Limits on interactions between weakly interacting massive particles and nucleons obtained with CsI(Tl) crystal detectors

The Korea Invisible Mass Search (KIMS) experiment presents new limits on the weakly interacting massive particle (WIMP)-nucleon cross section using data from an exposure of 3409 kg.d taken with low-background CsI(Tl) crystals at the Yangyang Underground Laboratory. The most stringent limit on the spin-dependent interaction for a pure proton case is obtained. The DAMA signal region for both spin-independent and spin-dependent interactions for the WIMP masses greater than 20 GeV/c2 is excluded by the single experiment with crystal scintillators.     

Limits on spin-independent interactions of weakly interacting massive particles with nucleons from the two-tower run of the cryogenic dark matter search

We report new results from the Cryogenic Dark Matter Search (CDMS II) at the Soudan Underground Laboratory. Two towers, each consisting of six detectors, were operated for 74.5 live days, giving spectrum-weighted exposures of 34 (12) kg d for the Ge (Si) targets after cuts, averaged over recoil energies 10-100 keV for a weakly interacting massive particle (WIMP) mass of 60 GeV/c2. A blind analysis was conducted, incorporating improved techniques for rejecting surface events. No WIMP signal exceeding expected backgrounds was observed. When combined with our previous results from Soudan, the 90% C.L. upper limit on the spin-independent WIMP-nucleon cross section is 1.6 x 10(-43) cm2 from Ge and 3 x 10(-42) cm2 from Si, for a WIMP mass of 60 GeV/c2. The combined limit from Ge (Si) is a factor of 2.5 (10) lower than our previous results and constrains predictions of supersymmetric models.     

New contribution to scattering of weakly interacting massive particles on nuclei

A weakly interacting massive particle (WIMP) is perhaps the most promising candidate for the dark matter in the Galactic halo. The WIMP detection rate in laboratory searches is fixed by the cross section for elastic WIMP-nucleus scattering. Here we calculate the contribution to this cross section from two-nucleon currents from pion exchange in the nucleus and show that it may, in some cases, be comparable to the one-nucleon current that has been considered in prior work and perhaps help resolve the discrepancies between the various direct dark-matter search experiments. We provide simple expressions that allow these new contributions to be included in current calculations.     

Property measurement of the CsI (Tl) crystal prepared at IMP

Large-sized CsI(T1) single crystals,～Φ100 mmx350 mm,have been grown successfully,and this CsI(Tl) coupled with PD has been successfully utilized at RIBLL (the Radioactive Ion Beam Line in Lanzhou)to measure the energy of heavy ions as a stopping detector.The performances of CsI(Tl) detector coupled with PD and APD have been tested and compared,including the temperature dependence of scintillating light yield.     

Property measurement of the CsI(Tl) crystal prepared at IMP

Large-sized CsI(Tl) single crystals, ～φ100mm×350mm, have been grown successfully, and this CsI(Tl) coupled with PD has been successfully utilized at RIBLL (the Radioactive Ion Beam Line in Lanzhou) to measure the energy of heavy ions as a stopping detector. The performances of CsI(Tl) detector coupled with PD and APD have been tested and compared, including the temperature dependence of scintillating light yield.     

Quenching of the spin-dependent scattering of weakly interacting massive particles on heavy nuclei

We present calculations of the quenching of the spin-dependent elastic scattering cross section for dark matter WIMPs on heavy nuclei. The theory of finite Fermi systems was used to describe the behavior of the nuclear spin matrix elements in the nuclear medium. The results of the calculations for planned dark matter detector nuclei are not only always smaller than corresponding single particle estimations but in some cases also differ from the ones obtained by using measured nuclear magnetic moments.One of the authors (M.A.N.) wants to thank the Max-Planck-Institut für Kernphysik (Heidelberg, Germany) for the hospitality during the time in which this work was carried out, and the Deutscher Akademischer Austauschdienst (DAAD) for financial support.     

Design and implementation of the NaI(Tl)/CsI(Na) detectors output signal generator

We designed and implemented a signal generator that can simulate the output of the NaI(Tl)/CsI(Na)detectors’pre-amplifier onboard the Hard X-ray Modulation Telescope(HXMT).Using the development of the FPGA(Field Programmable Gate Array)with VHDL language and adding a random constituent,we have finally produced the double exponential random pulse signal generator.The statistical distribution of the signal amplitude is programmable.The occurrence time intervals of the adjacent signals contain negative exponential distribution statistically.     

