PICO-LON Project for WIMPs search
Ken-Ichi Fushimi, Kensuke Yasudai, Yuuki Kamedai, Hiroyasu Ejiri, Ryuta Hazama, Kayoko Ichihara, Kyoshiro Imagawa, Hiroshi Ito, Norihiko Koori, Hidehito Nakamura, Shintaro Nakayama, Masaharu Nomachi, Tatsushi Shima, Saori Umehara, Sei Yoshida
aa r X i v : . [ nu c l - e x ] N ov PICO-LON Pro ject for WIMPs search
Ken-Ichi Fushimi , Kensuke Yasudai , Yuuki Kamedai , HiroyasuEjiri , , , Ryuta Hazama , Kayoko Ichihara , Kyoshiro Imagawa ,Hiroshi Ito , Norihiko Koori , Hidehito Nakamura , ShintaroNakayama , Masaharu Nomachi, , Tatsushi Shima , Saori Umehara ,Sei Yoshida
1. Faculty of Integrated Arts and Sciences, The University of Tokushima, Tokushima cityTokushima 770-8502, JAPAN2. Research Center for Nuclear Physics, Osaka University, Ibaraki city Osaka, 567-0043JAPAN3. National Institute of Radiological Sciences, Chiba city Chiba 263-8555, JAPAN4. Nuclear Science, Czech Technical University, Prague, Czech Republic5. Faculty of Engineering, Hiroshima University, Higashi Hiroshima city Hiroshima 739-8527,JAPAN6. Department of Physics, Osaka University, Toyonaka city Osaka, 560-0043 JAPAN7. Horiba Ltd., Kyoto city Kyoto,601-8510, JAPANE-mail: [email protected]
8. Research Center for Neutrino Science, Tohoku University, Sendai city Miyagi 980-8578,JAPAN
Abstract.
Highly segmented inorganic crystal has been shown to have good performance fordark matter search. The energy resolution of ultra thin and large area NaI(Tl) scintillator hasbeen developed. The estimated sensitivity for spin-dependent excitation of
I was discussed.The recent status of low background measurement at Oto Cosmo Observatory is reported
1. Introduction
Dark matter search is one of the most important subjects in nuclear- and astro-particle physics.The particle candidates for cosmic dark matter is a key subject not only for astrophysics butalso for particle physics since the particle candidates for dark matter is proposed by variousmodels the beyond standard model.The components of the universe have been studied by many cosmological observations.. Sincethe most of the matter in the universe should be the cold dark matter, searching for WIMPs(Weakly Interacting Massive Particles) is quite important. The dark matter in the galaxy hasbecome quite ensuring. One of the most promising candidate for WIMPs is the SUSY neutralino,which interacts with the matter via only weak interaction.The processes for WIMPs-nucleus interaction are divided into three types; spin-independentelastic scattering (SI), spin-dependent elastic scattering (SD) and spin-dependent inelasticexcitation (EX).In the case of the type-(SI) the scattering amplitude is summed coherently over all thenucleons. Thus the scattering cross section is enhanced by the factor A , where A is the massnumber of the target nucleus. In the case of the type-(SD), only one nucleon which carries theuclear spin contributes to the cross section. Thus the cross section is a few orders of magnitudesmaller than that of the SI case. The cross section depends on the nuclear spin-matrix element λ J ( J + 1) and has a large ambiguity because this matrix element has a large model dependencefor the heavy nuclei [3]. In the case of type-(EX), the target nucleus will be excited to the lowlying excited state, which is followed by the gamma ray emission. This process is arisen onlyif the WIMPs particle has enough kinetic energy. In this case, the matrix element is evaluatedfrom the measured rate of the gamma de-excitation[3, 4]. Thus the model dependence of thecross section is much smaller than the cross section of the SD case.The segmentation of the detector is shown to be the best way to enhance the positionsensitivity[1]. Coincidence measurements of nuclear recoils and γ rays for the inelastic excitationof I enhanced the sensitivity using the highly segmented NaI(Tl) detector. Recently, ionizedatomic electrons and hard X rays following WIMPs-nuclear interactions have been shown to beuseful for the exclusive measurement of nuclear recoils from the elastic scatterings of WIMPs offnuclei [5, 6]. On the other hand, the background events have their own characteristics of timingand spatial profiles. Because the event rate due to the background is reduced by segmentation,a probability of the accidental coincidence of individual background events is vastly reduced.It has been shown that piling up many thin scintillators enhances the sensitivity forWIMPs search[1]. The highly sensitive detector system PICO-LON (Planar Inorganic CrystalsObservatory for LOw-background Neutr(al)ino) has been developed. It consists of many thinNaI(Tl) crystals whose thickness is 0.05cm. Recently, PICO-LON-I which consists of threecrystals of thin (0.05cm in thickness) and wide area (6.6cm ×
2. Performance test and low background measurement
The energy resolution of the thin NaI(Tl) scintillator was measured for low energy gammarays. The gamma rays from the sources,
Am,
Ba and Co were irradiated without anycollimator. The typical energy resolution (FWHM) was obtained as small as 18% at 60keV [2].The energy dependence of the energy resolution was measured. The energy resolution changesinversely proportional to the square root of the energy. The obtained energy dependenceagrees with the relationship. From the relationship, the corresponding energy for the singlephotoelectron was calculated as 0.35keV.The low background measurement was started in September 2007 at Oto Cosmo Observatory.The observatory is located in the south of Nara prefecture and 150km east from Tokushima.There are three laboratories at the center of the unused railway tunnel whose length is 5km,and the thickness of the overburden rock is 450m. The flux of cosmic rays was reduced by fourorders of magnitude compared with the flux at the surface observatory. The fast neutron wasreduced by two orders of magnitude.The concentration of radioactive radon, which is most serious origin of the background againstdark matter search, is kept as low as a few Bq/m out of the air-tight container. The PICO-LON system was installed in the air-tight container in which pure nitrogen gas was used. Theconcentration of radon in the air-tight container is kept as low as 150mBq/m . The huge NaI(Tl)scintillators whose dimension of 10.6cm × × igure 1. Expected sensitivity for spin-dependent excitation. The cross section was normalizedto the proton-WIMPs cross section. The dotted line, dashed line, dash-dotted line and dash-double-dotted lines are the experimental obtained by CRESST, DAMA LXe, ELEGANT-V andNAIAD groups.
3. Prospect
The installation and the tuning of the PICO-LON detector will be done in the winter of 2007.The full measurement will be started in the beginning of 2008. The larger NaI(Tl) plates whosedimension of 15cm × × µ Bq/kg for
Pb and
Bi, 100 µ Bq/kg for
Pb [1].
4. Acknowledgment
The authors thank Prof.T.Kishimoto for continuous encouragement of the project. The presentwork was supported by “Toray Science and Technology Grant” and the University of Tokushima.
References [1] K.Fushimi et al., Journal of Physical Society of Japan, (2005) 3117.[2] K.Fushimi et al., Journal of Physical Society of Japan, (2006) 064201.[3] J.Ellis et al., Physics Letters B212 (1998) 375.[4] H.Ejiri, H.Ohsumi and K.Fushimi, Physics Letters
B317 (1993) 14.[5] Ch.C.Moustakidis, J.D.Vergados and H.Ejiri Nuclear Physics
B727 (2005) 406.[6] H.Ejiri, Ch.C.Moustakidis and J.D.Vergados, Physics Letters