Fast scintillation detectors for high-energy X-ray region

We have developed fast scintillation detectors for nuclear resonant scattering experiments using synchrotron radiation and a nuclear excited level existing in >30?keV. A fast x-ray detector using an organic-inorganic perovskite scintillator of phenethylamine lead bromide (PhE-PbBr₄) had a dominant light emission with a fast decay time of 9.9?ns. An x-ray detector equipped with a 0.9-mm-thick PhE-PbBr₄ crystal (size: ～8 × 7?mm²) was used to detect nuclear resonant scattering in ⁶¹Ni (the first excited level: 67.41?keV; half-life: 5.3?ns). We could successfully record the decaying gamma rays emitted from ⁶¹Ni with a relatively high detection efficiency of 24%. A lead-doped plastic scintillator (NE142, Pb ～5?wt% doped) had been known to have a faster decay time of 1.7?ns. Following a test of a single NE142 detector, a four-channel NE142 detector was fabricated and successfully applied to the synchrotron-radiation based M?ssbauer spectroscopy experiment on ⁶¹Ni.     

Excited state absorption and stimulated emission of Nd3+ in crystals III: LaSc3(BO3)4, CaWO4, and YLiF4

3+ ions in CaWO₄, LaSc₃(BO₃)₄, and YLiF₄ crystals are presented. The spectra were measured by a continuous-wave pump and probe technique. It is shown that ESA is a negligible loss mechanism in cw lasers emitting at 1.06 μm. In contrast, the effective emission cross sections in Nd:CaWO₄ and Nd:LaSc₃(BO₃)₄ around 1.35 μm are considerably diminished. Received: 8 May 1998/Revised version: 7 September 1998     

Synthesis and tunable luminescence properties of monodispersed sphere-like CaWO4 and CaWO4:Mo/Eu,Tb

Monodispersed CaWO₄ and CaWO₄:Mo/Eu, Tb microspheres have been obtained hydrothermally with excellent reproducibility. X-ray diffraction, field-emission scanning electron microscopy and photoluminescence were used to characterize the as-synthesized samples. The results uncover the formation process of the microspheres and the influence of ethylenediamine. Doping proper concentration of Mo into CaWO₄ micropheres can extend the excitation range as the incorporation induces obvious redshift. We have also tuned the color from blue to pink blue, white and green only by varying the relative content of Eu–Tb in the CaWO₄ host.     

Broadening of paramagnetic resonance lines by charged point defects in neodymium-doped scheelites

We study paramagnetic resonance linewidth in a series of CaWO₄ and CaMoO₄ crystals with different concentrations of neodymium ions (0.0031–0.81 at %). Experimental data are interpreted in the framework of the statistical theory of line broadening by charged point defects. In our calculations, three different contributions are singled out: arising from the local electric fields, electric field gradients and magnetic fields of the nearby point defects. The interaction parameters are determined from the spectroscopic data available for Nd:CaWO₄ crystal. Direct calculations of the linewidth are performed for different crystal orientations with respect to external magnetic field. We conclude that major contribution to the broadening comes from the interactions with random electric fields produced by neodymium and charge compensator ions.     

Untersuchung der Lumineszenzabklingung von Wolframatleuchtstoffen nach Anregung mit UV-Licht und Elektronen

The decay of the luminescence of CaWO₄, CdWO₄, MgWO₄, and ZnWO₄ after excitation with short pulses of light and electron beams has been investigated at different temperatures. Excitation with UV-light yields an exponential decay, and from the temperature dependence of the decay time it can be concluded that at high temperatures non-radiative transitions are prevalent. After excitation with electrons, there are additional effects by the heating up of the “excitation channel”. In this case the decay is not purely exponential and thermoluminescence is observed. This indicates the existence of traps, which are assumed to be located in the centers of the luminescence.     