Size-induced effect on cathode luminescence spectra of CsI(Na) and CsI(Tl) crystals

We investigated the cathode luminescence characteristics of CsI(Na)and CsI(Tl)crystals by the spectrum and structure properties at room temperature.We fabricated three different sizes of CsI(Na)and CsI(Tl)crystals and measured their luminescence spectra under cathode rays.We found that CsI(Na)cathode luminescence peaks appear at 420 and 305 nm,and CsI(Tl)cathode luminescence peaks are 540 and 410 nm,the grain size affects CsI(Na)luminescence significantly,and the Na-related420 nm luminescence intensified relatively when the average grain size reaches~20μm,which becomes weak when the grain size is down to nano-scale.But the cathode luminescence spectra of CsI(Tl)crystals with different size have no obvious changes.Our explanations for these phenomena are that the different impurities in the same host material CsI lead to different luminescence mechanisms.These cathode luminescence characteristics indicate the suitability of CsI(Na)and CsI(Tl)crystals to match photomultiplier tube for large area crystal detector development.     

CsI(Tl) layers spectroscopy - comparison with crystals

Abstract

Emission and excitation spectra at liquid helium and liquid nitrogen temperature of CsI(Tl) single crystals and evaporated thin layers have revealed the existence of new centers in such materials. In particular, an emission band is observed around 460nm excited near 325 nm correlates with the quality of the evaporated thin film in terms of light yield and temporal behavior. The results can be interpreted using a dopant ion-bound exciton model involving different kinds of perturbed and non-perturbed centers.     

Nonthermal production of weakly interacting massive particles and the subgalactic structure of the universe

There is increasing evidence that conventional cold dark matter (CDM) models lead to conflicts between observations and numerical simulations of dark matter halos on subgalactic scales, which rules out the favored candidates for CDM, namely weakly interacting massive particles (WIMPs). We propose a mechanism of nonthermal production of WIMPs and study its implications on the power spectrum. Our results show that, in this context, WIMPs as candidates for dark matter can work well both on large scales and on subgalactic scales.     

Photo- and radiation-chemical transformations of carbonate ions in CsI and CsI(Tl) crystals

It is shown that irradiation of CsI and CsI(Tl) crystals containing carbonate and hydroxyl ions induces radiation defects there in the form of color centers, HCO ₃ ^- ions, and H₂O molecules. HCO ₃ ^- ions are formed in the bulk of crystals, whereas water molecules are formed only in the surface layer. IR spectra offered no evidence of decomposition of CO ₃ ^2- ions into CO and CO ₂ ^- and of formation of CO ₃ ^- ions in the course of growth nor in the course of irradiation of CsI(CO₃) and CsI(Tl, CO₃) crystals. Electron activator color centers in CsI(Tl, CO₃) crystals are likely to be stabilized by hole near-activator centers and by HCO ₃ ^- ions produced in radiation-induced chemical reactions.     

Identification of weakly interacting massive particles through a combined measurement of axial and scalar couplings

We study the prospects for detecting weakly interacting massive particles (WIMPs) in a number of phenomenological scenarios, with a detector composed of a target simultaneously sensitive to both spin-dependent and spin-independent couplings, as is the case of COUPP (Chicagoland Observatory for Underground Particle Physics). First, we show that sensitivity to both couplings optimizes chances of initial WIMP detection. Second, we demonstrate that, in case of detection, a comparison of the signal on two complementary targets, such as in COUPP CF3I and C4F10 bubble chambers, allows a significantly more precise determination of the dark matter axial and scalar couplings. This strategy would provide crucial information on the nature of the WIMPs and possibly allow discrimination between neutralino and Kaluza-Klein dark matter.     

