Pulse Signal Detection

I Current is amplified and measured as series of pulses, with each pulse signaling detection of a radioactive particle or ray. 

Fig. 2.6.1 Schematic of an experiment with remote detection. The basic steps are (a) the polarization of the sensor medium, (b) NMR or MRI encoding using rf pulses and magnetic field gradients and (c) signal detection. The NMR or MRI information travels between the locations (b) and (c).

Bolometric signals, as we said, are modulated at a frequency co. Very rarely bolometers are used to detect pulsed signals or steady-state radiation levels. 

The signal detectable after a pulse of duration Tpulse, is given by (25), which for the time-independent Hamiltonian (i.e., under the static condition) yields... 

Re-evaluation of pulse delay times used to record fullerene 13C NMR spectra revealed that a 16 s pulse delay, twice the value for a standard detection, allowed the observation of a weak resonance in the sp3 region at 90.4 ppm in the 13C NMR spectrum of the unlabeled heterofullerene 114. Attempts were made to optimize the NMR experimental parameters for a long 7 i, i.e. the variation of delay times and pulse angles. Various conditions were tried on the labeled material without success. This is probably due to the mixture of the labeled and unlabeled 114 which give too low S/N for signal detection. Table 49 summarizes the NMR results obtained and illustrates a distinct pattern of the azafullerenes. 

Suppression of instrumental imperfections and/or selection of particular signal components are both based on the technique of phase cycling which exploits the dependence of NMR signals on the variations of the RF phases of the transmitter pulse(s) S (since phase-cycling is used in every branch of NMR, we assume that the reader is acquainted with the technique) (we will provide more information later, while discussing signal detection methods). At this point we just wish to point out that phase-cycling is extensively used in FFC and has to be supported by the console hardware - a requirement which implies pulser control of RF phases. 

Pull anti-ringing quadrature cycle is a bit more complex, while extension of the anti-ringing pulse technique to signal detection sub-sequences other than the simple FID is quite simple. 

In order to optimize the inversion, it is a good idea to make this pulse a composite one (except, maybe, in the case of rigid solid samples). As far as signal detection is concerned, all methods are acceptable so that, for example, IR preparation can be combined with a simple FID detection just as well as with the CPMG detection. Likewise, it is easy to combine IR with the balanced PP preparatory sub-sequence. 

The classical Jeener Broekaert sequence (133) is used to determine the dipolar-order relaxation time (in systems of spin 1/2 nuclides) and the Tiq relaxation time (in systems with spin 1 nuclides) of spin 1 nuclides with quadrupolar contributions to 7. Its FFC version is similar to the Inversion Recovery, except that the first 180° pulse is replaced by the sequence 90, — 5 — 45, the detection pulse becomes 45 and a special phase cycle is required. We shall not dwell on the details and purpose of the sequence since they go beyond the scope of this chapter. We wish to underline, however, the fact that sequences of this type require a close coordination of the preparatory sub-sequence with the signal-detection sub-sequence in order to isolate not just a particular magnetization component but a particular relaxation pathway. 

Fig. 2. N HSQC spectrum of a 75 mM solution of Pro -cyclosporin in CDCI3 at natural isotope abundance using the pulse sequence of fig. 1 without N decoupling during acquisition, t = 5.7 ms, SL = 2.5 ms. An additional, short spin-lock pulse was used right before signal detection [8]. The projections are shown at the top and on the left. (Reproduced by permission of Academic Press from...

In reality the individual lines obtained after the Fourier transformation are composed of both absorptive A(f) and dispersive D(f) components. This non-ideality arises because of a phase shift between the phase of the radiofrequency pulses and the phase of the receiver, PHCO, and because signal detection is not started immediately after the excitation pulse but after a short delay period A. Whereas the effect of the former is the same for all lines in a spectrum and can be corrected by a zero-order phase correction PHCO, the latter depends linearly on the line frequency and can be compensated for by a first-order phase correction PHCl. Both corrections use the separately stored real and imaginary parts of the spectrum to recalculate a pure absorptive spectrum. 

Fig. 2.4 outlines the concept of pulsed NMR, including the formation of transverse magnetization My, by the rf pulse (b), followed by the free induction decay (c) and the corresponding time-dependent signal detectable in the resonance and off-resonance situation (d, e). 

Most of the data are acquired in the same way as described above. K atoms are excited to the 29s and 27d states by the laser excitation, 4s —> 4p — 29s, 27d. The atoms are allowed to collide for 1 //s, after which a rapidly rising detuning pulse is applied, followed by the more slowly rising field ionization pulse. Atoms which have made the transition to the 29p state are selectively ionized by the field ionization pulse and detected. This signal is monitored as the small static tuning field is scanned. The amplitude and phase of the rf field are changed as parameters. 

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