Coincidence technique

Rather sophisticated applications of Mossbauer spectroscopy have been developed for measurements of lifetimes. Adler et al. [37] determined the relaxation times for LS -HS fluctuation in a SCO compound by analysing the line shape of the Mossbauer spectra using a relaxation theory proposed by Blume [38]. A delayed coincidence technique was used to construct a special Mossbauer spectrometer for time-differential measurements as discussed in Chap. 19. 

It is apparent that in both cases energy E is deposited in [AB+ +eej] and that, as in the case of excitation, the photon energy is analogous to the electron energy loss. However, since there are now two electrons sharing the excess energy in electron-impact ionization, it is necessary to use time correlation (coincidence techniques) for the simulation of photoionization... 

Figure 29. Relative cross sections for collisional dissociation of CHjBr in selected vibrational-energy states as function of kinetic energy. Data were obtained by photoion photoelectron coincidence technique and were analyzed by assuming only backscattering in center of mass. Maxima of curves were normalized to same relative cross section.86b...

Smith, A.J., Read, F.H. and Imhof, R.E. (1975). Measurement of the lifetimes of ionic excited states using the inelastic electron-photon delayed coincidence technique. J. Phys. B At. Mol. Phys. 8 2869-2879. 

Conventional decay-schemes studies do not seem appropriate, because of the complexity of the decay schemes, the errors to which they are subject (cf. the discussion of 8 Br above) and the large amount of time needed to carry them out. Direct measurement of the p-strength functions themselves, utilizing total-absorption y spectrometry and, where relevant, delayed-neutron-gamma coincidence techniques, promises to provide a means of producing the necessary information in a reasonable time. 

Lifetimes of levels in Ra, Ac, and Ac have been measured by delayed coincidence techniques and these have been used to determine the El gamma-ray transition probabilities. The reduced El transition probabilities in Z25Ra and 225Ac are about two orders of magnitude larger than the values in mid-actinide nuclei. On the other hand, the El rate in 2 7Ac is similar to those measured in heavier actinides. Previous studies suggest the presence of octupole deformation in all the three nuclei. The present investigation indicates that fast El transitions occur for nuclei with octupole deformation. However, the studies also show that there is no one-to-one correspondence between El rate and octupole deformation. 

Recently, the development of extremely high resolution solid state Ge(Li) gamma-ray detectors, the application of gamma-gamma coincidence techniques, and the availability of low cost 14 MeV neutron generators have renewed interest in activation analysis as a means for routine rapid and non-destructive elemental abundance determinations of both major and trace elements. Some of these new techniques will be discussed in the sections that follow. 

Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

A discussion of the coincidence technique with some general applications has been published by Wahlgren, Wing and Hines 71>. Many of the early applications of the technique made use of the fact that 64Cu is one of the few radionuclides produced by thermal neutron irradiation for which the 0.511 MeV positron annihilation photopeak is a prominent feature of the spectrum. Copper has been determined in meteorites 72> and copper ores 73,74) ]-,y coincidence counting of 04Cu annihilation radiation. The rapid and selective nature of the determination may have important applications in the on-line sorting of copper ores. 

Greenland 75> has discussed the application of cascade gamma-ray coincidence techniques to the determination of cesium and cobalt in silicate rocks. Bromine has been determined in stony meteorites by this technique, using the cascade gamma-rays of 82Br 76). 

The coincidence technique has also been applied to the activation analysis of biological materials 79 82) and forensic materials 83>. 

The use of 14 MeV neutron activation principally for major elements, Ge(Li) detectors for trace elements following thermal neutron irradiations, and gamma-gamma coincidence techniques for positron or cascade gamma-ray emitters as discussed in the previous sections, provide the analyst with powerful tools for devising schemes for non-destructive analysis. A few additional activation techniques which may be useful in special applications are discussed briefly below. In most of these cases rather sophisticated instrumentation is required. It is unlikely, therefore, that these techniques will be in routine use in a facility devoted principally to analytical applications. In some cases, however, arrangements may be made for part time use of a more extensive nuclear facility for a specific analytical problem. 

The positrons that arrive at the formation foil share the time structure of the electron accelerator, giving 2 ps long pulses of about 104 slow positrons at 600 Hz. Since a 7-ray detector would be saturated, the coincidence technique cannot be used, giving an order of magnitude worse signal-to-noise ratio than that in the previous experiments (due to 7 scintillations in the Lyman-a photo-multiplier), but the higher data rate more than compensates for this in total time to reach a given precision. 

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