: The precise level of this dark current was previously unknown, with only rough estimates suggesting it was less than 20 pA.
• The Need: Accurate measurement of this dark current is critical to:
  1. Validate the feasibility of the DASEL facility for LDMX.
  2. Inform the design requirements for the LDMX laser and spoiler systems needed to further deplete out-of-time bunches.
  3. Ensure the beam quality meets the strict "one electron per event" requirement.

2. Methodology

The authors conducted a measurement using a prototype Trigger Scintillator (TS) module from the LDMX detector, installed parasitically in the Sector 30 Transfer Line (S30XL) of the LCLS-II beamline.

Experimental Setup

• Detector: The TS prototype consists of 12 scintillating bars (EJ-200 PVT) arranged in two staggered rows ($2 \times 6$). Each bar is coupled to a Silicon Photomultiplier (SiPM) (Hamamatsu S13360-2050VE).
• Readout Electronics: The system uses a CMS HCal Readout module based on the QIE11 ASIC. It integrates current signals and provides:
  • ADC: 8-bit charge measurement.
  • TDC: 6-bit timing measurement with 0.538 ns resolution (sub-dividing the 26.9 ns sampling period).
• Timing & Control: A custom Zynq Clock and Control Module (zCCM) synchronizes the readout with the LCLS-II timing system, recovering the 186 MHz reference clock and delivering a 37.14 MHz clock (the 5th subharmonic) to the front-end.
• Beam Configuration: The S30XL kicker diverts dark current bunches (10 Hz) into the transfer line. The TS module was triggered by the 10 Hz kicker signal, recording 128 samples (3.44 $\mu$s window) per trigger.

Analysis Techniques

The team employed two complementary methods to quantify the dark current:

1. Charge Integration: Integrating the total charge within the kicker window (samples 30–60) and fitting the distribution to a Poisson-distributed sum of Landau peaks to estimate the average number of electrons ($\lambda$).
2. Direct Electron Counting: Identifying individual electron peaks above a 20 PE (photo-electron) threshold. Peaks were grouped into "clusters" based on spatial proximity (adjacent bars) and temporal coincidence (within the same 26.9 ns sample) to count distinct electrons.

3. Key Contributions

• First Precise Measurement: Provided the first precise measurement of LCLS-II dark current in the S30XL transfer line, moving beyond previous estimates of "< 20 pA."
• Parasitic Operation Demonstration: Successfully demonstrated the integration of LDMX detector technology (TS module and electronics) with the LCLS-II timing and control systems in a parasitic running mode.
• Dual-Method Validation: Validated the dark current measurement using two independent approaches (charge integration and electron counting), both yielding consistent results.
• Time-Resolved Characterization: Mapped the temporal structure of the dark current, confirming it aligns with the S30XL kicker flat-top duration and identifying the 186 MHz RF gun frequency structure within the dark current.

4. Results

• Dark Current Magnitude:
  • Both analysis methods consistently determined the dark current to be between 0.8 pA and 1.7 pA.
  • In terms of particle count, this corresponds to approximately 3.49 to 7.37 electrons per kicker flat-top window (approx. 700 ns).
  • This translates to an average of 0.05 to 0.25 electrons per 26.9 ns bunch.
• Beam Containment: The hit rate distribution across the scintillator bars confirmed that the beam spot is fully contained within the active area of the prototype in both horizontal and vertical directions.
• Timing Structure:
  • TDC data revealed clear peaks every ~5.38 ns, corresponding to the 186 MHz RF gun frequency.
  • The beam position stabilized for ~570 ns, matching the expected kicker flat-top timing.
• Stability: Over a month-long period, the dark current rate was found to be relatively stable, with only occasional discrete changes attributed to other LCLS-II activities or kicker interruptions.
• Background: Background noise was negligible (< 1 MIP-like signal per hour), corresponding to a background current on the order of 20 aA.

5. Significance

• Feasibility for LDMX: The measured dark current (0.8–1.7 pA) is well within the operational limits required for LDMX. It confirms that the DASEL facility can provide a sufficiently low-current beam for the missing momentum search.
• System Design: The precise quantification of the dark current allows the LDMX collaboration to optimize the design of downstream spoilers and collimators, ensuring that the final beam delivered to the target meets the "one electron per event" requirement.
• Operational Insight: The study confirmed that the dark current is not Poissonian in the high-charge tails (indicating some non-Poissonian behavior) and provided a baseline for future beam operations at LCLS-II.
• Technology Validation: The successful deployment of the LDMX TS prototype and its readout electronics serves as a critical pathfinder for the full-scale detector installation at SLAC's End Station A.