Studies on liquid argon S1 and S2 properties for low mass WIMP - - PowerPoint PPT Presentation

Masashi Tanaka, Waseda University

TAUP2019, Toyama International Conference Center

• Sep. 12th, 2019

Studies on liquid argon S1 and S2 properties for low mass WIMP search experiments

• Introduction

– Low Mass WIMP (1-10 GeV/c2) – Double phase argon detector – Waseda Liquid Argon Test-stand

• Recent Results
• 1. Improving S1 light collection efficiency
• 2. Scintillation and ionization efficiency measurement (nuclear recoil)
• 3. S2 electroluminescence mechanism
• Summary

Contents

Poster #364 by M. Kimura Measurement of the scintillation efficiency for low energy nuclear- and electronic-recoils in liquid argon detector for WIMP search Poster #398 by K. Aoyama Increasing light collection efficiency of liquid argon detector for low mass WIMP search Poster #429 by T. Takeda Study of luminescence mechanism by neutral bremsstrahlung in gaseous argon.

• Currently, argon is a top runner for SI WIMP (2 - 5 GeV/c2) search

– Darkside-50, S2 (ionization signal) only analysis

Phys.Rev.Lett. 121 (2018) no.8, 081307

• Extreme limitation: if 100% of recoil energy is converted to signal

– Ionization signal: Ar work function ~25 eV → WIMP mass ~ 0.3 GeV/c2 – Scintillation signal: 128 nm : ~ 10 eV

Argon Target for Low Mass WIMP Search

• M. Schumann J.Phys. G46 (2019) no.10, 103003

Good understanding of the low energy recoil for argon detector is very important

Basic studies on the argon detector at low energy recoil

• 1. S1 light collection efficiency

– Improve S1 PSD

• 2. Scintillation and ionization efficiency for NR

– calibration: S1, S2 → recoil energy

• 3. S2 electroluminescence mechanism

– New idea for future detector design

Double Phase Argon Detector

• R&D program for low mass WIMP dark matter search since 2012
• Recent publications

– “Measurement of the scintillation efficiency for nuclear recoils in liquid argon under electric fields up to 3 kV/cm”, Phys.Rev. D100 (2019) no.3, 032002 , (2019). – “Scintillation and Ionization Ratio of Liquid Argon for Electronic and Nuclear Recoils at Drift-Fields up to 3 kV/cm”,

• Nucl. Inst. & Meth. in Phys. Res. A, 910, 22-25 (2018)

– “Performance of VUV-sensitive MPPC for Liquid Argon Scintillation Light”,

• Nucl. Inst. & Meth. in Phys. Res. A, 833, 239-244 (2016).

Waseda Liquid Argon Group (ANKOK)

• Waseda university

– Tokyo, Nishi-Waseda campus – On surface

• Setup

– 200L cryostat – Liquefier (200W GM cryocooler) – Liquid and gas purification system – Radiation shield (10 cm Pb+ 1 cm Cu)

• Achievements

– ~1 month of stable operation – ~0.5 mm liquid surface control – Purity: scintillation light

• < 0.1 ppm N2 contamination

– Purity: drift electron

• < 0.1 ppb O2 contamination

– Establish 39Ar signal

Waseda Liquid Argon Test-stand

Ar Detector Hall of Fame

5 kg double phase detector (2017) Gas TPC for S2 study (2019) 0.25 kg single phase ultra high light yield detector (2018) 0.5 kg double phase high drift field (3 kV/cm) detector (2017)

• 1. Light collection efficiency

Poster #398 by K. Aoyama Increasing light collection efficiency of liquid argon detector for low mass WIMP search

• LAr scintillation light: 128 nm VUV

– Wavelength shifter TPB : 420 nm – Vacuum evaporation

• Evaporation optimization (PMT window)

– Adequate conversion efficiency (241Am GAr) – Higher transmittance (LED 420 nm) – Optimal amount of TPB: ~ 30 μg/cm2

TPB vacuum evaporation

Blue: Conversion efficiency 128 nm→420 nm Red: 420 nm transmittance

• 0.25 kg single phase detector

– 2 R11065 PMTs (Q.E. 30%) – NULL electric field

• 11.5 pes/keVee @137Cs 662 keV

– Almost reaches: 40 photons/keVee x Q.E. 30%

• Energy dependence measurement with various γ sources

– 2.8keV – 1275 keV – 25% lower yield @37Ar 2.7 keV – Results will be published in the paper

• Next step: SiPM PDE 65%, 25 pes/keVee can be achieved

– Study ongoing (Hamamatsu MPPC)

S1 light yield measurement

MPPC PMT

• 2. Liquid argon scintillation and ionization yield measurement

Poster #364 by M. Kimura Measurement of the scintillation efficiency for low energy nuclear- and electronic-recoils in liquid argon detector for WIMP search also

“Measurement of the scintillation efficiency for nuclear recoils in liquid argon under electric fields up to 3 kV/cm”, Phys.Rev. D100 (2019) no.3, 032002 , (2019).

• 0.5 kg double phase TPC

– φ6.4 cm × h10 cm – 2 PMTs (R11065).

• Drift field = 0.0 - 3.0 kV/cm
• 252Cf source + Time of Flight (TOF) .

