Scintillating GaAs for the Detection of Electron Recoils from Dark - - PowerPoint PPT Presentation

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Scintillating GaAs for the Detection of Electron Recoils from Dark - - PowerPoint PPT Presentation

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Scintillating GaAs for the Detection of Electron Recoils from Dark Matter in the MeV Mass Range Stephen E. Derenzo Lawrence Berkeley National Laboratory New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019 1

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Stephen E. Derenzo Lawrence Berkeley National Laboratory Scintillating GaAs for the Detection of Electron Recoils from Dark Matter in the MeV Mass Range

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Outline

  • DM direct search mass reach
  • Electron recoil spectrum (GaAs vs. CsI)
  • GaAs scintillation mechanism and threshold
  • X-ray emission spectrum
  • X-ray emission decay time
  • X-ray excited afterglow
  • Property summary table
  • Commercially grown 10 cm crystal
  • Si/TES readout
  • Future plans

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Experimental Mass Reach

(calculations by Essig et al.)

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From “Direct detection of sub-GeV dark matter with scintillating targets” Derenzo, Essig, Massari, Soto, and Tien-Tien Yu, PHYSICAL REVIEW D 96, 016026 (2017)

Lowest three are semiconductors Low band gap => low electron recoil threshold

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Electron Recoil Energy Spectra for GaAs and CsI

(calculations by Essig et al.)

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GaAs

5.32 g/cm3

CsI

4.51 g/cm3

Threshold 1.5 eV Threshold 6.4 eV

GaAs rate per kg year about 10x CsI rate Signal is one or a few photons Very low afterglow essential

From “Direct detection of sub-GeV dark matter with scintillating targets” Derenzo, Essig, Massari, Soto, and Tien-Tien Yu, PHYSICAL REVIEW D 96, 016026 (2017)

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

GaAs Optical Excitation/Emission Spectra at 10K

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0.0 0.2 0.4 0.6 0.8 1.0 800 850 900 950 Intensity Wavelength (nm) 1.52 eV 1.44 eV 1.33 eV

  • Shallow silicon donors

=> CB e– (even near 0 K)

  • Boron acceptors are

hole traps and radiative centers

  • Stokes shift 0.11 eV
  • Low self-absorption

Conduction band Valence band Si B– B0 1.52 1.44 1.33

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

GaAs X-ray Excited Emission Spectrum at 10K

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0.0 0.2 0.4 0.6 0.8 1.0 800 820 840 860 880 900 920 940 960 Emission Intensity Wavelength (nm) GaAs(Si) n-type crystal 40 keVp X-ray excitaton Eg = 1.52 eV 1.33 eV 1.46 eV

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Pulsed X-ray 850 nm Emission

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10/16/2018 https://htcf.lbl.gov/htcf/pxray/pxray_data_plot.php?fileID=47986&xMin=-1&xMax=20 https://htcf.lbl.gov/htcf/pxray/pxray_data_plot.php?fileID=47986&xMin=-1&xMax=20 1/1

HTCF Pulsed X-ray Raw Data Histogram

Specify X-axis boundaries: Minimum: Maximum: 1.2 ns (27%) 11 ns (57%)

Prompt escape cone + scatter?

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Afterglow: Thermally Stimulated Luminescence

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1) 50 keVp X-ray bombardment 10K for 30 minutes Saturate metastable radiative states 2) Record stimulated emission T => 450 C

Any metastable radiative states in n-type GaAs annihilated by conduction band electrons => no afterglow

16X expansion

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Properties of Scintillating GaAs for Direct Dark Matter Detection

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Property Value Comment Band gap 1.52 eV (cryogenic) Threshold for recoil electron detection Scintillation mechanism Donor band to acceptor emission Silicon (0.002 eV donor) Boron (0.2 eV acceptor) Luminosity > 100 photons/keV [1,2] Theoretical limit 300 photons/keV Operating temperature < 100K [1] Silicon donor free electrons above 8x1015/cm3 do not freeze out [1] Afterglow None detected [1] n-type e– annihilate metastable radiative states Stokes shift 0.11 eV [1] Overlap between emission and absorption bands is small Anti-reflective coatings Apparently not essential, even with polished faces, despite 3.5 refractive index [1,2] Apparent internal scattering from n-type free electrons Narrow-beam absorption length 20 cm at 1016 carriers/cm3 [3] Ratio between loss and scattering not known Typical size 5 kg Large-scale production for electronic circuits

[1] Derenzo et al. arXiv1802.09171, 2018 Cryogenic Scintillation Properties of n-Type GaAs for the Direct Detection of MeV/c2 Dark Matter [2] Vasiukov et al. arXiv 1904.09362 2019 GaAs as a bright cryogenic scintillator for the direct dark matter detection [3] Spitzer et al. Phys Rev 114 59-63, 1959 Infrared Absorption and Electron Effective Mass in n-Type Gallium Arsenide

