Instruments positron annihilation

Positron Annihilation Lifetime Spectroscopy (PALS). A fast-fast PALS system using cylindrical (40 mm diameter x 15 mm thick) Bap2 scintillators (14) arranged at 90 to each other to avoid pulse pile up problems. A count rate of 150-300 cps was achieved with a a source and a instrument resolution of 220-240 ps FWHM for a... 

Instrumentation used in the measurement of polymer crystallinity includes light scattering, positron annihilation lifetime spectroscopy, differential scanning calorimetry, differential thermal analysis, infrared spectroscopy, NMR spectroscopy and wide and small angle x-ray diffraction (Chapter 14). 

At present, most PET scanners can acquire in both a two-dimensional as well as a three-dimensional mode, whereas SPECT cameras measure in a three-dimensional mode. The physical property of the dual-positron gamma-rays emission lends itself to mathematical reconstruction algorithms to produce three-dimensional images in which the calculations are much closer to exact theoretical ones than those of SPECT. This is, in part, due to the two-photon as opposed to single-photon approach. PET can now achieve resolutions, for example in animal-dedicated scanners, in the order of 1 or 2 mm. The resolution is inherently limited theoretically only by the mean free path or distance in which the positron travels before it annihilates with an electron, e.g. those in biological water 2-8 mm. SPECT, although achieving millimeter resolution with the appropriate instrumentation, cannot quite achieve these levels. 

Figure 16.23. Two examples of neutralino models that provide a good fit to the excess of cosmic ray positrons observed by the HEAT collaboration. The two sets of data points (open and filled squares) are derived from two different instruments flown in 1994-95 and 2000. The lines represent (i) the best expectation we have from models of cosmic ray propagation in the galaxy ( bkg. only fit ), which underestimate the data points above 7 GeV (ii) the effect of adding positrons from neutralino annihilations (lines SUSY component , SUSY+bkg. fit , and bkg. component , the latter being the resulting background component when the data are fitted to the sum of background and neutralino contributions). (Figures from Baltz, Edsjo, Freese, Gondolo(2002)).

A small dose of a soluble fast-decay positron-emitting artificial radioisotope (produced as needed not too far from the PET instrument 6C11, 8015, 9F18 or 37Rb82) is put into human tissue (e.g., blood) the positron typically travels about 1 mm, meets an electron from within the human body, and the pair decays into two y photons of energy 0.51 MeV each, within microseconds to nanoseconds. Two spin states are possible for the positron— electron ion pair before their annihilation singlet and triplet. The annihilation rate for the triplet state depends sensitively on the electron density of the body tissue. Two y counters are set in coincidence mode, and several hundred thousand coincidence events are used to provide valuable tissue information (in addition to a CT scan). 

Figure 21.15 shows a patient undCTgoing a PET scan of the brain. The instrument actually detects gamma radiation. When a nucleus emits a positron within the body, the positron travels only a few millimeters before it reacts with an electron. This reaction is an example of the annihilation of matter (an electron) by antimatter (a positron). Both the electron and the positron disappear and produce two gamma photons. The gamma photons easily pass through human tissue, so they can be recorded by scintillation detectors placed around the body. You can see the circular bank of detectors in Figure 21.15. The detectors record the distribution of gamma radiation, and from this information a computer constructs images that can be used by the physician.

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