– NaI(Tl) for start signal – Event by event determination of neutron energy

Experimental setup

S2 S1

50 ns: En ~ 2 MeV 65 ns: En ~ 1 MeV 100 ns: En ~ 500 keV Backscattering edge ~ 200 keV

• Energy deposition model

– Geant4-10.1.1 QGSP_BERT_HP – Nuclear data library

• G4NDL4.5 modified by A. Robinson
• PRC 89 032801

– Full detector geometry

• Liquid argon response model

– Mei+TIB model – 2 variances

• recoil energy E0 (keV)
• Drift field F (kV/cm)

– 5 free parameters

Detector response model

• S1 (scintillation) and S2 (ionization) signal yields for

arbitrary nuclear recoil energy and drift field can be calculated

– Range of validity: 30 keV - 200 keV, 0 V/cm - 3 kV/cmNEW

• Full function form (with systematic uncertainty )

– http://www2.kylab.sci.waseda.ac.jp/ankok/LeffMaterials/

Fit Result

• 3. S2 electroluminescence mechanism

Poster #429 by T. Takeda Study of luminescence mechanism by neutral bremsstrahlung in gaseous argon.

• Well known S2 electroluminescence

– gas argon scintillation light – 128 nm(VUV) ~ 300 nm (UV) >700 nm (NIR)

• Additional mechanism: neutral bremsstrahlung (NBrS)

– Slow electrons(~10eV) are scattered on neutral atoms – Recently reported by A. Buzulutskov, et.al.,

• Astropart.Phys. 103 (2018) 29-40

– Continuous emission spectra 200 nm ~ 700 nm – Emission angle is correlated to the drift electron direction

• Measurement of S2 emission spectra at room temperature

– 1 kV/cm = 1.2 Td at 87K 1 bar = 4.1 Td at 300K 1 bar

Neutral Bremsstrahlung(NBrS)

Wavelength [nm]

NBrS

500 100 700 300

VUV UV

S2 Light yield [A.U.]

NIR

• A. Buzulutskov, et.al.,

Astropart.Phys. 103 (2018) 29-40

Experimental Setup (gas Ar, 1 bar, room temperature)

GAr in H-R11065: VL PMT Cathode

Drift E-Field 100V/cm 120mm 10mm

241Am 50mm

H-R6835 : VUV PMT

• VUV PMT(Hamamatsu-R6835)

MgF2 window, ϕ1 − 1/8inch QE : 100~190nm→S1, S2(VUV) Used for S1 signal tagging

• VL PMT(Hamamatsu-R11065)

Quartz window, ϕ3inch QE : 200~750nm →S1(UV), S2(UV-VL) S2 spectrum measurement w/WL Filter VUV PMT VL PMT λ [nm] Anode Grid Quartz window 6 cut-off filters for emission spectra measurement 380, 400, 460, 500, 540, 600 nm

Event Display(w/o cut-off filter)

S1 S2 S1 S2

241Am

𝛽 ray

S1 S2 e−

VL PMT VL PMT VUV PMT No filter

Cathode Offset Anode Luminescence E-Field 1.125kV/cm Drift E-Field 100V/cm

VUV PMT

Event Display(with 600 nm cut-off filter)

𝛽 ray

S1 S2 e−

Cut-off filter (600nm)

241Am

Cathode Offset Anode Drift E-Field 100V/cm

S2 S1 S2

VL PMT(600~700nm) VUV PMT VL PMT VUV PMT

Luminescence E-Field 1.125kV/cm

E-Field dependence measurement

VUV region ∶ 100 − 190nm

 Luminescence E-Field(offset-anode)

0.415~2.03kV/cm(modify anode voltage) Consistent with existence of NBrS VL region ∶ 600 − 700nm UV region ∶ 200 − 400nm

linear saturate

Scintillation dominant Scintillation + NBrS NBrS dominant

• A. Buzulutskov, et.al.,

Astropart.Phys. 103 (2018) 29-40

• Calculate observed emission spectra (red points)

– PMT Q.E. and cut-off filter transmittance are corrected

• Compare with the expectation

– Fig 9. in Astropart.Phys. 103 (2018) 29-40

• PMT acceptance of the NBrS emission

– emission angle distribution is not well known – 12% (uniform isotropic) ~ 50%

• Data and the NBrS model are in very good agreement

– Results will be published in the paper

Emission spectra measurement

Anode Grid Quartz window

• A. Buzulutskov, et.al.,

Astropart.Phys. 103 (2018) 29-40

• Argon detector is a good device for low mass WIMP search
• Waseda liquid argon group is performing R&D for basic properties
• f the argon detector

– Improve light detection efficiency

• 11.5 pes/keVee for 137Cs 662 keV peak (PMT Q.E 30%)
• Plan to replace PMT with SiPM (Hamamatsu MPPC Q.E. 65%)

– Measurement of scintillation and ionization yield

• Phys.Rev. D100 (2019) no.3, 032002 , (2019).

– S2 electroluminescence mechanism

• Data results are consistent with NBrS model

Summary

26 26

Summary

2019/9/12 LIDINE2019

Summary

• NBrS is additional electroluminescence mechanism. It has continuous spectrum from VUV to IR.
• We measure the S2 spectrum from 200nm to 600nm with room temperature gas argon for the first time.
• We observed NBrS like component.

Ongoing effort

• Spectrum measurement

➢ Measurement of the S2 spectrum with spectrometer.

• Measurement of photon emission direction

➢ NBrS light would have some directivity. ➢ Measurement of resolution of S2 light direction.

• We have begun preparing to write paper!

Top Channel Ratio : 𝑀𝑍

𝑈𝑝𝑞_𝑑𝑓𝑜𝑢𝑓𝑠/𝑀𝑍 𝑈𝑝𝑞

S2 26/14 centralized Uniform High E-Field (VUV dominant) Low E-Field (NBrS dominant)