Positrons source

Positrons can be used as particle probes, suitable to detect low concentrations of defects in materials. Positron physicists generally are in need of intense positron beams for applying positron annihilation techniques such as two dimensional (2D) Angular Correlation of Annihilation Radiation (ACAR) for investigating surfaces and interfaces of materials. The 2D-ACAR technique allows high resolution measurements of the electron momentum distribution for depth, localized defects, thin layer systems, and interfaces. In addition, a submicrometer size positron beam can be created for defect depth profiling on a lateral scale smaller than a micrometer. Vacancy type defects can be mapped in a three dimensional fashion. 

In the past, scientists were restricted to investigating bulk samples with lower intensity beams. The creation of intense positron beams ( 10 e s ) is feasible at research reactors under certain conditions. The main design problems are in coping with the hostile environment that hinders the sustainment of high quality vacuum and electrical fields in a position located close to the reactor core. 

The three methods for establishing a positron source at a research reactor are as follows  

To generate a 10 positrons per second, the minimum reactor power level is 100 kW with an available thermal neutron flux of lO n cm s and a gamma dose rate of 0.1 MGy h  

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When using an electron accelerator, fast positrons are produced by pair production from bremsstrahlung gamma-rays generated as the high energy electrons from the accelerator slow down in matter, whereas with cyclotrons and reactors, very intense primary positron sources are produced directly. Slow positron beams are then produced and transported using similar techniques to those described previously in this section. 

Recent progress in the study of positron scattering and positron annihilation processes is reviewed in Refs. [11,170-172]. Experimentally, the positron sources from radioisotopes and from electron accelerators are quite weak and have a broad spectrum of energies as compared with electron sources, making precise measurements of positron collision processes extremely difficult. Such measurements, however, have become possible recently due to the... 

Lanthanum laurates were prepared as in Ref. [2]. The spectra of the angular correlation of annihilation photons (ACAP) were measured in a standard long-slit geometry using 22Na radioactive isotope as a positron source. 

The rate of positronium formation has been increased by 2-3 orders of magnitude using recently developed accelerator based slow positron sources. This opens the possibility of improvements of precision experiments on the Ps atom as well as new experiments on excited states First evidence for enhanced metastable Ps formation is presented and future possibilities are discussed. 

The use of a moderator will also allow us to locate the positron source far enough from the trap to allow adequate shielding and minimze the effects of the decay gammas so that we will be able to use a 100 mCi source of 22Na. This is a 102 increase compared to the TJW experiment. A possible geometry of the source, traps and moderator is illustrated in Fig. 1. 

One of the main factors which needs to be considered in PAL analysis of polymers, is the affect which prolonged exposure to the positron source has on the lifetime parameters. It has been found that on prolonged exposure to a positron source, the o-Ps lifetimes are largely unchanged, but that there are significant variations in the o-Ps intensities for some polymers. Examples of these effects for a wide variety of polymers can be found polypropylene (PP), polyethylene (PE) [71], polystyrene [72], polycarbonates [73] poly(a-olefins) [49], poly(vinlyacetate) [74], poly(methyl methacrylate) [74] and a number of copolymers [75]. 

Figure 10.6 The dependence on exposure time to a positron source of the o-Ps intensity in polystyrene at different temperatures [72].

The positron source, 120 kBq of Na, was deposited onto a Kapton foil covered with identical foil and sealed. The foil 8 pm thick absorbed 10% of positrons in polyimides Ps does not form and annihilation in the source envelope gave one component only = 374 ps, which must be taken into account. The source was sandwiched between two samples of the material studied and placed into a container in a vacuum chamber. The source-sample sandwich was viewed by two Pilot U scintillators coupled to XP2020Q photomultipliers. The resolution of our spectrometer with a stop window broadened to 80% (in order to register the greatest number of three-quantum decays) was 300 ps FWHM. The finite resolution had no influence on the results of our experiment as FWHM was still comparable to the channel definition At = 260 ps.The positron lifetime spectra were stored in 8000 channels of the Tennelec Multiport E analyser. 

The experimental set-up consists of a positron source ( Na), a scintillation counter, to detect the y radiation from the positronium decay, and electronic peripheral equipment to analyse the time spectrum of the positron annihilation. 

The experimental arrangement of the Yale experiment is shown in Fig. 3. A Na positron source of about 15 mCi is placed inside a microwave cavity resonant in the TMno mode at 2.323 GHz and filled with N2 gas to a pressure between 0.25 and 3 atm. Eight Nal(Tl) detectors count in coincidence 0.5 MeV annihilation y rays emitted at 180°. The magnetic field of about 8 kG is varied across the resonance line as indicated in Fig. 4. The signal is the increase in Ps(2y) rate, and the linewidth is determined principally by the lifetime of the M=0 triplet state. 

Pair production eliminates the original photon, but two photons are created when the positron annihilates (see Sec. 3.7.4). These annihilation gammas are important in constructing a shield for a positron source as well as for the detection of gammas (see Chap. 12). 

In summary, it is clear that the o-Ps lifetime determined via the PALS technique provides accurate information on the apparent mean size of the nanoholes, which comprise the free volume in amorphous polymers. It also seems well established (see Chapter 11) that provided that the noise level in the PALS spectrum is sufficiently reduced, the distribution of o-Ps lifetimes can be obtained, which generates information regarding nanohole-size distribution. Concerns have been raised about the utility of the o-Ps intensity, I3, to characterize the number density of nanoholes and hence the fractional free volume via Eq. (12.2), because the value of I3 can be influenced significantly by the presence of species that inhibit or enhance positronium formation. We feel that we can utilize I3 values to evaluate fl actional free volumes via Eq. (12.2), provided either that the sample is rejuvenated by heating above Tg prior to measurement, and/or experiment indicates that the value of I3 remains constant within experimental error, during the time of exposure to the positron source. 

Figure 3. The experimental setup for the PALS measurements of sorption of liquid vapor in polymers. The positron source together with the sample polymer is contained in one arm of the glassware and the liquid to be sorbed is contained in the other arm. The positron lifetime measurement is performed by detecting the gamma-rays emitted from the source and from positron annihilation in the sample.

This work investigated PIM-1 membranes in the three states discussed above. Nickel-foil supported NaCl was used as a positron source and stacks of film samples, each about 1mm thick, were placed either side of the source. Annihilation lifetime decay curves were measured with an EG G Ortec fast-fast lifetime spectrometer. Measurements were made both in air and under an inert atmosphere (N2). However, o-Ps lifetimes in air were reduced due to quenching by oxygen, so only results obtained under N2 are discussed here. Results were analysed in terms of a four component lifetime distribution, which allowed obtaining better statistical fit. The two longest lifetimes, T3 and T4, for PIM-1 in... 

When positrons enter a material they annihilate with electrons in the material, giving rise coincidentally to two annihilation y-rays of -511 keV (equal to the electron or positron mass) at nearly 180° apart (Fig. 9.21). The annihilation occurs in a time (lifetime) after their emission from the positron source which depends on the density of electrons in the metal around the positron at the time of annihilation. The energy spread of the y-rays and the angle between them depend on the momentum of the annihilated electron. There are several experimental methods that can be used to measure these quantities including positron lifetime, positron annihilation... 

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