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1.
The ionization yield in a two-phase liquid xenon dark-matter detector has been studied in keV nuclear recoil energy region. The newly obtained nuclear quenching as well as the average energy required to produce an electron–ion pair from the measurement in Seguinot (1992) are used to calculate the total electric charges produced. To estimate the fraction of the electron charges collected, the Thomas-Imel model is generalized to describe the field dependence for nuclear recoils in liquid xenon. With free parameters fitted to experimentally measured 56.5 keV nuclear recoils, the energy dependence of ionization yield for nuclear recoils is predicted, which increases as recoil energy decreases and reaches the maximum value at 2∼3 keV. This prediction agrees well with existing data and may help to lower the energy detection threshold for nuclear recoils to ∼1 keV.  相似文献   

2.
The quenching factor of cesium and iodine nuclei recoiling in a CsI(Tl) scintillator is measured by scattering of 3 to 6 MeV neutrons. This factor increases when recoil energy decreases, from 7% at 150 keV to 15% at 25 keV. This relatively high efficiency could be useful in experiments dealing with very low recoil energies like the WIMP direct detection. These values are well explained by the Birks model. Pulse shape discrimination between electron and nuclei recoils is also investigated. Results are sufficiently good to allow a significant statistical rejection of radioactive background. This rejection capability is shown to be better than for NaI(Tl), at the same electron equivalent energy.  相似文献   

3.
A new method of searching for dark matter in the form of weakly interacting massive particles (WIMP) has been developed with the direct detection of the low energy nuclear recoils observed in a massive target (ultimately many tons) of ultra pure liquid argon at 87 K. A high selectivity for argon recoils is achieved by the simultaneous observation of both the VUV scintillation luminescence and of the electron signal surviving columnar recombination, extracted through the liquid–gas boundary by an electric field.

First physics results from this method are reported, based on a small 2.3 l test chamber filled with natural argon and an accumulated fiducial exposure of about 100 kg day, supporting the future validity of this method with isotopically purified 40Ar and for a much larger unit presently under construction with correspondingly increased sensitivities.  相似文献   


4.
We examine electron and nuclear recoil backgrounds from radioactivity in the ZEPLIN-III dark matter experiment at Boulby. The rate of low-energy electron recoils in the liquid xenon WIMP target is 0.75 ± 0.05 events/kg/day/keV, which represents a 20-fold improvement over the rate observed during the first science run. Energy and spatial distributions agree with those predicted by component-level Monte Carlo simulations propagating the effects of the radiological contamination measured for materials employed in the experiment. Neutron elastic scattering is predicted to yield 3.05 ± 0.5 nuclear recoils with energy 5-50 keV per year, which translates to an expectation of 0.4 events in a 1 yr dataset in anti-coincidence with the veto detector for realistic signal acceptance. Less obvious background sources are discussed, especially in the context of future experiments. These include contamination of scintillation pulses with Cherenkov light from Compton electrons and from β activity internal to photomultipliers, which can increase the size and lower the apparent time constant of the scintillation response. Another challenge is posed by multiple-scatter γ-rays with one or more vertices in regions that yield no ionisation. If the discrimination power achieved in the first run can be replicated, ZEPLIN-III should reach a sensitivity of ∼1 × 10−8pb · yr to the scalar WIMP-nucleon elastic cross-section, as originally conceived.  相似文献   

5.
The DEAP-1 low-background liquid argon detector was used to measure scintillation pulse shapes of electron and nuclear recoil events and to demonstrate the feasibility of pulse-shape discrimination down to an electron-equivalent energy of 20 keVee.In the surface dataset using a triple-coincidence tag we found the fraction of β events that are misidentified as nuclear recoils to be <1.4×10−7 (90% C.L.) for energies between 43–86 keVee and for a nuclear recoil acceptance of at least 90%, with 4% systematic uncertainty on the absolute energy scale. The discrimination measurement on surface was limited by nuclear recoils induced by cosmic-ray generated neutrons. This was improved by moving the detector to the SNOLAB underground laboratory, where the reduced background rate allowed the same measurement to be done with only a double-coincidence tag.The combined data set contains 1.23 × 108 events. One of those, in the underground data set, is in the nuclear-recoil region of interest. Taking into account the expected background of 0.48 events coming from random pileup, the resulting upper limit on the level of electronic recoil contamination is <2.7×10−8 (90% C.L.) between 44–89 keVee and for a nuclear recoil acceptance of at least 90%, with 6% systematic uncertainty on the absolute energy scale.We developed a general mathematical framework to describe pulse-shape-discrimination parameter distributions and used it to build an analytical model of the distributions observed in DEAP-1. Using this model, we project a misidentification fraction of approximately 10−10 for an electron-equivalent energy threshold of 15 keVee for a detector with 8 PE/keVee light yield. This reduction enables a search for spin-independent scattering of WIMPs from 1000 kg of liquid argon with a WIMP-nucleon cross-section sensitivity of 10−46 cm2, assuming negligible contribution from nuclear recoil backgrounds.  相似文献   

