Trace analysis of pesticides in water using pulsed photoacoustic technique

Trace concentration of pesticides such as Atrazine and Methyl parathion in water solutions has harmful health effects. In this work a methodology to detect trace concentrations for both pesticides has been developed. The pesticide solution contained into a quartz cell is irradiated by a pulsed laser at λ = 230 nm, this energy is absorbed depending on the pesticide concentration; the non absorbed light that sample allows to pass is absorbed by the NaCl:Mn²⁺ doped single crystal fixed to a piezoelectric sensor of 5 MHz to obtain the photo- acoustic (PA) signal. The results showed that is possible to detect concentrations around tenths of ng/l.     

Fluorescence quantum yield of rhodamine 6G using pulsed photoacoustic technique

Experimental method for measuring photoacoustic(PA) signals generated by a pulsed laser beam in liquids is described. The pulsed PA technique is found to be a convenient and accurate method for determination of quantum yield in fluorescent dye solutions. Concentration dependence of quantum yield of rhodamine 6G in water is studied using the above method. The results indicate that the quantum yield decreases with increase in concentration in the quenching region in agreement with the existing reports based on radiometric measurements.     

Evaluation of laser ablation threshold in polymer samples using pulsed photoacoustic technique

The acoustic signals generated in solids due to interaction with pulsed laser beam is used to determine the ablation threshold of bulk polymer samples of teflon (polytetrafluoroethylene) and nylon under the irradiation from a Q-switched Nd:YAG laser at 1.06μm wavelength. A suitably designed piezoelectric transducer is employed for the detection of photoacoustic (PA) signals generated in this process. It has been observed that an abrupt increase in the amplitude of the PA signal occurs at the ablation threshold. Also there exist distinct values for the threshold corresponding to different mechanisms operative in producing damages like surface morphology, bond breaking and melting processes at different laser energy densities.     

爆炸物检测实验系统研究

介绍了利用中子技术对爆炸物进行检测的原理,以及东北师范大学辐射技术研究所利用脉冲快热中子分析技术对爆炸物进行检测的实验装置,根据系统测量结果,讨论该系统存在的问题并提出一些解决方案.     

Electrostrictive limit and focusing effects in pulsed photoacoustic detection

We give an analytical solution in the time and frequency domain for the cylindrical pressure wave generated by a laser pulse traveling in a liquid, which is valid over a wide range of laser beam dimensions and pulse durations. This leads to a simple prediction of the ultimate limitation set by the electrostrictive coupling and an easy analysis of the focusing effects on the photoacoustic signal. Two separate detection schemes were considered and show different behavior.     

Thermal decomposition study of nitropyrazoles based on time-resolved pulsed photoacoustic spectroscopy

The paper reports the study of thermal decomposition of 3(5)-nitropyrozole, 4-nitropyrazole and 1-nitropyrazole using time-resolved pulsed photoacoustic spectroscopy technique for the first time. The second harmonic, i.e., λ = 532 nm, 7 ns pulse width at 10 Hz repetition rate, obtained from Q-switched Nd: YAG laser has been employed. In addition, we have studied the effect of pressure, temperature and incident laser energy on the thermal decomposition mechanism of nitropyrazoles. The strength of photoacoustic signal provides significant information about the multistep thermal decomposition mechanism of 3(5)-nitropyrazole and 4-nitropyrazole, while 1-nitropyrazole follows the path of direct decomposition based on single reaction, which is also verified by the thermal gravimetric–differential thermal analysis (TG–DTA) technique. We have also experimentally demonstrated the presence of free NO₂ before the melting point, which is due to the thermal dissociation of solid nitropyrazole compounds.     

A photoacoustic signal detection technique by using gated vector lock-in amplifier

An electronic gating technique is introduced in photoacoustic signal detection. It eliminates the classical effects of vibrational noise associated with mechanical scanners.

The new system samples the photoacoustic signal only during the time when the mechanical scanner is stationary. The signal is fed into the lock-in amplifier together with the reference signal. The amplifier output corresponding, to the sample characteristics is recorded by the computer during the above interval.

The new technique detects the subsurface features in metals similar to the conventional beam scanning techniques. Comparisons between the two shows that the photoacoustic signal amplitudes were identical for the same sample characteristics.

One outstanding advantage is its low cost relative to the cost of the beam scanners currently available on the market.     

用脉冲光声技术测量吸收系数

建立了由YAG激光器和超声探测器组成的利用脉冲光声技术测量样品吸收系数的实验系统,通过对测量的光声信号的拟合,可以计算出样品的吸收系数.用染色的琼脂进行了实验验证,测量误差为-5.2%～4.8%.     

