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Characterisation of Hamamatsu R11065-20 PMTs for use in the SABRE South NaI(Tl) Crystal Detectors

O. Stanley, W. J. D. Melbourne, P. Urquijo, E. Barberio, V. U. Bashu, L. J. Bignell, I. Bolognino, G. Brooks, S. S. Chhun, F. Dastgiri, M. B. Froehlich, T. Fruth, G. Fu, G. C. Hill, R. S. James, K. Janssens, S. Kapoor, G. J. Lane, K. T. Leaver, P. McGee, P. C. McNamara, J. McKenzie, L. J. McKie, M. Mews, L. J. Milligan, K. J. Rule, F. Scutti, Z. Slavkovska, A. E. Stuchbery, B. Suerfu, G. N. Taylor, A. G. Williams, Y. Xing, Y. Y. Zhong

TL;DR

This work presents a comprehensive pre-calibration of Hamamatsu R11065-20 PMTs for SABRE South NaI(Tl) detectors, detailing SPE-based gain extraction, in-situ gain monitoring via dark counts, and precise timing using a SiPM reference. It characterises PMT backgrounds through temperature-dependent dark rate modeling, spontaneous light emission, and after-pulsing, all crucial for low-energy backgrounds near 1 keV$_{\text{ee}}$. A Boosted Decision Tree (BDT) framework, including single- and paired-PMT analyses with CAP variables and RF templates, demonstrates significant improvements in discriminating scintillation from PMT-induced noise, enabling lower effective energy thresholds. A waveform-simulation tool (DOOM) is developed to convert optical Monte Carlo outputs into realistic PMT waveforms for training and performance studies at 1 keV$_{\text{ee}}$. Overall, the study provides essential calibration, background understanding, and advanced discrimination techniques to enhance SABRE South sensitivity to dark matter while enabling robust long-term PMT monitoring.

Abstract

The SABRE Experiment is a direct detection dark matter experiment using a target composed of multiple NaI(Tl) crystals. The experiment aims to be an independent check of the DAMA/LIBRA results with a detector in the Northern (Laboratori Nazionali Del Gran Sasso, LNGS) and Southern (Stawell Underground Physics Laboratory, SUPL) hemispheres. The SABRE South photomultiplier tubes (PMTs) will be used near the low energy noise threshold and require a detailed calibration of their performance and contributions to the background in the NaI(Tl) dark matter search, prior to installation. We present the development of the pre-calibration procedures for the R11065-20 Hamamatsu PMTs. These PMTs are directly coupled to the NaI(Tl) crystals within the SABRE South experiment. In this paper we present methodologies to characterise the gain, dark rate, and timing properties of the PMTs. We develop a method for in-situ calibration without a light injection source. Additionally we explore the application of machine learning techniques using a Boosted Decision Tree (BDT) trained on the response of single PMTs to understand the information available for background rejection. Finally, we briefly present the simulation tool used to generate digitised PMT data from optical Monte Carlo simulations.

Characterisation of Hamamatsu R11065-20 PMTs for use in the SABRE South NaI(Tl) Crystal Detectors

TL;DR

This work presents a comprehensive pre-calibration of Hamamatsu R11065-20 PMTs for SABRE South NaI(Tl) detectors, detailing SPE-based gain extraction, in-situ gain monitoring via dark counts, and precise timing using a SiPM reference. It characterises PMT backgrounds through temperature-dependent dark rate modeling, spontaneous light emission, and after-pulsing, all crucial for low-energy backgrounds near 1 keV. A Boosted Decision Tree (BDT) framework, including single- and paired-PMT analyses with CAP variables and RF templates, demonstrates significant improvements in discriminating scintillation from PMT-induced noise, enabling lower effective energy thresholds. A waveform-simulation tool (DOOM) is developed to convert optical Monte Carlo outputs into realistic PMT waveforms for training and performance studies at 1 keV. Overall, the study provides essential calibration, background understanding, and advanced discrimination techniques to enhance SABRE South sensitivity to dark matter while enabling robust long-term PMT monitoring.

Abstract

The SABRE Experiment is a direct detection dark matter experiment using a target composed of multiple NaI(Tl) crystals. The experiment aims to be an independent check of the DAMA/LIBRA results with a detector in the Northern (Laboratori Nazionali Del Gran Sasso, LNGS) and Southern (Stawell Underground Physics Laboratory, SUPL) hemispheres. The SABRE South photomultiplier tubes (PMTs) will be used near the low energy noise threshold and require a detailed calibration of their performance and contributions to the background in the NaI(Tl) dark matter search, prior to installation. We present the development of the pre-calibration procedures for the R11065-20 Hamamatsu PMTs. These PMTs are directly coupled to the NaI(Tl) crystals within the SABRE South experiment. In this paper we present methodologies to characterise the gain, dark rate, and timing properties of the PMTs. We develop a method for in-situ calibration without a light injection source. Additionally we explore the application of machine learning techniques using a Boosted Decision Tree (BDT) trained on the response of single PMTs to understand the information available for background rejection. Finally, we briefly present the simulation tool used to generate digitised PMT data from optical Monte Carlo simulations.

Paper Structure

This paper contains 19 sections, 18 equations, 21 figures, 8 tables.

Table of Contents

  1. Introduction
  2. Experimental Setup
  3. Signal
  4. Single Photo-Electron Response and Gain
  5. Gain calibration with dark counts
  6. Timing
  7. PMT Background
  8. Dark Rate
  9. Light Emission rate
  10. After-pulse Rate
  11. Noise -- Scintillation Discrimination via Machine Learning
  12. Data
  13. Selection
  14. Single PMT model
  15. Validation
  16. ...and 4 more sections

Figures (21)

  • Figure 1: The typical spectral response for the R11065 and R11410 PMTsR_Acciarri_2012. The shaded region shows where the emission spectrum of NaI(Tl) is above 50$\%$ of the peak emission amplitude. The vertical line indicates the emission peak at 415 nm.
  • Figure 2: Experimental set ups for (left) pico second pulsed laser system, (middle) thermal testing chamber with resistance temperature device sensors (right) SABRE detector mockup, operated with and without an encapsulated commercial NaI(Tl) crystal.
  • Figure 3: (left) The time-difference between the pulsed-laser signal and the peak start location within the waveform for PMT BC0174. The green region is the selected region (55-80 ns) for our SPE analysis, and the pink are our side band region to measure purity. (right) Single photo-electron charge observed for PMT BC0174, normalised to 1 at SPE peak.
  • Figure 4: The normalised average waveforms for BC0174 (left) and BC0175 (right) at 1500 V, aligned at the leading edge threshold position (200 ns). The shaded region shows the charge integration window.
  • Figure 5: Fitted charge spectra for PMT BC0174. At (left) 1500 V and (right) 1750 V.
  • ...and 16 more figures