6.
We perform a theoretical study of the scintillation efficiency of the low energy region crucial for liquid xenon dark matter detectors. We develop a computer program to simulate the cascading process of the recoiling xenon nucleus in liquid xenon and calculate the nuclear quenching effect due to atomic collisions. We use the electronic stopping power extrapolated from experimental data to the low energy region, and take into account the effects of electron escape from electron–ion pair recombination using the generalized Thomas-Imel model fitted to scintillation data. Our result agrees well with the experiments from neutron scattering and vanishes rapidly as the recoil energy drops below 3 keV.  相似文献   

7.
Recent developments of the nuclear emulsion technology led to the production of films with nanometric silver halide grains suitable to track low energy nuclear recoils with submicrometric length. This improvement opens the way to a directional Dark Matter detection, thus providing an innovative and complementary approach to the on-going WIMP searches. An important background source for these searches is represented by neutron-induced nuclear recoils that can mimic the WIMP signal. In this paper we provide an estimation of the contribution to this background from the intrinsic radioactive contamination of nuclear emulsions. We also report the neutron-induced background as a function of the read-out threshold, by using a GEANT4 simulation of the nuclear emulsion, showing that it amounts to about 0.06 per year per kilogram, fully compatible with the design of a 10 kg × year exposure.  相似文献   

8.
We provide a new way of constraining the relative scintillation efficiency Leff for liquid xenon. Using a simple estimate for the electronic and nuclear stopping powers together with an analysis of recombination processes we predict both the ionization and the scintillation yields. Using presently available data for the ionization yield, we can use the correlation between these two quantities to constrain Leff from below. Moreover, we argue that more reliable data on the ionization yield would allow to verify our assumptions on the atomic cross sections and to predict the value of Leff. We conclude that the relative scintillation efficiency should not decrease at low nuclear recoil energies, which has important consequences for the robustness of exclusion limits for low WIMP masses in liquid xenon Dark Matter searches.  相似文献   

9.
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10.
The UVIS dark matter detector, proposed by Spooner and Smith (Phys. Lett. B 314 (1993) 430), consists of < 1000 ppm Tl doped NaI scintillator operated at 100-200 K, with measurement of UV and visible scintillation components used to discriminate between electrons (gammas) and Na or I recoils. Presented here are results of measurements of the gamma/nuclear recoil discrimination power of a Kyropolous NaI (0.5 ppm Tl) UVIS test detector operated at 160 K, with monoenergetic neutrons used to induce nuclear recoils via elastic scattering. Defining the statistical gain factor for N events as , the coefficient Cm was measured as a function of photoelectron pulse height. At 40 photoelectrons Cm was found to be 0.5 (corresponding to 90% rejection of gammas with 20% loss of neutron events). The scintillation efficiency for Na and I recoils relative to electrons was also measured at 160 K and found to be 35 ± 5% for Na recoils and 10 ± 2% for iodine.  相似文献   

11.
The most mature directional dark matter experiments at present all utilize low-pressure gas Time Projection Chamber (TPC) technologies. We discuss some of the challenges for this technology, for which balancing the goal of achieving the best sensitivity with that of cost effective scale-up requires optimization over a large parameter space. Critical for this are the precision measurements of the fundamental properties of both electron and nuclear recoil tracks down to the lowest detectable energies. Such measurements are necessary to provide a benchmark for background discrimination and directional sensitivity that could be used for future optimization studies for directional dark matter experiments. In this paper we describe a small, high resolution, high signal-to-noise GEM-based TPC with a 2D CCD readout designed for this goal. The performance of the detector was characterized using alpha particles, X-rays, gamma-rays, and neutrons, enabling detailed measurements of electron and nuclear recoil tracks. Stable effective gas gains of greater than 1 × 105 were obtained in 100 Torr of pure CF4 by a cascade of three standard CERN GEMs each with a 140 µm pitch. The high signal-to-noise and sub-millimeter spatial resolution of the GEM amplification and CCD readout, together with low diffusion, allow for excellent background discrimination between electron and nuclear recoils down below ∼10 keVee (∼23 keVr fluorine recoil). Even lower thresholds, necessary for the detection of low mass WIMPs for example, might be achieved by lowering the pressure and utilizing full 3D track reconstruction. These and other paths for improvements are discussed, as are possible fundamental limitations imposed by the physics of energy loss.  相似文献   