Characterization of thermal, optical and carrier transport properties of porous silicon using the photoacoustic technique

In this work, the porous silicon layer was prepared by the electrochemical anodization etching process on n-type and p-type silicon wafers. The formation of the porous layer has been identified by photoluminescence and SEM measurements. The optical absorption, energy gap, carrier transport and thermal properties of n-type and p-type porous silicon layers were investigated by analyzing the experimental data from photoacoustic measurements. The values of thermal diffusivity, energy gap and carrier transport properties have been found to be porosity-dependent. The energy band gap of n-type and p-type porous silicon layers was higher than the energy band gap obtained for silicon substrate (1.11 eV). In the range of porosity (50-76%) of the studies, our results found that the optical band-gap energy of p-type porous silicon (1.80-2.00 eV) was higher than that of the n-type porous silicon layer (1.70-1.86 eV). The thermal diffusivity value of the n-type porous layer was found to be higher than that of the p-type and both were observed to increase linearly with increasing layer porosity.     

Laser photoacoustic spectroscopy helps fight terrorism: High sensitivity detection of chemical Warfare Agent and explosives

Tunable laser photoacoustic spectroscopy is maturing rapidly in its applications to real world problems. One of the burning problems of the current turbulent times is the threat of terrorist acts against civilian population. This threat appears in two distinct forms. The first is the potential release of chemical warfare agents (CWA), such as the nerve agents, in a crowded environment. An example of this is the release of Sarin by Aum Shinrikyo sect in a crowded Tokyo subway in 1995. An example of the second terrorist threat is the ever-present possible suicide bomber in crowded environment such as airports, markets and large buildings. Minimizing the impact of both of these threats requires early detection of the presence of the CWAs and explosives. Photoacoustic spectroscopy is an exquisitely sensitive technique for the detection of trace gaseous species, a property that Pranalytica has extensively exploited in its CO2 laser based commercial instrumentation for the sub-ppb level detection of a number of industrially important gases including ammonia, ethylene, acrolein, sulfur hexafluoride, phosphine, arsine, boron trichloride and boron trifluoride. In this presentation, I will focus, however, on our recent use of broadly tunable single frequency high power room temperature quantum cascade lasers (QCL) for the detection of the CWAs and explosives. Using external grating cavity geometry, we have developed room temperature QCLs that produce continuously tunable single frequency CW power output in excess of 300 mW at wavelengths covering 5μm to 12μm. I will present data that show a CWA detection capability at ppb levels with false alarm rates below 1:108. I will also show the capability of detecting a variety of explosives at a ppb level, again with very low false alarm rates. Among the explosives, we have demonstrated the capability of detecting homemade explosives such as triacetone triperoxide and its liquid precursor, acetone which is a common household liquid. This capability, deployed at airports and other public places, will go a long way towards increasing public safety and minimizing inconveniences faced in airline travel.     

Determination of thermal effusivity of solids by a photoacoustic scanning technique

A new method is proposed to determine the thermal effusivity of solid samples using a one dimensional photoacoustic scanning technique. The method employs a sample configuration in which the backing for a good light absorber layer is changed from a reference sample to the unknown sample by scanning the absorber surface with an incident modulated light beam. From the measured phase difference or amplitude ratio one can determine the thermal effusivity of the unknown sample, knowing the effusivity of the reference sample. The Rosencwaig-Gersho theory of photoacoustic effect has been extended to the present experimental situation and expressions have been derived for photoacousitc phase difference and amplitude ratio as the backing is changed. Values calculated using these expressions are found to agree well with measured values for different sample combinations except in amplitude ratio values when the thermal effusivities of the samples differ very widely. The reason for this disagreement is discussed.     

Fast calculation of pulsed photoacoustic fields in fluids using k-space methods

Two related numerical models that calculate the time-dependent pressure field radiated by an arbitrary photoacoustic source in a fluid, such as that generated by the absorption of a short laser pulse, are presented. Frequency-wavenumber (k-space) implementations have been used to produce fast and accurate predictions. Model I calculates the field everywhere at any instant of time, and is useful for visualizing the three-dimensional evolution of the wave field. Model II calculates pressure time series for points on a straight line or plane and is therefore useful for simulating array measurements. By mapping the vertical wavenumber spectrum directly to frequency, this model can calculate time series up to 50 times faster than current numerical models of photoacoustic propagation. As the propagating and evanescent parts of the field are calculated separately, model II can be used to calculate far- and near-field radiation patterns. Also, it can readily be adapted to calculate the velocity potential and thus particle velocity and acoustic intensity vectors. Both models exploit the efficiency of the fast Fourier transform, and can include the frequency-dependent directional response of an acoustic detector straightforwardly. The models were verified by comparison with a known analytic solution and a slower, but well-understood, numerical model.     

