Ultra-low Energy Calibration of LUX Detector using 127Xe Electron Capture

Dongqing HuangBrown UniversityOn behalf of the LUX collaborationAPS April Meeting 2015

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LUX Experiment

by Carlos Faham and Samuel Chan

S1

S2

Time

Drif

t ind

icat

es th

e de

pth

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LUX Experiment

by Carlos Faham and Samuel Chan

Time

S1

S2

S2

Particle

S2

127Xe decay scheme

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Electron capture: p + e- → n + νe

127Xe 127I

νe

cascade χ-rays or Auger

γ-ray52.7% 203keV17.2% 375keV

The binding energy and expected EC probability from that shell - K: 33.2 keV 83.4% - L: 5.2 keV 13.1% - M: 1.1 keV 2.9% - N: 190 eV 0.6%

- Cosmogenic activation- Half life: 36days- Based on the measurement of Apr222013 data, there are ~0.9million 127Xe atoms in LUX Xenon volume

127Xe EC Decay Event in LUX Data

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LUXEventViewerGUI by James Verbus

S1S2

S2

x-ray(EC site)

γ-ray

e-

e-

S2

S2

S1

K-shell χ-ray 33.2keV

γ-ray 203keV

TOP PMTs

BOTTOM PMTs

127Xe EC Events in data

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33.2 keV

5.2 keV

203k

eV

5.2

keV

1.1

keV

190e

V

Sin

gle

Ele

ctro

n

375keV

203keV

375k

eV

33.2

keV

LUXPreliminary Note:

Due to S2 threshold effect, N shell χ-ray events are not completed found for histogram; Detail analysis for N shell shown in later slides

µ = 74.5±0.9

µ = 183±1

µ = 856±2LUXPreliminary

190eV 1.1keV 5.2keV 33.2keV

(1electron = 10.6phe)

Ratio Analysis

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LUX Preliminary

K 33.2keV L 5.2keV M 1.1keV N 190eV

#events used for fitting 2515 375 96 16

Amplitude from fit to decay time histogram (y intercept) 7517±150 1208±62 245±25 45±11

Expected percentage(%) 83.4 13.1 2.9 0.6

Observed percentage(%) 83.4±2.3(Result + 0σ)

13.4±0.7(Result + 0.4σ)

2.7±0.3(Result - 0.6σ )

0.5±0.1(Result - 1σ )

Searching for N-shell χ-rays

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N-shell χ-ray(SE)

γ-ray

SE

SE

LUXPreliminary 203keV

2σ 2σ

BG

TOP PMTs

BOTTOM PMTs

Energy Reconstruction of Single-Vertex Events

N shell Fit

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● Fiducial r → 22cm● S2 threshold: 250phe

● All SE-type pulses between S1 and S2 in drift are histogrammed

○ In this case, more background will be presented; but no real signal is missed

● Total number of observed M-shell χ-ray events with fiducial r 22cm and S2 threshold 250phe

○ 501±22● Total expected number of N-shell

events based on the known ratio between M and N shell (see slide7):

○ 115±11● The best-fit N is within 1 sigma of

what is expected

LUXPreliminary

(1electron = 10.6phe)

µ = 15.9(1.2+/-1.3)σ = 7.7(+1.0/-0.8)N = 106

NESTv98*[1][2] & Data

10* NEST with simulation of detector effects such as the binomial light collection and extraction efficiency

LUXPreliminary

Energy (keVee)

(EC x-ray)127I

NEST S2c(phe)

NEST QY(e/keVee)

NEST QY width(e/keVee)

127Xe Data Fit S2c

(mean phe)

127Xe Data QY(e/keVee at 180

V/cm)

127Xe Data QY width

(e/keVee)

33.2 (K shell) 8121 23.1 3.42 9093±21 25.8±0.9 4.06±0.15

5.2 (L shell) 1830 33.2 5.18 1943±13 35.2±1.2 7.2±0.3

1.1 (M shell) 633 54.5 12.26 802±10 68.6±2.5 12.5±0.7

0.19 (N shell) 131 65.4 32.49

169(+11/-13) 83.7(+6.2/-7.1) 40.6(+5.6/-4.7)

[1] Matthew Szydagis, Adalyn Fyhrie, Daniel Thorngren, and Mani Tripathi. Enhancement of NEST Capabilities for Simulating Low-Energy Recoils in Liquid Xenon. JINST, 8:C10003, 2013. doi:10.1088/1748-0221/8/10/C10003.

[2] Brian Lenardo, Kareem Kazkaz, Aaron Manalaysay, Matthew Szydagis, Mani Tripathi. A Global Analysis of Light and Charge Yields in Liquid Xenon. arXiv:1412.4417 [astro-ph.IM]

Qy Comparison with Tritium[3] and NEST[1]

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LUXPreliminary

[3] Attila Dobi. Measurement of ER Fluctuations in Liquid Xenon with the LUX Detector Using a Tritium Calibration Source. APS April Meeting 2015, Volume 60 Number 4.

