Dark Matter Search with Direction Sensitive Scintillators IDM2004 September 8, 2004, Edinburgh Scotland Hiroyuki Sekiya Kyoto University M.Minowa, Y.Shimizu, Y.Inoue, W.Suganuma University of Tokyo

Table of Contents Properties of Stilbene Scintillators and its Response to WIMPs A Pilot Measurements in Kamioka Analysis and Results Prospects and Summary The Aim  Examination of a directional detector distinct from gaseous detectors.  Conducting a direction-sensitive WIMPs search experiment.

 Scintillation efficiency depends on direction of nuclear recoils with respect to crystallographic axes.  We focus on the stilbene crystals. Light yield : 30% of NaI λmax : 410 nm Decay time : 5 ns Stilbene has an advantage in realizing large single crystal In high energy regions, the response was precisely measured. Scintillation efficiency depends on only the angle with respect to c’ axis. P. Heckmann et al., Z. Phys 162(1961) 84 monoclinic system Organic Single Crystals stilbene

Response to ～ 100 keV carbon recoil 2cm×2cm×2cm c’ θ The variation is 7 % ～ annual modulation PLB 571 (2003) 132 Response to Carbon Recoils with Neutrons Recoil direction

Response to Isothermal WIMPs Halo c’ // V E c’ ┴ V E Stilbene crystal fixed on the earth diurnal modulation

Kamioka 36°N When the crystal installed with the c ’ axis towards the North, WIMP incident angle changes 80°over 12 sidereal hours Response in Kamioka Expected modulation of today in 4-6 keV window

５ｃｍ φ× ５ｃｍ １１６ｇ Cryos Beta Ltd., Ukraine 116g stilbene crystal 2 low BG PMT read out (Hamamatsu R8778MOD) PMTs cool down -7 ℃ by Peltier coolers 2 low BG NaI(Tl) active shields (Horiba, Ltd.) Pilot Measurements in Kamioka c’ N

OFHC Copper 10cm Lead 15cm Polyethylene 20cm N 2 gas for Rn purge Pilot Measurements in Kamioka Stilbene Shields Cold Finger

Obtained Spectrum The count rate for 4-6 keV of every one hour counts/keV/day/kg in 4-6 keV energy window 2003 November to December

Halo Model Independent Analysis Searching for the anisotropic signal synchronized with the earth ’ s rotation. white background noise is dominant… Lomb ’ s periodgram (ref. Y.Ramachers Astropart. Phys. 19 (2003) 419) 1 sidereal day

α Limits on σ χ-p in the Case of Isothermal Halo A S Expected Results A → conventional limits S → limit from direction-sensitivity

Limits on σ χ-p Conventional Limit “ Directional ” Limit MT  First limit from the directional signature.  The better limit will be obtained with higher statistics.

BG Rate Dependence of Expected Sensitivity (contemporary non-directional limits) 300 kg year Achievable

Radio isotopes in PMT 1.High background events 2. Stilbene itself. U-chainTh-chain ４０ K ６０ Co 1.8×10 -2 Bq6.9×10 -3 Bq1.4×10 -1 Bq5.5×10 -3 Bq Highly radio-pure, high quantum efficiency device is needed! Ge spectrometry Organic material contains only proton and 12 C and Kinematical factor Issues and some R&Ds Avalanche photodiode F-permutated organic crystal for SD WIMP search

Performance of APD Reverse Type Effective area 5mm× 5mm Anthracene with reflector Hamamatsu S K 1.5cm×1.5cm×1.5cm Comparable to PMT

Octafluolonaphthalene Far from scintillation grade … C 10 F 8 M.P. = 87 ℃ λmax= 350 nm

The anisotropic response of carbon recoils in low energy region in a stilbene crystal was proved and the sensitivity to WIMPs was investigated. We have performed pilot measurements with 116g stilbene in Kamioka Underground laboratory. No modulation signal was found because of the high white backgrounds yet to be eliminated. The limit was derived from the directional signature of WIMPs. Although the limit was rather looser than the present best limit, it demonstrated the effectiveness of the directional sensitivity. Summary

Expected Sensitivity Signal to Noise Ratio “ Directional ” Limit Conventional Limit n depends on C.L. Measured modulation amplitude Measured count rate

Lomb’s Periodgram where Identical to the least – squares fitting to the model The probability of obtaining a peak due to statistical fluctuation Significance level is defined as

Neutron Source  7 Li(p,n) 7 Be using 3.2 MV Pelletron Tokyo Institute of Technology Capacitive Time pick-off Unit

Measurements of Carbon Recoil Principle of the measurements θＮθＮ θ Incident neutron energy E n Scattered neutron energy E n ’ With fixed scattered angle θ n Recoil nucleus, recoil energy E R and recoil angle θ N are uniquely determined by kinematics. θ-dependence of the scintillation efficiency