12.
Dark matter direct search experiments with scintillators need an accurate knowledge of Light Relative Efficiency Factors (REF) between electron and nuclear recoils to estimate the energy of the recoiling nuclei from the scintillation signal (if the light signal is used with this purpose) or to implement an effective background rejection based on the comparison of the scintillation with ionization or heat signals (if the light signal is used for particle discrimination). The Light REF between α and γ particles is required in some nuclear physics applications of scintillators like rare α decay searches, internal radiopurity assessment and some double beta decay searches. Two scintillating bolometers of BGO and Al2O3 were operated at 20 mK and exposed to fast neutrons, gamma rays, α particles and heavy nuclei. We measured their Light REF between γ and α particles and between electron and neutron induced nuclear recoils as a function of the deposited energy. We also measured the Light REF for O and Np ions in BGO. Results obtained for the different Light REFs were unsuccessfully compared with calculations based on a simple semi-empirical approach (with only one fitting parameter) proposed by Tretyak.  相似文献   

13.
The NAIAD experiment (NaI Advanced Detector) for weakly interacting massive particle (WIMP) dark matter searches at Boulby mine (UK) is described. The detector consists of an array of encapsulated and unencapsulated NaI(Tl) crystals with high light yield. Six crystals are collecting data at present. Data accumulated by four of them (10.6 kg × year exposure) have been used to set upper limits on the WIMP–nucleon spin-independent and WIMP–proton spin-dependent cross-sections. Pulse shape analysis has been applied to discriminate between nuclear recoils, as may be caused by WIMP interactions, and electron recoils due to gamma background. Various calibrations of crystals are presented.  相似文献   

14.
As part of the DarkSide program of direct dark matter searches using two-phase argon TPCs, a prototype detector with an active volume containing 10 kg of liquid argon, DarkSide-10, was built and operated underground in the Gran Sasso National Laboratory in Italy. A critically important parameter for such devices is the scintillation light yield, as photon statistics limits the rejection of electron-recoil backgrounds by pulse shape discrimination. We have measured the light yield of DarkSide-10 using the readily-identifiable full-absorption peaks from gamma ray sources combined with single-photoelectron calibrations using low-occupancy laser pulses. For gamma lines of energies in the range 122–1275 keV, we get light yields averaging 8.887±0.003(stat)±0.444(sys) p.e./keVee. With additional purification, the light yield measured at 511 keV increased to 9.142±0.006(stat) p.e./keVee.  相似文献   

15.
The ionization deposited in a Ge crystal by the scattering of ≈ 1 MeV neutrons on Ge nuclei is measured and its lowenergy behavior is investigated down to recoil energies of 3 keV. This calibration study is fundamental for the discrimination of Weakly Interacting Massive Particles (WIMPs) from the radioactive background. Experimental results are compared with theoretical predictions.  相似文献   

16.
A systematic analysis of the energy losses of magnetic monopoles of different magnetic charges and of dyons in three types of detectors are investigated for a large range of β = υ/c (β > 10−5). More specifically we have computed the light yield in liquid scintillators, the ionization in streamer tubes and the Restricted Energy Loss in the CR39 nuclear track detectors. Also the total energy losses have been analyzed.  相似文献   

17.
The energy range above 60 keV is important for the study of many open problems in high energy astrophysics such as the role of Inverse Compton with respect to synchrotron or thermal processes in GRBs, non thermal mechanisms in SNR, the study of the high energy cut-offs in AGN spectra, and the detection of nuclear and annihilation lines. Recently the development of high energy Laue lenses with broad energy bandpasses from 60 to 600keV have been proposed for a Hard X ray focusing Telescope (HAXTEL) in order to study the X-ray continuum of celestial sources. The required focal plane detector should have high detection efficiency over the entire operative range, a spatial resolution of about 1mm, an energy resolution of a few keV at 500keV and a sensitivity to linear polarization. We describe a possible configuration of the focal plane detector based on several CdTe/CZT pixelated layers stacked together to achieve the required detection efficiency at high energy. Each layer can operate both as a separate position sensitive detector and polarimeter or work with other layers to increase the overall photopeak efficiency. Each layer has a hexagonal shape in order to minimize the detector surface required to cover the lens field of view. The pixels would have the same geometry so as to provide the best coupling with the lens point spread function and to increase the symmetry for polarimetric studies.  相似文献   

