Annihilation radiations

X-Rays and Annihilation Radiation. The interaction of y-rays with matter produces the x-rays that are characteristic of the atoms in the material in which the interactions take place. Such x-rays appear in measured spectmm. 

ZerstraUtmg, /. annihilation radiation, zerstreuen, v.t. disperse, scatter, disseminate, dissipate, diffuse divert, distract. — zer-streut, p.a. dispersed, etc. diffuse disperse abstracted, distracted, zerstreutporig, a. diffuse-porous. 

The other method for quality assurance inspection of pellet wt in the primer eliminates the need for a comparator oxygen-containing standard. Here, the Cu in the cup-anvil combination in the primer is used as an internal standard by comparing the 0.511 MeV positron annihilation radiation from 62Cu produced by the 63Cu(n,2n)62Cu reaction to the 6.1 MeV 7 from l6N produced by oxygen activation. In this case the actual determination of pellet wt is not required the ratio of Cu to O, which should be fixed for a pro-... 

Annihilation (Positron-Electron)—An interaction between a positive and a negative electron in which they both disappear their rest mass, being converted into electromagnetic radiation (called annihilation radiation) with two 0.51 MeV gamma photons emitted at an angle of 180° to each other. 

Kobayasi, T. (1994) Fourier inversion formalism for the calculation of angular correlation of positron annihilation radiation of semiconductors, Bull. Coll. Med. Sci. Tohoku Univ., 3, 11-22. 

Radionuclidic analyses are performed with either a lithium-drifted germanium or intrinsic germanium detector. The assay for Sr-82 is based upon its 777 keV photon of 13.6% abundance. Strontium-85, which is often present in amounts comparable to that of Sr-82, is assayed by its 514 keV photopeak, which must be resolved from prominent 511 keV annihilation radiation by a curve stripping procedure (12). 

Recent results of SMM on the galactic 511 keV annihilation radiation (1) exhibit a constant flux during 4-5 years, in total disagreement with post 1980 balloon... 

As far as we can see into the Universe, we don t observe any primordial antimatter. Within the limits of our present observational horizon the Universe is seen to contain only matter and no antimatter. The presence of cosmic antimatter would lead to observable traces of annihilation however the measurements of the extragalactic 7 ray flux indicate an absence of annihilation radiation, and the microwave background spectrum lacks a corresponding distortion. These findings preclude the existence of a significant amount of antimatter within tens of Megaparsecs, which is the scale of super-clusters of galaxies. 

In this section we introduce three techniques frequently encountered in positron physics, namely those used to measure annihilation lifetimes and the Doppler broadening (or Doppler shift) and angular correlation of the annihilation radiation. These techniques, or variants thereof, are encountered throughout the rest of this work, and here we briefly describe... 

Fig. 6.15. Cylindrically averaged angular correlation of annihilation radiation (ACAR) distributions for positron annihilation in the noble gases, (a) helium, (b) neon, (c) argon, (d) krypton and (e) xenon, from the work of Coleman et al. (1994). Reprinted from Journal of Physics B27, Coleman et al, Angular correlation studies of positron annihilation in the noble gases, 981-991, copyright 1994, with permission from IOP Publishing.

In this chapter we consider the physics of the positronium atom and what is known, both theoretically and experimentally, of its interactions with other atomic and molecular species. The basic properties of positronium have been briefly mentioned in subsection 1.2.2 and will not be repeated here. Similarly, positronium production in the collisions of positrons with gases, and within and at the surface of solids, has been reviewed in section 1.5 and in Chapter 4. Some of the experimental methods, e.g. lifetime spectroscopy and angular correlation studies of the annihilation radiation, which are used to derive information on positronium interactions, have also been described previously. These will be of most relevance to the discussion in sections 7.3-7.5 on annihilation, slowing down and bound states. Techniques for the production of beams of positronium atoms were introduced in section 1.5. We describe here in more detail the method which has allowed measurements of positronium scattering cross sections to be made over a range of kinetic energies, typically from a few eV up to 100-200 eV, and the first such studies are summarized in section 7.6. 

Gamma-ray spectral interferences. This is important in cases where the 0.511 MeV annihilation radiation from the product radionuclide is measured in the determination. Obviously, all other reactions yielding positron emitters would provide an interference. In the determination of oxygen a spectral interference is produced if the sample contains an appreciable amount of boron. The uBe produced by the 11B( , )11Be decays with the emission of an 11 MeV negatron and also gamma-rays... 

Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

A discussion of the coincidence technique with some general applications has been published by Wahlgren, Wing and Hines 71>. Many of the early applications of the technique made use of the fact that 64Cu is one of the few radionuclides produced by thermal neutron irradiation for which the 0.511 MeV positron annihilation photopeak is a prominent feature of the spectrum. Copper has been determined in meteorites 72> and copper ores 73,74) ]-,y coincidence counting of 04Cu annihilation radiation. The rapid and selective nature of the determination may have important applications in the on-line sorting of copper ores. 

Mayer-Hasselwander, H. A. et al. 1998. High-energy gamma-ray emission from the Galactic Center, A A335, 161 Merritt, D., Milosavljevic, M., Verde, L., Jimenez, R. 2002. Dark Matter Spikes and Annihilation Radiation from the Galactic Center, Phys. Rev. Lett.88, 191301... 

The fifth type of radioactive emission, gamma radiation, does not result in a change in the properties of the atoms. As a result, they are usually omitted from nuclear equations. Gamma emissions often accompany other alpha or beta reactions—any decay that has an excess of energy that is released. For example, when a positron collides with an electron, two gamma rays are emitted, a phenomenon usually referred to as annihilation radiation. 

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. 

---