Systematic Errors in Recoil Energy Recoil energy is calculated from the scattering angle and the TOF of the neutron energy Errors in scattering angle Errors in TOF Errors in Recoil angle is within 5keV i.e. within the bin width.

fitting Statistical Significance Best fit 2σ2σ 1σ1σ θ-dependent = Quenching parameter kBe Although statistics is not very high anisotropy of 7% is seen over the measured energy region

2cm×2cm×2cm θ dependence was confirmed. Response to 5.9 keV X-ray Response to ～ 100 keV carbon recoil θ = Recoil angle with respect to c’ c’ We measured the angle/energy dependence of the scintillation efficience of carbon recoils in a stilbene crystal using neutrons in energies of 30 keV to 1MeV. H. Sekiya et al., PLB 571(2003) 132 Measurements of Carbon Recoil

Naphthalene Z. phys. 165 (1961) 1 50% anisotropy Yield : 12% of NaI λmax : 348 nm Decay time : 96 ns Preparing NOW ！

Stilbene Scintillator Scintillation efficiency depends on only the angle with respect to c ’ axis. In high energy regions, the response was precisely measured. monoclinic system Cygnus Scintillation response to 6.9MeV αparticle P. Heckmann et al., Z. Phys 162(1961) 84 crystal rotation angle around c’ around a Light output around b We measured the angle/energy dependence of the scintillation efficience of carbon recoils in a stilbene crystal using neutrons in energies of 30 keV to 1MeV H. Sekiya et al., PLB 571(2003) 132

Obtained Visible Energy Spectra without NaI(Tl) shields with NaI(Tl) Total exposure 0.29 kg days without NaI(Tl) shields 2.58 kg days with NaI(Tl) shields 2000 counts/keV/day/kg in 4-6 keV energy window It is clear that the backgrounds originate from the PMTs are dominat.

V E =232 km/s Revolution 30km/s Sun DARK HALO (Maxwellian) Ｖ ０ ＝ 220km/s Cygnus Velocity distribution Motivation Realistic distinctive WIMP signals arise from the earth’s motion in the galactic halo. The annual modulation of event rate is one of the possible WIMP signal caused by earth ’ s revolution around the sun. The earth ’ s rotation through the galaxy provides WIMP wind. Most convincing signature would be identified with recoil direction sensitive detector. WIMP wind How to distinguish WIMPs from BGs?

Application to WIMPs Detector minumum at c’ // V E ｍ aximum at c’ ┴ V E Scattered WIMP Nuclear Recoil WIMP Wind γ VＥVＥ Light output should be cosγ distribution If the crystal is fixed to the earth, light outputs spectra would modulate synchronized to the earth ’ s rotation

Dark Matter Search with Direction Sensitive Scintillators IDM2004 September 8, 2004, Edinburgh Scotland H. Sekiya Kyoto University M.Minowa, Y.Shimizu, Y.Inoue, W.Suganuma University of Tokyo

The earth moves toward Cygnus (about 42°to the earth’s axis) Kamioka 36°N If we install the crystal with the c ’ axis towards the North, WIMP incident angle changes 80°over 12 sidereal hours Expected moduraltion of today in 4-6 keV window Moduraltion amplitude Response in Kamioka

Direction of the Wind α November to December Variation of the angle between the direction of c’ axis and the direction of the earth’s motion 2003

Direction Sensitivity is indispensable DM signal arise only from Earth’s motion in the galactic halo. That is, the signature depends on halo model As previous speakers say, directional detectors should Explore DM experiments! However, Directional Detector is very difficult. Therefore, various idea are favored!! Gas TPC? No detectors except this exp succeed in detect sub 100keV Nuclear recoil.. at the moment!

c’ Response to WIMPs V earth c’(α,β) R( γ, φ ) WIMP crystal Observable Signature θ Recoil nucleus E vis Light Output WIMP signal Detector Stilbene

Response to Isothermal Halo Isothermal halo model c’ // V E c’ ┴ V E

Dark Current of R8778MOD Without crystal inside the shield Temperature dependence + dual read out

PMT R8778MOD Low BG, metal bulb. Modified for stilbene based on the R8778 for Xmass. Hamamatsu R8778MOD-ASSY

エネルギー較正 光量が少なくポアソン分布を反映したガンマ分布 ５５ Fe 5.9 ｋ eV のスペクトル それぞれの PMT 足し合わせ Threshold 109 Cd 22.2 keV 241 Am 59.5 keV を用いた較正直線 １ p.e./keV

Stability 2003 年 11 月 1 日から 12 月 11 日まで測定 ほぼ一週間に一度エネルギー較正を行った。 ５５ Fe 5.9 keV のゲイン変動 ～ ±1 ％

イベント選択 低エネルギーでは NaI は “ 波形 ” を成さない オシロスコープで取得した 波形を領域に分けて 低エネルギーでの NaI （ Tl ）イベントを除去 CUT それぞれの領域の波形を積分して 領域 A での値との比に対して CUT をかける