Beam chopping, photoacoustic

Modulation frequency The amplitude of the measuring light beam chopped at required frequency to generate the photoacoustic signal at the same frequency. 

The monochromatic X-ray was obtained by silicon (111) channel cut double crystal using white X-ray (at Beam Line 4A (PF)). The ion chambers were set at the both side of the photoacoustic cell, in order to compare the sp trum of photoacoustic X-ray absorption spectroscopy (PAXAS) with usual absorption spectrum, simultaneously. The chopper at chopping frequency of 10 Hz was t at the up-stream of these detectors. Copper foil (5 pm thick) was used as a sample. 

Fig. 15a-c. Photoacoustic X-ray absorption imaging using focused beam at 1.48 A. a Photographic picture of the sample, b image of the signal amplitude, and c image of the phase. Chopping frequency 10 Hz... 

Photoacoustic spectra are recordings of the energy emitted as heat after absorption of monochromatic light. The sample is placed in a closed photoacoustic cell. The light beam, which is chopped at an audiofrequency, induces a periodic heating and cooling of the gas in contact with the sample in the cell. This is sensed as sound by a sensitive microphone. The... 

The simplest application of a piezoelectric transducer is its use as a strain indicator, since the strains applied to a piezoelectric element can be quantified simply by measuring the electrical field which is created across the boundaries of that element. In the case of photoacoustic detectors, pressure pulses, created by the absorption of light from a chopped beam, deform a thin plate and are transformed into a series of voltage pulses. In such applications the electrical signals created are either dc or low-frequency, so that the design of the device and interface electronics is straightforward. 

Photoacoustic spectroscopy is a type of absorption spectroscopy in which absorption of energy is detected by the generation of sound waves. A beam of monochromatic radiation is directed on the sample though a chopper, which is usually a rotating disk with several notches cut in the edge so that the beam is alternately passed and interrupted (chopped). 

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