XENONnT light sensors: performance and reliabilityGiovanni Volta, PhD student

September 17th, 2021

01/12

• Dark matter direct detection experiment

• Laboratori Nazionali del Gran Sasso (LNGS)

• Dual phase xenon time projection chamber

• 170 scientists, 27 institutions, 12 countries

2006 2014 2018 2021 2025

XENON1015 kg

XENON100161 kg

XENON1T3.2 t

XENONnT8.5 t

The XENON project

02/12

XENONnT experiment• 10 m × 10 m• 84 8’’ PMTs• 700 t water

• High reflectivity PTFE panels• 120 8’’ PMTs• Water based, doped with Gd (γi≈ 0.2%)

• 1.5 m × 1.3 m• High reflectivity PTFE panels• 494 3’’ R11410-21 PMTs• 8.5 t of liquid xenon of which 5.9 t instrumented• 5 electrodes• Two sets of field shaping rings

JCAP11(2020)031

02/12

XENONnT experiment

JCAP11(2020)031

• 10 m × 10 m• 84 8’’ PMTs• 700 t water

• High reflectivity PTFE panels• 120 8’’ PMTs• Water based, doped with Gd (γi≈ 0.2%)

• 1.5 m × 1.3 m• High reflectivity PTFE panels• 494 3’’ R11410-21 PMTs• 8.5 t of liquid xenon of which 5.9 t instrumented• 5 electrodes• Two sets of field shaping rings

03/12

XENONnT TPC photosensor

HV

signal ground(1) 0.024 W at -1500 V with total resistance of 92.5 MΩ

3’’ R11410-21 photomultiplier tubes

Low radioactivity components

Reliable at liquid xenon temperature

High performances at xenon scintillation light(𝑄𝐸|175 𝑛𝑚 = 34%)

Linearity conserved up to 1 MeV

Low power dissipation into the detector(1)

Behrens A. (PhD thesis, UZH)Mayani D. (PhD thesis, UZH)

04/12

(1) Evaluation and monitor of main features (e.g. single PE amplification, after-pulses, dark count)

PMT testing phaseOf the 494 XENONnT PMTs, 341 were new

Tests carried out among Max Planck Institute for NuclearPhysics in Heidelberg, Stockholm University and theUniversity of Zurich

2 weeks testing in liquid and gas xenon environment(1)

Problematic PMTs have been rejected and testing resultsused for PMT allocation in the TPC

JINST 16 P08033

Illustration of PMT Zurich facility (left) and the Stockholm PMTs holding structure (right)

See Luisa Hoetzsch’s talk:“Improved quality tests of R11410-21photomultiplier tubes”Sept 17th, 07:15 am PDT

05/12

Cleaning and assembly

(1) Maximum concentration of particulates less or equal to 100,000 per m3

ISO 14644-1 cleanroom standard

Cleaning and assembly carried out in ISO-5(1)

cleanroom

Different cleaning procedure according for eachmaterial

PMTs placed following a detailed cabling plan

Connectivity between the relevant point on a base(anode, cathode, ground) verified and PMT testperformed in light-tight box

Arrays stored in a clean environment underflowing nitrogen atmosphere to prevent externalcontamination, waiting for TPC assembly

05/12

Cleaning and assembly

(1) Maximum concentration of particulates less or equal to 100,000 per m3

ISO 14644-1 cleanroom standard

Cleaning and assembly carried out in ISO-5(1)

cleanroom

Different cleaning procedure according for eachmaterial

PMTs placed following a detailed cabling plan

Connectivity between the relevant point on a base(anode, cathode, ground) verified and PMT testperformed in light-tight box

Arrays stored in a clean environment underflowing nitrogen atmosphere to prevent externalcontamination, waiting for TPC assembly

05/12

Cleaning and assembly

(1) Maximum concentration of particulates less or equal to 100,000 per m3

ISO 14644-1 cleanroom standard

Top PMT array Bottom PMT array

Cleaning and assembly carried out in ISO-5(1)

cleanroom

Different cleaning procedure according for eachmaterial

PMTs placed following a detailed cabling plan

Connectivity between the relevant point on a base(anode, cathode, ground) verified and PMT testperformed in light-tight box

Arrays stored in a clean environment underflowing nitrogen atmosphere to prevent externalcontamination, waiting for TPC assembly

05/12

Cleaning and assembly

(1) Maximum concentration of particulates less or equal to 100,000 per m3

ISO 14644-1 cleanroom standard

Top PMT array Bottom PMT array

Cleaning and assembly carried out in ISO-5(1)

cleanroom

Different cleaning procedure according for eachmaterial

PMTs placed following a detailed cabling plan

Connectivity between the relevant point on a base(anode, cathode, ground) verified and PMT testperformed in light-tight box

