Scintillation light detection in the 6-m drift-length ProtoDUNE Dual Phase liquid argon TPC

Abstract

DUNE is a dual-site experiment for long-baseline neutrino oscillation studies, neutrino astrophysics and nucleon decay searches. ProtoDUNE Dual Phase (DP) is a 6 $\times$ 6 $\times$ 6 m ${}^3$ liquid argon time-projection-chamber (LArTPC) that recorded cosmic-muon data at the CERN Neutrino Platform in 2019-2020 as a prototype of the DUNE Far Detector. Charged particles propagating through the LArTPC produce ionization and scintillation light. The scintillation light signal in these detectors can provide the trigger for non-beam events. In addition, it adds precise timing capabilities and improves the calorimetry measurements. In ProtoDUNE-DP, scintillation and electroluminescence light produced by cosmic muons in the LArTPC is collected by photomultiplier tubes placed up to 7 m away from the ionizing track. In this paper, the ProtoDUNE-DP photon detection system performance is evaluated with a particular focus on the different wavelength shifters, such as PEN and TPB, and the use of Xe-doped LAr, considering its future use in giant LArTPCs. The scintillation light production and propagation processes are analyzed and a comparison of simulation to data is performed, improving understanding of the liquid argon properties.

Fig. 1

Views of ProtoDUNE-DP. Dimensions and positions of the major elements are indicated (in units of centimeters). The drift direction corresponds to the y-axis. The PMTs are represented with circles and, in the top view, the empty circles correspond to PEN PMTs and the filled circles to TPB PMTs

Fig. 2

ProtoDUNE-DP PDS formed of 36 PMTs installed at the bottom of the cryostat at CERN. The front left PMT has TPB coating and the front right PMT has PEN sheet

Fig. 3

Map of the drift field in ProtoDUNE-DP with cathode at − 50 kV simulated using COMSOL. A particular plane of the active volume is shown (vertical plane at 130 cm from a field cage wall). The drift direction is along the y-axis and the color scale represents the electric field strength. The discontinuity at y$\sim$100 cm is due to the HV failure described in the text

Fig. 4

Example of a PMT self-trigger event showing the waveforms corresponding to the 36 PMTs according to their relative position in the detector. The PEN PMTs are shown in black and TPB PMTs in red. The x-axis range is 0–16 $\mathrm{\mu }$s for all PMTs while the y-axis range (in ADC counts) varies for each PMT and is optimized to best display the S1 signal

Fig. 5

Example of a scintillation light event in a ProtoDUNE-DP PMT at a gain of ${10}^7$ in LAr without electric field

Fig. 6

Baseline mean STD during ProtoDUNE-DP operation. A fit to the mean value ($0.6\pm 0.1$ ADC) is shown in red and error bars represent the differences among the different PMTs

Fig. 7

Gain (G) stability with time for the 36 PMTs (one color per PMT) evaluated at 1500 V represented with respect to the average gain $<G>$

Fig. 8

Mean SPE amplitude from Gaussian fits as a function of the PMT gain, and linear fit. All PMTs are included but only data corresponding to gains that allow a proper fit are considered

Fig. 9

Example of SPE pulse in a PMT waveform (${10}^7$ gain)

Fig. 10

Example of the average scintillation waveform of a PEN PMT (blue). The fit to Eq. 6 is shown in red. The corresponding parameters are shown in the legend

Fig. 11

Monitoring of the ${\tau }_{\mathrm{slow}}$ component of the scintillation time profile during the detector operation. The error bars show the STD of the measurements for the different PMT waveforms. Red line at ${\tau }_{\mathrm{slow}}=1.46\mathrm{\mu }$s shows the average from September 2019 when stable operation began

Fig. 12

${\tau }_{\mathrm{int}}$ distribution obtained from the fit to the average waveform for PEN (blue) and TPB PMTs (red)

Fig. 13

Maps of the detected light (sum of PMT visibilities multiplied by efficiency, $E_{\mathrm{TPB}}=0.09$ and $E_{\mathrm{PEN}}=0.016$) per voxel for the baseline photon library in ProtoDUNE-DP: 4$·{10}^4$ photons per MeV, 99.9 cm of RSL and 26% VUV reflections. The PMT positions are represented with black dots and the active volume with a red rectangle

