基于WRF模式的长株潭城市群绿心通风廊道规划策略研究

在全球气候变暖和快速城镇化背景下,城郊自然开放空间在被城市空间替代的过程中,城市绿地面积的减少引发了一系列的气候和环境问题.而构建通风廊道可以通过提升城市通风能力有效缓解热岛效应和雾霾现象,近年来已成为各领域的研究热点.作为风景园林师,从改善城市风环境角度出发,在宏观层面的空间规划中考虑通风廊道构建方案,提升城市在发展...     

Large volume imaging arrays for gamma-ray spectroscopy

We report for the first time on the characteristics of large volume (4 cm×4 cm×0.5 cm) cadmium zinc telluride gamma-ray imaging arrays produced by IMARAD Imaging Systems. These arrays are shown to posses high uniformity of response to gamma-photons in the energy range of about 50 to over 600 keV. High resolution photopeaks have been obtained without any pulse processing or compensation techniques. Excellent peak-to-valley ratios, good efficiencies, and acceptable leakage currents have been measured. In addition measurements of the internal electric field indicate that the field is uniform and does not appear to be confined to only one region of the detector volume. Low temperature photoluminescence spectroscopy shows that the dominant peaks are different than those typically observed in high quality HPB material of the same composition obtained from other vendors. We believe that this material and the arrays that may be produced from it represent a significant step forward in the technology of room temperature semiconductor gamma and x-ray spectrometers.     

In-situ control of temperature and alloy composition of Cd1−xZnxTe grown by molecular beam epitaxy

In this work in-situ spectroscopic ellipsometry (SE) has been applied for the simultaneous determination of the growth temperature and alloy composition for the epitaxial Cd_1−xZn_xTe(211)/Si(211) structure. The optical dielectric functions of CdTe and Cd_0.96Zn_0.04Te (CZT) epilayers were studied as a function of temperature both ex-situ and in-situ in the range from 1.6 eV to 4.5 eV. We employed parametric models for the simulation of the optical properties of CZT at and between the critical points (CP) E₀, E₀ + Δ₀, E₁, E₁ + Δ₁, E₂(Σ) and E₂(Σ). Critical point energies and line widths for Cd_1−xZn_xTe were obtained through the fitting process, which included both zero order and higher order derivatives of the SE pseudo dielectric function. The dependence of the different critical points on Zn concentration x is discussed. It has been demonstrated that the energy of the weak E₀ + Δ₀ transition can be used to measure composition, while the E₁ energy can be used as a real-time temperature measure. The model parameters were optimized through the simultaneous analysis of multiple data sets, and the temperature dependent model was developed for in-situ application. Our analysis is estimated to produce uncertainties of only ±0.5°C in measuring the temperature and ±0.5% in measuring the composition of only the zero order dielectric function is being fitted. The effects of a surface overlayer, of reflected beam deflections, and of other experimental problems on the overall accuracy, are discussed as well as ways to improve the in-situ SE data quality.     

DC photoconductivity study of semi-insulating Cd1−xZnxTe crystals

Room temperature direct current (DC) photocurrents were measured for detector grade Cd_1−xZn_xTe (CZT) crystals in the spectral range of 400–1000 nm as a function of light intensity and voltage. The photocurrent data were analyzed and fit to a theoretical model to extract the electrical transport properties for high-resistivity detector grade CZT material. Using the DC photocurrent measurements, the mobility-lifetime () products and the surface recombination velocities for both electrons and holes were measured. For this study the CZT detectors had Au contacts, and the surfaces were treated in a standard 5% bromine in methanol etching solution. The correlation of the DC photocurrent measurements and detector performance is also reported.     

Characterization of etch pit formation via the Everson-etching method on CdZnTe crystal surfaces from the bulk to the nanoscale

A combination of atomic force microscopy, optical microscopy, and mass spectrometry was employed to study CdZnTe crystal surface and used etchant solution following exposure of the CdZnTe crystal to the Everson etch solution. We discuss the results of these studies in relationship to the initial surface preparation methods, the performance of the crystals as radiation spectrometers, the observed etch pit densities, and the chemical mechanism of surface etching. Our results show that the surface features that are exposed to etchants result from interactions with the chemical components of the etchants as well as pre-existing mechanical polishing.     

Characterization of charge collection in CdTe and CZT using the transient current technique

The charge collection properties of Cadmium–Telluride (CdTe) and Cadmium–Zinc–Telluride (CZT) in comparison with Silicon (Si) are presented using the transient-current technique (TCT) where the current pulses are generated by -particles emitted from an ²⁴¹Am source. From the recorded current pulse shapes, the charge collection efficiency, the charge carrier mobility and the electric field distribution inside the detectors are extracted. In particular, the signals of the compound semiconductors CdTe and CZT are interpreted with respect to the build-up of space–charges in the sensor volume and the subsequent deformation of the electric field. As high-quality CdTe and CZT samples are now commercially available, the knowledge of these material characteristics is of outmost importance for the application of CdTe and CZT in X-ray imaging.

In addition, the paper describes the influence of Ohmic and Schottky contacts on the current pulses in CdTe as well as the effects of polarization, i.e. the time-dependent degradation of the detector signals due to the accumulation of fixed charges within the sensor.     

室温核辐射CdZnTe像素阵列探测器的研制

报道了一种基于N型Cd0.9Zn0.1Te晶体材料的室温核辐射探测器.为充分发挥CdZnTe材料的优点,实验中采用了一系列工艺优化措施:如对材料表面进行处理,采用Pt/Au双层电极结构并优化器件表面钝化方法,改善退火条件等,从而制备出性能优良的7mm×7mm×5 mm阵列像素(3×3)探测器:其57Co12-2 ke光谱能量分辨率为23.7%(28.9 keV FWHM),137Cs 662 keV光谱分辨率为17.9%(17.8 keV FWHM);光谱曲线理想,呈高斯分布形状;低能带尾非常陡峭,这说明空穴的影响较小,可以略此外,该探测器在室温及不同测试条件下均能稳定工作,与传统探测器相比,省去了复杂昂贵的低温制冷系统,降低了成本.     

用于卫星探测X、γ射线的大灵敏面积CdZnTe探测器的研发

CdZnTe(CZT)探测器不需要低温制冷就可在30～600keV较宽的能量范围内得到较好的空间和能量分辨,已成为研究宇宙空间X、γ射线场重要的探测器。本工作研究将4个甄别级10mm×10mm×5mmCZT平面探测器进行改制,并将其并联拼接成20mm×20mm×5mm较大面积的CZT探测器。经测试,大面积CZT探测器对¹²⁵I、²⁴¹Am、⁵⁷Co、¹³³Ba、¹³⁷Cs具有较好的能量线性响应,对¹³⁷Cs的662keVγ射线有较好的能量分辨。     

Cadmium zinc telluride substrate growth, characterization, and evaluation

Cadmium zinc telluride substrates were grown by the vertical and horizontal Bridgman techniques, characterized, and evaluated. Evaluation included the fabrication and testing of infrared focal plane arrays. Correlations of substrate defects and FPA performance were made.     

Characteristics of a prototype CdZnTe detector

Cadmium zinc telluride(CZT)is a preferred material for X-ray and gamma-ray detector.Thanks to the relatively high atomic number,high density and wide band-gap,CZT detector possesses sharp energy resolution and high detection efficiency without cryogenic cooling.We have developed a CdZnTe detector with an energy resolution of 3.45%(FWHM)at 59.54 keV at room temperature,and it is used for X-ray fluorescence analysis.In this paper, leakage current,energy resolution and long term stability of the CZT detector are discussed.