18.
The discrimination capabilities of a 70 g heat and ionization Ge bolometer are studied. This first prototype has been used by the EDELWEISS dark matter experiment, installed in the Laboratoire Souterrain de Modane, for direct detection of WIMPs. Gamma and neutron calibrations demonstrate that this type of detector is able to reject more than 99.6% of the background while retaining 95% of the signal, provided that the background events distribution is not biased towards the surface of the Ge crystal. However, the 1.17 kg day of data taken in a relatively important radioactive environment show an extra population slightly overlapping the signal. This background is likely due to interactions of low energy photons or electrons near the surface of the crystal, and is somewhat reduced by applying a higher charge-collecting inverse bias voltage (−6 V instead of −2 V) to the Ge diode. Despite this contamination, more than 98% of the background can be rejected while retaining 50% of the signal. This yields a conservative upper limit of 0.7 event day−1 kg−1 keVrecoil−1 at 90% confidence level in the 15–45 keV recoil energy interval; the present sensitivity appears to be limited by the fast ambient neutrons. Upgrades in progress on the installation are summarized.  相似文献   

19.
The objective of this project is to develop and construct an innovative imaging system for nuclear medicine and molecular imaging that uses photon diffraction and is capable of generating 1–2 mm spatial resolution images in two or three dimensions. The proposed imaging system would be capable of detecting radiopharmaceuticals that emit 100–200 keV gamma rays which are typically used in diagnostic nuclear medicine and in molecular imaging. The system is expected to be optimized for the 140.6 keV gamma ray from a Tc-99m source, which is frequently used in nuclear medicine. This new system will focus the incoming gamma rays in a manner analogous to a magnifying glass focusing sunlight into a small focal point on a detector's sensitive area. Focusing gamma rays through photon diffraction has already been demonstrated with the construction of a diffraction lens telescope for astrophysics and a scaled-down lens for medical imaging, both developed at Argonne National Laboratory (ANL). In addition, spatial resolutions of 3 mm have been achieved with a prototype medical lens. The proposed imaging system would be comprised of an array of photon diffraction lenses tuned to diffract a specific gamma ray energy (within 100–200 keV) emitted by a common source. The properties of photon diffraction make it possible to diffract only one specific gamma ray energy at a time, which significantly reduces scattering background. The system should be sufficiently sensitive to the detection of small concentrations of radioactivity that can reveal potential tumor sites at their initial stages of development. Moreover, the system's sensitivity would eliminate the need for re-injecting a patient with more radiopharmaceutical if this patient underwent a prior nuclear imaging scan. Detection of a tumor site at its inception could allow for an earlier initiation of treatment and wider treatment options, which can potentially improve the chances for cure.  相似文献   

20.
X-ray reflection spectra from photoionized accretion discs in active galaxies are presented for a wide range of illumination conditions. The energy, equivalent width (EW) and flux of the Fe K α line are shown to depend strongly on the ratio of illuminating flux to disc flux,   F x/ F disc  , the photon index of the irradiating power law, Γ, and the incidence angle of the radiation, i . When   F x/ F disc≤2  a neutral Fe K α line is prominent for all but the largest values of Γ. At higher illuminating fluxes an He-like Fe K α line at 6.7 keV dominates the line complex. With a high-energy cut-off of 100 keV, the thermal ionization instability seems to suppress the ionized Fe K α line when  Γ≤1.6  . The Fe K α line flux correlates with   F x/ F disc  , but the dependence weakens as iron becomes fully ionized. The EW is roughly constant when   F x/ F disc  is low and a neutral line dominates, but then declines as the line progresses through higher ionization stages. There is a strong positive correlation between the Fe K α EW and Γ when the line energy is at 6.7 keV, and a slight negative one when it is at 6.4 keV. This is a potential observational diagnostic of the ionization state of the disc. Observations of the broad Fe K α line, which take into account any narrow component, would be able to test these predictions. Ionized Fe K α lines at 6.7 keV are predicted to be common in a simple magnetic flare geometry. A model that includes multiple ionization gradients on the disc is postulated to reconcile the results with observations.  相似文献   

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