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

Studies on liquid argon S1 and S2 properties for low mass WIMP search experiments Masashi Tanaka , Waseda University TAUP2019, Toyama International Conference Center Sep. 12 th , 2019

Contents Poster #398 by K. Aoyama • Introduction Increasing light collection efficiency of liquid argon detector – Low Mass WIMP (1-10 GeV/c 2 ) for low mass WIMP search – Double phase argon detector – Waseda Liquid Argon Test-stand Poster #364 by M. Kimura Measurement of the scintillation efficiency • Recent Results for low energy nuclear- and electronic-recoils in liquid argon detector for WIMP search 1. Improving S1 light collection efficiency 2. Scintillation and ionization efficiency measurement (nuclear recoil) 3. S2 electroluminescence mechanism • Summary Poster #429 by T. Takeda Study of luminescence mechanism by neutral bremsstrahlung in gaseous argon.

Argon Target for Low Mass WIMP Search • Currently, argon is a top runner for SI WIMP (2 - 5 GeV/c 2 ) 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/c 2 – Scintillation signal: 128 nm : ~ 10 eV Good understanding of the low energy recoil for argon detector is very important M. Schumann J.Phys. G46 (2019) no.10, 103003

Double Phase Argon Detector 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

Waseda Liquid Argon Group (ANKOK) • 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 Test-stand • 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 39 Ar signal

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

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

TPB vacuum evaporation • LAr scintillation light: 128 nm VUV – Wavelength shifter TPB : 420 nm – Vacuum evaporation • Evaporation optimization (PMT window) – Adequate conversion efficiency ( 241 Am GAr) – Higher transmittance (LED 420 nm) Blue: Conversion efficiency – Optimal amount of TPB: ~ 30 μg /cm 2 128 nm → 420 nm Red: 420 nm transmittance

S1 light yield measurement • 0.25 kg single phase detector MPPC – 2 R11065 PMTs (Q.E. 30%) – NULL electric field • 11.5 pes/keVee @ 137 Cs 662 keV – Almost reaches: 40 photons/keVee x Q.E. 30% PMT • Energy dependence measurement with various γ sources – 2.8keV – 1275 keV – 25% lower yield @ 37 Ar 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)

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).

Experimental setup • 0.5 kg double phase TPC – φ6.4 cm × h10 cm – 2 PMTs (R11065). • Drift field = 0.0 - 3.0 kV/cm • 252 Cf source + Time of Flight (TOF) . – NaI(Tl) for start signal – Event by event determination of neutron energy S1 50 ns: En ~ 2 MeV S2 65 ns: En ~ 1 MeV 100 ns: En ~ 500 keV Backscattering edge ~ 200 keV

Detector response model • 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 E 0 (keV) • Drift field F (kV/cm) – 5 free parameters

Fit Result • 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/cm NEW • Full function form (with systematic uncertainty ) – http://www2.kylab.sci.waseda.ac.jp/ankok/LeffMaterials/

3. S2 electroluminescence mechanism Poster #429 by T. Takeda Study of luminescence mechanism by neutral bremsstrahlung in gaseous argon.

Neutral Bremsstrahlung(NBrS) • 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 A. Buzulutskov, et.al., Astropart.Phys. 103 (2018) 29-40 – 1 kV/cm = 1.2 Td at 87K 1 bar = 4.1 Td at 300K 1 bar S2 Light yield [A.U.] VUV NBrS NIR UV 300 700 100 500 Wavelength [nm]

Experimental Setup (gas Ar, 1 bar, room temperature) H-R11065: • VUV PMT(Hamamatsu-R6835 ) VL PMT MgF2 window, ϕ1 − 1/8inch QE : 100~190nm → S1, S2(VUV) Used for S1 signal tagging Quartz window Anode • VL PMT(Hamamatsu-R11065 ) 10mm Quartz window, ϕ3inch Grid Drift E-Field QE : 200~750nm → S1(UV), S2(UV-VL) 50mm 100V/cm S2 spectrum measurement w/WL Filter 241 Am 120mm Cathode VL PMT H-R6835 : VUV PMT VUV PMT 6 cut-off filters for emission spectra measurement GAr in 380, 400, 460, 500, 540, 600 nm λ [nm]

Event Display(w/o cut-off filter) VL PMT VL PMT No filter Anode Luminescence E-Field S2 1.125kV/cm S1 Offset S2 e − Drift E-Field S1 100V/cm VUV PMT 241 Am 𝛽 ray Cathode S1 S2 VUV PMT

Event Display(with 600 nm cut-off filter) Cut-off filter VL PMT( 600~700nm ) VL PMT (600nm) Anode Luminescence E-Field S2 1.125kV/cm Offset S2 e − Drift E-Field S1 100V/cm VUV PMT 241 Am 𝛽 ray Cathode S1 S2 VUV PMT

A. Buzulutskov, et.al., E-Field dependence measurement Astropart.Phys. 103 (2018) 29-40  Luminescence E-Field(offset-anode) 0.415~2.03kV/cm(modify anode voltage) Consistent with existence of NBrS VUV region ∶ 100 − 190nm UV region ∶ 200 − 400nm VL region ∶ 600 − 700nm saturate linear Scintillation + NBrS NBrS dominant Scintillation dominant

A. Buzulutskov, et.al., Astropart.Phys. 103 (2018) 29-40 Emission spectra measurement • 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 Quartz window Anode – 12% (uniform isotropic) ~ 50% • Data and the NBrS model are in very good agreement Grid – Results will be published in the paper

Summary • Argon detector is a good device for low mass WIMP search • Waseda liquid argon group is performing R&D for basic properties of the argon detector – Improve light detection efficiency • 11.5 pes/keVee for 137 Cs 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

LIDINE2019 Summary 2019/9/12 26 26/14 26  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 Top Channel Ratio : 𝑀𝑍 𝑈𝑝𝑞_𝑑𝑓𝑜𝑢𝑓𝑠 /𝑀𝑍 𝑈𝑝𝑞 ➢ Measurement of the S2 spectrum with spectrometer. Uniform centralized • Measurement of photon emission direction ➢ NBrS light would have some directivity. Low E-Field (NBrS dominant) ➢ Measurement of resolution of S2 light direction. S2 High E-Field (VUV dominant) • We have begun preparing to write paper!