This work is supported by JSPS KAKENHI Grant Number 15K13478 (Started from Apr. 2015) ZnWO 4

Transcription

1 This work is supported by JSPS KAKENHI Grant Number 15K13478 (Started from Apr. 2015) ZnWO 4 Hiroyuki Sekiya Kamioka Observatory, ICRR, University of Tokyo Shunsuke Kurosawa, Akira Yoshikawa Research Lab. on Advanced Crystal Engineering, IMR, Tohoku University June CYGNUS2015 Occidental Collage

2 DAMA s report Directional response with MeV alpha particles Estimated quenching 5keV 40~50% difference ~30% difference 2

3 In case of stilbene crystal P.H. Heckmann et al.,z. Phys. 162 (1961) 122 Directional response with MeV alpha particles HS et al., Physics Letters B 571 (2003) 132 Measured quenching factor@40kev only 7 % difference 24% difference θ Only 7% 3

4 To go beyond the NC-ν wall Directional detectors must aim at <10-48 cm 2 arxiv: J.Billard et al. Hiroyuki Sekiya CYGNUS2015 Occidental College June2 4

5 Although direction is not detected directly it s a direction-sensitive detector How much ZnOW 4 is necessary in Kamioka? 5

6 Assuming 7% Expected diurnal modulation Including atmospheric ν & DSNB BG 10 tons x 1000 days exposure! c.f. Proton decay 22.5 ktons x 6000 days c axis // North-South c axis // East-West 6

7 Started to prepare (10 tons of) ZnWO 4! 2 x 2 x 2 mm 3 crystal made by Czochralski process At Tohoku University A 2mm cubic crystal was cut and polished. 2.0 mm 0.2 mm 2.0 mm To discern the direction of the crystal 7

8 Monoclinic crystal system Crystal structure of ZnWO 4 Schofield et al., Acta Crystallographica, Section B: Structural Science (1997) 53, p102-p112 Lattice type P Space group name P 2/c Space group number 13 Setting number 1 a [Å] b[å] c[å] α [deg] β [deg] γ [deg] Almost cubic! 8

9 Simulated Laue patterns X-ray irradiated from each axes of ZnWO 4 c-axis b-axis a-axis 9

10 Actual Laue pattern X-ray generator: RIGAKU RAD-IIC with Hamamatsu CCD 25kV, 30mA, Target:Cu exposure:240ns Distance between CCD and ZnWO 4 :4cm c-axis X-rays This plane is really perpendicular to c-axis! 10

11 ~b axis Actual Laue patterns ~a axis b-axis a-axis X-rays X-rays 11

12 Scintillation wavelength Scintillation wavelength was measure with 241 Am (5.5MeV α) Spectrometer: Edinburgh Instrument FLS920 Scintillation mechanism Charge transfer emission Peak There is a reproducible sharp structure around 450nm Under investigation c.f. Grabmaier IEEE TNS 31 (1984 )

13 Setup with PMT Hamamtsu R7600U-200 UBA PMT QE For fixed crystal position, Teflon + Al mounter were attached to the PMT ZnWO 4 Paper Black tape 241 Am Reflector/mounter ZnWO 4 optical grease Oken 6262A 10 mm R7600U-200 Al mounter 13

14 Scintillation waveform Decay constant :10µsec Oscilloscope Tektronix TDS 3034B Fitted function f = A1exp(-t/τ1) + C τ1: 10 μsec References Grabmaier IEEE TNS 31 (1984 ) µs Holl et al., IEEE TNS 35, (1988) µs Shkulkova et al., IEEE TNS 56 (2009) ns & 7,000 ns & 25,000 ns Depends on each crystal? Lattice Defect affects? Temperature dependence? Will be investigated 14

15 Setup for spectral measurement preamp ORTEC 113 C=1000pF shaper ORTEC 572A Shaping time: 10 µsec MCA AMPTEK8000A Will be tested with FADC 15

16 Reproducibility check 1 From ~a axis direction, attach remove attach cycled Both 59.5keV peak and 5.5MeV positions are same. 59.5keV X-ray 5.5MeV α Temperature o C 16

17 Reproducibility check 2 From ~a axis direction, but the other side of the crystal Both 59.5keV peak and 5.5MeV positions are same. 59.5keV X-ray 5.5MeV α Temperature o C 17

18 Main result From ~a axis direction, ~b axis direction, and c axis direction ~12% difference in 5.5MeV α peaks, while 59.5keV peaks are same. ~b axis c axis 59.5keV X-ray 5.5MeV α ~a axis Temperature o C 18

19 Summary of what we did so far Scintillation wavelength: peak ~480nm Scintillation decay time: ~10µs There are differences between previous studies. Lattice defect affects? Light yield must be checked. Directional response was confirmed. 12% with sys. error Lattice defect affects? More perfect crystal has smaller directional response? 19

20 To do Evaluation with another samples of the crystal Light yields check with gamma rays. 23% of NaI Holl et al., IEEE TNS 35, (1988) 105 Intrinsic background checks Temperature dependence of LY. F. Cappella et al., Eur. Phys. J. C (2013) 73:2276 Directional respose remains? Neutron beam test Prepare 10 tons of ZnOW 4 20

21 For larger crystals At research lab. on advanced crystal engineering, IMR, Tohoku University Many Czochralski machines that can make 3inch-size crystal. ~ 10kg x 1000 = 10 tons? It takes 2 weeks to make one. Optimization for ZnWO 4 is underway. 21

22 In Japan, a consortium for low BG techniques was organized by KAKENHI budget (Grants-in-Aid for Scientific Research, MEXT) ~15M$ for 5 years With PICO-LON and CANDLES Low BG crystal technologies are being developed. For Low BG crystal 22