Gamma photon generators

Apart from the highly specific case of deuterium and beryllium, whose (r,n) reactions are characterized by exceptionally low thresholds (2.23 MeV for the former and 1.67 MeV for the latter), the determination of all other elements by this method requires high energy gamma photons (10 to 40 MeV). These are obtained by bombarding a suitable metal target (Ta, W, Pt, Au, Hg etc) by accelerated electrons. This yields Brems- 

At present, most photon activation analysis laboratories use linear accelerators that generate very intense gamma photon beams with maximum energy adjustable between a few MeV and 30 to 50 MeV. 

Betatrons, microtrons and Van de Graaff accelerators are also used. The latter can generate electron beams of very high intensity (several mA) with energies limited to 10 MeV. 

This makes them unsuitable for (7,n) activation analyses, especially to determine carbon, nitrogen or oxygen. Van de Graaff accelerators are quite useful to determine deuterium or beryllium by direct detection of the neutrons generated by 

The microtron is a circular electron accelerator with particularly attractive possibilities but is still rarely used. It is rugged, easy to operate and incurs lower operating costs than a linear accelerator at equivalent performance. This gives it significant advantages over the latter. 

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At still higher energies, let us say 100 MeV and beyond, we move from nuclear physics to the domain of particle physics. In this arena unstable particles are created which release gamma photons as part and parcel of their decay process. When two protons of very high energy run into one another, they sometimes generate neutral pi mesons which then decay to gamma photons. 

The most commonly used photons in radiation chemical studies are the gamma rays generated by the disintegration of Co nuclei. At the average energy of these photons, 1.2 MeV, Compton absorption predominates. In Compton absorption, photon absorption is followed by ejection of the most loosely bound electron and emission of a photon of lesser energy ... 

In the other limiting case, one of the major components has an extremely large capture cross section, so all the neutrons are absorbed close to the surface in the sample, and every neutron generates gamma photons according to the emission probabilities of the capturing nuclide. The observed count rate does not depend on the cross section any more, but it is directly proportional to the number of neutrons, i.e., the real flux multiplied by the emission probability ... 

Figure 4.5 Position Annihilation Loss Spectroscopy (PALS) utilizes the gamma-ray photons generated by positron annihilation to interrogate the pore stmcture of ULK films.

Due to the conversion process an absorbed photon give rise to less than one electron generated in the CCD. This phenomenon, also called a "quantum sink" shows that the detector is degrading the S/N ratio of the image. The quality of an image being mainly limited by the quantum noise of the absorbed gamma this effect is very important. 

The Mossbauer effect, discovered by Rudolf L. Mossbauer in 1957, can in short be described as the recoil-free emission and resonant absorption of gamma radiation by nuclei. In the case of iron, the source consists of Co, which decays with a half-life of 270 days to an excited state of Fe (natural abundance in iron 2%). The latter, in turn, decays rapidly to the first excited state of this isotope. The final decay generates a 14.4 keV photon and a very narrow natural linewidth of the order of nano eV. 

In the strictest sense, the term gamma ray is applicable only to photons produced as a result of transitions in atomic nuclei. However, the term is also sometimes used to denote bremsstrahlung radiation produced when the high energy electrons in the beam of an electron accelerator, such as an electrostatic generator, a betatron, a synchrotron, or a linear accelerator, strike the target of that accelerator. 

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