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Large Crystals of High Purity Commercially Grown

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10 cm GaAs crystal (~5 kg) grown at the Leibniz-Institut für Kristallzüchtung (IKZ), Berlin, Germany

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Si/TES Readout

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  • e– recoils produce

scintillation photons

  • Si converts the photons

into athermal phonons

  • Phonon detection

using Al/TES CDMS technology

GaAs(Si,B) target TES Al Al Al Al Al Al Al TES TES TES TES TES Photon DM DM Recoil e– Si absorber Photon

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Future Plans

12 Thanks to

  • Rouven Essig (Stony Brook) asking me to find a low band gap scintillator for low-energy

electron recoils

  • Edith Bourret (LBNL) for providing the first successful GaAs samples
  • Matt Pyle (UC Berkeley) for raising the issues of afterglow and optical absorption
  • Maurice Garcia-Sciveres, Dan Mckinsey (LBNL) for support and encouragement

Near term: 1) Test 1 cm3 cubes with cryogenic InGaAs PMT Optimize silicon and boron doping 2) Test 1 cm3 cubes with Si/Al/TES readout Scale-up: Arrays of 8 cm (2 kg) cylinders

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Thank You for Your Attention

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

X-ray Excited Luminosity vs. Temperature

50 keVp X-ray excitation

  • Holes on B stable up to 120K
  • Above 120K => 12 meV barrier

for trapping by deeper traps (e.g. EL2)

  • Luminosity calibrated against

LSO and BGO crystals

  • >40,000 ph/MeV

(underestimate)

  • Miss >970 nm
  • No anti-reflection coating
  • Doping not optimized

100 1,000 10,000 40 80 120 160 200 Luminosity (ph/MeV) T(K) 30,000 3,000 300 850 nm peak kT = 4.3 meV 930 nm peak kT = 12 meV 10

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Donor and Acceptor Ionization Energies in Si, Ge, GaAs

Ge Ge Si Ga GaAs

B??

Benzaquen value 0.0023 eV

Possibly world record

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Mott Metal-Insulator Transition in n-type GaAs(Si)

1 2 3 4 5 6 0.0 5.0 x 1015 1.0 x 1016 1.5 x 1016 2.0 x 1016 Conductivity at 0K (Ω

  • 1 cm
  • 1)

Free carriers cm

  • 3

Data from Benzaquen et al. “Conductivity of n-type GaAs near the Mott transition“ Phys Rev B 36 (1987) Above 8x1015/cm3 (0.2 ppm) carriers do not freeze out at 0 K Coulomb repulsion => Conduction band 16

at 300K

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Scattering by n-Type Electrons

Photon scattering is proportional to the product of n (carriers/cm3) and path length Z (cm) At n = 2x1016/cm3 narrow-beam attenuation length is 8 cm Sa Same as ref

0.20 0.30 0.40 2x1016 4x1016 6x1016 8x1016 1x1017 1.2x1017

Transmission Model

Transmission nZ = carrers/cm

2

0.40 exp(-nZ/1.67x10

17)

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

GaAs Si B Concentration vs. Luminosity

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Sample No. Ne (cm–3) Si (cm–3) B (cm–3) C (cm–3) Lum (ph/keV) 13323* <0.01 13357** 3.83 x 1010 2.4 50075 3.0 x 1017 3.0 x 1017 1.5 x 1018 1.2 x 1016 19 13360 5.3 x 1016 2.0 x 1017 8.6 x 1018 3.7 x 1016 35 13365 2.2 x 10 17 3.8 x 10 17 2.5 x 1018 3.6 x 1015 49 13363 4.2 x 1017 7.0 x 1017 3.5 x 1018 6.0 x 1015 57 13359 1.9 x 1017 2 x 1017 8.0 x 1018 1.0 x 1017 150*** * Semi-insulating (excess As on Ga anti-sites) ** Intrinsic *** Scattering on n-type electrons reduces internal trapping??

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New Directions in the Search for Light Dark Matter Particles Fermilab, June 4-7, 2019

Pulsed X-ray Facility

(LBNL Scintillator Research Group, Bldg 55)

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Sample X-ray Tube Stop Start

+ 30 kV

Ti-sapphire laser Nd:YAG Pump laser Doubler crystal Fluorescent Emissions Microchannel photomultiplier tube Time to Digital Converter Photodiode Data acquisition computer Discriminator Discriminator Overall timing resolution 100 ps fwhm 200-850 nm