Delayed coincidence experiments

O.M. Williams, W.J. Sandle, A pile-up gate generator for removing distortion in multichannel delayed coincidence experiments, Phys. Sci. Instrum. 3, 741-743 (1970)... 

It was then realized that these different decay rates can be measured directly in delayed-coincidence experiments if suitably polarized light is used. Time-resolved experiments by Dodd et ai.(1970) and also by Deech and Baylis (1971), which are similar to those described in section 15.8, have confirmed the original measurements made by Omont. 

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.

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. 

The techniques used in the three measurements of the 23S —23Pj, J = 0,1,2 intervals are summarized in Figure 8. In all of these experiments the initial state is the 23S i state formed from positrons striking a metal target with about 100 eV kinetic energy. The first two measurements [15] [16] detected the transition as a 243 nm Lyman-a photon in delayed coincidence with a detected 7 ray from the annihilation of orthopositronium. The most recent and most precise experiment [17], which we detail below, uses only the Lyman-a detection. 

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). 

The basic principle of the experiment of Canter, Mills and Berko (1975) was to collide low energy positrons with a surface and to look for coincidence between a Lyman-a photon and a delayed gamma-ray arising from the subsequent annihilation of a 13S positronium. The presence of the Lyman-a signal was verified by the use of three interference filters with pass bands centred on, just above, and just below, 243 nm. An enhanced coincidence rate was found with the 243 nm filter in place. A similar Lyman-a gamma-ray technique has been adopted by all subsequent workers in this field (e.g. Laricchia et al., 1985 Hatamian, Conti and Rich, 1987 Ley et al., 1990 Schoepf et al., 1992 Steiger and Conti, 1992 Hagena et al., 1993 Day, Charlton and Laricchia, 2000). 

While solid matrices have been employed successfully, they may be less than ideal for controlled mechanistic studies. A more appropriate technique for controlled doublet photochemistry appears to be two-photon excitation in solution. In this experiment, the first photon is used to initiate radical ion formation, whereas the second photon, appropriately delayed to coincide with the maximum concentration of the radical cation so generated and tuned to its absorption maximum, serves to excite these intermediates. However, we hasten to add that the benefits of this technique have yet to be demonstrated. The photoinduced rearrangement of radical cations very likely will benefit substantially from a mismatch between (quartet vs. doublet) potential surfaces, much as triplet sensitized isomer-izations can be ascribed to mismatches between triplet and ground state surfaces. 

Figure 5.2b is the pulse sequence for a 13C experiment. The sequence in the 13C channel is exactly the same as the sequence in the H channel in Figure 5.2a. The protons are decoupled from the 13C nuclei by irradiating the protons during the experiment that is, the proton decoupler is turned on during the entire experiment. In other experiments, the decoupler for a given nucleus can be turned on and off to coincide with pulses and delays in another channel (i.e., for another nucleus). This process is termed gated decoupling. (See Sections 4.2.5 and 4.4)...

I found that now, too, I had a qualitatively different sensation in my fingertips. Then as I tried stronger stimulation of the finger ends, I experienced a peculiar phenomenon that I had never noted before nor have I noted it since, under any conditions. If you watch as you touch a tabletop with your finger, you will notice that the time when you hit it, as determined visually, and the time when you feel it are in essential coincidence. However, under this drug, I found that I first hit the table, and then felt it the feeling was a very definitely delayed phenomenon. I experimented with this for a half hour or more.. 

The values of /i, determined from Equation 5.25 may than be used in Equation 5.26 to determine the effective diffusivity. Note that the first moment of Equation 5.26 coincides with the time delay in the step injection experiments (Equation 5.24). 

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