Dejun Han

Surface enhanced Raman scattering (SERS) spectra of trinitrotoluene in silver colloids prepared by microwave heating method.

Surface enhanced Raman spectroscopy (SERS) has been demonstrated for the detection of trace levels of explosives due to its high sensitivity, speed of detection and fingerprint feature. 2,4,6-Trinitrotoluene (TNT), a leading example of nitroaromatic explosives, is causing wide concern. In this study, SERS spectra of TNT solution in silver colloids have been successfully measured and a comparison was drawn with the normal Raman spectra of bulk TNT. The silver colloids were prepared by the microwave heating method and characterized by UV-Vis spectra and the scanning electron microscopy (SEM). NaCl and pH value have a great impact on SERS intensity of TNT, the corresponding experimental research results and theoretical interpretations were further illustrated to a certain extent. Moreover, the detection limit of TNT in aqueous solution was achieved as low as 10(-10) mol L(-1) and some preliminary experiments of detecting TNT vapor (about 10 μg/L) using SERS have been carried out. Our results demonstrated the potential of SERS for probing TNT with high sensitivity, and suggest SERS as a powerful method for detection of TNT and similar species at trace levels.

High-gain bipolar detector on float-zone silicon

Since the float-zone (FZ) silicon has lower contaminations and longer minority-carrier lifetime than those in Czochralski silicon and other semiconductor materials, it has potential advantages to fabricate bipolar detectors on the high-purity FZ silicon substrate to achieve a high gain at ultra-low-signal levels. The authors present preliminary experimental results on a bipolar detector fabricated on an unusual high-purity FZ silicon substrate. A backside gettering layer of phosphorus-doped polysilicon was employed to preserve the long carrier lifetime of the high-purity FZ silicon. The device has been investigated in the detection of a continuous flux of X-ray and infrared light. The bipolar detector with a circular emitter of 2 mm diameter has demonstrated high gains up to 3820 for 22 keV X-ray from a 1 mCi Cd radioactive source (the X-ray photon flux, received by the detector is estimated to be B7.77 � 10 4 /s). High gain up to 4400 for 0.17 nW light with a wavelength of 0.83mm has been observed for the same device. r 2003 Elsevier B.V. All rights reserved.

Fast identification of trace substance by single-photon detection of characteristic Raman scatterings with gated coincidence technique and multipixel photon counters

We report a method for fast identification of trace substances. It is based on the single-photon detection of characteristic Raman scatterings by using a gated dual-wavelength coincidence technique and multipixel photon counters. The objective of this study is to overcome the drawback of the low-speed detection of trace substances by Raman spectroscopy and other ultraweak spectra. Analysis indicates that a trace substance in the single-molecule level can be identified within 1 ms. As an example, a fast measurement of a CCl4 concentration with a high intrinsic peak-to-background ratio was demonstrated using this method.

Progresses in large dynamic range silicon photomultipliers with bulk quenching resistor

Silicon photomultiplier (SiPM) with bulk quenching resistor is one of the main SiPM technologies now under researches and developments. The device was first demonstrated at the Novel Device Laboratory (NDL). It has advantages of high micro cell density, short recovery time and has potential to alleviate the conflict between the high photon detection efficiency (PDE) and large dynamic range encountered by conventional SiPM, and it is beneficial to be applied in HEP calorimeters, PET, scintillator readout, radiation monitors and spectroscopy analysis, where high space resolution, large dynamic range and adequate high PDE are mandatory needed simultaneously. In the past years, with the optimization of the fabrication technology and structure of the device, the active area of the device has been increased from 1 mm × 1 mm to 2.2 mm × 2.2 mm, the peak PDE has been increased from 8% to 13%, while the density of the micro cells remains as large as 9000/mm2 and the recovery time remains to as low as ~5ns. Application of the device to the detection of natural background radiation by using a lutetium orthosilicate (LSO) scintillator has been performed. Energy resolution of 1-to-1 coupling of this device and a 2 mm × 2 mm × 10 mm lutetium yttrium oxyorthosilicate (LYSO) crystal yielded an energy resolution of 11.8% by using radiation sources of 137Cs (662keV photons) without any high speed preamplifiers. These latest developments of SiPM with bulk quenching resistor will be presented in this record.

Time-Correlated Raman and Fluorescence Spectroscopy Based on a Silicon Photomultiplier and Time-Correlated Single Photon Counting Technique

We report a time-correlated Raman spectroscopy technique based on a silicon photomultiplier (SiPM) and a time-correlated single photon counting (TCSPC) technique to exploit the natural temporal separation between Raman and fluorescence phenomena to alleviate the high fluorescence background with conventional Raman detection. The TCSPC technique employed can greatly reduce the effect of high dark count rate (DCR) and crosstalk of SiPM that seriously hinder its application in low light level detection. The operating principle and performance of the 400 ps time resolution system are discussed along with the improvement of the peak-to-background ratio (PBR) for bulk trinitrotoluene (TNT) Raman spectrum relative to a commercial Raman spectrometer with charge coupled device (CCD). The fluorescence lifetime for solid TNT and Surface Enhanced Raman Scattering (SERS) spectrum for 10−6 mol/L trace TNT have also been obtained by this system, showing excellent versatility and convenience in spectroscopy measurement.

Feasibility Study on Silicon Photomultiplier With Epitaxial Quenching Resistors as the Readout for PET Detectors

The silicon photomultiplier (SiPM) with epitaxial quenching resistors (EQR) is a novel SiPM technology currently under research and development. It has distinguished features of simple device structure, high fill factor, high microcell density, large dynamic range, and fast recovery time. In this paper, we report its latest developments aiming at positron emission tomography (PET) applications. The EQR type SiPM with active area of 2.2 mm ×2.2 mm demonstrated a peak PDE of 11.6% at 460 nm, PDE greater than 8.0% at 420 nm, temperature coefficient for breakdown voltage of 16 mV per ° C, with the fill factor of 40% and microcell density as high as ~ 9000/mm2. We measured a time resolution better than 357 ps FWHM and a 17.5% FWHM energy resolution at 511 keV by 1-to-1 coupling of the device with a 2 mm ×2 mm ×10 mm lutetium yttrium oxyorthosilicate (LYSO) crystal without any preamplifiers. Those preliminary results show the potential of using such SiPM to build a high spatial resolution and high timing resolution PET system.

Fast identification of substance by measuring two Raman peaks with dual strip silicon photomultipliers and gated photon counting technique.

In this report, we use carbon tetrachloride as an example to demonstrate that substance can be quickly identified through the measurement of the two Raman peaks simply by an integrated module of dual strip silicon photomultipliers operating at room temperature in conjunction with a gated photon counting technique. Both the peak positions and the relative intensity ratio of the two Raman peaks are used to identify the substance with a reduced false acceptance rate. A complete Raman spectrum of the substance can also be measured by using this method combined with a scanning monochromator.

Low-threshold-switch phototransistor based on Kirk effect and float-zone silicon

Based on the Kirk effect, a n+pn−n+ phototransistor with low optical threshold switching on high purity float-zone silicon was demonstrated. Under a collector-emitter voltage range from 3to15V, the phototransistor can turn on from a lower current state (range from 0.29to0.65mA) to a higher current state (range from 2.47to13.20mA) when it was triggered by illumination with a small critical optical power (range from 2.0to3.6μW). The theoretical calculations on critical collector current density were compared with the experimental results. Negative differential resistance and latch characteristics were observed and discussed.