Ultra-low Energy Calibration of LUX 127 Detector using Xe Electron Capture Dongqing Huang Brown University On behalf of the LUX collaboration APS April Meeting 2015 1 Time S1 Drift indicates the depth S2 LUX Experiment by Carlos Faham and Samuel Chan 2 S2 S2 LUX Experiment Time S2 S1 Particle by Carlos Faham and Samuel Chan 3 127 Xe decay scheme Electron capture: p + e- → n + νe cascade χ-rays or Auger 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% 127 127 Xe I γ-ray νe - Cosmogenic activation - Half life: 36days - Based on the measurement of Apr222013 data, there are ~0.9million 127Xe atoms in LUX Xenon volume 52.7% 203keV 17.2% 375keV 4 127 Xe EC Decay Event in LUX Data S2 TOP PMTs S2 e- S1 S1 S2 S2 x-ray(EC site) eγ-ray γ-ray 203keV K-shell χ-ray 33.2keV BOTTOM PMTs LUXEventViewerGUI by James Verbus 5 33.2 keV 5.2 keV 1.1 keV 190eV Xe EC Events in data Single Electron 127 190eV 1.1keV 203keV 33.2keV µ = 856±2 LUX Preliminary 375keV 5.2keV µ = 183±1 33.2 keV µ = 74.5±0.9 203keV LUX Preliminary 375keV 5.2 keV Note: (1electron = 10.6phe) 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 6 Ratio Analysis K 33.2keV L 5.2keV LUX Preliminary #events used for fitting M 1.1keV N 190eV 2515 375 96 16 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 ) Amplitude from fit to decay time histogram (y intercept) 7 Searching for N-shell χ-rays Energy Reconstruction of Single-Vertex Events TOP PMTs LUX Preliminary 203keV BG SE 2σ N-shell χ-ray(SE) 2σ SE γ-ray BOTTOM PMTs 8 N shell Fit ● ● 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 LUX Preliminary µ = 15.9(1.2+/-1.3) σ = 7.7(+1.0/-0.8) N = 106 Total number of observed M-shell χray 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 (1electron = 10.6phe) 9 NESTv98* Energy (keVee) (EC x-ray) 127I NEST S2c (phe) [1][2] NEST QY (e/keVee) & Data 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) LUX Preliminary 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: 10.1088/17480221/8/10/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] NEST with simulation of detector effects such as the binomial light collection and extraction efficiency 10 Qy Comparison with Tritium[3] and NEST[1] ● ● ● ● The lowest energy point of 127 Xe 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 127 Xe points are not used for run3 re-analysis LUX Preliminary [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. 11 Acknowledgment Special thanks to My advisor Professor Richard Gaitskell Special thanks to Alastair Currie(Imperial College London) Thanks to James Verbus, David Malling, Jeremy Chapman, Simon Fiorucci 12 Appendix slides: 13 127 Xe EC Decay There 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 14 127 Xe EC Event Schematics ● 127 ● In the left two schematics, events appear as double-vertice Xe EC events appear in three scenarios in LUX detector as shown in figure on right. electron drift direction e- e- ● In the right schematic, χ-ray pulse and γ-ray pulse merge into one and events appear as single-vertex γ-ray x-ray(EC site) e- eγ-ray x-ray(EC site) ex-ray(EC site) ● 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 eγ-ray 15 Gamma S2c Histogram ● ● The first peak is the γ-203keV S2c histogram The second peak is the γ-375keV S2c histogram LUX Preliminary 16 Z-Separation ● ● Figure on the right shows the scatter plot of z-separation(us) btw χ-ray and γ-ray of each event. 0.19keV 0.19keV 1.1keV 1.1keV 5.2keV 5.2keV 33.2keV 33.2keV LUX Preliminary 1μs = 0.15cm in z direction 17 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 γ203keV. ● 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 ● ● LUX Preliminary 18 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. electron drift direction N-shell x-ray(EC site) SE ● 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 γ-ray SE γ-ray ● 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 x-ray(EC site) SE N-shell χ-ray(SE) SE γ-ray ● γ-ray signal helps determine what single-vertex events are needed to be looked into to find Nshell χ-ray signal 19 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 LUX Preliminary 20 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 ○ ○ ● The third peak is the 127Xe EC events with γ-203keV pulse merged with K-shell χ-ray pulse ○ ○ ● ● data: mean = 215.4keV; sigma = 8.2keV expected: 203 + 5.2 = 208.2keV predicted by theory 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): ● The energy cut for background region: ○ ○ [195.8 228.6] keVee [240 260] keVee 21
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