Beam chopper

Fig. 3.8. Experimental set-up to examine interaction of atom particles with the surface of a solid body by means of atom beam reflection. I - Chamber with atom particles source installed II, III - Intermediate and main chambers / -Pyrolysis filament 2 - Collimation channel 3 - Beam chopper 4 - Titanium atomizer 5 - Collimation slot 6 - Target 7 - Deflector 8 - To vacuum pump pipe 9 - Filament 10 - ZnO semiconductor sensor...

Such measurement provides the magnitude of birefringence, but not its sign. In addition, identical transmission values will be observed for multiple birefringence orders, that is, whenever the optical path difference, dAn, becomes a multiple of X. The main interest of this method arises from its excellent time resolution, below 1 ms, that is readily achieved using a low-power (e.g., 5 mW) continuous-wave laser and a photodiode. If the sample is initially isotropic, it is possible to follow the birefringence order to obtain quantitative results. For improved accuracy, a second (reference) photodiode or a beam chopper and a lock-in amplifier can be used. 

Fig. 13.18 Periodic on/off heating of a MLR using a C02 laser beam and a beam chopper. Reprinted from Ref. 23 with permission. 2008 Institute of Electrical and Electronics Engineers...

Multiple sprayer ion source with rotatory ion beam chopper (MUX) (performed by instrument maker) Place two to four sprayers in same interface housing each sprayer receives effluent of HPLC spinning chopper protects orifice of MS to allow ions only from one sprayer to enter at a time special MS program separates data from sprayers HPLCs operated in parallel and simultaneously... 

The design of a conventional atomic absorption spectrometer is relatively simple (Fig. 3.1), consisting of a lamp, a beam chopper, a burner, a grating monochromator, and a photomultiplier detector. The design of each of these is briefly considered. The figure shows both single and double beam operation, as explained below. 

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

As with all other types of spectrometers operating in the UV/visible region of the spectrum, it is advantageous to modulate the primary beam using a mechanical beam chopper, and detect it at the same frequency, to reduce background noise. This is usually done with a rotating beam chopper, shaped... 

Figure 3.2 Beam chopper in AAS. In a single beam instrument it is mounted vertically off-centre, so that it chops the beam. In a dual-beam instrument it is angled and mirrored so that it alternately allows the sample beam through and reflects the reference beam along the secondary path.

The beam chopper forces radiation to come alternately from the sample and the reference cell. 

Figure 3. Illustration of the cross-beam machine. N is the nozzle source for the molecular beam, C is the buffer chamber with a beam chopper (not shown), H is the hexapole electric field quantum state selector, U are the homogeneous electric field plates, Q is an on-axis quadrupole mass filter, O is the fast atom beam source, and Q and C,8o are channeltrons.

Figure 21-21 Operation of a beam chopper for subtracting the signal due to flame background emission, (a) Lamp and flame emission reach detector, (b) Only flame emission reaches detector, (c) Resulting square wave signal.

Figure 2 Schematic view of the apparatus used in studies of the steric effects in gas-surface scattering. A detail of the crystal mount with die orientation rod at 1 cm in front of the surface is shown in die right hand corner. A detailed drawing of the hexapole state selector is given below the main figure. The voltage is applied to die six small rods indicated by an arrow. Key Q quadrupole mass spectrometer R Rempi detector M, crystal manipulator SI, beam source for state selected molecules H electric hexapole state selector C mechanical beam chopper V pulsed gas source S2, continuous molecular beam source. From Tenner et al. [34].

Molecules that are ionized by electron impact in the ion source are accelerated, sent through a conventional 90° magnetic sector analyzer, postaccelerated by a few thousand volts, and arrive at the electron multiplier detector. The output of the electron multiplier detector consists of pulses of about lO- coulomb per ion. The pulses are amplified and sent through a gated amplifier and an electronic switch which is synchronized with the beam chopper so that one of the ion counters records ions only when the beam chopper is open, the other only when the beam chopper is closed. The difference between the two ion counts represents the ion intensity contributed by the molecular beam, while the square root of the sum of the two ion counts is approximately equal to the standard deviation of the measurement and serves as a useful indicator of the quality of the data being obtained. 

Figure 3-5 illustrates schematically a typical double-beam-in-space system in which all components are dupH-cated except the light source. Another approach is a double-beam-in-time instrument that uses a light-beam chopper (a rotating wheel with alternate silvered sections and cutout sections) inserted after the exit slit (Figure 3-6). A system of mirrors passes the portions of the light reflected...

An electric beam chopper and a tuned amplifier are incorporated into most AA instrument. Operationally, the power to the hoUow-cathode lamp is pulsed so that the light is emitted by the lamp at a certain number of pulses per second. On the other hand, aU of the light coming from the flame is continuous. When light leaves the flame, it is composed of pulsed, unabsorbed light from the lamp and a small amount of unpulsed flame spectrum and sample emission. The detector senses all light, but the amplifier is electrically tuned to accept only pulsed signals. In this way, the electronics in conjunction with the monochromator discriminate between the flame spectrum and sample emission. 

M Modulation of the source by using a beam chopper or by pulsing it electronically is widely used to convert the source radiation to an alternating form. 

Fig. 10.15 Three-channel AAS coupled with a microcomputer for processing of data. H hollow-cathode sources C rotating sector B burner M monochromator units P detectors J beam chopper and start/reset unit I spectrometer control panel A analogue/digital converter K remote terminal V video monitors. (Reproduced from [43] with permission of Elsevier).

Figure 2a. Schematic diagram of TIRF optical apparatus. The beam chopper is used to minimize the possibility of photobleaching during experiments. A thin layer of cyclohexanol is used to optically couple the prism to the glass slide. The fluorescent light is collimated and condensed by the lenses and detected by the photomultiplier. The color filter in front of the photomultiplier blocks scattered incident light and selectively transmits the fluorescence emission. (Reproduced with permission from Ref. 17. Copyright 1983 Academic Press.)...

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