Sum peaks

More sophisticated pulse sequences have been developed to detect nuclear modulation effects. With a five-pulse sequence it is theoretically possible to obtain modulation amplitudes up to eight times greater than in a tlnee-pulse experunent, while at the same time the umnodulated component of the echo is kept close to zero. A four-pulse ESEEM experiment has been devised to greatly improve the resolution of sum-peak spectra. 

Fig. 4.27. Artifacts in energy-dispersive X-ray spectra. Occur renceof(a) escape and (b) sum peaks.

Once we have the appropriate nuclide, we must separate the radiation of interest from all other radiation present. A typical gamma spectrum is shown in Figure 3 for cobalt-57 in palladium. The radiations which can be identified include the 6-k.e.v. x-ray, the 14-k.e.v. y-ray of interest, and a sum peak and palladium x-ray peak, both lying at about 21 k.e.v. If one now sets the single-channel analyzer window correctly, one observes essentially only the 14-k.e.v. peak, but all of this is not recoil-free radiation it includes other radiation which falls into the window from various gamma quantum de-excitation processes. 

Hoffmann and Knozinger therefore state that the free hydroxyl vibration in fact is a superimposition of the free hydroxyl vibration and the geminal vibration. The absence of geminal hydroxyls at high temperatures explains the peak narrowing of this sum-peak . ... 

Figure 8.31 Artifacts in an EDXRF spectrum. The spectrum of pure iron, measured with a Si(Li) detector, shows a peak lower in energy than the Fe Kq. peak by an amount exactly equal to the energy of the Si Kq, line. Some of the Fe photon energy is transferred to the Si detector atoms the amount of energy absorbed by an Si atom has escaped from the Fe photon. This t)rpe of peak is called an escape peak. Sum peaks also appear in EDXRF spectra when two intense photons arrive at the detector simultaneously. A sum peak from two Kq, photons is shown along with a sum peak from one Kq and one Kys photon. [Courtesy of Thermo ARE (www.thermo ARL.com).]...

Sum peaks in the EDXRF spectrum occur when two high-intensity peaks arrive so close in time that the signal processing electronics cannot separate them. A single peak is registered at an energy that is the sum of the two peaks. Figure 8.31 displays this type of artifact The major elements in the sample (e.g., iron in steel) are usually the source of the sum peaks. 

Most EDXRF systems come with software that automatically corrects for escape and sum peaks. 

Predict what sum peaks you might see as artifacts in the EDXRF spectmm of pure Cu. 

The previously cited sum peaks occur for two or more coincident gamma rays, for example, at 2505 keV for °Co. Interactions outside the detector commonly are detected as a peak at 511 keV due to annihilation radiation and at about 200 keV due to Compton scattering at 180°. Gamma rays produced by cosmic-ray interactions in or near the detector are observed as discussed in Section 8.2.2. 

Solomon (16,has uset a different method to obtain extinction coefficients. Essentially, total hydrogen content from elemental analysis and hydroxyl content from measurements of the area of the 0-H stretching band near 3450 cm were used in conjunction with the peak areas of aliphatic and aromatic bands to obtain a plot from which extinction coefficients can be determined. In principle, this approach appears to be sound, but there are a number of problems. One difficulty, discussed above, is general to all infrared methods that have been employed so far what errors are introduced by summing peak areas over a number of bands, each of which has an individual extinction coefficient, and essentially averaging such coefficients for the total area Other problems involve the correct use of curve resolving techniques and the measurement of hydroxyl groups, which we will now consider in more detail. 

Another subject which will be of interest for those who wish to apply nuclear chemistry for analytical purposes, is the sum peak method . The prindple of this method is based on a perturbed angular correlation (PAC) for two y-emissions in cascade decay from a radioactive nucleus. The emission angle betweoi the two y s has a distribution pattern which reflects the mode of radiative decay, as well as depetuling on the environmental conditions. The sum peak which is seen in a y-ray spectrum as a result of simultaneous detection of the twoy-rays as one event, is therefore influenced by the environments in which the source is placed. In the sum peak method, intendty ratios of the stun peak to the single peak can be used and chan in the ratios due to the environments can be observed. 

The sum peak method, if carefully used in the systems accompanied by chemical or biochemical change, is a useful tool for detecting such change and it may also be a convenient method to determine perturbed angular correlation (PAC) parameters mthout complicated electronic circuits. 

Sum peak formation is observed when two y-rays emitted from a nucleus enter simultaneously into the detector. This is because cascade emission of the y-rays in the disintegration of the nucleus is measured as one coincidence count within the detector resolution time. Figure 19 shows a y-ray spectrum of in which displays two single peaks ofy (171.3 and 245.4 keV) and a sum peak at 416.7 keV. The sum peak... 

A description of some basic considerations on the formation of a sum peak in a detector of cylindrical type may be useful. When a point source and a cylindrical detector are jdacol as in the geometrical arrangement in Fig. 20, the sum peak intensity ratio is given by the following relation... 

Recently, Kudo et al. proposed that when the time differential PAG function W(0, t) is taken into account, the observed sum peak intensity is approximately expressed as follows ... 

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