Fig. 14

Normalized distributions of the collected S1 charge for CRT triggered light data and simulation

Fig. 15

Collected S1 charge versus track-PMT distance for PEN PMTs: Light data (top) and MC sample (bottom). The color maps contain all the S1 signals detected by the PMTs and passing the event selection. A Gaussian fit of each charge-distribution every 10 cm is performed and the mean values are plotted in black over the map. The vertical error bars correspond to errors from the fits and a 5-cm distance uncertainty (horizontal error bars) is included

Fig. 16

Average S1 charge per channel (in PE) in CRT-trigger mode: top view of PEN-PMT array for data (top panel) and simulation (bottom panel). Each color square represents one PMT channel at its position in the detector. The approximate positions of the CRT frames are indicated with black lines

Fig. 17

Top panel: Average S1 charge collected by the PEN PMTs as a function of the track-PMT distance without drift field (black) and with cathode at −50 kV (red). The larger error bars at short distance are due to a lack of statistics. Bottom panel: Ratio between the two distributions. The ratio is fitted to a constant value (result: 0.833 ± 0.007)

Fig. 18

Light yield reduction with drift field measured by various experiments (red triangles from [49], black squares from [50], blue inverted triangles from [35]), and the ProtoDUNE-DP result (orange circles) discussed in this section

Fig. 19

Evolution of ${\tau }_{\mathrm{slow}}$ with the cathode voltage for CRT-trigger events. The results are obtained averaging PEN PMTs and the error bars indicate the STD among them

Fig. 20

Top panel: average S1 charge collected by the PEN PMTs as a function of the track-PMT distance. Two MC samples with different RSL values (61.0 cm and 99.9 cm) are compared to data. For each distribution, an exponential fit is plotted as a solid line. Bottom panel: data-MC ratio for the two previous MC samples

Fig. 21

Cosmic muon flux determined in ProtoDUNE-DP at CERN at 455 m above mean sea level (red square) compared to the measurements reported in [52] (filled black dots) and [53] (open black dot)

Fig. 22

Normalized distributions of the total S1 charge per cosmic muon event (sum of all detecting PMTs) in random trigger. Both data and MC (TPB and PEN) cases are displayed. The ADC saturation in data that limits the charge range is fairly reproduced in the MC

Fig. 23

Example of an event with S1 and S2 signals showing the waveforms corresponding to the 36 PMTs according to their relative position in the detector. PEN PMTs are shown in black and TPB PMTs in red. The x-axis range is 0.8−3 ms for all PMTs while the y-axis range varies for each PMT and is optimized to best display the S2 signal

Fig. 24

Example of a scintillation light event in a ProtoDUNE-DP PMT at a gain of ${10}^7$ in LAr with drift, amplification and extraction fields (S1 and S2 signal). The acquired waveform is shown in black and the resulting waveform after the overshooting correction in red

Fig. 25

Average waveform of PEN PMTs with a S2 signal identified. The y-axis is zoomed to focus on the S2 signal

Fig. 26

Evolution of the average S1 charge per event detected in all the PMTs for PMT(CRT)-trigger data in black (red) during the evaporation, filling and N${}_2$ injections. The processes are described in Sect. 2

Fig. 27

Ratio of the average S1 amplitude and S1 charge in the three doping concentrations relative to pure LAr. PMT-trigger data are shown in black, and CRT-trigger in red. Only a selection of PMTs is considered for the average. Selected PMTs are marked as black (red) circles in the small diagram for the PMT(CRT)-trigger data. A blue circle marks the trigger PMT in the PMT-trigger diagram. Errors show the STD among the selected PMTs. Xe and N${}_2$ concentrations for each situation are summarized in Table 1

Fig. 28

Top panel: average collected S1 charge per PMT as a function of the track-PMT distance for the different doping concentrations. For each data-set, an exponential fit is performed (see results in Table 9). Bottom panel: ratios between the data shown in the top panel. Each data-set is fitted with an exponential function

Fig. 29

PMT average waveform normalized to same amplitude in LAr (black), LAr + Xe + N${}_2$ (red), after the 1${}^{\mathit{st}}$ N${}_2$ injection (blue) and after the 2${}^{\mathit{nd}}$ N${}_2$ injection (magenta). Concentrations are described in Table 9