Constant-acceleration drives

X-Ray Diffraction (XRD) patterns were recorded on a Philips HTK-KC diffractometer with a CuKa X-ray source, linked to Philips 386 computer. Thermo-Gravimetry (TG), was recorded on a Setaram TG-DTA 92 thermobalance. Diffuse Reflectance Spectra (DRS) were recorded on a Cary-5 spectrofotometer with a BaS04 integration-sphere in the UV-VIS-NIR region. Mossbauer spectra are recorded on a vertical constant acceleration drive in transmission geometry with a 28mCi Co(Rh) source. Isomer shift data are expressed relative to metallic Fe at 293 K which has an isomer shift of8 = -0.0888 mm/s relative to natural a-Fe. 

One of the more difficult experimental aspects of Mossbauer spectroscopy is the accurate determination of the absolute velocity of the drive. The calibration is comparatively easy for constant-velocity instruments, but most spectrometers now use constant-acceleration drives. The least expensive method, and therefore that commonly used, is to utilise the spectrum of a compound which has been calibrated as a reference. Unfortunately, suitable international standards and criteria for calibration have yet to be decided. As a result, major discrepancies sometimes appear in the results from different laboratories. The problem is accentuated by having figures quoted with respect to several different standards, necessitating conversion of data before comparison can be made. However, calibration of data from an arbitrary standard spectrum will at least give self-consistency within each laboratory. 

Fig. 2.12 Absolute calibration of constant-acceleration drives using (a) diffraction grating, (b) optical interferometer.

When applying Newton s law to a moving automobile, acceleration depends on the excess of power over that required for constant-speed driving, namely P -P,.. From this it follows that the instantaneous acceleration (a) of the vehicle at a given road speed (V) is... 

Mossbauer spectra are usually recorded in transmission geometry, whereby the sample, representing the absorber, contains the stable Mossbauer isotope, i.e., it is not radioactive. A scheme of a typical spectrometer setup is depicted in Fig. 3.1. The radioactive Mossbauer source is attached to the electro-mechanical velocity transducer, or Mossbauer drive, which is moved in a controlled manner for the modulation of the emitted y-radiation by the Doppler effect. The Mossbauer drive is powered by the electronic drive control unit according to a reference voltage (Fr), provided by the digital function generator. Most Mossbauer spectrometers are operated in constant-acceleration mode, in which the drive velocity is linearly swept up and down, either in a saw-tooth or in a triangular mode. In either case. 

Fig. 3.2 Triangular velocity reference signal top) and drive error signal bottom) of a Mossbauer drive operating in constant acceleration mode. The error signal is taken from the monitor output F of the drive control unit (see Fig. 3.1). Usually it is internally amplified by a factor of 100. Here, the deviations, including hum, are at the 2%o level of the reference. The peaks at the turning points of the triangle are due to ringing of the mechanical component, induced by the sudden change in acceleration (there should be no resonance line at the extremes of the velocity range)...

The Mossbauer transmission spectra were recorded in the constant acceleration mode with an Elscint Mossbauer drive unit and a model MFG 3A Elscint function generator, an MVT-3 linear velocity transducer and an MD-3 transducer driving unit, y-ray detection was done with a Reuter-Stokes Kv-CH4 proportional counter driven by an Ortec 401A/456 high voltage power supply. Voltage pulses were introduced into an Ortec 142 PC preamplifier and an Ortec 571 spectroscopy amplifier. Data were collected on a Tracor-Northern NS-701A multichannel analyzer. The data were later analyzed on an IBM 360/370 computer. 

Fe Mossbauer spectra were obtained using a digital constant acceleration spectrometer having a symmetrical triangular velocity drive waveform. A 10 m Ci 5 Fe in Pd source was used and all experiments were carried out at room temperature. The... 

The normal experimental technique is to scan rapidly through the velocity range and repeat this scan many times imtil data of the required accuracy has been accumulated. The Doppler motion is provided by an electromechanical drive system controlled by a servo -amplifier. Usually, the source is attached to the drive shaft and driven either in a saw-tooth or a triangular constant acceleration wave form. The transducer is coupled to a multichannel analyser operating in the multiscaler mode, and the servo-amplifier is controlled by the channel advance frequency. The dwell time in each channel, corresponding to a specific velocity increment, is 100 ps, and while the channel gate is open it accepts pulses from the detector. 

The Mossbauer measurement requires the generation of a precise, controllable relative motion between the source and the absorber. A large variety of drive systems has been developed. The majority of drives work on electromechanical, mechanical, hydrauHc, and piezoelectric principle. The spectrometers can be classified into constant-velodty spectrometers and velocity-sweep spectrometers. The mechanical drives, hke a lead screw or a cam, move with constant velocity. They have advantages for the thermal scan method and because their absolute velocity calibration is straightforward. The velocity-sweep spectrometers are usually of electromechanical nature (like loudspeaker-type transducers) and normally used in conjunction with a multichannel analyzer. The most commonly used M(t) functions are rectangular (constant velocity), triangular (constant acceleration), trapezoidal, and sinusoidal. A typical Mossbauer spectrometer is shown schematically in O Fig. 25.24. 

The product of force F and the rolling radius (R) of the tires on the drive wheels is the wheel torque (T). Power depends on both torque and rotational speed (N). By definition, power is given by P = 2tiNFR = 27tNT. Wlien driving at constant speed, the driver adjusts the accelerator pedal so the drive-wheel power exactly matches the power required (P,) to overcome the resistance of the vehicle (discussed later in this article). To accelerate the vehicle, the driver further depresses the accelerator pedal so that the power available at the drit c wheels (PJ exceeds P,.. 

From no load to full load, the drop in speed of compound-wound motors is approximately 25%. Compound-wound motors are used where reasonably constant speed is required and for loads where high starting torque is needed to accelerate the drive machine. 

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

To simplify the discussion, let us reconsider the velocity profiles shown in Fig. 17.3 for the basic cyclone configuration where little or no rear drive flow is present. As reactants are entrained out of the front jet, conservation of mass and momentum requires that the axial velocity of the outer jet decrease with increasing distance from the front drive. But, as the reactants are converted to products when they cross the flame sheet, mass is added at fixed area to the product flow. This causes the axial velocity of the products to accelerate toward the exhaust nozzle as in classic constant-area heat addition. Thus, one expects... 

Let s say you are driving your car onto the expressway. To increase the speed of your car from 30 to 65 mph, you press down on the accelerator pedal. Having reached a velocity of 65 MPH, you ease off the accelerator pedal to maintain a constant speed. Why Well, according to Newton s second law of motion, it takes more energy to accelerate your car than to keep it in motion. 

Simulations of RNA secondary structure landscapes provide insight into the necessary mutation rate to drive adaptation. Huynen etal. (1996) found that the ability of a population to adapt is determined by the error threshold of the fitness and not the sequence. Indeed, they found that any mutation rate greater than zero will cause the population to drift on the neutral network [The error threshold on landscapes with high neutrality approaches zero (Derrida and Peliti, 1991).] A second, higher mutation threshold causes the fitness information to be lost. To accelerate the diffusion of the population on the neutral network, it is necessary to be above the sequence error threshold and as close to the fitness error threshold as possible. Under these criteria, the population will diffuse rapidly without losing fitness information. On a flat landscape, the diffusion constant D0 for a population of M sequences of length N can be approximated by Eq. (37). 

---