Delayed coincidence technique

As in the previous method the delayed-coincidence technique also uses short laser pulses for the excitation of selected levels. However, here the pulse energy is kept so low that the detection probability Fo of a fluorescence photon per laser excitation pulse remains small (Fd 0.1). If Fo(0df is the probability of detecting a fluorescence photon in the time interval Mo / -h d/ after the excitation then the mean number npi(0 of fluorescence photons detected 

The repetition rate / of the excitation pulses is chosen as high as possible since the measuring time for a given signal-to-noise-ratio is proportional to 1//. An upper limit for / is determined by the fact that the time T between two successive laser pulses should be at least three times the lifetime Xk of the measured level k). This technique is therefore ideally suited for excitation with mode-locked or cavity-dumped lasers. There is, however, an electronic bottleneck the input pulse rate of a TAC is limited by its dead time to and should be smaller than 1/td. It is therefore advantageous to invert the functions of the start and stop pulses. The fluorescence pulses (which have a much smaller rate than the excitation pulses) now act as start pulses and the next 

The experimental realization is shown schematically in Fig. 11.38. Part of the laser pulse is sent to a fast photodiode. The output pulse of this diode at t = to starts a Time-Amplitude Converter (TAC) which generates a fast rising voltage ramp U(t) = (t-to)Uo. A photomulitplier with a large amplification factor generates for each detected fluorescence photon an output pulse that triggers a fast discriminator. The normalized output pulse of the discriminator stops the TAC at time t. The amplitude U(t) of the TAC 

More information about the delayed-coincidence method can be found in [11.101]. 

This method also uses short excitation pulses. Contrary to the previous method, however, the detection probability is kept below one fluorescence photon per excitation pulse and the repetition rate of the excitation pulse is chosen as high as possible. Single photon counting techniques are used (see Sect.4.5.4), which measure the time distribution of the probability P l (t)dt that a fluorescence photon is emitted within a time interval dt between t and t + dt after the excitation pulse at t = 0 [11.18a]. 

Schematic diagram of lifetime apparatus with mode-locked laser 

bcnemacic diagram or ntet ana singTe-photon counting techniques 

Such high pulse frequencies cannot be used to start the TPC which has a dead time of at least 100 ns after a start pulse. Therefore the fluorescence photons (counting rate about 10-100 KHz) are used to start the ramp and the subsequent laser pulse stops it. This corresponds to a time reversal and yields the probability distribution Pj (T-t) if T is the constant time interval between successive mode-locked pulses. 

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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. 

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. 

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. 

A more direct method for lifetime measurements is the delayed coincidence technique [6] in which the time between an initiation event and the emission of a decay product is measured. A schematic diagram of an apparatus used for the measurement of atomic lifetimes is shown in figure BLIP.5. The slope of the graph of the natural log of the number of decay events as a function of time delay gives the lifetime directly. The... 

For ionization by cw lasers, quadrupole mass spectrometers are generally used. Their disadvantage is the lower transmittance and the fact that different masses cannot be recorded simultaneously but only sequentially. At sufficiently low ion rates, delayed coincidence techniques in combination with time-of-flight spectrometers can even be utilized for cw ionization if both the photoion and the corresponding photoelectron are detected. The detected electron provides the zero point of the time scale and the ions with different masses are separated by their differences Afa = ion — tQ in arrival times at the ion detector. 

Fig.9.23. Principle of lifetime measurements using the delayed-coincidence technique... 

Time-resolved fluorescence spectroscopy is also a valuable tool in biological and medical research [10.143-147]. Since the lifetimes involved are normally short, picosecond spectroscopy techniques are frequently employed (Sect. 9.4). Examples of fluorescence decay curves for tissue recorded with delayed coincidence techniques employing a frequency-doubled picosecond dye laser are depicted in Fig. 10.42. The decay characteristics allow the discrimination between tumour and normal tissue, and atherosclerotic plaque and normal vessel wall, respectively. General surveys of the use of LIF for medical diagnostics can be found in [10.148,149]. 

Delayed-Coincidence Techniques. This method operates in the extremely low intensity regime, in which single-photon counts axe recorded. The principle is illustrated in Fig. 9.22. 

Principle of the technique. The delayed-coincidence technique, which is well known in nuclear physics, was first applied to the measurement of the lifetimes of excited atoms by Heron et al, (1954, 1956). However, it was not widely used until Bennett (1961) improved the method... 

This multicliannel delayed-coincidence technique was developed by Bennett (1961) and has since been applied to detailed studies of the lifetimes and collision-induced relaxation of excited levels of many different atoms, molecules, and ions. An excellent review of this method is given by Bennett et al. (1965). In the following sections we discuss various details of the experimental technique, methods used for the reduction of the data, and experimental difficulties. 

In beam-foil experiments the velocities would be so great that no decay would be observed in any apparatus of convenient laboratory size. Similarly in the single-photon delayed-coincidence technique, the time required to obtain sufficient data would become quite prohibitive. The few reliable lifetime measurements that do exist have been made by the static afterglow technique. This was originally developed for experiments on the collisional destruction and diffusion of metastable atoms, which are discussed in detail in section 7.6. The difficulties encountered in the application of the afterglow and other methods to the experimental determination of the transition probabilities of forbidden lines have been reviewed by Corney (1973) and Corney and Williams (1972). 

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