Ba2CaWO6-Sr2CaWO6赝二元系的研究

用差热分析、X射线物相分析和点阵常数的精确测定,研究了Ba₂CaWO₆-Sr₂CaWO₆赝二元系的相平衡和相变。发现在高温(860℃以上)时,形成连续固溶体。在室温时,在富Ba₂CaWO₆一侧,形成以Ba₂CaWO₆为基的固溶体;在富Sr₂CaWO₆一侧,形成以Sr₂CaWO₆为基的固溶体,相变点在成分为Ba₂CaWO:Sr₂CaWO₆=25:75(克分子比)处。同时研究了作为高压钠灯灯丝涂层材料的Ba_xSr_2-xCaWO₆的几个重要性质:电子发射和多次启动时的灯压升高等。发现成分为BaSrCaWO₆时,性质最好。 关键词：     

Low-temperature light detectors: Neganov–Luke amplification and calibration

The simultaneous measurement of phonons and scintillation light induced by incident particles in a scintillating crystal such as CaWO₄ is a powerful technique for the active rejection of background induced by γ?s and β  ?s and even neutrons in direct Dark Matter searches. However, ?1%$?1\text{\%}$ of the energy deposited in a CaWO₄ crystal is detected as light. Thus, very sensitive light detectors are needed for an efficient event-by-event background discrimination. Due to the Neganov–Luke effect, the threshold of low-temperature light detectors based on semiconducting substrates can be improved significantly by drifting the photon-induced electron–hole pairs in an applied electric field. We present measurements with low-temperature light detectors based on this amplification mechanism. The Neganov–Luke effect makes it possible to improve the signal-to-noise ratio of our light detectors by a factor of ∼9 corresponding to an energy threshold of ∼21 eV$\sim 21\text{eV}$. We also describe a method for an absolute energy calibration using a light-emitting diode.     

The influence on intrinsic light emission of calcium tungstate and molybdate powders by multivalence Pr codoping

For trivalent praseodymium (Pr³⁺) and quadrivalent praseodymium (Pr⁴⁺) codoped CaMO₄ (M = W, Mo) powders, the luminescence propriety of matrix is obviously influenced by carrier concentration. The light emission intensity of CaWO₄ matrix decreases exponentially with increasing of Pr concentration because oxygen-deficient (WO₃·V_O^··\mathrm{WO}_{3}\cdot V_{\mathrm{O}}^{\bullet \bullet}) obtains an electron supplied by Pr³⁺ (5d). However, the light emission intensity of CaMoO₄ is enhanced by Pr codoping because the quasi-free electrons increase the probability of radiative combination. The difference of photoluminescence properties in the two materials are attributed to the bonding character of M and O in the CaMO₄ structure.     

Luminescence properties of europium-terbium double activated calcium tungstate phosphor

Double incorporation of Eu³⁺ and Tb³⁺ ions into a CaWO₄ crystalline lattice modifies the luminescence spectrum due to the formation of new emission centers. Depending on the activators concentration and nature, as well as on the interaction between the activators themselves, the luminescence color can be varied within the entire range of the visible spectrum. Variable luminescence was obtained when CaWO₄:Eu,Tb phosphors with 0-5 mol% activator ions were exposed to relatively low excitation energies as UV (365 and 254 nm). Under high energy excitation such as VUV (147 nm) radiation or electron beam, white light has been observed.This material with controlled properties seems to be promising for the applications in fluorescent lamps, colored lightning for advertisement industries, and other optoelectronic devices.     

CaWO4晶体中F型色心电子结构的研究

运用相对论的密度泛函离散变分法(DV-Xα)研究了CaWO₄晶体中F型色心的电子结构. 计算结果表明，F和F⁺心在禁带中引入了新的施主能级；分析了晶体内可能存在的光学跃迁模式，并通过过渡态的方法计算了F，F⁺心跃迁到导带底的能量分别为1.92eV和2.42eV. 因此，从理论上推断了F和F⁺心在CaWO₄晶体中可能引起650nm和515nm的吸收，由此说明CaWO₄晶体中650nm和515nm吸收带起源于晶体中的F和F⁺心. 关键词： 4晶体')" href="#">CaWO₄晶体 +心')" href="#">F和F⁺心 DV-Xα     

