dark matter search with direction sensitive scintillators idm2004 september 8, 2004, edinburgh...
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Dark Matter Search withDirection Sensitive
Scintillators
IDM2004 September 8, 2004, Edinburgh Scotland
Hiroyuki SekiyaKyoto University
M.Minowa, Y.Shimizu, Y.Inoue, W.SuganumaUniversity of Tokyo
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Table of Contents
Properties of Stilbene Scintillators and its Response to WIMPsA 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.
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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 crystalIn 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
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Response to ~ 100 keV carbon recoil
2cm×2cm×2cmc’
θ
The variation is 7 % ~ annual modulation
PLB 571 (2003) 132
Response to Carbon Recoils with Neutrons
Recoil direction
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Response to Isothermal WIMPs Halo
c’ // VE
c’ ┴ VE
Stilbene crystal fixed on the earth diurnal modulation
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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
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5cm φ× 5cm 116g
Cryos Beta Ltd., Ukraine
116g stilbene crystal2 low BG PMT read out (Hamamatsu R8778MOD)PMTs cool down -7℃ by Peltier coolers2 low BG NaI(Tl) active shields (Horiba, Ltd.)
Pilot Measurements in Kamioka
c’
c’
N
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OFHC Copper 10cmLead 15cmPolyethylene 20cmN2 gas for Rn purge
Pilot Measurements in Kamioka
Stilbene
Shields
Cold Finger
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Obtained Spectrum
The count rate for 4-6 keV of every one hour.
2000 counts/keV/day/kg in 4-6 keV energy window
2003
November 8 2003 to December 11 2003
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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
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α
Limits on σχ-p
in the Case of Isothermal Halo
A
S
Expected
Results
A → conventional limits
S → limit from direction-sensitivity
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Limits on σχ-p
Conventional Limit“Directional” Limit
MT
First limit from the directional signature.The better limit will be obtained with higher statistics.
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BG Rate Dependence of Expected Sensitivity
(contemporary non-directional limits)
300 kg year
Ach
ieva
ble
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Radio isotopes in PMT1.High background events
2. Stilbene itself.
U-chain Th-chain 40 K 60 Co
1.8×10-2Bq 6.9×10-3Bq 1.4×10-1Bq 5.5×10-3Bq
Highly radio-pure, high quantum efficiency device is needed!
Ge spectrometry
2ASIp
Organic material contains only proton and 12C and Kinematical factor
Issues and some R&Ds
Avalanche photodiode
F-permutated organic crystal for SD WIMP search
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Performance of APD
Reverse Type Effective area 5mm× 5mm
Anthracene with reflector
Hamamatsu S8664-55K
1.5cm×1.5cm×1.5cm
Comparable to PMT
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Octafluolonaphthalene
Far from scintillation grade…
C10F8
M.P. = 87 ℃λmax= 350 nm
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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
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Expected Sensitivity Signal to Noise Ratio
“Directional” Limit
Conventional Limit
n depends on C.L.
Measured modulation amplitude
Measured count rate
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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