Temperature Effects of CsI(Tl) Crystal Detector with APD Readout

The temperature dependences of the light output of CsI(Tl) crystal grown at IMP and of the gain of the Hamamatsu S8664-1010 avalanche photodiode (APD) have been investigated systematically. The light output of the CsI(Tl) crystal increases with temperature by 0.67%/℃ in the region from －2℃ to 8℃, and by 0.33%/℃ in the region from 8℃ to 25℃, while the gain of the tested APD decreases by －3.68%/℃ (working voltage 400V) on average in the room temperature range. The best energy resolution 5.1% of the CsI(Tl) with APD was obtained for the 662keV γ ray from ¹³⁷Cs radiation source.     

Experimental Studies on Radiation Damages of CsI(Tl) Crystals

The results of experimental studies on eadiation damage of CsI(Tl) crystal were reported.There are radiation damage effects on CsI(Tl) crystal.Experimental studies on recovery of damaged CsI(Tl) crystals were made.It seems that after heating at 200℃ for 4 hours,the damaged crystals could be recovered completely.     

CsI(Tl)晶体探测器APD读出的温度效应

对中国科学院近代物理研究所自行生长的铊激活碘化铯闪烁晶体CsI(Tl)光输出及Hamamtsu公司生产的S8664-1010型雪崩光二极管(APD)增益对温度的依赖关系做了系统研究. 结果表明, CsI(Tl)晶体光产额在室温范围内随着温度的增加而增加, 在－2℃—8℃温度范围内的平均温度系数为0.67%/℃, 在8℃—25℃温度范围内的平均温度系数为0.33%/℃. 而对所使用的APD, 其增益在室温范围内的温度系数为－3.68%℃(工作电压400V). APD结合CsI(Tl)晶体在室温下对¹³⁷Cs的662keV γ射线的能量分辨可达5.1%.     

Limits on masses and number of neutral weakly interacting particles

Limits on the masses and number of neutral weakly interacting particles are derived using cosmological arguments. No such particles with a mass between 120 eV and 3 GeV can exist within the usual big band model. Similar, but much more severe, restrictions follow for particles that interact only gravitationally. This seems of importance with respect to supersymmetric theories.     

Performance assessment of CsI(Tl) screens on varioussubstrates for X-ray imaging

Thallium-doped cesium iodide (CsI(Tl)) screens are widely used in X-ray imaging devices because of the columnar structure of the CsI(Tl) layer, but few reports focus on the optical role of the substrate in the screen system. In this paper, four substrates including fused silica (SiO₂), silver-film coated SiO₂, graphite (C) and fiber optic plate (FOP) are used to fabricate CsI(Tl) screens by thermal evaporation. Their imaging performance is evaluated by relative light output (RLO), modulation transfer function (MTF), normalized noise power spectrum (NNPS) and noise equivalent quanta (NEQ). The results reveal that although CsI(Tl) film on graphite plate yields images with the lowest light output, it presents relatively higher spatial resolution and better signal-to-noise characteristics. However, films on SiO₂ plate obtain low MTF but high NNPS curves, whether they are coated with silver film or not. Furthermore, scintillation screens on FOP have bright images with low NNPS and high NEQ, but have the lowest MTF. By controlling the substrate optical features, CsI(Tl) films can be tailored to suit a given application.     

X光高分辨探测用CsI(Tl)晶体的蒙特卡罗模拟研究

提出了一种基于CsI(Tl)闪烁晶体和面阵CCD器件、采用光纤和光纤面板进行光耦合及传输、以扇形束线阵扫描方式实现对X光高分辨探测的方案。CsI(Tl)晶体的尺寸大小将直接影响到晶体的发光效率及X光的高分辨探测,据此开展了蒙特卡罗模拟研究。模拟研究了X射线能量、X射线源到探测晶体的距离(源距)、CsI(Tl)晶体的厚度与X射线能量分布、全能峰效率与CsI(Tl)闪烁晶体转换效率之间的关系。结果表明,当X射线能量为120~450 keV,CsI(Tl)晶体尺寸厚度为0~1.5 cm变化时,全能峰效率的变化范围为31.34~96.74%,CsI(Tl)闪烁晶体的转换效率的变化范围为12.8~97.43%。可见,X射线的能量及CsI(Tl)闪烁晶体尺寸的厚度,是决定X光高分辨探测的重要参量,这对优化X光高分辨探测用CsI(Tl)晶体的尺寸设计具有一定的参考价值。