Positron beam, slow

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. 

Although there have been many technical advances in this area, no one facility has yet emerged as significantly superior to any other. A brief overview of such facilities around the world can be found in the Proceedings of the Sixth International Workshop on Slow 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.

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

SLOPOS-2 Proceedings of the MURR Slow Positron Beam Workshop (held at Columbia, Missouri, USA, 1985) edited by D.C. Reichel and W.B. Yelon... 

SLOPOS-3 Proceedings of the International Workshop on Slow Positron Beams for Solids and Surfaces (held at Norwich, UK, 1986) edited by P.G. Coleman and A.B. Walker... 

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)... 

Mills Jr., A.P. (1980). Brightness enhancement of slow positron beams. Appl. Phys. 23 189-191. 

All of the systematic tests [l, 10] to date support the results of reference [1]. The 1900 ppm difference with theory remains unresolved at this time. A new and systematically very different experiment designed to reach 100 ppm accuracy [12] using a slow positron beam with Ps formation in an evacuated cavity is now underway. Results are expected within one year. 

The first approach has been made by LYNN et al. 17/, who used a neutron activated Cu source with half-life of 12.8h to obtain a high specific activity of 600 Ci/g and produced a slow positron beam of 4 x 107 e+/s. The moderation of high energy positrons appearing in the bremsstrahlung of electron accelerators has been investigated at first in Mainz /8/ and Livermore /9/. A picture of the present setup at Mainz is shown in Fig. 1. 

The first reactor-based slow positron beam was developed by Lynn at Brookhaven [13]. In this system, housed in the reactor building, a copper ball was irradiated in the reactor core (63Cu(n,y)64Cu) and transferred automatically into the source chamber. It was dropped into a cmcible and evaporated on to a tungsten backing. The strong MCu source therefore acted as a self-moderator for the production of slow positrons. At the Munich reactor positrons are produced by pair production by gamma rays emitted in the reaction "3Cd(n,Y)"4Cd [14]. 

It is of great interest to note that after Cherry s observation of positron emission from a solid surface the first beam system—developed in the late 60 s—was based at a LINAC facility [15]. Bremsstrahlung gamma radiation from the energetic (50MeV) LINAC electrons create electron-positron pairs in a Ta target the fast positrons thus created are then moderated (see section 8) to form the slow positron beam. The efficiency of this process clearly depends on the LINAC electron energy and the thickness of the converter. 

Suzuki, R., Ohdaira, T., Uedono, A. and Kobayashi, Y. (in press) Positron annihilation in Si02-Si studied by a pulsed slow positron beam , Appl. Surf. Sci. 

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. 

Hamada, E., Oshima, N., Katoh, K., Suzuki, T., Kobayashi, H., Kondo, K., Kanazawa, I., Ito, Y. (2001) Application of a pulsed slow-positron beam to low-density polyethylene film . Acta physicapolonicaA. 99,373. 

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... 

AR293 Ion and slow positron beam utilization, OECD Proceedings, Workshop proceedings, Costa da Caparica, Portugal, 15 17 September 1998. 

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). 

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