Delayed protons

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Delayed Proton and Neutron Decays. By means of a variety of nuclear reactions, as weh as the spontaneous fission of synthetic nucHdes, large numbers of isotopes of some elements have been produced. For example, whereas the only stable isotope of Cs (Z = 55) is Cs (JV = 78), ah of the Cs isotopes from Cs where 77 = 59 and = 0.57 s, to Cs where N = 93 and = 0.13 s, have been observed. At the low mass end of this series, the last proton is only loosely bound, and at the high mass end, the last neutron is only loosely bound. 

The population of high-lying unbound states by (3 decay is an important feature of nuclei near the driplines. 3-Delayed proton emission and (3-delayed neutron emission have been studied extensively and provide important insight into the structure of exotic nuclei. 

Laurie was one of the first to apply two-dimensional (2D) NMR to carbohydrates. With students Subramaniam Sukumar and Michael Bernstein, and visiting scientist Gareth Morris, he demonstrated and extended the application of many of the directly observed 2D NMR techniques of the time. These included the homo- and hetero-nuclear 2D /-resolved techniques, delayed proton /-resolved NMR that allowed broad resonances to be suppressed, for example, those of dextran in the presence of methyl /Lxvlopyranoside. proton-proton chemical shift correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), proton-carbon chemical shift correlation (known later as HETCOR), and spin-echo correlated spectroscopy (SECSY). Trideuteriomethyl 2,3,4,6-tetrakis-<9-trideuterioacetyl-a-D-glucopyranoside served as a commonly used model compound for these studies. 

Alpha decay is observed for heavy nuclei with atomic numbers Z > 83 and for some groups of nuclei far away from the line of P stabihty. Radionuclides with very long half-lives are mainly a emitters. Proton emission has been found for nuclei with a high excess of protons far away from the line of P stability and more frequently as a two-stage process after p decay P delayed proton emission). 

More frequently, p emission occurs after decay in a two-stage process P decay leads to an excited state of the daughter nuclide, and from this excited state the proton can easily surmount the energy barrier. This two-stage process is called jff -delayed proton emission. It is observed for several P emitters from to " Ti with N = Z - 3, with half-lives in the range of 1 ms to 0.5 s. Simultaneous emission of two protons has been observed for a few proton-rich nuclides, e.g. Ne ti/2 10 ° s). Proton decay from the isomeric state is observed in case of Co (probability 1.5%, q/2 0.25 s). 

Radioactive decay by proton emission is a very seldom observed decay mode for very neutron deficient nuclides because decay by /S or EC normally has a very much shorter partial half-life ( 4.14). Decay by p" has been observed for " 0 E 1.55 MeV, ti/ 0.25 s, -1.5%). However, jS decay sometimes leads to a proton-unstable excited state which immediately (< 10 s) emits a proton. Several 0 emitters from to Ti with N = Z — 3 have /S delayed proton emission with half-lives in the range 10 — 0.5 s. Also radioactive decay by simultaneous emission of two protons has been observed for a few proton rich nuclides, e.g. Ne, 6 10 ° s. 

On the proton-rich side there appear P-delayed proton and alpha emissions. From these the delayed proton emission occurs more often, with >120 cases known. P-delayed two-proton emission has also been observed in several (>8) cases. 

A number of uncommon decay modes exist which are of little direct relevance to gamma spectrometrists and I will content myself with just listing them delayed neutron emission, delayed proton emission, double beta decay (the simultaneous emission of two 3 particles), two proton decay and the emission of heavy ions or clusters , such as and Ne. Some detail can be found in the more recent general texts in the Further Reading section, such as the one by Ehmann and Vance (1991). 

Alexiev, U., MoUaaghababa, R., Scherrer, R, Khorana, H.G., and Heyn, M.R, Rapid long-range proton diffusion along the surface of the purple membrane and delayed proton transfer into the bulk, Proc. Nad. Acad. Sci. USA, 92, 372, 1995. 

