Positron beam technique

C International conferences on slow positron-beam techniques... 

SLOPOS-5 Proceedings of the Fifth International Workshop on Slow Positron Beam Techniques for Solids and Surfaces (held at Jackson Hole, Wyoming, USA, 1992) edited by E. Ottewitte and A.H. Weiss, American Institute of Physics Conference Proceedings 303, New York (1993)... 

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. 

More recent experiments by Overton, Mills and Coleman (1993) may have gone some way towards resolving this dispute in favour of theory. As mentioned above, this group used the Texas technique to measure erPS but paid particular attention to obtaining a well-controlled positron beam,... 

Fig. 7.3. Schematic illustration of the timed and gated slow positron beam used by Nico et al. (1990) to measure the vacuum decay rate of ortho-positronium. Reprinted from Physical Review Letters 65, Nico et at, Precision measurement of the ortho-positronium decay rate using the vacuum technique, 1344-1347, copyright 1990 by the American Physical Society.

We now briefly review experimental evidence for the existence of some simple positronium compounds more detailed accounts have been given for early lifetime experiments by Goldanskii (1968) and for the liquid phase by Mogensen (1995). In the case of PsCl we shall see how traditional positron experiments using lifetime and ACAR techniques have provided strong evidence for the stability of this compound, in accord with theory. The first direct experimental evidence of the existence of PsH came from a positron-beam experiment (Schrader et al, 1992). 

A broad overview of traditional methods and recent developments in experimental positron spectroscopy is presented. A discussion of the generation and detection of positrons and their annihilation radiation is followed by a survey of techniques used for positron lifetime measurement, Doppler broadening spectroscopy and angular correlation of annihilation radiation, and the opportunities presented by combining these methods (e.g. in age-momentum correlation) and/or extending their capabilities by the use of monoenergetic positron beams. Novel spectroscopic and microscopic techniques using positron beams are also described. 

Suzuki, R., Ohdaira, T. and Mikado, T. (1998) Low-energy pulsed positron beam at the ETL linac facility Proc. Int. Workshop on Advanced Techniques of Positron Beam Generation and Control, RIKEN, Wako, Japan (Committee of Crossover Research Program for Basic Nuclear Science). 

The aim of this chapter is to introduce the reader to the application of positron annihilation techniques to polymers. An extensive review of the large volume of publications related to positron studies in polymers will not be presented. Rather it is intented to introduce the reader to the theory and techniques used in polymer studies and indicate the types of information that can be obtained about different polymer systems. The main focus of this chapter will be on the use of positron annihilation lifetime spectroscopy (PAL) in polymer studies. Chapter 11 discusses the use of monoenergetic slow positron beams used to study polymers surfaces. One of the interesting new developments in the application of positron annihilation techniques in polymers is the positron age-momentum correlation technique (AMOC). This technique promises to shed new light on the mechanisms of positronium formation and annihilation in polymer systems. A more detailed discussion of this technique can be found elswhere in this text. 

As more complex polymers systems have been studied it is evident that subtle effects in the interpretation of data and the underlining theory have come to the fore. Advances in polymerization chemistry, as well as, the enormous advances in techniques used in polymer characterization, has meant that PAL polymer studies can now be conducted on well characterized and controlled systems. There is no doubt that PAL will continue to be a valuble tool for the polymer scientist. The application of monoenergitic slow positron beams to polymers surfaces will be discussed in the next chapter. 

Speed and performance of positron age-momentum-correlation (AMOC) measurements have been improved significantly by using an MeV positrons beam and by taking advantage of the beam-based p+yAf r-coincidence technique [1-5]. 

Positron annihilation lifetime spectroscopy (PALS) allows the quantitative investigation of the polymer free volume [1, 2]. Additionally, the PALS beam technique makes a direct depth resolution possible, by implanting the probe - the positron - within a definite sample depth interval depending on the positron kinetic energy [3]. It is one of the very few nondestructive techniques for investi-... 

A particularly interesting field is the study of surface and near-surface properties using a slow, monoenergetic positron beam [see, e.g., Krause-Rehberg and Leipner, 1998]. This technique requires a variation of the energy of incident monoenergetic... 

P. K. Pujari, D. Sen, G. Amarendra, S. Abhaya, A. K. Pandey, D. Dutta, S. Mazumder, Study of pore structure in grafted polymer membranes using slow positron beam and small-angle X-ray scattering techniques, Nucl. Instrum. Meth. B., 254, 278-282 (2007). 

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. 

Pujari, P. K., et al. (2007). Study of Pore Structure in Grafted Polymer Membranes Using Slow Positron Beam and Small-Angle X-ray Scattering Techniques. Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms, 254(2), 278-282. 

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