1. WIMPs Direct Search with Dual Light-emitting Crystals Xilei Sun IHEP International Symposium on Neutrino Physics and Beyond 2012 1

2. Contents Introduction Difference between nuclear and electron recoils Dual Light-emitting Crystals CINMDS experiment Summary 2

3. Some evidences for Dark Matter Rotation curves of galaxies Gravitational lenses 3 What is Dark Matter????????Bullet Cluster

4. DM candidates from particle physics Most popular candidates ▫ Stable weakly interacting massive particles (WIMPs) of mass GeV – TeV ▫ Massive neutrinos ▫ Axions WIMPs is preferred by most of theorist. two reasons: WIMPs occur in several theories beyond the Standard Model,such as ▫ Neutralino-Supersymmetry ▫ KK-Extra dimension WIMPs can be naturally produced as a thermal relic of the Big Bang, with a relic density consistent with that required of dark matter. It is called “WIMP miracle”. 4 Our detector is focus on WIMPs WIMP miracle

5. Detection mechanisms of WIMPs Indirect detection: if dark matter annihilated in the early Universe, it must also annihilate now through and the annihilation products may be detected. Direct detection: dark matter may scatter on normal matter through interactions, depositing energy that may be observed in sensitive, low background detectors. Particle colliders: dark matter may be produced at particle colliders through The massing mass provides signatures of dark matter. 5

6. Status of direct search 6

7. Main challenge of direct detection Low background, Low threshold Large mass, good n/e recoils separation 7 Sensitivity is determined by the power of background discrimination exposure and threshold. event rate< 0.1 events/kg·d WIMP-n cross section O (10 -44 ) cm 2 DM density 0.3 GeV/cm 3 the main challenge for the direct detection of WIMPs is the identification of nuclear recoils from electron recoils produced by γ-rays and betas from radioactive decays. The lower the threshold the higher the count rate

8. WIMPs detection technology These techniques can be broadly divided into two categories. Two channels technology: Scintillation-Phonon Scintillation-Ionization Ionization-Phonon background discrimination power is good, but relatively complex. One channel technology: Scintillation Ionization Phonon are relatively simple, but the ability to identify the β, γ bk presents weak. Dual Light-emitting Crystals One scintillation channel and has good background discrimination. How dose it work?? 8 a proposal

9. The difference between nuclear recoils and electron recoils Nuclear recoil electron 9 The main differences: Nuclear recoil has higher dE/dx, the rang is small Electron recoil has lower dE/dx, the rang is large Range of 10keV electron in CsI Range of 10keV Cs in CsI by SRIMby CASINO from J. I. Collar Dual Light-emitting Crystals can Reflect the different ranges of different particles by the ratio of different scintillation components.

10. Dual Light-emitting Crystal As an example of CsI(Na) CsI(Na) crystal is transparent for the scintillation of pure CsI, but CsI(Tl) is not. So there are two components of light for CsI(Na). Na + pure CsI The ratio of the two luminescence is different for nuclear recoils and electron recoils.  Average spacing of Na + is so large ~77Å.  Nuclear recoils cover less Na + because of small range ~ n*100Å  electron recoils cover more Na + because of large range ~ n*1000Å 10 300420 nm Electron recoils 300420 nm Nuclear recoils Emitting spectrum pure CsI

11. Waveform of n/gamma for CsI(Na) A typical waveform of a γ-ray (60 keVee) from 241 Am and the waveform profile of 200 waveforms (50-60 keVee). A typical waveform of a neutron (10 keVee) from neutron beam and the waveform profile of 50 waveforms (5-10 keVee). We found the CsI(Na) crystal has very different response to n/gamma Decay time 600ns Decay time 20ns 11

12. n/γ separation by PSD Scatter plot of ADC2/ADC versus energy for n and γ. Dot is γ-ray from 241 Am and plus is neutron Pulse Shape Discrimination (PSD) technique could be used for n/γ separation based on different waveforms of nuclear recoils and γ-rays Define: ADC is the integration of the slow component 2us ADC2 is the integration of the fast component 100ns the ratio ADC2/ADC can used for n/gamma separation 12

13. 2M gamma events With surface effect Without surface effect Rejection factor and threshold Only one event (in the small round circle) was seen in the region of ADC2/ADC > 0.7 and energy >12 keVee. It may be an environmental fast neutron. Hence the rejection factor is 10 7 or more with a threshold about 10keVee. the separation is not good at low energies. There are mainly two reasons:  one is the statistical fluctuation due to too few photoelectrons;  the other is the surface effect. A total of ~3.8 million γ-ray events from 241 Am were acquired 13 threshold is >50keVnr considering the quenching factor 10- 20% at room temperature. It can be improved at low temperature.

