Backscattering lifetime

In the case of a single lifetime with no interference from ambient or backscattered light only a single frequency is necessary to determine the lifetime of the luminescence. Determination of the phase and modulation at multiple frequencies is necessary to characterize complex decays in fiberoptic sensors. 

Platinum and palladium porphyrins in silicon rubber resins are typical oxygen sensors and carriers, respectively. An analysis of the characteristics of these types of polymer films to sense oxygen is given in Ref. 34. For the sake of simplicity the luminescence decay of most phosphorescence sensors may be fitted to a double exponential function. The first component gives the excited state lifetime of the sensor phosphorescence while the second component, with a zero lifetime, yields the excitation backscatter seen by the detector. The excitation backscatter is usually about three orders of magnitude more intense in small optical fibers (100 than the sensor luminescence. The use of interference filters reduce the excitation substantially but does not eliminate it. The sine and cosine Fourier transforms of/(f) yield the following results ... 

A sharp peak at about 6 ns, occurs when backscattered positrons pass the grid and reach the CEMA and trigger timing pulses without the secondary electron time of flight. This 6 ns peak vanishes in statistical noise in the case of samples that cause longer lifetimes. In the lifetime analysis, the data in the 6 ns peak region are ignored in the present discussion. 

More information can be extracted when the data are deconvoluted from the experimental resolution and the backscattering component and separate lifetimes are extracted with PATFIT to obtain the experimental resolution function and MELT to obtain lifetime distributions as shown in Fig 7.19 for the case of 23% and 80% porogen load. The probabilities were scaled to the peak value for the component at 0.5 ns and are enhanced by factors of 10 and 200 for lifetimes larger than 2 ns and 7 ns respectively. 

The Mossbauer apparatus consists of an emitter, an absorber, and a y-ray detector. In a typical Mossbauer experiment, which can be performed either in transmission or in backscattering mode, a radioactive source is mounted on a velocity transducer which imparts a smoothly varying motion to the souree of the y-rays (relative to the absorber, which is held stationary), up to a maximum of several cm/s (Fig. 1.42). In practice, a source is needed which decays to the excited state of the nucleus of interest with a sufficiently long lifetime such that experiments are practical. The source usually consists of nuclei in the excited state which are obtained... 

---