Coincidence experiment

Coincidence experiments explicitly require knowledge of the time correlation between two events. Consider the example of electron impact ionization of an atom, figure Bl.10.7. A single incident electron strikes a target atom or molecule and ejects an electron from it. The incident electron is deflected by the collision and is identified as the scattered electron. Since the scattered and ejected electrons arise from the same event, there is a time correlation... 

Figure Bl.10.7. Electron impact ionization coincidence experiment. The experiment consists of a source of incident electrons, a target gas sample and two electron detectors, one for the scattered electron, the other for the ejected electron. The detectors are coimected tlirough preamplifiers to the inputs (start and stop) of a time-to-amplitiide converter (TAC). The output of the TAC goes to a pulse-height-analyser (PHA) and then to a nuiltichaimel analyser (MCA) or computer.

Figure Bl.10.8. Time spectrum ftom a double coincidence experiment. Tln-ough the use of a delay in the lines of one of the detectors, signals that occur at the same instant in botii detectors are shifted to tlie middle of the time spectrum. Note the unifonn background upon which the true comcidence signal is superimposed. In order to decrease the statistical uncertainty in the detemiination of the true coincidence rate, the background is sampled over a time Aig that is much larger than the width of the true coincidence signal. Ax.

Figure Bl.10.9. Plot of the reduced relative uncertainty of a double coincidence experiment as a fimction of the signal-to-background ratio. Note that the relative uncertainty decreases as the signal-to-background rate decreases.

Figure Bl.10.11. Electron impact double ionization triple coincidence experiment. Shown are the source of electrons, target gas, tluee electron detectors, one for the scattered electron and one for each of the ejected...

Figure Bl.10.12. Schematic diagram of a two-dimensional histogram resulting from the triple coincidence experiment shown in figure BLIP. 10. True triple coincidences are superimposed on a imifomi background and tliree walls corresponding to two electron correlated events with a randomly occurring third electron.

Optical Oscillator-Strength Measurements By Electron Spectroscopy E. Continuum Effects and (e,2e) Coincidence Experiments... 

Van der Wiel12 has pointed out that there are two inherent limitations to intensity (with given statistics) in the coincidence experiment. First, it can be seen (Fig. 11) that the statistical relative error... 

A number of optical photoion-photoelectron coincidence experiments have been reported107 giving qualitative information on molecular breakdown. However, in these experiments little or no attempt has been made to correct for either the transmission efficiency of the photoelectron spectrometer or the ion kinetic-energy discrimination. Fragment ions usually... 

Figure 31. Oscillator strength for dissociative ionization of N2 , From Wight et al. 25 broken curves, partial oscillator strengths for three dissociative ion states, with C state dissociative only from o = 3 up, which makes total C-state spectrum approximately 10% higher than spectrum shown here. The three broken curves combined equal total spectrum and give a best fit with other data points dissociative double ionization (N + + N+) is expected to set in at 48 eV o and A, from branching ratios measured in an electron-electron coincidence experiment.171 Chain curve (N2+) from El-Sherbini and Van der Wiel.108...

Brion94 and of Hamnett et al.m (2) the PES experiments of Samson and Gardner172 and Plummer et al.18 (3) the photofluorescence experiments of Judge and Lee189 and the recent extended measurements by Lee et al.190 and (4) a different type of measurement, the triple coincidence experiment by Backx et al.191 In this work coincidences were recorded between electrons of variable energy loss, CO+ ions, and photons from the process CO+(B22- A 22). The results of the various experiments shown in Fig. 39 indicate excellent quantitative agreement between the five different electron-spectroscopic methods. However, the photofluorescence measurements... 

An a-y coincidence experiment was performed using a cooled Si(Li) detector for the detection of photons and a Si detector for the detection of a-particles. Three parameter events were collected on tape and one dimensional spectra were later generated in coincidence with various gates. The spectra showed that the and a3Q are in prompt coincidence with L X-rays and the delay occurs at the 27.4 keV level. The analysis of the time spectrum between the group and the 27.4 keV photopeak gave a half-life of 38.3 - 0.3 ns, in agreement with previous measurements. 

In order to demonstrate the convenient properties of lenses combined with an electrostatic energy analyser, three examples will be discussed first the important case in which an acceleration/retardation lens is placed in front of the analyser which allows an operation mode with a constant pass energy in the analyser second the case in which increased acceptance angles are used, which is important for coincidence experiments and third the case in which 0 eV electrons are handled and analysed with high resolution and high acceptance. 

In order to derive information on dynamical quantities from such a coincidence experiment, one has to select angles d at which other components of the alignment tensor also contribute, because then the different dependences on the dipole matrix elements, including their relative phases, are involved. As can be seen from... 

Most of the notation used in these expressions is explained in connection with equ. (2.37) for the case of single ionization, but three further comments are in order which are specific for coincidence experiments ... 

From this relation it follows that a good, i.e., large, ratio of true to accidental coincidences requires a small coincidence resolving time At and a small source strength (f J<7). However, for small values of N all counting rates, Ix, l2, and /true, are small, and therefore the ratio r is not well suited as a criterion of the quality or feasibility of coincidence experiments. Indeed, a more appropriate figure of merit follows if the relative error a of true coincidences, defined by... 

Equs. (4.114) then describe the time required for an actual coincidence experiment to reach an accuracy a of true coincidences. This time proves to be a better figure of merit than the ratio r. Noting that N depends on the ratio r, one can take the difference between TcoU (r) and TcoU (r = 1), using equ. (4.114b), and normalize this quantity against coll. (r = 1), using equ. (4.114a). This yields... 

After this detailed treatment of true and accidental coincidences, all the statements derived so far for a good performance for coincidence experiments can be summarized ... 

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