Radio frequency cavity

Fig. 3. Principle of linear accelerator (linac). Partially accelerated electrons from a source, such as a Cockcroft-Walton generator, arc further accelerated by stages as rhe electrons pass through radio-frequency cavities, powered by if oscillators. Each paiticle receives a small push as it passes from one cavity to the next until the final desired accelerated beam is produced Tile machine must be carefully synchronized CSG = Cockcroft-Walton generator, RFO = radio-frequency oscillator RFC = radio-frequency cavity...

The field strength and the frequency are dependent on the equipment, while the dielectric constant, dissipation factor, and loss factor are material-dependent. The electric field strength is also dependent on the location of the material within the microwave/radio-frequency cavity (Turner and Ferguson, 1995), which is one reason why domestic microwave ovens have rotating turntables (so that the food is exposed to a range of microwave intensities). This mechanism is the major one for the generation of heat within materials by these electromagnetic fields. 

Moisture analyzers include a large variety of designs listed here. The list includes their inaccuracies (1) electrolytic hygrometer (2-5% FS), (2) capacitance (3% FS), (3) impedance (3% FS), (4) piezoelectric (10% AR or 2 ppm by volume), (5) heat of adsorption, (6) infrared (0.5-1% FS), (7) microwave (for a 1-15% moisture range, error is within 0.5%, less if corrected for density), (8) Karl Fischer titrator (0.5-1% FS), (9) drying oven (0.5-1% FS), (10) dipole, (11) cavity ring down, (12) fast neutron (0.2% in solid s density corrected), and (13) radio-frequency absorption (5 ppm). 

A radio-frequency (rf) cavity that restores the energy the particles lose while emitting SR and, thus, maintains constant beam energy. 

The next big advance towards higher precision was the 1997 phase-coherent measurement of the frequency gap with an optical frequency interval divider chain [27]. The 2.1 THz gap was no longer measured by counting cavity fringes, but divided down to the radio frequency domain by a phase-locked chain of five optical frequency interval dividers [56] (see Fig. 5). The accuracy of this approach was limited by the secondary frequency standard to 3.4 parts in 1013, exceeding the accuracy of the best previous measurements by almost two orders of magnitude. 

Figure 18-5 shows a block diagram of the ENDOR apparatus. This equipment was first used in conjunction with a Varian E-12 EPR spectrometer, but is meant to show the general features of apparatus that could be used with any EPR spectrometer. The EPR cavity (Figure 18-1) was modified for the ENDOR experiment with the inclusion of a hair-pin loop around the Cryo-Tip. The loop is fed with a 50 0 transmission line from a 50-100 Watt broadband radio-frequency (rf) amplifier and is terminated with a non-inductive, water cooled 50 O resistance. 

Hydration of biopolymers is a mechanism for stabilizing these materials (Fig. 2.78). When proteins are conpletely dry, they tend to decompose. One way of evaluating hydration in polyions is to measure the dielectric constant of a solution containing a dissolved protein as a function of concentration at radio frequency. The dielectric constant falls with increase in concentration and the water per polyion can be calculated by assuming that water bound to the protein no longer makes any contribution to the dielectric constant. Thus, Buchanan calculated the irrotationally bound water from such expaiments. Some of this water is hidden in cavities within the structure of the protein molecule. 

Working with an NMR spectrometer is slightly more difficult than, for instance, with an electron microprobe. As the NMR frequencies are all in the radio frequency range, NMR spectrometers are computer operated radio transmitting and receiving systems controlled by a computer. A receiver and transmitter antenna surrounds the sample, which is located in the cavity of a super-conducting cryomagnet. 

MRI is noninvasive to the body, requires no use of radioactive substances, and is quick, safe, and painless. A person is placed in a cavity surrounded by a magnetic field, and an image (based on the extent of radio frequency energy absorption) is generated, stored, and sorted in a computer. Differences between normal and malignant tissue, atherosclerotic thickening of an aortal wall, and a host of other problems may be seen clearly in the final image. 

---