Lifetime start signal

Figure Bl.10.5. Lifetime experiment. A pulser triggers an excitation source that produces excited species in the sample. At the same time the pulser provides the start signal for a time-to-amplitude converter (TAG). Radiation from the decay of an excited species is detected with a photomultiplier at the output of a monochromator. The signal from the photomultiplier is amplified and sent to a discriminator. The output of the discriminator is the stop signal for the TAG. The TAG output pulse amplitude is converted to a binary number by a pulse height analyser (PHA) or analogue-to-digital converter (ADG). The binary number serves as an address for the multichannel analyser (MGA) that adds one to the number stored at the specified address. The pulser rate must accommodate decay times at least a factor of 10 longer than the lifetime of the specie under study. The excitation source strength and sample density are adjusted to have at most one detected decay event per pulse.

In the course of the positronium lifetime measurements, radioactive isotopes of positron decay serve as sources, e.g., sodium-22, copper-64. At the moment of the emission of the positron from this source a y-photon is also released. (In the case of, e.g., sodium-22 its energy is 1.28 MeV.) This y-photon serves as the start signal in the coincidence equipment used. The y-photon produced by the 2y-annihilation process to be studied (0.51 MeV) is the stop signal. The magnitude of the time measured between the start and stop signals (the positronium lifetime) is in the range 10 —10 s. To get a lifetime curve of adequate statistics, the apparatus repeats the time measurement about 10 — 10 times. For the details of the experimental technique see, e.g., refs. [De 53, Fe 56, Go 71a]. 

In olden times the lifetime distribution of short-lived nuclides was measured without a multichannel analyzer using the method of delayed coincidences. In this case, the start signals were delayed and the coincidence rate was plotted against the time of delay. The resulted graph showed the characteristic features of the exponential law, fi om which the decay constant could be determined by calculating the slope of the semilogarithmic (semilog) plot. 

Woodward achieved his first signal success of a lifetime devoted to the preparation of increasingly complex natural products by total synthesis by the successful preparation of quinine. Despite its elegance, this synthesis did not provide a commercially viable alternative to isolation of the drug from chincona bark. A rather short synthesis for this drug from readily available starting materials has been only recently developed by the group at Hoffmann-LaRoche. (The economics of this synthesis are,... 

The output from the TAC is an analog signal that is proportional to the time difference between the start and stop pulses. The next step consists of digitizing the TAC output and storing the event in a multichannel analyzer (MCA). After repeating this process many times, a histogram of the arrival times of photons is accumulated in the memory of the MCA. In fluorescence lifetime spectroscopy the histogram usually contains 512-2048 channels... 

---