XMASS: a large single-phase liquid-xenon detector

Transcription

1 XMASS: a large single-phase liquid-xenon detector Katsuki Hiraide, the university of Tokyo for the XMASS Collaboration October 3 rd, 2016 IPRD16@Siena, Italy 1

2 XMASS project XMASS: a multi purpose experiment with liquid xenon Aiming for Direct detection of dark matter Observation of low energy solar neutrinos (pp/ 7 Be) Search for neutrino-less double beta decay Features Low energy threshold (~0.5keV) Sensitive to e/g events as well as nuclear recoil Large target mass and its scalability XMASS-1 (total ~1ton) XMASS-1.5 (total ~6tons) XMASS-2 (total ~24tons) 2

3 Experimental site Located 1,000 m underground (2,700 m water equivalent) in Kamioka mine in Japan. Japan KamLAND Kamioka Mine XMASS CANDLES E-GADS Super-Kamiokande NEWAGE CLIO Tokyo 3

4 11m Single-phase liquid Xenon detector: XMASS-I 10m 80cm Liquid xenon detector 832 kg of liquid xenon (-100 o C) inch PMTs (Photocathode coverage >62%) Each PMT signal is recorded by 10-bit 1GS/s waveform digitizers Water Cherenkov detector 10m diameter, 11m high inch PMTs Active shield for cosmic-ray muons Passive shield for n/g 4

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6 History of XMASS-I Dec. May Nov. Construction Commissioning data taking Detector refurbishment Data taking (ongoing) Today 6

7 Inner calibration system Various RI sources can be inserted Used for light yield monitoring, optical parameter tuning, energy and timing calibrations etc. RI Energy [kev] Diameter [mm] Geometry 55 Fe pi source 109 Cd 8, 22, 25, pi source 241 Am 17.8, pi/4pi source 57 Co 59.3 (W X-ray), pi source 137 Cs cylindrical 57 Co source 19mm 0.21mm 4mm Source rod (Ti) Active region is concentrated on the 1.8 mm edge region 7

8 Detector response Total number of photoelectrons data MC 59.3keV (W X-ray) 122keV ~4% rms 136keV Photoelectron yield is monitored by the 57 Co source placed at the detector center. High photoelectron yield ~15 PE/keV The distribution is reproduced by simulation well. 8

9 Self-shielding of gamma-ray background Traces of U-chain gamma-rays from PMT Fiducial volume Owing to high atomic number Z=54, external gamma-ray background can be shielded by liquid xenon itself. By selecting events occurred in the restricted inner volume (fiducial volume), low background can be achieved. 9

10 Vertex reconstruction Event vertex position and energy are reconstructed using PE in each PMT P i (n) : probability that the i-th PMT detects n PE Reconstructed vertex DATA MC 57 Co 122keV 10

11 Comparison of background rate in fiducial volume including both nuclear recoil and e/g events XMASS achieved low background rate of O(10-4 ) event/day/kg/kev in a few 10s kev including e/g events XMASS-I Low background rate for e/g events is good for searching for dark matter other than WIMPs. Original figure taken from D. C. Mailing, Ph.D (2014) Fig

12 Prospects for pulse shape discrimination (PSD) using LXe scintillation LXe scintillation processes Excitation: Singlet ( 1 S u+ ): t~ a few ns Triplet ( 3 S u+ ): t~ 20 ns Recombination: t~30 ns or more Singlet/triplet ratio, recombination time depend on ionization density PSD study with a small setup Electron rejection power for 50% NR efficiency New! Early study of PSD with a small set up Electron rejection power of ~8x10-2 for 50% NR efficiency at kevee Detail measurement of scintillation time profile for low energy e/g has been conducted NIM A659 (2011)

13 Measurement of LXe scintillation time profile for low energy gamma-ray induced events Using 55 Fe, 241 Am, and 57 Co sources (Eg= kev) Waveforms are decomposed into single PE pulses MC simulation takes into account optical parameters (absorption, scattering, ), electronics response Timing distributions of data and MC are compared to obtain intrinsic decay time parameters. 3 PEs 11 PEs 13

14 Measurement of LXe scintillation time profile for low energy gamma-ray induced events Fast decay component is needed to reproduce our calibration data. t 1 =2.2 ns (fixed) F 1 : 0.05~0.15 (increase at low energy) Energy dependence of decay time was studied as a function of mean kinetic energy of electrons induced by g-ray Time profile for neutron under study Nucl. Instrum. Meth. A834 (2016)

15 Diversity of physics target with XMASS Dark matter searches Low mass WIMP search Phys. Lett. B719 (2013) 78 WIMP- 129 Xe inelastic scattering PTEP (2014) 063C01 Solar axion search Phys. Lett. B724 (2013) 46 Possibility of supernova neutrino detection arxiv: XMASS XMASS (90%CL) DAMA LXe (90%CL) XMASS Supernova at 10kpc (Livermore model) Expected signal Background Bosonic super-wimps search Phys. Rev. Lett. 113 (2014) Annual modulation search Phys. Lett. B759 (2016) 272 Search for 2n double electron capture on 124 Xe, 126 Xe Observed data Expected signal 214 Pb background Phys. Lett. B759 (2016) 64 XMASS 15

16 Summary XMASS-I is a large single-phase liquid-xenon detector for astro-particle and particle physics located in Kamioka in Japan. The detector construction was completed in Dec. 2010, and data-taking is ongoing. Low background rate of O(10-4 ) event/day/kg/kev was achieved in the energy region of a few 10s kev. Recently, detail measurement of scintillation time profile for low-energy e/g has been conducted. Pulse shape discrimination is under study. 16

17 Backup slides 17

18 Next step: XMASS-1.5 Total ~6 tons (Fiducial mass ~3 tons) Newly developed low-background 3-inch dome-shape PMT Background level of ~1x10-5 dru (pp solar neutrinos) assumed Sensitivity to WIMP SI cross section (1-3)x10-47 cm GeV/c 2 18

19 Sensitivity to low mass WIMPs/bosonic super-wimps Low mass WIMPs Bosonic super-wimps XMASS 1.5 Analysis threshold ~1 kevee Whole volume (no vertex reconstruction) Expect to reduce BG to 1/500 (<10-2 dru) Higher energy O(100 kev) e/g events Background level 10-5 dru assumed Ultimate BG: pp solar neutrino & 136 Xe 2nbb 19