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

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.

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

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

Response to Isothermal WIMPs Halo

c’ // VE

c’ ┴ VE

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 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

OFHC Copper 10cmLead 15cmPolyethylene 20cmN2 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.

2000 counts/keV/day/kg in 4-6 keV energy window

2003

November 8 2003 to December 11 2003

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 σχ-p

in the Case of Isothermal Halo

A

S

Expected

Results

A → conventional limits

S → limit from direction-sensitivity

Limits on σχ-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

Ach

ieva

ble

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

Performance of APD

Reverse Type Effective area 5mm× 5mm

Anthracene with reflector

Hamamatsu S8664-55K

1.5cm×1.5cm×1.5cm

Comparable to PMT

Octafluolonaphthalene

Far from scintillation grade…

C10F8

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