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

Elizabeth Berzin, Lene Kristian Bryngemark, Robert Craig Group, Joesph Kaminski, Timothy Nelson, Rory O'Dwyer, Jessica Pascadlo, Emrys Peets, Benjamin Reese, Lauren Tompkins, Kieran Wall, Andrew Whitbeck

TL;DR

The study measures the LCLS-II dark current in the Sector 30 Transfer Line using a prototype LDMX Trigger Scintillator, demonstrating parasitic operation with the LCLS-II timing and delivering a time-resolved dark current profile. The detector setup combines a scintillator bar array with SiPMs, a QIE11-based readout, and a zCCM timing system, enabling precise charge and timing measurements. Results show a dark current of about 0.8–1.7 pA within the kicker window and an average of ~7 electrons per kicker window, with stable performance across hours to weeks, and timing peaks aligned to the 186 MHz RF structure. These findings constrain the expected dark current for LDMX and inform laser operation, spoiler design, and trigger strategies to achieve the targeted one electron per event, fixed-target missing momentum search.

Abstract

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.

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

TL;DR

The study measures the LCLS-II dark current in the Sector 30 Transfer Line using a prototype LDMX Trigger Scintillator, demonstrating parasitic operation with the LCLS-II timing and delivering a time-resolved dark current profile. The detector setup combines a scintillator bar array with SiPMs, a QIE11-based readout, and a zCCM timing system, enabling precise charge and timing measurements. Results show a dark current of about 0.8–1.7 pA within the kicker window and an average of ~7 electrons per kicker window, with stable performance across hours to weeks, and timing peaks aligned to the 186 MHz RF structure. These findings constrain the expected dark current for LDMX and inform laser operation, spoiler design, and trigger strategies to achieve the targeted one electron per event, fixed-target missing momentum search.

Abstract

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.
Paper Structure (15 sections, 1 equation, 14 figures)

This paper contains 15 sections, 1 equation, 14 figures.

Table of Contents

  1. Introduction
  2. Detector and experimental setup
  3. Trigger Scintillator
  4. SiPMs
  5. Readout Electronics
  6. Timing and Control
  7. Triggering and Data Acquisition
  8. Mechanics
  9. Results
  10. SiPM Calibration
  11. Dark Current Measurement
  12. Electron Counting
  13. Timing Structure
  14. Dark Current Stability
  15. Conclusions

Figures (14)

  • Figure 1: Layout of LESA and End Station A in relation to the SLAC Linac. Figure is from markiewicz2022slaclinacesalesa.
  • Figure 2: LCLS-II timing structure, showing primary FEL pulses, consisting of $10^8$ electrons, and bunches from the S30XL kicker at 37.14 MHz.
  • Figure 3: Configuration of scintillating bars in the trigger scintillator prototype installed in the S30XL beam line, with the beam direction along the $z$-axis.
  • Figure 4: TS module installed in the S30XL beam line, with S30XL beam pipe, readout electronics housing, and the S30XL dump indicated. A closer view of the TS and electronics housing is shown on the right, with the backplane labeled.
  • Figure 5: Distributions of charge integrated over 10 samples for two TS bars, during a period with no active dark current. Distinguishable peaks corresponding to individual photoelectrons are identified by red points.
  • ...and 9 more figures