● The lowest energy point of 127Xe is higher than what can be reached with w value 13.7eV

● It indicates that the w value is not necessary to be a constant

● It implies w value ~12.1eV with only ion-electron pairs generated at 190eV

● 127Xe points are not used for run3 re-analysis

Acknowledgment Special thanks to My advisor Professor Richard GaitskellSpecial thanks to Alastair Currie(Imperial College London)

Thanks to James Verbus, David Malling, Jeremy Chapman, Simon Fiorucci

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Appendix slides:

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127Xe EC DecayThere are two components in Electron Capture decay: - γ-ray or I.C. Electron associated with 127I excited state - χ-ray cascade (in daughter atom) from atomic electron vacancy in the K,L,M or even N shell.

- Note that Auger Electron processes are subdominant in high-Z atoms.

If a γ-ray is emitted, the mean free path in Xe, will lead to a reasonable probability that a second vertex separated from the first created.

- 203 keVee → ~1.0cm

- 375 keVee → ~2.2cm

- Note that events where γ-ray escapes the active region completely become low

energy ER background events.

Binding energies of atomic electrons in 127I and percentages: - K: 33.2 keVee 83.4% - L: 5.2 keVee 13.1% - M: 1.1 keVee 2.9% - N: 0.19 keVee 0.6% - Note that percentages are calculated by including neutrino-phase space

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127Xe EC Event Schematics● 127Xe EC events appear in three

scenarios in LUX detector as shown in figure on right.

● In the left two schematics, events appear as double-vertice

● In the right schematic, χ-ray pulse and γ-ray pulse merge into one and events appear as single-vertex

● Events in the bottom left show as a big pulse following a small pulse in data, which are used for the major part of the analysis

x-ray(EC site)

x-ray(EC site)

x-ray(EC site)

γ-ray

γ-ray

γ-ray

e-

e-

e- e-

e-

e- electron drift direction

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Gamma S2c Histogram● The first peak is the γ-203keV S2c

histogram

● The second peak is the γ-375keV S2c histogram

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LUXPreliminary

Z-Separation 0.19keV 1.1keV 5.2keV 33.2keV

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0.19keV 1.1keV 5.2keV 33.2keV● Figure on the right shows the

scatter plot of z-separation(us) btw χ-ray and γ-ray of each event.

● 1μs = 0.15cm in z direction

LUXPreliminary

Z-Separation Histogram● Events with γ-203keV S2 size less than the

mean value are used for histogram to mitigate γ-375keV impact on its mean free path(see slide16)

● The histograms show a semi-exponential trend due to photon attenuation of γ-203keV.

● Data are fitted from the bins which are fully efficient.

● The mean free path of the best fit:○ 1.08±0.06cm

● The theory predicted mfp:○ 0.928cm

● The discrepancy(16%) is due to uncertainty in electron drift velocity and liquid xenon density

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LUXPreliminary

N-shell Event Schematics● When the size of N-shell χ-ray pulse is too small

(less than S2 threshold), it is then classified as SE-type pulse; and thus 127Xe EC event that happens with N-shell χ-ray appears as single vertex event.

● Bottom-left schematic shows an N-shell event with real SEs and χ-ray pulse; and χ-ray pulse is classified as SE-type pulse due to its small size

● Events shown on the right schematic become background events because the χ-ray pulse merges into γ-ray pulse; the combined pulse has size close to the size of γ itself since the size of γ pulse is significantly greater than that of χ-ray

● γ-ray signal helps determine what single-vertex events are needed to be looked into to find N-shell χ-ray signal

N-shell x-ray(EC site)

γ-ray

x-ray(EC site)

γ-ray

SE

SE

electron drift direction

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N-shell χ-ray(SE)

γ-ray

SE

SE

Energy Histogram of Single-Vertex events● S2 threshold: 250 phe

● Energy reconstruction model:● g1 = 0.120±0.002 (phe/ph)● EE = 0.431±0.015● SE = 24.66±0.02 (phe)● g2 = EE*SE

● E = 0.0137 *(S1/g1 + S2/g2)● Esys = ~1.6% at the energy

range shown in the plot

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LUXPreliminary

Energy Histogram of Single-Vertex events● The second peak is the 127Xe EC events with γ-203keV pulse merged with L-shell χ-ray pulse

○ data: mean = 215.4keV; sigma = 8.2keV○ expected: 203 + 5.2 = 208.2keV predicted by theory

● The third peak is the 127Xe EC events with γ-203keV pulse merged with K-shell χ-ray pulse○ data: mean = 244.2keV; sigma = 8.0keV○ expected: 203+33.2 = 236.2keV predicted by theory

● The peak means are systematically off from expected values by ~3% which is a factor of 2 greater than the systematics caused by uncertainty in g1 and g2; the extra systematics is due to the field variation in detector.

● Set up a new energy scale relative to the third peak (with most statistics) ○ γ-203keV peak:

■ mean = 212.2 keV; sigma = 8.2keV● N-shell χ-rays, which have been classified as SE-type pulse, expects to be found in γ-203keV single-vertex

events

● The energy cut for signal region (2 sigma):○ [195.8 228.6] keVee

● The energy cut for background region:○ [240 260] keVee

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