Because both spins are in the transverse plane and transition energy levels are matched, energy can be transferred from the protons to the nuclei. In this manner the rate of repolarization is controlled by rather than by Because the protons can interchange energy by spin-diffusion only a single-proton exists and its value is usually on the order of 1 s. As a result the preparation delay can be reduced from 10 s to about 5 s increasing the number of transients, which can be acquired by two or more orders of magnitude. 

Fig. 7. A C-13 relaxation time measurement of solid state wetted cellulose acetate (6% by weight water) using the inversion recovery (IR) method at 50.1 MHz and spinning at 3.2 kHz at the magic angle (54.7 deg) with strong proton decoupling during the aquisition time (136.3 ms), (upper part of the Figure). Tau represents the intervals between the 180 deg (12.2 us) inverting and 90 deg (6.1 us) measuring pulse. 2200 scans were collected and the pulse delay time was 10 s, Cf. Table 3 and Ref.281...

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

The relaxation rates of the individual nuclei can be either measured or estimated by comparison with other related molecules. If a molecule has a very slow-relaxing proton, then it may be convenient not to adjust the delay time with reference to that proton and to tolerate the resulting inaccuracy in its intensity but adjust it according to the average relaxation rates of the other protons. In 2D spectra, where 90 pulses are often used, the delay between pulses is typically adjusted to 3T] or 4Ti (where T] is the spin-lattice relaxation time) to ensure no residual transverse magnetization from the previous pulse that could yield artifact signals. In ID proton NMR spectra, on the other hand, the tip angle 0 is usually kept at 30°-40°. 

It is important to avoid saturation of the signal during pulse width calibration. The Bloch equations predict that a delay of 5 1] will be required for complete restoration to the equilibrium state. It is therefore advisable to determine the 1] values an approximate determination may be made quickly by using the inversion-recovery sequence (see next paragraph). The protons of the sample on which the pulse widths are being determined should have relaxation times of less than a second, to avoid unnecessary delays in pulse width calibration. If the sample has protons with longer relaxation times, then it may be advisable to add a small quantity of a relaxation reagent, such as Cr(acac) or Gkl(FOD)3, to induce the nuclei to relax more quickly. 

The whole sequence of successive pulses is repeated n times, with the computer executing the pulses and adjusting automatically the values of the variable delays between the 180° and 90° pulses as well as the fixed relaxation delays between successive pulses. The intensities of the resulting signals are then plotted as a function of the pulse width. A series of stacked plots are obtained (Fig. 1.40), and the point at which the signals of any particular proton pass from negative amplitude to positive is determined. This zero transition time To will vary for different protons in a molecule. 

Hence it is clear that if the two delay periods before and after the 180° pulses are kept identical, then refocusing will occur only when a selective 180° pulse is applied. This can happen only in a heteronuclear spin system, since a 180° pulse applied at the Larmor frequency of protons, for instance, will not cause a spin flip of the C magnetization vectors. 

The INEPT experiment can be modified to allow the antiphase magnetization to be precessed for a further time period so that it comes into phase before data acquisition. The pulse sequence for the refocused INEPT experiment (Pegg et al., 1981b) is shown in Fig. 2.13. Another delay, A. is introduced and 180° pulses applied at the center of this delay simultaneously to both the H and the C nuclei. Decoupling during data acquisition allows the carbons to be recorded as singlets. The value of Z), is adjusted to enable the desired type of carbon atoms to be recorded. Thus, with D, set at V4J, the CH carbons are recorded at VsJ, the CH2 carbons are recorded and at VeJ, all protonated carbons are recorded. With D3 at %J, the CH and CH ( carbons appear out of phase from the CH2 carbons. 

The basic INEPT spectrum cannot be recorded with broad-band proton decoupling, since the components of multiplets have antiphase disposition. With an appropriate increase in delay time, the antiphase components of the multiplets appear in phase. In the refocussed INEPT experiment, a suitable refocusing delay is therefore introduced that allows the C spin multiplet components to get back into phase. The pulse sequences and the resulting spectra of podophyllotoxin (Problem 2.21) from the two experiments are given below ... 

Fiffire 5.38 Pulse sequence for delayed COSY—a modification of the COSY experiment. The fixed delays at the end of the evolution period t and before the acquisition period <2 allow the detection of long-range couplings between protons. 

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