14. Surface effect Surface effect is known for CsI(Na) crystal due to deliquescence, which will limit the light from Na + in the surface layer. It is clear that if the thickness of the surface layer (lacking Na + ) is more than 1 µm, α with energy less than 100 keV or electron with energy less than 10 keV cannot interact with Na +. So the light of surface-α or electron is dominated by the pure CsI. We can use alpha to test the surface effect. The average projected ranges of different ions injected into CsI crystal 14 1 µm A vacuum quartz tube is used to avoid deliquescence

15. The result of Surface effect test A typical waveform of 5.2 MeV α from 239 Pu and the waveform profile of 200 waveforms with surface effect. A typical waveform of 5.2 MeV α from 239 Pu and the waveform profile of 200 waveforms without surface effect. Measured with old surface there is surface effect Measured with new surface after grinding there is no surface effect 15

16. Wavelength should different Transmittance of the wavelength filter as a function of wavelength lights of the fast and slow components should have different wavelengths, Na + is 420nm slow pure CsI is 310 nm fast we use a filter to test. PMT1PMT2 CsI(Na) filter Experimental Set-up 16 Transmittance: more than 80% at 310 nm less than 1% above 400 nm. we confirmed that: fast component has peak wavelength 310nm slow component has peak wavelength 420nm Scatter plot of ADCfilter/ADC for γ and surface-α.

17. Performance at low temperature Relative light output as a function of temperature for gamma and surface alpha. waveform as a function of temperature for gamma and surface alpha. Low temperature: Waveform of gamma gradually flattened with decreasing temperature Waveform of surface alpha gradually widened with decreasing temperature At -100 degree, the waveform reaches the maximum difference. 17 With the Light yield increasing, Threshold is expected to be ~10keVnr from >50keVnr. gammaS-alpha gamma S-alpha

18. CsI(Na) Dark Matter Search Experiment CINDMS CsI(Na) Dark Matter Search (CINDMS) experiment is proposed based on CsI(Na). Pulse Shape Identification (PSD) and filter technology is used to identify nuclear recoils. One detector module: 18cmx18cm x30cm crystal 40kg Packaged by Copper Vacuum chamber Each quartz window read out by 4 3” PMTs. Detector concept design Water tank: To shield environment neutron Cherenkov light mark cosmic ray copper low-temperature box: To provide a low-temperature environment Shield environment gamma 18 Copper Vacuum chamber

19. 19 Ideal Sensitivity of CINDMS CINDMS 1T To consider 10 photoelectron 10 -8 rejection power, sensitivity of CINDMS 1T is expected to be the order of 10 -11 pb by one year exposure 。 Background rejection power as a function of temperature. Calculated by waveform sample according waveform of g/sa.

20. The progress of the experiment Back ground simulation and shielding design Detector simulation and design Electronics Design Mechanical Design Low temperature neutron scattering experiments Search other Dual Light-emitting Crystals are under way. Device of Low temperature neutron scattering experiments Initial stage of a 100 kg scale experiment (CINDMS100) will be under construction at Daya Bay underground laboratory for verification of this technology. (2013-2015) 20

21. Summary Dual Light-emitting Crystals is proposed for WIMPs search.  One scintillation channel  background discrimination by range The test of CsI(Na) has been done:  waveforms of n/gamma are different  wavelengths of fast and slow light are different  surface effect is confirmed and need to be removed  performance is improved at low temperature CINDMS is proposed  CsI(Na) crystal, low temperature, Modular, water shielding  R&D is underway 21

22. Thank you 22

23. CsI:Na 的进展 23

24. CsI:Tl 情况 常温：纯 CsI 的发光波长在 300nm 无法透 出晶体，没有双发光性能； 低温：纯 CsI 的发光波长向 340ns 移动， 可透出晶体，具有双发光性能。 实验结果表明，低温下波形分辨能力变 强，但与 CsI:Na 波形分辨相差较大。 24

25. NaI:Eu 的情况 波长在 300nm 附近有一个透明带， 正好可透过部分纯 NaI 的发光， 掺杂 Eu 的发光在 450nm 附近，并 且衰减时间为 2.1µs ，其波形为扁 平形状，明显区别于纯 NaI 的窄高 形状，利于波形分辨。 25

26. 常用卤化物闪烁晶体 gamma-alpha 波形比较 CsI(Na) 晶体对 gamma 、 alpha 波形明显不同： alpha 激发明显的快发光， gamma 没有 明显的快成分 其它晶体对 alpha-gamma 的波形没有此明显区别 26

27. 波形鉴别 — 时间窗优化 优化时间窗口宽度使 ADC2/ADC 两者差别最大 归一化波形 ADC2/ADC 的差别随时间窗 的变化 不同温度时 ADC2/ADC 差 别最大的时间窗 时间窗确定后 ADC2/ADC 随温度的变化 -100 o C 时差别最大

28. 波形鉴别 — 优化 CUT 优化 ADC2/ADC CUT ，保证效率 >60% 不同温度时的 CUT 不同温度时的效率

29. 波形鉴别 — 排斥比 波形抽样计算结果显示 -100 o C 排 斥比最好，与实验数据符合的 很好。 此时光产额 : gamma- 慢成分为常温 45% alpha- 快成分为常温 6.5 倍 实际计算本底时能谱有 45% 的压 缩，使得探测窗口内的本底数 目发生变化。 -100 o C 实验数据散点图

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31. 排斥比测量 —gamma 事例挑选 放射源： 241 Am  衰变到  Np 产生的 59.5keV  射线 Time of decay:  信号后 5ns-200ns 为事例挑 选窗口