Experiment to demonstrate separation of Cherenkov and scintillation signals

The ability to separately identify the Cherenkov and scintillation light components produced in scintillating mediums holds the potential for a major breakthrough in neutrino detection technology, allowing development of a large, low-threshold, directional detector with a broad physics program. The CHESS (CHErenkov/Scintillation Separation) experiment employs an innovative detector design with an array of small, fast photomultiplier tubes and state-of-the-art electronics to demonstrate the reconstruction of a Cherenkov ring in a scintillating medium based on photon hit time and detected photoelectron density. This paper describes the physical properties and calibration of CHESS along with first results. The ability to reconstruct Cherenkov rings is demonstrated in a water target, and a time precision of $338\pm 12\mathrm{ps}$ FWHM is achieved. Monte Carlo–based predictions for the ring imaging sensitivity with a liquid scintillator target predict an efficiency for identifying Cherenkov hits of $94\pm 1\%$ and $81\pm 1\%$ in pure linear alkyl benzene (LAB) and LAB loaded with 2 g/L of a fluor, PPO, respectively, with a scintillation contamination of $12\pm 1\%$ and $26\pm 1\%$.

Figure 1

PMT H11934 quantum efficiency (QE) [13] and ultraviolet-transmitting (UVT) acrylic absorption length compared to the Cherenkov and scintillation emission spectra for pure LAB [14] and LAB loaded with 2 g/L of PPO as a fluor [15]. The normalization of the emission spectra are shown in arbitrary units.

Figure 2

The CHESS apparatus. (a) Detailed schematic view with dimensions and (b) demonstration of ring-imaging concept. The PMT array is designed to hold up to 53 PMTs. The dozen slots occupied for this study are color coded by radius: red and orange for those hit primarily by scintillation photons and blue for those in the expected Cherenkov ring for LAB and LAB-PPO. Due to the lower refractive index, the ring from a water target is detected in the middle (orange) PMTs.

Figure 3

Layout of veto panels around the CHESS apparatus.

Figure 4

A schematic diagram of how the DAQ and hardware are connected together with signal and data paths labeled. Omitted are the high voltage supplies, which are connected to the VME backplane, and all PMTs.

Figure 5

SPE charge distribution for a single PMT. Data are shown as black points with error bars, and the red dashed line is the result of the fit used to extract the shape of the SPE distribution.

Figure 6

PMT pulse shape characterization. (a) Mean and RMS spread for one PMT, showing excellent stability in the pulse region. (b) Averaged pulse shapes for SPE pulses for each individual PMT. (c) Log-normal fit to the average pulse shape across all PMTs. In all cases, the pulses are normalized to unit area, so the voltage is reported in arbitrary units.

Figure 7

(a) Time distribution for a single channel relative to a reference channel, overlaid with the Gaussian fit used to extract the relative time delay. (b) Mean and uncertainty on the mean for the per-channel time delays.

Figure 8

(Top to bottom, left to right) Charge distribution of events on the upper and lower cosmic tags ($Q_U$ and $Q_L$) and the four muon panels ($Q_{V1}–Q_{V4}$). Panel 1 is located directly below the CHESS apparatus. Events are separated into ring (blue, lower line) and background (red, upper line) according to the criterion in Sec. 6a. Vertical black dashed lines show the cut values in each case with arrows indicating the acceptance region. Distributions are shown before (dashed) and after (solid) application of cuts on these six parameters.

Figure 9

Monte Carlo simulation of the summed PE distribution on the inner PMT group due to cosmic muon events with perfect rings (turquoise, right diagonal hatching), muon contamination (orange, left diagonal hatching), electron contamination (purple, right diagonal dashed hatching), and the total (red, solid). (a) Water target, with water data overlaid to show the agreement, and (b) LAB-PPO target. The vertical black dashed line represents the chosen cut value, with arrows to illustrate the acceptance region.

Figure 10

Distribution of hit time residuals for ring candidate events in water for middle PMTs, where the Cherenkov ring is expected. Data points are shown with statistical errors, with the Monte Carlo prediction overlaid (dashed lines). (a) Hit times with respect to the lower muon tag time and (b) hit times with respect to the reconstructed event time.

Figure 11

Typical ring event in the water data set. (a) Estimated number of detected PEs and (b) hit time residuals.

Figure 12

(a) Averaged number of detected PE and (b) averaged first-photon hit time residual per individual PMT for ring candidates in water data.

Figure 13

Distribution of hit time residuals for ring candidate events in water for each PMT grouping. Data points are shown with statistical errors, with the Monte Carlo prediction overlaid (dashed lines).

Figure 14

Projected hit time residual distributions for LAB as predicted from the Monte Carlo simulation.

Figure 15

Projected hit time residual distributions for LAB-PPO as predicted from the Monte Carlo simulation.