Silicon detector technology development in India for the participation in international experiments

A specific research and development program has been carried out by BARC in India to develop the technology for large area silicon strip detectors for application in nuclear and high energy physics experiments. These strip detectors will be used as pre-shower detector in the CMS experiment at LHC, CERN for π ⁰/λ rejection. The fabrication technology to produce silicon strip detectors with very good uniformity over a large area of ∼40 cm², low leakage currents of the order of 10 nA/cm² per strip and high breakdown voltage of >500 V has been developed by BARC. The production of detectors is already under way to deliver 1000 detector modules for the CMS and 90% production is completed. In this paper, research and development work carried out to develop the detector fabrication technology is briefly described. The performance of the silicon strip detectors produced in India is presented. The present status of the detector technology is discussed.      

Eu,Li doped CaWO4 nanorods: Synthesis,emission-decay curves and effective-refractive index

In the present paper, CaWO₄: Eu³⁺, Li⁺ nanorods have been successfully synthesized via an oleic acid (OA)-assisted solvethermal route. The transmission electron microscope (TEM) photograph shows that the CaWO₄: Eu³⁺, Li⁺ nanorods are monodisperse and uniform nanorods with average diameter of 26 nm. The optical properties of Eu³⁺ in CaWO₄ samples, including photoluminescence (PL) excitation spectra and luminescent decay curves, are investigated in detail. Due to the form of the nanorods, the relative contribution of CTB to the nanorods sample is greater than that to the bulk counterparts. The decay time of the ⁵D₀ level (ranging from 0.94 to 0.65 ms, depending on the filling factor of the nanorods) of the nanorods is longer than that of the bulk counterpart mainly due to the reduction in the size of the nanorods, which introduces an effective-refractive index smaller than the refractive index of CaWO₄.     

Raman spectroscopic study of structural disordering in YVO<Subscript>4</Subscript>, GdVO<Subscript>4</Subscript>, and CaWO<Subscript>4</Subscript> crystals

The Raman spectra of single-crystal YVO₄, GdVO₄, and ZrSiO₄ with a zircon structure, as well as CaWO₄ and BaWO₄ with a scheelite structure, are studied in detail over a wide temperature range 14–800 K. An inhomogeneous splitting of the A _1g (ν₂) vibrational lines in the Raman spectra of YVO₄ and GdVO₄ and the A _g (ν₁) vibrational lines in the spectrum of CaWO₄ is detected. It is shown that the profiles of these lines can be decomposed into two components, whose integrated intensities are redistributed with temperature and also depend on the matrix kind in which they are detected. The phenomenon observed is associated with the thermally activated processes of disorientation of the tetrahedral anions in the zircon and scheelite structures.     

Polarization selection rules for the allowed optical transitions in europium-terbium double activated calcium tungstate phosphor

The paper is devoted to the problem of the optical anisotropy of the rare-earth ions occupying low-symmetry positions in crystals. The crystal field multiplets arising from LSJ terms of Eu³⁺ and Tb³⁺ ions in the crystal field of calcium tungstate scheelite (CaWO₄) are analyzed (S₄ point symmetry). The selection rules, in particular, polarization rules for the allowed electric dipole optical transitions in the electronic shells of the Eu³⁺ and Tb³⁺ in CaWO₄ host lattice are discussed. Special attention is paid to the study of the angular (polarization) dependence of the two-photon absorption that seems to be an effective tool for the understanding of the complicated optical pattern. The peculiarities of the anisotropy of the two-photon absorption prove to be specific for each allowed dipole transition in S₄ symmetry center.     

