Excited nuclei

Neutron capture always is exothermic, because the neutron is attracted to the nucleus by the strong nuclear force. Consequently, neutron capture generates a product nuclide in a metastable, excited state. These excited nuclei typically lose energy by emitting either y rays or protons ... 

Greatly enhanced sensitivity with very short measuring time is the major advantage of PFT (pulse Fourier transform) experiments. In the CW (continuous wave) experiment, the radiofrequency sweep excites nuclei of different Larmor frequencies, one by one. For example, 500 s may be required for excitation over a 1-KHz range, while in a PFT experiment a single pulse can simultaneously excite the nuclei over 1-KHz range in only 250 jits. The PFT experiment therefore requires much less time than the CW NMR experiment, due to the short time required for acquisition of FID signals. Short-lived unstable molecules can only be studied by PFT NMR. 

When the pulse is switched off, the excited nuclei return slowly to their original undisturbed state, giving up the energy they had acquired by excitation. This process is known as relaxation. The detector is switched on in order to record the decreasing signal in the form of the FID (free induction decay). You can observe the FID on the spectrometer s computer monitor, but although it actually contains all the information about the NMR spectrum we wish to obtain, it appears completely unintelligible as it contains this information as a function of time, whereas we need it as a function of frequency. 

In spin-lattice relaxation, the excited nuclei transfer their excitation energy to their environment. They do so via interaction of their magnetic vectors with fluctuating local fields of sufficient strengths and a fluctuation frequency of the order of the Larmor frequency of the nuclear spin type. Depending upon the atomic and electronic environment of a nucleus in a molecule and the motion of that molecule, there are five potential mechanisms contributing to spin-lattice relaxation of the nucleus. 

Fourier transform NMR spectroscopy, on the other hand, permits rapid scanning of the sample so that the NMR spectrum can be obtained within a few seconds. FT-NMR experiments are performed by subjecting the sample to a very intense, broad-band, Hl pulse that causes all of the examined nuclei to undergo transitions. As the excited nuclei relax to their equilibrium state, their relaxation-decay pattern is recorded. A Fourier transform is performed upon this relaxation-decay pattern to provide the NMR spectra. The relaxation-decay pattern, which is in the time domain, is transformed into the typical NMR spectrum, the frequency domain. The time required to apply the Hl pulse, allow the nuclei to return to equilibrium, and have the computer perform the Fourier transforms on the relaxation-decay pattern often is only a few seconds. Thus, compared to a CW NMR experiment, the time can be reduced by a factor of 1000-fold or more by using the FT-NMR technique. 

It is very important that there be sufficient time between pulses in FT-NMR experiments so that the nuclei can return to the original equilibrium state. If the equilibrium state has not been reached before the next H pulse, the still-excited nuclei will not participate in the transition and thus will produce a decreased signal intensity relative to the previous signal. As the experiment proceeds and more pulses are applied, more nuclei will remain in the exited state until eventually none of the nuclei will be in the lower energy state when pulsed. At this point the sample is saturated and will not produce a signal. The length of time required for the nuclei to relax is called the spin-lattice or T relaxation time. 

Excited nuclei that have attained statistical equilibrium will decay into different products in proportion to the number of states available to the whole system after the decay. The different decays are often called channels, and we speak of the probability to decay into a given channel. A very schematic representation of the energy levels and the energies involved in the decay of an excited nucleus into various channels is shown in Figure 6.20. The total sum of the probabilities for decay into all channels is, of course, one. We can simply count the number of states available for a decay channel and obtain a general expression for the relative probability, P(e, n), for an excited nucleus to emit a portion with size n, requiring an energy e. The expression is... 

The processing of all foods by high-energy radiation beams (electron, x-ray, or 7-ray beams) will produce excited nuclei of atoms that will emit... 

In an on-flow NMR experiment, the excited nuclei leave the flow cell whereas fresh nuclei enter. Due to the decrease of the apparent Tmow rates, faster pulse repetition rates can be used and more transients in a distinct time-period can be accumulated (Figure 1.4). The theoretical maximum sensitivity is obtained... 

Figure 1 depicts a typical sequence of events started by absorption of an incident photon with an energy near the nuclear excited state energy Eq. The Fe nucleus has an excited state lifetime of 141ns, and excited nuclei have two decay channels. About 10% of them reemit a 14.4kev photon. For recoilless absorption, where no vibrational levels are excited, time-resolved measurements of 14.4kev photons scattered in the forward direction reveal information on hyperfine interactions comparable to conventional Mossbauer spectroscopy (see Mossbauer Spectroscopy). The remaining nuclei expel electrons from the atomic K shell, followed by... 

---