Fluorescence time profile measurement of LAB based liquid scintillator in response to medium relativistic ion particles

TL;DR

The study addresses how LAB-based liquid scintillator time profiles depend on the ionization density $dE/dX$ by measuring fluorescence timing for high-energy ions ($Z=1$, $Z=2$, and Krypton) and comparing to MeV-scale radioactive sources. A thin 5 mm LAB-based scintillator sample read out by fast PMTs, coupled with a downstream energy detector, is used to capture detailed time spectra, which are analyzed with a four-component exponential model convolved with the PMT response. GEANT4 simulations guide particle identification and energy deposition, enabling separation of Z=1 and Z=2 samples and assessment of the role of secondary electrons in Kr interactions. The results reveal that ions with similar $dE/dX$ can exhibit timing close to electron-like responses, while Kr shows reduced long-time tails due to secondary-electron contributions, underscoring the importance of $dE/dX$ in scintillation timing and offering a path toward a unified time model for PSD in large detectors like JUNO and for DSNB searches.

Abstract

Liquid scintillator is widely used in particle physics experiments due to its high light yield, good timing resolution, scalability and low cost. Certain liquid scintillators exhibit pulse shape discrimination capabilities because of difference in fluorescence timing properties induced by different particles. Its fluoresence timing properties have been measured mostly for radioactive decay sources at MeV energies. We present a novel measurement of fluorescence time properties of LAB based liquid scintillator in response to high-energy ions of hydrogen (Z = 1), helium (Z = 2) and Krypton at around 200-300 MeV/u for the first time. We compared the results to those from radioactive sources and observed a distinct $dE/dX$ dependence, regardless of the particle type. These findings are essential for physics searches such as the diffuse supernova neutrino background in large liquid scintillator detectors like JUNO, and are also critical towards understanding the underlying scintillation timing mechanism.

Figures (12)

• Figure 1: Schematic layout of the experiment. The upper panels show the full view of the RIBLL2 beamline and the External Target Facility (ETF), showing the plastic scintillators (SC1 and SC2) and the optional targets (see Table \ref{['tab:beam']}). The bottom panel presents a zoom of ETF region, highlighting the elements used in this study. The downstream plastic scintillator (SC2) and dE liquid scintillator (LS) provide the start and the end signals for time of flight in this research. The LS samples to be measured are located at the dark box region whose details are shown in Figure \ref{['fig:Experimental Setup in Dark Box']}.
• Figure 2: The left panel shows the schematic view of the LS detector in the dark box, and the right panel is the photo of the setup. The detector contains two parts, including time profile and deposit energy measurement.
• Figure 3: Visualization of the Geant4 simulation of the secondary particles that could pass through the dE LS sample. Protons with varying energies were ejected from different angles ($0^\mathrm{o}-21^\mathrm{o}$) and the magnetic field was set as 0.9 Tesla in accordance with the actual scenario. The blue lines represent the simulated tracks of protons. Particles within a certain range of scattering angles can be directed to the LS sample.
• Figure 4: $\Delta E$ vs. E distribution. The 2D histogram represents the data, while the lines are medians from MC predictions for Z=1,2,3 particles. The two prominent clusters in the data align with the median of Z=1 and Z=2 particles in MC.
• Figure 5: Single-event waveform from all channels of an R7600U PMT. The bottom channel (ch3) serves as the signal channel, while the other channels exhibit crosstalk noise at roughly 20% of the signal amplitude and occur simultaneously with the signal.