Magnetic monopole search at high altitude with the SLIM experiment

The SLIM experiment was a large array of nuclear track detectors located at the Chacaltaya high altitude Laboratory (5230 m a.s.l.). The detector was in particular sensitive to intermediate mass magnetic monopoles, with masses 10⁵ GeV <M_M< 10¹² GeV. From the analysis of the full detector exposed for more than 4 years a flux upper limit of 1.3×10^-15 cm^-2 s^-1 sr^-1 for downgoing fast intermediate mass monopoles was established at the 90% C.L. PACS 14.80.Hv; 29.40.Wk; 29.90.+r     

Transverse beam asymmetries measured from <Superscript>4</Superscript>He and hydrogen targets

The HAPPEX Collaboration at Jefferson Lab has measured the transverse beam spin asymmetries (A_T) for elastic electron scattering from proton and ⁴He targets. The experiment was conducted using a vertically polarized electron beam of energy ∼ 3 GeV, at a Q ² ∼ 0.1 GeV^2 and a scattering angle θ_lab ∼ 6^° . The preliminary results are reported here. The ⁴He measurement is the first measurement of A_T from a nucleus. A_T for ⁴He is non-negligible; therefore, it will be necessary to make measurements of A_T for future parity-violating experiments using nuclear targets.     

Scintillating double-beta-decay bolometers

We present the results obtained in the development of scintillating double-beta-decay bolometers. Several Mo and Cd based crystals were tested with the bolometric technique. The scintillation light was measured through a second independent bolometer. A 140-g CdWO₄ crystal was run in a 417-h live time measurement. Thanks to the scintillation light, the α background is easily discriminated, resulting in zero counts above the 2615-keV γ line of ²⁰⁸Tl. These results, combined with an extreme easy light detector operation, represent the first tangible proof demonstrating the feasibility of this kind of technique. The text was submitted by the authors in English.     

On the intrinsic nature of the blue luminescence in CaWO4

Luminescence and thermoluminescence properties of nominally pure LiF and CaWO₄ crystals are compared with those of the crystalline system LiF/CaWO₄. Evidence is found which clearly indicates that from the various emission bands of CaWO₄ crystals only the blue one is of intrinsic nature. Luminescence decay measurements are reported. Within the spectral range of the blue tungstate band in LiF/CaWO₄ four different decay times are obtained. This result is interpreted as arising from WO^2-₄ luminescence centres with different environments in the LiF host.     

Relaxation dynamics of electronic excitations in CaWO4 and CdWO4 crystals studied by femtosecond interferometry technique

The relaxation of electronic excitations in CdWO₄ and CaWO₄ crystals was studied using the method of time-resolved interferometry with 100-fs temporal resolution at temperatures 15–295 K. The electronic system was excited in the one-photon and two-photon regime within the excitonic band in CaWO₄ and in the electron-hole continuum in CdWO₄. Immediate trapping of charge carriers was detected under pumping in the excitonic band of CaWO₄. This result is in agreement with decay kinetics measurements with nanosecond time resolution under direct creation of excitons by 100-fs laser pulses. Fast relaxation of charge carriers followed by formation of excitons was observed in CdWO₄. The comparison with previous work allows suggesting the formation of bulk excitons and surface-perturbed excitons in the multi-photon and one-photon regime. The corresponding models of self-trapped exciton creation in tungstate crystals are discussed.     

Refinement of the (NH4)3H(SO4)2 crystal structure: II. X-ray and neutron single-crystal diffraction from phase II at room temperature

The crystal of (NH₄)₃H(SO₄)₂ is of special interest due to the possible influence of ammonium ion on the series of phase transitions: I↔II↔ III↔IV↔V↔VII. Earlier, the x-ray single-crystal diffraction study of phase II of the (NH₄)₃H(SO₄)₂ crystal showed that the crystal structure of this compound had two crystallographically independent groups of the ammonium ions NH₄(1) and NH₄(2) but orientational positions of these ammonium ions were not determined exactly. The refinement of the NH₄(1) and NH₄(2) orientational positions in phase II was carried out by the x-ray and neutron single-crystal diffraction study. The analysis of difference Fourier maps of electron charge density and nuclear density suggested the possibility of disordering of NH₄(2) ammonium ions. The text was submitted by the authors in English.