Arrays stored in a clean environment underflowing nitrogen atmosphere to prevent externalcontamination, waiting for TPC assembly

05/12

Cleaning and assembly

(1) Maximum concentration of particulates less or equal to 100,000 per m3

ISO 14644-1 cleanroom standard

Top PMT array Bottom PMT array

Cleaning and assembly carried out in ISO-5(1)

cleanroom

Different cleaning procedure according for eachmaterial

PMTs placed following a detailed cabling plan

Connectivity between the relevant point on a base(anode, cathode, ground) verified and PMT testperformed in light-tight box

Arrays stored in a clean environment underflowing nitrogen atmosphere to prevent externalcontamination, waiting for TPC assembly

05/12

Cleaning and assembly

(1) Maximum concentration of particulates less or equal to 100,000 per m3

ISO 14644-1 cleanroom standard

Top PMT array Bottom PMT array

Cleaning and assembly carried out in ISO-5(1)

cleanroom

Different cleaning procedure according for eachmaterial

PMTs placed following a detailed cabling plan

Connectivity between the relevant point on a base(anode, cathode, ground) verified and PMT testperformed in light-tight box

Arrays stored in a clean environment underflowing nitrogen atmosphere to prevent externalcontamination, waiting for TPC assembly

05/12

Cleaning and assembly

(1) Maximum concentration of particulates less or equal to 100,000 per m3

ISO 14644-1 cleanroom standard

Top PMT array Bottom PMT array

Cleaning and assembly carried out in ISO-5(1)

cleanroom

Different cleaning procedure according for eachmaterial

PMTs placed following a detailed cabling plan

Connectivity between the relevant point on a base(anode, cathode, ground) verified and PMT testperformed in light-tight box

Arrays stored in a clean environment underflowing nitrogen atmosphere to prevent externalcontamination, waiting for TPC assembly

06/12

PMT cabling planSignal read out through coaxial cables. Highvoltage and grounding provided by kaptoninsulated wires

Two cryogenic pipes host the cables. Vacuum-tight breakout chamber allows connection fromxenon side to air side

CAEN A7030LN/A1536LN boards provides highvoltage supply

Coaxial cables are routed directly to the custom-made dual channel amplifiers(1) and then toanalog-to-digitizer converted CAEN v1724

Picture from Brown A. (PhD thesis, UZH)

Wulf J. (PhD thesis, UZH)

(1) Amplification ×10 and ×0.5

07/12

PMT calibration system

(1) STReam Analysis for Xenon TPCs

Blue LEDs (λ ~ 460nm) coupled withcryogenic optical fibers for PMT calibration

Data acquisition externally triggered.Acquisition windows and trigger frequencyadjustable for different analysis

Weekly calibration and monitoring of mainfeatures, e.g. single PE amplification orafter pulses rate

Raw data processing based onSTRAX(1)/STRAXEN software

Picture adapted from Capelli C. (PhD thesis, UZH)

08/12

Noise conditionDedicated noise hunting campaign before starting of data taking, good noise level achieved

Weekly PMT noise calibration: monitor the time stability

09/12

Single PE amplification

(1) Hard cut-off at 1500 V

Weekly PMT calibration. Single photoelectron amplification computed using model independent method

Time-dependent gain model for performance monitoring and offline data processing input

Intense PMT performance optimization campaign during commissioning phase. Individual PMT high voltage(1)

chosen to maximize the performances

Nucl.Instrum.Meth.A 863 (2017) 35-46

10/12

Single PE acceptance and resolutionDedicated low illumination runs for single PE featuresanalysis:

• Single PE acceptance – defined as the fraction ofsingle photoelectron spectrum above a given ADCthreshold

• Single PE resolution – measure the ability todistinguish noise and signals

Fundamental for determining the self-trigger thresholdfor each PMT as well as the high voltage setting

Monitor of single PE acceptance over the time

SPE shape extracted from these runs. Input forsimulator framework

11/12

Afterpulse rateHigh illumination runs for afterpulse(1) analysis

Monitor of residual gases concentration: special interest ontime evolution of Xe+ and Xe++ rate

Individual afterpulse model per PMT, fundamental input forsimulation and analysis

(1) Among the possible origin of afterpulse, we are interested on those produced by ionization of gas contamination inf the PMT body

12/12

ConclusionThe XENONnT PMTs have been working stably since the beginning of the commissioning (for over one year now).

The high voltage setting have been chosen such optimize single PE amplification as well as the single PE acceptance andminimize after pulses rate. High voltage threshold at 1500 V to minimize performance degradation over time.

The uniform single PE amplification (~ 2 million) and single PE acceptance, overall above 90%, are showing good data quality.

OutlookImportant PMT features to characterize, e.g. double PE emission probability and dark count rate.

Keep monitoring PMT performances to optimize the data quality during the science run.

Thank you for the attention!