Positron annihilation spectrometry

Another relatively recent technique, in its own way as strange as Mossbauer spectrometry, is positron annihilation spectrometry. Positrons are positive electrons (antimatter), spectacularly predicted by the theoretical physicist Dirac in the 1920s and discovered in cloud chambers some years later. Some currently available radioisotopes emit positrons, so these particles arc now routine tools. High-energy positrons are injected into a crystal and very quickly become thermalised by... 

Spectroscopy, 490. See also 13C NMR spectroscopy FT Raman spectroscopy Fourier transform infrared (FTIR) spectrometry H NMR spectroscopy Infrared (IR) spectroscopy Nuclear magnetic resonance (NMR) spectroscopy Positron annihilation lifetime spectroscopy (PALS) Positron annihilation spectroscopy (PAS) Raman spectroscopy Small-angle x-ray spectroscopy (SAXS) Ultraviolet spectroscopy Wide-angle x-ray spectroscopy (WAXS)... 

It is more than likely that the electron with which the positron annihilates will be bound to an atom. It is necessary, therefore, for some energy to be shared with the atom in order to remove the electron. This means that the energy available to be shared between the annihilation quanta will be lower than expected. For example, in aluminium the annihilation radiation has been estimated to be 510.9957 keV instead of the theoretical 511.0034keV. In everyday gamma-ray spectrometry, the difference is unlikely to be noticed. What is certainly noticeable is the extra width of annihilation gamma-ray peaks due to Doppler broadening, the reason for which I explained in Chapter 1 (Section 1.2.2). 

The techniques most commonly used in thermo-oxidative studies on polymers are mainly based on thermal analysis methods such as thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and on pyrolysis-gas chromatographic studies (particularly if they are linked to complimentary techniques such as mass spectrometry or infrared spectroscopy). Other techniques that have been used to a much lesser extent include chemiluminescence analysis, nuclear magnetic resonance (NMR) spectroscopy, electron spin resonance, and positron annihilation lifetime mass spectrometry. 

The principle techniques used in thermooxidalive studies are based on thermal analysis methods such as thermogravimetric analysis and differential scanning calorimetry and on methcxds based on polymer pyrolysis followed by gas chromatography and mass spearometry and/or infrared spectroscopy of the volatiles produced. Other techniques which have have been include nuclear magnetic spectroscopy, electron spin resonance spectroscopy and meilxids based on chemiluminescence and positron annihilation lifetime mass spectrometry. 

Free volume of a polymer is widely recognized to be a factor of crucial importance for the gas transport in polymeric membranes. The techniques of spin probes [58-60], photo- and electrochromic probes [49, 52, 60], inverse gas chromatography [41, 62-64] and positron annihilation [50-51,55, 65-66] were employed in the investigations of the free volume of glassy polymers. Below we report the results provided by low temperature nitrogen adsorption and Xe NMR spectrometry techniques [67]. 

MacKenzie, I.K. (1983). Experimental methods of annihilation time and energy spectrometry. In Positron Solid-State Physics, Proc. Int. School of Physics Enrico Fermi , Course 83, eds. W. Brandt and A. Dupasquier (North-Holland) pp. 196-264. 

Isotopic. Fluorine-18 exhibits a half-life of 112 minutes and emits 0.65-M.e.v. positrons, with which are associated the 0.51-M.e.v. 7-annihilation radiation. Samples are most simply assayed as solids or liquids with a Geiger counter, but positive identification of the isotope is best carried out by spectrometry of the 7-radiation combined with decay measurements over a period of at least 8 hours (i.e., four half-lives of F ). 

The steep decline up to 2000 keV, the normal spectrometry range, is due to Compton events, backscatter and bremsstrahlung resulting from the decay of muons into high-energy electrons and positrons. On this are superimposed the 511 keV annihilation radiation and all the gamma-ray peaks from the background nuclides and the peaks from activations described below. 

ANNIHILATION In the context of ganuna spectrometry, the meeting of a positron and an electron resulting in their disappearance, their mass being converted to photon energy. 

---