Investigation of glucose concentration measurement based on tunable pulsed laser induced photoacoustic technique

A tunable pulsed laser induced photoacoustic （PA） measurement set-up is established in the forward mode to monitor in vitro glucose concentration. A series of experiments are investigated to verify the feasibility of this set-up and scheme. Peak-to-peak values （PPVs） of several glucose aqueous solutions are recorded and averaged 512 times at each wavelength. Experimental results demonstrate that the time-resolved PA profile of glucose solutions has a good agreement with the PA theories. The characteristic wavelengths of glucose solution are determined via differential method. The root-mean-square error （RMSE） of predicted concentration reaches 3.15 mg/dl at the optimum wavelength of 1 510 nm via least square （LS） fitting algorithm.     

压电光声技术检测非均匀材料热物性的理论研究

利用压电光声技术研究了测量非均匀材料性能的理论和实验方法,通过两种基本的数学变换,得到了非均匀材料和相邻介质的温度场的表达式,进而在非均匀材料的热扩散率按指数规律变化的前提下,得到了非均匀材料温度场的解析表达式。根据压电光声检测理论,得到了非均匀材料的压电信号的解析式,这样即可通过压电光声检测系统,在不同的频率下测得非均匀材料的光声信号,通过最小二乘法的非线性拟合,确定出非均匀材料热扩散率的分布。该方法具有计算量小、实验简单等特点,并给出了有关的理论模拟和实验结果,验证了理论的正确性。     

Measurement of the normal thermal diffusivity of a dielectric film by a pulsed photoacoustic method

A method of determining the thermal diffusivity of a thin film of a transparent dielectric in the direction normal to the surface has been developed. It is based on excitation of reflecting dynamic gratings. The effectiveness of the method is checked experimentally with the example of a thermally oxidized submicron SiO₂ film on a silicon substrate. The temperature dependence of in the range 290–420 K is measured. The possibilities of setting up thermal measurements of films tens of nanometers thick and its problems are discussed.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 793–797, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Analog integrating detection technique for pulsed muon beams

A new analog integrating detection technique for time resolvedSR measurements at high intensity pulsed muon beams has been developed. Data acquisition is realized by digitizing the integrated anode signal for each burst by means of a fast transient recorder. Details of setup and performance are described.     

Studies on selected polymeric materials using the photoacoustic spectroscopic technique

Polymethylmethacrylate-graft-polybisphenol-A-carbonate (PMMA-G-PC) with 50% grafting is synthesized. The graft co-polymerization of methylmethacrylate (0.036 mol · lit-1) onto polybisphenol-A-carbonate (0.5 g) in the presence of a redox couple formed from potassium persulphate (40 mol · lit-1) and thio-urea (30 mmol · lit-1) in aqueous nitric acid (0.18 M, 100 ml) in air at (45±2)°C for 3.0 h. Condensation of (PMMA-G-PC) with N- [p-(carboxyl phenyl amino acetic acid)] hydrazide (PCPH) affords polybisphenol-A-carbonate-graft-polymethylmethacrylate hydrazide (PCGH). The photoacoustic (PA) spectra of (PCGH) are recorded in a wavelength range from 200 nm to 800 nm at a modulation frequency of 22 Hz, and compared with those of pure polybisphenol-A-carbonate (PC), (PMMA-G-PC) and (PCPH). In the present work, a non-destructive and non-contact analytical method, namely the photoacoustic technique, is successfully implemented for optical and thermal characterization of selected polymeric materials. The indigenous PA spectrometer used in the present study consists of a 300-W xenon arc lamp, a lock-in amplifier, a chopper, a (1/8)-m monochromator controlled by computer and a home-made PA cell.     

Radiation-induced decomposition of explosives under extreme conditions

We present high-pressure and high temperature studies of the synchrotron radiation-induced decomposition of powder secondary high explosives pentaerythritol tetranitrate (PETN) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) using white beam synchrotron radiation at the 16 BM-B and 16 BM-D sectors of the HP-CAT beamline at the Advanced Photon Source. The radiation-induced decomposition rate TATB showed dramatic slowing with pressure up to 26.6 GPa (the highest pressure studied), implying a positive activation volume of the activated complex. The decomposition rate of PETN varied little with pressure up to 15.7 GPa (the highest pressure studied). Diffraction line intensities were measured as a function of time using energy-dispersive methods. By measuring the decomposition rate as a function of pressure and temperature, kinetic and other constants associated with the decomposition reactions were extracted.